Snowden, D. P.

Wikner, E. G.

English

NASA Technical Reports Server

. 
GENERAL DYNAMiCS 
General Atomic Division 
SPECIAL NUCLEAR EP FECTS LABORATORY 
P.O. BOX 600. SAN DILGO. CALIFORNIA 02 I12 
RADIATIOM EFF'EC'IS ON SIUCCOM 
Fourth Quarterly Progress R e p o r t  Covering the Period 
September 25, 1965, through December 31, 1965 
Work done by: 
N. Brown 
H. Horiye 
J. A. Naber 
Do P. Snowden 
V. A. J. van Lint 
E. G. W i k n e r  
D. K. Nichols 
Report written by: 
E. G. Wikner  
D. P. Snowden 
(THRUI 
National Aeronautics and Space Administration 
Contract NAs7-289 
Project 430 January 10, 1966 
https://ntrs.nasa.gov/search.jsp?R=19660020896 2020-03-16T20:53:01+00:00Z
1. IIQRODIICTION 
This fourth quarterly progress report on Contract TJAST-289, "Radiation 
Effects i n  Silicon Solar C e l l s , "  covers the period September 25, 1965, 
through December 311 1965. On July 21, 1965, a Final Report") w a s  written 
covering the work performed under this contract from June 1, 1964, through 
m y  3, 1965. That Flml Report should be considered an Annual S.lmrmary 
Report, and the work discussed i n  the present progress report is a con- 
tinuation of the same contract. 
During this period the experimental setup t o  study microwave conductiv- 
i t y  w a s  modified t o  obtain improved law-temperature operation of the sample 
holder and bet ter  accuracy i n  conductivity measurements. 
have also been made  i n  the cavity t o  be used for the microwave resonance 
studies. 
production of the divacancy i n  p-type silicon. 
Wdifications 
Preliminary experiments have begun t o  determine the indirect  
( l )V .  A. J. van U n t  and D. P. Snowden, "Radiation Effects on Silicon - 
Final Report, " Contract NAS7-289, General Atomic Report GA-6556, July 1965. 
1 
2. MI CROW^ CONDUCTIVITY 
Work during this quarter has concentrated on improvement of the 
experimental setup i n  order t o  obtain improved low-temperature operation 
of the sample holder and also better accuracy i n  conductivity measurements. 
2.1 SAMPIE HOLDER 
The sample holder has been redesigned i n  order t o  obtain better 
The thermal contact with the coolant a t  l iquid helium temperatures. 
stainless steel sheath which surrounded the waveguide (see Fig. 5 ,  R e f .  1) 
has been eliminated. A vacuum-tight seal i s  made a t  the bottom end of the 
waveguide by soldering the sample t o  a 0.002 in .  thick copper sheet which 
i s  then soldered t o  the waveguide a f t e r  the sample i s  inserted in to  it. 
The waveguide i s  evacuated through an 0.008 in.  wide s l o t  cut  i n  the broad 
face of the guide ju s t  below the upper flange and pressure window. 
Styrofoam plug i s  provided with a diagonal hole through it t o  allow 
evacuation of the lower past of the waveguide. 
The 
Temperature sensors are attached t o  the bottom of the copper sheet; 
the copper-constantan thermocouple i s  soldered t o  the sheet and the carbon 
res i s tor  i s  glued with a low-temperature varnish (General Electr ic  7031). 
I n  order t o  e f fec t  annealing s tud ie s ,  a nichrome heater i s  wound around 
tne lower part of t h e  waveguide and insulated from it. 
ment, temperatures between 4 and U O 0 K  are easi ly  obtainable. Tempera- 
tures above those of the l iquid coolants (e i ther  l iquid helium or liquid 
nitrogen) are obtained aI"ter expelling the coolant by adding s m a l l  pulses 
of room-temperature gas t o  the top of the working space i n  the dewar 
which contains the sample holder. 
top plate through which passes the waveguide, e lec t r ica l  feedthrus and 
f i l l  and vent l ines .  
reservoir which provides adequate thermal stabil i ty for  several minutes 
between such stepwise temperature increases. 
