Chapter Surface Studies of Pulsed Laser Irradiated Semiconductors

CHAPTER 7 SURFACE STUDIES SEMICONDUCTORS

OF

PULSED LASER IRRADIATED

D. M. Zehner

. . . . .. .. .. .. .. .. .. .. .. .. .. . . . . . .. .. .. .. .. . .. .. .. .. .. .. .. .. .. .. .. .. ... ... ........

I. INTRODUCTION. 11. EXPERIMENTAL APPROACH 1. Sampl e P r e p a r a t i o n 2. C h a r a c t e r i z a t i o n Techniques. 111. PRODUCTION OF ATOMICALLY CLEAN SURFACES 3. S i l i c o n . 4. Germanium. 5. Group 111-V Compounds. GEOMETRIC SURFACE STRUCTURE IV. 6. Ordered Surfaces 7. Metastable Surfaces. 8. V i c i n a l Surfaces 9. Defects. SURFACE AND SUB-SURFACE STUDIES OF V. ION-IMPLANTED SILICON 10. S u b s t i t u t i o n a l Implants. 11. I n t e r s t it i a1 Imp1a n t s APPLICATIONS. VI. CONCLUSIONS VII. REFERENCES.

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............... . . . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . ................ ................ I.

Introduction

The process of pulsed l a s e r annealing p r o v i d e s a way o f very r a p i d l y t r e a t i n g t h e near-surface

r e g i o n o f semiconductors.

t h i s a d i a b a t i c mode o f thermal processing, 1 iquid-phase

epitaxial

In

m e l t i n g f o l l o w e d by

regrowth from t h e s u b s t r a t e occurs w i t h

growth v e l o c i t i e s o f t h e o r d e r o f meters/s.

Thus t h e h e a t i n g and

c o o l i n g r a t e s achieved by t h i s form o f processing are o r d e r s o f

405

Capyright 01984 b i Academic Press, Inc. All rights of reproduction in any form reserved.

ISBN 0-12-752123.2

406

D. M. ZEHNER

magnitude f a s t e r t h a n t h o s e achieved by more c o n v e n t i o n a l t r e a t ments.

i t has been shown t h a t

With proper annealing conditions,

r e g i o n s f r e e o f extended d e f e c t s can be formed and s u b s t i t u t i o n a l i m p u r i t i e s can be i n c o r p o r a t e d i n t o t h e l a t t i c e f a r i n excess o f t h e e q u i l i b r i u m s o l u b i l i t y l i m i t s (see Chapters 1-4).

The f i n a l

a c t o f s o l i d i f i c a t i o n i s t h e f r e e z i n g o f t h e surface.

I n view o f

the

region,

i t may be

(impurities,

geometric

results

expected

that

structure, ductors

obtained f o r the

the

surface

near-surface

properties

e l e c t r o n i c energy l e v e l s ) o f l a s e r - a n n e a l e d semicon-

can

be

significantly

altered

with

respect

to

those

o b t a i n e d by c o n v e n t i o n a l h e a t i n g treatments. R e s u l t s o f experiments d i s c u s s e d i n t h i s c h a p t e r show t h a t p u l s e d l a s e r a n n e a l i n g can be used t o produce a t o m i c a l l y c l e a n s u r f a c e s , remove damage i n t h e outermost s u r f a c e l a y e r s , and a l t e r t h e e l e c t r o n i c p r o p e r t i e s i n t h e s u r f a c e region.

D e t a i l s con-

cerned w i t h p r o c e s s i n g o f t h e s u r f a c e i n o r d e r t o achieve these conditions

and t h e measurement

discussed i n Section

11.

of

the surface properties are

R e s u l t s which show t h a t

unwanted i m p u r i t i e s i n semiconductors (0, C, etc.) to

near

the

practical

detection

limits

of

levels of

can be reduced

surface

sensitive

I n Section I V the

s p e c t r o s c o p i e s a r e presented i n S e c t i o n 111.

s u b j e c t o f o r d e r i n t h e outermost l a y e r s f o r b o t h f l a t and v i c i n a l s u r f a c e s i s discussed, and

changes

in

along w i t h the production o f defects

stoichiometry

for

compound

semiconductors.

S e c t i o n V d e a l s w i t h t h e changes i n b o t h geometric and e l e c t r o n i c p r o p e r t i e s o f t h e s u r f a c e r e g i o n which occur when i o n i m p l a n t a t i o n

is

combined w i t h

laser

annealing.

Finally

in

Section

YI,

examples o f how t h e unique s u r f a c e p r o p e r t i e s achieved w i t h l a s e r a n n e a l i n g can discussed.

be a p p l i e d t o o t h e r s u r f a c e

investigations

are

7.

407

PULSED LASER IRRADIATED SEMICONDUCTORS

11.

Experimental Approach

For most i n v e s t i g a t i o n s concerned w i t h l a s e r p r o c e s s i n g o f semiconductors,

t h e i r r a d i a t i o n o f t h e sample has been performed

i n a standard atmospheric environment.

It i s g e n e r a l l y assumed

t h a t t h e m o d i f i c a t i o n o f t h e subsurface p r o p e r t i e s i s u n a f f e c t e d by i n t e r a c t i o n s a t t h e g a s - s o l i d i n t e r f a c e .

However, when one i s

concerned w i t h o b t a i n i n g i n f o r m a t i o n about t h e p r o p e r t i e s o f t h e s u r f a c e r e g i o n (1-20 A ) and t h e changes which occur due t o l a s e r annealing,

t h e i r r a d i a t i o n o f t h e sample and subsequent a n a l y s i s

must t a k e p l a c e i n an u l t r a h i g h vacuum (UHV) environment Torr).

I n t h i s s e c t i o n , t h e experimental d e t a i l s concerned w i t h

b o t h l a s e r annealing o f semiconductors i n UHV and t h e subsequent s u r f a c e c h a r a c t e r i z a t i o n a r e presented.

1.

SAMPLE PREPARATION

A v a r i e t y o f l a s e r s has been used i n i n v e s t i g a t i o n s concerned w i t h surface s t u d i e s o f laser-annealed semiconductors.

The most

f r e q u e n t l y used l a s e r s are e i t h e r p u l s e d ruby o r p u l s e d Nd:YAG, a l t h o u g h UV excimer and t u n a b l e dye l a s e r s have a l s o been employed. The procedures f o l l o w e d i n p e r f o r m i n g t h e l a s e r a n n e a l i n g i n a

UHV environment are very s i m i l a r i n a l l i n v e s t i g a t i o n s and w i l l be i l l u s t r a t e d by d i s c u s s i n g t h e approach used w i t h p u l s e d ruby l a s e r s (Zehner e t al.,

1980a,b).

A f t e r bakeout,

t h e background

p r e s s u r e i n t h e chamber which c o n t a i n e d t h e sample was t y p i c a l l y l e s s than 2 x 10-10 Torr.

The l i g h t from a Q-switched ruby l a s e r

( A = 694 nm,

FWHM), t r a n s m i t t e d i n t o t h e UHV system

T

= 15 nsec,

t h r o u g h a glass window, vacuum environment.

was used t o i r r a d i a t e t h e sample i n t h e

The samples were p o s i t i o n e d so t h a t any evap-

o r a t e d m a t e r i a l o r s c a t t e r e d l i g h t was c o n t a i n e d i n an enclosure which s h i e l d e d a1 1 s u r f a c e a n a l y s i s

instruments.

i r r a d i a t e d u s i n g t h e single-mode (TEMoo)

Samples were

o u t p u t o f t h e ruby l a s e r

a t energy d e n s i t i e s t h a t c o u l d be v a r i e d between -0.2 J/cm2.

The beam diameter was t y p i c a l l y between 3.0

and -4.0

and 6.0 mn.

Energy d e n s i t i e s , which have been c o r r e c t e d f o r t h e r e f l e c t i v i t y

408

D. M. ZEHNER

( - 10%) o f t h e g l a s s window,

were determined by measuring t h e

photon energy d e l i v e r e d through an a p e r t u r e o f known diameter positioned i n f r o n t o f a calorimeter. was

measured

with

an

in-1 i n e

against the calorimeter.

The energy o f each p u l s e

photodi ode

assembly

c a l ib r a t e d

Implanted samples were prepared i n a

separate i o n i m p l a n t a t i o n f a c i 1it y which was a1 so equipped f o r making R u t h e r f o r d b a c k s c a t t e r i n g (RBS) measurements.

T h i s tech-

n i q u e was used t o determine t h e i m p l a n t p r o f i l e and t o charact e r i z e t h e changes i n t h e subsurface r e g i o n t h a t occurred w i t h An e x t e n s i v e d i s c u s s i o n o f these r e s u l t s i s

l a s e r annealing.

c o n t a i n e d i n Chapter 2.

2.

CHARACTERIZATION TECHNIQUES Many o f t h e s u r f a c e s e n s i t i v e s p e c t r o s c o p i c techniques used t o

i n v e s t i g a t e t h e s u r f a c e r e g i o n o f laser-annealed semiconductors employ e i t h e r e l e c t r o n s o r photons as t h e i n c i d e n t probe. t h e s e cases t h e d e t e c t e d p a r t i c l e i s an e l e c t r o n .

In

As a con-

sequence o f t h e s h o r t mean f r e e path o f e l e c t r o n s w i t h energies between 20 and 1000 eV, o n l y t h e outermost s u r f a c e region, -20 A , i s probed. Auger

Several d i f f e r e n t techniques have been used. electron

spectroscopy

(AES)

was

used t o m o n i t o r t h e

l e v e l s of b o t h i m p u r i t i e s and implanted species i n t h e s u r f a c e r e g i o n of t h e sample. elements w i t h Z > 3. in

terms

of

the

T h i s technique i s capable o f d e t e c t i n g a l l L e v e l s o f i m p u r i t y c o n t a m i n a t i o n a r e quoted

ratios

of

the

peak-to-peak

signals

of

the

i m p u r i t y Auger t r a n s i t i o n s t o a p r i n c i p a l Auger t r a n s i t i o n o f t h e substrate. this

Although one must be c a r e f u l

technique

to

make

quantitative

i n a t t e m p t i n g t o use

measurements,

in

many

s i t u a t i o n s reasonable estimates o f t h e upper l i m i t o f t h e amount o f a p a r t i c u l a r species present

i n t h e s u r f a c e r e g i o n can be

made. Low-energy e l e c t r o n d i f f r a c t i o n (LEED) was employed t o d e t e r mine geometric o r d e r i n t h e s u r f a c e r e g i o n o f t h e sample.

By

examining t h e p o s i t i o n s o f t h e r e f 1e c t e d beams ( s p o t p a t t e r n s ) ,

7.

409

PULSED LASER IRRADIATED SEMICONDUCTORS

t h e symmetry and s i z e ( i n t r a - a t o m i c spacing) o f t h e s u r f a c e u n i t c e l l can be determined. with

Thus, any changes i n these spot p a t t e r n s are a

surface modification

r e f l e c t i o n o f changes

i n the

geometric arrangement o f atoms i n t h e outermost s u r f a c e l a y e r s . P h o t o e l e c t r o n spectroscopy

(PES)

was

used t o o b t a i n i n f o r -

m a t i o n about t h e e l e c t r o n i c p r o p e r t i e s o f t h e s u r f a c e r e g i o n o f t h e sample.

I n f o r m a t i o n about t h e valence and conduction bands o f

t h e s o l i d can be obtained

by employing photons w i t h energies

t y p i c a l l y l e s s than 50 eV.

With t h e use o f angle-resolved tech-

niques, i t i s p o s s i b l e t o map o u t bands and c h a r a c t e r i z e t h e symmetry o f s u r f a c e s t a t e s .

By employing photons o f h i g h e r energies

i t i s p o s s i b l e t o measure c o r e - l e v e l

b i n d i n g energies f o r b o t h

s u r f a c e and subsurface atoms.

111.

P r o d u c t i o n o f A t o m i c a l l y Clean Surfaces

The c r e a t i o n o f an a t o m i c a l l y c l e a n s u r f a c e i s one o f t h e obvious b u t f r e q u e n t l y d i f f i c u l t t a s k s t h a t must be performed p r i o r t o conducting experiments i n t h e f i e l d o f s u r f a c e science. I n v e s t i g a t i o n s concerned w i t h examining t h e p h y s i c a l and chemical properties

o f s u r f a c e s i n o r d e r t o understand s u r f a c e - r e l a t e d

phenomena r e q u i r e t h a t t h e l e v e l o f unwanted contaminants i n t h e f i r s t few monolayers be <1 at.%.

S i m i l a r requirements e x i s t i n

many areas o f device t e c h n o l o g i e s where t h e presence o f s u r f a c e contaminants e i t h e r d u r i n g f a b r i c a t i o n o r d u r i n g a p p l i c a t i o n can c o n t r i b u t e g r e a t l y t o t h e degradation o f device performance.

In

t h e s e areas, t h e near-surface r e g i o n w i l l become even more import a n t as d e v i c e dimensions become i n c r e a s i n g l y smaller. The t r a d i t i o n a l methods o f g e n e r a t i n g a t o m i c a l l y c l e a n s o l i d s u r f a c e s i n UHV i n c l u d e p h y s i c a l s p u t t e r i n g ,

reactive sputtering,

chemical r e a c t i o n s , e l e c t r o n scrubbing, thermal desorption, s i t i o n o r growth o f a f i l m i n s i t u , t u r e (Roberts,

1963).

depo-

and vacuum c l e a v i n g o r f r a c -

For a s i n g l e c r y s t a l

it i s

frequently

410

D. M. ZEHNER

necessary t o anneal t h e c r y s t a l a t h i g h temperatures i n o r d e r t o remove damage produced i n t h e s u r f a c e r e g i o n by c l e a n i n g t e c h niques such as s p u t t e r i n g .

However, i m p u r i t i e s may d i f f u s e from

t h e s u r f a c e i n t o t h e b u l k , o r indeed from t h e b u l k t o t h e surface; i n many cases, this

such as i n t h e study o f semiconductor

surfaces,

r e d i s t r i b u t i o n can be p a r t i c u l a r l y troublesome.

Another

disadvantage i s t h a t t h e t i m e r e q u i r e d t o c l e a n a s u r f a c e can be measured i n hours and sometimes days when c y c l i n g between sputt e r i n g and annealing i s required.

Furthermore,

f o r experiments

i n which t h e c r y s t a l i s h e l d below room temperature,

adsorption

o f background gases d u r i n g t h e t i m e t h e sample c o o l s anneal

at

an

tamination.

elevated

temperature

Therefore,

can produce

from an

unwanted

con-

improved ways o f p r e p a r i n g atomical l y

c l e a n and ordered surfaces are needed. I t i s w e l l known t h a t a focused l a s e r beam can remove macro-

scopic

quantities

(Ready,

1965),

o f material

from a s u r f a c e by v a p o r i z a t i o n

and i t has been shown t h a t l a s e r s are capable o f

r a i s i n g t h e temperature i n t h e near-surface conductor t o t h e m e l t i n g p o i n t (see, f o r e.g., Wang e t al.,

White e t al.,

1978;

1978, Chapter 4) i n a w e l l c o n t r o l l e d manner.

From

t h e s e observations,

i t was c l e a r t h a t l a s e r s might be u s e f u l i n

g e n e r a t i n g c l e a n surfaces. pose

in

r e g i o n o f a semi-

1969 (Bedair,

Although f i r s t u t i l i z e d f o r t h i s pur-

1969),

only

recently

has a

number

of

d e t a i l e d i n v e s t i g a t i o n s o f l a s e r c l e a n i n g been c a r r i e d out (see f o r e.g. al.,

Zehner e t al.,

1980;

Cowan e t al.,

1980a; McKinley e t al., 1980).

1980; Rodway e t

While most s t u d i e s have been

concerned w i t h S i , o t h e r elemental and compound semiconductors, as w e l l as metals, have a l s o been examined.

3.

SILICON

To i l l u s t r a t e t h e a p p l i c a t i o n o f l a s e r i r r a d i a t i o n i n t h e p r o d u c t i o n o f a t o m i c a l l y c l e a n surfaces, r e s u l t s obtained from S i

7.

411

PULSED LASER IRRADIATED SEMICONDUCTORS

surfaces are shown i n Fig.

1 (Zehner e t al.,

1980a).

The Auger

e l e c t r o n spectrum o b t a i n e d from t h e n a t i v e o x i d e o f an air-exposed S i sample, a f t e r i n s e r t i o n i n t o a UHV system and f o l l o w i n g bakeout,

i s shown a t t h e t o p o f Fig. 1. carbon (272 eV),

and S i

s i l i c o n dioxide,

Auger s i g n a l s from oxygen (510 eV),

(70-100

eV),

characteristic of S i

in

a r e r e a d i l y d e t e c t e d on t h e s u r f a c e o f t h e asMeasurements made by RBS i m p l y a

i n s e r t e d sample.

o x i d e t h i c k n e s s , t y p i c a l o f air-exposed S i .

M A native

A substantial reduction

i n t h e l e v e l s o f 0 and C present i n t h e s u r f a c e r e g i o n i s observed a f t e r i r r a d i a t i o n w i t h one l a s e r p u l s e (-2.0

1.

Fig.

J/cmz),

as shown i n

A f t e r exposing t h e same area t o f i v e l a s e r pulses, t h e

Auger e l e c t r o n spectrum shown i n Fig. same d e t e c t i o n c o n d i t i o n s , noise level.

1 indicates that f o r the

t h e 0 and C s i g n a l s a r e w i t h i n t h e

The Auger e l e c t r o n spectrum obtained from t h e same

area a f t e r i r r a d i a t i o n w i t h t e n l a s e r pulses i s shown a t t h e b o t 1.

Although t h e 0 and C s i g n a l s a r e n o t observable

i n t h i s trace,

by i n c r e a s i n g t h e e f f e c t i v e s e n s i t i v i t y o f t h e

tom o f Fig.

e l e c t r o n d e t e c t i o n system, these i m p u r i t i e s were determined t o be p r e s e n t i n s u r f a c e c o n c e n t r a t i o n s o f
o f a monolayer.

It

should a l s o be noted t h a t t h e l i n e shape o f t h e S i (70-100 eV) Auger

transition

is

that

expected

from

a clean S i

surface.

Although a hydrogen Auger t r a n s i t i o n cannot be d e t e c t e d w i t h AES, PES r e s u l t s (Zehner e t al.,

f r e e o f hydrogen.

