JOURNAL OF APPLIED PHYSICS 107, 014311 共2010兲
Combined electron backscatter diffraction and cathodoluminescence
measurements on CuInS2 / Mo/ glass stacks and CuInS2 thin-film solar cells
D. Abou-Ras,1,a兲 U. Jahn,2 M. Nichterwitz,1 T. Unold,1 J. Klaer,1 and H.-W. Schock1
1
Helmholtz-Zentrum Berlin für Materialien und Energie, Glienicker Strasse 100, 14109 Berlin, Germany
Paul-Drude Institute for Solid-State Electronics, Hausvogteiplatz 5-7, 10117 Berlin, Germany
2
共Received 24 June 2009; accepted 24 November 2009; published online 7 January 2010兲
Electron backscatter diffraction 共EBSD兲 and cathodoluminescence 共CL兲 measurements in a
scanning electron microscope were performed on cross sections of CuInS2 thin films and
ZnO/ CdS/ CuInS2 / Mo/ glass thin-film solar cells. The CuInS2 layers analyzed for the present study
were grown by a rapid thermal process. The regions of the CuInS2 layers emitting high CL intensity
of band-band luminescence are situated near the top surface 共or close to the interface with ZnO/
CdS兲. This can be attributed to an enhanced crystal quality of the thin films in this region. The
phenomenon may be related to the recrystallization via solid-state reactions with CuxS phases,
which is assumed to run from the top to the bottom of the growing CuInS2 layer. The distribution
of CL intensities is independent of the sample temperature, the acceleration voltage of the electron
beam, and of whether or not the ZnO/CdS window layers are present. When comparing CL images
and EBSD maps on identical sample positions, pronounced intragrain CL contrast is found for
individual grains. Also, it is shown that at random grain boundaries, the decreases in CL intensities
are substantially larger than at ⌺3 grain boundaries. © 2010 American Institute of Physics.
关doi:10.1063/1.3275046兴
I. INTRODUCTION
I
III VI
Chalcopyrites with the chemical formula A B C 2 have
been investigated as absorbers in thin-film solar cells for
more than 30 years. However, only recently, such solar cells
have been commercialized by a rapidly growing industry.
One of these chalcopyrite-type compounds is CuInS2, which
may be deposited by a so-called rapid thermal process 共RTP兲
when aiming for high throughput in an industrial baseline.
The best thin-film solar cells with RTP-CuInS2 absorbers
have exhibited conversion efficiencies of up to 11.4%.1 Although the research on and development of these solar cells
have been conducted for several decades, the detailed knowledge of the optoelectronic properties of CuInS2 thin films is
still limited.
For semiconductor devices, cathodoluminescence 共CL兲
performed in a scanning or transmission electron microscope
represents an important technique to obtain information on
optical and electronic properties at a high spatial resolution
down to several tens of nanometers. A general introduction
into CL can be found in the review by Yacobi and Holt.2
Another technique applied in scanning electron microscopy 共SEM兲 is electron backscatter diffraction 共EBSD兲,
which gives information on the size and orientation of individual grains, also at a high spatial resolution of several tens
of nanometers. Since the orientations of two adjacent grains
can be determined, the EBSD technique allows for the classification of individual grain boundaries. A more extended
overview on EBSD maps from chalcopyrite-type thin films
in solar cells can be found in Ref. 3.
The present contribution reports on CL analyses performed on cross sections of CuInS2 / Mo/ glass stacks and
a兲
Electronic mail: daniel.abou-ras@helmholtz-berlin.de.
0021-8979/2010/107共1兲/014311/8/$30.00
also on completed solar cells with the stacking sequence
ZnO/ CdS/ CuInS2 / Mo/ glass, where the p-n junction is
formed at 共or in the vicinity of兲 the interface between the
n-type ZnO/CdS and the p-type CuInS2. The cross-section
configuration for the analyses was chosen in order to characterize directly spatially extended features that may impact
the current transport and voltage in the solar cells. The influences of various acquisition parameters, as well as those of
diverse cross-section sample preparation methods, on the CL
signals were investigated. It will be shown that the acquisition of EBSD and CL maps on identical areas is useful if
conclusions on the electronic properties of individual grains
or grain boundaries are to be drawn.
