Anion vacancies in CuInSe2

Thin Solid Films 387 Ž2001. 129᎐134

Anion vacancies in CuInSe 2 S. Niki a,U , R. Suzuki a , S. Ishibashi a , T. Ohdairaa , P.J. Fons a , A. Yamadaa , H. Oyanagi a , T. Wadab , R. Kimurac , T. Nakadad a

Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, lbaraki 305-8568, Japan b Ryukoku Uni¨ ersity, Seta, Otsu, Shiga 520-2194, Japan c Teikyo Uni¨ ersity Science and Technology, 2525 Yatsuzawa, Kitatsuru, Yamanashi 409-0193, Japan d Aoyama Gakuin Uni¨ ersity, 6-16-1 Chitosedai, Seatagaya, Tokyo 157, Japan

Abstract Effects of the Cu 2 ᎐ x Se surface phase, the post-growth air-annealing and the Na incorporation on the growth and properties of CuInSe 2 films have been systematically investigated by various defect-sensitive characterization techniques such as low temperature photoluminescence and positron annihilation. The presence of the Cu᎐Se surface phase, the post-growth air-annealing and the Na incorporation all provided significant changes in photoluminescence spectra. Decrease in positron lifetime and reduction of twin density were found to occur simultaneously, along with the changes in photoluminescence spectra. Change in photoluminescence spectra and the corresponding decrease in positron lifetime indicate the annihilation of Se-vacancies; the control of Se-vacancy is a key issue to be addressed for improving the electrical, optical and structural properties of CuInSe 2 films. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Photovoltaic material; CuInSe 2 ; Defects; Molecular beam epitaxy; Photoluminescence; Positron annihilation

1. Introduction CuInSe 2 ŽCIS.-based solar cells have attracted much attention for high-efficiency low-cost thin-film solar cells and a significant improvement in solar cell performance with conversion efficiencies of ; 19% has been reported w1x. The characteristics of CIS-based solar cells were known to improve by various techniques such as the use of Cu 2 ᎐ x Se second phase w2x, Na interdiffusion from the soda lime glass substrate w3x and the post-growth air-annealing w4x. The cause of such effects has been considered to be strongly related to defects in CIS and a great deal of effort has been focused on the understanding of the defects in bulk and at interface w5,6x. Kronik et al. w5x investigated the

U

Corresponding author. Tel.: q81-298-61-5610; fax: q81-298-615615. E-mail address: [email protected] ŽS. Niki..

chemical effects of oxygenation of the CuInGaSe 2 ŽCIGS. interface and showed that the passivation of Se-vacancies and Cu removal are involved. Rau et al. w6x, investigated the reversible changes of electrical properties of CIGS-based heterojunctions and the persistent conductivity were found to be the origin of the reversible change. In our previous work, we have demonstrated reproducible growth of high-quality CIS epitaxial films by molecular beam epitaxy ŽMBE. and investigated the properties of defects in the films by means of positron annihilation ŽPA. technique in addition to conventional characterization techniques such as photoluminescence ŽPL. spectroscopy, electrical characterization, etc. Cuvacancies were found to be dominant in the films grown under Cu-excess conditions ŽCu-excess films., while Se-vacancies were formed and Cu᎐Se divacancies became dominant when grown under In-excess conditions ŽIn-excess films.. A significant change in film quality has been observed though CurIn ratios of the