1981c) show t h e s u r f a c e r e g i o n t o be

Consequently,

by i r r a d i a t i n g t h e c r y s t a l w i t h

s e v e r a l (t5) l a s e r pulses, t h e r e l a t i v e c o n c e n t r a t i o n s o f 0 and C i n t h e n e a r - s u r f a c e r e g i o n have been reduced by f a c t o r s of a t l e a s t 500 and 50,

respectively.

Thus,

contaminant l e v e l s com-

p a r a b l e t o those o b t a i n e d by repeated s p u t t e r i n g and c o n v e n t i o n a l thermal annealing over a p e r i o d o f several days can be o b t a i n e d i n a l a s e r processing t i m e o f <1 s. Because p h y s i c a l s p u t t e r i n g i s commonly employed as p a r t o f cleaning

procedures

i n s u r f a c e science,

the e f f e c t o f

pulsed

l a s e r i r r a d i a t i o n on samples t h a t had been s p u t t e r e d w i t h A r + i o n s (1000 eV,

5 PA,

30 min) was a l s o i n v e s t i g a t e d .

The Auger

412

D.M. ZEHNER I

I

I

I

c

AS INSERTED

-

f PULSE

9

5 PULSES

-

V

10 PULSES L A S E R ANNEALED

AES Si (100) PRIMARY BEAM: 2 keV, 5 p A MODULATION: 2 V p - p

I

0

Fig. 1 .

I00

I

I

200 300 400 ELECTRON ENERGY ( e V 1

I

500

600

Auger electron spectra obtained from an uncleaned Si ( 1 0 0 ) surface

and a f t e r pulsed laser annealing a t -2.0

J / c m 2 for 1 , 5 , and 10 pulses.

7.

413

PULSED LASER IRRADIATED SEMICONDUCTORS

e l e c t r o n spectrum a c q u i r e d f o l l o w i n g s p u t t e r i n g s t i l l showed t h e presence o f 0 and C, a l t h o u g h s u b s t a n t i a l l y reduced i n i n t e n s i t y , as w e l l

as t h e presence o f implanted Ar.

sample w i t h l a s e r pulses o f -2.0 tion

i n t h e contaminant-level

I r r a d i a t i o n o f the

J/cm2 produced a s i m i l a r reducAuger

signals

observed f o r t h e u n s p u t t e r e d surface.

for

0 and C as

Complete e l i m i n a t i o n o f

t h e A r Auger s i g n a l s occurred a f t e r two o r t h r e e pulses. Contamination m a i n l y CO, H,

of

the

Si

surface

background

These contaminants can be e a s i l y removed by I n addi-

i r r a d i a t i o n o f t h e sample w i t h one o r two l a s e r pulses. tion,

i f an a t o m i c a l l y

(1-1000 L),

gases,

i s i n e v i t a b l e even with a

H20, and hydrocarbons,

10-10 T o r r vacuum.

from

c l e a n s u r f a c e i s exposed t o 0,

o r CO

t h e s u r f a c e can be r e t u r n e d t o an a t o m i c a l l y c l e a n

s t a t e by i r r a d i a t i o n w i t h about f i v e l a s e r pulses. Increases

i n t h e UHV chamber,

i n t h e background pressure

measured by b o t h a nude i o n i z a t i o n gauge and a quadrupole mass spectrometer,

indicate

laser irradiation.

removal

of

s u r f a c e contaminants d u r i n g

Using l a s e r pulses o f -2.0

J/cmz and s t a r t i n g

w i t h a background pressure o f 2 x 10-1° T o r r , t h e f i r s t l a s e r p u l s e on an a s - i n s e r t e d o r f r e s h l y s p u t t e r e d sample caused a t r a n s i e n t p r e s s u r e r i s e i n t o t h e 5 x 10-8 t o 3 x

T o r r range.

Subsequent

p u l s e s on t h e same area were accompanied w i t h pressure r i s e s i n t o 10-9

the

Torr

range,

and these

c o n t i n u e d t o drop u n t i l

the

p r e s s u r e stayed i n t h e 10-10 T o r r range by about t h e f i f t h pulse. Because o f t h e p o s i t i o n o f t h e i o n i z a t i o n gauge r e l a t i v e t o t h e sample l o c a t i o n and t h e sampling r a t e o f t h e mass spectrometer, these

increases

did

not

provide

an

a c c u r a t e measure

of

the

p r e s s u r e i n c r e a s e i n f r o n t o f t h e sample, which was probably much h i g h e r than measured. The e f f e c t o f energy d e n s i t y on t h e e f f i c i e n c y o f removal o f i m p u r i t i e s was a1 so qua1 i t a t i v e l y i n v e s t i g a t e d . sity

range

i n v e s t i g a t e d was

one p u l s e a t -0.3

J/cm2,

from -0.3

to

3.2

The energy denJ/cm2.

After

t h e 0 and C s i g n a l s were reduced by a

f a c t o r o f o n l y 2 r e l a t i v e t o those observed on t h e a s - i n s e r t e d

414

D. M. ZEHNER

sample, and on a f r e s h l y s p u t t e r e d surface, embedded A r was s t i l l detected.

Two

general

energy d e n s i t y .

t r e n d s were

First,

observed

as

a function

of

t h e h i g h e r t h e energy d e n s i t y t h e more

e x t e n s i v e t h e removal o f i m p u r i t i e s by t h e f i r s t pulse.

Second,

a t any p u l s e energy d e n s i t y t h e l a r g e r t h e number o f pulses t h e more

complete

the

removal

of

impurities.

Energy d e n s i t i e s

g r e a t e r than 1.0 J/cm2 were r e q u i r e d t o produce a t o m i c a l l y c l e a n s u r f a c e s u s i n g ruby l a s e r i r r a d i a t i o n .

V i s i b l e s u r f a c e damage,

as observed o p t i c a l l y , occurred a t an energy d e n s i t y o f 3.2 J / c d b u t was n o t observed f o r energy d e n s i t i e s ~3.0 J/cm2. The process o f l a s e r annealing, which i n v o l v e s r a p i d m e l t i n g and r e s o l i d i f i c a t i o n (on a - 1 - u ~ t i m e s c a l e ) , suggests t h a t removal of

impurities

mechanisms,

from t h e s u r f a c e

r e g i o n can t a k e p l a c e by two

d e s o r p t i o n o f t h e v o l a t i l e contaminants and absorp-

t i o n and d i f f u s i o n o f o t h e r i m p u r i t i e s through t h e l i q u i d l a y e r . Both

mechanisms-desorption

and

diffusion-probably

influence

t h e r e d i s t r i b u t i o n o f a given i m p u r i t y , w i t h t h e r e l a t i v e import a n c e o f t h e two

processes depending on experimental c o n d i t i o n s .

F o r t h e c o n d i t i o n s discussed above,

t h e f a c t t h a t a pronounced

p r e s s u r e r i s e i s observed d u r i n g t h e f i r s t l a s e r p u l s e suggests that

t h e 0 and C contaminants a r e desorbed

during irradiation.

from t h e s u r f a c e

However, i t i s d i f f i c u l t t o q u a n t i f y these

o b s e r v a t i o n s and determine t h e amount o f m a t e r i a l desorbed from o n l y t h e c r y s t a l surface.

A b s o r p t i o n and d i f f u s i o n deeper i n t o

t h e sample occur predominantly f o r i m p u r i t i e s t h a t have a r e l a t i v e l y h i g h segregation c o e f f i c i e n t from t h e m e l t (White e t a l . , 1980a; Wood e t al., t i o n coefficient

1981a).

I m p u r i t i e s which have a low segrega-

from t h e m e l t have been shown t o segregate t o

t h e s u r f a c e d u r i n g annealing (White e t al., and 0 t h e e q u i l i b r i u m c o e f f i c i e n t s tively,

suggesting

that

these

1979,

a r e 0.07

1980a).

and 0.5,

For C respec-

i m p u r i t i e s may be r e d i s t r i b u t e d

over a depth i n t e r v a l e q u i v a l e n t t o t h e m e l t depth (-5000 A f o r l a s e r energy d e n s i t i e s o f -2.0

J/cm2).

Complete r e d i s t r i b u t i o n

o f t h e i m p u r i t i e s found on t h e surface, as i n s e r t e d i n t o t h e UHV

7. chamber,

415

PULSED LASER IRRADIATED SEMICONDUCTORS

over t h i s depth would g i v e r i s e t o a remaining s u r f a c e

c o n c e n t r a t i o n o f -0.3%

o f a monolayer f o r 0 and -0.1% of a mono-

b o t h of which a r e near t h e d e t e c t i o n l i m i t s f o r

l a y e r f o r C,

Auger e l e c t r o n spectroscopy.

These values are c o n s i s t e n t w i t h

t h e measured values discussed above.

Whether these i m p u r i t i e s

a r e desorbed from t h e s u r f a c e o r r e d i s t r i b u t e d i n depth can be a s c e r t a i n e d by experiments designed t o determine C and 0 depth p r o f i l e s o r t h e i r t o t a l c o n c e n t r a t i o n s i n t h e near-surface r e g i o n b e f o r e and a f t e r i r r a d i a t i o n . An i n v e s t i g a t i o n o f oxygen i n d i f f u s i o n d u r i n g l a s e r anneali n g o f s i l i c o n has been performed u s i n g RBS and t h e 160(a,ao)160 resonance s c a t t e r i n g a t 3.042 MeV (Westendorp e t a l . , e t al.,

1983).

T h i s technique,

1982; Wang

i n c o n j u n c t i o n w i t h AES, was used

t o determine t h e oxygen c o n c e n t r a t i o n on t h e s u r f a c e and i n t h e near-surface (3000 A ) region, both b e f o r e and a f t e r pulsed l a s e r annealing.

From t h e oxygen peak

area i n t h e b a c k s c a t t e r i n g

spectrum shown i n Fig. 2a, t h e i n i t i a l n a t i v e oxide l a y e r a t t h e s u r f a c e was determined t o have an a r e a l c o n c e n t r a t i o n o f 5.2 x 1015 atoms/cm2.

A f t e r i r r a d i a t i o n of t h e

Si(100) sample i n UHV w i t h

e i g h t pulses a t an energy d e n s i t y o f -1.5

J/cm2 (x=694 nm), Auger

measurements showed an oxygen c o n c e n t r a t i o n a t t h e s u r f a c e o f ~0.3% The oxygen c o n c e n t r a t i o n a t a depth o f 1200 A

o f a monolayer. (depth i n t e r v a l atoms/cm3,

500-1700 A ) was determined t o be 63.1

u s i n g t h e data shown i n Fig.

2b.

x lo1*

Assuming t h a t t h e

s i l i c o n s u r f a c e l a y e r i s m e l t e d completely d u r i n g t h e l a s e r p u l s e and t h a t t h e oxygen atoms i n t h e n a t i v e o x i d e l a y e r are homogeneously d i s t r i b u t e d over t h e depth o f t h e molten l a y e r (-3000 A ) , an oxygen Concentration o f 1.6 x 1020 atom/cm’ would be achieved i n t h a t molten l a y e r .

Thus, these r e s u l t s show no evidence f o r oxy-

gen i n d i f f u s i o n from t h e n a t i v e oxide l a y e r d u r i n g pulsed l a s e r annealing o f s i l i c o n .

I n fact,

t h e amount o f oxygen i s found t o

be equal t o o r l e s s than t h e s o l i d s o l u b i l i t y o f oxygen i n s i l i con,

which i s 5-8 x 1018 atomsjcm3 ( d e Kock,

1980).

This r e s u l t

can be made p l a u s i b l e by c o n s i d e r i n g t h e t i m e needed f o r l i q u i d

416

D. M. ZEHNER

CHANNEL N U M B E R

CHAW N iL NIJ M B E R

F i g . 2.

Resonance at ( a ) center o f native oxide l a y e r , ( b ) a t 1200

pulsed laser cleaned silicon.

BL = 164O.

in

7. SiO,

t o be d i s s o l v e d i n s i l i c o n .

i n t h e range o f 10-5-10-4

The d i s s o l u t i o n v e l o c i t y l i e s

cm/min f o r s i l i c o n s u b s t r a t e tempera-

t u r e s between 1700-1775 K (Chaney e t a1 a 30 A

Thus,

a thermal

., 1976; H i r a t a e t a1 ., 1980).

n a t i v e o x i d e l a y e r would d i s s o l v e a t about t h e

m e l t i n g p o i n t o f s i l i c o n i n 1.8-0.18 of

417

PULSED LASER IRRADIATED SEMICONDUCTORS

m e l t i n g model,

s.

Consequently,

a native

oxide

i n terms

l a y e r cannot be

d i s s o l v e d i n s i l i c o n s i n c e t h e m e l t d u r a t i o n a f t e r pulsed l a s e r annealing, f o r t h e c o n d i t i o n s used, i s measured t o be -100 ns. I f t h e n a t i v e o x i d e l a y e r on t o p o f s i l i c o n does n o t d i f f u s e

i n t o t h e bulk, it has t o evaporate d u r i n g t h e c l e a n i n g procedure. This

can

be

shown

to

be

reasonable

by

calculating

the

Si02 m o l e c u l a r e v a p o r a t i o n r a t e f o r d i f f e r e n t s i l i c o n s u b s t r a t e temperatures.

Using equi 1 ib r i u m thermodynamics, t h e r a t e a t 2000 K

i s c a l c u l a t e d t o be 3.4 x 1012 mol/cm2 ns (Westendorp e t al., Wang e t al.,

Since a t y p i c a l a r e a l d e n s i t y o f a n a t i v e

1983).

o x i d e on s i l i c o n i s 2-5 x

lo15

mol/cm2,

t h e complete removal by

e i g h t l a s e r pulses r e q u i r e s 5 x 1014 mol/cm2 d u r i n g each pulse.

1982;

t o be evaporated

T h i s e v a p o r a t i o n r a t e r e q u i r e s a m e l t dura-

t i o n o f t h e s u r f a c e o f -150 ns, which i s t h e same o r d e r o f magnit u d e as obtained from thermal m e l t i n g model c a l c u l a t i o n s . When

a

silicon

surface

is

covered w i t h

an o v e r l a y e r

of

i m p u r i t i e s much t h i c k e r t h a n t h e t y p i c a l n a t i v e o x i d e t h i c k n e s s (20 A ) ,

complete e l i m i n a t i o n o f t h e i m p u r i t i e s by l a s e r i r r a d i a -

tion i s difficult. (Bermudez,

Experiments on h e a v i l y o x i d i z e d S i surfaces

1982) have shown t h a t t h e minimum a t t a i n a b l e l e v e l o f

c o n t a m i n a t i o n i s dependent on t h e i n c i d e n t energy d e n s i t y and on t h e i n i t i a l i m p u r i t y coverage.

From these r e s u l t s i t i s con-

c l u d e d t h a t although d e s o r p t i o n may be t h e dominant mechanism f o r removal o f i m p u r i t i e s i n t h i n o v e r l a y e r s , d i f f u s i o n and r e d i s t r i b u t i o n o f i m p u r i t i e s i n t h e melted r e g i o n occur when t h i c k l a y e r s a r e i n i t i a l l y present. I n v e s t i g a t i o n s have a l s o been performed t o examine t h e i n c o r p o r a t i o n o f oxygen i n t o S i d u r i n g p u l s e d l a s e r annealing i n an atmospheric environment.

While some i n v e s t i g a t o r s c l a i m evidence

418

D. M. ZEHNER

f o r i n c o r p o r a t i o n ( G a r u l l i e t al.,

1980; Hoh e t al.,

a1

incorporation

.,

1981),

others

(Westendorp e t a1

argue

., 1982;

that

Wang e t a1

1980; L i u e t

does

., 1983).

not

occur

To minimize con-

c e r n about t h e p o s s i b l e r e d i s t r i b u t i o n o f i m p u r i t i e s i n a UHV t h e s a f e s t procedure t o f o l l o w i s t o s p u t t e r t h e

environment,

s u r f a c e m i l d l y t o reduce s u b s t a n t i a l l y t h e c o n c e n t r a t i o n o f these i m p u r i t i e s and then t o l a s e r anneal t h e c r y s t a l t o produce an a t o m i c a l l y clean surface region.

4.

GERMANIUM I n v e s t i g a t i o n s concerned w i t h t h e removal o f t h e n a t i v e o x i d e

l a y e r from Ge samples w i t h pulsed l a s e r i r r a d a t on have produced r e s u l t s s i m i l a r t o those o b t a i n e d w i t h S i samples (Zehner e t a1

.,

1 9 8 0 ~ ) . A t o m i c a l l y c l e a n surfaces can be o b t a i n e d by i r r a d i a t i o n w i t h about f i v e l a s e r pulses (-1.9

J/cm2).

As observed w i t h S i ,

s p u t t e r i n g can be used i n c o n j u n c t i o n w i t h l a s e r i r r a d i a t i o n t o produce a t o m i c a l l y c l e a n surfaces also.

The range o f energy den-

s i t y t h a t can be used t o remove i m p u r i t i e s from t h e s u r f a c e w i t h o u t p r o d u c i n g macroscopic damage was found t o be -0.5

t o -2.1

J/cm2.

Damage t o t h e s u r f a c e occurred above an energy d e n s i t y o f 2.2 J/cm*,

and complete removal o f s u r f a c e i m p u r i t i e s d i d n o t occur

below an energy

5.

d e n s i t y o f -0.5

J/cm2.

III-V COMPOUNDS

GROUP

The p r o d u c t i o n o f a t o m i c a l l y c l e a n surfaces on GaAs c r y s t a l s was i n v e s t i g a t e d

u s i n g procedures s i m i l a r t o those p r e v i o u s l y

d e s c r i b e d (Zehner e t a1

.,

1982).

The Auger e l e c t r o n spectrum

o b t a i n e d from an a s - i n s e r t e d sample and p l o t t e d a t t h e t o p o f F i g . 3 shows t h a t l a r g e amounts o f C (272 eV) and 0 (510 eV) are present,

as w e l l as a small t r a c e o f Ca (291 eV).

impurities

from t h e s u r f a c e r e y i o n

m u l t i p l e pulses,

irradiation with

and t h e e f f i c i e n c y o f c l e a n i n g i n c r e a s e d w i t h

i n c r e a s i n g eneryy d e n s i t y . following multiple-pulse F i g . 3.

required

Removal o f

An example o f a spectrum o b t a i n e d

i r r a d i a t i o n o f -0.6

J/cm2 i s shown i n

The carbon and oxygen s i g n a l s are w i t h i n t h e n o i s e l e v e l

7.

