The defect structure of processed edge defined film-fed growth (EFG) silicon ribbons was studied using a variety of electron microscopic techniques. Comparison between the present results and previous studies on as-grown ribbons has shown that solar cell processing introduces additional defects into the ribbons. The creation of point defects during high temperature phosphorus diffusion induces dislocation climb, resulting in the formation of dislocation helices in the diffused layer

Ast, D. G.

Cunningham, B.

English

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Report #4
PROCESSED-INDUCED DEFECTS IN EFG RIBBONS
(NASA -Cit-169205) PEOCESSED-INDUCED DEFECTS 	 H82-29724
IN EFG RIBBONS (Cornell Univ., Ithaca, N.
Y.)
	
14 p HC A02 /MF A01
	 CSCL 10A
Unclas
G3/44 28594
May 1982
JPL Contract No.956046
_
av6, o
aE^E
by
B.Cunningham and D.G.Ast
Materials Science and Engineering
Bard Hall, Cornell University
Ithaca, New York 14853
The JPL low-cost solar array project is sponsored by the U.S. Department
of Energy and forms part of the Solar Photovoltaic Conversion Program to
initiate a major effort toward the development of low-cost solar arrays.
This work was performed for the Jet Propulsion Laboratory, California
Institute of Technology by agreement between NASA and DOE.
PROCESS-INDUCED DEFECTS IN EFG SILICON RIBBONS
B. Cunningham and D.G. Ast
Department of Materials Science and Engineering
Cornell University, Bard Hall
Ithaca, N.Y. 14853
Abstract
The defect structure of processed edge defined film-
fed growth (EFG) silicon ribbons has been studied using a
variety of electron microscopic techniques. Comparison
between the present results and previous studies on as-
grown ribbons has shown that solar cell processing intro-
duces additional defects into the ribbons.
The creation of point defects during high temper:.-
ture phosphorus diffusion induces dislocation climb,
resulting in the formation of dislocation helices in
the diffused layer.
The base regions of the processed ribbons contain
low-angle tilt boundaries which are formed during pro-
cessing. Small precipitates, ,.20nm in size, are found
at the low-angle boundaries. In addition, larger pre-
cipitates, tilvm in size, have been identified. Possible
formation mechanisms for these precipitates are discussed.
Introduction
Silicon ribbons grown by the edge defined film-fed growth (EFG)
technique are currently being used as substrate material for photovol-
taic cells (1). These ribbons generally contain high densities of
structural defects, most of which act as efficient minority carrier
recombination centers. As-grown EFG ribbons have been studied in con-
siderable detail (e.g. 2,3) and are found to contain first order twin
boundaries, higher order twin boundaries and lattice dislocations.
Large temperature gradients in the ribbons and temperature fluctuations
across the solid-liquid interface are responsible for the generation
of most of the structural defects. So far it has not been possible to
eliminate these defects by altering the growth conditions.
Carbon concentrations in EFG ribbons are ti10 18 atoms/cm3,
typically a factor of ten or more higher than those in Czochralski
single crystal silicon. The influence of such a high carbon content
on the electrical prop«ties and defect structure of EFG ribbons is
not well understood. Oxygen is also present since CO/CO 2
 is intro-
duced into the die area. It is known that during annealing of
Czochralski silicon, carbon-oxygen complexes are formed (4), and that
carbon can provide nucleation sites for oxygen precipitation (5). The
processing time of EFG ribbons is short ti30 mins, compared to the
times at which Czochralski silicon is annealed in precipitation
studies. The high supersaturation of carbon in EFG ribbons (i.e. a
large driving force for precipitation) and short diffusion time favor
the formation of numerous, but small precipitates.
Experimental
The growth of EFG ribbons is described in ref. (1). The pro-
cessing of the ribbons was performed by Mobil Tyco Solar Energy Cor-
poration. A phosphene (PH3) gas diffusion at 950'C for 30 rains was
used to produce a shallow p-n junction about 0.5um below the ribbon
surface.
Samples were selected at random from several processed ribbons.
Discs, 3mm in diameter, were removed from the ribbons and ion beam
milled to a thickness suitable for electron microscopy, typically
<Ipm. Specimens were milled from the backside only to study the
diffused region, and from both sides to study the base region.
Electron microscopic observations were carried out on a Siemens 102
transmission electron microscope (TEM) and on a JEOL 2000X, scanning
transmission electron microscope (STEM) equipped for x-ray energy
dispersive spectroscopy (IDS). Non-dispersive energy spectroscop ., was
carried out on a JEOL 733 scanning electron microscope (SEM).
Results