0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 7 1 8 - 1

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In-excess films were found to be similar to those of the KCN-treated Cu-excess films. Based upon such experimental results, we believe that the suppression of Sevacancies is a critical issue to be addressed for improving the film quality. In this work, effects of the Cu 2 ᎐ x Se second phase, the post-growth air-annealing and the Na incorporation will be discussed in terms of Sevacancies. 2. Cu 2 – x Se second phase 2.1. Presence of surface Cu 2 ᎐ x Se phase Four MBE-grown Cu-rich specimens with molecularities ŽCurIn ratio. of m s 2.1᎐2.6 were dipped in a 5-wt.% KCN aqueous solution at room temperature for 0.5᎐3 min. The detailed description of the MBE growth conditions can be found elsewhere w7x. The results of electron micro-probe analysis ŽEPMA. indicated the compositions of all the as-grown films changed drastically from m s 2.1᎐2.6 Žheavy Cu-excess. to m s 0.92᎐0.93 Žslight In-excess .. The molecularity did not change with increasing etching time, suggesting that most of the surface phase can be removed within 30 s and that the Cu᎐Se phase is formed near the surface. The van der Pauw measurements at RT showed the resistivity of the as-grown films drastically increased with increasing etching time. The Hall voltages of the as-grown films were found to be too small and were beyond the detection limit of our system, suggesting hole concentrations of the as-grown films to be p4 1 = 10 19 cmy3 . The hole concentration decreased with increasing etching time, and the 1-min KCN-etched sample showed a modest hole concentration of ps 7.5 = 10 16 cmy3 with a mobility of 23 cmrŽVrs., values suitable for solar cell applications. The low resistivity of the as-grown films was considered to be due to the surface Cu 2 ᎐ x Se phase, not due to the intrinsic properties of the Cu-excess CIS film. No change in the PL spectra was observed before and after 1 min of KCNetching except for a slight enhancement of a broad emission centered at Es 0.80᎐0.85 eV and of the excitonic emissions at Es 1.03᎐1.04 eV. This suggests that the sharp PL emissions represent the intrinsic properties of CIS and not those of the Cu᎐Se phase. No PL emission was observed from Cu 2 ᎐ x SerGaAs also supporting those emissions being associated with the intrinsic defects in CIS. AFM images of a CIS film with m s 1.4 before and after a 1-min KCN-etching are shown in Fig. 1a,b, respectively. A large number of dark spots were observed in the as-grown films that were removed by KCN-etching, leaving numerous holes with depths of 200᎐300 nm. The dark spots observed in the as-grown films are believed to be Cu᎐Se grains, this also shows that the Cu᎐Se grains are not uniformly distributed.

Fig. 1. Atomic force microscopic images of a CuInSe 2 film with m s 1.4 Ža. before and Žb. after 1 min KCN-etching.

Lattice constants determined by X-ray diffraction indicated that the Cu᎐Se grains are most likely the low temperature monoclinic Cu 1.5 Se phase grown epitaxially on the CIS film w8x. In-situ RHEED analysis showed that the diffraction patterns specific to the chalcopyrite structure became weaker with increasing thickness, though they were observed throughout the growth. A significant fraction of indium on the surface of the as-grown film was identified by means of X-ray photoemission spectroscopy w9x. The above results indicate that the surface of CIS is not fully covered by the thick Cu 2 ᎐ x Se grains visible in the AFM images. Fons et al. observed wedge-shaped Cu 2 ᎐ x Se grains in the Cuexcess CIS epitaxial layers by cross-sectional TEM in good agreement with our AFM results w10x. The volume fraction of the surface Cu᎐Se phase was estimated to be several percent from the acceleration energy dependence of molecularity by means of EPMA. The low resistivity of the as-grown films, despite a partial coverage of the CIS surface by the Cu 2 ᎐ x Se grains strongly suggests that a thin Cu-excess layer ŽCuSe or Cu-excess CIS. with its thickness no more than several monolayers Žless than the vertical penetration depth of the RHEED electron beam. co-exists with the Cu 2 ᎐ x Se the as-grown films. 2.2. Effects of Cu᎐Se Fig. 2a,b shows typical PL spectra and positron lifetime obtained from CIS films with various molecularities. Well-defined emission lines were observed from the Cu-excess films, while the In-excess films showed broad peaks. A significant blue-shift of a broad peak at E; 0.85 eV with increasing excitation power indicates that the In-excess films were heavily compensated consistent with the results of electrical characterization which showed that the films became highly resistive for molecularity less than unity. The positron lifetime in the bulk and various defects were calculated theoretically using the superposed-neutral-atom model w11x modified for semiconductors w12x. The positron lifetime for CIS grown under Cu-excess condition was ␶ ; 270 ps, while the lifetime increases with decreasing CurIn

S. Niki et al. r Thin Solid Films 387 (2001) 129᎐134

Fig. 2. Ža. Photoluminescence spectra and Žb. positron lifetime of CuInSe 2 films as a function of CurIn ratio.

ratio for films grown under an In-excess. The lifetime values for films grown under an In-excess with good variance of the fit of single component analysis indicates that a high density of vacancy-type defects exists and most of the positrons are trapped in the same kind of defects w13x. The results of PL and positron annihilation can be categorized into three groups as shown in Table 1. The origin of the intrinsic defects which dominates the properties of the CIS films have been identified in a consistent manner primarily by means of PL spectroscopy and positron annihilation techniques as follows w14x. High-quality CIS films with photoluminescence spectra dominated by a conduction band to acceptor emission have been grown under Cu-flux excess conditions. The acceptor-type defects were assigned to Cu-vacancies Ž VCu . though the films were grown under Cu-flux excess conditions: Ž1. the measured positron lifetime Ž␶ ; 270 ps. is similar to those calculated for single vacancies; Ž2. the p-type conductivity of the films indicates the defects are cation vacancies; and Ž3. the compositions of the CIS films after selective etching of the Cu 2 ᎐ x Se surface phase were found to be slightly In-rich Ž m s 0.90᎐0.93.. On the other hand, films grown under In-flux excess conditions tend to be heavily compensated with their photoluminescence spectra dominated by broad Table 1 Correlation between the positron lifetime and photoluminescence Category

