Photoluminescence characterization of polycrystalline CuGaSe2 thin films grown by rapid thermal processing

Solar Energy Materials and Solar Cells 51 (1998) 371—384

Photoluminescence characterization of polycrystalline CuGaSe thin films grown by 2 rapid thermal processing J.H. Scho¨n*, O. Schenker, L.L. Kulyuk1, K. Friemelt, E. Bucher University of Konstanz, Faculty of Physics, P.O. Box 5560, D-78434 Konstanz, Germany Received 31 December 1996; received in revised form 15 August 1997; accepted 15 August 1997

Abstract CuGaSe thin films have been prepared by rapid thermal processing of stacked elemental 2 layers on different substrates. The film homogeneity across the depth and the influence of the substrate used have been investigated mainly by means of photoluminescence spectroscopy. The photoluminescence spectra could be divided into five spectral ranges: emissions from Ga-rich phases (above 1.75 eV), band edge emissions (1.72 eV), emissions involving shallow levels (V , V ), a broad donor-acceptor-pair transition (1.4—1.55 eV), and emissions from deep C6 S% levels (below 1.4 eV). All films grown from the conventional precursor stack showed inhomogeneities, which could be avoided by modifications of the precursor stack. Investigations on the growth on different substrates revealed the best crystalline properties for films grown on sapphire. In contrast to films grown on floating glass the difference in quality to CuGaSe on 2 Mo was rather small. This underlines the suitability of Mo-coated floating glass as cheap substrates for thin-film solar cells. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Thin films; Thermal processing; Homogeneity

1. Introduction Ternary Cu-III—VI semiconducting chalcopyrite compounds have proved to be 2 very efficient materials for thin-film solar cells. Small-area cells of CuIn Ga Se 1~x x 2 * Corresponding author. 1 Permanent address: Institute of Applied Physics, Academy of Sciences of Moldova, MD-2028 Kishinev, Moldova. 0927-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 2 5 6 - 0

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have reached efficiencies exceeding 17% [1]. Although efficiencies of CuGaSe thin2 film cells are limited to 9.3% up to now [2], this material exhibits some advantages with respect to module integration due to its higher band gap (1.72 eV). Furthermore, a tandem arrangement of CuInSe and CuGaSe could increase efficiencies above 2 2 33% [3]. In addition to that, wide-gap Cu-III—VI compounds are of great interest for 2 the realization of light-emitting devices operating in the spectral range from visible to ultraviolet [4]. Thin films of this compound, which are a prerequisite to the development of cost-effective device structures, have been fabricated by many techniques [5—9]. This study will deal with the examination of photoluminescence (PL) properties of CuGaSe thin films prepared by rapid thermal processing of stacked elemental layers 2 (RTP-SEL). The advantages of this formation process are the possibility of upscaling, low temperatures around 500°C allow the use of cheap glass substrates, and short process times of only a few minutes, which gives the possibility of a high throughput [9]. Although many studies on structural and optoelectronic properties of polycrystalline CuGaSe thin films have been published [7—13], there has been little reported 2 work on photoluminescence properties of such films [7,14,15]. Some photoluminescence studies on heteroepitaxial CuGaSe layers [16—18] and single crystals [19—24] 2 have been published. In this study the changes of the PL properties due to different substrates and the variation as a function of film thickness have been examined. Using the results of these investigations the conventional precursor has been changed in order to improve crystalline quality and homogeneity across the depth of the films. The improvement of these parameters was shown for two different precursor arrangements.

2. Experimental procedure CuGaSe thin films were fabricated starting with a stack of elemental layers and 2 then using the RTP method. The elemental layers were deposited by thermal evaporation in an Edwards E306A unit at a background pressure of 10~6 mbar. The Cu and the Ga layers were separated by an intermediate Se layer to prevent the formation of the CuGa compound during evaporation [25]. Furthermore, the substrate was 2 heated up to 100°C during the evaporation sequence to prevent the formation of cracks in the processed films and to obtain a better mixing of Ga and Se. The precursors were annealed in a graphite box at 550°C for 5 min in Ar-atmosphere using a commercial RTP-furnace SHS-100 supplied by AST equipped with tungsten halogen lamps. Four different precursor stacks were investigated: a substrate//Ga/ Se/Ga/Se/Cu-stack optimized for direct illumination from both sides [25] (without graphite box) and three advanced precursor stacks in order to obtain improved crystalline properties using the graphite box. The total film thickness was of the order of 1.5 lm. Secondary phases as Cu Se were removed by chemical etching (10% 2~x KCN for 3 min). X-ray diffraction patterns of the CuGaSe thin films were acquired 2 using a Siemens D5000 diffractometer (CuK -radiation). They indicated polycrystala1 line single-phase thin films. The morphology and composition of the films were

