The purity, crystalline perfection, and electrical properties of n- and p-type GaAs crystals grown by the liquid encapsulated Czochralski (LEC) technique are evaluated. The determination of the dislocation density, incidence of twinning, microstructure, background purity, mobility, and minority carrier diffusion length is included. The properties of the LEC GaAs crystals are generally comparable to, if not superior to those of small-diameter GaAs material grown by conventional bulk growth techniques. As a result, LEC GaAs is suitable for application to minority carrier devices requiring high-quality and large-area substrates

Chen, R. T.

Holmes, D. E.

Kirkpatrick, C. G.

English

NASA Technical Reports Server

edge (Region 1 ,  or " r i n g "  region); ( 2 )  intermediate EPD occurs i n  'the center (Region 
2 ,  o r  "center" region); (3)  maximum EPD occurs a t  the edge (Region 3, or "edge" 
reg ion) .  In  addition, the EPD i n  the r i n g  and edge regions i s  greater along the 
< 100 > t h a n  the < 110 > direction (See figure 4 ) .  Figure 5 indicates measured EPD 
d i s t r i b u t i o n s  across the fu l l  diameter of wafers which typically followed a "W"-  
shaped prof i le ,  and invariably increased from front t o  t a i l ,  indicating tha t  the 
overall level of s t r e s s  increased along the c rys ta l ,  o r  t h a t  the dislocations 
multiplied a f t e r  growth, or  both.  
the center and r i n g  regions represent over 75% of the area of the wafer. 
the averaged EPD from these two areas i s  indicative of the EPD value for  the en t i r e  
wafer. O u r  experimentally determined radial EPD dis t r ibut ions a re  consistent w i t h  
theoretical thermoelastic analyses of Czochralski crystals  by Penning ( re f .  13) 
and lJordan e t  a l .  ( refs .  15, 16 ) .  
> 5000 cm- ), shallow cone shaping (- 30°),  thicker B2O3 encapsulant layers,  and 
good diameter controls ( < +  3mm), we have routinely produced 3-inch undoped LEC 
GaAs crystals  w i t h  s ignif icant ly  reduced dislocation densit ies a s  shown i n  table 
I. 
s l igh t ly  As-ri h me t ,  low ambient (- 3 atm) pressures (see table I ) ,  or Se-do ing 
were observed, which is  the lowest density reported for  3-inch diameter LEC GaAs 
wafers. Finally, substantial reduction in dislocation density (- 3.5 x 104cm-2) 
has a l s o  been achieved a t  the t a i l  o f  the crystal through a slow pull-free process, 
as shown in table  I .  In contrast t o  the regular f a s t  pull-free process used t o  t e r -  
minate the crystal growth  process, the pull-rate did n o t  increase i n  the slow 
pull-free process i n  order t o  s ignif icant ly  reduce the thermal shock during the 
pull-free process. 
I t  i s  important t o  note tha t  the EPD values a t  
Therefore, 
Throuah the use of good quali ty seeds (EPD < 5000 seed necking ( i f  E P D  
Further reductions have been attained a t  the front of the crystal  using a 
([Se] - 3 x 10 7 i  8cm- ) ( see  table 11). Dislocation densit ies a s  low as 6000 cm' B 
No dislocation reduction e f fec t  was observed for  Si or  Zn doping  up t o  
2 x 
a higher dislocation density was observed in Si-doped LEC GaAs crystals  a s  com- 
pared t o  that  of the undoped crystals .  
reduction i n  the B2O3 encapsulant ( i . e .  increasing thermal s t r e s s )  due t o  the 
increase of Si  content in B2O3 ( r e f .  18). 
Our EPD resul ts  observed i n  the present study (see tables I and 11) are  
comparable t o  commerical 2-inch, undoped D-shaped Bridgman-grown GaAs, as shown 
in tnbleII1.  Moreover, the observed values are  a t  l ea s t  5 t o  10 times lower than 
commercial 2-inch and 3-inch LEC GaAs crystals  (see table 111). 
and 1 x 1019cm-3, respectively. In contrast t o  Bridgman growth ( r e f .  1 7 ) ,  
The resu l t  can be explained by the viscosity 
