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Thin Solid Films 480–48

Structural peculiarities of CCSVT-grown CuGaSe2 thin films

M. Rusu*, S. Doka, A. Meeder, R. Wqrz, E. Strub, J. Rfhrich, U. Blfck, P. Schubert-Bischoff,W. Bohne, Th. Schedel-Niedrig, M. Ch. Lux-Steiner

Hahn-Meitner Institut, Glienicker Strasse 100, D-14109 Berlin, Germany

Available online 24 December 2004

Abstract

The microstructure of the CuGaSe2 (CGSe) thin films deposited by a novel chemical close-spaced vapour transport (CCSVT) technique

on clean and Mo-coated soda lime glass (SLG) substrates has been investigated by transmission electron microscopy (TEM). The CGSe bulk

and the interface between the CGSe and Mo films have been investigated. The as-grown CGSe films possess high bulk crystalline quality. At

the CGSe/Mo interface, a MoSe2 interfacial layer (~20–40 nm) has been observed and also an excess of Ga. Additionally, composition

measurements and depth profiling of the elements were performed by elastic recoil detection analysis (ERDA). It has been found that the

CGSe constituent elements are homogeneously distributed in the bulk, whereas the surface composition is influenced by the [Ga]/[Cu] ratio

in the film. With the [Ga]/[Cu] ratio increase, the CGSe surface composition changes from Ga- and Cu-poor, and Se-rich to Cu-poor, and Ga-

and Se-rich. Photoluminescence (PL) spectroscopy has been used as a complementary technique to study the defect profiles at the CGSe front

and rear sides as a function of the [Ga]/[Cu] ratio. The PL data support the results of structural investigations, pointing out higher Ga

concentration at the films rear side.

D 2004 Elsevier B.V. All rights reserved.

Keywords: CuGaSe2; Microstructure; CCSVT; TEM; ERDA; PL

1. Introduction

CuGaSe2 (CGSe) is one of the most promising materials

for the preparation of thin film solar cells with high open-

circuit voltage, owing to a larger band gap of 1.68 eV. This

fact makes this material suitable for application in photo-

voltaic modules. Also, CGSe matches perfectly for a

Cu(In,Ga)Se2/CuGaSe2 tandem device. Consequently, sci-

entific interest in this material is continuously increasing.

Research effort is made to improve the film quality which

is known to have a decisive impact on the final photo-

voltaic properties of the solar cell devices. Usually,

physical vapour deposition (PVD) technique is used for

the preparation of high quality polycrystalline chalcopyrite

thin films. Solar cells with an efficiency of 9.5% were

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.11.091

* Corresponding author. Tel.: +49 30 8062 2604; fax: +49 30 8062 3199.

E-mail address: rusu@hmi.de (M. Rusu).

prepared from PVD-grown CGSe films [1]. This technique,

however, is designed mainly for the preparation of thin

films for fundamental investigations. Recently, we have

successfully implemented a new chemical close-spaced

vapour transport (CCSVT) technique for the deposition of

binary, e.g., Ga2Se3 [2], and ternary CuGaSe2 [3] com-

pounds on 10�10 cm2 substrates. Solar cell devices

prepared from CCSVT-grown CGSe absorber have shown

efficiencies of 8.7% [3].

In this contribution, we report on the microstructure of

the CCSVT-deposited CGSe thin films on Mo/soda lime

glass (SLG) substrates as well as on the microstructure of

the CGSe/Mo interface. Additionally, elastic recoil detection

analysis (ERDA) has been applied for the depth profiling of

the elements on samples with various compositions. Photo-

luminescence (PL) spectroscopy has been employed to

study the evolution of the defects across CGSe films as a

function of the [Ga]/[Cu] ratio. For this purpose, PL

measurements have been carried out from the top and

backside of the CGSe/SLG samples.

