Oxygen and carbon related defects in silicon, particularly as related to high-efficiency silicon solar cells were studied. A summary of oxygen processes in silicon versus process temperature was shown along with experimental results. The anamolous diffusion of oxygen was explained by the dissociation of the center allowing O sub i to move through the lattices

Corbett, J. W.

English

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HIGH-EFFICIENCY DEVICE RESEARCH N 8 6 - 2 9 3 _' 8 "
STUDIES OF OXYGEN-RELATED AND CARBON-RELATED
DEFECTS IN HIGH-EFFICIENCY SOLAR CELLS
STATE UNIVERSITY OF NEW YORK AT ALBANY
James W. Corbett
'- We are studying oxygen- and carbon-related defects in
I
silicon, particularly as related to high-efficiency silicon
solar cells. We are carrying out a survey of these process-
induced defects, of life-time measurement techniques, and of
defect aggregates in general. We are carrying out coordinated
experimental and theoretical studies of process-induced
defects, and have i]itiated a new series of annealing experi- :
-- ments, including flash-lamp heating of "as-received," pre- ,
heat-treated and homogenized samples, using DLTS, IR, TEM,
positron annihilation, and as needed, EPR and x-ray i,
scattering studies.
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iHIGH-EFFICIENCY DEVICE RESEARCH j_
Isolated oxygen is an interstitial (0 i) in a puckered
bond-centered position as shown in Fig. 1. This defect is NOT
electrical active. It has several infra-red bands, the one 4
Figure 1. Configuration of interstitial oxygen in the silicon
i lattice. t"
i t_
4 I
that will concern us being at ca. ii00 cm -I. But oxygen in i
Czochralski-grown sillcon is supersaturated with oxygen at4
all practical temperatures for processing and use, and oxygenI
does a number of strange and wonderous things. These are
outlined in Fig. 2. There appear now to be (at least) two
! lines of precipitati_- For oxygen. One line progresses
I
through the original thermal donors (found first by Fuller e__t
al. in 1955, before it was known that oxygen is in the
lattice), to the <ii0> rods (shown by Sourret to be coeslte,
a high pressure phase of silica - SiO 2) with their associated!
I extrlnslc/Interstltlal dislocation dipoles. It is presumedi
that Interstitials are emitted in this precipitation process;
we have shown that the emission of SiaO is energetically
favorable vs a silicon interstitial, and that this option may
be operative at high temperatures and in oxidation processes.
_iete that this llne of precipitation does not persist to high
34O
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1986019875-339
|HIGH-EFFICIENCY DEVICE RESEARCH 'i
temperatures, i.e., the rods (and dislocations dipoles)
dissolve in favor of the second line of precipitation•
6.
Isolated oxygen
TEMP. . .
• • !
450°C (OLD) THERMAL DONORS
TD-1,2,...,9
I
I [RECOMBINATION CENTERS] .
I I :
600°C <ii0> RODS (coesite) BLACK DOTS
+ DISLOCATION DIPOLES (NEW DONORS)
(EXTRINSIC-INTERSTITIAL) I
REDISSOLVE J
I
800°C {100} a-SiO x PLATELETS
to
900oc
(DISLOCATION DONORS)
1000oc a-SiO TRUNCATED OCTAHEDRA
X
SURROUNDING STACKING FAULT :
-- (EXTRINSIC-INTERSTITIAL)
I
i
1300°C REDISSOLVE
Figure 2. Summary of oxygen processes in silicon.
"eo_
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-- • O / _,__';"
1986019875-340
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• ' HIGH-EFFICIENCY DEVICE RESEARCH 'i_
r
The second line of precipitation is first evident with
"black dots" in transmission electron microscopy experiments. 4
• These grow into amorphous SiO x defects, {i00} platelets at
higher temperatures and into truncated octahedra at even
higher temperatures. Then at temperatures approaching the
melting point of silicon (e.g., 1300°C) the precipitated
"?
oxygen dissolves into the isolated interstitials that we
started with. Depending on the thermal cycling that the
samples experience, the precipitates can give rise to
prismatic dislocation loops, and other dislocation networks.
\
At 4500C recombinations centers appear along with the
_.- thermal donors. It is not clear which of the lines of preci-
pitation they belongs to, or if either, but these centers are
L
oxygen dependent. {
Thus oxygen introduces shallow donors and centers deep
w
in the forbidden gap. But the defects produced by oxygen _=
precipitation also provide gettering centers for recombina- *
tion centers, e.g., the iron-group transitions elements,
which tend to be fast diffusers. It is not exactly clear how
this gettering works, but it is known that iron and copper
precipitate at different sites.
And the means by which oxygen precipitates is still
not certain. Kaiser, Frisch and Reiss argued that the main
' features of the formation of the (old) therm_l donors could
be explained as the sequential agglomeration of oxygen atoms.
The difficulty was that the mobility of the oxygen is not
sufficient to account for the rate of thermal donor forma-
tion. In verifying our earlier measurements of the diffusion
coefficient of oxygen with an activation energy of 2.54 eV,
Stavola et al. found that in "as-received" samples the oxygen
• notion occurs with an activation energy of 1.9 eV. This
.i anomalous diffusion has been presumed to be defect-assisted.
.l
342
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1986019875-341
HIGH-EFFICIENCY DEVICE RESEARCH .I
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In irradiated silicon we had earlier established the
