- Email: [email protected]
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.
12.S.R. Steele, S.P. Runowicz, K.R. Johnson in: GaAs and Related Cmpds. lOP London 45 (1978) 45. 13.H.Bruch, H. M a r t i n i , K.H. Bachem, P. Balk: Rev. Phys. App. (France) 13 {1978) 783. 14.1.H. Goodridge in: J. de Phys. Colloque C5 43 (1982) 249.
in:
2nd e d i t Physics of SemiDevices, Wiley U.S.A. (1981)
2. J.R. Knight, D. E f f e r , P. Evans, Solid State Elec. 8 (1965) 178. 3. L.R. Weisberg, F.D. Rosi, P.G. Herkart, in: M e t a l l u r g i c a l Soc. Conf. Vo.5, Interscience, New York (1959). 4. J . Parsons, F.G. Krajenbrink: Electronchem. Soc. 130 (1983) 1782.
J.
(France),
15.L.J. van der Pauw: P h i l i p s R e s . Rep. 13 (1958) i . 16.S.J. Bass, P.E. O l i v e r in-Proc. GaAs and Related Cmpds. lOP London 33b (1976} i . 17.Y.G. Sidorov, L.F. Vasileva, I.V. Sabinina, S.A. Dvoretsky, A.V. Sidorova: J. Electrochem. Soc. 123 (1976) 698 18.D.M. Brookbanks, B.J. Buck in: IEEE MTT-S I n t e r n a t . Microwave Symp. Digest, IEEE Mew Jersey (1983) 215.
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.
12.S.R. Steele, S.P. Runowicz, K.R. Johnson in: GaAs and Related Cmpds. lOP London 45 (1978) 45. 13.H.Bruch, H. M a r t i n i , K.H. Bachem, P. Balk: Rev. Phys. App. (France) 13 {1978) 783. 14.1.H. Goodridge in: J. de Phys. Colloque C5 43 (1982) 249.
in:
2nd e d i t Physics of SemiDevices, Wiley U.S.A. (1981)
2. J.R. Knight, D. E f f e r , P. Evans, Solid State Elec. 8 (1965) 178. 3. L.R. Weisberg, F.D. Rosi, P.G. Herkart, in: M e t a l l u r g i c a l Soc. Conf. Vo.5, Interscience, New York (1959). 4. J . Parsons, F.G. Krajenbrink: Electronchem. Soc. 130 (1983) 1782.
J.
(France),
15.L.J. van der Pauw: P h i l i p s R e s . Rep. 13 (1958) i . 16.S.J. Bass, P.E. O l i v e r in-Proc. GaAs and Related Cmpds. lOP London 33b (1976} i . 17.Y.G. Sidorov, L.F. Vasileva, I.V. Sabinina, S.A. Dvoretsky, A.V. Sidorova: J. Electrochem. Soc. 123 (1976) 698 18.D.M. Brookbanks, B.J. Buck in: IEEE MTT-S I n t e r n a t . Microwave Symp. Digest, IEEE Mew Jersey (1983) 215.