1 (2005) 352–357

M. Rusu et al. / Thin Solid Films 480–481 (2005) 352–357 353

2. Experimental

CuGaSe2 thin films are prepared by annealing the Cu

precursors deposited on clean and Mo-coated soda lime

glass (SLG) substrates under gaseous GaClx /H2Se atmos-

phere in the CCSVT system. The Ga2Se3 employed as

source material is stoichiometrically volatilised by a

controlled amount of HCl/H2 agent. A two-stage deposi-

tion process is applied for the preparation of the CGSe

thin films with a required [Ga]/[Cu] ratio. In the first

stage, Cu-rich CGSe films are grown at a deposition rate

of 230–240 nm/min. The substrate and source temper-

atures in this stage are maintained at 450 and 550 8C,respectively. A second growth stage was implemented to

dissolve the remaining Cu2�xSe phases at a lower CGSe

deposition rate of 10 to 60 nm/min. The substrate and

source temperatures in the second stage are increased up

to 530 and 580 8C, respectively. The [HCl]/[H2] ratio is

varied from 10/1 in the first stage to 2/1 in the second

one. After the second stage, an annealing step at 530 8Cis used in the presence of H2 ambience only. During the

process, the reactor pressure is maintained constant at 800

mbar. In this way, a fine control of the CGSe film

thickness and chemical composition is realized. Further

details of the CCSVT process can be found elsewhere [3].

The absorber films are deposited on uncoated SLG for the

PL characterisation and on Mo/SLG structures for micro-

structural characterisation. The typical thickness of the

CGSe thin films ranges from 1.6 to 1.9 Am. The [Ga]/

[Cu] ratio of the films is adjusted within the range of

0.9–1.3.

The phase analysis of the as-grown CGSe films was

performed by X-ray diffraction (XRD) with a Bruker D8

diffractometer using Cu Ka radiation. The composition of

the as-grown films was determined using a Philips MagiX

Pro X-ray fluorescence (XRF) spectrometer. Elastic recoil

detection analysis (ERDA) technique using 350 MeV197Au ions as projectiles was applied at the Ionenstrahl-

Labor (ISL) of the Hahn-Meitner-Institut to obtain the

absolute atomic concentrations as a function of film depth.

Details about the ERDA principle and the experimental

setup can be found in Ref. [4]. The microstructure of the

films was observed by transmission electron microscopy

(TEM) using a Philips CM12, and the local compositions

were analyzed using an energy dispersive X-ray spectro-

scopy (EDX) system within the TEM. For the TEM

analysis, the sample was cut and fixed with conductive

glue face-to-face and subsequently thinned, first by polish-

ing with a diamond abrasive wheel and alumina paste, and

finally by Ar ion milling. For the photoluminescence

measurements, the 514.5 nm emission line of a Coherent

Innova–90 Ar+ laser was used as an excitation source. The

samples were cooled down to 10 K in a helium bath

cryostat (Oxford). The measurements were carried out on

the top and rear sides of the samples at a laser excitation

power of 20 mW.

3. Results and discussion

3.1. Structure and elemental profiles

For the CGSe thin films grown in the compositional

range of 1.0V[Ga]/[Cu]V1.3, no second phases such as

Cu2�xSe and/or Ga2Se3 were detected by means of XRD

analysis within the CGSe bulk and on the layer surface [3].

For the TEM studies, a CGSe/Mo sample was chosen with a

[Ga]/[Cu] ratio of 1.14 within CGSe. This composition

matches for the preparation of ZnO/CdS/CuGaSe2/Mo solar

cells devices with high efficiencies. Fig. 1a shows an

overview of the CGSe/Mo cross-section. The CGSe film

consists of large grains. However, relatively small ones with

a submicron size are also observed. Their structure,

however, is not affected by the grain size; it corresponds

to that of a tetragonal chalcopyrite structure. Indication of

the crystallite perfection is available from the two-dimen-

sional fast Fourier transformation (FFT) images, e.g., in the

inset to Fig. 1b. In the CGSe grains, the spacing between the

(112), (200), and (220) planes is found to be 3.2, 2.8, and

2.0 2, respectively, in good agreement with tabulated

JCPDS-35-1100 data. The grain boundaries show highly

defective regions, but no other phases are observed (Fig.