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studied using a scanning electron microscope (SEM, JSM-840A, Jeol) and energydispersive X-ray analyses (EDX). With respect to solar cell applications Mo-coated (1 lm thickness) floating glass was mainly used as a substrate material, but also Si (1 0 0) and Si (1 1 1) could be interesting as substrate material with respect to tandem-cell applications with Si. Furthermore, the growth of CuGaSe thin films on 2 floating glass, Al O (0 0 0 1), and MgO (1 0 0) was investigated. A 40 mW krypton 2 3 ion laser (Spectra Physics model 165 ion laser) at wavelengths of 629, 568, and 476 nm was used as excitation source for steady-state PL-measurements. The laser beam was focused onto the sample with a diameter of about 100 lm and the luminescent light was detected by a GCA/McPherson Instrument 1 m scanning monochromator (Czerny—Turner type) and a photomultiplier of S1 characteristics or a liquid-nitrogen-cooled Ge-detector (North Coast EO-817L), respectively. In order to investigate the homogeneity of the films two different investigation methods were used. On the one hand, different excitation energies of the laser and therefore penetration depths were used to obtain depth profiles and, on the other, CuGaSe samples of the same 2 batch were etched in a solution of bromine and methanol for different times and for thicknesses ranging from 0.1 to 2 lm. Since the excitation intensities of the red (629 nm) and the green—blue (476 nm) line were limited, the investigations of depth profiles were mainly focused on measurements of etched samples with excitation at 568 nm. Additionally, PL excitation through the substrate (glass, MgO, Al O ) was 2 3 performed to examine the properties close to the substrate/CuGaSe interface. 2 3. Results and discussion Fig. 1 shows PL-spectra of polycrystalline CuGaSe thin films grown on different 2 substrates recorded at 40 K. The influence of the different substrates on the PLproperties of the CuGaSe films is obvious. Furthermore, the films prepared from the 2 conventional precursor stack showed some inhomogeneities. The PL-emissions of the CuGaSe thin films can be classified as follows: (i) emissions above the band gap of 2 CuGaSe (above 1.72 eV) due to Ga-rich phases, (ii) band-to-band or excitonic 2 emissions (1.72 eV), (iii) emissions due to V - and V -levels (1.55—1.68 eV), (iv) emisC6 S% sions involving Ga or Ga (1.40—1.55 eV), and (v) emissions due to deep levels * C6 (below 1.4 eV). All films showed an increased PL-intensity above the band gap of CuGaSe close to 2 the substrate/CuGaSe interface, as shown in Fig. 2. Similar emissions have been 2 observed in Ga-rich epitaxial films [18]. Since Ga and Se layers were at the bottom of the precursor stack, we assume the formation of gallium selenides like GaSe or Ga Se [7], although these phases could not be detected by conventional X-ray 2 3 diffraction measurements. The films grown on MgO-substrates exhibited some sharp peaks in the range between 1.7 and 2.0 eV. Similar radiative transitions have been found for Cu-doped GaSe [26,27]. Some films showed emissions close to the band gap of CuGaSe . At low temperatures the emission line could be resolved to consist of two 2 peaks. Fig. 3 shows these radiative transitions of a film grown on Al O at 2 K. The 2 3 peak at 1.720 eV showed a superlinear dependence on excitation power and is

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Fig. 1. PL-spectra of CuGaSe films grown on different substrates at 40 K. 2

Fig. 2. PL-emissions above the band gap of CuGaSe due to Ga-rich phases. 2

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Fig. 3. Band-edge emissions of CuGaSe grown on Al O at 2 K (FX: free exciton, B—B: band-to-band 2 2 3 transition).