419

PULSED LASER IRRADIATED SEMICONDUCTORS

AES GaAs (100) PRIMARY BEAM: 2 keV, 5 p A

I

-

15 PULSES -0.6 J/cm2

4

AFTER Ar' SPUTTERING

-

15 PULSES -0.3 J/cm2 0

100

200 300 400 ELECTRON ENERGY (eV)

500

F i g . 3. Auger electron spectra obtained from an uncleaned GaAs ( 1 0 0 ) surf a c e , a f t e r sputtering and a f t e r pulsed laser annealing a t 4 . 6 and 4 . 3 J /cm2.

o f t h e Auger detected.

e l e c t r o n spectrum,

and no o t h e r i m p u r i t i e s were

However, t h e r e i s a d i f f e r e n c e i n t h e Ga (55 eV) t o As

( 3 1 eV) Auger t r a n s i t i o n i n t e n s i t y r a t i o and i n t h e Ga Auger t r a n s i t i o n l i n e shape when compared w i t h t h a t obtained a f t e r A r + i o n s p u t t e r i n g , a l s o shown i n t h i s f i g u r e ( A r Auger s i g n a l a t 215 eV). Complete removal o f t h e C and 0 i m p u r i t i e s from an a s - i n s e r t e d sample c o u l d n o t be o b t a i n e d when u s i n g energy d e n s i t i e s o f t0.5

420

D. M. ZEHNER

J/cm2.

However,

by f i r s t s p u t t e r i n g w i t h Ar+ i o n s , i t was found

t h a t a s u r f a c e w i t h no i m p u r i t i e s present c o u l d be produced by laser J/cm2,

i r r a d i a t i o n w i t h energy d e n s i t i e s between 0.15 as i l l u s t r a t e d a t t h e bottom of Fig.

f i f t e e n pulses a t -0.3 intensity

J/cm2.

and 0.4

3 f o r t h e case o f

Although t h e Ga/As Auger t r a n s i t i o n

r a t i o observed f o l l o w i n g

such

surface treatment

is

s i m i l a r t o t h a t observed a f t e r s p u t t e r i n g , t h e Ga Auger l i n e shape showed t h e same t y p e o f change as t h a t observed a t h i g h e r energy densities.

T h i s change, a r e d u c t i o n i n t h e degree o f s p l i t t i n g

w i t h i n t h e l i n e shape (due t o t h e s p i n - o r b i t s p l i t ,M,,

level),

observed a f t e r i r r a d i a t i o n a t a l l energy d e n s i t i e s i s due t o t h e presence o f Ga i n l o c a l regions which a r e n o n s t o i c h i o m e t r i c . I n o r d e r t o c o n f i r m t h i s e x p l a n a t i o n f u r t h e r , a f t e r pulsed l a s e r a n n e a l i n g i n UHV, t h e samples were t r a n s f e r r e d i n a i r t o a highvacuum (HV)

chamber, where t h e y were analyzed u s i n g 1.5-MeV

ion scattering.

Het

For purposes o f comparison, s i m i l a r measurements

were made on samples t h a t were n o t exposed t o l a s e r i r r a d i a t i o n (virgin).

A r e p r e s e n t a t i v e r e s u l t o f such a comparison i s shown

i n Fig. 4 f o r t h e (110) face o f GaAs.

The s p e c t r a obtained from

t h e v i r g i n sample can be i n t e r p r e t e d as s c a t t e r i n g from an ordered crystal

covered w i t h an a i r - f o r m e d

<111> c h a n n e l i n g spectra,

oxide.

shown i n Fig.

4,

Comparison o f t h e before ( s o l i d l i n e )

and a f t e r (dashed l i n e ) annealing w i t h f i v e l a s e r pulses a t an energy d e n s i t y o f -0.3 J/cm2 shows t h a t (1) t h e c o n c e n t r a t i o n o f As atoms a t t h e surface i s e s s e n t i a l l y t h e same i n t h e v i r g i n and laser-annealed

samples;

( 2 ) t h e y i e l d from Ga s u r f a c e atoms

i n c r e a s e s about a f a c t o r o f 2 a f t e r annealing; and ( 3 ) t h e scatt e r i n g y i e l d versus depth ( d e c r e a s i n g energy) increases s l i g h t l y a f t e r a n n e a l i n g w i t h no n o t i c e a b l e change i n t h e r a t e o f dechanneling.

The measured y i e l d s can be a t t r i b u t e d t o s c a t t e r i n g from

a s u r f a c e which c o n t a i n s r e g i o n s w i t h t h e c o r r e c t s t o i c h i o m e t r y i n c o n j u n c t i o n w i t h r e g i o n s which c o n t a i n excess Ga, c o n s i s t e n t w i t h t h e i n t e r p r e t a t i o n o f t h e measured Ga Auger l i n e shape. cleaning

results

are q u a l i t a t i v e l y

similar

t o those

The

obtained

7.

PULSED LASER IRRADIATED SEMICONDUCTORS

421

Backscattering-channeling spectra from a GaAs ( 1 1 0 ) surface. (---) a f t e r pulsed laser annealing at -0.3 J / c r n 2 .

(3

to4

103

v)

I-

$

8

402

10'

10° 1

Fig. 4 . As grown,

u s i n g glass-bonded [ 100) GaAs t r a n s m i s s i o n photocathodes (Rodway e t al.,

1980), and t h e d e t e r m i n a t i o n o f s t o i c h i o m e t r y i s c o n s i s t e n t

w i t h RBS t e s t s of GaAs decomposition due t o p u l s e d l a s e r i r r a d i a t i o n [de Jong e t al.,

1982a).

422

D. M. ZEHNER

S i m i 1a r

problems w i t h

respect

to

s t o i c h i ometry

have

been

encountered i n i n v e s t i g a t i o n s o f l a s e r - i r r a d i a t e d cleaved InP (110) s u r f a c e s exposed t o a i r (McKinley e t a1

., 1980).

Clean s u r f a c e s

were produced, and t h e AES s p e c t r a o b t a i n e d were q u i t e s i m i l a r t o t h o s e f o r c l e a n cleaved surfaces.

However, t h e s u r f a c e s were n o t

ordered, and d e t a i l e d s t u d i e s u s i n g scanning AES ( w i t h a 1-pin spatial

resolution)

indicated that,

although the overall

surface

showed a s t o i c h i o m e t r y c o n s i s t e n t w i t h t h a t o f t h e cleaved face, l o c a l r e g i o n s e x i s t e d where t h e d e v i a t i o n from s t o i c h i o r n e t r y was large.

The d e p a r t u r e from s t o i c h i o m e t r y subsequent

t o laser

i r r a d i a t i o n has been observed i n i n v e s t i g a t i o n s o f InP (100) s u r f a c e s a1 so (Moison e t a1 , 1982).

IV.

Geometric Surface Structures

The p r o c e s s i n g o f semiconductor m a t e r i a l s w i t h l a s e r i r r a d i a t i o n has been i n v e s t i g a t e d e x t e n s i v e l y .

O f m a j o r importance i s t h e f a c t

t h a t l a s e r a n n e a l i n g can be used t o anneal c o m p l e t e l y d i s p l a c e ment

damage

1978).

in

ion-imp1 anted

semiconductors

(Narayan

et

a1

.,

I n t h i s a p p l i c a t i o n t h e l a s e r r a d i a t i o n causes t h e s u r -

f a c e r e g i o n o f t h e c r y s t a l t o be m e l t e d t o a d e p t h o f several thousand

angstroms.

The m e l t e d

l a y e r then

regrows

from t h e

u n d e r l y i n g s u b s t r a t e by means o f l i q u i d - p h a s e e p i t a x i a l regrowth, and t h e regrown r e g i o n has t h e same c r y s t a l l i n e p e r f e c t i o n as t h e substrate.

I n s u r f a c e - r e l a t e d experiments, depending on t h e m e l t i n g

p o i n t , t y p e s and q u a n t i t i e s o f i m p u r i t i e s p r e s e n t i n t h e b u l k , reac-

t i v i t y t o background gases , and o t h e r f a c t o r s , c o n v e n t i o n a l thermal a n n e a l i n g o f c r y s t a l s i n UHV, e i t h e r t o c l e a n o r t o remove s p u t t e r damage,

i n v o l v e s h e a t i n g and subsequent c o o l i n g o f t h e sample f o r

p e r i o d s t h a t can range from minutes t o hours.

Calculations indicate

x=

694 nm, s e v e r a l t e n s o f

t h a t f o r p u l s e energies o f 1-2 J/cm2 a t

microseconds elapse between t h e i r r a d i a t i o n o f t h e sample a t room t e m p e r a t u r e w i t h t h e l a s e r beam and t h e r e t u r n o f t h e sample t o -600 K.

Thus, t h i s a n n e a l i n g t e c h n i q u e p r o v i d e s t h e c a p a b i l i t y f o r

7.

423

PULSED LASER IRRADIATED SEMICONDUCTORS

d o i n g experiments i n which t h e t i m e f o r thermal p r o c e s s i n g i s r e duced t o a minimum.

Since t h e p r e v i o u s s e c t i o n has shown t h a t

p u l s e d l a s e r i r r a d i a t i o n can be used t o produce a t o m i c a l l y c l e a n surfaces, i t i s then o f i n t e r e s t t o determine t h e annealing capab i l i t y o f t h i s technique w i t h respect t o o r d e r i n t h e s u r f a c e region o f a single crystal. 6.

ORDERED SURFACES I n o r d e r t o i n v e s t i g a t e t h i s q u e s t i o n , r e s u l t s o b t a i n e d from

samples o f s i l i c o n c r y s t a l s a r e presented (Zehner e t al.,

1980b).

These samples r e c e i v e d no c l e a n i n g t r e a t m e n t o t h e r t h a n a r i n s e i n a l c o h o l p r i o r t o i n s e r t i o n i n t o t h e UHV system.

Examination o f t h e

a s - i n s e r t e d samples w i t h LEED showed t h a t d i f f r a c t i o n p a t t e r n s c o u l d be observed o n l y a t r e l a t i v e l y h i g h energies (>250 eV) and t h a t t h e y c o n t a i n e d an i n t e n s e backround r e s u l t i n g from d i f f u s e s c a t t e r i n g . This

o b s e r v a t i o n i s c o n s i s t e n t w i t h t h e presence o f a n a t i v e

o x i d e l a y e r c o n t a i n i n g 0 and C as determined by AES and shown i n F i g . 1.

The LEED p a t t e r n s shown i n Fig. 5 i l l u s t r a t e t h e e f f e c t s

o f m u l t i p l e - p u l s e i r r a d i a t i o n on a S i ( 100) sample. d i a t i o n w i t h one l a s e r p u l s e o f -2.0

J/cm2,

A f t e r irra-

a (2x1) LEED p a t t e r n

w i t h moderate background i n t e n s i t y due t o d i f f u s e s c a t t e r i n g was obtained,

as shown a t t h e t o p o f Fig.

5.

Improvements i n t h e

q u a l i t y o f t h e d i f f r a c t i o n p a t t e r n occurred w i t h subsequent l a s e r pulses.

After

diffraction observed,

f i v e pulses,

reflections

a LEED p a t t e r n e x h i b i t i n g

and very

as shown i n F i g .

5.

sharp

low background i n t e n s i t y was

The f a c t t h a t w e l l - d e f i n e d LEED

p a t t e r n s can be obtained i n d i c a t e s t h a t c r y s t a l l i n e o r d e r extends to

t h e outermost monolayers

regrowth process.

after

the

1iquid-phase

epitaxial

No d e t e c t a b l e change i n t h e LEED p a t t e r n s was

observed w i t h a d d i t i o n a l pulses, as can be seen by comparing t h e patterns

for

five

and t e n pulses

shown i n Fig.

5.

Similar

r e s u l t s were o b t a i n e d from samples t h a t were i n i t i a l l y s p u t t e r cleaned by A r + bombardment.

Although t h e LEED p a t t e r n o b t a i n e d

subsequent t o one l a s e r p u l s e on a s p u t t e r e d s u r f a c e was

of

7.

PULSED LASER IRRADIATED SEMICONDUCTORS

h i g h e r q u a l i t y t h a n t h a t shown i n Fig.

5,

425

m u l t i p l e pulses were

a l s o r e q u i r e d t o o b t a i n t h e sharpest d i f f r a c t i o n p a t t e r n s . The LEED p a t t e r n s obtained from t h e t h r e e low-index o r i e n t a t i o n s o f S i subsequent t o l a s e r i r r a d i a t i o n w i t h f i v e pulses a r e shown i n F i g . 6.

A l l p a t t e r n s show sharp d i f f r a c t i o n r e f l e c t i o n s accompanied

by low background i n t e n s i t y , and i n a l l cases t h e q u a l i t y o f t h e p a t t e r n obtained improved w i t h m u l t i p l e - p u l s e f i v e shots.

i r r a d i a t i o n up t o

The (2x1) and ( 1 x 2 ) LEED p a t t e r n s o b t a i n e d from t h e

(100) and (110) surfaces,

respectively,

are s i m i l a r t o those ob-

t a i n e d u s i n g conventional thermal t r e a t m e n t s and show t h e presence o f r e c o n s t r u c t e d surfaces.

These o b s e r v a t i o n s i n d i c a t e t h a t t h e

atoms i n t h e outermost l a y e r s have enough t i m e a t a temperature, under t h e l a s e r annealing c o n d i t i o n s used, t o r e o r g a n i z e i n t o t h e r e c o n s t r u c t e d arrangements f r o m w h i c h t h e LEED p a t t e r n s a r e obtained. T h i s i s c o n s i s t e n t w i t h t h e proposed s u r f a c e s t r u c t u r e models f o r t h e (100) s u r f a c e t h a t i n v o l v e o n l y small l a t e r a l and v e r t i c a l d i s placements o f t h e atoms i n t h e f i l l e d outermost monolayers.

Laser

a n n e a l i n g o f e i t h e r (100) o r (110) S i samples cooled t o 100 K (Zehner e t a1

., 1980d)

produced surfaces from which LEED p a t t e r n s

i d e n t i c a l t o those shown i n Fig. 6 were obtained. The LEED p a t t e r n o b t a i n e d from t h e (111) s u r f a c e suggests t h a t as a r e s u l t o f l a s e r annealing t h e normal s u r f a c e s t r u c t u r e ( t r u n c a t i o n o f t h e b u l k ) i s obtained,

and t h e r e i s no evidence o f any

ordered l a t e r a l r e c o n s t r u c t i o n (Zehner e t a1

., 1 9 8 0 ~ ) . T h i s p o i n t

w i l l be discussed i n more d e t a i l l a t e r . Although a ( 2 x 1 ) LEED p a t t e r n can be obtained from a cleaved S i ( l l 1 ) surface, t h e (7x7) p a t t e r n shown i n Fig. 7 i s always observed on a clean, t h e r m a l l y annealed c r y s t a l surface.

A f t e r i r r a d i a t i o n w i t h t h e l a s e r and structure,

t h e S i sample was

t h e r m a l l y annealed a t e l e v a t e d temperatures.

The o b s e r v a t i o n o f

production o f the (1x1)

surface

1/7-order d i f f r a c t i o n spots, i n d i c a t i v e o f t h e r e c o n s t r u c t e d surface, occurred a f t e r annealing a t temperatures g r e a t e r t h a n -800 K. By h e a t i n g f o r a s u f f i c i e n t t i m e (>30 rnin) a t these temperatures,

a well-defined

( 7 x 7 ) p a t t e r n s i m i l a r t o t h a t shown i n Fig. 7 was

426

Fig. 6.

D. M. ZEHNER

LEED patterns from clean ( a )

(loo),

( b ) ( 1 1 0 ) , and ( c ) ( 1 1 1 ) Si

surfaces a t primary beam energies o f ( a ) 4 9 , ( b ) 9 2 , and ( c ) 47 eV. are shown subsequent to laser annealing at -2.0

Patterns

J / c m 2 for 5 pulses.

7.

Fig. 7.

PULSED LASER IRRADIATED SEMICONDUCTORS

427

LEED pattern from a clean thermally annealed ( 1 1 1 ) Si surface at a

primary beam energy o f 1 1 1 eV.

observed.

Subsequent i r r a d i a t i o n a t room temperature w i t h t h e

l a s e r resulted i n a (1x1) surface structure, possible t o cycle Moreover,

back and f o r t h between t h e two s t r u c t u r e s .

i t was determined t h a t t h e sample c o u l d be h e l d a t a

temperature between -100

and 700 K,

and a f t e r i r r a d i a t i o n w i t h a

l a s e r p u l s e t h e (1x1) s t r u c t u r e was observed.

LEED

defined

showing t h a t i t i s

patterns

single-crystal (Zehner e t a1

were

obtained

from

s u r f a c e s o f Ge subsequent

., 1 9 8 0 ~ ) .

As w i t h S i , w e l l low-index-oriented

t o laser irradiation

Mechanisms o f energy a b s o r p t i o n and r e d i s t r i b u t i o n i n t h e s u r f a c e r e g i o n , as w e l l as t h e development o f a comprehensive unders t a n d i n g o f t h e s t a t e o f t h e s u r f a c e under l a s e r annealing conditions,

have r e c e i v e d c o n s i d e r a b l e a t t e n t i o n i n r e c e n t years.

The

f a c t t h a t ordered surfaces can be produced w i t h l a s e r annealing, as j u s t discussed,

suggests t h a t a n a t u r a l probe o f t h e s u r f a c e

r e g i o n which would y i e l d s t r u c t u r a l i n f o r m a t i o n on t h e f i r s t few atomic l a y e r s i s t i m e - r e s o l v e d LEED.

By measuring t h e i n t e n s i t y

o f a d i f f r a c t e d beam d u r i n g t h e l a s e r a n n e a l i n g process, i n f o r m a t i o n about t h e s t a t e o f t h e s u r f a c e can be obtained.

Such measurements

in conj u n c t i on w i t h time- r e s o l ved opt ic a l r e f 1 e c t iv i t y measurements have r e c e n t l y been made u s i n g a 'Ge (111) sample (Becker e t al., 1984a,b).

The LEED i n t e n s i t y was measured i n a temporal window

428

D.M. ZEHNER

e x t e n d i n g from a few nanoseconds b e f o r e t h e l a s e r p u l s e t o 1000 ns a f t e r t h e l a s e r pulse.