A B C

Positron lifetime Žps. 270᎐280 280᎐300 G 300

Photoluminescence

Sharp emission lines wD, Ax␣ dominant or co-existence of wD, Ax␣ and wD, Ax␤ wD, Ax␤ dominant

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donor᎐acceptor pair emissions. Our model which associates the presence of Cu᎐Se divacancies Ž VCu ᎐ Se . in the as-grown In-rich films and the subsequent annihilation of VSe by oxygen during air-annealing is consistent with the following results: Ž1. the measured positron lifetimes Ž␶ s 290᎐320 ps. are similar to those calculated for divacancies; Ž2. the photoluminescence spectrum changed drastically after air-annealing and became similar to those of films grown under Cu-flux excess conditions indicating the presence of VCu in the as-grown films and the annihilation of donor-type vacancies during air-annealing; Ž3. the decrease in positron lifetime of ; 20 ps observed after air-annealing was consistent with the proposed model of oxygen annihilating the anion vacancies Ž VCu ᎐ Se ª VCu y O Se .; and Ž4. a significant increase in oxygen concentration after air-annealing was confirmed by secondary ion mass spectroscopy. Results related to the post-growth annealing will be discussed in more detail in the next section. The Cu-excess CIS films with the as-grown CurIn ratios of m G 1.0 were found to become slightly In-excess compositions of m s 0.90᎐0.93 after KCN-etching, indicating that the molecularities of CIS films are Inrich regardless of the growth conditions. However, the film grown under Cu-excess conditions are superior to those grown under In-excess conditions. It is clear that the Cu᎐Se second phase covering the surface of CIS suppresses the formation of Se-vacancies ŽCu᎐Se divacancies.. Transmission electron microscopy and RHEED analysis showed that the Cu-excess films are almost twin free, implying there is a correlation between the formation of Se-vacancies and twins. 3. Effects of post-growth air-annealing Films grown under both a Cu-excess ŽCu-excess films. and an In-excess ŽIn-excess films. were used in this work and all films were grown by molecular beam epitaxy ŽMBE. on GaAs Ž001. at a substrate temperature of Ts s 450⬚C. Post-growth annealing experiments as a function of temperature have been carried out in various atmospheres in order to understand the behavior of the defects. Fig. 3 shows the PL spectra as a function of annealing temperature for Ža. m s 1.04 and Žb. m s 0.80. No significant change in PL spectra was observed for the film with m s 1.04. On the other hand, the intensity of PL emission decreases with increasing annealing temperature for CIS with m s 0.80 and the PL spectrum annealed at 400⬚C became similar to those of the films with m s 1.04. This indicates that CIS grown under either a Cu- or an In-excess has the same acceptor-type defect; Cu-vacancies. Post-growth annealing of the CIS films in different gas atmospheres such as nitrogen and argon or in vacuum, did not exhibit similar change of

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PL spectrum, suggesting that oxygen is responsible for the effects. A significant change of ; 20 ps in positron lifetime was also observed as shown in Fig. 2b. A significant change in PL and a decrease in positron lifetime have simultaneously occurred, suggesting the presence of Se-vacancy related defects and the annihilation of such defects by oxygen as discussed in the previous section. The structural properties of the CIS films were characterized by a Philips MRD X-ray diffractometer. The twin density was evaluated using the ratio between the integrated intensity of Ž1, y1, 2.␶ with respect to Ž1, y1, 2. diffraction peak using the Gaussian approximation for curve fitting. Structural properties of the CIS films have been also investigated. Fig. 4 shows a change in the ratio Ž r . between the integrated intensity of Ž1, y1, 2.␶ with respect to that of Ž1, y1, 2. after the post-growth annealing in air and Ar᎐H 2 ŽAr 96%, H 2 4%.. Air-annealing effectively reduced the twin density than the Ar᎐H 2 annealing and r was reduced to approximately 20% of the asgrown samples despite the fact that the annealing temperature of Ts s 400⬚C is below the growth temperature of Ts s 450⬚C. In addition to a decrease in twin density, changes in crystalline quality in terms of linewidth and integrated intensity of CIS Ž008. were investigated by using a GeŽ220. monochromator. The integrated intensity increased for all the samples after annealing in both air and Ar᎐H 2 . The linewidth decreased approximately 20% for the samples annealed in air, while no clear change in linewidth was confirmed for the sample annealed in Ar᎐H 2 . Such a significant decrease in linewidth indicates the reduction of mosaic and an increase in integrated intensity indicates the reduction of twin density. The results are consistent with those discussed in the effects of Cu 2 ᎐ x Se surface phase.