W i t h  this arrange- 
0 
This working space i s  enclosed with a 
The gas enclosed i n  the dewar acts  as a thermal 
2 -2 MICROWAm CIRCUITRY 
As Illentioned i n  Reference 1, small leakage signals through the magic 
tee could combine i n  random phase and produce serious errors  i n  the 
measured conductivity. W e  have carefully investigated the nature of such 
2 
signals and have found that, i n  addition t o  leakage through the magic 
tee, small reflections off of various other components i n  the microwave 
bridge can also create standing waves which m i x  with the desired signal 
and give erroneous results.  
The seriousness of small spurious reflections on quantitative meas- 
urement of conductivity by the microwave-reflection technique can be 
easi ly  seen. 
wave components typically ranges from 1.02 t o  1.15, with the lower 
values correspondlng t o  simple components, such as bends, and the higher 
values t o  components, such as isolators and directional couplers. 
lower value of 1.02 corresponds to  a reflection coefficient of T' - 0.01. 
Such a reflection mixing i n  arbitrary phase with a signal of amplitude 1 
can yield an uncertainty i n  the naeasured microwave voltage of 2 percent. 
A t  values of sample conductivity which yield high ref lect ivi ty ,  large 
errors are possible because of the flatness of the curve relating conduc- 
t i v i t y  t o  reflected voltage. Using the curves of Fig. 7 of Ref. 1 w e  can 
see tha t  a t  -log A = 0.1 a 2 percent error i n  the reflected voltage can 
yield an error as high as 30 percent i n  the conductivity. 
of' ref lect ivi ty  (i.e., a t  higher values of -log A),  since the curves are 
steeper, the errors i n  conductivity are reduced. Clearly, spurious 
reflections, especially a t  high values of sample ref lect ivi ty ,  must be 
reduced t o  a very l o w  value i n  order t o  obtain accepbble errors i n  
conductivity measurements. 
The voltage standing-wave r a t io  (VSWR) of various micro- 
The 
A t  lower values 
W e  have measured the VSWR of al l  microwave components available i n  
our laboratory t o  use i n  our bridge and have selec-d those with the 
smallest spurious reflections. 
circuitry, we have adopted the philosophy of using the minimum number of 
microwave components. 
arm which served t o  measure the incident signal has been eliminated. 
i t s  place a waveguide switch (Hewlett-Packasd X930A) has been substituted. 
In addition, i n  redesigning the bridge 
lCI0 this end, the directional coupler i n  the sample 
In 
The incident power 
from the switch i n  
reduce unnecessary 
the sample chamber 
i s  now determined by measuring the signal reflected 
the short-circuit position. In  addition, t o  further 
components, the magic tee  i s  positioned direct ly  above 
with no bends between it and the sample. 
3 
The microwave c i r cu i t  now employed is  shown i n  Fig. 1. 
of the various important components of the bridge are as follows: 
The VSWR 
Matched load: 1.006 
Magic tee: between 1.09 and 1.12 for the 
Input isolator: 1.09 
Output isolator: 1.07 
Waveguide switch (open): 1.02. 
various arms 
The cross coupling of the magic tee between the E-plane and H-plane arms 
i s  -45 d ~ .  
Since the spurious reflected signals and the cross coupling could 
r e s u l t  i n  errors larger than desirable, tuners were constructed and 
included i n  the microwave circui t  as sham i n  Fig. 1. 
sist of two screws (provided w i t h  lock nuts) which penetrate the center 
of the broad face of a length of waveguide and are separated by one- 
fourth of a guide wavelength at  the operating frequency. 
penetration of these two screws, and possibly by mving this tuner for- 
ward or  backward one-quarter of a wavelength with respect t o  the ref lect ion 
being tuned out, it i s  possible t o  cancel any spurious reflected signal. 
