Melting points of GaP-metal systems and growth of GaP by vertical liquid epitax

Pearson, G. L.

English

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Q u a r t e r l y  Progress Report 
For P e r i o d  
October 1 - December 31, 1969 
FUNDAMENTAL STUDIES OF THE METALLURGICAL, 
ELECTRICAL AND OPTICAL PROPERTIES OF 
GALLIUM ARSENIDE 
G r a n t  No .  NGL-05-020-043 
P r e p a r e d  For 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
LEWIS RESEARCH CENTER 
CLEVALAND , OHIO 
Work Per fo rmed  By 
S o l i d - s t a t e  E l e c t r o n i c s  L a b o r a t o r i e s  
S t a n f o r d  U n i v e r s i t y  
S t a n f o r d ,  C a l i f o r n i a  
https://ntrs.nasa.gov/search.jsp?R=19700010407 2020-03-12T00:59:51+00:00Z
PROJECT 5115: SEMICONDUCTOR DEVICES FOR HIGH TEMPERATURE USE 
National Aeronautics and Space Administration 
Grant NGL-05-020-043 
P r i n c i p a l  Inves t iga to r :  G. L. Pearson 
S t a f f :  A .  J. Domenico and H. Nakatsuka 
The purpose of t h i s  p r o j e c t  i s  t o  develop power r e c t i f i e r s  and 
0 s o l a r  b a t t e r i e s  which w i l l  ope ra t e  a t  temperatures up t o  500 C. 
t h i s  q u a r t e r ,  f u r t h e r  r e s u l t s  have been obtained i n  t h e  melting 
temperatures of Gap-metal systems ( t o  a id  i n  t h e  formation of 0hmi.c 
contac ts )  and t h e  growth of GaP by v e r t i c a l  l i q u i d  ep i taxy  has been 
continued. I n  t h e  l a t t e r  case,  a new system has been designed, b u i l t ,  
and t e s t e d .  It i s  f e l t  t h a t  t h i s  system o f f e r s  d i s t i n c t  advantages 
over previous equipment used a t  Stanford.  
During 
Melting TemDerature of Gap-Metal Svstems 
To determine p o t e n t i a l l y  u s e f u l  metals f o r  ohmic con tac t s  t o  Gap, 
t h e  mel t ing temperature of Gap-Au, Gap-A1, Gap-Ag, and Gap-Ni were 
measured. I n  each case,  t h e  m e t a l  was evaporated on a GaP (111) 
wafer which Was t e l l u r i u m  doped t o  a c a r r i e r  concent ra t ion  of 
2 x 1017 c m  . The sample was then  heated on a molybdenum s t r i p - h e a t e r  
i n  a forming gas atmosphere and t h e  a l loy ing  process was observed under 
a microscope. The temperature of t h e  sample was measured by a t t ach ing  
t o  t h e  s t r i p - h e a t e r  a thermocopule which had previous ly  been c a l i b r a t e d  
by t h e  known e u t e c t i c  temperatures of Si-Au, Si-A1, and Si-Ag systems. 
The measured melting temperatures are shown i n  Table I.  A l l  measurements 
were repeated a t  l e a s t  once with n e a r l y  i d e n t i c a l  r e s u l t s .  
-3 
I n  t h e  cases  of Gap-Au and Gap-A1, t h e  m e l t  w e t t e d  t h e  GaP su r face  
s a t i s f a c t o r i l y .  However, due t o  t h e  na tu re  of t h e  experiment, t h e  
samples were held a t  temperatures a t  or above t h e  a l loy ing  temperatures 
f o r  a period of t i m e .  During t h i s  t i m e ,  t h e  meta l  formed i n t o  a smooth 
h i l l - l i k e  s t r u c t u r e  on t h e  Gap. 
poorer wet t ing  c h a r a c t e r i s t i c s  were found. Upon heating, t h e  previously 
mentioned bunching or c l u s t e r i n g  became more severe  w i t h  small  i s l a n d s  
of metal  formed on t h e  otherwise uncovered GaP sur face .  Under a l l  
For t h e  cases  of Gap-Au and Gap-Ni, 
\ 
1 
circumstances, t h e  r e c r y s t a l l i z a t i o n  d i d  not produce t h e  fine-grained 
s t r u c t u r e  which i s  c h a r a c t e r i s t i c  of a e u t e c t i c  but r a t h e r  y i e l d e d  
a l a y e r  with a smooth m e t a l l i c  appearance. 
nonuniform a l loy ing  on the  su r face  a s  evidenced by t h e  c lus t e r ing ,  none 