Reduction of Twinning 
A major problem which has affected the yield of GaAs material sui table  for  
We have found that  device processing has been the incidence of t w i n  formation. 
the melt stoichiometry i s  an important parameter in controlling the formation of 
twins i n  3-inch-diameter, (IO0)LEC GaAs crystals.  
show t h a t  the incidence of t w i n n i n g  i s  considerably reduced when undoped G r  doped 
crystals  are grown from As-rich,near stoichiometric melts. 
undoped crystals grown from Ga-rich melts were single. On the other hand, 11 of 
13 (85%) undoped crystals  and 8 of  9 (90%) doped crystals  grown from As-rich melts 
were single. 
other growth parameters, such as the wetness of B2O3 ( fo r  [H20] < 500 ppm), the 
The resul ts  o f  our  study 
Only 4 or 12  (33%) 
Furthermore, the incidence o f  t w i n n i n g  could not be correlated w i t h  
111 
https://ntrs.nasa.gov/search.jsp?R=19830007548 2020-03-22T00:07:04+00:00Z
cone angle, the fluctuation i n  the diameter of the c rys ta l ,  or the type of the 
crucible. The achievement of h i g h  s ingle  crystal  yield (- 90%) i n  the LEC GaAs 
crystals  indicates a s ignif icant  advantage over the conventional Bridgman growth 
technique, i n  which the single crystal yield is  usually substantially reduced by 
the crystal  st icking t o  the boat. 
Microstructures Observed by TEM 
I n  general, microstructures f ree  of stacking f au l t s ,  low-angle grain boundaries 
and dislocation loops, except a few dislocations, can be observed fo r  a l l  und ed 
3 x lo1' and 1 x 10 cmm3 respectively. However, as the Se doping increases to  
greater than 2 x 1018cm-3: stacking f au l t s  and dislocation loops, as shown i n  
figure 6,  can be observed. Our resul ts  are  consistent with that  of high-quality 
Bridgman-grown materials. 
LEC GaA crystals ,  73 well as crystals  w i t h  Se, Si  and Zn doping up to  2 x 10 ?8 , 
Background Impurities 
The averaged concentrations of background shal low-donor ( S i ,  S,  Se and Te) 
W i t h  the exception of Si and B the impurity concentration levels a re  
and metal (Mg, Cr, Mn, Fe and B )  impurities i n  the 3-inch-diameter LEC GaAs grown 
from both quartz and PBN crucibles as determined by SIMS measurements are  shown i n  
table  IV. 
exceedingly low. Many measurements a re  e i ther  a t  or below the typical background 
sens i t iv i ty  of SIMS technique, suggesting tha t  LEC material i n  some cases i s  purer 
element-by-element than the standard used t o  check the SIMS back roun sensi t ivi ty .  
The concentration both S i  and B range from 1 x 1014 to  3 x 10y6cm-g, and from 
6 x 1014 t o  2 x 10 ~ m - ~ ,  respectively. 
no indication of the electr ical  ac t iv i ty  of B in LEC GaAs has been observed. 
C as determin d by LVM measurements i s  present a t  evels varying from about less  
t o  be a dominant acceptor i n  LEC material. 
B is  the isoelectronic impurity i n  GaAs, 
than - 1 x 10 e 5cm-3 (detection l imit)  t o  1.3 x 101'cm-3. Therefore, C i s  expected 
We have found t h a t  the var iab i l i ty  i n  S i  and B levels i s  dependent on va r i a -  
t ions i n  the H20 content of the B2O3 ( re f .  19). 
transport of S i  through the B2O3 from the quartz crucible to  the melt. 
the presence of H20 reduces the pick-up by the melt of B from ghe B2O3. 
n-type LEC GaAs w i t h  an electron density less  than 5 x 1016cm- can, therefore, 
only be grown from quartz crucibles u s i n g  "dry" B203 ([H20]< 500 ppm). 
shows v i r tua l ly  no difference between LEC and Bridgman material, as indicated i n  
t a b l e  IV. 
Although the Si concentration varies i n  LEC material grown from quartz crucibles,  
S i  contamination is  v i r tua l ly  eliminated by growing from PBN crucibles. As a 
resu l t ,  the background Si  concentration i n  LEC GaAs i s  typically more than one 