1b). The EDX point measurements within the TEM showed

slightly different compositions for different grains. For the

sample under consideration, [Ga]/[Cu] ratios between 1.12

and 1.16 are observed. These values agree well with a mean

value of 1.14 determined from the XRF measurements.

These compositional deviations suggest that different grains

started to grow at different times.

Fig. 1c shows the TEM micrograph of the CGSe/Mo

interface. The TEM picture reveals the presence of a ~20- to

40-nm-thick interfacial layer at the rear interface between

CGSe and Mo. The spacing between the layers is ~6.5 2,which corresponds to the distance between the (100) planes

of the hexagonal unit cell of MoSe2, according to JCPDS-

20-757 data. Although no MoSe2 presence is indicated in

the structure of the CGSe-based record cell [1], the

interfacial MoSe2 at the rear contact is reported to have a

beneficial effect on the high efficiency Cu(In,Ga)Se2 device

performance, ensuring the ohmic contact [5]. The presence

of a ~150-nm-thick layer at the CGSe/Mo interface has been

reported in Ref. [6]. It should be mentioned that the MoSe2layers in our structures are oriented mostly parallel to the

Mo surface. The EDX measurement at the CGSe/Mo

interface (Fig. 2b) reveals a Mo- and Se-rich composition

confirming the TEM observations of the MoSe2. However,

in the EDX spectra from the CGSe/Mo interface (Fig. 2b), a

significantly higher Ga signal is recorded compared to that

in the CGSe bulk (Fig. 2a), while the Cu signal remains

close to the background level (Fig. 2b). The same behaviour

of the Ga and Cu intensities is revealed by XRF measure-

ments on the Mo surface after the lifting-off of the CGSe

film. As no other phases different from CGSe tetragonal and

MoSe2 rhombohedral are observed at the CGSe/Mo inter-

Fig. 1. (a) TEM cross-section of a CuGaSe2/Mo/SLG structure. (b) TEM

micrograph of the CuGaSe2 layer involving a grain boundary between two

crystallites. The inset shows an electron diffraction pattern of a CuGaSe2crystallite. (c) Enlarged TEM micrograph of the CuGaSe2/Mo interface—

point B in panel a.

Fig. 2. EDX spectrum of a point (a) in the middle of the CuGaSe2 thin film

and (b) at the CuGaSe2/Mo interface, which correspond to points A and B

in Fig. 1a, respectively. The insets show the calculated concentrations of the

corresponding elements.

M. Rusu et al. / Thin Solid Films 480–481 (2005) 352–357354

face, we conclude that Ga metallic clusters of nanometre

dimensions are present between CGSe and Mo. The exact

location of such clusters is difficult to indicate due to the

limited resolution. Although the spot size of the electron

beam is ~9 nm, the lateral extension of the fluorescent

region within the material is much larger, which leads to a

resolution of the point measurement of about 50 nm, which

is larger than the MoSe2 thickness. We believe that the

above-mentioned Ga clusters are located in the bright

regions seen in Fig. 1a between the CGSe and Mo layers.

By means of the ERDA technique, the depth profiling of

the elements concentration from CGSe films with a [Ga]/

[Cu] ratio in the range of 1.06–1.26 was extracted. Fig. 3a

and b shows the concentration of the elements vs. distance

from the films top surface. The bulk concentration of the

elements Cu, Ga, and Se is nearly constant throughout the

films, while the composition of the CGSe top surface

changes in function of the integral [Ga]/[Cu] ratio of the

films. With increasing [Ga]/[Cu] ratio, the CGSe surface

composition changes from Ga- and Cu-poor, and Se-rich

(Fig. 3a) to Cu-poor, and Ga- and Se-rich (Fig. 3b). It should

be mentioned that in the compositional range investigated,

the films surface is always Cu-poor. It is well known for this

case that a Cu-deficient composition leads to the formation

of CuGa3Se5 compounds on the Ga-rich film surface [7].

