Surface vs. volume stoichiometry of MBE grown CuInS2 films on Si

Thin Solid Films 431 – 432 (2003) 317–320

Surface vs. volume stoichiometry of MBE grown CuInS2 films on Si Wolfram Calvet*, Hans-Joachim Lewerenz, Christian Pettenkofer Hahn-Meitner-Institut Berlin, Glienicker Straße 100, 14109 Berlin, Germany

Abstract Molecular beam epitaxy CuInS2 layers were grown on Si single crystals of different orientation. The stoichiometry was varied between 0.5 and 1.5 for the nominal CuyIn ratio and the film thickness was limited to 150 nm. Composition analysis was done by in situ photoelectron spectroscopy (PES), Rutherford backscattering (RBS) and X-ray diffraction (XRD). PES was used to determine the surface composition, RBS monitored the volume contribution and XRD probed phase formation. It was found that the surface composition differed significantly from that of the volume: instead of the expected variation along the Cu2S–In2S3 binary, the actual composition varied along the Cu2 S–InS line at the surface (S-deficiency). The role of sulfur incorporation into the growing film is discussed. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Chalcopyrite; CuInS2; MBE; Solar cell

1. Introduction Recently, the investigation and development of thin film solar cells based on chalcopyrite absorber material of the type Cu–IIIa–VIa2 (IIIasIn, Ga, and VIasS, Se) is in the focus of interest. Solar-to-electrical conversion efficiencies of approximately 18% have been obtained for Cu(In,Ga)(S,Se)2 cells w1x promising economic fabrication perspectives for terrestrial photovoltaic applications. In contrast to Cu, In, Ga, S and Se containing CIGSSe cells, devices without Ga and Se (CIS) have not yet exceeded 13% w2,3x. Comparably low photovoltages indicate non-ideal heterojunctions with lower contact potentials than expected. Until now this behaviour is unclear and has not been clarified in detail. One possible attempt to get more information about the influence of the growth conditions on film formation is the use of molecular beam epitaxy (MBE) as preparation method. In Refs. w4,5x it has been successfully demonstrated that heteroepitaxy of the Si(1 1 1)y CuInS2 system is possible. Besides the expected chalcopyrite structure of CuInS2 the polytypic CuAu-structure has been observed w6x as well as the *Corresponding author. Tel.: q49-30-80-62-25-75; fax: q49-3080-62-24-34. E-mail address: [email protected] (W. Calvet).

presence of sphalerite ordered film portions cannot be excluded. In this context the following aspects concerning the growth of thin crystalline films must be regarded: (i) point symmetry of substrate and deposit, (ii) lattice mismatch between substrate and deposit and (iii) growth conditions influencing interface and surface chemistry. Thus, for a better understanding of crucial parameters in the preparation of epitaxial CuInS2 films or polycrystalline CuInS2 absorbers for photovoltaic solar cells it is necessary to analyse these aspects in more detail. The determination of electronic, compositional and structural characteristics of the deposited CuInS2 films with photoelectron spectroscopy (PES) and low energy electron diffraction (LEED) methods in combination with MBE techniques therefore appears appropriate w7x. 2. Experimental 2.1. Preparation Thin CuInS2 films were deposited by simultaneous evaporation of Cu, In and S from elemental sources in a standard MBE process. As substrates w1 1 1x-, w1 1 0xand w1 0 0x-orientated single crystalline silicon wafers of 10=12 mm2 size and different p-doping level (1015 – 1017 cmy3) were used. Before transfer into the ultra high vacuum (UHV) system the substrates were treated wet chemically with HF containing solutions w8,9x.

0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00160-3

W. Calvet et al. / Thin Solid Films 431 – 432 (2003) 317–320

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Typical deposition parameters were: growth temperature ˚ sy1 and growth time 30 520 8C, growth rate below 1 A min resulting in approximately 150 nm thick CuInS2 films. Additionally, the cation ratio Iny(InqCu) was varied by adjustment of the In or Cu flux while the sulfur flux was kept constant. The working pressure in the MBE-chamber during growth was 10y5 h Pa determined by the partial pressure of sulfur. The CuInS2 preparation started after the substrate has been heated within 10 min to the growth temperature by first opening the S-shutter and then opening the Inand Cu-shutters simultaneously. After deposition, the substrate was cooled down within 10 min to 200 8C and subsequently transferred under UHV-conditions into a surface analysis chamber. 2.2. Characterisation The CuInS2 films were characterised in situ by PES and LEED. Due to the small escape depth of electrons both methods provide information on the surface region up to a depth of a few angstrom. The PES system consists of a Leybold LHS EA 11 analyser with a Mg Ka X-ray source (hns1253.6 eV) for chemical analysis and an ultraviolet light source (hns21.2, 40.8 eV) for valence band spectroscopy (UPS). As LEED system a standard Omicron NLG 10 optic was used to get structural information of the surface. Ex situ characterisation was performed by standard X-ray diffraction (XRD) and Rutherford backscattering (RBS) using Heq projectiles of 1.8 MeV energy. In contrast to the used in situ methods these methods are bulk sensitive. 3. Results The results in this paper are dealing with structural aspects concerning epitaxial relations and compositional aspects concerning surface and volume. Table 1 shows