The c rys ta l  arm (consisting O f  
tuner No. 1, the isolator,  the transmission cavity and the crystal  detector) 
i s  attached t o  a s lot ted l i n e ,  and tuner No. 1 is adjusted t o  minimize 
the VSWR looking in to  the am. The VSWR obtained was 1.007. Next, with 
the c rys ta l  arm i n  place and a matched load terminating the sample 
chamber, tuner Bo. 2 i n  the reference arm i s  adjusted t o  minimize the 
s ignal  i n  the crys ta l  arm. 
the signal arm was down more than 70 dB. Next, with a sliding short 
terminating the sample chamber, tuner No. 3 i s  adjusted t o  minimize the 
variation of the microwave signal i n  the c rys ta l  a r m  as the phase of the 
ref lect ion i n  the sample am i s  varied. The function of this  tuner i s  t o  
cancel components of the incident signal which are derived from the  s ignal  
reflected from the short (or from the sample during an actual measurement), 
which are reflected a second t i m e  from the various components. These 
spurious components of the incident s ignal  w i l l ,  of course, depend on 
the magnitude of the signal reflected from the sample, and therefore 
!I!hese tuners con- 
By adjusting the 
The tuning procedure i s  as follows; 
After adjusting tuner No. 2, the coupling t o  
4 
x 
U 
k 
0 
rtl 
0 
k 
0 
5 
. 
this last tuning can be expected t o  be perfect a t  only one level  of the 
reflected signal. Since, as discussed previously, the maximum error i n  
conductivity occurs when the re f lec t iv i ty  of the sample i s  a maximum, we 
make this tuning with 100 percent ref lect ion from the sample arm. 
t ion  of the microwave voltage i n  the crys ta l  arm with variation of the 
s l iding short was not detectable a f t e r  tuner No. 3 was adjusted, and 
therefore was l e s s  than 0.2 percent. W i t h  a sl iding mismatch which attenu- 
ated the reflected signal by 9 dl3, this variation was 4.6 percent. 
maximum uncertainty i n  conductivity from spurious reflections has therefore 
been reduced by th i s  tuning procedure t o  below 4 percent.for values of 
-log A > 1, an acceptable value., 
Varia- 
The 
2.3  EXPERIMENTS € " T E D  
The f irst  experiment t o  be perfomed w i l l  be an investigation of the 
change of lifetime on i r r a d i a t i o n  and annealing of ultrapure si l icon. 
Irradiations will be perfomed a t  l iquid helium temperature, and the 
change of lifetime due t o  amealing w i l l  be studied up t o  400%. 
sample of n-type s i l icon with a room-temperature conductivity of -10 
A 
-4 
(ohm-cm)" and a carrier lifetime of 7 mSec has been prepared for  mounting 
i n  the microwave sample chamber and will be investigated first. 
have a boule of ultrapure p-type s i l icon  which will be studied. 
W e  a l so  
6 
. . 
3. MICROWAVE RES0"X EXPEBIMNTS 
. 
The main emphasis i n  the microwave resonance studies has been i n  
preparing for  future experiments, i n  particular the 80% irradiations, 
the transfer of samples a t  80°K, and the subsequent step annealing t o  room 
temperature. 
t ion to  the incorporation of a heater for the annealing experiments. 
These experiments will be to masure the indirect  production of the 
divacancy i n  p-type sil icon and the vacancy-phosphorus complex i n  n-type 
silicon. 
contract'') the energy dependence of the introduction rates at  3OO0K 
was reported and w i l l  be used for  comparison with the 80°K irradiations 
and subsequent 30O0K anneals. 
This involved a modification of the present cavity i n  addi- 
In the report written on work done previously under this 
Two pulled 0.1 ohm-cm B-doped samples have been irradiated a t d O ° K  
2 with 2 x 
w i l l  be ascertained i n i t i a l l y  without allowing the sample to exceed 8o°K 
i n  temperature. 
to find i n  w h a t  temperature regions additional divacancies form, pre- 
sumably reflecting the temperatures a t  which the vacancy moves i n  p-type 
silicon. 