II. EXPERIMENTAL DETAILS
The CuInS2 / Mo/ glass stacks and the CuInS2 thin-film
solar cells studied in the present work were produced in the
following way. Cu and In layers 共with thicknesses of 550 and
650 nm兲 were sputtered on soda-lime glass substrates coated
with a 0.5 ␮m Mo back contact layer. The Cu/In stack was
exposed to sulfur during a rapid thermal annealing step
共heating-up rate 10 K/s兲 with a final temperature of nominally 530 ° C held for about 3 min, resulting in the formation
of a 2 – 3 ␮m thick CuInS2 film. Since the 关Cu兴/关In兴 ratio
was about 1.6, Cu–S forms on top of the CuInS2 layer, which
is removed by a KCN etch after film preparation.
The solar cells were completed by chemical bath deposition of a CdS buffer layer 共50 nm兲, sputtering of a transparent ZnO/ZnO:Al bilayer 共100 nm/500 nm兲 front contact,
and a Ni/Al contact grid. For further details on the solar-cell
production, the reader is referred to Ref. 1. The solar-cell
performances of all samples studied for the present work are
summarized in Table I.
107, 014311-1
© 2010 American Institute of Physics
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
014311-2
J. Appl. Phys. 107, 014311 共2010兲
Abou-Ras et al.
TABLE I. The solar-cell parameters 共open-circuit voltage Voc, short-circuit
current density jsc, fill factor ff, and solar-cell efficiency ␩兲 of the completed
solar cells used for the present work.
␩
Sample No.
Voc
共mV兲
jsc
共mA/ cm2兲
FF
共%兲
共%兲
1
2
3
692
563
684
21.4
22.7
21.8
68
66
65
10.1
8.4
9.7
For CL and EBSD analyses, various preparation methods
were applied.
共1兲 Cutting stripes from the CuInS2 / Mo/ glass stacks and
the CuInS2 solar cells, which were glued together face to
face by the use of epoxy glue. The cross sections were
polished mechanically using diamond lapping foils and
also by the use of an Ar-ion polishing machine, using
incident Ar-ion beam angles as low as 4°. Graphite layers of nominally 4–5 nm thickness were deposited on
top of the cross-section samples in order to reduce the
drift during the CL and EBSD measurements 共sample 1兲.
共2兲 Cutting a cross section at the edge of a
ZnO/ CdS/ CuInS2 / Mo/ glass stack by the use of a
ZEISS CrossBeam 1540 scanning electron microscope,
which is equipped with a focused ion beam 共FIB Ga
ions兲, at an ion beam voltage and current of 30 kV and 2
nA 共sample 2兲.
共3兲 Fracturing a CuInS2 / Mo/ glass stack in order to obtain a
cross section. For comparison, CL was also performed in
plan-view configuration on this CuInS2 / Mo/ glass stack
共sample 3兲.
The CL analyses were performed on two setups:
共I兲
共II兲
a ZEISS DSM962 scanning electron microscope
equipped with a tungsten filament and a MonoCL2
CL system 共CL setup I兲 and on
a ZEISS ULTRA55 scanning electron microscope
equipped with a field-emission gun 共FEG兲 and a MonoCL3 system 共CL set up II兲.
In setup I, the CL signal is detected by a liquid-nitrogen
cooled photomultiplier tube 共PMT兲 R5509-72 with an InP/
InGaAs photocathode 共spectral range: 300–1700 nm兲, while
setup II consists of a PMT R943-02 with a GaAs photocathode 共spectral range: 160–930 nm兲. Both scanning electron
microscopes are equipped with a He-cooling stage allowing
for sample temperatures between 6 and 300 K. The reason of
combining setups I and II in the present study is that setup II
共FEG兲 provides an enhanced spatial resolution compared
with setup I 共tungsten cathode兲, whereas setup I covers a
larger spectral range than setup II.
The photoluminescence 共PL兲 spectrum was recorded at
300 K using a 668 nm semiconductor laser as an excitation
source. The luminescence radiation is analyzed by a grating
monochromator and a low-noise InGaAs linear image sensor.