Fig. 1. LEED image of a near stoichiometric epitaxial CuInS2 film prepared on Si(1 1 1). The kinetic energy of the impinging electrons was 60.5 eV.

a compilation of epitaxial relations for various substrate orientations and preparation regimes derived from XRD and LEED measurements. On the Cu-rich side surface and volume of the CuInS2 films are mainly {1 1 2} orientated independent on the substrate orientation (indices are related to the tetragonal system for CuInS2 and to the cubic system for Si). Whereas on the In-rich side the epitaxial relations are directed by the concept of corresponding planes with additional formation of {1 1 2} facets on Si(1 1 0) and Si(1 0 0). The near stoichiometric range displays the transition from the Curich to the In-rich side showing a (2=1) CuInS2 surface on a Si(1 1 1) substrate (Fig. 1). The surface composition was determined with XPS evaluating Cu 2p3y2, In 3d5y2 and S 2p core level intensities. Fig. 2 depicts a set of these emission lines for three samples with varying CuyIn-ratio (adjusted through the In source temperature TIn). All peaks are

Table 1 Compilation of epitaxial relations derived from XRD-measurements for various CuInS2 films prepared on differently orientated Si substrates dependent on the CuyIn-ratio Substrate

Regime of preparation Cu-rich

Near stoichiometric

In-rich

Si(1 1 1)

Si{1 1 1}NNCuInS2{1 1 2} CuInS2(1 1 2)(1=1)

Si{1 1 1}NNCuInS2{1 1 2} CuInS2(1 1 2)(2=1)

Si{1 1 1}NNCuInS2{1 1 2} CuInS2(1 1 2)(1=1)

Si(1 1 0)

Si{1 1 0}NNCuInS2{1 1 2}

Si{1 1 0}NNCuInS2{1 1 0} Si{1 1 0}NNCuInS2{1 1 2} CuInS2(0 1 2)(1=1)

Si{1 1 0}NNCuInS2{1 1 0} Si{1 1 0}NNCuInS2{0 1 2} CuInS2(0 1 2)(1=1)

CuInS2(1 1 2)(1=1) Si(1 0 0)

(1 1 2)-,(1 1-2)-faceted

Si{1 0 0}NNCuInS2{1 1 2}

Si{1 0 0}NNCuInS2{1 0 0} Si{1 0 0}NNCuInS2{1 1 2}

No LEED pattern

CuInS2(0 0 1)(1=1)

The appropriate surface mesh was measured by LEED-experiments.

(1 1 2)-,(1 1-2)-faceted

Si{1 0 0}NNCuInS2{1 0 0} Si{1 0 0}NNCuInS2{0 0 2} CuInS2(0 0 1)(1=1) {1 1 2}-faceted

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Fig. 2. XP-spectra of Cu 2p, In 3d and S 2p core levels for three epitaxial CuInS2 films with different CuyIn ratio (adjusted through the In source temperature TIn) grown on Si(1 1 1). All peaks are background subtracted and the binding energies refer to the onset of the valence band which depends on the CuyIn ratio as well.

background subtracted and the binding energies refer to the onset of the valence band. The evaluation of the independent molecular abundances wCux y(wCuxqwInx) and wCux y(wCuxqwSx) was performed using the tabulated sensitivity factors of the Leybold LHS EA 11 analyser. These factors include the photoionization cross-section, the analyser transmission function and the inelastic mean free path which depend on the kinetic energy of the emitted photoelectrons. The sensitivity factors were cross checked for CuS and InS prepared in the same system. Additionally, the spectra in Fig. 2 show that with increasing In content the In 3d peaks shift slightly to higher binding energies while the Cu 2p and the S 2p peaks stay nearly fixed. In contrast to the surface composition the bulk composition has been analysed with RBS. In Fig. 3 a typical spectrum of a CuInS2 film on Si is shown. It is obvious

Fig. 3. RBS-spectrum of a near stoichiometric epitaxial CuInS2 layer with a thickness of approximately 130 nm and a roughness of approximately 20 nm. The simulation was performed with the software package RUMP.

that within the Cu–In–S system, RBS analysis can be applied with high accuracy as each elemental peak is well separated. Fig. 4 represents the Gibb’s phase triangle of the Cu– In–S system. The surface and the volume composition of approximately 30 epitaxial films with variable Cuy In-ratio is shown based on XPS- and RBS-measurements. Orientation dependencies are intentionally neglected since it has been shown above that the main growth direction of the CuInS2 films is mainly along w1 1 2x. In the phase triangle it is clearly seen that there are differences between the phase formation on the surface following the Cu2S–InS cut and the phase

Fig. 4. Gibb’s phase triangle of the Cu–In–S system. The composition of epitaxial films with variable CuyIn-ratio is drawn in based on XPSand RBS-measurements. The given denotations represent characteristic points in the Gibb’s phase triangle and they do not correlate to any of the measured points.

