Impact of Cu/III ratio on the near-surface defects in polycrystalline
M. M. Islam,1,a?A. Uedono,1S. Ishibashi,2K. Tenjinbayashi,1T. Sakurai,1A. Yamada,3
S. Ishizuka,3K. Matsubara,3S. Niki,3and K. Akimoto1
1Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
2Nanosystem Research Institute (NRI) “RICS,” National Institute of Advanced Industrial Science and
Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
3Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology
(AIST), Tsukuba, Ibaraki 305-8568, Japan
?Received 9 November 2010; accepted 23 February 2011; published online 17 March 2011?
Polycrystalline CuGaSe2thin films grown with various Cu/III?=Cu/Ga? ratios were investigated by
positron annihilation spectroscopy ?PAS?. The line-shape parameter S of the spectra was used to
characterize defects in CuGaSe2films. The S-parameter in positron annihilation spectra increased
with decreasing bulk Cu/III ratio in the CuGaSe2film. Experimental results combined with
theoretical calculation show the formation of multiple vacancy-type defect complexes in the
near-surface region of the CuGaSe2film when Cu-content in the film is decreased. These point
defects appear to cause the higher S-parameter in PAS measurement. © 2011 American Institute of
Chalcopyrite Cu?In1−xGax?Se2, abbreviated as CIGS, is a
promising material for realizing high-efficiency, low-cost
thin-film solar cells. Efficiency of 19.9% has been reported
for solar cell fabricated with CIGS.1Ideal CIGS band gap for
highest conversion efficiency is speculated theoretically to be
around 1.4 eV.2Therefore, CuGaSe2?x=Ga/In+Ga=1.0?
with a band gap of 1.68 eV can be considered as a leading
material to enable the highest possible efficiency.3Neverthe-
less, the efficiency of solar cells based on CuGaSe2seldom
exceeds 10%.4,5A better understanding of the material prop-
erties and actual defect physics of CuGaSe2is needed to
realize efficiency beyond the current level. In CIGS material,
12 native point defects are likely to exist and can affect the
overall electrical, optical and microstructural properties of
the films.6,7In this letter, we discuss our application of pos-
itron annihilation spectroscopy ?PAS? to wide gap CuGaSe2
thin films to investigate the point defects generated in this
material when there is a deviation from the stoichiometric
composition. PAS is a nondestructive technique suitable for
studying vacancy-type defects in semiconductors to gain in-
formation on the structures of the defects.8,9PAS evaluates
changes in the Doppler broadening of annihilated gamma
rays resulting from the trapping of positrons in defects. The
resultant changes in the spectra are characterized by the
line-shape parameter S, which mainly reflects changes due
to the annihilation of positron–electron pairs with a low-
momentum distribution, and the W-parameter, which mainly
characterizes changes due to the annihilation of pairs with a
high-momentum distribution. In general, the characteristic
value of S?W? for the annihilation of positrons trapped by
vacancy-type defects is larger ?smaller? than that for posi-
trons annihilated from the free-state. So far, only a few pos-
itron annihilation studies on CIGS thin films have been
done.10,11In this study, we used this technique to characterize
active defects in CuGaSe2thin films.
Polycrystalline CuGaSe2films with a typical thickness
of 2 ?m were grown over Mo-coated soda lime glass sub-
strates through three-stage evaporation process using mo-
lecular beam epitaxy system.12Evaporation was done at a
base pressure of ?10−6Pa from three Knudsen-cells that
were the respective Cu, Ga, and Se sources. Growth tempera-
ture of the first stage was kept at 400 °C during coevapora-
tion of Ga and Se. Temperature was increased to 520 °C at
the second and third stage when Cu, Se and Ga, Se coevapo-
ration was done, respectively. Substrate was being rotated at
10 rpm during the growth to maintain uniform composition.
All the samples were grown at constant flux rate of Cu, Ga,
and Se. Flux was measured as the beam equivalent pressure
?BEP? which is the difference of material flux before and
after the opening of the shutter, monitored, using a separate
pressure gauge. Se BEP was kept constant at 5.75
?10−3Pa during growth of all the samples. Several
CuGaSe2samples with various bulk Cu/III ratio were grown
by changing the third stage growth time. To avoid any sur-
face oxidation effect during the PAS experiment, films were
potassium cyanide ?KCN?-etched followed by the deposition
of a thin ?25 nm? CdS layer through chemical bath process.