of these m e t a l  systems produced r e l i a b l e  ohmic contacts .  However, t he  
present  plans a r e  t o  use an n-type dopant with t h e  metal system, e.g., 
t h e  N i - G e  mixtures previously descr ibed i n  t h e  l a s t  q u a r t e r l y  r epor t ,  
t o  achieve repea tab le  r e s u l t s .  Therefore,  t hese  mixtures w i l l  next 
be t r i e d  on a s t r i p - h e a t e r  where t h e  a l loy ing  process can be observed 
c lose ly  under t h e  microscope, F ina l ly ,  information so  obtained 
w i l l  be used t o  a l l o y  t h e  metal-dopant mixture i n  a hydrogen-atmosphere 
furnace where t h e  times and temperatures a r e  c lose ly  cont ro l lab le .  
Possibly because of t h e  
V e r t i c a l  Liquid Epitaxy 
As described i n  t h e  l a s t  q u a r t e r l y  repor t ,  work has continued on 
t h e  growth of t h i n  high p u r i t y  n-type l aye r s  of GaP by v e r t i c a l  l i qu id  
epi taxy.  The goa l  of t h i s  aspect of t h e  work i s  t o  be ab le  t o  grow 
cons i s t en t ly ,  l aye r s  10  t o  20 microns i n  th ickness  of a c a r r i e r  
concentrat ion i n  t h e  low l O I 5  cm region upon n subs t ra tes .  The 
i n i t i a l  e f f o r t s  t h i s  q u a r t e r  used e s s e n t i a l l y  t h e  same system as  had 
been developed l a s t  year (Quar t e r ly  Prpgress Report f o r  t h e  period 
October 1 t o  December 13, 1968). That system had incorporated a new 
qua r t z  seed holder  of our  design which el iminated a source of poss ib le  
contamination. A s  a r e s u l t ,  considerably better ma te r i a l  than previously 
ava i l ab le  was grown. However, our present  work has been unable t o  
repeat  t h e  r e s u l t s .  Four consecutive attempts were made with t h e  
r e s u l t a n t  c a r r i e r  concentrat ions being from 2 x 10 cm t o  
1 x 10 cm a The problem was t r aced  t o  a s u b s t a n t i a l  melt-back of 
t he  seed ma te r i a l  with subsequent regrowth. The suspected reason f o r  
t h e  melt-back is  an incomplete s a t u r a t i o n  of t h e  m e l t  a t  t he  growth 
temperature due t o  t h e  l o s s  of phosphorus during t h e  s a t u r a t i o n  procedure. 
T h i s  problem has always ex is ted  i n  our  growth attempts due t o  t h e  na ture  
of t h e  process. 
-3 + 
17 -3 
18 -3 
S p e c i f i c a l l y ,  i n  t h e  process, s a t u r a t i o n  i s  accomplished by placing 
2 
source ma te r i a l  (here high p u r i t y  po lyc rys t a l l i ne  Gap) i n  t h e  s e e d  
holder,  r a i s i n g  t h e  temperature of t h e  gall ium m e l t  t o  near t h e  growth 
temperature,  immersing t h e  source ma te r i a l  f o r  an extended t i m e  
(approximately 4 hours), withdrawing the  source 'mater ia l ,  and cooling 
t h e  m e l t  t o  room temperature. The system i s  then  opened and t h e  source 
ma te r i a l  i s  replaced by t h e  e p i t a x i a l  seed mater ia l .  The m e l t  i s  now 
reheated t o  t h e  growth temperature, t h e  seed  ma te r i a l  immersed, and t h e  
m e l t  cooled u n t i l  s u f f i c i e n t  growth has been accomplished. The seed is  
then  withdrawn, t h e  system is  cooled t o  room temperature, and the  s e e d  
with t h e  growth is  removed. The phosphorus l o s s  occurs during t h e  second 
hea t ing  of t h e  m e l t .  
a temperature of 5 t o  15  C above t h e  d e s i r e d  growth temperature t o  
compensate f o r  t h e  loss. However, t h e  method i s  unre l iab le .  Therefore 
w e  have designed a new system which e l imina tes  t h e  need f o r  t h e  second 
hea t ing  of t h e  m e l t .  