These i n t e n s i t i e s were then compared

t o those o b t a i n e d from a sample r a i s e d t o successive s t e a d y - s t a t e temperatures by r a d i a t i v e h e a t i n g from a f i l a m e n t - t y p e Results

show an e x t i n c t i o n

o f the diffracted

sistent

with

increase

the

observed

in

heater.

intensity,

optical

con-

reflectivity,

c l e a r l y i n d i c a t i n g t h a t t h e Ge s u r f a c e i s n o n c r y s t a l l i n e d u r i n g t h e l a s e r a n n e a l i n g process.

These changes i n i n t e n s i t y can be

c o m p l e t e l y accounted f o r i n t h e m e l t i n g model. To i l l u s t r a t e t h e a p p l i c a t i o n o f l a s e r a n n e a l i n g i n producing o r d e r e d s u r f a c e s t r u c t u r e s on c r y s t a l faces o f compound semicond u c t o r s , e s p e c i a l l y those i n which one o f t h e components i s v o l a tile,

r e s u l t s o b t a i n e d from t h e low-index faces o f GaAs c r y s t a l s

w i l l be presented (Zehner e t a1

., 1982).

A l l r e s u l t s t o be d i s -

cussed were o b t a i n e d from surfaces t h a t were i n i t i a l l y s p u t t e r e d i n o r d e r t o remove t h e C and 0 i m p u r i t i e s ,

s i n c e t h i s procedure

p e r m i t t e d t h e use o f r e l a t i v e l y low l a s e r p u l s e energy d e n s i t i e s i n o r d e r t o o b t a i n c l e a n surfaces. the

(loo),

The LEED p a t t e r n s obtained f o r

(110), and (111) o r i e n t a t i o n s o f GaAs c r y s t a l s f o l l o w i n g

i r r a d i a t i o n a t an energy d e n s i t y o f -0.3

J/cm2 a r e shown i n Fig. 8.

The q u a l i t y o f t h e s u r f a c e s t r u c t u r e r e s u l t i n g from l a s e r annealing, as r e f l e c t e d i n these p a t t e r n s , d i f f e r e d s i g n i f i c a n t l y from t h a t o b t a i n e d from elemental semiconductors.

The h i g h e s t qua1 i t y p a t -

t e r n s were o b t a i n e d from t h e (110) o r i e n t a t i o n ,

and reasonable

q u a l i t y p a t t e r n s were o b t a i n e d from both A- and B-type (111) o r i e n t a tions.

Very poor q u a l i t y p a t t e r n s were o b t a i n e d from t h e (100)

orientations.

A l l LEED p a t t e r n s were b a s i c a l l y (1x1)

,

suggesting

no long-range ordered r e c o n s t r u c t i o n as n o r m a l l y observed a f t e r c o n v e n t i o n a l thermal annealing.

I n a l l cases t h e o b s e r v a t i o n o f

d i f f u s e background i n t e n s i t y and/or s t r e a k i n g i n d i c a t e d t h e presence

o f d i s o r d e r i n t h e s u r f a c e region.

These o b s e r v a t i o n s a r e consis-

t e n t w i t h b o t h AES and RBS r e s u l t s , which i n d i c a t e t h e e x i s t e n c e o f excess Ga i n l o c a l r e g i o n s which a r e n o n s t o i c h i o m e t r i c i n t h e n e a r - s u r f a c e region.

Although a range o f energy d e n s i t i e s and a

7.

Fig. 8.

PULSED LASER IRRADIATED SEMICONDUCTORS

LEED patterns from clean laser-annealed

429

( a ) ( l o o ) , ( b ) 1 1 0 ) , and

( c ) ( 1 1 1 ) GaAs surfaces a t primary beam energies o f ( a ) 1 1 3 eV, ( b ) 1 2 3 eV, and ( c ) 95 eV.

D. M. ZEHNER

v a r i a t i o n o f t h e number o f pulses were t r i e d , i t was n o t p o s s i b l e t o produce surfaces from which b e t t e r q u a l i t y LEED p a t t e r n s c o u l d be observed.

Similar

r e s u l t s have been o b t a i n e d from t h e InP

( 100) s u r f a c e (Moison e t a1 7.

., 1982).

METASTABLE SURFACES The o b s e r v a t i o n t h a t a (1x1) LEED p a t t e r n i s o b t a i n e d from t h e

( 1 1 1 ) s u r f a c e o f S i a f t e r l a s e r i r r a d i a t i o n i n a UHV environment and t h a t t h e s u r f a c e i s a t o m i c a l l y c l e a n a f t e r such t r e a t m e n t suggests t h a t t h i s s u r f a c e p r o v i d e s t h e o p p o r t u n i t y f o r i n v e s t i g a t i n g a c l e a n semiconductor s u r f a c e t h a t e x h i b i t s no ordered l a t e r a l reconstruction.

The understanding o f t h i s s t r u c t u r e i s o f v i t a l

importance i n view o f t h e o r e t i c a l d e s c r i p t i o n s o f t h e S i (111) surface.

I f t h e s u r f a c e i s t r u l y b u l k - l i k e except f o r s u r f a c e r e -

l a x a t i o n , i t should d i f f e r from t h e d i s o r d e r e d high-temperature (1x1)

., 1981) and i m p u r i t y - s t a b i l i z e d (1x1) (Eastman e t a1 ., F l o r i o e t a1 ., 1971) surfaces. Furthermore, o t h e r i n v e s t i -

( B e n n e t t e t a1 1980a,b;

g a t i o n s o f t h e (111) s u r f a c e subsequent t o i r r a d i a t i o n w i t h l a s e r p u l s e s have i n d i c a t e d t h a t some degree o f d i s o r d e r i s present. T h i s s u b j e c t w i l l be discussed i n d e t a i l l a t e r i n t h i s s e c t i o n . W h i l e i n f o r m a t i o n about t h e symmetry and s i z e o f t h e twodimensional

unit

cell

diffraction

patterns,

on

the

surface

information

about

can

be

obtained

surface

from

relaxations

r e q u i r e s t h e measurement o f t h e i n t e n s i t i e s o f t h e d i f f r a c t e d e l e c t r o n beams as a f u n c t i o n o f i n c i d e n t e l e c t r o n energy ( I - V profile).

The e x p e r i m e n t a l l y measured p r o f i l e s must t h e n be com-

pared w i t h r e s u l t s o b t a i n e d from f u l l y converged dynamical LEEU c a l c u l a t i o n s assuming v a r i o u s s t r u c t u r a l models f o r t h e geometric arrangement i n t h e outermost l a y e r s . between t h e experimental

A measure o f t h e agreement

r e s u l t s and t h e p r e d i c t i o n o f model

c a l c u l a t i o n s i s p r o v i d e d by t h e R f a c t o r ( t h e lower t h e R f a c t o r value, t h e

b e t t e r t h e agreement).

A d e t a i l e d LEED a n a l y s i s f o r

l a s e r - a n n e a l e d (111)-( 1x1) s u r f a c e s o f S i has been performed, and t h e r e s u l t s are discussed below (Zehner e t al.,

1981a).

7.

431

PULSED LASER IRRADIATED SEMICONDUCTORS

A S i (111) s u r f a c e t h a t had been i r r a d i a t e d w i t h t h e o u t p u t o f t h e l a s e r a t an energy d e n s i t y o f -2.0 investigations.

J/cmz was used i n these

The i n t e n s i t i e s o f t h e d i f f r a c t e d beams were

measured as a f u n c t i o n o f e l e c t r o n energy u s i n g a Faraday cup operated as a r e t a r d i n g f i e l d analyzer.

Data were obtained f o r

a l l o f t h e { l o } , {01}, {20}, and {02} beams and f o r t h r e e each o f t h e { l l } and {21} beams.

Based on o b s e r v a t i o n s and c o n c l u s i o n s

drawn from p r e v i o u s s t u d i e s , s y m m e t r i c a l l y e q u i v a l e n t beams were averaged t o p r o v i d e a data base c o n t a i n i n g s i x average p r o f i l e s . The experimental data base has been compared w i t h t h e r e s u l t s o b t a i n e d from f u l l y converged dynamical LEED c a l c u l a t i o n s .

Details

o f these c a l c u l a t i o n s can be found elsewhere, and o n l y t h e r e s u l t s

w i l l be summarized here. t h e dynamical

Comparison o f p r o f i l e s o b t a i n e d from

LEED c a l c u l a t i o n s t o t h e measured I - V

suggests t h a t t h e f i r s t i n t e r l a y e r spacing, d,

profiles

i s c o n t r a c t e d by

25.5 a 2.5% w i t h respect t o t h e b u l k value and t h a t t h e second i n t e r l a y e r spacing, d,, b u l k value.

i s expanded 3.2 r 1%w i t h respect t o t h e

P r o f i l e s c a l c u l a t e d u s i n g these values a r e shown i n

Fig. 9, which a l s o c o n t a i n s t h e corresponding experimental p r o f i l e s and single-beam r e l i a b i l i t y f a c t o r s ( R ) determined f o r each comparison.

The six-beam R f a c t o r corresponding t o Fig. 9 i s 0.115.

T h i s value i n d i c a t e s a very good agreement between c a l c u l a t e d and experimental p r o f i l e s i n a conventional LEED a n a l y s i s and suggests t h a t t h e proposed s t r u c t u r a l model i s h i g h l y probable.

Furthermore,

t h i s R value i s s i g n i f i c a n t l y lower than any r e p o r t e d value o b t a i n e d i n a LEED a n a l y s i s o f any semiconductor surface.

The changes i n

i n t e r l a y e r spacings determined from t h i s a n a l y s i s correspond t o n e a r e s t - n e i g h b o r bond l e n g t h changes o f -0.058 and t0.075 A.

These

r e s u l t s are c o n s i s t e n t w i t h a t o t a l energy c a l c u l a t i o n f o r such a s u r f a c e which g i v e s an inward r e l a x a t i o n o f t h e outermost l a y e r ( N o r t h r u p e t al.,

1981).

I n a separate

investigation

of

a Si

(111) s u r f a c e l a s e r

annealed w i t h pulses from a doubled Nd:YAG l a s e r ( A = 530 nm), a V i d i c o n camera was used t o scan t h e LEED p a t t e r n recorded on

432

D. M. ZEHNER

(40)BEAM R = 0.466

( 0 2 ) BEAM R = 0.095

I

I

(24) BEAM R = 0.088

-

CALCULATED AVERAGE EXPERIMENTAL

4 20

00

00

(60

420

460

200

ENERGY (eV)

Fig. 9 .

A comparison o f the averaged experimental I-V p r o f i l e s with calcu= -25.5%

lated results for Adl2

and Ad23 = 3.2%.

P o l a r o i d f i l m i n o r d e r t o o b t a i n t o o b t a i n angular i n t e n s i t y prof i l e s (Chabal e t al., weak peak [-0.02 ha1 f - o r d e r

1981a).

I n these measureriients, a broad and

t i m e s t h e (11) i n t e n s i t y ]

position,

characteristic

was p r e s e n t a t t h e

o f a (2x1)

reconstruction.

From these data i t was concluded t h a t no long-range o r d e r e x i s t s b u t t h a t d i s o r d e r e d domains w i t h a buckled ( 2 x 1 ) - l i k e r e c o n s t r u c t i o n are present.

The absence o f such o b s e r v a t i o n s i n t h e pre-

v i o u s l y discussed LEED a n a l y s i s suggests t h a t surfaces prepared w i t h d i f f e r e n t l a s e r a n n e a l i n g parameters may d i s p l a y d i f f e r e n c e s i n t h e d e t a i l s o f s u r f a c e order. To examine t h e q u e s t i o n o f s u r f a c e order, scattering

medium energy i o n

combined w i t h channel i n g and b l o c k i n g ,

a technique

which is a l s o s e n s i t i v e t o geometrical s t r u c t u r e i n t h e s u r f a c e r e g i o n , has been used t o i n v e s t i g a t e t h e S i ( l l 1 ) s u r f a c e (Tromp

433

7 . PULSED LASER IRRADIATED SEMICONDUCTORS e t a1

., 1982).

I n t h i s study,

data were obtained b o t h from a

s u r f a c e e x h i b i t i n y a sharp (7x7) LEED p a t t e r n ,

prepared by con-

v e n t i o n a l procedures, and from a s u r f a c e e x h i b i t i n g a sharp (1x1) LEED p a t t e r n , prepared by i r r a d i a t i n g t h e sample w i t h a s i n g l e p u l s e from a ruby l a s e r .

From an a n a l y s i s o f t h e data i t was concluded

t h a t t h e atomic displacements on both s u r f a c e s a r e r e s t r i c t e d t o two monolayers,

probably t h e f i r s t double l a y e r o f t h e c r y s t a l .

T h i s c o n c l u s i o n i s c o n s i s t e n t w i t h t h e r e s u l t s o f t h e LEE0 a n a l y s i s . However, i n t h i s model t h e atoms i n t h e f i r s t two monolayers occupy w e l l - d e f i n e d p o s i t i o n s and should g i v e r i s e t o a s t r o n g b l o c k i n g effect.

This blocking e f f e c t

i s n o t reproduced i n t h e data,

suggesting t h a t t h e atoms may occupy d i f f e r e n t l a t e r a l p o s i t i o n s and g i v e r i s e t o less e f f i c i e n t and smeared-out b l o c k i n g .

Thus, t h e

r e s u l t s are i n c o n s i s t e n t w i t h a simple r e l a x a t i o n model and i n d i c a t e some degree o f d i s o r d e r i n t h e s u r f a c e region.

A s i m i l a r LEED a n a l y s i s has been performed on a laser-annealed Ge (111) s u r f a c e (Zehner e t a1

., 1981b).

As w i t h S i , t h e b e s t agree-

ment i s o b t a i n e d f o r a s t r u c t u r a l model i n which atoms i n t h e o u t e r most l a y e r are d i s p l a c e d inward and t h o s e i n t h e second l a y e r a r e d i s p l a c e d outward r e l a t i v e t o t h e i r b u l k p o s i t i o n s , r e s p e c t i v e l y . The corresponding nearest-nei ghbor bond l e n g t h changes are -0.037 and +0.066 A . An examination o f t h e e l e c t r o n i c s t r u c t u r e i n t h e s u r f a c e r e g i o n o f t h e laser-annealed S i (111) and Ge (111) s u r f a c e s i s o f i n t e r e s t i n view o f t h e r e s u l t s o f both t h e LEED analyses and i o n s c a t t e r i n g r e s u l t s j u s t discussed.

P h o t o e l e c t r o n spectroscopy

d i r e c t l y y i e l d s i n f o r m a t i o n about t h e l o c a l bonding b u t i s l e s s s e n s i t i v e t o t h e long-range o r d e r than LEED.

Therefore,

resolved

studies

and

anyle-i ntegrated

photoemi s s i o n

valence band s u r f a c e s t a t e s and s u r f a c e c o r e - l e v e l been performed f o r t h e f o l l o w i n g s u r f a c e s :

of

angleboth

s h i f t s have

( 1 ) laser-annealed S i

and Ge (111)-(1x1)

surfaces prepared as f o r t h e LEED s t u d i e s and

(2) S i (lll)-(7x7)

and Ge ( l l l ) - ( 2 x 8 )

s u r f a c e s prepared by t h e r -

m a l l y annealing t h e (1x1) surfaces (Himpsel e t al.,

1981).

The

434

D. M. ZEHNER

measurements were made u s i n g t h e d i s p l a y - t y p e spectrometer a t t h e s y n c h r o t r o n r a d i a t i o n source, Tantalus I. I n Fig.

10, a n g l e - i n t e g r a t e d photoemission s p e c t r a a r e pre-

sented f o r laser-annealed

(1x1) surfaces ( f u l l

curves) and f o r

t h e t h e r m a l l y annealed surfaces (dashed curves) o f Ge (111) and Si

The d o t t e d l i n e s show t h e s p e c t r a o b t a i n e d a f t e r a

(111).

hydrogen exposure, which r e s u l t s i n about a s a t u r a t i o n monolayer coverage o f hydrogen.

Below -4 eV, hydrogen induces e x t r a s t a t e s

t h a t are w e l l understood b u t n o t i m p o r t a n t i n t h i s c o n t e x t . difference

between

the

solid

(dashed)

curves

The

and t h e d o t t e d

A l l four

curves above -3 eV r e p r e s e n t s s u r f a c e - s t a t e emission.

s u r f a c e s have a d o u b l e t o f s t a t e s near t h e t o p o f t h e

clean

valence band which i s quenched by hydrogen exposure.

Relative t o

t h e t o p o f t h e valence band, these s t a t e s l i e a t -0.4

and -1.3

f o r t h e annealed S i (111) s u r f a c e s and a t -0.7 annealed Ge (111) surfaces. dependent

photoelectron

and -1.3

By u s i n g a n g l e - r e s o l v e d p o l a r i z a t i o n -

spectroscopy,

the

surface

states

determined t o have d i s t r i b u t i o n s i n momentum (Ell)-space

i n Fig.

are

and sym-

m e t r i e s which are s i m i l a r f o r a l l f o u r annealed surfaces. results

eV

f o r the

These

a r e summarized r e l a t i v e t o t h e hexagonal B r i l l o u i n zone

11.

I t is remarkable t h a t t h e predominant s u r f a c e s t a t e s f o r t h e

t h e r m a l l y annealed Ge (111) and S i (111) surfaces match t h e (1x1) s u r f a c e B r i l l o u i n zone and show no i n d i c a t i o n o f t h e small r e c i p r o cal

(2x8)

that

or

(7x7)

photoemission

unit

cells.

(This

can indeed sense

by t h e l a r g e r (1x1) u n i t c e l l i n b,, space.) t i o n f o r the S i ( l l l ) - ( 7 x 7 ) appears near t h e Fermi l e v e l

surface:

, which

observation

confirms

t h e short-range o r d e r given There i s one excep-

a weak t h i r d s u r f a c e s t a t e

makes t h i s s u r f a c e m e t a l l i c , i n

c o n t r a s t t o t h e o t h e r t h r e e surfaces.

This exception i s consistent

w i t h a band p i c t u r e , wherein t h e S i ( l l l ) - ( 7 x 7 )

s u r f a c e has t o be

m e t a l l i c because t h e r e i s an odd number o f e l e c t r o n s i n t h e ( 7 x 7 ) unit cell. tially

Each band holds two e l e c t r o n s ,

filled

band.

The

extra

surface

which leaves a parstate

for

the

Si

7.

F i g . 10.