30 MeV electrons/cm . The number of divacancies formed 
Then annealing experiments t o  WOK win be performed 
This experiment i s  now under way. 
In addition, a major goal of the program i s  t o  correlate the lifetima 
and resonance results.  Wte r i a l  similar to  tha t  used i n  the lifetime 
experiments on n-type siUcon, i.e., 10 ohm-cm P-doped, has been irradiated 
to  flux levels between 1015 and 1017 electrons/c$. 
been observed. 
but none can be correlated as yet with the recombination center. 
No resonances have 
Resonances were observed i n  more heavily doped material, 
The recombination center i n  n-type s i l icon (to be called the X-center) 
is  at  an energy of more than 0.40 e V  from the conduction band. Its intro- 
duction ra te  i s  large (- 1 an-') and i ts  charge s t a t e s  are -1 and 0. 
pointed out above, a difference i n  the resonance and recombination studies 
has been that  the experiments are performed on dissimilar material. In 
10 ohm-cm P-doped sil icon there are about 3 x 1014 P/m3, and, with our 
sample sizes, thSs means about 10 P donors. The X-center may remove an 
electron from the conduction band, but even i f  a l l  the 
populated the X-centers and were observable with resonance techniques, 
14 
electrons 
7 
1 
. 
a 
14 they would be d i f f i cu l t  to observe i n  our experiments because 10 spins 
can be seen only under the most ideal conditions. One way to circumvent 
this i s  t o  use larger samples, essentially building samples which ac t  as 
one w a l l  of the cavity and can increase the signal by a factor of 5 or 
1G. This xi= be tried i n  the new future. 
There are other questions which ar ise  due t o  the properties of the 
X-center, and these w i l l  be discussed on the basis that the original donor 
concentration is  not. a si,dftcnnt parameter. 
With the Fermi level abwe the A-center, the X-center responsible for 
It is  introduced a t  - 5 thes  the recanbination i s  charged negatively. 
the rate of the A-center, bxt o w  the A center i s  seen under these con- 
ditions i n  the resonance experiments. It cannot be said that it is  not 
present i n  sufficient quantities, unless the introduction rate  changes 
with flux, for instance a t  10 1wrm2 , becoming much lower (say 2 orders 
of magnitude). However, there seems t o  be no good reason for  the intro- 
duction ra te  t o  change when the densily of radiation-induced centers i s  
small and when the Fermi level is far above 0.40 eV. 
Another possibil i ty is that the g-value of the center may be q d t e  
different from 2 .  This is  possible, but up t o  now a l l  centers in si l icon 
are near g = 2, and it i s  very diff icul t  t o  understand why the spin-orbit 
coupling, which determines deviation of g-value from 2, should be very 
different for the X-center. 
The electron-spin la t t ice  relaxation time may be different. 
Relaxation times are long fo r  all centers i n  si l icon so far studied, 
and possibly this center has not been seen because of unusual passage 
conditions. However, the X-center i s  appazently present in high concentra- 
t ion  and many samples have been studied under m a ~ y  conditions of passage. 
Therefore, it is surprising that it has not been seen. 
possibil i ty i s  that this center has a very broad l i n e  w i d t h  which has not 
been noticed. 
One other 
Most of our measurements have concentrated on looking for  
centers with a width of a few gauss, but even using f i e l d  modulation 
amplitudes of only a gauss or two ,  we probably would have seen a center 
present i n  high concentration even i f  it were 100 gauss wide. 
Thus a t  this time there is a qidstionas to  why the resonance experi- 
ments and the recombination experimnts do not show a positive correlation. 
8 


Radiation effects on silicon Fourth quarterly progress report, Sep. 25 - Dec. 31, 1965

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