Properties of a-boron films prepared by low pressure chemical vapour deposition

783

Journal of Non-Crystalline Solids 114 (1989) 783-785 North-Holland Section 19: III- V compounds and other materials

PROPERTIES OF a-BORON FILMS PREPAREDBY LOW PRESSURE CHEMICAL VAPOUR DEPOSITION C.W. ONG, K.P. CHIK and H.K. WONG Department of Physics, The Chinese University of Hong Kong, N.T., Hong Kong. Properties of LPCVD a-B f i l m s are found to depend strongly on substrate temperature Ts. Hardness increases but H content and ESR linewidth (5) decrease with increasing T@. A decreases with increasing measuring temperature, T. Narrowing of A may be due to the hlgher charge c a r r i e r m o b i l i t y (and therefore motional narrowing) at higher T. I. INTRODUCTION

I

Boron is an elemental semiconductor of both fundamental

results

and

application

on a-B:H fi l m s

temperature (Ts) several

prepared at

Some

These films

amount of hydrogen.

,

,

sD

Ts(°C)

r=

~

100-66 :75

substrate

~ 400°C have been reported by

authors 1'2'3

substantial

interest.

l

t(pm)

~ m

contain

~

We expect that

~

6~~~~~~~_

:80 so?~ -

'

~

~

0 282/~/~5"~

~'-~

films with low hydrogen content can be prepared by rai s i n g Ts with substantial change in f i l m

300 to 800°C is

presented.

It

is

electrical

c o n d u c t i v i t y (o)

i 2000

l 1500

I 1000

I

500

WAVE NUMBER(cm"I)

found that

infrared (IR) absorption spectra, microhardness (HV),

~

i 3000

4000

properties. In t h i s report, a study on a-B films prepared by LPCVD method with Ts ranging from

and electron

spin resonance (ESR) are a l l affected by T s"

FIGURE I Infrared absorption spectra of various T s

a-B films

with

For films with Ts = 300°C, three bands can be identified.

2. SAMPLE PREPARATION The LPCVD system elsewhere4.

In

used has

brief,

been described

5% B2H6 in

argon was

Band I

at 2560cm-I

is

sharp and

originates from B-H terminal bonds5, i n d i c a t i n g that the f i l m contains a conspicuous amount of H

admitted into a quartz tube at a flow rate of

atoms.

e i t h e r 10 or 20sccm. During deposition, pressure

broad and covers

was kept at 0.12 to 5 Torr.

modes at 1602, 1755, 1915 and 2109cm-I f o r B2H6

Films of thickness

Band IT ranging from 1600 to 2200cm-I is

I-9.5~m were deposited at Ts ranging from 300 to

molecules 6.

800°C.

for

For IR absorption experiment, s i l i c o n

a-B:H

the

B-H-B bridge-stretching

C . C . Tsai 3 observed a s i m i l a r band films

and a t t r i b u t e d

it

to

B-H-B

For other measurements, Corning 7059 glass was chosen when Ts <

bridge bonds. Band I l l

460°C, while quartz was used f o r

band covers the 485, 550, 614, 680, 768, 848 and

wafer was used as substrate.

T ~ 540°C. s

below 1000cm-I with a minimum near 780cm- I . This 925cm-I

bands of

crystalline

B-rhombohedral

boron 7. The absorption at 780cm-I also coincides

3. IR ABSORPTION SPECTRA Fig. I shows IR absorption spectra fo r films prepared at various T . For c l a r i t y , s of each spectrum was s u i t a b l y s h i f t e d .

is also broad and s et t les

baseline

0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

with

the

788cm-I

double-hydrogen Thus band I l l

B-B stretching bridge

in

B2H6

mode across molecules6 .

is related to B-B bondings.

784

C.W. Ong et M./Properties of a-boron films

With r i s i n g II

Ts, the strength of bands I and

-1

and of the dip at 780cm-I drops and f i n a l l y

vanishes f o r

T

~> 540°C. This i n d i c a t e s t h a t s hydrogen content is highest at the lowest T s and

-3

becomes n e g l i g i b l y

small at T ~> 540°C. The s- I . behaviour of the dip near 780cm ms consistent with a decrease of B-B s t r e t c h i n g mode across

'7

E o

higher Ts. about

0

g,

broadens at

_At Ts=800°C, the band extends to

1250cm

which

1235cm- I mode7.

includes

the

-.q, ..~oooc

-~ ".2". .\ ",",.

"7

double-hydrogen bridge as H atoms are driven o f f by r a i s i n g Ts. Furthermore, band I l l

•.'..

C

,,, %. a 4 6 0 ° C • . • 620oC

-9

• o

crystalline

% 300°C

-11

t h a t of c-B at high Ts.

I 3

,

values

(HV)

of

the

films

I 5

,

still

(°K'l)

The e l e c t r i c a l

for

I

i

I

L

600

from

o vs I / T cannot

plots be

is

important

contribution as a f u n c t i o n of

for

o at room

].Ox10-10(£-cm) - I for

show t h a t

described

t h e r m a l l y a c t i v a t e d process.

Ts (oC)

FIGURE 2 Microhardness HV of a-B f i l m s substrate temperature T

with Ts (Fig. 3).

increases

conduction

I

500

o of the f i l m s

Ts=300°C to 3.9x10-4(£-cm) - I

The log 400

conductivity

varies d r a s t i c a l l y temperature

~

(5000Kgf/mm2).