PULSED LASER IRRADIATED SEMICONDUCTORS

435

Angle-integrated photoelectron spectra f o r the annealed G e ( 11 1 )

and S i ( l l 1 ) surfaces showing emission from two surface states near the top o f the valence band which i s quenched by hydrogen exposure (dotted l i n e s ) . denotes the valence-band

maximum.

Ev

436

D. M. ZEHNER

LOWER STATE

UPPER STATE

n

EXTRA STATE

EF

AT

F OR

Si (111)- (7x7)

Fig. 1 1 . Characteristic locations (dashed areas) of different surface states in the ( 1 x 1 ) surface Briliouin zone (hexagon) for the annealed G e ( l l 1 ) and S i ( l l 1 ) surfaces. At the zone center, the lower surface state has A 3 ( P ~ , ~character ) and the upper state has A, ( s , p z ) character.

(lll)-(7x7)

i s c o n c e n t r a t e d near t h e m i d d l e o f t h e edges o f a

( 7 x 7 ) surface B r i l l o u i n zone as shown i n Fig.

11, and i t s i n t e n -

s i t y i s s e n s i t i v e t o t h e long-range (7x7) order. Additional

information

about

the

surface

geometry

can be

o b t a i n e d by measuring t h e s h i f t s i n energy and i n t e n s i t y of c o r e l e v e l s f o r s p e c i f i c surface

atoms.

The s u r f a c e - s e n s i t i v e angle-

i n t e g r a t e d photoemission s p e c t r a f o r Ge(3d) and S i ( 2p) core l e v e l s ( w i t h experimental mean-free paths o f 5.9 S i , r e s p e c t i v e l y ) a r e shown i n Fig.

12.

and 5.4

A f o r Ge and

By comparing s p e c t r a f o r

c l e a n ( f u l l l i n e s ) and hydrogen-covered ( d o t t e d l i n e s ) surfaces, i t i s c l e a r t h a t t h e r e are c o r e l e v e l s a t lower b i n d i n g energies which are c h a r a c t e r i s t i c o f t h e c l e a n s u r f a c e (marked by arrows

7.

PULSED LASER IRRADIATED SEMICONDUCTORS

Si(111) hv=120 eV

Ge(ll1) h v = 7 0 eV

7x7

2x8

-1.0

Fig.

12.

437

- j.0 0 1 .O 0 1.0 I N I T I A L STATE ENERGY RELATIVE TO BULK (eV)

Surface-sensitive

core-level

spectra f o r the, annealed Ge( 1 1 1 )

and S i ( l l 1 ) surfaces showing s h i f t e d c o r e levels f o r special surface atoms. The Ge data consits o f spin-orbit-split

3 d 3 / 2 and 3 d g / 2 levels, whereas in

t h e S i data the 2 p 1 / 2 levels have been removed by spin-orbit D o t t e d lines are f o r hydrogen-covered Si(ll1)

-

( 2 x 1 ) + H, r e s p e c t i v e l y ] ,

l o w e r binding energies are removed.

deconvolution. ( 1 x 1 ) + H and wherein the surface core levels at

surfaces [ G e ( l l l )

-

438

D. M. ZEHNER

i n Fig.

The r e s u l t s o f a l e a s t - s q u a r e s f i t t o t h e data a r e

12).

g i v e n i n Table 1 and can be summarized as f o l l o w s :

t h e annealed

Ge (111) and S i (111) s u r f a c e s have r o u g h l y 1/4 o f a monolayer o f s u r f a c e atoms,

w i t h l a r g e core-level

l o w e r b i n d i n g energy.

s h i f t s (0.6-0.8

L i t t l e difference

eV) t o w a r d

i s observed between

t h e r m a l l y annealed and l a s e r - a n n e a l e d surfaces. TABLE I S p e c i a l S u r f a c e Atoms f o r t h e Annealed Ge( 111) and S i ( 111) Surfaces Core-level s h i f t (towards lower binding energy, M.1 e v ) (ev)

Number o f atoms in v o l ved (20.05 l a y e r ) ( 1dyer)

Ge( 111)-( 2x8)

0.75 0.35

0.28 >O. 25

Ge( 111)-( 1x1)

0.60

0.37

S i ( 111)-(7 x 7 )

0.70

0.16

S i ( 111)-( 1x1)

0.80

0.23

The s t r o n g s i m i l a r i t y o f t h e valence band s u r f a c e s t a t e s and surface core-level s p e c t r a f o r b o t h t h e l a s e r - a n n e a l e d and t h e r m a l l y annealed S i and Ge s u r f a c e s i n d i c a t e s t h a t these s u r f a c e s have very s i m i l a r l o c a l bonding geometries and d i f f e r m a i n l y i n long-range o r d e r i n v o l v i n g g e o m e t r i c a l arrangements t h a t a r e o n l y a p e r t u r b a t i o n o f t h e average l o c a l bonding geometry.

An i n t e r -

e s t i n g q u e s t i o n t h e n i n v o l v e s t h e LEED analyses (Zehner e t al., 1981a; Zehner e t al.,

1 9 8 l b ) , which g i v e such good agreement w i t h

d a t a u s i n g a model ( 1 x 1 ) geometry t h a t appears t o be d i f f e r e n t f r o m t h a t needed t o d e s c r i b e t h e s u r f a c e e l e c t r o n i c s t r u c t u r e . One p o s s i b l e e x p l a n a t i o n i s t h a t LEED i s n o t p a r t i c u l a r l y s e n i t i v e t o long-range d i s o r d e r i f i t i s p r e s e n t on t h e ( 1 x 1 ) surface. Thus,

t h e i n t e r l a y e r displacements determined may be considered

439

7 . PULSED LASER IRRADIATED SEMICONDUCTORS

t o be averages over t h e coherence l e n g t h o f t h e e l e c t r o n beam. Another relaxed,

explanation

is

that

photoemission

can

rule

out

the

ordered ( 1 x 1 ) geometry o n l y i f t h e s u r f a c e s t a t e s a r e

band-1 ike as assumed i n one-el e c t r o n band c a l c u l a t i ons [ Pandy e t 1974; S c h l i i t e r e t a1

al.,

calculations

predict

., 1975;

that

such

C i r a c i e t al.,

1975).

These

a s u r f a c e would be m e t a l l i c ,

w i t h a h a l f - f i l l e d band o f d a n g l i n g bond s t a t e s a t t h e Fermi energy, EF, and t h i s i s i n c o n s i s t e n t w i t h t h e data, which show no emission near EF f o r t h e (1x1) surfaces.

However,

correlation

e f f e c t s might be very i m p o r t a n t f o r these narrow s u r f a c e l e v e l s .

A number o f researchers (Duke e t al., al.,

1981; Lannoo e t al.,

f o r t h theoretical

1981,

1982; L o u i s e t al.,

proposals

1982; Del Sole e t 1982, 1983) have p u t

t h a t would make t h e photoemission

d a t a from t h e laser-annealed S i (111) s u r f a c e c o n s i s t e n t w i t h t h e u n r e c o n s t r u c t e d r e l a x e d s u r f a c e p r e d i c t e d by t h e LEED a n a l y s i s . I n t h e s e models i t i s assumed t h a t s t r o n g c o r r e l a t i o n s dominate the

surface

state

band

structure,

and

they

predict

a

low-

temperature a n t i f e r r o m a g n e t i c ground s t a t e and downward d i s p e r s i o n o f t h e d a n g l i n g bond s t a t e s along r-J. have n o t been t e s t e d e x p e r i m e n t a l l y .

These p r e d i c t i o n s

Nevertheless,

e f f e c t s cannot e x p l a i n t h e s i m i l a r i t y i n c o r e - l e v e l

correlation shifts for

b o t h laser-annealed and t h e r m a l l y annealed surfaces. B o t h a n g l e - i n t e g r a t e d (McKinley e t a l . (Chabal e t al.,

, 1981) and angle-resolved

1981a) photoemission data have been obtained from

laser-annealed S i (111) s u r f a c e s u s i n g d i f f e r e n t annealing conditions.

I n agreement w i t h t h e r e s u l t s j u s t discussed and w i t h

r e s u l t s o b t a i n e d i n an independent i n v e s t i g a t i o n u s i n g a ruby l a s e r (Dernuth e t al.,

1984), no occupied s t a t e s a t EF are observed.

However, t h e energies of t h e s u r f a c e s t a t e s and t h e i r d i s p e r s i o n , o b t a i n e d a f t e r i r r a d i a t i o n w i t h e i t h e r a XeCl o r frequency-doubled Nd:YAG d i f f e r somewhat from t h e r e s u l t s presented u s i n g a ruby laser.

I n fact,

i t i s argued t h a t t h e laser-annealed

surface

examined i n these s t u d i e s i s buckled w i t h no long-range o r d e r b u t w i t h a short-range ( 2 x 1 ) r e c o n s t r u c t i o n .

From these r e s u l t s and

440

D. M. ZEHNER

t h o s e o b t a i n e d from t h e r m a l l y

quenched S i

(111)

surfaces,

it

appears t h a t d i f f e r e n t l a s e r a n n e a l i n g c o n d i t i o n s (depth o f m e l t , r e g r o w t h v e l o c i t y ) can r e s u l t i n d i f f e r e n t l o c a l bonding arrangements.

8.

VICINAL SURFACES The

chemical

influence o f

steps

reactivity of

on t h e e l e c t r o n i c

properties

semiconductor s u r f a c e s a r e well

and

known.

Stepped ( v i c i n a l ) s u r f a c e s can be prepared by i n s i t u c l e a n i n g o r i o n etching,

but t h e control o f step density,

step height,

and

ease o f r e p r o d u c i b i l i t y has proved d i f f i c u l t u s i n g t h e s e convent i o n a l procedures.

The r a p i d m e l t i n g and regrowth achieved w i t h

l a s e r a n n e a l i n g suggest t h a t t h i s procedure can be used w i t h vicinal

surfaces t o produce s u r f a c e s c o n t a i n i n g monatomic steps

and u n i f o r m t e r r a c e widths. To demonstrate t h a t such s u r f a c e s can be produced, o b t a i n e d from a S i ( l l 1 ) f r o m a (111) plane (Zehner e t a1

c r y s t a l whose s u r f a c e was c u t a t 4.3'

toward t h e

., 1980b).

results

[ i i 2 ] d i r e c t i o n w i l l be discussed

F o r t h i s d i r e c t i o n , t h e edge atoms have

o n l y two n e a r e s t neighbors.

The w e l l - d e f i n e d (1x1)

LEED p a t t e r n

o b t a i n e d from t h e c l e a n s u r f a c e and shown i n Fig. 13 ( a ) was observed after

i r r a d i a t i n g t h e s u r f a c e with f i v e p u l s e s a t -2.0

J/cm2.

The p a t t e r n i n d i c a t e s t h e e x i s t e n c e o f a stepped s u r f a c e which can be indexed [ 1 4 ( 1 1 1 ) x ( i i 2 ) ] . energy, and [ O l ]

By v a r y i n g t h e p r i m a r y e l e c t r o n

t h e t h r e e f o l d spot s p l i t t i n g a l t e r n a t e s between t h e r e f l e c t i o n s a t s p e c i f i c e l e c t r o n energies.

[lo]

The energies

a t which a g i v e n r e f l e c t i o n i s s p l i t or n o n s p l i t g i v e s p e c i f i c i n f o r m a t i o n on t h e s t e p h e i g h t , and t h e angular s e p a r a t i o n between s p l i t spots provides i n f o r m a t i o n on t h e t e r r a c e width.

An a n a l y s i s

o f t h e spot s p l i t t i n g s i n t h i s p a t t e r n u s i n g o n l y a k i n e m a t i c t r e a t m e n t o f s i n g l e s c a t t e r i n g from t h e t o p l a y e r (Henzler, 1970) i n d i c a t e s t h a t t h e s u r f a c e c o n s i s t s o f monatomic s t e p s w i t h an average s t e p h e i g h t o f one double l a y e r (3.14 w i d t h s -45 A as i l l u s t r a t e d i n Fig. 14.

A)

with terrace

The absence o f f r a c t i o n a l

7.

Fig.

13.

441

PULSED LASER IRRADIATED SEMICONDUCTORS

LEED

patterns from clean vicinal

beam energies o f ( a ) 40 and ( b ) 68 eV.

Si(ll1 )

surfaces a t primary

( a ) Laser annealed, (b) thermally

annealed.

LASER ANNEALED

-

( 4 x 4 ) WITH SPLIT SPOTS

4;3"

THERMALLY ANNEALED

-

(7 x 7 )

4.30

Fig.

14.

Schematic

the (710) plane.

illustration o f the vicinal

Top view i s for the laser-annealed

surface projected into surface.

Bottom view

illustrates a possible configuration obtained with thermal annealing.

442

D. M. ZEHNER

order r e f l e c t i o n s ,

i n d i c a t i v e of

reconstruction,

suggests t h a t

t h e l o c a l atomic arrangement produced by t h i s a n n e a l i n g procedure may be s i m i l a r t o t h a t produced on f l a t (111) surfaces. I n o r d e r t o achieve such a h i g h step d e n s i t y c o n f i g u r a t i o n , a l a r g e amount o f atom motion has t o t a k e place.

T h i s movement can

be accomplished e i t h e r by e v a p o r a t i o n o f s u r f a c e atoms o r by d i f f u s i o n i n t h e molten phase.

R e s u l t s o f r e c e n t experiments w i t h

stepped s u r f a c e s (Osakabe e t al., evaporation 1475

K.

1980, 1981) show t h a t some

t a k e s p l a c e a t step edges a t temperatures as low as

Assuming m e l t i n g occurs d u r i n g t h e l a s e r a n n e a l i n g con-

d i t i o n s used, about

lo9

atoms/cm2 evaporate i n a 10-ns p u l s e f o r

an e v a p o r a t i o n r a t e o f 1017 atoms/cmzs [vapor p r e s s u r e 5 x 10-3 T o r r (Chabel e t al., monolayer,

T h i s corresponds t o o n l y 10-6 o f a

1982)].

which i s n o t enough t o account f o r t h e l a r g e atomic

rearrangements over hundreds o f angstroms. mechanism must dominate, be e s t i m a t e d D

2

Thus,

the diffusion

and a s u r f a c e d i f f u s i o n c o e f f i c i e n t can’

(100 A ) 2 / ( 10 ns) -loe4 cm2/s.

This high d i f -

f u s i o n c o e f f i c i e n t would be q u i t e i n c o m p a t i b l e w i t h a nonthermal model

of

l a s e r a n n e a l i n g b u t i s c o n s i s t e n t w i t h experimental

measurements ( N i shizawa e t a1 the melting

point,

the

., 1972).

surface

A t temperatures c l o s e t o

arrangement

i s dominated

by

e n t r o p y , which i s r e s p o n s i b l e f o r a s t e p - s t e p r e p u l s i o n (Gruber et al., disorder

1967).

is

As t h e c r y s t a l c o o l s down and t h e e n t r o p y - d r i v e n

reduced,

the

surface d i f f u s i o n

decreases

t o the

e x t e n t t h a t t h e steps cannot recombine; t h e y remain f r o z e n i n t h e high-temperature c o n f i g u r a t i o n . The s t a b i l i t y o f t h e r e g u l a r a r r a y o f steps was i n v e s t i g a t e d by s u b j e c t i n g t h e laser-annealed s u r f a c e t o a s e r i e s o f thermala n n e a l i n g t r e a t m e n t s a t h i g h e r and h i g h e r temperatures. f o r f l a t (111) S i surfaces,

As observed

thermal annealing o f t h e c r y s t a l t o

temperatures g r e a t e r than -800 K r e s u l t e d i n a s u r f a c e from which t h e ( 7 x 7 ) d i f f r a c t i o n p a t t e r n shown i n Fig. 13 ( b ) was o b t a i n e d i n accord w i t h p r e v i o u s o b s e r v a t i o n s (Olshanetsky e t a l .

, 1979).

The

absence o f s p l i t t i n g of i n d i v i d u a l spots i n d i c a t e s t h e e l i m i n a t i o n

7.

443

PULSED LASER IRRADIATED SEMICONDUCTORS

o f t h e r e g u l a r a r r a y o f monatomic steps, and t h e sharpness o f t h e integral-order

reflections

i s c o n s i s t e n t w i t h a s u r f a c e having

t e r r a c e s wider than -200

A.

macroscopic

inclination,

multilayer

illustrated

i n Fig.

14.

I n o r d e r t o m a i n t a i n t h e average steps must be present

as

A s u r f a c e c o n t a i n i n g monatomic steps

c o u l d be regenerated by i r r a d i a t i n g t h e t h e r m a l l y annealed surface with the laser.

These o b s e r v a t i o n s i n d i c a t e t h a t i t i s

p o s s i b l e t o produce r e p e a t e d l y a p a r t i c u l a r s t e p arrangement by i n i t i a l l y c u t t i n g the crystal t o the desired orientation. I n v e s t i g a t i o n s o f v i c i n a l S i (111) s u r f a c e s c u t a l o n g t h e [ i i 2 ] d i r e c t i o n have produced r e s u l t s very s i m i l a r t o those discussed above (Chabal e t al.,

1981b).

Steps along t h i s d i r e c t i o n c o n t a i n

edge atoms t h a t have t h r e e nearest neighbors.

Although d e t a i l e d

s t u d i e s on t h e angular p r o f i l e s show t h e step h e i g h t t o be 3.06 A i n t h i s d i r e c t i o n , somewhat l e s s than t h e d o u b l e - l a y e r separation, t h e o v e r a l l behavior f o r laser-annealed v i c i n a l surfaces i s t h e same f o r b o t h types o f steps.

9.

DEFECTS As a consequece o f m e l t i n g d u r i n g t h e l a s e r annealing process,

atoms a r e evaporated from t h e s u r f a c e region.

I n f a c t , measure-

ment o f S i p a r t i c l e emission d u r i n g e v a p o r a t i o n u s i n g a c l a s s i c a l t i m e - o f - f l i g h t technique has been used t o determine t h e l a t t i c e temperature and t o demonstrate t h a t me1 t i n g occurs ( S t r i t z k e r e t al.,

1981).

I n a d d i t i o n t o n e u t r a l p a r t i c l e emission, b o t h i o n

and e l e c t r o n e j e c t i o n s have been d e t e c t e d (Moison, e t a1

., 1982).

