Radiation effects in lithium-doped silicon solar cell

Carter, J. R.

Scott, R. E.

English

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FIRST QUARTERLY REPORT 
15 August 1968 
STUDY OF RADIATION EFFECTS IN LI-DOPED 
SILICOIY SOLAR CELLS 
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ff 653 July 65 
10971-6005-R0-00 
J. R. Carter 
R. E. Scott 
Contract 952251 
Jet Propulsion Laboratory 
California Institute of Technology 
Pasadena, California 
TRW Systems 
One Space Park 
Redondo Beach, California 90278 
https://ntrs.nasa.gov/search.jsp?R=19680027026 2020-03-12T06:40:21+00:00Z
i 
FIRST QUARTERLY REPORT 
15 August 1968 
STUDY OF RADIATION EFFECTS IN LI-DOPED 
SILICON SOLAR CELLS 
Prepared w :  
__-. 2,, ,' ,: . 
.I . ' < i  i I - 
R. G. Downing ,/' 
10971-6005-RO-00 
J. R. Carter 
R. E. Scott 
A- , 
Contract 952251 
Jet Propulsion Laboratory 
California Institute of Technology 
Pasadena, California 
TRW Systems 
One Space Park 
Redondo Beach, California 90278 
This work was performed,for the Jet Propulsion Laboratory, California 
Institute of Technology, as sponsored by the National Aeronautics and 
Space Administration under Contract 952251. 
ii 
TABLE OF CONTENTS 
I. INTRODUCTION 
11. PROGRESS IN THIS REPORT PERIOD 
A. Diffusion Sources 
B. Stability Studies 
C. Removal Rate Studies 
D. Annealing 
111. FUTURE WORK 
10971.6005-R0-00 
FIG* 1 Topography of Lithium Doped Float Zone Silicon 6 
(0.1 ohm-cm), As Received 
FIG. 2 Topography of Lithium Doped Float Zone Silicon 7 
(21 ohm-cm), after 18 Hrs. at 400°C, Slow Cooled 
FIG, 3 Majority Carrier Removal in Lithium Doped 
Silicon under Electron Irradiation 
8 
FIG. 4 Annealing of Carrier Removal Damage in Lithium 9 
Doped Silicon 
iii 
ABSTRACT 
This report covers the initial three months of effort on JPL 
Contract No. 952251 concerning radiation effects in Li-doped silicon, 
Inasmuch as considerable additional work remains to be performed, the 
data and conclusions presented herein are considered tentatlve. 
efforts utilizing LiA1H4 as a diffusion source are promising, but to 
date have failed to yield surface lithium concentrations in excess of 
lo1’ atoms/cm . Lang x-ray topography has been attempted to determine 
whether lithium instabilities at higher temperatures are due t0 precip- 
itation, outdiffusion, or some other mechanism. Results to date indicate 
that lithium precipitation producing strained regions greater than 0.1 mm 
are not observed, 
removal as a function of electron fluence in Li-doped float-zone silicon, 
but a linear dependence on electron fluence in crucible grown silicon. 
Finally, isochronal annealing studies indicate a variety of material 
dependent effects including relatively low temperature annealing stages 
and reverse annealing stages. 
have not yet been obtained to determine their dependence on material 
parameters, 
Initial 
3 
Removal rate studies indicate exponential carrier 
Sufficient data on these annealing phenomena 
‘ - 1 -  10971.6005-R0-00 
T . INTRODUCTION 
This r epor t  covers  a c t i v i t i e s  during t h e  first t h r e e  months of per for -  
mance on .TPL Contract  952251. This  work has  included s t u d i e s  of new methods 
of d i f f u s i n g  l i t h ium i n t o  s i l i c o n ,  thermal s t a b i l i t y  of l i t h i u m  doped 
s i l i c o n ,  r a d i a t i o n  caused carrier removal i n  l i t h ium doped s i l i c o n ,  anneal ing 
of c a r r i e r  removal damage, and recovery k i n e t i c s  s t u d i e s .  As t h i s  per iod 
covered t h e  s tar t  of t he  con t r ac t ,  cons iderable  e f f o r t  was expended on 
prepara t ion ,  planning and equipment maintenance. 