therefore attributed to the decay of the free exciton. Band-to-band recombination is assumed for the transition at 1.725 eV. The result for the binding energy of the free exciton of 5 meV is in good agreement with data reported in the literature [28]. In comparison to single crystals these two peaks are shifted to lower energies which could be attributed to internal stress in these films [17]. The emissions in the range between 1.55 and 1.68 eV can be described using the model of Masse´ [23]. This is based upon investigations on single crystals and contains an acceptor level (50 meV, Cu-vacancy) and two donor levels (80 and 110 meV, Se-vacancy). Therefore five transitions could occur in this spectral range: three free-to-bound transitions (1.675 eV, V /1.645 eV, V /1.615 eV/V ) and two donor—acceptor-pair transitions C6 S% S% (1.62 and 1.59 eV assuming a coulomb interaction of approximately 25 meV). The intensity of the V -related emissions increased close to the substrate (for all different S% substrates), which is shown for films grown on Mo-coated soda-lime glass in Fig. 4. The reason for this lack of Se near the interface is not clearly understood. In the fourth spectral range, between 1.40 and 1.55 eV, a broad emission was detected. The position of the peak shifted to smaller energies with decreasing excitation power, which is a clear sign of a donor—acceptor-pair transition. Single crystals annealed in Gaatmosphere at 600°C showed similar broad emissions in this range [24,29]. They were attributed to Ga on interstitial lattice sites rather than Ga-antisite defects. This is in

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Fig. 4. V -related emissions of films grown on Mo as function of thickness (as grown and etched) at 40 K. S%

good agreement with the increase of these emissions close to the surface (Fig. 5), where the Ga/Cu ratio is larger than one. Measurements with the photomultiplier showed a broad, deep emission at approximately 1.3 eV. Similar emissions have been found in CuGaSe single crystals and were attributed to the incorporation of the transport 2 agent iodine [20]. Since the thin films were grown without iodine this interpretation cannot hold for these deep states. Because of the very low sensitivity of the S1photomultiplier in this spectral range further investigations were carried out using a liquid-nitrogen-cooled Ge-detector. These measurements revealed that this broad band consists of at least two transitions (Fig. 6). Excitation intensity dependent measurements indicated the character of donor—acceptor-pair transitions. The PLintensity of these peaks increased from the “bottom” to the “top” of the film, a region which should show Cu excess due to the initial precursor stack. Therefore, the origin of these transitions could be deep Cu-related defect levels like Cu or Cu . This deep * S% emission line vanished in vacuum-annealed single crystals [20]. A possible explanation could be a thermal healing of the defects due to the following reactions: Cu #V PCu , * C6 C6 Cu #V PCu #V . S% C6 C6 S%

(1) (2)

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Fig. 5. Broad donor—acceptor-pair transition for films grown on MgO (excited from front and back).

These processes could explain the reduction of the acceptor density (V ) and inC6 creased formation of Se-vacancies due to vacuum annealing reported in the literature [23,30]. X-ray diffraction measurements also clearly revealed different structures of the films grown on different substrates, as shown by PL-measurements (see Fig. 1). All samples showed a preferred (1 1 2)-orientation, but the diffracted X-ray intensity and the relative ratio of the (1 1 2)-peak to the other peaks were higher for thin films grown on single-crystalline substrates than for films grown on glass. The strongest texture was seen for samples grown on (0 0 0 1)-Al O and (1 1 1)-Si. This could be ascribed to the 2 3 fact that the atoms on the (1 1 2)-surface of CuGaSe have as nearly a hexagonal 2 surrounding as the hexagonal surface structure of these substrates. Furthermore, the average grain size of the films G was approximated from XRD-measurements indicating the largest grains for films on Al O . If one takes the half-width of the PL2 3 emissions and the intensity of the band-edge emission as a direct measure for the crystalline quality, the results are in good agreement with the XRD-investigations indicating best crystalline properties for films grown on single-crystalline substrates, especially Al O , similar results for CuGaSe on Mo, and broad emissions and small 2 3 2 grains for thin films on floating glass. SEM-micrographs also clearly revealed a more

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Fig. 6. Deep-level luminescence of CuGaSe grown on glass at 40 K measured with a Ge-detector. 2

