Epitaxial growth and assessment of p-type GaAs by chloride VPE

408

Ph.',,sica 129B ( 15)85 ) 4 0 8 4 1 2

Nolth-HoIland, Amsterdam 11-5:

III-V SEMICONDUCTOR TECHNOLOGY

EPITAXIAL GROWTH AND ASSESSMENT OF P-TYPE GaAs BY CHLORIDE VPE I.H. Goodridge and P.M. Edwardson Plessey Research (Caswell) L i m i t e d , Allen Clark Research Centre, Caswell, Towcester, NNI2 8EO, England. A p r e l i m i n a r y study of p-type doping in the AsCI3-GaAs-H 2 system has been made w i t h the o b j e c t i v e of achieving close c o n t r o l of laver t h i c k n e s s , growth r a t e , doping l e v e l , and u n i f o r m i t y to enable double d r i f t GaAs IMPATT layers to be grown in high y i e l d . Because of t h i s well defined o b j e c t i v e , the gaseous dopant dimethyl zinc in hydrogen has been used as a doping source as i t o f f e r s the p o t e n t i a l of a large doping range coupled w i t h close c o n t r o l . Our r e s u l t s confirm t h a t DM7 is a useful dopant source in t h a t layers of I016 through 3.1019 cm- 3 have been grown w i t h good surface morphology and high m o b i l i t y . However there is a problem of n o n - r e p r o d u c i b i l i t y at l e v e l s below mid 10L7 cm-3. We b e l i e v e t h i s to be associated with r e s i d u a l water and oxygen w i t h i n the r e a c t o r i t s e l f . These problems have been circumvented by the novel use of a doped source to produce a Ibase' or r e s i d u a l p-type background doping level of i0 i~ cm- ~. Double d r i f t layers have s u c c e s s f u l l y been produced using a combination of these doping techniques.

llsing the t r i c h l o r i d e

1. INTRODUCTION

method ~ we have, over

several years,

developed techniques to produce

m a t e r i a l s c a p a b i l i t y for the production of high

single

IMPATT lavers

power pulsed

sulphide as a donor dopant precursor.

The

object

of

this

work

is

to

IMPATTs at J-band.

provide For

a

device

drift

hydrogen For the

a p p l i c a t i o n s reasons, t h i s is best s a t i s f i e d by

double d r i f t

the double d r i f t

GaAs IMPATT l as the presence

e x a c t l y m i r r o r e d by the p - l a y e r s .

regions (n and p) halves the

t h a t the p e r m i t t e d tolerances of thickness and

of the two d r i f t

structures,

using

the n - l a y e r s must be This means

doping level of

individual

layers is d r a s t i c -

i n c r e a s i n g the i n p u t power density due to the

ally

Therefore

in

higher o p e r a t i n g v o l t a g e .

correct

device

capacitance

whilst

at

the

same time

Such a s t r u c t u r e

is

reduced.

selection

incorporation

s c h e m a t i c a l l y represented in Figure 1.

In

this

of

the

this

work,

dopant

technique is c r u c i a l l y

paper

we

affect

the choice

using

the

discuss of

chosen

the

dopant,

dopant,

and

its

important

factors

present and

the

that

results

discuss

its

limitations. 2. DOPANT SELECTION .

J

The most GaAs are

m

rich

O'3

x

FIG.1. DOUBLE DRIFT L o - H i - Lo STRUCTURE

from

Be,Cd,Ge,Mg,Si

and Zn.

in

doping

The arsenic

a m i n i m i s a t i o n of arsenic vacancies

means t h a t Group

gallium.

IV

the

two amphoteric

substitute

dopants

preferentially

Thus under these c o n d i t i o n s ,

and germanium act mainly 0378-4363/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

used in

c o n d i t i o n s of the AsCI3-VPE growth system

result which

common acceptors

as donors.

for

silicon Of the

LH. Goodridge and P.M. Edwardson / Epitaxial growth and assessment of p-type GaAs remainder, beryllium and magnesium appear to

This

have been ignored in VPE. This has probably

involve controlling the vapour pressure of,

been due to their lack of a v a i l a b i l i t y in a

say, elemental zinc I° or its

suitable precursor form coupled with concern

rely upon transportation with flowing hydrogen

for

or halogen12.

the

highly

toxic

nature of

beryllium.