The excitation spot had a diameter of approximately
100 ␮m, and the excitation intensity was about
100 W / cm2.
FIG. 1. 共Color online兲 EBSD pattern quality map with ⌺3 grain boundaries
highlighted by intensified 共red兲 lines, acquired on a polished cross section
共see method 1 above兲.
The EBSD maps were acquired on the identical sample
areas as the corresponding CL measurements by using a LEO
GEMINI 1530 scanning electron microscope equipped with
a FEG and a NordlysII-S EBSD detector from Oxford Instruments HKL. The acceleration voltage applied was 20 kV, and
the probe current was about 1 nA. The EBSD patterns were
acquired and evaluated using the Oxford Instruments HKL
software package CHANNEL5. EBSD maps were recorded
with point-to-point distances of 50–100 nm and with recording durations of about 80–120 ms at each point.
III. RESULTS AND DISCUSSION
A. The RTP-CuInS2 microstructure
An exemplary EBSD pattern quality map from a polished cross-section sample is given in Fig. 1. The high relative frequency of ⌺3 共twin兲 boundaries 共the ⌺ value is introduced in Ref. 4兲, given as 共red兲 lines, can be attributed to the
rapid growth rate of the RTP-CuInS2 layer, which leads to
large strains in the thin film. Since the chalcopyrite-type
crystal structure of CuInS2 exhibits a low enthalpy for
stacking-fault formation,5 this mechanism 共where twin
boundaries are special cases of stacking faults兲 may help to
reduce the strain in CuInS2 thin films during growth and
cooling down. Note that there is not any significant difference between the average grain sizes for CuInS2 regions
close to the ZnO/CdS layers and for those situated near the
Mo back contact.
B. Influence of the stacking sequence and sample
preparation methods on CL intensities
A comparison of CL spectra obtained from cross sections
of a CuInS2 / Mo/ glass stack and a completed solar cell 共with
stacking sequence ZnO/ CdS/ CuInS2 / Mo/ glass兲, both prepared by method 1, at identical acquisition parameters 关room
temperature 共RT兲 and 10 kV acceleration voltage兴 is given in
Fig. 2. Background noise from the glass substrate was subtracted. Also included is a PL spectrum recorded at RT on a
CuInS2 / Mo/ stack. Apparently, there are two intensity
maxima, one at about 820 nm, which can be related to the
band-band transition in CuInS2, and the other one at about
1050 nm, which can be attributed to transitions via deep
defects, which have not been identified so far.6 It is evident
that the two spectra do not differ significantly in the vicinity
of the peak at 820 nm; i.e., the properties of the luminescence spectrum of CuInS2 in this wavelength range can be
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
014311-3
Abou-Ras et al.
J. Appl. Phys. 107, 014311 共2010兲
FIG. 2. CL spectra 共normalized to the intensity at about 820 nm兲 acquired
by the use of setup I at RT on a completed solar cell 共sample 2兲 and on a
CuInS2 / Mo/ glass stack, as well as a PL spectrum acquired on a
CuInS2 / Mo/ glass stack produced in the same run as sample 2 共see Table I兲.
considered independent of whether ZnO/CdS layers are
present or not in the layer stack. The positions of the dominant peaks of these CL spectra agree well with those of the
PL spectrum acquired on a CuInS2 / Mo/ stack comparable to
the samples studied by CL.
Figure 3 shows the CL spectra acquired by the use of
setup II, for cross sections prepared by methods 1–3 as well
as for the plan-view sample. The CL acquisition parameters
were kept identical for all these measurements 共RT, 10 kV,
and identical acquisition window size兲. Apparently, the peak
intensities at about 820 nm 共band-band transitions兲 are larger
for the cross section prepared by polishing and for the planview sample. The fractioned cross section and the one prepared by FIB exhibit peak intensities that are substantially
lower than that of the polished cross section. Furthermore, it
is found that the CL intensities at 820 nm decrease from
about 5000 共10 kV兲 to 100 共5 kV兲 for the FIB-prepared cross
section 共not shown here兲, whereas this decrease is much
FIG. 4. 共Color online兲 Superimposed SE and CL images, acquired at RT and
10 kV on the identical sample region 共two solar cells glued together face to
face兲. Monochromatic CL images 共a兲 at 820 nm, acquired by means of CL
setup II, 关共b兲 and 共c兲兴 by the use of CL setup I, at 820 and 1050 nm; 共d兲
superposition of CL images shown in 共b兲 and 共c兲.
smaller for the polished cross section with a graphite layer
共from about 13 000 at 10 kV to about 10 000 at 5 kV; data
not shown here兲.