a) Diffused region
Figure 1 shows TEM micrographs of dislocation helices situated
in the diffused --pion of an EFG ribbon. Dislocation helices have
never been observed in as-grown ribbons, but have previously been
observed in processed semiconductor devices (6), where they were
ascribed to the absorption of diffusion created interstitial point
defects. The helix shown in Figure 1(a) is typical of those found in
the thicker regions of TEM foils. The abrupt change in the direction
and helical content of the dislocation, may reflect a gradient in the
point defect concentration. Figure 1(b) was taken from a thinner part
of the foil and clearly shows the helical nature of the dislocations.
It was expected that misfit dislocations would be seen in some of the
foils milled from the backside. These misfit dislocations are
observed after phosphorus diffusion in single crystal silicon, but
none were observed in the present study. In the samples milled from
the backside only, dislocation helices were the only defects which
were identified as originating from the diffusion process.
b) Base region
The following section describes the defect structure in the base
region of processed EFG ribbons.
.	 . ,	 -
v Figure 2 shows STEM and secondary mlectron images (SEI) of large
precipitates, %lum in size, in an EFG silicon ribbon. The SEI con-
tract from the arrowed precipitate in figure 2(b) indicates that it is
completely embedded in the matrix. TD. analysis shows that the pre-
cipitates are surrounded by dense dislocation tangles, figure 3.
Figure 4 is a typical x-ray spectrum froP one of the precipitates and
shows that they contain several metallic impurities such as Fe, Mo,
and Ti. The Cu peak is due to scattering f .ori the specimen support
grid. Note that the Fe peak is the largest lipurity peak of the 7P:DS
spectra. Non-dispersive x-ray analysis shower that in addition to the
above impurities, the precipitates also contained n large amount of 	 F
carbon. The precipitates described above have been ubserved only	 l
occasionally in processed ribbons, and it is not certain if they form
during processing. The precipitates have never been observed in as-
grown ribbons, but it is possible that they were not detected because
of their low concentration.
TEM micrographs of several low-angle tilt boundaries are shown
in figures 5-12. These boundaries invariably have a <0 1 1> tilt
axis, which do not normally coincide with the <0 1 1> surface o ,.ien-
tation of the ribbons.
Figure 5 shows a boundary with a nominal 0.j° tilt, in which
intrinsic (I) and extrinsic (E) dislocations have been labelled. The
intrinsic dislocations lie along the tilt axis and are the disloca-
tions which take up tiie misorientation between the grains. The
extrinsic dislocations are lattice dislocations which have slipped
into the boundary. Two types of dislocation nodes are formed as a
result of dislocation interactions and these can be seen more clearly
in figure 6. The dislocations are out of contrast in figure 6(b) and
contrast from the stacking faults of dissociated dislocations and dis-
sociated nodes is observed. Small precipitates,--20nm in size, have
been identified in the boundary, figure 7. The Moire fringes in the
precipitates allow them to be detected fairly easily. Figure 8 is a
higher magnification micrograph of the area shown in figure 6. The
dislocation nodes, N, contain precipitates, while no precipitates are
observed at the dissociated nodes, D. The precipitates shown by the
arrows lie on dislocation segments between nodes. The above observa-
tions indicate that the precipitation process is heterogeneous. The
precipitates are too small to obtain x-ray spectra and attempts to
obtain micro-diffraction patterns have so far been unsuccessful.
Figure 9 is a TEM micrograph of a 4° tilt boundary and the
intrinsic dislocations are, therefore, much closer spaced than those
comprising the boundary in figure 5. Figures 10 and 11 show enlarged
regions of the boundary, taken with different diffraction conditions.
Extrinsic dislocations can also be seen in this boundary. The intrin-
sic dislocations are only ti5nm apart and it is unlikely that pre-
cipitation would occur on a scale which could be observable by TEM
techniques.
Many boundaries of the types discussed above have been observed
in processed EFG ribbons. All of the boundaries have <0 1 1> tilt
axes with tilt angles up to n-5'.
A recent observation has provided additional evidence that the
low-angle tilt boundaries are formed during processing. Figure 12 is
a TIId micrograph of what appears to be a tilt boundary in an earlier
stage of formation than those shown in figures 5-11. Dislocation
dipoles are visible and many of the dislocations contain either
per'odic kinks or jogs, implying the adsorption of point defects.
Discussion
a) Introductory remarks
