The effort reported here represents data of lithium properties in bulk-silicon samples before and after irradiation for analytical information required to characterize the interactions of lithium with radiation-induced defects in silicon. A model of the damage and recovery mechanisms in irradiated-lithium-containing solar cells is developed based on making measurements of the Hall coefficient and resistivity of samples irradiated by 1-MeV electrons. Experiments on bulk samples included Hall coefficient and resistivity measurements taken as a function of: (1) bombardment temperature, (2) resistivity, (3) fluence, (4) oxygen concentration, and (5) annealing time at temperatures from 300 to 373 K

Brucker, G. J.

English

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DEPENDENCE OF DEFECT INTRODUCTION ON TEMPERATURE
AND RESISTIVITY AND SOME LONG-TERM ANNEALING EFFECTS
G. J. Brucker
Astro-Electronics Division of RCA
Princeton, New Jersey
I. INTRODUCTION
The effort reported here represents data which
was obtained after the termination (June 1970) of
our contractual investigation of lithium properties
in bulk-silicon samples before and after irradia-
tion. The objective of this phase of the study was
to obtain the analytical information required to
characterize the interactions of lithium with
radiation-induced defects in silicon, and hopefully
to develop a model of the damage and recovery
mechanisms in irradiated-lithium-containing solar
cells. The approach to the objectives was based
on making measurements of the Hall coefficient
and resistivity of samples irradiated by 1-MeV
electrons. Experiments on bulk samples included
Hall coefficient and resistivity measurements
taken as a function of: (1) bombardment tempera-
ture, (2) resistivity, (3) fluence, (4) oxygen con-
centration, and (5) annealing time at temperatures
from 300 to 373 K.
II. RESULTS
A. Carrier Removal
Previous work of several investigators (Refs.
1, 2) demonstrated that the defect introduction
rate in phosphorus-containing silicon decreases as
the bombardment temperature is decreased and
also as the dopant concentration is increased at a
fixed bombardment temperature. One of the
objectives of our study was to determine the de-
pendence of carrier-removal rate in irradiated
silicon on bombardment temperature and lithium
concentration. To achieve this objective, several
Hall bars fabricated from 1500 Q- cmand 5000 Q2 -cm
float-zone refined silicon and from 30 to 50 Q-cm
quartz-crucible silicon were diffused with lithium
to concentrations from 2 X 1020 to 3 X 1023 Li/M 3
and irradiated at bombardment temperatures from
79 to 200 K. The rates of carrier-removal were
determined after each bombardment. These rates
measured in float zone (FZ) silicon after annealing
to a temperature of 200 K versus the reciprocal of
the bombardment temperature are shown in Fig. 1.
Lithium concentrations of the samples used in the
measurements are shown as a parameter. The
carrier-removal rates were normalized at a bom-
bardment temperature of 200 K, and the ordinate
of the curves is labeled as the probability of defect
formation. The curves shift along the temperature
axis toward lower temperatures as the lithium
density is decreased over three orders of magni-
tude. These concentrations of lithium correspond
to resistivities from 0. 03 to 20 Q-cm. This shift
of the temperature dependence portion of the
curves is in qualitative agreement with the pre-
diction of the interstitial-vacancy close-pair
model (Refs. 1,-2) of radiation damage in silicon
at low temperatures. It was previously reported
(Ref. 3) that the saturated value of carrier-
removal rate measured at a bombardment temper-
ature of 200 K appeared to increase with lithium
concentration. This conclusion is in agreement
with the results obtained on the samples containing
concentrations to 3 X 1023 Li/m 3 .
A similar set of curves is shown in Fig. 2 for
the samples fabricated from quartz-crucible sili-
con with lithium concentrations from 2 X 1021 to
3 X 1023 Li/m 3 . The saturated value of carrier-
removal rate obtained for the samples doped
with 3 X 1023 Li/mn 3 is equal to 10 m- 1 which is
JPL Technical Memorandum 33-491 35
https://ntrs.nasa.gov/search.jsp?R=19720002408 2020-03-23T13:54:06+00:00Z
the same value obtained for the samples doped
with 2 X 1022 Li/m3.