These differences hint to substantially differing influences of the cross-section surfaces on the radiative recombination for the samples prepared by methods 1–3. The cross
sections prepared by FIB are known to contain a Ga contamination layer,7 which may be responsible for enhanced
surface recombination. Although the thickness of the Ga contamination layer, in principle, may be reduced considerably
by reducing the ion beam energy, this is to be optimized for
future works. On the other hand, the polished cross sections
exhibit the highest CL yield, which may be due to a thin
graphite layer 共4–5 nm兲 deposited on the sample surface.
This graphite layer may modify the surface potential and/or
change the charging of the surface caused by the impinging
electron beam. In either case a change in the band bending at
the surface of the p-type CuInS2 absorber layer is expected,
resulting in a reduced surface recombination velocity for minority carriers.
C. Spatial distribution of CL signals
FIG. 3. CL spectra 共as taken兲 acquired by the use of setup II on the samples
prepared by methods 1–3. Identical CL acquisition settings were used for all
spectra 共RT and 10 kV兲.
A comparison of CL images obtained at 820 nm by CL
setups I and II is given in Figs. 4共a兲 and 4共b兲, which show
results from a stack formed by gluing two RTP-CuInS2 solar
cells face to face together. There, the CL signals are super-
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
014311-4
Abou-Ras et al.
J. Appl. Phys. 107, 014311 共2010兲
FIG. 6. Schematics of the RTP-CuInS2 growth mechanism. Sputtered Cu
and In layers 共where Cu diffuses into the In layer, forming a Cu–In alloy兲
are exposed to sulfur molecules in a closed reaction chamber. CuInS2 forms
on top of the Cu/Cu–In stack. Sulfur diffuses from the surface to the bottom
of the layer stack, whereas Cu and In diffuse into the opposite direction.
Since the 关Cu兴/关In兴 ratio is larger than 1, CuxS forms on top of the CuInS2
layer.
FIG. 5. 共Color online兲 共a兲 CL image at 820 nm from a completed solar cell
at RT and 10 kV, superimposed on the SE image acquired on the identical
sample area; 共b兲 CL image on the identical sample area as 共a兲, acquired at 5
K; and 共c兲 CL image at RT and 10 kV from a glass/ Mo/ CuInS2 stack,
superimposed on the SE image acquired on the identical sample area.
imposed on the secondary electron 共SE兲 image in order to
illustrate where in the layer stack the CL signals are emitted
from. Apparently, the lateral CL intensity distribution is similar for both detectors. The SE images in Figs. 4共b兲 and 4共c兲
are somewhat blurred since a high beam current was chosen
on the SEM with the tungsten cathode in order to improve
the CL signals, which deteriorated the SE image quality.
In Fig. 4, CL emission can be detected mainly from the
CuInS2 layer 共the glass substrate also emits very weak CL
signals兲. Most of the regions in the CuInS2 layer with high
CL intensity at 820 nm 关Fig. 4共b兲兴 are situated close to the
interface with the ZnO/CdS layers. In contrast, most of the
regions in the CuInS2 layer with high CL intensity at 1050
nm 关Fig. 4共c兲兴 are rather close to the interface with the Mo
back contact. This behavior becomes apparent particularly
when the CL signals in Figs. 4共b兲 and 4共c兲 are superimposed
on each other 关Fig. 4共d兲兴.
