Electrical conductivity and electron spin resonance experiments in study of radiation effects on silico

Naber, J. A.

Wikner, E. G.

English

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Genera/ Atomic Ohision 
SPECIAL NUCLEAR Ef PECTS LABORATORV 
P.O. BOY 608. SAN DIEGO. CALIFORNIA a a i u  
RADIATION EFFECTS ON SILICON 
Seventh Quarterly Progress Report Covering the Period 
July 1, 1966, through September 30, 1966 
Work done by: 
-J. 3, Bryant 
H. Horiye 
J. A, Naber 
E. G. Wikner 
Report written by: 
J. A. Naber 
E. G. W i k n e r  
National Aermautics and Space Administration 
Contract NAN-289 
General Atomic Project 430 October 12, 1966 
GpQ pff'cE $- 
_ .  
CFSTI PRICE(S) $ 
~ 6 7  12164 
-Ru'/ 
ULCCESBION NU R 3 4 
https://ntrs.nasa.gov/search.jsp?R=19670002835 2020-03-16T17:00:16+00:00Z
1. INTRODUCTION 
This seventh quarterly progress report on Contract NAS7-289, "Radiation 
Effects i n  Silicon Solar Cells," ccwers the period July 1, 1966, through 
September 30, 1966. 
silicon was investigated by the microwave technique af ter  irradiation by 
30 MeV electrons at room temperature. 
made on high-resistivity material t o  establish a correlation between the 
microwave resonance and carrier lifetime studies. 
performed on the formation of the Si-@ center at 8OoK, with subsequent 
annealing t o  30OoK. 
D u r i n g  this period, e lectr ical  conductivity in  high-purity n-type 
Electron-spin resonance studies were 
Further experiments were 
A portion of the results presented here were obtained under Contract 
DA-49-186-M-65 (X) . 
1 
2, 
Electrical  conductivity i n  high-purity n-type silicon was investigated 
by the microwave technique, 
temperature by 30 MeV electrons and then the effects of these high-energy 
electrons were investigated at the flash x-ray facil i ty.  
The n-type silicon was first irradiated at room 
The sample used was n-type sil icon cut from the same bou1.e as the 
sample which was  reported i n  GACD-6249 (6/30/66) and irradiated at 4.2'K. 
The electrical conductivity and lifetime data were reproducible and showed 
the same injection-level deFendence at temperatures from room temperature 
t o  l iquid nitrogen temperature , 
The Shockley-Read recombination theory was used t o  analyze the 
indection-level dependence of this n-type sil icon sample before irradiation. 
The Shockley-Read recombination theory for  a single-level defect i s  based on 
the assumption that the concentration of recombination centers i 8  suffi- 
ciently smztll relative t o  the excess-carrier dens i ty tha t  a negligible 
fraction of the excess carriers are used t o  readjust the occupation frac- 
t ion of the recombination center, 
the recombination center and the excess electrons and holes must have equal 
densities and lifetimes i n  either steady s ta te  or transient recambination. 
The lifetime is then a function of excess-carrier concen-kration and is  
given by : 
The excess carriers must f l o w  through 
An -
T + T  n 4 h o  
An 1.t- 
7 =  
"0 
nl p1 
='I' ( 1 + - ) + 7  - 
0 0 
il. no n P O  
7 = c i  + T  h no PO 
where the above symbols are defined in  GA-3872 (15 February 1963). Figure 
1 shows the plot of r(l + E )  versus at liquid nitrogen -';emperature. 
0 
2 
Figure 2 shows the inverse temperature dependence of T~ and T 
pretation of t h i s  data has not yet been determined. 
The inter-  h' 
2.2 LINAC IRRADIATION 
This sample was irradiated at the electron l inear accelerator f ac i l i t y  
(Linac) w i t h  0.1 Vsec, 30 MeV electrons. 
ature the carrier lifetime at room temperature was about 700 psec, but the 
first measurements at Linac showed a lifetime i n  the neighborhood of 70 psec. 
This decrease in  lifetime was no doubt due t o  scattered electrons during 
tuning of the Linac even though precautions were taken t o  guard against 
these stray electrons. This portion of the experiment w i l l  be rerun in  
the immediate future. 
Before irradiating at room temper- 
Although the carrier lifetime was unexpectedly changed, the e lec t r ica l  
conductivity as a function of t o t a l  integrated flux was recorded. This i s  
plotted on Figure 3 .  
-1 irradiation, then the calculated carrier removal rate is about 0-11 cm 
After th i s  irradiation of 1.53 x lou electrons/cm2, the injection-level 
dependence of the lifetime at room temperature is  not present. 
ature dependence of the lifetime below room temperature shows tha t  the 