The inhomogeneous distribution of defects in EFG ribbons together
with the fact that only very small volumes can be investigated in the
transmission electron microscope requires that a large number of
specimens has to be studied before reliable information can be
obtained on the defect structures. In the present case, many speci-
mens of as-grown and processed ribbons have been studied and con-
clusions about process induced defects have been made based on sta-
tistical differences observed in the two types of specimens.
b) Diffused region
Development of helical dislocations is linked to the generation
of point defects during PH 3 diffusion. The number of point defects
absorbed per unit length of the dislocation is given by (7):
N	 irr2b
L ava
where r is the radius of the helix, b is the Burgers vector, v a
 the
atomic volume and 1/a is the number of turns per unit length. The
sense of the helix can be analyzed to decide if vacancies or inter-
stitials are absorbed (8); such an analysis was carried out in (6)
with the result that the point defects introduced during phosphorus
diffusion were interstitials.
The chemical potential G of the point defects in equilibrium
with a helix is within the framework of the line tension concept,
inversely proportional to 47r r +	 (7). An analysis of the different
section of the helix in Fig. lb along these lines indicates that G
in the lower part of the helix is about a factor n-2 lower; i.e. that
the point defect concentration is roughly n-1/3 of that in the upper
part.
The absence of TEM evidence for diffusion induced misfit dislo-
cations is not understood at present. It is possible, but unlikely,
that the failure to detect misfit dislocations is an artifact of the
specimen preparation.
i
{
• •w
The factors governing the gaaeratiun of misfit dislocations have
been studied in detail (see for example ref. [9)). Experimentally,
one finds that high doping levels of P do not generate misfit dislo-
cations when one simultaneously dopes with As (9). The mechanism for
suppression is not, as one may suspect, strain compensation (P has a
smaller and As has a larger covalent radius than Si), but rather
suppression of dislocation motion and trapping of point defects (9). 	 j
It is conceivable that C plays a similar role, for example by solute	 i
drag on dislocations, but more experimental information is required 	 {i
before this question can be decided. 	 y,,
c) Base region
The origin of the large precipitates is unknown at present.
Because of their relatively rare occurrence, it is not possible to
state unambiguously that they are process induced (although never
observed in unprocessed EFG, they may have been absent in the small
areas investigated by TEM).
The precipitates may have been formed in the melt, in which case
their melting point must be higher than \,1410°C, the melting point
of Si. The melting point of SiC is 2830°C and that of FeSi is 1410%
(10). Thus it appears likely that stable Si-C-Fe 	 precipitates can
be formed in the melt.
Alternatively, we consider the case that the precipitates may
have been formed in the solid during processing. In this case an
estimate of the diffusion required to generate micro-meter sized
particles shows that the particles cannot have formed by bulk diffusion.
The particles are, however, surrounded by a dense network of disloca-
tions (Fig. 3) which extends to about 5um from the particle center. If
pipe diffusion occurred, this network of dislocation could conceivably
collect and transport sufficient C to form Si-C-X particles at the
processing temperatures and time. Thus, although formation in the
liquid is more likely, formation in the solid cannot be ruled out with
certainty.
The particles appear to act as gettering centers for transition
metals, such as Mo, which likely originates from the heat shield from
where it is transported as volatile oxide to the ribbon. Cl, which has
also been identified, can be traced to the C1 bakeout of the graphite
dies.
Since transition metals reduce solar cell performance, formation
of these precipitates is likely beneficial.
The small angle boundaries are almost certainly formed during pro-
cessing. The formation is not linked to the P diffusion since the net-
works are formed far away from the junction. Unprocessed ribbons con-
tain high internal stresses which are partially relieved by dislocation
motion and multiplication at the temperatures required for processing.
It appears that the boundaries are formed by this process. Disloca-
tion nodes in these boundaries offer nucleation sites for precipita-
tion. Note that C does not seem to segregate to stacking faults
(Suzuki segregation) since the extended nodes do not act as nuclei.
This observation is in agreement with the general rule that elements
which increase the energy difference between the staggered and