The CL intensity distribution in CuInS2 at 820 nm with
high intensities mainly close to the ZnO/CdS layers was confirmed on several other sample regions; see Fig. 5共a兲 for an
example. The situation is still similar when lowering the
nominal sample temperature from RT to 5 K 关Fig. 5共b兲兴. CL
maps obtained at both RT and 5 K resolve small features
down to about 50–100 nm, and the resolution does not
change substantially at very low temperatures. This indicates
that the grain-boundary recombination velocity is large, and
that the effective minority-carrier diffusion lengths in the
RTP-CuInS2 are also in the order of 100 nm. A more detailed
analysis of the relationship between CL contrast and
minority-carrier recombination properties will be given in a
forthcoming publication.
Also for the CuInS2 / Mo/ glass stack without ZnO/CdS,
most of the regions in the CuInS2 layer with high CL intensity at 820 nm seem to be situated close to the CuInS2 surface, similar as for the completed solar cells. This result
shows that the ZnO/CdS layers apparently do not influence
the spatial distribution of the CL intensity.
D. Implications for RTP-CuInS2 growth mechanism
The emission of CL signals in a semiconductor is a process induced by the incident electron beam, which generates
electron-hole pairs. These charge carriers diffuse and then
recombine. Recombination of electrons and holes may be
associated with the emission of photons, as expected if an
electron and a hole from states close to the edges of the
conduction and the valence bands recombine. However,
when the recombination occurs via deep levels positioned
close to the middle of the band gap of the semiconductor, the
process is likely to be nonradiative 共e.g., phonon assisted or
accompanied by Auger-electron emission兲.2
Therefore, the CL intensity depends on the generation
rate of electron-hole pairs as well as on the probability of
radiative and nonradiative recombinations. Assuming laterally homogenous generation rates, the CL intensities at RT
and 820 nm 共band-band transitions兲 can be expected to be
high where only a few deep defects are present, as, e.g., in
crystals with small densities of crystal defects. This may be
the case for the major part of the near-surface regions 共or
those close to ZnO/CdS for completed solar cells兲 in the
RTP-CuInS2 layers, as shown in Figs. 4 and 5 共note that
there are some regions exhibiting high CL intensity at 820
nm also close to the Mo back contact兲.
On the other hand, from the results shown above, it can
be concluded that the density of crystal defects is larger for
the major part of the regions in RTP-CuInS2 close to the Mo
back contact. Rudigier et al.8 found a correlation between the
peak intensity around 1050 nm in the PL spectra with an
increased full width at half maximum 共FWHM兲 of the A1
Raman mode by combined PL and Raman spectrometry on
RTP-CuInS2 layers. The increase in the FWHM of the A1
mode again can be attributed to a decrease in the phonon
correlation length, caused by a larger density of localized and
spatially extended defects in the lattice.9
In order to understand why the crystalline quality of the
sample regions close to the surface 共close to the ZnO/CdS
layers兲 may be enhanced compared with those close to the
Mo back contact, it is helpful to consider the RTP-CuInS2
growth process1 more in detail. Sputtered Cu and In layers
are deposited on Mo-coated glass substrates. Cu diffuses into
the In top layer to form a Cu–In alloy.10 This layer stack is
exposed to sulfur molecules in a closed reaction chamber
共Fig. 6兲. For high sulfur partial pressures, it was found that
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
014311-5
Abou-Ras et al.
FIG. 7. 共Color online兲 Monochromatic CL images at 820 nm and EBSD
maps from the identical sample area. 共a兲 CL image from a completed solar
cell at RT and 10 kV, superimposed on the SE image acquired on the identical sample area; 共b兲 CL image acquired at 6 K and 5 kV; 共c兲 EBSD pattern
quality-map with ⌺3 grain boundaries highlighted by intensified 共red兲 lines;
and 共d兲 EBSD orientation-distribution map 共greyvalues/colors represent local orientations; see legend兲.