5. ELECTRICAL CONDUCTIVITY

"I-

300

with

smaller than the bulk value of

c r y s t a l l i n e boron

{i

0 0 0

I 7

FIGURE 3 E l e c t r i c a l c o n d u c t i v i t y o of a-B f i l m s various Ts vs r e c i p r o c a l temperature which is

E

• .380oC

340°C

on

Vickers scale are p l o t t e d against Ts in Fig. 2.

e~

•

..

1000/T

4. MICROHARDNESS

1

• I

• e

Thus f i l m s t r u c t u r e approaches

Microhardness

'~',~'" • "<

all

by

a

simple

Hopping conduction cases,

becomes less

Ts=800°C. electrical

though

significant

as

its T

s

is lowered.

S

6. ELECTRON SPIN RESONANCE Below Ts=400°C, the f i l m s

are r e l a t i v e l y

HV increases g r a d u a l l y with T s. these f i l m s

soft.

The ESR signals have a g-value of 2.003 which

F r o m IR data,

is independent of Ts and measuring temperature.

have high hydrogen content.

The H

This

value

is

atoms probably weaken the linkage of B atoms and

electron

spin.

soften the f i l m s .

features.

(A)

sharply.

Above Ts=400°C, HV increases

This strong

attributed

to

atoms.

Ts=620°C,

At

the

increase is more s u i t a b l y

stronger HV is

800%,

very

close The

With

electron

to

that

signal

increasing

of

has

a free

following

Ts from

300 to

spin density Ns increases from

bondings between B

3x1018cm-3 to

3xlO19cm-3. These high Ns values

around 3600Kgf/mm 2

indicate

hopping conduction

that

is

important

C.W. Ong et al,/ Properties of a-boron films

t•

.T.~. v••

the mechanism of motional narrowing.

300o C vv • •••

•v

v~Al('vv " . . . . v "... o vvv "'. 34000

30

VV

~'&

786

.*

increasing

Ts,

temperature

dependent

AHp_p

20

=t

v

a= o

o 620oc • 460oC 10

~ ~

0 V

~ o v

6-rhombohedral boron 8. lJh can also

i

I

100

i

1

200

400

It

Mechanical and e l e c t r i c a l with

increase of

N

But the

alone cannot account f o r

S

increase in

o at

room temperature

films

to

become harder.

over 5 orders of magnitude. increases from 3x1018cm-3 to

Spin concentration 3x1019cm-3 while

us to

increasing temperature f o r Ts ~ 340°C.

suppose that

mobility

lJh is

higher Ts.

ESR signals may o r i g i n a t e

charge

carriers

higher f o r

width

narrowing is

more pronounced with

whose hopping

films

deposited at

The measured g-value supports t h i s

argument.

This is

accompanied by an increase of room temperature o

line

trapped

and

properties of a-B

(more than f i v e orders of magnitude). This leads from

T,

by the substrate temperature. With increasing T s from 300 to 800°C, hydrogen content is reduced causing

be enhanced by r a i s i n g Ts.

large

supposed that

films deposited by LPCVD are g r e a t l y influenced

S

small

is

be increased by r a i s i n g

7. CONCLUSION

FIGURE 4 Line width of ESR signals of a-B films various T as a function of temperature

the

(Fig. 4).

deposited at 800°C,

AHp-p is thus narrowed.

i

300 T(°K)

and w i l l

by

narrowing has also been observed on c r y s t a l l i n e

~£OoV ~ o~.v

A 80000

films

strongly

narrowed

AHp-p decreases from 27 to 4G when T increases from -160 to 120°C. A s i m i l a r l i n e width

v 380o0.

0

"I"

is

increasing measuring temperature (T) As an example, f or

&

becomes

and

(C) With

(B) Peak to peak l i n e width

AHp_p

measured at room temperature decreases from 35 to 13G when Ts increases from 300 to 800°C.

The

large AHp_p f o r films deposited at low Ts may be

REFERENCES I. L.J. Dimmey, H. Park, P.L. Jones and F.H. Cocks, J. of Electronic Materials, 10 (1981) 111.

2. B.G. Bagley, D.E. Aspnes, A.C. Adams and R.E. Benenson, J, of Non-Crystalline Solids 35 & 36 (1980) 441.

due to H atoms within the films which a l t e r the local

environment

broaden

the

influence

of

of

spin

g-value.

centers

For

H atoms is

Consequently, carrier

the

thus

Ts,

the

due to

the

higher

reduced

reduction of i t s concentration. gap states are produced.

and

Meanwhile, more

This can enhance lJh.

average time

for

a charge

to be trapped by a l o c a l i z e d state is

reduced, and the environmental v a r i a t i o n f e l t by the trapped charge c a r r i e r s may average out more s u b s t a n t i a l l y due to frequency causing

the

increase of

AHp-p to decrease.

hopping This is

3. C.C. Tsai, Physical Review B 19 (1979) 2041. 4. C.W. Ong, H.K. Wong, K.S. Sin, S.T. Yip, and K.P. Chik, Rev. Sci. Instrum. 60 (1989) 1174. 5. N.A. Blum, C. Feldman and F.G. Satkiewicz, Physica Status Solidi(A) 41 (1977) 481. 6. Walter J. Lehmann and I. Shapiro, Spectrochimica Acta 17 (1961) 396. 7. O.A. Golikova, M. Zhubanov and D.N. M i r l i n , Soviet Phys.-Solid State 11 (1969) 1341. 8. G. Gewinner, L. Kubler, J.J. Koulmann and A. Jaegl~, Physica Status Solidi( B) 59 (1973) 395.