CdS/CuGaSe2/Mo/SLG. A monoenergetic positron beam
was used to measure the Doppler broadening spectra of the
annihilation radiation as a function of incident positron en-
ergy, E ranging from 0 to 30 keV. The bulk Cu/III ratio in the
film used in this study was determined with an electron
probe microanalyzer ?EPMA? at 15 kV.
Figure 1 shows the results of the Doppler broadening
measurement of several CuGaSe2samples; each grown with
a different Cu/III ratio in the film. In the high energy region
of the S-E curve, the S-parameter for all the films approached
a value close to that corresponding to the S-value from the
substrate region of each film. However, significant differ-
ences were observed in the S-parameter in the lower energy
a?Electronic mail: firstname.lastname@example.org. Present address: Re-
search Center for Advanced Science and Technology ?RCAST?, The Uni-
versity of Tokyo, 4-6-1 Komaba, Meguro-Ku, Tokyo-153-8904, Japan.
APPLIED PHYSICS LETTERS 98, 112105 ?2011?
0003-6951/2011/98?11?/112105/3/$30.00 © 2011 American Institute of Physics
regions; i.e., the surface region and the subsurface region of
the film. In these regions, CuGaSe2films grown with high
Cu/III ratios showed low S-values while films with low Cu/
III ratios showed considerably higher S-values. The relation-
ship between the reciprocal S-values and the Cu/III ratios
was almost linear. This indicates the formation of different
types of defects in the CuGaSe2film when grown with a low
Cu/III ratio. Moreover, the peak of the S-E curve shifted to a
deeper region of the film with decreasing Cu/III ratio. This
indicates that the defect-rich region became wider with de-
creasing Cu/III ratio. In three-stage CuGaSe2-growth system,
to make the sample more Cu-deficient ?low Cu/III?, duration
of the third stage was kept longer when only Ga and Se are
coevaporated. It makes a wide Cu-poor region on the near-
surface region of the film. Thus, wide defect-rich region ob-
served in the Cu-poor CuGaSe2presumably due to the Cu-
vacancy related defects. As, samples were KCN-etched prior
to the PAS experiment, we can discard Cu2−xSe, as the origin
of the defects on low energy region of the spectra. EPMA
applied to all the samples shows that a decrease in the Cu/III
ratio introduces a proportional Cu deficiency along the depth
in the film ?inset of Fig. 1?. This result further supports the
assumption that Cu-vacancy related defects might be prima-
rily responsible for the increasing S-value in the S-E curve.
Since, samples were not KCN-treated for the EPMA mea-
surement, composition of the Cu-rich film ?Cu/III=1.12?
may include a little over estimation of Cu-content on the
surface region that may arise from any segregated Cu2−xSe
phase. Nevertheless, the tendency of the Cu-content in the
studied films with variation in Cu/III ratio is assumed to be
reasonable, as this tendency generally remains similar, irre-
spective of growth condition of the CIGS material. Con-
versely, higher S-values in the deep region of the film with
Cu/III=1.12 can be explained by the formation of a Cu2−xSe
secondary phase, which is likely to be formed inside grain of
the CuGaSe2films grown under Cu-rich conditions, thereby
cannot be etched by KCN-etching.13
To identify the type of defects at the surface region of
the film, we plotted ?S,W? values corresponding to the
positron annihilation process at E=5 keV in the CuGaSe2
samples ?Fig. 2?. Plotting S versus W enables us to determine
whether a defect type remains unchanged under external
influences.14,15The ?S,W? values obtained through first-
principles calculations using our in-house Quantum MAteri-
als Simulator ?QMAS? code16are shown in the same figure,
where the calculation was performed for the annihilation of
positrons from the delocalized state ?defect-free? of single
crystal CuGaSe2and for the annihilation of positrons trapped
by VCu, VSe, and VGa?i.e., monovacancy of Cu, Se, and Ga
respectively? or by multiple vacancies such as ?VCu?2,
?VCu?2VSe, or ?VCuVSe?2?i.e., Cu divacancy, ?VCu?2−VSeva-
cancy pair and divacancy of Cu–Se vacancy pair respec-
tively?. Although our grown CuGaSe2films are polycrystal-
line, scanning electron microscopy showed that the grain size
of all the films lie in the range of 500–700 nm which is fairly
large compared to the diffusion length of positron. Thus, we
can ignore the effect of grain boundaries in our experimental