11. PROGRESS I N  THIS REPORT PERIOD 
Recent s t u d i e s  have been d i r e c t e d  a t  a b e t t e r  understanding of t he  
behavior of l i t h ium i n  s i l i c o n .  More knowledge is needed regarding t h e  
thermal s t a b i l i t y  of l i t h ium doped s i l i c o n  and t h e  r o l e  of l i t h ium i n  the  
r a d i a t i o n  damage processes.  The results repor ted  he re  are in te rmedia te  i n  
na tu re  and w i l l  be  f u r t h e r  e labora ted  before  drawing s i g n i f i c a n t  conclusions.  
A. Di f fus ion  Sources 
A series of d i f f u s i o n  experiments have been performed t o  eva lua te  
t h e  usefu lness  of L i A l H  as a d i f f u s i o n  source.  The material was used i n  
t h e  form of a 5 molar s o l u t i o n  i n  d i e t h y l  e the r .  The s i l i c o n  wafers  were 
painted wi th  a t h i c k  coa t ing  of so lu t ion .  
use  of t h i s  material i s  t h e  evaporat ion of t h e  so lven t  t o  o b t a i n  a uniform 
coa t ing  of t he  hydride.  There i s  a s t rong  tendency f o r  t h e  coa t ing  t o  bubble 
i n  some areas. A second problem is the  tendency of t h e  hydride t o  e a s i l y  
oxidize.  The ox ida t ion  problem w a s  e l iminated by evacuat ing t h e  d i f f u s i o n  
furnace before  heat ing.  
s u f f i c e .  The one c l e a r  advantage of t h i s  material is l a c k  of phys ica l  a t t a c k  
4 
The f i r s t  problem involved i n  
Use of an i n e r t  atmosphere would probably a l s o  
This  r e p o r t  conta ins  information prepared by TRW Systems Group under JPL 
subcontract .  I ts  content  is  not  n e c e s s a r i l y  endorsed by t h e  Jet  Propuls ion 
Laboratory,  Ca l i fo rn ia  I n s t i t u t e  of Technology, o r  t h e  Nat ional  Aeronautics 
and Space Administration. 
- 2 -  10971.6005-R0-00 
of the silicon surface which is common with a lithium-mineral oil paste. 
surface of the hydride diffused wafers remained flat with no pitting and 
the residue is easily removed. 
any surface concentrations in excess of Li atom/cm . Additional 
work should improve this problem. 
The 
So far it has not been possible to produce 
3 
B. Stability Studies 
When lithium doped silicon is raised to temperatures above 300°C and 
cooled to room temperature, the resistivity increases. Such a change 
could be caused by precipitation or outdiffusion of lithium. The times 
and temperatures involved suggest outdiffusion. To further investigate 
this behavior lithium doped float zone silicon wafers were annealed at 
various temperatures and slow cooled to room temperature. The resistivities 
of these wafers were measured with a four point probe after sufficient 
material was removed to insure uniform lithium distribution. The resistivity 
results are shown below: 
3 
3 
A s  received 0.035 R-cm 5 . 1 ~ 1 0 ~ ~  Li/cm 
1 hr @ 200°C 0.035 R-cm 5 . 1 ~ 1 0 ~ ~  Li/ cm 
3 
3 
1 hr @ 30OOC 0.035 R-cm 5.1~10'~ Li/cm 
18 hr @ 400°C 21.2 R-cm 2. 4x1OI4 Li/cm 
These wafers were examined by the Lang x-ray topography technique 
after all treatments. This x-ray technique uses Bragg reflection from a 
given set of crystal planes to give an unmagnified image of the crystal, 
Stressed areas of the crystal reflect the X rays more strongly and appear 
darker on a photographic plate, or lighter on a positive print such as 
those shown. Figures 1 and 2 are 9x enlargements of (220) topographs 
of the as-received and 18-hrs-at-400"C specimens, respectively. The 
topographs are taken in transmission through the specimens, using Mo 
characteristic radiation, and show a projection of the structural defects 
throughout the volume of the specimens, not just the surface. 
resolution is less than 10 microns in the direct specimen image, which is 
less than 0.1 mm on the enlargements, 
Instrumental 
- 3 -  10971.6005-R0-00 
The h ighly  d i s loca ted  s t r u c t u r e  normally a s soc ia t ed  wi th  f loat-zone 
s i l i c o n  i s  c l e a r l y  shown i n  F igures  1 and 2. 
are e a s i l y  seen  i n  many p a r t s  of t h e  topographs. 