eclipsed bond configuration will not segregate to stacking faults. C
with its higher bond strength and bandgap is expected to increase this
difference.
If the precipitates are SiC, they contain about 5 . 104C atoms.
Since the bulk C content is .1 . 1018/cm3 , format#on of the small pre-
cipitates requires diffusion over distances of 1,0.4P m at the process-
ing time (1, 30 min)and tempperature ( 1,950°C), or a diffusion coeffi-
cient of roughlyy 1 . 10-12cmz/sec. The diffusion coefficient of C at
950°C is 1,1 . 10-12cm2 /sec (11) and the observed size of the precipitates
is, therefore, compatible with the hypothesis that they are formed
during processing and that they are related to C. Ox en may also be
involved since the diffusion coefficient (about 1 . 10-11cm2/sec at
950°C [111) and bulk concentration (0.3 - 1.2 . 1018/cm3 [121) are
roughly similar to carbon. The influence of the small precipitates
on the efficiency of solar cells fabricated from EFG ribbons has not
been studied. EFG based cells behave somewhat differently from
standard single crystal cells in that the electrical properties depend
on the ititensity of the incident light. A study of the relation
between the electrical properties and the size and density of the
small precipitates is planned.
Conclusions
TEM of processed EFG has identified a variety of defects which
are introduced during processing; i.e. point defects, large precipi-
tates, and small, heterogenously nucleated precipitates. The latter
form at dislocation nodes of process induced sub-boundaries with
<0 1 1> tilt components. The large precipitates are probably formed
in the liquid although pipe diffusion models can be constructed to
account for their formation in the solid. The size of the small pre-
cipitates is compatible with estimates that their formation is con-
trolled by the bulk diffusion of C or 0. Undissociated dislocation
nodes in sub-boundaries, formed during processing, act as nucleation
sites for the precipitates.
Acknowledgment
This research was sponsored by JPL under Contract No. 954852 and
by DOE under Contract No. DE-ACO2-76ER02899. The specimens, which
were grown by Mobil-Tyco, were supplied by JPL as part of the LAS
program. The Materials Science Center at Cornell University provided
facilities for research.
i
1
References
1. J.P. Kaleja, B.H. Mackintosh and T. Surek, J. Cryst. Growth,
50, 175 (1980).
2. B. Cunningham, H.P. Strunk and D.G. Ast, Appl. Phys. Lett.,
40, 237 (1982).
3. H.P. Strunk, B. Cunningham and D.G. Ast, "Defect Structure of
EFG Silicon Ribbons," JPL/DOE Report No. 954852-6 (1980).
4. G.S. Oehrlein, J.L. Lindstrom and J.W. Corbett, Appl. Phys. Lett.,
40, 241 (1982).
5. W.K. Tice and T.Y. Tan, Defects in Semiconductors, eds. J. Narayan
-ind T.Y. Tan, North-Holland Inc. (1981).
6. H.P. Strunk, V. Goesele and B.O. Kolbesen, J. Microsc., 118,
35 (1980).
7. J.P. Hirth and Y. Lothe, Theory of Dislocations, McGraw Hill,
576 (1968).
8. Ibid, p. 571
9. T. Yonezewa et. al., in Semiconductor Silicon
eds. R. Huff and E. Sirtl, Proc. 77-2, Electrochemical Soc.,
Inc., P.O. Box 2071, Princeton, N.Y. 08540, 658 (1977).
10. W.R. Runyan, Silicon Semiconductor Technology, McGraw Hill,
247 ff (1965).
11. Ibid, p. 185 ff
12. J. Hanoka, private communication
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Figure 2. a) STE4 and b) second
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ORIGINAL PALL- IS
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Figure 1	 Dislocation helices in the diffused region of an EFG ribbon.
OR"19114 L PAGE
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Figure 3.	 TEM micrograph of the Figure 4.	 X-ray energy disper-
dislocation tangle sive spectrum from
surrounding a precipitate one of the precipitates shown
in figure ?.
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Figure 5.
	 Low angle tilt boundarv. Intrinsic
	 (I) and extrinsic
	 (E)
dislocations are shown.
. M
ORIGINAL PAGE
OF p00R QUALITY
Figure 6. Weak-beam micrographs showing two types of dislocation nodes
in a low-angle boundary. g=diffraction vector
Figure 1. Precipitates at dislocation nodes in Low-angle boundary.
g=diffraction vector
ORIGINAL PAGE fS
OF POOR QUALITY
..,r	 . —, .
ORIGINAL PAGE IS
OF POOR QUALITY
Figure 9. 4 0 tilt boundary in processed ribbon.
1'
^- 9111
50 nm -
.^	 1
♦ 	 ^^	 S•"	
^''^ • ♦ 	 1
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Figure 10. Higher magnification microgra?h of boundary shown in
figure 9.
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for detai.Is.
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0
ORIGINAL PAGE IS
OF POOR QUA'_1TY
Figure 11. Same area as figure 10 imaged with different diffraction
vector.
Figure 12.
1
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1	 1	 1
Low-angle boundary
A


Processed-induced defects in EFG ribbons

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