I n  f a c t ,  common procedure has been t o  s a t u r a t e  a t  
0 0 
I n  t h e  new system, two seed  holders  have been incorporated such 
t h a t  immediately following s a t u r a t i o n  and without cooling t o  room 
temperature, growth may be s t a r t e d .  Previous attempts t o  design such 
a system had been unsuccessful  due e i t h e r  t o  cons t ruc t ion  or alignment 
d i f f i c u l t i e s .  The new design uses a coaxia l  p r inc ip l e  t o  overcome these  
problems. The e s s e n t i a l  f e a t u r e s  a r e  given i n  Fig. 1. Each s e e d  holder ,  
i n  t h i s  case two a r e  used, may be ro t a t ed  ind iv idua l ly  i n t o  pos i t i on  
i n  t h e  cen te r  of t h e  r eac t ion  tube and lowered i n t o  t h e  m e l t .  When 
not i n  use, a holder  remains i n  t h e  system near one of t he  s i d e  wal ls .  
T h i s  approach d i f f e r s  from e a r l i e r  ones i n  t h a t  coaxia l  tubing is  used 
r a t h e r  than  ind iv idua l  rods being placed adjacent t o  each other .  As a 
r e s u l t ,  t h e  alignment of t h e  seed holders  with respec t  t o  each o ther  
need only be maintained over t h e  r e l a t i v e l y  s h o r t  d i s tance  t h a t  they  
a r e  separated ins tead  of a s i g n i f i c a n t  f r a c t i o n  of t he  r eac t ion  tube 
length  a s  would be t h e  case with adjacent rods. Therefore, t h e  main 
d i f f i c u l t y ,  namely t h e  seed holders  i n t e r f e r i n g  with each o t h e r  i n  t h e i r  
movement, i s  eliminated. Further ,  t h e  end cap having only one main 
s leeve  r a t h e r  than  two or more i s  much s t ronger .  The coaxia l  design a l so  
e l imina tes  a poss ib le  source of contamination i n  t h a t  t he  system need 
not be opened t o  t h e  atmosphere between s a t u r a t i o n  and growth. 
3 
,..- 
The design shown i n  Fig. 1 has been constructed and t h e  f i r s t  
attempt was made t o  grow mater ia l .  The r e s u l t s  were successfu l  i n  
t h a t  sa tura t ion-  of t h e  m e l t  was e a s i l y  obtained and maintained p r i o r  
t o  immersion of t h e  e p i t a x i a l  seed. No melt-back was observed and a 
uniform l a y e r  approximately 20 microns i n  th ickness  was grown. 
t h e  c a r r i e r  concent ra t ion  was d isappoin t ing ly  high a t  2 x 10 cm 
The poss ib l e  causes f o r  t h i s  a r e  not  believed t o  be connected with t h e  
new design and w i l l  be  inves t iga ted  i n  t h e  immediate fu tu re .  
However, 
17  -3 
I n  summary of t h e  c r y s t a l  growing e f f o r t s  f o r  t h i s  quar te r ,  a 
new coaxia l  design has been developed which i s  believed t o  o f f e r  
d i s t i n c t  improvements over former methods. Problem remain t o  be 
solved, but  t h e  r e s u l t s  a r e  encouraging. 
PI ans f o r  Next Quar t e r  
Work w i l l  continue i n  order  t o  pe r fec t  t h e  growing technique w i t h  
t h e  new coaxia l  design f o r  t h e  v e r t i c a l  l i q u i d  ep i taxy  system and t o  
produce r e l i a b l e  ohmic contac ts  through t h e  p a r a l l e l  a l loy ing  
inves t iga t ion .  
4 
TABLE 1 
Melting Temperatures of Gap-Metal Systems 
System 
Gap-Au 
Gap-A1 
Gap-Ag 
Melting Temperature 
520° + - 2OoC 
635O - + 2OoC 
690° - + 2OoC 
Gap-Ni 760° + - 20°C 
5 
FIGURE TITLE 
Fig.  1 V e r t i c a l  l i q u i d  ep i t axy  system showing t h e  dua l  seed holder  
design. 
6 



Fundamental studies of the metallurgical, electrical and optical properties of gallium arsenide Quarterly progress report, 1 Oct. - 31 Dec. 1969

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