n i ck  is d i s t o r t i o n  a s soc ia t ed  wi th  a zone s p i l l  dur ing zone-refining of 
t h e  c r y s t a l ,  
due t o  a s c r a t c h  during handl ing of t h e  photographic p l a t e ,  
area i n  F igure  2 near  t h e  edge and 90 degrees  from t h e  zone s p i l l  i s  due 
t o  t h e  specimen mounting pos t ,  which d id  no t  appear i n  F igure  1 because 
t h a t  specimen w a s  mounted a t  t h e  unseen edge. The prominent whi te  s p o t  
i n  Figure 2 appeared wi th  t h e  f i r s t  hea t  t reatment  and is  due t o  improper 
Ind iv idua l  d i s l o c a t i o n s  
The s p o t  a t  t h e  edge 
The small whi te  spo t  i n  F igure  1 is spur ious ,  and is  probably 
The l i g h t  
handling by tweezers. 
No sepa ra t e  p r e c i p i t a t e  p a r t i c l e s  which might be in t eg ra t ed  as l i t h i u m  
metal p r e c i p i t a t e s  are seen. I f  p re sen t ,  such p a r t i c l e s  should be 
v i s i b l e  because of t h e  extremely high s t r a i n  s e n s i t i v i t y  of t h i s  technique. 
However, i t  i s  e n t i r e l y  p o s s i b l e  t h a t  l i t h ium may p r e c i p i t a t e  a t  e x i s t i n g  
d i s l o c a t i o n s ,  causing very l i t t l e  change i n  specimen appearance. This 
specimen w i l l  be  analyzed f o r  l i t h i u m  by atomic absorp t ion  spectrometry 
t o  check f o r  ou td i f fus ion  of l i th ium.  
ment us ing  Lopex s i l i c o n ,  which is  r e l a t i v e l y  f r e e  of d i s loca t ions .  
It is  p l a n n e d t o  r epea t  t h i s  experi-  
C. Removal R a t e  S tudies  i 
Recent work has ind ica t ed  t h a t  t h e r e  are s i g n i f i c a n t  d i f f e rences  
i n  t h e  na tu re  of t h e  carrier removal process  during i r r a d i a t i o n  of f l o a t i n g  
zone and c r u c i b l e  grown l i t h i u m  doped s i l i c o n .  One d i f f e r e n c e  i s  i n  t h e  
anneal ing behavior and a second i s  found i n  t h e  i r r a d i a t i o n  removal rate 
f o r  major i ty  carriers. W e  have previous ly  discussed t h e  manner i n  which 
t h e  major i ty  carrier concent ra t ion  changes exponent ia l ly  wi th  e l e c t r o n  
f luence  i n  heavi ly  l i t h ium doped f l o a t i n g  zone s i l i c o n .  This e f f e c t  is  
aga in  shown i n  Figure 3. 
s i t u a t i o n  is very  d i f f e r e n t  i n  s i m i l a r l y  doped c r u c i b l e  grown s i l i c o n .  In  
such material t h e  decrease  of carrier concent ra t ion  is l i n e a r l y  r e l a t e d  t o  
t h e  e l e c t r o n  f luence.  The removal r a t e  cons tan t  f o r  t h i s  material appears  
t o  be 1.8 cm . This  i n d i c a t e s  t h a t  t h e  product ion rate of t h e s e  d e f e c t s  
It can a l s o  be  seen i n  t h i s  f i g u r e  t h a t  t h e  
-1 
is cons tan t  and independent of t h e  l i t h ium concentrat ion.  This  observa t ion  
- 4 -  10971.6005-R0-00 
is in contrast to that of float zone material which is proportional to 
the remaining ionized lithium concentration. Since the oxygen concentra- 
tion in the crucible silicon is much higher than the lithium concentration, 
it is likely that the defects forming do not consume lithium but rather 
involve some combination of displacement products with oxygen. 
is necessary to clarify the dati; and allow a proper interpretation of 
these effects. 