B. Annealing Properties-High Lithium
Concentrations
Following the completion of the bombardments,
the float-zone samples doped to 3 X 1023 Li/m 3
were annealed at temperatures ranging from 250 to
300 K. Hall and resistivity measurements were
obtained on these samples as a function of time
and measurement temperature. The time to half
recovery, tR of the mobility (TM = 79 K) was
determined from these annealing cycles, and the
reciprocal of this quantity is shown in Fig. 3. The
error in the measurement is large at the higher
temperatures since the sample recovered to its
original value of mobility in less than a minute at
300 K. Nevertheless, the activation energy deter-
mined from the slope of the curve is 0. 68 eV which
is close to the activation energy (0. 66 eV) for lith-
ium diffusion in oxygen-lean silicon measured by
Pell (Ref. 4) and also reported (Ref. 5) in mea-
surements made on solar cells.
C. Long-Term Annealing Effects
Results reported (Ref. 6) on the long-term
annealing effects in lithium-containing quartz-
crucible and float-zone samples with low lithium
concentrations showed a disturbing property,
namely, the continuous loss of carrier density
(measured at low and high temperature) as a
function of time in irradiated samples annealing
at room temperature. These samples were re-
cently remeasured, and the carrier densities mea-
sured as a function of time are shown in Fig. 4 for
three samples. Two float-zone samples were
doped to a low and a high concentration, respec-
tively, and the quartz-crucible sample contained
a high concentration of lithium. It appears that
the runaway loss of carriers has finally ceased to
occur in the quartz-crucible (>2 yr) and low-
lithium-density sample of float-zone silicon. Thus,
the precipitation-like process which produces a
charge-neutral defect complex and which was pre-
viously postulated (Ref. 6) as one of the annealing
mechanisms appears to equilibrate. The behavior
of the other sample of float-zone silicon is typical
of the more heavily doped samples, namely, an
initial dissociation of defects with an increase of
carrier density, and then they reach equilibrium
over the long-term period.
III. DISCUSSION OF RESULTS
The irradiation temperature dependence of the
production rate of impurity complexes in silicon
has been explained (Refs. 1, 2) by a model based
on the formation of close-spaced interstitial-
vacancy pairs by electron bombardment. This
model yields a probability PC, of vacancy libera-
tion from the interstitial to form vacancy-impurity
defects described by
PC = 1 + g exp · ·F  (1)C [F KT
where g is the ratio of the number of ways the
state can be occupied to the number of ways the
state can be unoccupied, EF(n0, T) is the Fermi
level for electrons which is a function of initial
carrier concentration n o and temperature T, and
EM is the energy level of the metastable interstitial-
vacancy pair. Equation (1) was used in an attempt
to fit the data of Figs. 1 and 2. The preliminary
results indicate that there is not a unique pair of
values for the parameters g and EM which will
satisfactorily fit the data obtained on the float-zone
samples over the three orders of magnitude range
in lithium concentration. The best value of E
appears to be =0. 09 eV, but several values of g
are required to fit the entire range of concentra-
tions. In addition, the values of g are small (as
low as 10- 3 ) and thus not physically reasonable.
This model does not appear to be the complete
description of defect production mechanisms in
lithium-doped float-zone silicon over this range of
concentrations.
The attempts at curve fitting were more suc-
cessful with the quartz crucible samples. In this
case, EM = 0. 07 eV and g = 0. 1 gave a reason-
able fit over the two orders of magnitude in lithium
concentration of these samples. This higher value
of g is more physically reasonable, and the lower
value of EM is in agreement with the value used by
previous investigators (Ref. 1) in fitting their data
on phosphorus-doped silicon. Our investigations
have shown (Ref. 4) that the oxygen-vacancy defect
is the dominating carrier-removal center in the
lithium-containing silicon. Thus, it is reasonable
to expect that these samples would behave simi-
larly to the phosphorous-doped (5 X 1020 P/M3 ,
p = 10 Q2-cm) quartz-crucible silicon as our data
indicates.