damage intToduced by the Linac irradiation i s  a trapping center. 
gives the carrier lifetime as a f'unction of 10 /T( K)'l wer a l imi ted  
temperature range. 
temperature did not produce any noticeable change i n  the lifetime of the 
excess carrriers at room kmperature. 
grated flux of 2.84 x lox3 electrons/cm . A t  these high flux levels w i t h  
high-energy electrons at room temperature, it is  known that the introduced 
defects are not of a simple nature but m a y  be quite complicated and there- 
fore difficult t o  identify. 
increasing 10 /T( K)'l) seems t o  be the same center that was introduced 
with l iquid helium temperature irradiations (GACD-6249, 6/33/66). 
If the mobility i s  assumed t o  be constant during th i s  
., 
The temper- 
Figure 4 
3 0  
Further Linac irradiation on this  sane sample at room 
The sample was irradiated t o  an inte- 
2 
The trapping center (increasing T with 
3 0  
2.3 COMMENTS 
Even though the room-temperature irradiations of the high-purity 
n-type silicon are not finished, it is  possible to comment on the nature 
of the introduced damage, The trapping center introduced by the room- 
3 
temperature irradiation is very much like the trapping center istroduced 
by the irradiation at liquid helium temperature. 
dependence of the lifetime of the excess carriers is the same after the 
liquid-helium-temperature and room-temperature irradiations, and there is 
no injection-level dependence of lifetime of the excess caxriers for this 
trapping center. Continued electron irradiation at room temperature does 
not change the nature of the trapping center. 
The inverse temperature 
2,4 FUTURE: PLANS 
In the immediate future, the high-resistivity n-type silicon sample 
w i l l  again be irradiated at room temperature, but this time the flux will 
be controlled so that the radiation-inducci3 recombination center can be 
detected. The recovery of the excess-cmzier lifetime as a f’unction of 
annealing temperature above room temperature w i l l  be investigated. In 
this manner, the recombination and trapping center may be associated with 
one of the known centers as identified by electron-spin resonance wurk . 
Future plans a-lso call for the investigation of the type of defects intro- 
duced at liquid nitrogen temperature in high-resistivity n-type silicon by 
30 MeV electrons. 
high-resistivity p-type silicon. 
(1) 
The rest of the program calls for the investigation of 
1) G. D. Watkins, Radiation Damage in Semiconductors, Academic 
PTws, p. 98, 1364. 
4 
3. 
The electron-spin resonance studies have concentrated on establishing 
a correlation between the microwave resonance and carrier lifetime studies, 
Most of the lifetime studies must be carried out on high-resistivity mat.er- 
ial, namely i n  the res i s t iv i ty  region between 10 t o  10 ohm-cm. "herefore, 
it was considered necessary t o  perform the resonance experiments on s i m i l a r  
material with the major effor t  being placed on the more available 10 ohm-cm 
samples. 
resonance techniques have been at l ea s t  an order of magnitude below that  
observed w i t h  the galva,nomwetic techniques, including lifetime measure- 
ments, Consideration is  now being given to the role of the r a t io  of the 
number of i n i t i a l  chemical impurities t o  the number of radiation-induced 
defects i n  determining the lifetime degradation in  order t o  plan future 
experiments. 
center (the vacancy phosphorus complex) at 8OoK, and the influence on the 
resonance as the sample was annealed t o  300°K. The experimental procedure 
w a s  t o  irradiate the sample at 8OoK and, before warming it, measure the 
resonance intensity at 5OoK, using both pulled and floating-zone material. 
Again, as explained in  the last quarterly report, no change in  the nmber 
of Si-& centers was observed upon annealing except that due t o  a change i n  
the charge s t a t e  of this center, 
4 
Thus fm, the introduction ra tes  of the observed centers seen by 
Additional experiments were performed on the formation of the Si-G8 
5 
W 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
CI 
e3 
0 
0 
0 
0 
0 
0 
0 
0 0 
0 
0 0 
1 I I oq 1 1  I I I I I  I 1- 
7 

lo-5 
fig. 4--~rtcess carrier lifetime as a function 
of 103/~ (Oic)'' 
9 


Radiation effects on silicon Quarterly progress report, 1 Jul. - 30 Sep. 1966

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