Furthermore there is conjecture 3 that magnesium

could be external to

409

the reactor and arsenide 11 and

Alternatively, the dopant could

be inside the reactor i t s e l f , for example, zinc

could react with a s i l i c a work tube to release

dissolved in the gallium source6, 13.

silicon and compensated layers would be formed.

unfortunately been proved impractical

This has

We have no evidence for this but i t is evident

process since the dopant is preferentially lost

that the hot wall AsCI3-VPE system would tend

from the source.

as a

to maximise this effect; whereas in the cold wall MOVPE system, where alkyl derivatives of these elements are currently being used4,5 i t

3. EXPERIMENTAL The epitaxial layers have been grown in a

would tend to be minimised. For these reasons

production size reactor 14 using H~ and DMZ as

we have, for the present, rejected usage of Mg

dopant precursors.

and Be. Cadmium, on the other hand, has been used in

structures

a AsCI3-VPE system both in elemental form6 and

assessment d i f f i c u l t i e s , to adopt a two stage

gaseous form7.

growth process i n i t i a l l y .

However in an MOVPE system

In the growth of the double d r i f t it

IMPATT

has been necessary, because of Following the growth

cadmium (as dimethyl cadmium) has demonstrated8

of the n-side, the layer is removed from the

its usefulness as a dopant in InP.

reactor, assessed and then reinserted for the

Zinc has almost universally been used as the p-type

dopant

diffusion

but

has

fears

resulted

of in

zinc

in

In the case of p++ con-

tacted single d r i f t layers, i t

interest

grow the structure in a single growth run.

in

alternative p-type dopants. However, the high s o l u b i l i t y 9 of

p-side to be grown.

enhanced Zn

GaAs is

a powerful

reason for selecting zinc as a dopant i f contact resistance layers are required.

low

Since

we have no evidence that diffusion of zinc in

The low barrier height

is possible to

(N 0.4 eV) of a

metal-GaAs(p) junction means that conventional C-V (Hg) profiling is not possible.

To over-

come this problem, we have used two techniques. Firstly a Polaron electrochemical C-V p r o f i l e r

GaAs grown in the AsCI3-GaAs-H2 system is a

has been used and secondly, Neff has been

problem, we decided, in

measured by reverse biasing the p/n junction.

the

absence of

a

suitable supply of DMCd, either in concentrated

In the l a t t e r case, mesas are defined litho-

form or as a gaseous mixture, to use zinc in

graphically but, because of the ohmic nature of

the form of dimethyl zinc in H2 (BOC Special

the p++ contact, metallisation is unnecessary.

Gases).

This means that

Recently, however, DMCd has become

available from both Alfa

Products and BOC

Special Gases. The

decision

possible.

direct

Since

n

surface probing

has

is

been .previously

determined (or is the level of the substrate) to

use a gaseous dopant

precursor is based upon our successful usage of hydrogen sulphide. Doping from a gas mixture is theoretically simply a matter of metering the correct flow of dopant into the reactor. The alternative would be to use a solid dopant.

and Nef~lmeasured, p can be calculated from IJ equation 1 Neff

=1+1 p

n

(1)

410

1. H. Goodridge and P.M. Edwardso~t ,; Epitaxial growth and us'~'['sst'~zL{ll[(,/ l~-t~ pc (;~, 1~

Figure

2 shows

structure

such

a Nef f p r o f i l e

of the

s c h e m a t i c a l l y shown in F i g u r e i . 1019 f

C-2

I

N A- N D

5

This work / " Bass

Sidorov *"

V(V)

ND-N A

0 N EFFECTIVE DRIFT

Below 5.1017 cm- ~ doping

C-V

PROFILE OF D O U B L E

Lo-Hi-Lo

dramatically. residual

LAYER

levels

Figure

This

moisture of

3

gives

the

Hall

grown using DMZ.

data

of

p-type

These were o b t a i n e d

standard van der Pauw techniques zs.