The t h r e s h o l d f l u e n c e r e q u i r e d f o r d e t e c t i o n o f such p a r t i c l e emission bas been determined f o r a number o f m a t e r i a l s (Moison e t al.,

1983).

R e s u l t s o b t a i n e d f o r InP and GaAs s i n g l e c r y s t a l s

a r e c o n s i s t e n t w i t h AES and RBS observations, erential Si,

l o s s o f t h e more v o l a t i l e component.

indicating a prefI n t h e case o f

t h e amount o f m a t t e r removed was observed t o be orders o f

magnitude l e s s .

444

D. M. ZEHNER

The o b s e r v a t i o n t h a t e v a p o r a t i o n occurs d u r i n g l a s e r a n n e a l i n g i n d i c a t e s t h a t t h e c r e a t i o n o f d e f e c t s i s p o s s i b l e and t h a t d u r i n g t h e quenching p e r i o d a c o m p e t i t i o n t a k e s p l a c e between t h e e l i m i n a t i o n o f d e f e c t s c r e a t e d a t t h e m e l t i n g temperature and t h e growth o f an ordered s u r f a c e

region.

T h i s p o s s i b i l i t y may be par-

t i c u l a r l y i m p o r t a n t i n t h e case o f r e c o n s t r u c t e d s u r f a c e l a y e r s . I f t h e d e f e c t s a r e n o t e l i m i n a t e d f a s t enough,

t h e y may impede

growth o f t h e s u p e r s t r u c t u r e by v a r i o u s mechanisms.

Thus,

the

r e g r o w t h v e l o c i t y o f t h e m e l t f r o n t may p l a y an i m p o r t a n t r o l e i n the d e t a i l s o f the

f i n a l geometric o r d e r i n g .

been suggested (Chabel e t al.,

I n fact,

i t has

1982) t h a t t h e d i f f e r i n g photo-

emission r e s u l t s o b t a i n e d i n independent laser-anneal i n g s t u d i e s can be i n t e r p r e t e d as a consequence o f d i f f e r e n t f i n a l

state

geometric o r d e r i n g due t o d i f f e r e n t regrowth v e l o c i t i e s . I n order

t o e x p l o r e t h e dependence on regrowth v e l o c i t y ,

measurements have been made subsequent t o annealing w i t h a pulsed XeCl excimer l a s e r ( A = 308 nm) (Zehner e t a1

., 1984a).

Measurements

were made a f t e r l a s e r a n n e a l i n g t h e c r y s t a l w i t h an energy d e n s i t y i n t h e range 1.0-4.0 (Wood and G i l e s ,

J/cm2.

Standard heat f l o w

calculations

1981) have been used t o e s t a b l i s h t h a t a v a r i a -

t i o n i n regrowth v e l o c i t y from 1 m/s a t 4.0 J / c d t o 4.5 1.0

J/cm2

m/s a t

can be o b t a i n e d w i t h t h e excimer l a s e r used i n t h i s

experiment (see Chapter 4).

R e s u l t s o b t a i n e d from p h o t o e l e c t r o n

spectroscopy

determine

were

used

to

s t r u c t u r e o f v a r i o u s laser-annealed annealed S i ( l l 1 )

-

(7x7),

the

surface

-

S i ( 111)

and cleaved S i ( l l 1 )

-

electronic

(1x1)

,

thermally

( 2 x 1 ) surfaces.

The s u r f a c e s t a t e s near t h e t o p o f t h e band are i m p o r t a n t s i n c e t h e y have c h a r a c t e r i s t i c energies and angular d i s t r i b u t i o n s t h a t have been s t u d i e d p r e v i o u s l y (Zehner e t al.,

1 9 8 1 ~ ) . I n Fig.

15

t h e energy d i s t r f b u t i o n s o f s u r f a c e s t a t e s near t h e t o p o f t h e valence band a r e shown f o r v a r i o u s S i ( l l 1 ) surfaces.

As evidenced

by t h e s e n s i t i v i t y t o hydrogen exposure ( n o t shown), s u r f a c e s e x h i b i t t h r e e dominant Fig.

the (7x7)

s u r f a c e s t a t e s l a b e l e d 1-3 i n

15 and t h e ( 2 x 1 ) s u r f a c e i s dominated by two s u r f a c e s t a t e s

7.

445

PULSED LASER IRRADIATED SEMICONDUCTORS

-6

-5

-3

4

-2

-1

0

1

INITIAL ENERGY ( RELATIVE TO VALENCE BAND MAXIMUM )

Fig.

15.

Angle-integrated

spectra from freshly cleaved S i ( 1 1 1 )-( 2x1 )

,

UV (308 nm XeCI) laser-annealed Si( 1 1 1 )-( 1x1 ) produced with 1, 2, 3 , and 4 J / c m 2 pulses, ruby (694 n m ) laser-annealed Si( 1 1 1 )(lxl) produced with a 2 J /cm2 pulse, and

Si (1 1 1 )-(7x7 )

obtained by thermal annealing.

446

D. M. ZEHNER

l a b e l e d 4 and 5.

For t h e v a r i o u s laser-annealed surfaces,

two

s u r f a c e s t a t e s which c l o s e l y resemble s t a t e s 1 and 2 on t h e ( 7 x 7 ) s u r f a c e and a r e t o t a l l y d i f f e r e n t from those observed on t h e c l e a v e d (2x1) s u r f a c e were i d e n t i f i e d . change

of

variation

this of

surface

state

regrowth v e l o c i t y

Moreover, no s i g n i f i c a n t

structure from

was

1-4.5

observed over

m/s,

apart

a

from a

weakening o f t h e s u r f a c e s t a t e s f o r t h e f a s t e s t regrowth velocity.

T h i s weakening c o u l d be due t o t h e onset o f d i s o r d e r when

t h e energy d e n s i t y employed approaches t h e me1t t h r e s h o l d . I t i s known t h a t t h e S i ( l l 1 ) s u r f a c e undergoes a s t r u c t u r a l

change from (7x7) ( B e n n e t t e t al.,

t o (1x1) a t a c r y s t a l temperature o f 1150 K 1981).

For very low c o o l i n g r a t e s t h e s t r u c -

t u r a l t r a n s i t i o n i s reversible,

b u t i f quenching r a t e s exceed

approximately

lo2

irreversible.

Consequently, t h e quenching r a t e a t t h e t r a n s i t i o n

K / s (Hagstrum e t al.,

1973), t h e t r a n s i t i o n i s

temperature, subsequent t o l a s e r i r r a d i a t i o n , may be i m p o r t a n t i n When S i ( 111) i s quenched

d e t e r m i n i n g t h e s u r f a c e s t a t e spectra.

t h r o u g h t h e t r a n s i t i o n temperature a t 102 K/s, s u r f a c e s t a t e s p e c t r a s i m i l a r t o those f o r t h e laser-annealed surfaces shown i n Fig. 15 a r e observed (Eastman e t al.,

1980b).

Heat f l o w c a l c u l a t i o n s f o r

l a s e r a n n e a l i n g a t 1.0 J/cm2 p r e d i c t a quenching r a t e o f 1010 K / s a t 1150 K.

Thus,

s i m i l a r s u r f a c e s t a t e s p e c t r a are observed f o r

quenching r a t e s between 102 and 1010 K/s.

I f t h e quenching r a t e

a t t h e t r a n s i t i o n temperature i s o f importance i n d e t e r m i n i n g t h e surface

state

spectra,

rates

i n excess

of

1010 K / s

will

be

necessary t o produce s u r f a c e s t a t e f e a t u r e s s i m i l a r t o those o f t h e (2x1) surface.

Furthermore, f o r regrowth v e l o c i t i e s g r e a t e r

t h a n 15 m/s, where an amorphous l a y e r i s formed ( C u l l i s e t a l . , 1982),

one would expect t o see s u b s t a n t i a l

differences i n the

s u r f a c e s t a t e spectra. Additional

i n f o r m a t i o n about b o t h t h e s i m i l a r i t i e s and d i f -

ferences i n geometric s u r f a c e s t r u c t u r e f o r s u r f a c e s prepared by d i f f e r e n t t r e a t m e n t s can be obtained by i n v e s t i g a t i n g a d s o r p t i o n phenomena.

The technique o f h i g h - r e s o l u t i o n i n f r a r e d spectroscopy

7.

447

PULSED LASER IRRADIATED SEMICONDUCTORS

(Chabel , 1983) has been used t o study t h e v i b r a t i o n a l spectrum o f hydrogen chemisorbed on S i ( l l l ) - ( 7 x 7 ) prepared by thermal a n n e a l i n g and S i ( l l 1 ) - ( 1 x 1 )

prepared by l a s e r annealing.

T h i s technique

g i v e s d i r e c t i n f o r m a t i o n on t h e number, p o s i t i o n , and p o i a r i z a t i o n o f d a n g l i n g bonds, which a r e present a t t h e s u r f a c e o f a semiconductor.

For coverages as low as 1.5% o f a monolayer o f hydrogen

on t h e S i ( l l l ) - ( 7 x 7 ) observed.

surface,

two d i s i i n c t a d s o r p t i o n peaks are

Each peak corresponds t o a S i - H s t r e t c h i n g v i b r a t i o n

f o r hydrogen chemisorbed a t d i f f e r e n t s i t e s .

By i n v e s t i y a t i n g

t h e change i n i n t e n s i t y and energy o f these v i b r a t i o n s i t i s concluded t h a t a unique chemisorption s i t e e x i s t s on t h i s s u r f a c e and i s recessed from t h e outermost plane.

R e s u l t s o b t a i n e d from

t h e laser-annealed S i ( 1 1 1 ) - ( 1x1) s u r f a c e show o n l y one a d s o r p t i o n peak.

The peak a s s o c i a t e d w i t h t h e unique a d s o r p t i o n s i t e i s

absent.

This

observation

strongly

suggests

that

the

unique

a d s o r p t i o n s i t e on t h e (7x7) s u r f a c e i s a r e s u l t o f long-range rearrangement

which

i s absent

on t h e

1aser-anneal ed surface.

Since b o t h a (1x1) u n r e c o n s t r u c t e d b u t r e l a x e d s u r f a c e as d e t e r mined i n t h e LEED a n a l y s i s and a m o s t l y d i s o r d e r e d s u r f a c e as determined by PES would n o t c o n t a i n such a w e l l d e f i n e d hole, t h e s e r e s u l t s cannot be used t o d i s c r i m i n a t e between t h e proposed structures. Rare gas t i t r a t i o n i s another technique used t o i n v e s t i g a t e geometric s t r u c t u r e .

The approach employed i s based on t h e concept

t h a t d i f f e r e n t geometric a d s o r p t i o n s i t e s f o r r a r e gas atoms can have d i f f e r e n t l o c a l work f u n c t i o n s .

Such l o c a l work f u n c t i o n

d i f f e r e n c e s produce d i f f e r e n t e l e c t r o n b i n d i n g energies re1a t i v e t o EF f o r these adsorbed atoms,

which a l l o w t h e d e l i n e a t i o n o f

v a r i o u s s i t e s as w e l l as t h e d e t e r m i n a t i o n o f t h e i r r e l a t i v e conc e n t r a t i o n s when examined w i t h PES. Si(lll)-(7x7) show

Recent i n v e s t i g a t i o n s o f t h e

s u r f a c e f o r xenon a d s o r p t i o n (Demuth e t a1

coverage-dependent

changes

i n t h e measured

., 1984)

PES b i n d i n g

e n e r y i e s a t b o t h h i g h and low coverages i n e i t h e r a d s o r p t i o n (as l o n g as near e q u i l i b r i u m a d s o r p t i o n c o n d i t i o n s a r e maintained) or

448

D. M. ZEHNER

d e s o r p t i o n experiments.

The sequence and number o f a d s o r p t i o n

s i t e s found f o r t h i s s u r f a c e are c o n s i s t e n t w i t h ( 1 ) a s p e c i a l h i g h b i n d i n g energy s i t e a t low coverages,

(2) a majority o f

nearly

equivalent

sites

over

surface

higher

coverages

where

rare-gas

most o f

the

adatom

(including

interactions

become

and ( 3 ) another t y p e o f m i n o r i t y s i t e p r i o r t o f o r -

important),

m a t i o n o f condensed o r m u l t i l a y e r s .

These r e s u l t s are c o n s i s t e n t

w i t h proposed s t r u c t u r a l models f o r t h e (7x7) s u r f a c e which have adatoms. (ruby

S i m i l a r measurements have been made on a laser-annealed S i ( 111)-(1x1)

laser)

surface.

The s i m i l a r i t i e s

i n the

r e s u l t s o b t a i n e d from t h i s s u r f a c e and those from t h e (7x7) surf a c e suggest t h e e x i s t e n c e o f adatoms.

This conclusion i s i n

c o n t r a s t t o t h e LEED r e s u l t s s u p p o r t i n g a f l a t ,

compressed s u r -

face.

A s t e p can be t r e a t e d as a d e f e c t and ordered a r r a y s o f such d e f e c t s produced by l a s e r a n n e a l i n g have been considered i n t h e discussion o f v i c i n a l

It i s w e l l

surfaces.

known t h a t l a s e r -

annealed s u r f a c e s have a r i p p l e d topography when examined on a macroscopic s c a l e (Leamy e t al.,

1978).

T h i s i m p l i e s t h a t steps,

randomly d i s t r i b u t e d , must e x i s t on such surfaces.

The p o s s i b i l -

i t y t h a t t h e S i ( 111)-(1x1) s u r f a c e s t r u c t u r e observed a f t e r l a s e r

annealing

can

be

associated

(Haneman,

1982; Moisum e t al.,

m i n i m i z e i t s f r e e energy.

with

steps

1983).

has

been

A surface reconstructs t o

The l o w e r i n g i n f r e e energy achieved

by r e c o n s t r u c t i o n can be e s t i m a t e d t h e o r e t i c a l l y , g r e a t accuracy, entropy.

considered

but not w i t h

due t o d i f f i c u l t i e s w i t h c o r r e l a t i o n e f f e c t s and

The presence o f s t r a i n w i l l t e n d t o oppose t h i s e f f e c t .

Based on

results

suggested (Haneman,

from a v a r i e t y

o f experiments

it

has been

1982) t h a t a s t r a i n e d r e g i o n a t t h e base o f

s t e p s on laser-annealed ( 111) surfaces causes s u r f a c e r e c o n s t r u c t i o n t o be i n h i b i t e d ,

r e s u l t i n g i n a (1x1)

surface structure.

Furthermore, i t i s suggested t h a t t h e behavior o f (100) surfaces, where t h e laser-annealed s t r u c t u r e i s t h e same as t h a t produced by thermal

annealing,

i s then

not

unexpected s i n c e t h e s t e p

7.

449

PULSED LASER IRRADIATED SEMICONDUCTORS

s t r u c t u r e s are o f d i f f e r e n t c r y s t a l l o g r a p h y and t h e r e i s no s i m i l a r evidence f o r step-associated s t r a i n .

V.

Surface and Subsurface S t u d i e s o f Ion-Implanted S i l i c o n

P r e v i o u s i n v e s t i y a t i o n s (White e t al.,

1980b) have shown t h a t

group I11 o r V i m p l a n t s occupy s u b s t i t u t i o n a l s i t e s subsequent t o l a s e r annealing and t h a t , as a consequence o f b o t h t h e h i g h l i q u i d phase d i f f u s i v i t i e s and t h e h i g h values o f d i s t r i b u t i o n c o e f f i c i e n t s , t h e y are a b l e t o d i f f u s e i n t o t h e c r y s t a l d u r i n g t h e regrowth I n c o n t r a s t , i t has been shown (White

process a f t e r i r r a d i a t i o n . e t al.,

1980c) t h a t those i m p l a n t s which do n o t form c o v a l e n t

bonds e x h i b i t , dependiny on t h e i m p l a n t dose, s e g r e g a t i o n t o t h e s u r f a c e as w e l l as t h e f o r m a t i o n o f a c e l l s t r u c t u r e subsequent t o l a s e r annealing.

The RBS and secondary i o n mass spectroscopy

( S I M S ) techniques employed i n these i n v e s t i g a t i o n s p r o v i d e d e t a i l e d

i n f o r m a t i o n about t h e d i s t r i b u t i o n w i t h respect t o depth b u t prov i d e no i n f o r m a t i o n about t h e c o n c e n t r a t i o n i n t h e s u r f a c e r e g i o n (<20 A )

and t h e changes which occur w i t h m u l t i p l e - p u l s e

diation.

Thus,

irra-

it i s o f i n t e r e s t t o u t i l i z e surface s e n s i t i v e

techniques i n o r d e r t o p r o v i d e complementary i n f o r m a t i o n . By u s i n g m u l t i p l e - p u l s e

irradiation,

the levels o f the prin-

c i p a l contaminants on s u r f a c e s o f i o n - i m p l a n t e d S i c r y s t a l s , 0 and C, can be reduced t o t h e p o i n t where t h e y a r e n o t d e t e c t a b l e i n Auger

s p e c t r a o b t a i n e d from t h e - s u r f a c e region.

However,

s i n c e t h e e x t e n t o f d i f f u s i o n o r s e g r e g a t i o n o f t h e implanted species i s known t o be a f u n c t i o n o f t h e number o f l a s e r pulses, a more a p p r o p r i a t e procedure i s t o s p u t t e r t h e samples i n i t i a l l y . T h i s r e s u l t s i n removal o f most o f t h e 0 and C s u r f a c e contaminants, and t h e l e v e l o f c l e a n l i n e s s a f t e r one p u l s e i s h i g h e r than

that

obtained without

sputteriny.

material affected during sputteriny,

-50 A,

Since t h e depth o f i s much s m a l l e r than

t h e depth o f t h e i m p l a n t e d r e g i o n , t y p i c a l l y -1000-2000

A,

the

s p u t t e r i n g process has l i t t l e o r no e f f e c t on t h e subsequent chanyes

i n t h e i m p l a n t r e d i s t r i b u t i o n t h a t occur d u r i n g l a s e r

450

D. M. ZEHNER

irradiation.

This

has

been

verified

by

comparing

results

o b t a i n e d f o l 1owing l a s e r anneal ing o f imp1 anted c r y s t a l s t h a t had been s p u t t e r e d w i t h t h o s e t h a t had not. sputtering

species

laser irradiation,

i s removed

Moreover,

from t h e s u r f a c e

since t h e

reyion during

as discussed i n S e c t i o n 111, i t cannot have

any e f f e c t on t h e r e d i s t r i b u t i o n o f t h e implanted species i n t h e s u r f a c e region.

