The stable surfaces in chalcopyrites are the polar ͕112͖ surfaces. We present an electron microscopy study of epitaxial films of different compositions. It is shown that for both CuGaSe 2 and CuInSe 2 the ͕001͖ surfaces form ͕112͖ facets. With increasing Cu excess the faceting is suppressed. This indicates a lower surface energy of the ͕001͖ surface than the energy of the ͕112͖ surface in the Cu-rich regime, but the ͕001͖ surface is higher in energy than the ͕112͖ surface in the Cu-poor regime. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2192638͔The chalcopyrite structure of the I-III-VI 2 compound semiconductors can be considered as a double zinc blende structure. The availability of two different cation species has important consequences: a tetragonal distortion of the original cubic structure 1 and greatly reduced defect formation energies.2 Another difference has only recently been discovered; while in the zinc blende structure of either III-V or II-VI compounds the most stable surfaces are the nonpolar ͕110͖ surfaces, in the chalcopyrite structure the stable surfaces are the polar ͕112͖ tet surfaces ͕͑111͖ cub ͒. By the subscript "cub" we denote indices corresponding to the ͑cubic͒ zinc blende, while "tet" structures correspond to the ͑tetrag-onal͒ chalcopyrite structure. First it was observed that the cleavage planes in CuInSe 2 are the ͕112͖ tet and the ͕101͖ tet ͕͑111͖ cub and ͕201͖ cub ͒, 3 unlike in zinc blende compounds where the cleavage planes are the ͕110͖ cub planes. When growing zinc blende compounds epitaxially on the stable ͕110͖ GaAs surface, the surface of the growing film can be obtained ideally flat. 4 On the contrary, when growing Cu͑In, Ga͒Se 2 epitaxially on ͕110͖ GaAs no flat ͑220͒ tet or ͑204͒ tet surface is obtained, but it breaks up completely into ͕112͖ tet facets.5 These results are rather unexpected. In the zinc blende materials the nonpolar ͕110͖ cub surfaces are more stable than the polar ͕111͖ cub or ͕100͖ cub surfaces, since at the polar surfaces massive surface reconstructions are necessary to create an areal charge that prevents the catastrophic increase in electrostatic energy ͑e.g., Ref. 6͒. These surface reconstructions increase the surface energy considerably. But it was shown that due to the low defect formation energies in chalcopyrite the massively reconstructed ͕112͖ tet ͕͑111͖ cub ͒ surfaces are stabilized compared to the ͕110͖ tet surfaces. 8,9 Depending on the composition of the crystal the cation terminated ͕112͖ tet surface is stabilized mostly by the formation of Cu vacancies V Cu in the Cu-poor case or by Cu In antisites on the Cu-rich side.This letter has investigated the stability of the ͕220͖ tet surface compared to the ͕112͖ tet surface. It has been argued that the ͕112͖ tet surface should be the most stable one in the chalcopyrite system. In the current letter we investigate the stability of the ͕001͖ tet surface. It has been described previously that the ͕001͖ tet surfaces of CuInSe 2 ͑Ref. 10͒ or CuGaSe 2 ͑Ref. 11͒ show small trenches along th...
Cu͑In 1−x Ga x ͒Se 2 ͑CIGS͒ films were grown on ͑001͒ GaAs at 570 or 500°C by means of metal organic vapor-phase epitaxy. All films were Cu-rich ͓Cu/ ͑In+ Ga͒ Ͼ 1͔ with pseudomorphic Cu 2 Se second phase particles found only on the growth surface. During growth, diffusion of Ga from the substrate and vacancies generated by the formation of CIGS from Cu 2 Se at the surface occurred. The diffusion processes lead to the formation of Kirkendall voids at the GaAs/CIGS interface. Transmission electron microscopy and nanoprobe energy dispersive spectroscopy were used to analyze the diffusion and void formation processes. The diffusivity of Ga in CIGS was found to be relatively low. This is postulated to be due to a comparatively low concentration of point defects in the epitaxial films. A reaction model explaining the observed profiles and voids is proposed.
The photoluminescence excitation spectra are presented of weakly and highly compensated CuGaSe 2 , grown under Cu-excess and under Cu-deficiency, respectively. An overlap is observed between the photoluminescence and the excitation spectrum in the Cu-poor material, indicating fluctuating potentials due to high compensation, whereas no overlap is observed in material grown under Cu-excess, indicating flat bands. The photoluminescence excitation spectra can be used as measure for the absorption spectrum in the case of flat bands, but not in the case fluctuating potentials, where different emission energies probe different depths of the fluctuations. Methods for the determination of the fluctuation amplitude from the photoluminescence and the excitation spectra are discussed.
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