be due to

in

the

Moisture past

been

To circumvent

As

has

source.

been

covered

with

adequate

mobility.

made

use

of

to

one a f t e r

!

earlier

10

10

10 3

cm

20 10

permits

one o f

discs

resulted;

the

the

that

if

interesting

too result

of was

curve

the < I i i >

five

As can be seen, onset of

the d i f f e r e n t

saturation

occur

It

appears

to

doping

at

behaviour

5.10 z7 through 2.10 z9 cm-3

- 3.1019 cm-3 occurs

purity

a p-doped

undoped n - t y p e

far

the that

downstream

of

i0 15 cm-3 indicates

3 mm too

'fine

from

was

p-level

the was

tuning'

A further could

be

the A or B face of

This

reflects

perhaps

etch r a t e s of these o r i e n t a t i o n s .

found of

then n-type

the degree of

source r e a c t i o n .

doped s l i c e .

f o r a DMZ/H2 m i x t u r e . reasonable

high

doping source comprised

achieved bY exposing e i t h e r the c a l i b r a t i o n

GaAs

strategy

n o r m a l l y make up our source.

The l a t t e r

efficiency finding

essentially

the

be exchanged f o r

Initially

obtained.

FIG.3. HALL MOBILITY VS NA N D

F i g u r e 4 is

solid

growth

near the i n p u t AsCI3, high doping c o n c e n t r a t i o n

19

10 N A -N D

doped

Optimum p o s i t i o n was found e m p i r i c a l l y - i f

T h e solid line represents results of Sze Et Irvin shown here for comparison

18

to

zinc stage

o f a <111> o r i e n t e d zinc doped s u b s t r a t e lodged

thick

17

the growth of

a s u i t a b l e stock of n - s i d e s have been

grown.

against

16

a

two

undoped source

I

in

the problem we have success-

The

referred

10

effects

n - t y p e GaAs. fully

and

to

the equipment.

ppma have

have any d e l e t e r i o u s

can be seen the range I0 16 through 3 . 1 0 . 19 cm-3

10

in

a few

behaviour changes

we b e l i e v e

q u i t e normal in such r e a c t o r s and appear not to

4. RESULTS

I

5

2.5 (um)

Depth

oO

6

10 10 10 PDMZ over substrates (atm) D O P I N G R A N G E O F A D M Z / H 2 MIXTURE

FIG.4.

layers

7

10

FIG.2.

:

10

(cm 3) 16 10

using

.

i "-

. 17~_ ]U F

U 0

17

18I

(cm 3)

(pF -2) .02

10

10

.i~// ////

that

the

optimised

1016 cm-3 could

~bout - 20 growth runs.

background

be m a i n t a i n e d f o r

Work is at present in

LH. Goodridge and P.M. Edwardson / Epitaxial growth and assessment of p-type GaAs

411

hand to use independent sources, one undoped

with the measuredmoisture levels of a few ppma

and one doped to

and the

improve the

longevity even further. 5 show that sources,

control

and

The results in Figure

by metering AsCl3 to two such

the

p-level

can

be easily

and

reproduci bly adjusted.

hydrolysis

inference before

is

that

entering

OMZ undergoes the

work tube.

Although these concentrations of dopant over the substrate may appear small, i t

should be

remembered that in n-type doping, where [H ~ ] is often i-2 orders of magnitude lower, no such problems exist.

18

I t can be seen from Figure 4 that there is

°I

an

f

Slope = 1

essentially

linear

relationship

between

log [PDMz] and log [NA-ND) for doping levels above ~ 2.1017 cm-3 and that our results, in this region, are similar to those obtained by

I<

Z

1016

Bass16, using DMZ in

5 0 0 s c c m MFC

an MOVPE reactor and

Sidorov, using Zn3As2 in an AsCI~VPE reactor.

20sccm MFC

The effective distribution coefficients appears 10151

,

,

, , ....

I

.

.

.

.

.

.

.

.

I

.

to be in the range 15-50. Neither Bass16 or

. . . . . . . .