Fig. 3. Photoluminescence spectra as a function of annealing temperature in air Ža. CurIn s 1.04 and Žb. CurIn s 0.80.

Fig. 4. Change in the ratio Ž r . between the integrated intensity of Ž1, y1, 2.T with respect to that of Ž1, y1, 2. after the post-growth annealing in air and Ar᎐H 2 .

4. Effects of Na incorporation CIS films were deposited on Corning 7059 glass substrates by a co-evapolation method. A Cu-rich layer was first deposited at a CurIn flux ratio of 1.28 at a substrate temperature of Ts s 500⬚C, then In-rich layer was deposited at a CurIn flux ratio of 0.43 at Ts s 550⬚. The CurIn ratio was measured by quartz monitors. In the Na-incorporated samples, 10 mg of Na 2 Se Ž99.9%. were evaporated during the first stage of CIS deposition. The samples were not rinsed with distilled water for removing Na since, no difference in resistivity before and after the rinsing was found. An 0.3 at.% of Na incorporation is expected for 10 mg of Na 2 Se. Fig. 5 shows the PL spectra of the CIS films with and without Na 2 Se measured at 2 K. The line shape and emission energies are similar to those reported for In-rich CIS films w15x. The samples grown without Na 2 Se incorporation are shown as solid lines. A peak labeled wD, Ax ␣ appeared for the films with m s 0.94 and a peak labeled wD, Ax ␤ appeared for the films with m F 0.90. The significant energy difference Žapproximately 80 meV. between wD, Ax ␣ and wD, Ax ␤ suggest that different radiative recombination processes are involved for these two emissions. The slight difference

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in the emission energies of the films with m F 0.90 can be attributed to the CurIn ratio dependence of the band-gap energy w16x. On the other hand, for the CIS films with Na 2 Se Žshown in dotted lines in the figure., only wD, Ax ␣ was observed regardless of the CurIn ratio, with a small peak energy variation. Both wD, Ax ␣ and wD, Ax ␤ disappeared when annealed in air thus, both of these donor related defects were considered to be associated with Se-vacancies because of a significant indiffusion of oxygen during air-annealing w15x. A dominant donor-type defect for the film with m s 0.94 is considered to be Se-vacancy whereas, that of the CIS film with m F 0.90 correspond to Cu᎐Se divacancies w11,13x. The resistivity of the CIS films was measured in order to investigate the relationship between the Na incorporation and electrical properties, with an attempt to correlate the relation with the PL results. Fig. 6 shows the measured resistivity of the samples discussed above. The solid squares represent CIS without Na 2 Se and open squares represent CIS with Na 2 Se. The CIS films with Na 2 Se showed lower resistivity than those without Na 2 Se for the whole CurIn range, suggesting that the incorporation of Na 2 Se suppressed carrier compensation, i.e. of the formation of the donor-type

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Fig. 6. Resistivity of CuInSe 2 films with Na 2 Se Žopen square. and without Na 2 Se Žsolid square..

defects, VSe and VCu ᎐ Se . These results were consistent with those obtained from Kronik et al. w17x. The films with resistivities below Žabove. 2 = 10 3 ⍀ cm predominantly showed wD, Ax ␣ , ŽwD, Ax ␤ . emission, implying that the degree of carrier compensation can be estimated by the energies of predomonant PL emissions. 5. Summary The presence of the Cu᎐Se surface phase, the postgrowth air-annealing and the Na incorporation all were found to suppress the Se-vacancy formation. Control of Se-vacancy related defects such as VSe and VCu ᎐ Se , were found to be critical for improving structural, electrical and optical properties of CIS ŽCIGS. films. Development of the methods to in-situ control anion vacancies can lead to further improvement of the solar cell performance and to simpler and more reliable solar cell fabrication processes. References

Fig. 5. Photoluminescence spectra of CuInSe 2 films with Na 2 Se Ždotted line. and without Na 2 Se Žsolid line..

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