CuInS2 forms at an early stage of the process.10 Sulfur diffuses from the surface to the bottom of the layer stack,
whereas Cu and In diffuse into the opposite direction. Since
the 关Cu兴/关In兴 ratio is larger than 1, CuxS forms on top of the
CuInS2 layer. These two compounds exhibit crystal structures that resemble each other. It is assumed that the CuInS2
grain growth is substantially assisted by an exothermic solidstate reaction between CuxS and CuInS2, which includes the
exchange of cations while the sulfur substructures remain
nearly unaffected.11–13 The reaction front is considered close
to the CuInS2 near surface in the beginning and then moves
further down into the CuInS2 layer. Rodriguez-Alvarez et
al.14 showed by EBSD maps acquired on cross sections of
annealed CuInS2 / CuxS stacks 共where the as-grown CuInS2
layers exhibit rather small average grain sizes兲 that the average grain size of the recrystallized CuInS2 共after annealing兲
increases for larger thicknesses of the CuxS layer.
It is important to point out here that EBSD maps of
various RTP-CuInS2 layers 共see Fig. 1 for an example兲 do
not reveal any significant difference in average grain size
between the CuInS2 regions close to the ZnO/CdS layers and
those situated near the Mo back contact. Therefore, it can be
J. Appl. Phys. 107, 014311 共2010兲
FIG. 8. 共Color online兲 Magnified areas from Figs. 7共b兲 and 7共d兲. 共a兲 Monochromatic CL image at 820 nm, and 共b兲 EBSD pattern quality map with ⌺3
grain boundaries highlighted by intensified 共red兲 lines. The contrast in the
CL image was increased substantially 关as compared with that in Fig. 7共b兲兴 in
order to make the details close to the Mo back contact visible. A grain is
highlighted by 共yellow兲 circles in the CL image and the EBSD map. 共c兲
Profiles extracted from the positions marked with dotted lines, across a ⌺3
共1兲 and a random grain boundary 共2兲.
assumed that after crystallization 共running from the top to the
bottom兲, point and extended defects 共and not smaller average
grain sizes than at the CuInS2 surface兲 are to be found in the
vicinity of the Mo back contact. On the contrary, CuInS2
grains present in the near-surface region correspond to a
crystallization seed layer; thus, they may exhibit an enhanced
crystal quality with a significantly smaller density of crystal
and electronic defects.
E. CL and EBSD measurements on identical sample
positions
From the CL images shown in Figs. 4 and 5, it may be
concluded that the intensity distributions represent closely
the microstructure in the RTP-CuInS2 thin films. In order to
verify this assumption, identical sample positions in several
ZnO/ CdS/ CuInS2 / Mo/ glass and CuInS2 / Mo/ glass stacks
were analyzed by means of CL and EBSD. Figure 7 gives an
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
014311-6
Abou-Ras et al.
J. Appl. Phys. 107, 014311 共2010兲
FIG. 9. 共Color online兲 共a兲 CL image from a completed solar cell at 5 K and 5 kV, and 共b兲 EBSD orientation distribution map 共greyvalue/colors represent local
orientations; see legend兲 from the identical sample area as for 共a兲. Two grains are highlighted by 共yellow兲 circles. Across the features in these grains, linescans
were extracted 共dotted lines兲 and shown in 共c兲.
overview of the various details, which can be extracted from
the CL images and EBSD maps. When reducing the sample
temperature to 6 K and the acceleration voltage to 5 kV 关Fig.
7共b兲兴, much smaller details of down to 50 nm are resolved as
compared to the CL map acquired at RT and 10 kV 关Fig.
7共a兲兴. Note that the improvement of the resolution is attributed to the decrease in acceleration voltage, not to that of the
sample temperature 共see also Fig. 5兲. The distribution of the
local orientations perpendicular to the Mo substrate 关Fig.
7共d兲兴 appears to be random, as it was also confirmed by
EBSD texture measurements performed on large areas 共not
shown here兲. It was not possible to correlate the CL intensity
distribution at 820 nm to that of the local orientations.
In CuInS2 thin films, grain boundaries can be divided
roughly into 共near兲 ⌺3 共twin兲 boundaries and other grain
boundaries, in the following called “random” 共see Ref. 3 for
details兲. The ⌺3 共twin兲 boundaries exhibit the most frequent
grain-boundary type in RTP-CuInS2, as visible in Fig. 7共c兲.
From the EBSD data, their relative frequency can be estimated to about 75%–80%. Also, ⌺3 共twin兲 boundaries can
be expected to be less electrically active than random boundaries, due to a lower density of dangling and distorted atomic
bonds.