compact structure for films grown on sapphire than on glass, as shown in Fig. 7. In addition, the lattice constants were calculated using a least-squares fit [31]. The results are shown in Table 1. Although solar cells prepared on CuGaSe layers grown on Mo reached efficiencies 2 up to 3%, there is need for further optimization, since the results of the PLmeasurements showed, that all thin films prepared from the precursor optimized for illumination from both sides exhibited some gradient in composition. Since the low annealing temperature and the short annealing time are two advantages of the RTP-method, we tried to keep the RTP-parameters constant and to improve the precursor structure for the application of the graphite box. However, the number of elemental layers was kept as low as five to ensure experimental simplicity. Mo-coated floating glass was used as substrate material. In order to reduce the Ga-rich emissions near the interface two advanced precursor stacks were used: (a) substrate//Ga/ Se/Cu/Se/Ga and (b) substrate//Ga/Se/Cu/Ga/Se. Since the diffusion of Cu is the fastest diffusion process, this should ensure a better homogeneity of the Cu/Ga ratio throughout the films. Fig. 8 shows the PL-spectra of the films grown by the conventional and the two modified precursor stacks. Due to the upper Ga-layer of stack (a) emissions above the band gap were also detected on the top of the film. In contrast

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Fig. 7. SEM planar view micrographs for CuGaSe grown on Al O (a) and glass (b). 2 2 3

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Table 1 Lattice constants of CuGaSe films grown on different substrates compared with the values for melt grown 2 crystals (JCPDS 35-1100 [33]) Substrate

a (A_ ) 0

c (A_ ) 0

c /a 0 0

Glass Mo MgO (1 0 0) Al O (0 0 0 1) 2 3 Si (1 1 1) Si (1 0 0) JCPDS-standard

5.621 5.619 5.625 5.618 5.618 5.618 5.612

11.014 11.100 11.023 11.001 11.021 11.002 11.032

1.960 1.959 1.960 1.958 1.962 1.958 1.966

Fig. 8. Comparison of the PL-spectra of films grown from conventional and modified (a/b) precursors.

to that films prepared by precursor (b) did not show any emissions in this range. In addition, the emissions close to the interface were suppressed. The difference between the spectra of both modified films could be due to the formation of Cu—Ga alloys during the evaporation of Ga onto the Cu layer. Therefore, the Cu/Ga gradient was significantly reduced. Moreover, the amount of crystalline selenium prior to annealing was much higher for stack (a). This could lead to less diffusion of Cu resulting in

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Fig. 9. SEM planar view micrographs for CuGaSe grown from precursor (a) and (b). 2

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Fig. 10. Comparison of the PL-spectra of films grown from the conventional and modified precursors (type c).

a Cu/Ga gradient throughout the sample. Fig. 9 shows SEM-micrographs of films grown from precursor (a) and (b). Stack (a) showed much smaller crystallites, which is typical for Ga-rich films [32]. Furthermore, the conventional precursor was changed to overcome the increased V -related PL-intensity close to the interface. This was done by increasing the S% thickness (1.5 times) of the first elemental Se layer resulting in the stack: (c) Ga/Se(1.5)/Ga/Se/Cu. Fig. 10 shows the PL-spectra of conventional and modified CuGaSe films from the front and close to the interface. The significant reduction of 2 the Se-related emissions in the modified film structure is obvious and, furthermore, more or less no inhomogeneity regarding the Se-content was detected. These results obviously show the improvement of the quality of the CuGaSe layers due to the 2 modification of the precursor based on the better knowledge of the defects. 4. Conclusion and outlook This study showed that PL-measurements is a powerful tool to investigate homogeneity and structure of CuGaSe thin films grown by RTP on different substrates. 2

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The analyses of the PL-spectra revealed five spectral ranges. They were identified as emissions from Ga-rich phases (above 1.75 eV), band-edge emissions, emissions involving shallow V (50 meV) and V (80 and 110 meV) levels. Furthermore, a broad C6 S% band at energies from 1.40 to 1.55 eV was ascribed to a donor—acceptor-pair transition involving Ga and V . A model to explain emissions from deep levels * C6 (below 1.4 eV) was presented. All films grown from the conventional precursor stack (substrate//Ga/Se/Ga/Se/Cu) showed inhomogeneities of their PL-properties. This was reduced by the modification of the precursor stack. Investigations on the growth on different substrates revealed the best crystalline properties for films grown on sapphire (0 0 0 1) substrates. In contrast to films grown on floating glass the difference to CuGaSe on Mo was rather small. Therefore, Mo-coated floating glass appears to 2 be very good and cheap material for substrates of thin film solar cells. Future experiments will focus on the correlation between precursors, PL- and photovoltaic properties. Since the crystalline properties of these films could be improved, we are hopeful that solar-cell performance can be improved based on our findings.

Acknowledgements The authors gratefully acknowledge financial support by the Energiestiftung Baden-Wu¨rttemberg (Contract No. A 5193).

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