Band structure investigation of chalcopyrite CuInSe 2 (001) by angle-resolved photoelectron spectroscopy
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Band structure investigation of chalcopyrite CuInSe2(001) by angle-resolved photoelectron spectroscopy
Ralf Hunger1*, Christian Pettenkofer2 1
Fachgebiet Oberflächenforschung, Institut für Materialwissenschaft, TU Darmstadt, Petersenstr. 23, 64287 Darmstadt, Germany 2 Abteilung SE 6, Bereich Solarenergieforschung, Hahn-Meitner-Insitut Berlin GmbH, Glienicker Str. 100, 14109 Berlin ABSTRACT Clean and ordered chalcopyrite CuInSe2 surfaces are a precondition for the study of the electronic structure by angle-resolved photoelectron spectroscopy. The preparation of welldefined CuInSe2(001) surfaces by the combination of molecular beam epitaxy and a selenium capping and decapping process is described. The surface structure of CuInSe2 epilayers with different bulk composition is compared and analysed by low-energy electron diffraction. Employing near-stoichiometric surfaces, the valence electronic structure of CuInSe2 was investigated by angle-resolved photoelectron spectroscopy at the synchrotron source BESSY 2. This is the first study of the valence band structure of a copper chalcopyrite semiconductor material by photoelectron spectroscopy. The valence band dispersion along ΓΤ, i.e. the [001] direction, was investigated by a variation of the excitation energy from 10 to 35 eV under normal emission, and the band dispersion along ΓΝ, i.e. the [110] direction, was analysed by angular scans with hν = 13 eV. The valence bands derived from antibonding and bonding Se4p-Cu3d hybrid orbitals, nonbonding Cu3d states and In-Se hybrid states are clearly indentified. The strongest dispersion is found for the topmost valence band with a bandwidth of ~0.7 eV from Γ to Τ. From Γ to Ν, the observed dispersion was 0.5 eV. The experimental valence bands are discussed in relation to calculated band structures in the literature. INTRODUCTION The copper chalcopyrite semiconductors CuInSe2, CuGaSe2 and CuInS2, and their solid solutions are widely investigated for their application in thin film solar cells [1]. In an early fundamental work of Jaffe and Zunger, the equilibrium structure and electronic band structure of these materials was calculated within the density functional theory framework [2]. Employing a more recent code, the atomic and electronic band structure was recalculated lately [3, 4]. From the experimental side, the method of choice for the measurement of a valence electronic structure is angle-resolved photoelectron spectroscopy (ARPES) [5]. However, ARPES studies of CuInSe2, CuGaSe2, or CuInS2, have not been reported, yet. The presumable reason for the lack of experimental data lies in the difficulty of the preparation of clean, ordered (single crystalline) and oriented surfaces of these materials [6] [7]. *
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Recently, we could however demonstrate the possibility of the preparation of UHV-clean and ordered CuInSe2(001) surfaces by a selenium capping and decapping process in combination with molecular beam epitaxy [7]. We employed this new technique for our ARPES study present
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