The mobility recovery shown in Fig. 3 demon-
strated again in agreement with the detailed mea-
surements obtained on solar cells (Ref. 5) that the
diffusion of free lithium is definitely related to the
recovery process. This conclusion applies to the
short-term recovery period that is within the first
few minutes or hours following bombardment.
The long-term annealing effects shown in
Fig. 4 are important, since they indicated that the
carrier density changes will finally equilibrate in
lightly doped float-zone silicon or in the more
heavily doped quartz-crucible silicon after a suf-
ficiently long annealing time. This behavior is
observed only under the conditions of these irradi-
ations, namely the damage level was not large
compared to the lithium concentrations. The
author and other investigators (Ref. 7) have ob-
served that after large fluences, the loss of car-
riers does not equilibrate, and the samples will
return to the starting resistivity of the silicon
before diffusion with lithium. This applies to both
float-zone and quartz-crucible silicon.
IV. CONCLUSIONS
The simple close-pair vacancy-interstitial
model does not completely describe the dependence
of defect introduction on temperature and resistiv-
ity in lithium containing float-zone silicon. Better
agreement of the model with experimental results
is obtained in samples of lithium-containing quartz-
crucible silicon. Further evidence of the relation-
ship between diffusion of free lithium and recovery
of damage was obtained. This was deduced from
the activation energy for recovery of 0. 68 eV mea-
sured in lithium-containing float-zone silicon.
JPL Technical Memorandum 33-49136
Equilibrium and stability of the carrier density is
possible after long-term annealing at 300K in
irradiated quartz-crucible (>2 yr) or lightly doped
(2 to 5 X 1020 Li/m 3 ) float-zone silicon (<1 yr).
REFERENCES
1. Stein, H. J., and Vook, F. L., Phys. Rev.,
Vol. 163, p. 790, 1967; also in Radiation
Effects in Semiconductors, pp. 99 and 115.
Edited by F. L. Vook. Plenum Publishing
Corp., New York, 1968.
2. Gregory, B. L., and Barnes, E. E., in
Radiation Effects in Semiconductors, p. 124.
Edited by F. L. Vook. Plenum Publishing
Corp., New York, 1968.
3. Brucker, G. J. , et al. , Third Quarterly
Report, RCA, under JPL Contract No. 952555
April 10, 1970.
4. Pell, E. M. , Phys. Rev., Vol. 119, p. 1222,
1960; see also J. Appl. Phys. Vol. 32, p. 6
1961.
5. Brucker, G. J., et al., Fourth Quarterly
Report, RCA, under JPL Contract No. 952555
July 10, 1970.
6. Brucker, G. J., IEEE Trans. Nucl. Div.,
Vol. NS-17, p. 144, 1970.
7. Goldstein, B., Final Contract Report, RCA
under Contract No. F19628-68-C-0133, July 1968.
JPL Technical Memorandum 33-491 37
3 5 7 9 11
103 /T B,K-
- 2x1 20 Li/M3
\ 5x 1020 Li/M3
\ 3 x 102 1 Li/M3
2 ), 1022 Li/M3
\ 3 x 1023 LU/M3
13 15
Fig. 1. Probability of defect formation versus
reciprocal bombardment temperature
for electron irradiated float-zone silicon
measured at 79 K after annealing to
200 K. Initial lithium concentration is
the curve parameter
100
Z
O
0
2
O
U
w
LI-
bU
>.-
0
coe
a-
10 - 1
10-2
2 x102 1 L/JM3
5 6 7 8 9 10 11 12 13 14
103 /TB, K
-
1
Fig. 2. Probability of defect formation versus
reciprocal bombardment temperature
for electron irradiated quartz-crucible
silicon, measured at 79 K after anneal-
ing at 200 K. Initial lithium concentra-
tion is the curve parameter
JPL Technical Memorandum 33-491
100
10-1 FLOAT-
ZONE
SILICON
Z
O
O
U
U-
0
a-o
p!
I I I I I l
38
FLOAT-ZONE SILICON
10-2
q ' - \ EA = .68 eV
10- 3
10- 4 i i t I I 
3.0 3.2 3.4 3.6 3.8 4.0 4.2
103 /TA, K- 1
Fig. 3. Reciprocal half recovery time of the
mobility in electron irradiated float-
zone silicon versus the reciprocal
annealing temperature. Measure-
ments of mobility at 79 K. Activation
energy for the recovery process is
indicated. Initial lithium concentra-
tion 3 X 1023 Li/M 3
C-,_
LU4
UC3-
c.,
0
C
1 .7
1.5
c-
E
1.3
wL
1.1 o
04
OC
0.9
0.7
100 101
TIME AFTER IRRADIATION, days
Fig. 4. Carrier density versus time after electron irradiation for samples of float-zone and
quartz-crucible silicon, measured at 300 K. Initial lithium concentration is
indicated for each sample
JPL Technical Memorandum 33-491
10- 1
39


Dependence of defect introduction on temperature and resistivity and some long-term annealing effects

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