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formation in the volume following the Cu2S–In2S3 cut. Also, the spread of surface related measurement points on the Cu2S–InS cut around the stoichiometric point is comparatively larger than for the volume. The shown error area is derived from maximum error estimations assuming "3% uncertainty in determining each elemental peak intensity for both experimental analysis methods. For simplicity, the error area in the Gibb’s phase triangle is represented through a circle with the maximum error as radius and not through a distorted rhomb which would be expected evaluating the independent molecular abundances wCux y(wCuxqwInx) and wCux y (wCuxqwSx). 4. Discussion The evaluation of the epitaxial conditions show that for Cu-rich preparation a tendency of {1 1 2} orientated growth of the deposited CuInS2 films exists. From XRD spectra (not shown here) it can be deduced that on non(1 1 1) substrates this phenomenon starts already in the initial growth phase which means that the origin of this change in growth direction is probably related to effects concerning interface chemistry and morphology. This leads to high defect densities in the interface region. On the In-rich side the formation of facets with {1 1 2} outer planes indicates that the CuInS2(1 1 2) surface is energetically the most stable even if the main film orientation fits to the substrate orientation. A simple explanation can be given based on atomic level considerations: on (1 1 2) surface planes, the amount of dangling bonds per atom is restricted to one (steps are not regarded here) whereas on other surface planes the outermost atoms partially do have more than one dangling bond. Energetically, these configurations seem to be less stable. As a result of the compositional analysis different phase formation paths for the surface and for the volume exist. The high partial pressure of S and the reduction of the background pressure (S-related) in the MBE chamber after completion of the growth could result in sulfur depletion during cooling down possibly forming sphalerite type vacancy compounds. This could also be the reason why the spread of the surface composition along the Cu2S–InS line is larger than for the volume composition along the Cu2S–In2S3 line. Also the invariance of the LEED images of CuInS2 grown on Si(1 1 1) with respect to the CuyIn ratio supports this supposition. Additionally, the occurrence of an ordered (2=1) surface in the near stoichiometric range (as seen in Fig. 1) indicates that the growth of CuInS2 is either influenced by the substrate forcing the impinging atom species on specific lattice positions during the nucleation phase or

that thermodynamic conditions during the growth process are responsible for the mentioned type of (2=1) surface ordering which is also compatible to the presently discussed CuAu-ordering in the volume of polycrystalline or epitaxial CuInS2 films. Further, the slight shift of the In 3d peaks to higher binding energies with increasing In content may hint to an increase in the oxidation state of In. A possible explanation for this effect could be the formation of an ordered defect compound with a changed valence band configuration influencing the chemical shift of the core level peaks. A closer examination of this effect considering the Auger parameters of the respective lines and the evaluation of the valence band structures as obtained by UPS is beyond the scope of this article and will be published elsewhere. The consequences of the compositional analysis are interesting. As it is known that sulfur poor CuInS2 shows n-conduction the preparation of Cu-rich films results in p-type material. Consequently, the change in conduction type at the surface leads to an according band bending with possible influence on the growth mechanism. For example, ionised defect atoms migrating in the internal electrical field may lead to ordered defect structures. Also the formation of buried junctions seems to be possible. Acknowledgments The authors gratefully acknowledge technical support by S. Kubala, A. Porsinger, and U. Pettenkofer and a valuable discussion with Dr. S. Tiefenbacher and Dr. Dino Tonti. References w1x M. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, F. Hasoon, R. Noufi, Prog. Photovolt. 7 (1999) 311. w2x R. Scheer, T. Walter, H.W. Schock, M.L. Fearheiley, H.J. Lewerenz, Appl. Phys. Lett. 63 (1993) 3294. w3x M.C. Lux-Steiner, A. Ennaoui, C.-H. Fischer, A. Jager-Waldau, ¨ ¨ J. Klaer, R. Klenk, R. Konenkamp, T. Matthes, R. Scheer, S. Siebentritt, A. Weidinger, Thin Solid Films 361–362 (2000) 533. w4x H. Metzner, T. Hahn, J.-H. Bremer, J. Conrad, Appl. Phys. Lett. 69 (1996) 1900. w5x R. Hunger, R. Scheer, K. Diesner, D.S. Su, H.J. Lewerenz, Appl. Phys. Lett. 69 (1996) 3010. w6x D.S. Su, W. Neumann, R. Hunger, P. Schubert-Bischoff, M. Giersig, H.J. Lewerenz, R. Scheer, E. Zeitler, Appl. Phys. Lett. 73 (1998) 785. w7x R. Hunger, C. Pettenkofer, R. Scheer, Surf. Sci. 477 (2001) 76. w8x H. Angermann, K. Kliefoth, H. Flietner, Appl. Surf. Sci. 104y 105 (1996) 107. w9x G.S. Higashi, Y.J. Chabal, G.W. Trucks, K. Raghavachari, Appl. Phys. Lett. 56 (1990) 656.