Indeed, it has already been shown15 that the CL intensity
in RTP-CuInS2 decreases more strongly at random than at
⌺3 共twin兲 boundaries. This result is confirmed by the present
work 关see, e.g., Fig. 8共c兲兴. Moreover, it is shown in Fig. 8
that individual grains, as determined by EBSD 关Fig. 8共b兲兴,
consist of regions with different CL intensity 关Fig. 8共a兲兴. In
the example given in Fig. 8, the CL signal from the grain
highlighted by a 共yellow兲 circle appears divided into a highintensity part close to the ZnO/CdS layers and a lowintensity part near to the Mo back contact. The latter region
seems to be again divided into smaller areas by lines of further decreased CL intensities. These phenomena may be attributed to planar crystal defects such as dislocations or other
extended defects 关probably not twin boundaries, which do
not lead to strong decreases in CL intensity, as shown in Fig.
8共c兲兴. Obviously, these planar defects are not detected by
EBSD but may contain a certain density of electronic defect
states, which affect the CL signal.
A further example of how easily CL images may be misinterpreted without the knowledge of the corresponding microstructure is given in Fig. 9. At several positions, the CL
map 关Fig. 9共a兲兴 exhibits regions containing lines of reduced
CL intensity 共two of such regions are highlighted by yellow
circles兲. It appears that these lines can be attributed to grain
boundaries. However, the EBSD map in Fig. 9共b兲 shows that
the regions within the yellow circles are indeed single crystals without any grain boundary. By examining SEM images
from the identical position 共not shown here兲, it was also verified that the lines of reduced CL intensity are not due to
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
014311-7
J. Appl. Phys. 107, 014311 共2010兲
Abou-Ras et al.
However, this may explain only for some regions why
EBSD detects a contiguous grain, while the CL results indicate extended defects at the identical sample position. Especially in view of the large grains present in Figs. 8–10 with
2 – 4 ␮m in diameter, this explanation seems not sufficient.
Rather, it may be assumed that, in general, the CL intensity is
affected considerably even by small defect densities that do
not involve a change in crystal orientation, which again is the
only information to be gathered by EBSD, apart from
changes in crystal structure.
IV. CONCLUSIONS
FIG. 10. 共Color online兲 共a兲 Monochromatic CL image at 820 nm from a
CuInS2 / Mo/ glass stack at RT and 5 kV, and 共b兲 EBSD orientation distribution map 共colors represent local orientations; see legend兲 from the identical
sample area as for 共a兲. A region containing a contiguous grain 共right side兲 is
highlighted by a yellow circle. Across the ⌺3 共twin兲 boundaries 共marked 1
and 2兲, profiles were extracted 共dotted lines兲 and shown in 共c兲.
mechanical defects 共e.g., scratches兲 remaining from the
sample preparation. Also, these mechanical defects would
reduce the local quality of the EBSD patterns substantially,
which is not the case 共Fig. 9兲.
Also for a CuInS2 / Mo/ glass stack 共Fig. 10兲, it was
found that ⌺3 共twin兲 boundaries 关1 and 2 in Fig. 10共c兲兴 do
not exhibit a strong decrease in the CL signal, as well as that
CL images from contiguous grains may contain areas of reduced intensity, as it is the case for the grain in the region
highlighted by the circle on the right. Therefore, this result is
again independent of whether the ZnO/CdS layers are
present on top of the CuInS2 or not.
It is important for the interpretation of the EBSD maps
and CL images of identical sample positions to take into
account that EBSD has an information depth of about 20 nm
共most of the backscattered electrons do not reach the EBSD
detector, not even at an acceleration voltage of 20 kV兲, while
the excitation and information depths of CL images at 5 and
10 kV are larger than about 150 and 500 nm 共if additional
diffusion of the generated charge carriers is considered, these
values may be even 1 ␮m or larger兲. I.e., the EBSD technique is much more surface sensitive.