(vacancy+oxygen) center, (V.O), as shown in Fig. 3. [We also
\
: Fig. 3. The (vacancy + oxygen) center in silicon.
found (v2.o), (v2.02), (v3.0), (v3.o2), and (V3.O 3) centers
• but they will not concern us here. ] When the (V.O) center
disappears on annealing, the (V.O 2) center is created (see
.:ii /
"L
Fig. 4. The (vecancy + two-oxygen) center in silicon.
343
1986019875-342
!HIGH-EFFICIENCY DEVICE RESEARCH i
Fig. 4), as we have recently verified by studying the depen-
dence of the formation of this defect on the oxygen concent-
ration. Subsequent annealing of this center leads to a (V.O 3)
center, or what might be called a (V.O2)+O i center, as shown
, in Fig. 5.
968ore"I 904 cm-',I
,, Fig. 5. (V.O 2) + O i center in silicon.
In recent studies we have found that the growth of the
(V.O 2) band is characterized by a ~ 1.9 eV activation energy,
i.e., essentially the same as the anomalous diffusion of
Stavola et al. (See _ig. 6) This argues that the anomalous
diffusion occurs by the dissociation of the (V.O) center,
" 344
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1986019875-343
i• HIGH-EFFICIENCY DEVICE RESEARCH }[
350"C 325"C 30o_
1.0 • _ ._
o.o _'_ 1.86 eV
/
'_ "_.0
c
-- .%topic .o +
• t
t
-Z.O
""_,','60 1.6S 1.:/0 1.7S
1/T ( 10.3 K"1)
Fig. 6. Arrhenius plot for the growth of the (V.O 2) defect.
with that process causing the O i to move through the lattice,
e.g.,
'9i) + (v) _ (v.o)• (0 i) + (v)
and the growth of the (V.O 2) by the free vacancy encountering
two adjacent oxygens, e.g.,
(V) + (Si-O-Si-O-Si) • (V.O2).
This important advance needs further experiments to test its
several implications, azldthese experiments are being done.
,,._*
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1986019875-:344
4HIGH-EFFICIENCY DEVICE RESEARCH
In related studies we have found that infra-red bands
associated with thermal donors can be found in floating-zone
silicon (i.e., low-oxygen) silicon that has been neutron- ,_
irradiated in transmutation doping. Thus the vacancies
created in that process can substantially accelerate defects
agglomeration, a point that also is important in ion-implant-
ation.
As is seen in Fig. 2, there are nine different thermal
donors which have been resolved by infra-red studies, where
they have Rydberg transitions characteristic of double
donors. What then are these thermal donors? The consensus is
that they consist of a core (or perhaps there are several
such cores) which has a double plus charge state, that the
Rydberg states arise from the attraction of electrons to this
double plus center, and that successive donors occur as
additional oxygens are added to, or adjacent to, this core.
We have developed a theory that describes the electrical
behavior of this hierarchy of thermally-induced double
donors, including a core and an electronically repulsiv_
oxygen-rich region, which repulsive region grows in size as
oxygens are added to the defect. We have succeeded in fitting
the perturbation to the ground state energies all nine double
donors as shown in Fig. 7.
346
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1986019875-345
HIGH-EFFICIENCY DEVICE RESEARCH -i
/
5
(ev)
10z_.--
L
I
5-
r-- !
2 3 4 5 6 7 8
q
Fig. 7. Perturbation energy for the thermal donor ground tt
states versus added oxygens. _
What then i$ the core (or are the cores)? We have
: identifiec the most likely core for the homogeneously-
" the saddle-pointnucleated oxygen precipitate as the "ylid,
for oxygen diffusion, stablized by the presence of two or
more additional oxygens, We conclude _s well that the
precipitation straln-energy can also cause the emission of an
interstitial leaving the core as the (V.O 2) center. These
processes are consistent with precipitation processes which
yield coesite plus interstitial dislocation dipoles. They
also provide a mechanism for th_ loss of thermal donor
activity, as i$ observed, and for the creation of recombJna-
tion centers. But a number of lines of investigation need to
be pursued to be certain of these models.
'-i
_ 347 i
1986019875-346
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HIGH-EFFICIENCY D£VICE RESEARCH i'
b;
Why then the two line of precipitation? Our working
h_?othesis is that the "black dot" line involves carbon.
4
Sinco carbon contracts the lattice, it will relieve the
strain associated with oxygen agglomeration, delaying the
formation of the strain-induced formation of the thermal
donors which we employed for the "old" thermal donors. This
i permits the "new" thermal donors to be associated with
" ca bonl as is observed, and to be more stable But these;
matters must be pursued further to achieve certainty.
.!
What about the recombination centers? As we indicated
the processes that we have discussed can create recombination
centers, e.g. the (_2.0), (V2.02), etc. centers, which wel
I passed over. But the experimental results are becoming quite
J incisive Suezawa and Sumino have argued persuasively that
one of the thermal donors involves eight oxygens. But just as
i persuasively Glinchuk et al. have observed a recombination
} center that contains eight oxygens!
Li Clearly the agglomeration can follow several paths,
and much needs to be done to sort out these mutters. Still we
i have made great progress. ,_
t
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1986019875-347


Studies of oxygen-related and carbon-related defects in high-efficiency solar cells

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