results. PAS measurement for polycrystalline CuInSe2thin
films grown by various methods has already been reported
by several authors.11,17
As the figure shows, the calculated S-value of Cu-
vacancy pair, ?VCu?2is higher than that of monovacancy VCu
and VSe. Moreover, triple-vacancy, ?VCu?2VSeis associated
with more higher S-value and right-shifting of ?S,W? value,
as indicated by the arrow in the figure. Now, the experimen-
tal ?S,W? values of samples with Cu/III=1.12 and 0.95 lie
close to the line connecting the theoretical ?S,W? values of
defect-free CuGaSe2and those of ?VCu?2VSeand ?VCuVSe?2
defects, suggesting that these defect-complexes exist in the
samples. Careful observation shows a rightwards shift of the
experimental ?S,W? values with decrease in Cu/III ratio in
the film. We can explain this observation as follows: from the
calculated CuInSe2and CuGaSe2phase diagrams,18,19it was
found that the maximum In ?or Ga?-rich growth conditions
correspond, at the same time, also to maximum Se-poor con-
ditions. Therefore, as VCuis always abundant irrespective of
growth condition due its very low formation energy,20con-
siderable VSeconcentration would also be present even in In
?or Ga? rich growth condition, i.e., in Cu-poor CuGaSe2.
Lany and Zunger19calculated the binding energy, Ebof
?VCuVSe? complex which shows that VSebinds VCu. Consid-
ering the very mobile nature of VCuin CIGS,21and the law
of mass action, i.e., ??VSe−VCu??=?VSe??VCu?exp?Eb/kT?,
FIG. 1. ?Color online? S-parameter as a function of incident positron energy
E for various CuGaSe2thin films fabricated with various Cu/III?=Cu/Ga?
ratio in the film. Mean implantation depth of the positrons in the film is
shown at upper horizontal axis. Solid line indicates fitting of the S-E curve
by done by the computer program, VEPFIT. Inset shows the Cu at. % in the
corresponding CuGaSe2films determined by EPMA at several acceleration
FIG. 2. ?Color online? theoretical ?S,W? values ?solid symbols? of different
point defects in CuGaSe2, calculated by first principle method. Notation of
the defects are mentioned next to the corresponding ?S,W? values. Experi-
mental ?S,W? values ?see legend for the notation? of CuGaSe2samples
grown with different Cu/III ratio are also shown. Here, measured ?S,W?
values correspond to the positron energy, E=5 keV, which reflects defect
information at the near-surface region of the film.
112105-2 Islam et al.Appl. Phys. Lett. 98, 112105 ?2011?
they concluded that practically all Se vacancies will be Download full-text
present as ?VSe−VCu? complexes if the VCuconcentration
exceeds a certain limit. Furthermore, formation of ?VSe
−VCu? in In-rich material and suppression of it in Cu-rich
material has been explained by the positron life-time experi-
ment of Niki et al.22Therefore, from the Fig. 2, while
?VCu?2VSeand ?VCuVSe?2 defects already existed in the
slightly Cu-rich and near-stoichiometric samples, a further
decrease in Cu in the film would tend to generate more mul-
tiple vacancy-complexes of Cu and Se, which is consistent
with the rightward shift in experimental ?S,W? values with
decreasing Cu-content. This multiple vacancy-complex of
?VCuVSe? in Cu-poor samples might be associated with
higher S-values in our positron experiment. EPMA results in
this study shows an increasing tendency of Ga at. % in the
CuGaSe2with decreasing Cu-content. Therefore, formation
very low formation energy.23However, we did not calculate
?S,W? values for this defect in this study, so further discus-
sion of this defect is beyond the scope of this paper.
In conclusion, we have investigated defects in CuGaSe2
thin films through positron annihilation technique. The value
of S-parameter corresponding to the annihilation of positrons
in the surface region of the films was found to increase with
decreasing Cu/III ratio. Theoretical calculation of various
point defects along with the experimental values of the
?S,W? parameters of CuGaSe2samples grown under various
conditions revealed the formation of Cu–Se-related multiple
vacancy-complexes in Cu-deficient samples. These defects
appear to be responsible for higher S-value in Cu-poor
+2? defect complex is highly plausible due to
This work was partly supported by the Next Generation
Super Computing Project, Nanoscience Program, MEXT, Ja-
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