The composition of the front side of the film with a [Ga]/

[Cu]=1.26 is Cu/Ga/Se=8:33:56 (Fig. 3b), i.e., corresponds

approximately to the CuGa3Se5 composition. Carbon and

oxygen are detected with a concentration in the range 0.09–

0.18 at.% and 0.49–0.87 at.%, respectively. A higher

oxygen concentration is observed closer to the CGSe/Mo

interface in both ERDA (Fig. 3) and EDX (Fig. 2)

measurements. This fact shows that oxygen diffuses from

the substrate. In addition, Na diffuses from the SLG glass. A

higher Na amount is found closer to the SLG substrate. The

Na concentration gradually decreases in the bulk. At the

film surface, Na is found with a concentration of 1 at.%. A

correlation between oxygen and sodium profiles is

observed: the higher the oxygen content is at the CGSe

backside, the lower the sodium concentration is detected at

the same Mo thickness. This can be an indication that the Na

Fig. 3. The ERDA depth-resolved elemental concentrations of the CuGaSe2thin films on Mo/SLG substrates with the ratios of (a) [Ga]/[Cu]=1.11 and

(b) [Ga]/[Cu]=1.26. The lines are guides to the eye. The insets show the

averaged elemental concentrations.

Fig. 4. Logarithmical photoluminescence spectra of the top (full triangles)

and back (open circles) sides of the CuGaSe2 thin films on SLG substrates

as a function of the [Ga]/[Cu] ratio. (T=10 K, k=514.5 nm, pexc=20 mW).

M. Rusu et al. / Thin Solid Films 480–481 (2005) 352–357 355

transport through the Mo/SLG interface is controlled by the

molybdenum oxide and not by the Mo layer itself. A similar

conclusion has been made in Ref. [8]. The Cl concentration

decreases gradually with increasing distance from the rear

side and becomes undetectable at a distance of c400 nm

from the surface side. Such a Cl concentration behaviour

correlates well with the CCSVT process stages. Cl is

incorporated in the CGSe during the first growth stage when

the deposition occurs at high HCl concentration (see Section

2). In the second growth stage, the Cl concentration is five

times lower than that in the first one, resulting in Cl

incorporation below the ERDA detection limit. An insig-

nificant amount of H (b0.05 at.%) is found in all samples

investigated.

3.2. Defect profiles

PL spectra—measured at T=10 K—of the CCSVT-

grown CGSe thin films as a function of composition are

presented in Fig. 4. We observe an almost perfect correlation

between PL-spectra of CCSVT-grown CGSe and PL-spectra

published in Refs. [9,10] for the case of chemical vapour

deposition (CVD)-grown CGSe single crystals and thin

films. Ref. [10] gives a radiative recombination model

consisting of two intrinsic donor (D1, D2) and three intrinsic

acceptor levels (A1, A2, and A3) sufficient to explain the

observed composition dependent PL at 10 K of CCSVT-

grown CGSe thin films. The defect-chemical nature of the

A1 and A3 acceptors suggested by most of the reports found

in the literature is the Cu vacancy (VCu) [11] and the Ga

vacancy (VGa) [9], respectively.

The observed PL does not show any new emission lines

caused by extrinsic impurities (e.g., Na, O, Cl, C, H) which

have been observed by means of EDX and ERDA in these

films. Therefore, we assume that the above-mentioned

impurities have an amphoteric behaviour in the films under

study.

Roughly summarized, the PL spectra of stoichiometric

samples show a structure which includes an excitonic peak

at 1.72 eV, a shallow donor-to-acceptor pair recombination

(D1A1) emission line at 1.67 eV and its corresponding

Fig. 5. Photoluminescence maximum (PLM) of the shallow defect emission

(D1A1) of the top (E) and back (o) sides as a function of the [Ga]/[Cu]

ratio. The inset represents the difference between the D1A1 PL maxima of

the both sides.