For most i m p l a n t c o n d i t i o n s , as a consequence o f

t h e Gaussian-like

distribution,

the concentration o f

implanted

species i n t h e s u r f a c e r e y i o n i s n o t d e t e c t a b l e w i t h AES e i t h e r a f t e r insertion

10.

or a f t e r s p u t t e r i n g .

SUBSTITUTIONAL IMPLANTS R e s u l t s obtained on a Si(100) sample i m p l a n t e d w i t h 75As(100 keV,

8.3 x 1.0’6/cm2)

u s i n g t h e RBS technique are shown i n Fig. 16 and

i l l u s t r a t e t h e r e d i s t r i b u t i o n o f implanted species t h a t occurs w i t h multiple-pulse

irradiation.

Auger data were a l s o obtained

from t h i s sample f o l l o w i n g s i m i l a r i r r a d i a t i o n c o n d i t i o n s .

With

t h e s e d a t a t h e r a t i o o f t h e i n t e n s i t y o f t h e As(31 eY) Auger t r a n s i t i o n t o t h a t o f t h e S i ( 9 l eV) t r a n s i t i o n i s shown i n F i g . 17, where i t i s p l o t t e d as a f u n c t i o n o f t h e number o f l a s e r pulses. The data show t h a t t h e r e l a t i v e amount o f As i n t h e s u r f a c e r e g i o n decreases w i t h an i n c r e a s i n g number o f pulses, s i m i l a r t o t h e RBS r e s u l t s obtained f o r t h e subsurface r e y i o n .

Although n o t shown i n

t h i s f i g u r e , l i t t l e change i s observed i n t h e s u r f a c e c o n c e n t r a t i o n a f t e r a l a r g e number (>15) o f pulses where RBS r e s u l t s i n d i c a t e uniform concentration liquid-solid

interface.

from t h e subsurface

r e y i o n down t o t h e

I t i s d i f f i c u l t t o q u a n t i f y t h e AES

r e s u l t s t o t h e same degree as can be done w i t h t h e RBS data. Thus, a l t h o u g h i t can be concluded t h a t a r e d u c t i o n i n concentrat i o n occurs w i t h m u l t i p l e - p u l s e i r r a d i a t i o n , t h e r e c o u l d s t i l l be a p o s s i b l e chanye i n c o n c e n t r a t i o n i n going from t h e s u r f a c e t o subsurface r e g i o n t h a t

i s a consequense o f t h e surface-vacuum

451

7 . PULSED LASER IRRADIATED SEMICONDUCTORS

5

2

5

Fig.

16.

in S i ( 1 0 0 )

E f f e c t o f laser annealing on dopant profiles for As implanted as determined by RBS.

Profile

results are

implanted condition and subsequent to laser annealing at -2.0

shown

for

J/cm2.

as-

452

D. M. ZEHNER

gJ O

2

I

I

I

I

I

I

I

4 6 0 10 12 NUMBER OF PULSES (E0-2.1 J/cm2)

14

Fig. 17. Plot of the ratio of the As M W ( 3 1 e V ) to Si L W ( 9 1 eV) 4,5 283 Auger transition intensities as a function of the number of laser pulses.

interface. v a r i e t y of

Similar

Auger

results

have

been

obtained

for

a

i m p l a n t e d doses o f s u b s t i t u t i o n a l dopants i n S i ( l 0 U )

and (111) c r y s t a l s .

To examine t h e e f f e c t o f t h e i m p l a n t e d s p e c i e s on s u r f a c e o r d e r , LEED p a t t e r n s have been o b t a i n e d from t h e same 7%-implanted S i ( 1 0 0 ) sample.

Only a very weak,

was observed a f t e r

one l a s e r pulse.

p o o r l y d e f i n e d LEED p a t t e r n F o l l o w i n g two p u l s e s o f

i r r a d i a t i o n , t h e p a t t e r n shown a t t h e t o p o f F i g . 18 was obtained. I n t e g r a l o r d e r beams a r e observed, as w e l l as weak s t r e a k s between them.

With a d d i t i o n a l l a s e r pulses t h e s t r e a k s b e g i n t o coalesce

7 . PULSED LASER IRRADIATED SEMICONDUCTORS

Fig. 18.

LEED patterns from an As-implated Si( 100) surface a t a primary

beam energy of 49 eV.

-2.0

453

Patterns are shown subsequent to laser annealing at

J / m 2for ( a ) 2 , ( b ) 5 , and ( c ) 10 pulses.

454

D. M.ZEHNER

i n t o half-order reflections, surface structure.

i n d i c a t i n g t h e f o r m a t i o n o f a (2x1)

They c o n t i n u e t o become sharper and more

i n t e n s e w i t h a d d i t i o n a l pulses, as shown i n t h e f i g u r e .

However,

t h e p a t t e r n observed a f t e r t e n l a s e r pulses i s n o t as good as t h a t o b t a i n e d from a v i r g i n Si(100) c r y s t a l as shown i n Fig. 5 and t h u s i n d i c a t e s t h e presence o f d i s o r d e r i n t h e s u r f a c e region. Nevertheless,

i t i s i n t e r e s t i n g t o note t h a t t h e (2x1) LEE0 pat-

t e r n shows t h e e x i s t e n c e o f t h e r e c o n s t r u c t e d surface, s i m i l a r t o that

obtained

results

have

from a been

virgin

obtained

Si(100) for

a

crystal.

variety

Similar

of

LEED

substitutional

dopants i n Si(100) w i t h t h e q u a l i t y o f t h e LEED p a t t e r n o b t a i n e d f o r a s p e c i f i c l a s e r annealing c o n d i t i o n decreasing w i t h i n c r e a s i n g i m p l a n t dose.

I n c o n t r a s t t o these o b s e r v a t i o n s , (1x1) LEED p a t t e r n s were o b t a i n e d from S i ( l l 1 ) c r y s t a l s i m p l a n t e d w i t h a group I11 o r V dopant and then l a s e r annealed.

The p a t t e r n s are o f much h i g h e r

q u a l i t y a f t e r a g i v e n number o f l a s e r pulses when compared w i t h t h o s e obtained from t h e (100) surfaces, and t h e y show no evidence o f ordered l a t e r a l r e c o n s t r u c t i o n . The o b s e r v a t i o n t h a t l a s e r anneal i n y can be combined w i t h i o n i m p l a n t a t i o n t o p r o v i d e semiconductor s u r f a c e r e g i o n s c o n t a i n i n g n o v e l doping c o n c e n t r a t i o n s ( s u p e r s a t u r a t e d a l l o y s ) suggests t h a t t h e s e t e c h n i q e s may be used t o a l t e r o r t a i l o r t h e e l e c t r o n i c s t r u c t u r e i n t h i s region.

To examine t h i s p o s s i b i l i t y , photoemis-

s i o n techniques have been used t o i n v e s t i y a t e h i g h l y degenerate n-type S i ( l l 1 )

-

(1x1) surfaces as a f u n c t i o n o f As c o n c e n t r a t i o n

up t o - 5 x lO21/cm3 (-10 a t . %) and degenerate p-type S i ( l l 1 )

-

( 1 x 1 ) surfaces as a f u n c t i o n o f B c o n c e n t r a t i o n up t o -1 x 1021/cm3

( - 2 at.

% ) (Eastman e t a1

centrations electrically

are

about

., 1981).

10 and

These maximum doping con-

3 times

the concentrations o f

a c t i v e As and B a c h i e v a b l e by c o n v e n t i o n a l t e c h -

niques, r e s p e c t i v e l y . Angl e - i n t e g r a t e d photoemi s s i on s p e c t r a f o r t h e valence bands a r e presented i n F i g . 19 f o r i n t r i n s i c S i ( l l 1 )

-

( l x l ) , degenerate

7.

455

PULSED LASER IRRADIATED SEMICONDUCTORS

I

I

I

I

h u = 21 eV s / p POL. ANGLE-INTEG.

I

I

I

1 I

A.R.

A

7% AS 1.1 eV

INTRINSIC^

X- POCKEl

I

-I!

‘\ \“‘z”

(EF-EvIs =0.5

1 -8

-6

-2

-4

ENERGY (eV) Fig. valence

19.

Photoemission spectra ( p a r t i a l density o f states PDOS) for the

bands

of

highly doped Si. states.

laser-annealed

( 1 11 )-( 1 x 1 )

The levels near -0.4

ES, ES, and E denote the v c F

band minimum, and Fermi-level

surfaces o f

and -1 .3

valence-band

intrinsic

and

eV are due to surface maximum,

positions at the surface.

conduction-

456

D. M.ZEHNER

n-type As-doped ( 4 and 7 a t . %) S i ( l l 1 ) ( 1 a t . %) S i ( l l 1 )

p-type B-doped are

normalized

to

constant

-

-

( l x l ) , and degenerate

(1x1) surfaces.

total

emission

The s p e c t r a

within

5

eV

of

EF, and e n e r y i e s are g i v e n r e l a t i v e t o t h e valence-band maximum a t the

surface

(E:).

EF i s seen t o eV above E:

from 0.25

(i.e.,

shift

for the

markedly w i t h

doping

B-doped sample t o t h e con-

d u c t i o n band minimum Ec = 1.1 eV f o r t h e 7% As-doped sample). Relative t o i n t r i n s i c Si, doping,

f o r h i g h l y degenerate ( 1 a t .

%) B

t h e two s u r f a c e s t a t e s are u n a l t e r e d , and t h e p r i n c i p a l

changes a r e t h a t EF moves down by 0.25 eV and t h e s u r f a c e becomes metallic. at.

More dramatic e f f e c t s are seen w i t h As doping.

At 4

% As doping,

t h e s u r f a c e s t a t e s have become s i g n i f i c a n t l y

EF

has i n c r e a s e d by 0.1 eV r e l a t i v e t o t h e i n t r i n -

altered, while sic Si.

That i s ,

t h e upper "sp,-like"

d a n g l i n g bond s t a t e has

become much weaker and s h i f t e d upward i n energy by -0.3 l o w e r -1.4

eV; t h e

eV s t a t e has i n c r e a s e d s i g n i f i c a n t l y i n i n t e n s i t y , b u t

i t i s u n s h i f t e d i n energy;

w i t h new s t a t e s near

EF.

and t h e s u r f a c e has become m e t a l l i c

As t h e dopiny i s f u r t h e r i n c r e a s e d from

4 t o 7 a t . %, EF r a p i d l y s h i f t s and becomes pinned a t t h e conduct i o n band minimum Ec.

Also,

t h e upper sp,-like

surface s t a t e

c o n t i n u e s t o d i m i n i s h i n i n t e n s i t y so as t o be n e a r l y impercept i b l e by 7 a t .

% doping,

extremely intense. become occupied,

and t h e lower s u r f a c e s t a t e becomes

The conduction-band minima

( A min)

near X

and emission from these minima i s observed as

i n t e n s e e l l i p t i c a l lobes i n angle-resolved photoemission s p e c t r a ( d o t t e d l i n e l a b e l e d "AR"

i n Fig. 19).

By d e p o s i t i n g a t h i n Au

f i l m on t h i s s u r f a c e i t was p o s s i b l e t o show v i a S i 2p c o r e - l e v e l

measurements t h a t EF remained unchanged ( w i t h i n -50 meV). a "zero-barrier-height" e l e c t r i c a l purposes,

Thus,

Schottky b a r r i e r was formed , a l t h o u g h f o r

t h e Au-Si

i n t e r f a c e i s undoubtedly shorted

because o f t h e extreme degenerate n-type doping.

457

7. PULSED LASER IRRADIATED SEMICONDUCTORS 11.

INTERSTITIAL IMPLANTS I n o r d e r t o determine t h e e f f e c t s o f i n t e r s t i t i a l i m p l a n t s on

surface properties,

i n v e s t i g a t i o n s o f t h e segregation and zone

r e f i n i n g o f i m p u r i t i e s t o t h e s u r f a c e r e g i o n f o l l o w i n g pulsed 1 aser anneal ing have been performed. a c q u i r e d i n these s t u d i e s , implanted w i t h lOl5/cm2,

To i11u s t r a t e t h e r e s u l t s

data o b t a i n e d u s i n g S i ( l l 1 ) samples

Fe t o doses o f 1.13

x 1015 atoms/cm2,

x

6.0

and 1.8 x 10’6 atoms/cm2 and w i t h Cu t o a dose o f 6.9 x

101s atoms/cm2 and l a s e r annealed a t -2.0

J/cm2 (Zehner e t a1

.,

1984b) w i 11 be discussed.

As mentioned p r e v i o u s l y , examination w i t h AES showed t h a t a l l samples were covered w i t h insertion

i n t h e UHV

large quantities

system,

as

shown f o r

i m p l a n t e d w i t h Fe a t t h e t o p o f Fig. 20.

o f 0 and C a f t e r a Si(ll1)

sample

(Compare w i t h s i m i l a r

o b s e r v a t i o n s f o r v i r g i n S i c r y s t a l s as shown i n Fig.

1.)

The

s u r f a c e s were then s p u t t e r e d with 1000 eV Ar+ ions, which r e s u l t e d i n t h e removal o f most o f t h e 0 and C s u r f a c e contaminants as shown i n Fig. 20.

Auger s i g n a l s from t h e implanted species c o u l d

n o t be detected a f t e r t h i s t r e a t m e n t .

Following i r r a d i a t i o n w i t h

one l a s e r pulse, AES s p e c t r a showed t h e i m p l a n t e d species t o be p r e s e n t i n t h e s u r f a c e region.

T h i s i s i l l u s t r a t e d i n Fig. 20,

where Fe Auger s i g n a l s a t 46 and 703 eV are r e a d i l y detected. F o r t h e low dose case, l i t t l e i n c r e a s e i s observed i n t h e i n t e n s i t y o f t h e Fe Auger s i g n a l o b t a i n e d from a s u r f a c e i r r a d i a t e d w i t h a d d i t i o n a l pulses.

A1 though a t i n t e r m e d i a t e doses several

p u l s e s (two o r t h r e e ) are s u f f i c i e n t t o produce t h e s u r f a c e conc e n t r a t i o n t h a t r e s u l t s i n t h e maximum Fe Auger s i g n a l i n t e n s i t y , i n t h e h i g h dose case a t l e a s t f i v e pulses are r e q u i r e d t o produce t h e same r e s u l t . w i t h multiple-pulse 20.

An example o f t h e i n c r e a s e t h a t occurs

i r r a d i a t i o n i s shown a t t h e bottom o f Fig.

These observations are c o n s i s t e n t w i t h p r e v i o u s KBS r e s u l t s ,

showing a dependence o f t h e s e g r e g a t i o n t o t h e s u r f a c e t h a t i s a f u n c t i o n o f t h e i m p l a n t dose and number o f l a s e r pulses used f o r a n n e a l i n g (White e t a1

., 1 9 8 0 ~ ) .

458

D. M.ZEHNER

-

Jt-

-

AFTER Ar" SPUTTERING

AES 56Fe (150 keV. 6 X 1015/cm 2) IN (111) Si PRIMARY BEAM: 2 keV, 5 p A MODULATION: 2 Vp-p

w

e z U

r/l

-

-

z

+

1 PULSE

5 PULSES

+ +

Fe Si

Fig.

+

a +

Ar

1

I

I

0

100

200

20.

Auger

+

0

C

1

I

I

500 ELECTRON ENERGY (eV)

300

electron spectra

400

Fe I

600

I 700

from an uncleaned S i ( l l 1 ) surface

implanted with 56Fe ( 1 5 0 KeV, 6 ~ 1 0 ~ ~ / c m af~ t e )r , sputtering and a f t e r pulsed laser annealing at -2.0 J / c m 2 .

7.

459

PULSED LASER IRRADIATED SEMICONDUCTORS

The e f f e c t o f segregation on s u r f a c e o r d e r was determined by LEE0 observations. Fe-implanted

The LEED p a t t e r n s o b t a i n e d from each o f t h e

samples

subsequent

l a s e r pulses are shown i n Fig. 21.

t o the

irradiation with

five

For purposes o f comparison, a

LEED p a t t e r n o b t a i n e d from a v i r g i n S i ( l l 1 ) c r y s t a l f o r t h e same i n c i d e n t e l e c t r o n energy i s a l s o shown i n t h i s f i g u r e .

Although

( 1 x 1 ) LEE0 p a t t e r n s were obtained a f t e r one p u l s e o f i r r a d i a t i o n on each sample, a h i g h e r backyround i n t e n s i t y was always observed r e l a t i v e t o t h a t obtained from t h e v i r g i n c r y s t a l . increased

segregation,

at

intermediate

and

The e f f e c t o f

h i g h doses,

with

m u l t i p l e l a s e r pulses was t o degrade t h e q u a l i t y o f t h e LEED patterns.

I n yeneral

i n Fig.

21,

, the

background i n t e n s i t y increased , as shown

although t h e symmetry o f t h e p a t t e r n observed was

s t i l l (1x1). I n c o n t r a s t t o t h e r e s u l t s o b t a i n e d from Fe-implanted samples, t h e LEED p a t t e r n o b t a i n e d from t h e Cu-implanted sample a f t e r one l a s e r p u l s e was a (1x1) w i t h h e x a g o n a l - l i k e

r i n g s around each

i n t e g r a l o r d e r r e f l e c t i o n as shown i n Fig. 22.

T h i s i s t o be com-

pared w i t h t h e

(5x5)

pattern,

shown a t t h e t o p o f F i g .

22,

o b t a i n e d from a t h e r m a l l y annealed (111) s u r f a c e which contained Cu e i t h e r due t o s e g r e g a t i o n from t h e b u l k o r as a r e s u l t o f beam deposition.

The r i n g s around t h e i n t e g r a l

became more i n t e n s e and sharp w i t h a d d i t i o n a l shown a t t h e bottom o f Fig.

22,

order

reflections

l a s e r pulses,

although a well-defined

as

(5x5)

LEED p a t t e r n was never obtained. T h i s sugyests t h a t t h e domains c o n t a i n i n g Cu on t h e laser-annealed s u r f a c e a r e n e i t h e r as w e l l o r d e r e d nor as l a r g e as those on t h e t h e r m a l l y annealed surface. Subsequent

examination o f t h e ion-imp1 anted laser-anneal ed

c r y s t a l s w i t h RBS (2.5-meV f o l l o w i n g features:

He+ i o n b a c k s c a t t e r i n g )

( 1 ) For t h e Cu-implanted c r y s t a l

showed t h e

, one

pulse

o f l a s e r r a d i a t i o n caused t h e t r a n s p o r t o f a l l Cu t o t h e near-

s u r f a c e region, and ( 2 ) f o r t h e low dose Fe-implanted c r y s t a l , one p u l s e i s s u f f i c i e n t t o cause t h e complete t r a n s p o r t o f Fe t o t h e near s u r f a c e region. A t i n t e r m e d i a t e doses, s u b s t a n t i a l segregation

460

Fig. 21. (a) Si(ll1 ) S6Feat ( b ) ( Patterns are j lcrn2.