order of magnitude lower than i n  Bridgman material. 
had a concentration of to ta l  donors and pjceptors ( N D  + N A )  o f  about 4 x 1015cm-3, 
b u t  the typical purity is  about 1-2 x 10 
H20 i n  the encapsulant reduces the 
In addition, 
Si-doped, 
O u r  analysis of the background metal impurities, such as Fe, Cr, and Mn, 
The background concentration of the residual donor S i s  also comparable. 
Our  h ighes t  purity material 
112 
Mobi 1 i ty  
The dependence of the electron mobility on the f ree  car r ie r  concentration of 
our material i s  shown in f igur  7. The Se-doped LEC samples have a mobility of 
1 x 10 A s l igh t ly  lower mobility i s  observed 
for  the Si-doped samples indicating higher compensation. In general, the mobility 
curves show a peak fo r  an electron density of - 1017cm-3. The reduced mobilities 
a t  lower electron concentrations are  probably due t o  compensation controlled by 
background impurities o r  native defects. Our mobility resul ts  are consistent w i t h  
s t a t i s t i c a l  analysis o f  hundreds o f  crystals  grown by conventional bulk-grown 
methods reported recently by Mullin e t  a l .  ( re f .  20). In  addition, a comparison 
of our  resul ts  t o  the theoretical mobility'electron concentration relationship for  
GaAs, indicates tha t  our  material i s  characterized by low compensation ra t ios  
(0.3 t o  0.4) consistent w i t h  the s t a t i s t i c a l  behavior of other b u l k  GaAs materials. 
approxj7ately 4000 and 2300 cm 9 /Vsec corresponding to  electron densit ies of 
and 1 x 1018cm' , respectively. 
Good mobility was also observed for  the p-type LEC GaAs, as  shown irsfigyre 8. 
Hole mobilities as  high as 330 cm2/Vsec (for  hole concentration - 1 x 10 cm' ) 
were observed for  the LEC material grown from undoped Ga-rich melt using a PBN 
crucible. 
the p-type conduction of the undoped LEC GaAs i n  terms of the 77 meV acceptor 
( re f .  22) .  The origin of this acceptor i s  probably the Ga a n t i s i t e  defect. Our 
available,  high-purity, small-diameter Bridgman-grown GaAs, a s  shown i n  f igure 8. 
Minority Carrier Diffusion Leng th  
Mo ' l i t i e s  as h i g h  as 210 cm2/Vsec were observed fo r  Zn-doped material 
w i t h  - 4 x 10 Y? cm-3 hole concentration. E l l io t t  e t  a l .  ( re f .  21) have explained 
mobility results fo r  the p-type LEC GaAs are  also comparab eS e to  the comemrcially- 
Good hole diffusion lengths (as h i g h  as 1.3 pm) have been observed for  n-type 
(Se- or Si-doped), 3-inch-diameter LEC GaAs crystals ,  as shown in figure 9. 
measured values are comparable to  those of n-type bulk GaAs grown by conventional 
methods reported by Sekela e t  a l .  (ref. 1 2 ) .  The electron diffusion length has 
only been determined for  one p-type undoped LEC GaAs crystal grown from Ga-rich 
melt i n  a PBN crucible ( table  V ) .  
i n  the material. 
for  b o t h  p-type high-purity MOCVD and LPE layers ( re fs .  10, 11 and 23). 
The 
A diffusion l e n g t h  as  h i g h  as 5.3 pm i s  observed 
T h i s  value is  close t o  the 8 pm electron diffusion length reported 
No correlation was observed between the dislocation density and diffusion 
length for  e i the r  n- or p-type LEC GaAs. A similar observation has been reported 
by Sekela e t  a l .  ( ref .  1 2 )  for  n-type bulk GaAs. 
note that  the hole diffusion l eng th  as well as the hole concentration across the 
full-diameter wafer decrease towards the edge, as  shown in table V .  
constant hole mobility was observed across the wafer and no correlation between 
the diffusion length and dislocation density was detected, the decrease of the 
diffusion length may be at t r ibuted t o  an increase of an unknown donor concentration 
toward the edge of the wafer. 