1.0 10 100 1000 Arsenic trichloride flow rate (sccm)

Sidorov 17

reported

results

b e l o w 2.10 17

cm-3where our results show a dramatic f a l l off.

FIG.5. LOW NAN D VALUES USING A P - T Y P E SOLID GaAs SOURCE

Extrapolation of

both workers results

NA-NO = 10 16 cm-3 predicts

the

to

required

partial pressure of zinc to be 1-7.10-7 atm. Using

a combination of

uniformly

doped, hybrid

structures

these techniques

and double l o - h i - l o

have been prepared.

There s t i l l

Since this is within the control range of our gas handling system, the inference is that DMZ w i l l indeed be a suitable dopant precursor at

remains a problem with the l a t t e r structure

the 1016 cm-3 level provided care is taken to

~hich is reproducing the p-type hi region with

provide a dry environment. As said e a r l i e r , DMCd is now available and

the correct amount of charge, i . e . =m-2 ± 5%.

1.3.1012

Again this we believe this to be

associated with

DMZ usage at

low concen-

with

a calculated e f f e c t i v e

distribution

coefficient of 2-3 orders of magnitude lower

tration and not the epitaxial system i t s e l f .

than that of zinc

No similar d i f f i c u l t i e s would be experienced in

it

producing matching 200 A layer when using H~.

precursor.

(using Sidorov's l? data),

offers the potential of a better dopant HoweverDMCd w i l l also require a

dry environment before optimum doping performance can be achieved.

5. DISCUSSION The h i g h reactivity of DMZ with moisture means that moisture, worse

a constant

leak

rate

of

low

OMZ concentrations.

6. CONCLUSION The main conclusion of this work is that

Such

dimethyl zinc is a suitable doping precursor

behaviour could account for the sudden change

but that optimum usage of i t or indeed of any

in doping e f f i c i e n c y seen at PDMZof ~ 5.10-7

metal organic, can probably only be achieved

atm.

at

for

the effect on DMZ concentration is

T h i s pressure represents a [DMZ] of

5-10 ppma in the reactor.

This equates well

in very low backgrounds of H20 and 0 2. However, these problems have been circumvented in this

412

L H. Goodridge and P M. Edwardson / t:pitaxial growth and assessment ol p-tvpc (;a~t~

work by usage of a doped source.

By using a

combination

techniques,

double

of

drift

fabricated

the

two

doping

Read IMPATT layers

and

h a v e been

have y i e l d e d 18 the

power and e f f i c i e n c y

predicted

18% e f f i c i e n c y

J.

6. D.J. Ashen, P.J. Dean, D.T.J. Hurle, J.B. M u l l i n , A . M . White, P.D. Green: J. Phys. Chem. Solids 36 (1975) 1041.

values concomitant with

t h e i r c o n f i g u r a t i o n , e.g. 10.3 Watts at 15 GHz with

5. C.R. Lewis, W.T. Dietze, M.J. Ludowise: Elect. Mat. 12 (1983) 507.

for

the

uniformly

7. H.M. Manasevit, A.C. Thorsen: J. Electrochem Soc. 119 (1972) 99.

doped

structure.

8. A.W. Helson, L.D. Westbrook in: Abstracts oflCMOVPE-2 S h e f f i e l d , England, 1984.

ACKNOWLEDGEMENTS

9. H.J. Sol.

The authors would l i k e

D.M.

Brookbanks

Phys. Chem.

to thank Mrs. G.M.

Goodridge, S. Cotton and A. Odell for providing the assessment data.

Queisser, M.B. Panish: J. 28 (1967) 1177.

IO.M. Ettenberg, C.J. Nuese: J. (1975) 3500.

App. Phys. 46

We also thank B.J. Buck,

and P.L.

Giles

for

helpful

I I . K . H . gachem, M. Heyen in: Proc. GaAs and Related Cmpds. IDP London 56 (1980) 65.

discussions of the subject matter and R.T.Blunt for reviewing t h i s manuscript. This work was c a r r i e d out with the support of Procurement Executive, Ministry of Defence, sponsored by DCVD. REFERENCES I.S.M. Sze conductor 566.

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