D. M. ZEHNER

LEED patterns, at primary beam energy of 110 eV, from a surface and from ( 1 1 1 ) surfaces of crystals implanted with 1 . 3 ~ 1 0 ~ ~ / c, m ( c ~) () 6 . 0 ~ 1 O ~ ~ / c and m ~ ()d,) ( 1 . 8 x 1 0 1 6 / c m 2 ) . shown subsequent to five pulses o f laser annealing at -2.0

7.

Fig. 22.

PULSED LASER IRRADIATED SEMICONDUCTORS

461

LEED patterns, a t a primary beam energy o f 71 eV, from ( a ) a

thermally annealed S i ( l l 1 ) surface a f t e r -1

-

monolayer deposition o f Cu and

from a ( 1 1 1 ) surface o f a crystal implanted with 6 . 9 ~ 1 0 ~ ~ / and c m laser ~ annealed with ( b ) 1 and ( c ) 5 pulses at

2.0 J / c m 2 .

462

D. M. ZEHNER

t o t h e surface occurs d u r i n g t h e f i r s t pulse,

b u t two pulses

a r e r e q u i r e d t o c o m p l e t e l y segreyate t h e Fe t o t h e near-surface region.

F i n a l l y a t h i g h doses,

even a f t e r f i v e l a s e r pulses,

s u b s t a n t i a l q u a n t i t i e s o f Fe remain i n t h e f i r s t 1000 A o f t h e c r y s t a l a t an averaye c o n c e n t r a t i o n o f -2 x 1021/cm3.

Furthermore,

c h a n n e l i n g s t u d i e s showed t h a t Fe i n t h e b u l k o f t h e c r y s t a l i s not i n s o l i d solution. From t r a n s m i s s i o n e l e c t r o n microscopy s t u d i e s that,

i t i s known

i n t h e case o f a high-dose Fe-implanted c r y s t a l ,

a well-

d e f i n e d c e l l s t r u c t u r e (see Chapters 1 and 4) i s observed i n t h e n e a r - s u r f a c e r e y i on subsequent t o l a s e r anneal ing (White e t a1 1980~).

The i n t e r i o r o f each c e l l i s an e p i t a x i a l

.,

column of

s i l i c o n e x t e n d i n g t o t h e s u r f a c e (average c e l l diameter -250 A ) . Surrounding each column o f s i l i c o n i s a c e l l w a l l and e x t e n d i n g t o a depth o f -1000 A,

,

650 A t h i c k

c o n t a i n i n g massive quan-

t i t i e s o f segreyated Fey p o s s i b l y i n t h e form o f Fe s i l i c i d e s . These r e s u l t s show t h a t subsequent t o l a s e r a n n e a l i n g t h e Fe (and Cu) i s n o t u n i f o r m l y d i s t r i b u t e d i n t h e plane o f t h e near-surface reyion but instead i s h i g h l y concentrated i n the w a l l s o f t h e c e l l structure. Fe-implanted

Thus,

t h e (1x1) LEE0 p a t t e r n s observed f o r t h e

samples a r i s e from t h e b u l k t e r m i n a t i o n o f (111)

planes i n t h e columns o f s i l i c o n a t t h e surface.

The absence o f

any o t h e r w e l l - d e f i n e d d i f f r a c t i o n f e a t u r e s from t h e Fe-implanted r e g i o n shows t h a t no long-range o r d e r e x i s t s i n t h e t e r m i n a t i o n o f t h e c e l l w a l l s a t t h e surface. rings

i n t h e Cu-implanted

The presence o f h e x a g o n a l - l i k e

crystals

order e x i s t s i n those c e l l walls, scale.

The

high

background

o b t a i n e d from t h e v i r g i n c r y s t a l

shows t h a t ,

i t i s on an w+xxmel_y small

intensities,

,

i f long-range

relative

to

that

observed f o r a l l i m p l a n t con-

d i t i o n s f o r Fe and Cu i n d i c a t e t h e presence o f d i s o r d e r ( p o s s i b l y s t r a i n i n t h e r e y i o n o f t h e c e l l w a l l boundaries) i n t h e o u t e r most l a y e r s , which increases w i t h i n c r e a s i n g i m p l a n t dose. F o r t h e s e samples , s p u t t e r i n g f o l 1owi ng 1aser

ir r a d i a t i o n

r e s u l t e d i n t h e removal o f some o f t h e i m p l a n t from t h e s u r f a c e

7. region.

463

PULSED LASER IRRADIATED SEMICONDUCTORS

However,

subsequent i r r a d i a t i o n w i t h t h e l a s e r again

r e s u l t e d i n t h e segregation o f l a r g e q u a n t i t i e s o f t h e i m p l a n t t o t h e s u r f a c e region.

Furthermore, f o r samples i n which i n t e r s t i -

t i a l species such as Cu a r e present i n t h e b u l k as a r e s u l t o f t h e growth process, l a s e r i r r a d i a t i o n can be used t o zone r e f i n e t h e s e species t o t h e s u r f a c e r e g i o n from a depth e q u i v a l e n t t o t h e maximum m e l t

penetration.

These i m p u r i t i e s can t h e n be

removed from t h e s u r f a c e w i t h l i g h t i o n S p u t t e r i n g ,

l e a v i n g an

i m p u r i t y - f r e e subsurface r e g i o n ( t o a depth determined by t h e melt

front

penetration),

l a s e r annealing.

which

remains

such a f t e r

subsequent

I n many d e v i c e a p p l i c a t i o n s i n v o l v i n g s i l i c o n ,

Cu and Fe i m p u r i t i e s a c t as very e f f i c i e n t recombination c e n t e r s and adversely a f f e c t m i n o r i t y - c a r r i e r l i f e t i m e .

The above obser-

v a t i o n s show t h a t l a s e r annealing combined w i t h s p u t t e r i n g can be used as a r a p i d p u r i f i c a t i o n t r e a t m e n t i n o r d e r t o produce an i m p u r i t y - f r e e s u r f a c e region.

VI.

Applications

I n v e s t i g a t i o n s discussed i n S e c t i o n I11 and I V concentrated on examining s p e c i f i c s u r f a c e p r o p e r t i e s a s s o c i a t e d w i t h l a s e r annealing changes

while in

S e c t i o n V was p r i n c i p a l l y

these

properties

that

i m p l a n t a t i o n w i t h l a s e r annealing.

occurred

concerned w i t h t h e by

combining

ion

I n t h i s section the u t i l i z a -

t i o n o f laser-annealed surfaces i s discussed.

The most p r o m i s i n g

a p p l i c a t i o n o f t h e l a s e r annealing t e c h n i q u e f o r producing atomic a l l y c l e a n surfaces i n d e v i c e processing appears t o be i n preparing

surfaces

application,

for

molecular

beam e p i t a x y

(MBE).

In this

t h e high-temperature t r a n s i e n t induced by t h e l a s e r

o f f e r s a very a t t r a c t i v e and e f f i c i e n t a l t e r n a t i v e t o t h e present prolonged preheat t r e a t m e n t a t t h e moderate temperature r e q u i r e d t o c l e a n t h e semiconductor s u r f a c e t o t h e h i g h standard e s s e n t i a l f o r good q u a l i t y e p i t a x y . a p r o d u c t i o n technique.

T h i s b r i n g s MBE a s t e p nearer t o being

464

D. K.ZEHNER

I n a r e c e n t i n v e s t i g a t i o n (de J o n j e t al.,

1983) LEED was

used t o study t h e i n i t i a l stages o f e p i t a x i a l growth o f s i l i c o n on s i l i c o n .

Both thermal a n n e a l i n g and l a s e r i r r a d i a t i o n were

used f o r s u r f a c e p r e p a r a t i o n , 1-10 nm.

and S i d e p o s i t i o n s were t y p i c a l l y

Using LEEO p a t t e r n s , t h e e p i t a x i a l growth temperature

was d e f i n e d as t h a t p a r t i c u l a r s u b s t r a t e temperature a t which an e p i t a x i a l overlayer, same q u a l i t y

grown on t h e c l e a n s u b s t r a t e ,

of diffraction

p a t t e r n as t h e s u b s t r a t e

R e s u l t s o b t a i n e d from 1aser-anneal ed vicinal

exhibits the

(loo),

itself.

(110) , ( 111) , and

(111) S i o r i e n t a t i o n s showed t h a t e p i t a x i a l growth can

t a k e p l a c e on surfaces prepared by t h i s procedure.

The growth

temperature f o r t h e (100) s u r f a c e was i d e n t i c a l t o t h a t o b t a i n e d u s i n g t h e r m a l l y prepared surfaces.

For t h e (111)

surface the

growth temperature determined f o r t h e thermal l y annealed s u r f a c e was h i g h e r t h a t t h a t determined f o r t h e l a s e r - a n n e a l e d s u r f a c e and a l s o f o r generally

the

laser-annealed

vicinal

surface.

accepted growth mechanism i n Si:MBE

growth by s t e p f l o w ,

the

r e s u l t s obtained f o r

Since t h e

above 870 K i s (111)

surfaces

sugyest t h e presence o f steps on t h e laser-annealed surface. Using an approach s i m i l a r t o t h a t j u s t described, t h e growth o f epitaxial

m u l t i l a y e r f i l m s o f v a r y i n g t h i c k n e s s on s i l i c o n

s u r f a c e s has been i n v e s t i y a t e d (de Jong e t a l .

Laser-

(loo), ( l l o ) ,

(111) and v i c i n a l (111)

A f t e r preparation,

s i l i c o n f i l m s were de-

annealed and t h u s c l e a n S i s u r f a c e s were used.

, 1982b,c).

p o s i t e d and subsequently l a s e r annealed a t i n c r e a s i n y energy dens i t i e s i n o r d e r t o determine t h e t h r e s h o l d f o r growth. determined by LEEU t o be -0.9 these

experiments.

After

T h i s was

J/cm2 f o r t h e ruby l a s e r used i n

determining t h e threshold,

silicon

l a y e r s were s e q u e n t i a l l y d e p o s i t e d and l a s e r annealed on a l l s u r faces.

I n t h i s way e p i t a x i a l

l a y e r s up t o 800 nm were yrown,

b u i l t up out o f 1 t o 20 sublayers.

The reappearance o f a LEEU

p a t t e r n a l l over t h e annealed area a f t e r each i r r a d i a t i o n i n d i cated e p i t a x i a l

regrowth o f a l a y e r .

o r i e n t e d samples,

I n particular,

on S i ( l l 1 )

annealed d e p o s i t e d l a y e r s e x h i b i t e d a (1x1)

7.

465

PULSED LASER IRRADIATED SEMICONDUCTORS

p a t t e r n which

i n t h e case o f t h e v i c i n a l

s u r f a c e had charac-

t e r i s t i c spot s p l i t t i n y i n t h e same c r y s t a l l o g r a p h i c d i r e c t i o n and t o t h e same amount as a nondeposited sample.

T h i s means t h a t

t h e steps i n t h e s u r f a c e are preserved by d e p o s i t i o n and pulsed Spectra obtained w i t h RBS show t h e e p i t a x i a l l y

l a s e r annealing.

grown r e g i o n s t o be o f good q u a l i t y . e x t r a r o u t e t o three-dimensional

T h i s method may p r o v i d e an

s i l i c o n structures.

By combininy t h e i o n i m p l a n t a t i o n , l a s e r annealing techniques discussed i n S e c t i o n V with m o l e c u l a r beam e p i t a x y , i t i s p o s s i b l e t o produce b u r i e d doped l a y e r s .

T h i s approach has been f o l l o w e d

i n a recent i n v e s t i g a t i o n ( S m i t e t a1 was f i r s t implanted w i t h As.

., 1982).

A Si(100) wafer

A f t e r subsequent i n s e r t i o n i n t o a

UHV system,

t h e sample was i r r a d i a t e d w i t h f i v e pulses from a

ruby l a s e r .

I n a d d i t i o n t o producing a clean, ordered surface, as

determined by LEED, As was r e d i s t r i b u t e d i n depth, as p r e v i o u s l y i l l u s t r a t e d i n Fig.

16.

The sample was t h e n heated ( t y p i c a l l y

K), and s i l i c o n was deposited a t a r a t e on t h e o r d e r o f 0.1

-800 nm/s.

A t y p i c a l l a y e r t h i c k n e s s was 100 nm.

The samples were

R e s u l t s showed (1) good e p i t a x y w i t h i n

t h e n examined w i t h RBS.

t h e d e p o s i t e d r e g i o n and ( 2 ) t h e e x i s t e n c e o f a b u r i e d As l a y e r w i t h an abrupt doped-undoped substrate-epitaxy with

specific

interface.

dopant

i n t e r f a c e (<15 nm),

located a t the

D i f f e r e n t l y doped s i l i c o n l a y e r s

concentration

profiles

and w e l l - d e f i n e d

i n t e r f a c e s are an i n t e g r a l p a r t o f most semiconductor devices. The above r e s u l t s , a b u r i e d l a y e r o f As-doped s i l i c o n produced by low-temperature

silicon

a n n e a l i n y and cleaning,

MBE

in

combination

with

pulsed-laser

show t h a t such devices can be produced.

With respect t o devices, t h e r e s u l t s j u s t discussed may prove t o be i m p o r t a n t f o r very h i g h frequency devices,

because MBE i s

one o f t h e few techniques t h a t o f f e r s t h e p o s s i b i l i t y o f making t h e sequences o f very s h a l l o w l a y e r s (-2000 A) w i t h w e l l - c o n t r o l l e d dopiny l e v e l s and abrupt changes i n dopant c o n c e n t r a t i o n s (-200 A) required

by

some

of

the oscillator

and a m p l i f i e r

structures

designed t o operate a t frequencies i n excess o f 200 GHz.

A major

466

D.M.ZEHNER

problem w i t h

Schottky

diodes

i s that

t h e metal-semiconductor

i n t e r f a c e forms t h e p o t e n t i a l b a r r i e r , and so device o p e r a t i o n i s v e r y s u s c e p t i b l e t o t r a c e contamination o f t h e semiconductor surface.

Laser c l e a n i n g o f such s u r f a c e s p r i o r t o metal d e p o s i t i o n

may w e l l minimize r e p r o d u c i b i l i t y problems i n Schottky devices.

VII.

Conclusions

It has been shown t h a t l a s e r annealing p r o v i d e s a new method f o r c l eani ny semi conductor surfaces.

There are f o u r advantayes

(1) no f o r e i g n atoms are i n t r o d u c e d i n t o t h e

o f t h i s technique:

s u r f a c e o r subsurface region;

( 2 ) t h e c l e a n i n y procedure does n o t

s p o i l t h e vacuum c o n d i t i o n s s i n c e t h e l a s e r i s l o c a t e d o u t s i d e t h e system and t h e beam i s i n t r o d u c e d t h r o u g h an o p t i c a l l y t r a n s p a r e n t window; irradiation,

(3) s i n c e o n l y t h e s u r f a c e r e g i o n i s heated d u r i n g b u l k i m p u r i t i e s cannot m i g r a t e t o t h e surface;

and

( 4 ) w i t h t h e t o t a l p r o c e s s i n g t i m e being on t h e o r d e r o f tl s, investigation

can

begin

immediately

after

cleaning,

thereby

a v o i d i n y t h e p o s s i b i l i t y o f r e c o n t a m i n a t i o n by background gases d u r i n g t h e c o o l i n g phase.

The a b i l i t y t o remove t h e n a t i v e o x i d e

l a y e r from a semiconductor s u r f a c e opens up t h e p o s s i b i l i t y f o r u s i n g t h i s technique t o w r i t e on a wafer. d u c i n g a t o m i c a l l y c l e a n surfaces,

I n a d d i t i o n t o pro-

i t has been shown t h a t l a s e r

a n n e a l i n g r e s u l t s in r e s t o r a t i o n o f o r d e r t o t h e s u r f a c e r e g i o n o f a damaged c r y s t a l . faces

containing

The m e t a s t a b l e s u r f a c e s t r u c t u r e s and sur-

ordered

arrays

of

steps

produced

by

laser

a n n e a l i n y can be used i n i n v e s t i g a t i o n s aimed a t understanding r e c o n s t r u c t i o n and growth w i t h i n t h e outermost monolayers.

In

t h e area o f b a s i c research concerned w i t h t h e physics and chemi s t r y of

surfaces,

these o b s e r v a t i o n s i n d i c a t e t h a t i t may be

p o s s i b l e t o modulate s u r f a c e coverage ( c l e a n , adsorb-desorb) i n a d s o r p t i o n experiments and a l t e r ordered s u r f a c e s t r u c t u r e s f o r k i n e t i c s studies.

467

7 . PULSED LASER IRRADIATED SEMICONDUCTORS When combined w i t h i o n i m p l a n t a t i o n , laser

annealing

(interatomic region.

can be used t o t a i l o r

i t has been shown t h a t

t h e geometric

lattice

spacings) and e l e c t r o n i c s t r u c t u r e i n t h e s u r f a c e

T h i s combination p r o v i d e s a way o f i n v e s t i g a t i n g a l l o y s

t h a t cannot be obtained u s i n g conventional c r y s t a l growth techniques.

For t e c h n o l o g i c a l a p p l i c a t i o n s , t h e p r o d u c t i o n o f a l l o y s

(submicrometer r e g i o n s ) w i t h t a i l o r e d s u r f a c e p r o p e r t i e s ( b u r i e d 1 ayers) should prove t o be very u s e f u l . From t h e r e s u l t s presented i t i s obvious t h a t l a s e r annealing o f semiconductor surfaces i n UHV has a tremendous p o t e n t i a l as a tool

for

both s u r f a c e science and p r a c t i c a l

v a r y i n g t h e wavelength,

energy d e n s i t y ,

application.

and p u l s e d u r a t i o n ,

By it

should be p o s s i b l e t o c h a r a c t e r i z e and understand t h i s processing t e c h n i q u e move completely.

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