M. Rusu et al. / Thin Solid Films 480–481 (2005) 352–357356

phonon replica (LO-D1A1) as well as a deep donor-to-

acceptor pair recombination (D2A3) emission line at 1.27

eV. As shown in Ref. [10], the broad luminescence features

in the intermediate energy region between 1.4 and 1.6 eVare

due to overlapping effects of shallow to deep donor-to-

acceptor pair recombinations. With increasing [Ga]/[Cu]

ratio the (D1A1) peak shifts to lower energies. A model,

describing this effect in detail based on potential fluctua-

tions, is given in Ref. [11]. Thus, the (D1A1) peak position

is a measure of the [Ga]/[Cu] ratio: the higher the [Ga]/[Cu]

ratio of the sample, the larger the energy red shift of the

(D1A1) peak. Because 80% of the exciting photons are

absorbed within the first 100 nm of the film depth, a

measure of the D1A1 peak energy of the film front side and

film backside (through the glass substrate) is a measure of

the film composition at both surfaces. When plotting the

energy peak position of the shallow luminescence (D1A1)

of the both CGSe sides as a function of [Ga]/[Cu] ratio, see

Fig. 5, we can monitor the evolution of the [Ga]/[Cu]-ratio

near the substrate/surface side.

As mentioned above, the energy peak position of the

shallow luminescence (D1A1) at both top and back sides

gradually shifts towards lower energies as Ga increases. In

addition, with the Ga content increase the overall PL

intensity goes down. However, the energy difference DPLM

between the PL maximum (PLM) positions of the (D1A1)

peak at the both sides (which is zero at the stoichiometry-

point) increases and reaches a maximum value at about

[Ga]/[Cu]=1.08 (Fig. 5). Up to this film composition, we

assume a gradual increase of the shallow defect concen-

tration along the film depth. With further [Ga]/[Cu] ratio

increase, DPLM decreases to a value of ~5 meV. The

shallow defect concentration becomes homogeneous. At the

same time, the intensity of the deep defect luminescence

I(D2A3) decreases significantly, pointing out less Ga

vacancies [9,10]. This result agrees with the CCSVT

process stages [3]. In fact, the second CGSe growth stage

could be regarded as an annealing in GaClx/H2Se vapours of

the Cu-rich CGSe films prepared in the first stage. For this

case, it is known that the deep defect concentration related

to Gallium deficiency decreases with increasing annealing

time [12]. From the above observations, we conclude that all

optically active defects are almost homogeneously distrib-

uted in the stoichiometric films as well as in samples with

[Ga]/[Cu]z1.24. At intermediary compositions, a gradient

in the bulk defect structure from back- to topside is formed

in the films considered.

These results agree well with our EDX observations and

the XRF measurements (see Section 3.1), and with the data

of Klenk et al. [13] on CGSe samples prepared by rapid

thermal processing (RTP). As for our samples, the Ga

accumulation towards the substrate occurs for layers

deposited directly on glass as well as for films prepared

on Mo/SLG structures.

4. Conclusion

CGSe thin films possess high crystalline bulk quality.

Cu, Ga and Se are homogeneously distributed inside the

CGSe bulk. The CGSe surface composition is dependent on

the [Ga]/[Cu] ratio of the film. Ga- and Cu-poor, and Se-rich

surfaces are observed for nearly stoichiometric samples,

whereas with increasing [Ga]/[Cu] ratio, the films topside

becomes Cu-poor, and Ga- and Se-rich, corresponding to a

composition of a CuGa3Se5 compound. At the CGSe/Mo

interface, a MoSe2 interfacial layer is formed with a

thickness of ~20–40 nm. A Ga accumulation in the region

of the CGSe/Mo interface is also observed. Na transport

through the Mo/SLG interface into the CGSe film is limited

by molybdenum oxides.

The PL studies support the results of structural inves-

tigations, pointing out higher Ga concentration at the films

rear side. The distribution of the intrinsic defects across the

CGSe films depends on the films integral composition. The

defects are almost homogeneously distributed in the

stoichiometric films as well as in samples with [Ga]/

[Cu]z1.24. At intermediary compositions, the defects are

gradually distributed from the films back- to the topside.

Acknowledgements

The authors thank Prof. E. Arushanov for discussions.

This work was supported by the German Ministry of

Research BMWi (No. 0329740B).

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