Finally, i t  is of in te res t  t o  
Since a 
Further work i s  s t i l l  needed in th i s  area. 
113 
CONCLUSION 
We have shown that large-diameter, n- and p-type LEC GaAs can be grown with 
A high single crystal yield (- 90%) and clean microstructures 
a low-dislocation-density, high purity, long minority carrier diffusion lengths, 
and high mobility. 
have also been achieved in these materials. 
sniall-diameter GaAs crystals grown by conventional bulk growth techniques. Our 
results showing low dislocation densities and long diffusion lengths indicate that 
the dislocation density will not be a limiting factor for the application of 
3-inch-diameter LEC GaAs crystals to minority carrier devices, such as solar cells. 
The low background impurities, consistent with high mobility and long diffusion 
length, also ensure the use of these materials as both passive and active substrate 
materials in these devices. We, therefore, conclude that for minority carrier 
devices requiring high-quality and large-area substrates, the 3-inch-diameter LEC 
GaAs crystals are indeed an excellent material for such applications. 
The properties are comparable to 
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115 
TABLE I. DISLOCATION DENSITY REDUCTION IN UNDOPED 3-INCH LEC GaAs 
Front Tail I 
(cm-') 
1 )  12 x 103(6 x lo3*) 
(cm-') 
9.0 lo4 (3.3 lo4+) 
TABLE 11. DISLOCATION DENSITY REDUCTION IN DOPED 3-INCH LEC GaAs* 
2) 1.8 104(1.3 lo4*) 
3) 2.0 x 105(9 x IO4*) 
1.0 lo5 (3.5 lo4+) 
2.2 lo5 (1.1 lo5+) 
TABLE 111. DISLOCATION DENSITY FOR CMERICALLY AVAILABLE 2-OR 3- 
INCH BRIDGMAN AND LEC GaAs CRYSTALS 
Front 
(cm-') 
1) ti lo3 
2)  1.1 lo4 
3) 1.7 lo5 
E.P.D. (cm-') 
- 2 - 3  x lo4 
2 INCH - 
Bridgman (D-shaped) 
LEC 
Tail 
(cm-') 
1) 6.1 lo4 
2)  1.2 lo5 
3) 1.7 lo5 
I 
3 INCH - 
LEC > 4  x lo4 (Front) 
> 3 x lo5 (Tail) 
116 
TABLE I V .  BACKGROUND IPlPURITY ANALYSIS OF LEC GaAs 
L E C ~  ?EN 1.1.15 <Sa14 <yl3 b14 (6.14 <1~16 <a15 2-13.15 <le16 4114- 
2817 
BRIWMAN' OUARTZ 3116 3.14 h i 3  6114 <Sa14 6.14 6.16 NQ 2018 CL14 
r L 
* 7 CRYSTA(s ANALYZED AND AVERAGED. 
4 CRYSTALS ANALYZED AND AVERAGEb. 
12 CRYSTALS AUALYZED AND AVARAGED. 
*CARBON DETSRM1NED BY LVM. 
TABLE V, ELECTRON DIFFUSION LENGTH I N  P-TYPE LEC GaAs 
*ELECTRON MFFUjlON LENOTHN8#m REPORTED FOR BOTH p T W E  MDCVD AND LPE LAYERS. 
117 
* . . ' .  
. . . .  
.GaAs MELT 
, ./ 
1 
.~ 
Figure 1.  Configuration f o r  LEC Growth * -  
Res/ 
Figure 
System. 
Figure 2. A 3.6 Kg, 3-inch Diameter, 
(700) LEC GaAs Crystal  w i t h  Diameter 
Variation Less Than 2mm. 
ion 1 
3, Radial Dislocat ion Density Map f o r  3-in 
(100 > 
\Region 2 
3 
ch GaAs LEC wafers. 
118 
llMCH 
WIRDC 82-16276 
< 110> - 
U 
200 I.tm 
Figure 4. Etch P i t  Density Map f o r  a (100) LEC GaAs Wafer. 
119 
RADIAL DISLOCATION DENSITY 
IN 3-INCH LEG CRYSTAL 
2 6 
DlSTlNCE ALONG SLlCr IN 110 DIRECTlON mi 
Figure 5. Radial and Longitudinal Dislocation Density 
f o r  Large-diameter LEG GaAs Crystal .  
Figure 6. B.F. Micrograph f o r  a Se-doped LEC GaAs 
sample w i t h  [Se] - 7 x 1018cm-3. 
I20 
(QUARTZ) 
-+-Se-DOPED (QUARTZ) 
I I 
1000 - 
0 I I 1  I I , , I  1 I I I I  1 1 1  
1015 1016 1018 
FREE ELECTRON CONCENTRATION (cm-3) 
F igure  7. M o b i l i t y  vs. Free E lec t ron  Concentration 
f o r  n-type LEC GaAs. 
1 
1 I I I I 1  I I 1 1 1  I 1 1 1 1  I I I l l  I 1 I 1  
FREE HOLE CONCENTRATION (cm-3) 
I 015 10’6 1018 1 0 1 ~  
Figure 8. M o b i l i t y  vs. Free Hole Concentration 
f o r  p-type LEC GaAs. 
121 
r I S  
@Si-DOPED (QUARTZ) 
OSe-DOPED (QUARTZ) 
1 
0.1 I I , I t  
10’6 10’8 10’9 
I I # I /  0 ,  
FREE ELECTRON CONCENTRATION, ( ~ r n - ~ )  
Figure 9. Hole Diffusion Length vs. Free Electron 
Concentration f o r  n-type LEC GaAs. 
122 


Advances in large-diameter liquid encapsulated Czochralski GaAs

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