CuInS2 thin films grown by a RTP in ZnO/ CdS/
CuInS2 / Mo/ glass and CuInS2 / Mo/ glass stacks were analyzed by means of CL and EBSD. It was shown that the
regions in the CuInS2 layers emitting high CL intensity at
820 nm 共band-band transitions兲 are situated near to their surface and close to the interface with ZnO/CdS, while at 1050
nm, the high intensities were detected rather close to the Mo
back contact. This behavior is attributed to a better crystal
quality close to the top surface of RTP-deposited CuInS2
films. The spatial CL intensity distribution detected at 820
nm is independent of the measurement temperature, the acceleration voltage of the electron beam, and the cross-section
preparation method. The CL intensity itself is found to be
affected by the preparation method and hints toward differing levels of surface recombination for the samples. When
combining monochromatic CL and EBSD on identical
sample positions, it was found that individual grains exhibit
additional CL features on several positions, which do not
seem to be related to grain boundaries but to other kinds of
extended defects.
ACKNOWLEDGMENTS
This work was supported in part by the EFRE project
ANTOME. The authors would like to thank N. Blau, C.
Kelch, M. Kirsch, P. Körber, and T. Münchenberg for solarcell processing, and also J. Bundesmann for the specimen
preparation by FIB-SEM. Special thanks are due to R. Mainz
and H. Rodriguez-Alvarez for fruitful discussions.
1
K. Siemer, J. Klaer, I. Luck, J. Bruns, R. Klenk, and D. Bräunig, Sol.
Energy Mater. Sol. Cells 67, 159 共2001兲.
2
B. G. Yacobi and D. B. Holt, J. Appl. Phys. 59, R1 共1986兲.
3
D. Abou-Ras, S. Schorr, and H.-W. Schock, J. Appl. Crystallogr. 40, 841
共2007兲.
4
H. Grimmer, W. Bollmann, and D. H. Warrington, Acta Crystallogr., Sect.
A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 30, 197 共1974兲.
5
N. I. Medvedeva, E. V. Shalaeva, M. V. Kuznetsov, and M. V. Yakushev,
Phys. Rev. B 73, 035207 共2006兲.
6
K. Töpper, J. Bruns, R. Scheer, M. Weber, A. Weidinger, and D. Bräunig,
Appl. Phys. Lett. 71, 482 共1997兲.
7
J. Mayer, L. A. Giannuzzi, T. Kamino, and J. Michael, MRS Bull. 32, 400
共2007兲.
8
E. Rudigier, T. Enzenhofer, and R. Scheer, Thin Solid Films 480–481, 327
共2005兲.
9
C. Camus, E. Rudigier, D. Abou-Ras, N. A. Allsop, T. Unold, Y. Tomm, S.
Schorr, S. E. Gledhill, T. Köhler, J. Klaer, M. C. Lux-Steiner, and Ch.-H.
Fischer, Appl. Phys. Lett. 92, 101922 共2008兲.
10
H. Rodriguez-Alvarez, I. M. Koetschau, C. Genzel, and H. W. Schock,
Thin Solid Films 517, 2140 共2009兲.
11
R. Klenk, T. Walter, H. W. Schock, and D. Cahen, Solid State Phenom.
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
014311-8
37–38, 509 共1994兲.
R. Scheer and H. J. Lewerenz, J. Vac. Sci. Technol. A 13, 1924 共1995兲.
13
H. Rodriguez-Alvarez, R. Mainz, A. Weber, B. Marsen, and H. W.
Schock, in Thin-Film Compound Semiconductor Photovoltaics—2009,
Materials Research Society Symposium Proceedings No. 1165, edited by
A. Yamada, C. Heske, M. Contreras, M. Igalson, and S. J. C. Irvine 共Ma12
J. Appl. Phys. 107, 014311 共2010兲
Abou-Ras et al.
terials Research Society, Warrendale, PA, 2009兲, p. M02-07-1-7.
H. Rodriguez-Alvarez, D. Abou-Ras, R. Mainz, B. Marsen, and H.-W.
Schock 共unpublished兲.
15
D. Abou-Ras, C. T. Koch, V. Küstner, P. A. van Aken, U. Jahn, M. A.
Contreras, R. Caballero, C. A. Kaufmann, R. Scheer, T. Unold, and H.-W.
Schock, Thin Solid Films 517, 2545 共2009兲.
14
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp