The electrical and structural properties of low and medium angle tilt grain boundaries in silicon bicrystals were studied in order to obtain insight into the mechanisms determining the recombination activity. The electrical behavior of these grain boundaries was studied with the EBIC technique. Schottky barriers rather than p-n junctions were used to avoid annealing induced changes of the structure and impurity content of the as-grown crystals. Transmission electron spectroscopy reveals that the 20 deg boundary is straight, homogeneous, and free of extrinsic dislocations. It is concluded that, in the samples studied, the electrical effect of grain boundaries appears to be independent of the boundary misorientation. The dominant influence appears to be impurity segregation effects to the boundary. Cleaner bicrystals are required to study intrinsic differences in the electrical activity of the two boundaries

Ast, D. G.

Gleichmann, R.

English

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https://ntrs.nasa.gov/search.jsp?R=19830027618 2020-03-21T01:09:40+00:00Z
r- - •	 JPL NO. 9950-867
Quarterly Report
DRL No. 162
	
	
DOE/JPL-956046-83/X 7
Distribution Category UC-63
TEN A14D SE11 (ERIC) INVESTIGATIONS OF SILIC0P1 BICRYSTALS
Quarterly Report
Period: April 1, 1983 to June 31, 1983
JPL Contract No. 956046, Report 117
Prepared by:
R.Gleichmann and D.G.Ast
Department of Materials Science and Engineering
Cornell University
Ithaca, NY 14853.
July 1983
The JPL Flat Plate 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 towards 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.
(NASD-CR-173107) TEM AND SEN (EPIC)
	 N83-35E89
INVESTIGATIONS OF SILICON EICR?STALS
Quarterly Report, 1 Apr. - 31 Jun. 1983
(Cornell Univ.) 13 p HC A02/MF A01 C SCL 20B
	 Unclas
G3/76 36726
a
TEM AND SEM (EBIC) INVESTIGATIONS OF SILICON BICRYSTALS
Introduction
The production of low cost solar silicon involves a compromise between cell
efficiency and crystalline perfection. An optimum trade-off between these two
properties requires knowledge of the electrical effects of crystal defects, such
as grain boundaries, in order to estimate their influence on cell efficiency.
In most solar grade materials the dominant structural defects are dislocations
and first order twins and the electrical properties of these defects have been
studied previously. Due to the high growth speed, ribbon materials contain
large residual stresses. The relaxation F these stresses can cause the
generation of additional (sub) grain tilt boundaries and may even result in
boundaries with large mis-orientation angles. Such boundaries usually are known
to have a strong influence on the minority carrier lifetime due to their
recombination activity. The aim of the present report was to study the
electrical and structural properties of low and medium angle tilt grain
boundaries in silicon bicrystals grown at JPL in order to obtain insight into
the mechanisms determining the recomination activity. The electrical behavior
of these grain boundaries was studied with the EBIC technique. Schottky
barriers rather than p-n junctions were used to avoid annealing induced changes
of the structure and impurity content of the as-grown crystals. The structure
of the boundary was studied with transmission electron microscopy.
Experimental
Both bicrystal samples were grown at JPL and were oriented to obtain tilt
boundaries with {100) median planes and <100> tilt axes. Tilt angles were about
200 , to study the effect of larger misorientations, and about 5 0
 to study the
electrical properties of low angle tilt boundaries. The starting material was
Czochralsky grown silicon (p-type, 4 n cm). The samples supplied by JPL were
in she form of 1-112 inch diameter wafer of lmm thickness. The wafers were
mounted on glass plates and cut by a diamond saw into 8x12 mm stripes parallel
to the boundaries. The stri pes were mounted on a stainless steel polishing
block with thermosetting wax and ground down with silicon carbide powder,
startiry with 600 grit and progressing to a 1200 grit, to a thickness of 100 um
for TEM samples and 300 um for EBIC samples. After a final polish with M305
powder, the TEM stripes were mounted on glass carriers and discs with 3mm
diameter were cut by ultrasonic drilling. These discs were then thinned down by
V	 ion milling since the chemical etching preferential attacked the grain boundary
plane. Stripes used for EBIC investigations were additionally polished on one
side with • syton. The stripes were cleaned using the RCA procedure and Schottky
diodes were formed by evaporating 500 A of pure aluminum in a vacuum of better
than 10-6 Torr. To limit the capacity of the Schottky diodes, 3x3 mm square
dots were prepared by marking the samples with a metal grid carefully aligned
parallel to the boundary. The TEM analysis was carried out with a JEM 200 CX
operated at 160 keV and equipped with a side-entry double tilt stage for
diffraction contrast experiments and tilt angle determinations. The EBIC
investigations were carried out in a JEOL 733 SEM equipped with an additional
low impedence amplifier (Keithley, model 427) and a special EBIC holder. To
avoid image contributions from the depletion layer, i.e. to establish diffusion
rather than drift field conditions, beam voltages above 20 kV were applied. The
use of high beam voltages has the additional advantage that the onset of high
injection conditions is shifted to higher beam currents. In solar material,
high injection conditions can be reached relatively easily since the doping
level is relatively low. The beam currents used in the present investigation
were usually 0.1 nA. The EBIC image is formed by collecting the beam induced
u
minority charge carriers at the Schottky diode and using this current to
modulate the intensity of a CRT display scanned synchronously with the specimen
beam. Crystal defects reducing the lifetime of the material are characterized
by an enhanced recombination activity and thus appear dark on the EBIC image.
Results and Discussion
TEM allows only the inspection of short pieces of the boundary which, at
most, some ten micrometers in length. Within these limitations, the 200
boundary was found to be surprisingly straight, homogeneous and free of
extrinsic dislocations. These features can be seen in the brightfield images of
different parts of the boundary - Fig. 1. Since the density of the intrinsic
O
dislocations is expected to be high with spacings in the order of lA A, it was
not surprising that individual dislocations could not be resolved :ven in
weak-beam micrographs.
The relative orientation of the adjoining crystals was determined by
diffraction analysis of adjacent parts of the bicrystal on either sides of the
boundary and by selected area diffraction at the boundary itself. Weak
convergent beam conditions were selected in order to enhance the visibility of
the Kikuchi lines. One example of such pairs of diffraction pattern is given in
Fig. 2. The upper crystal in the image was tilted close to a [001] orientation.
The crystal orientation of the lower left part corresponding to this special
orientation of the bicrystal was close to [OT3]. The exact determination of the
orientation relationship using Kikuchi lines gave the result that the tilt
angle of the boundary was 17.2 0 ±0.1 0 . The bicrystal contained no detectable
The dislocations should lie - paralell to the surface of the specimen.
Unfortunately, a-fringes (haXAing about the same orientation as the dislocation
lines) cannot be avoided using diffraction contrast imaging. Some minor
disturbances in the a-fringe appea rance indicate some density variations in the
dislocation array.	 ,
twist component. However, the selected area diffraction at the boundary plane
(near edge on position) shows an additional tilt component about the [010]
axis orthogonal to the wafer surface of 1.3°±0.2°. 	 An analysis of the
diffraction streak corresponding to the boundary plane indicated a boundary
0
width in the order of 50 A. This suggests that the boundary, whose orientation
is half way between E=25 and E-37, may be facetted on a fine scale. It was
difficult to orient the boundary precisely into an edge-on position. For this
reason, the symmetric position of the boundary plane relative to the adjoining
crystals was proved by exciting identical imaging conditions in both crystal
_
	 parts (symmetric tilting out of the edge-on position) and measuring the image
width at the same position of the boundary. Within the accuracy of the
technique, about one degree, the boundary plane was identical to the median
plane. The EBIC investigation showed that the boundary over a distance
investigated, about one centimeter, was essentially straight (compare e.g. Fig.
3). Only minor variations in the order of few microns could be observed and the
recombination efficiency along the boundary plane appeared to be homogeneous.
The material adjacent to the boundary contained, however, a high density of
extremely recombination active dislocations. The density of these dislocations
was in the order of several 105cm -2 .  The high dislocation density makes it
difficult to obtain quantitative data on individual dislocations. However,
strong recombination undoubtedly occurs at these dislocations. The strong
recombination activity of the dislocations can be explained by their TEM
appearance. All dislocations investigated were found to be heavily decorated
with small precipitates and appear as "pearl chains". One brightfield and one
darkfield image of the dislocations is given in Fig. 4. Fig. 4a shows in
addition the presence of an isolated small precipitate. The weak-beam
micrograph of Fig. 4b demonstrates that the decorating pa rticles are composed
Ag
o
of a large number of smaller precipitates in the size range 20...100 A
decorating the dislocations asymmetrically. Unfortunately the amount of
decoration is too small to obtain an X-ray signal of sufficient intensity to
analyze their chemical composition, even when the STEM is highly focussed.
However, the appearance of the precipitates resembles somewhat early
precipitation of oxygen in silicon and may be a result of the low cooling rate
during the growth of the bicrystal. Although no decorating precipitates could
be Ptserved at the boundary plane itself, it is likely that impurity segregated
to the 'boundary and are present below the TEM detection limit.
The investigation of the second bicrystal which contained a low angle tilt
boundary was carried out in an identical manner. The measured tilt angle was
5.5°±0.1°. Agai p , an additional tilt was observed around an axis orthogonal to
the wafer surface ([010] orientation within th,: boundary plane) with a tilt
angle of 2.1°±0.2°. Finally, a twist component of 0.7`k0.1° was also present in
this boundary. The boundary appeared straight and homogeneous in the TEM and
O
dislocations with varying spacings, from 100 to 1000 A, could be observed (ste
Fig. 5). No evidence for impurity segregation could be found either at the
boundary or at dislocations in adjacent crystal regions. The dislocation
density in the bulk crystal was lower and about 1.10 5cm-3 . This TEM result was
verified by investigating larger areas with the EBIC technique. One example of
a relative straight segment of the boundary is given in Fig. 6. The "sideways
movements" or kinks of the boundary usually are below 10um. The boundary seems
to be instabla, see Fig. 7, which shows a dissociation reaction of the boundary.
The additionally introduced boundaries of dotted EBIC contrast correspond to the
intersections of {111} planes with the surface and thus "intersections' appear
to be first order twin boundar i es. Dissociation of a low angle grain boundary
containing twist and tilt components occur relatively frequently and similar
reactions have been observed previously by Foell and Ast i and by Carter i
First order coherent twins are known to have usually a very low
—
!rences in the electrical activity of the two boundaries.
recombination activity. Their main contrast arises from the presence of
dislocations in the boundary pland (Strunk and Ast). This interpretation is
supported by the dotted appearance of the boundaries and the partly vanishing
contrast, particularly at the start of the twin plane in the lower part. Though
all the dislocations appeared to be clean within the resolution of the TEM, the
high EBIC contrast of the dislocations indicates that ft this sample, too, the
dislocations are decorated. Decoration of the 5.5 0 boundary could account for
.he observed high recombination activity. The EBIC contrast depends on the
imaging conditions but usually exceeds 50% as can be seen from the line scan
wr.	
across the boundary shown together with the zero current line, see Fig. 8. The
line profile was taken at 30 keV (5.10-10 0 A) to minimize the influence of the
Schottky diode depletion region. The electrical asymmetry of the boundary is
typical and could be observed at different positions and imaging conditions.
Possible explanations are that the stress field of the boundary is asymmetric
and/or that the impurity level in the adjoining crystals (at least in the
neighborhood of the boundary) is different. As a general result of the
investigation one can conclude that in these particular bicrystal samples the
electrical effect of grain boundaries appears to be independent of the boundary
misorientation. The dominant influence appears to be impurity segregation
effects to the boundary. Cleaner bicrysals would be required to study intrinsic
nrrrnrunrr
1. H. Foll and D.G. Ast, Phil. Mag. A 40 (1979) 589.
2. C.B. Carter and H. Foll "Dislocation Modelling in Physical Systems" Eds.
M.F. Ashby, R. Bullough, C.S. Hartley and J.P. Hirth, 1980 (Pergamon,
Oxford).
3. H. Strunk and D.G. Ast, 38 Proc. Electron Microscopy Soc. Amer. Eds. B.W.
Bailey, 1980, p. 332.
FIGURE CAPTIONS
Fig. 1.	 Brightfield TEM micrographs of the 17.2° tilt boundary, taken from
several TEM specimen prepared from the bicrystal (tilt axis [100]).
Fig. 2.	 Determination of the orientation relationship of the 17.2° boundary by
convergent beam diffraction.
Fig. 3. EBIC micrograph of the 17.2° tilt boundary at 20 keV.
Fig. 4. Brightfield (a) and darkfield weak-beam image of the dislocations in
the bulk section of the 11.2° bicrystal.
Fig. 5. Darkfi eld weak-beam image of the 5.5 0 tilt boundary with the common
(100) reflections excited.
Fig. 6. Straight segment of the 5.5 0 tilt boundary imaged with EBIC, beam
energy is 20 keV.
Fig. 7.	 Dissociation reaction of the 5.5° boundary (EBIC, 30 keV).
Fig. 8.	 Line scan across the 5.5 0
 tilt boundary with corresponding zero line
(area corresponding to Fig. 6) taken at 30 keV.
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TEM and SEM (EBIC) investigations of silicon bicrystals

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