More work 
D. Annealing 
As previously mentioned, the study of annealing of carrier removal 
damage has indicated some significant differences in various lithium 
doped silicon crystals. 
of various irradiated lithium doped crystals are shown in Figure 4 .  One 
interesting phase of this data is the low temperature annealing stage in 
the 0.1 ohm-cm lithium doped floqt zone silicon. This stage occurs rapidly 
at 175°C and appears to be a discrete chemical reaction. 
such samples with different electron fluence are mown in Figure 4. 
both cases it was possible to fit the data to a theoretical function which 
describes the isochronal anneal of a first order process, the time constant 
The results of the isochronal (15 min.) annealing 
Data from two 
In 
of which is limited by an activation energy of 0.32 eV. A different value 
of thepre-exponentialconstant was required for each of the samples, since 
this sample with a 3 fold higher fluence annealed at a slightly higher 
temperature. It is interesting to note that, in both cases, this stage 
restores roughly 40% of the carrier removal damage and the remainder is 
stable to 350°C. 
are nulified by the previously discussed "outdiffusion" effect which rapidly 
removes carriers at 400°C. 
to that of the Si-E center, but that center anneals with an activation 
energy of 0.94 ev. 
because ohosphorus concentration of this material (approx. lox4 ~ m - ~ )  is 
not sufficient to produce the required defects. 
0.32 eV is curiously low. Interestingly this value is half that of the 
activation energy for lithium diffusion. 
the energy of solution of lithium in silicon (0.54 eV). 
necessary to confirm the data. 
Attempts to continue the annealing at higher temperatures 
The annealing is similar in temperature range 
It is also unlikely that Si-E centers would be produced 
The activation energy of 
This value is even smaller than 
Further work is 
- 5 -  10971.6005-R0-00 
The 0.07 ohm-cm qua r t z  c r u c i b l e  s i l i c o n  and t h e  2 ohm-cm f l o a t  zone 
s i l i c o n  both exhib i ted  similar anneal ing behavior a f t e r  i r r a d i a t i o n  
d e s p i t e  t h e i r  very  d i f f e r e n t  impuri ty  concent ra t ion .  
cha rac t e r i zed  by 
is  t h e  formation of a d d i t i o n a l  carrier removal d e f e c t s  from o t h e r  i n c i p i e n t  
o r  l a t e n t  d e f e c t s  which were prevented from forming a more s t a b l e  d e f e c t  
by some thermally a c t i v a t e d  l i m i t i n g  process.  
is  an anneal ing s t a g e  observed i n  both materials a f t e r  hea t ing  t o  150OC. 
I n  t h e  2 ohm-cm f l o a t  zone t h e  anneal ing s t a g e  removes approximately a l l  
of "reverse anneal" damage produced a t  100°C. 
s i l i c o n  t h e  15OOC anneal ing s t a g e  removes over 10% of t h e  o r i g i n a l  carrier 
damage i n  a d d i t i o n  t o  t h e  " reverse  anneal" damage, The reason f o r  t h e  
d i f f e r e n t  behavior i n  these  two samples may be  r e l a t e d  t o  t h e  re la t ive 
l i t h ium and oxygen concent ra t ions  which are more nea r ly  equal  i n  t h e  
case of t h e  l as t  descr ibed samples. 
Both samples are 
a "negat ive annealing" s t a g e  a t  100'C. This  i n  e f f e c t  
The second po in t  of i n t e r e s t  
I n  t h e  qua r t z  c r u c i b l e  
111, FUTURE WORK 
We have rece ived  enough l i t h ium s o l a r  cel ls  t o  start t h e  appl ied  
po r t ion  of t h e  program. Evaluat ion s t u d i e s  w i l l  be  performed and a d d i t i o n a l  
s t u d i e s  w i l l  be  made t o  support  t h e o r e t i c a l  work on t h e  model f o r  t h e  
k i n e t i c s  of recovery. 
w i l l  be  analyzed f o r  u se  i n  these  s tud ie s .  
I n  add i t ion ,  gold b a r r i e r  diodes now being s tud ied  
- 6 -  10971,6005-R0-00 
FIG, 1 TOPOGRAPH OF LITHIUM DOPED FLOAT ZONE 
SILICON (0.1 ohm-cm), AS RECEIVED 
10971.6005-R0-00 - 7 -  
FIG, 2 TOPOGRAPH OF LITHIUM DOPED FLOAT ZONE SILICON 
(21 ohm-cm), AFTER 18 HRS. AT 4OOOC SLOW COOLED 
- 8 -  10971-6005-R0-00 
1 MEV ELECTRON FLUENCE (10’6e/cm2) 
FIG. 3 MAJORITY CARRIER REMOVAL IN LITHIUM DOPED 
SILICON UNDER ELECTRON IRRADIATION 
- 9 -  10971-6-05-R0-00 
FIG. 4 ANNEALING OF CARRIER REMOVAL DAMAGE IN 
LITHIUM DOPED SILICON 


Study of radiation effects in Li-doped silicon solar cells

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