Low-temperature electron-paramagnetic-resonance study of extrinsic and intrinsic defects inCuGaSe2

Abstract

The low-temperature electron-paramagnetic resonance ~EPR! of the compound semiconductor CuGaSe2 was investigated. Spectra of as-grown lumps and powders with a defined maximum grain size were measured inX band. The spectra of two transition-metal impurities, Ni 1 and Fe 21 , could be identified and their appropriate EPR parameters are presented. The position of the Fermi-level energy within the forbidden zone as indicated by the occurrence of nickel and iron with these oxidation states is discussed by analogy with previous EPR investigations on other chalcopyrite semiconductors. An aging effect was observed, associated with the occurrence of EPR signals typical for Cu 21 . The nature of the aging process is discussed with respect to the electronic properties of CuGaSe2 for thin-film solar cell applications. @S0163-1829~99!01019-X# CuGaSe2 is a representative of the class of I-III-VI2 semiconducting chalcopyrites. The chalcopyrite structure is a tetragonal derivative from the cubic sphalerite structure. Two different bond lengths occur in the crystal lattice between metal and chalcogen atoms giving rise to a variety of interesting physical properties. 1 CuGaSe2 exhibits its first fundamental transition via a direct band gap of 1.68 eV at RT. The optical absorption coefficient is larger than 10 4 cm 21 for hn.1.7 eV. Electrical conduction in both crystals and thin films of CuGaSe 2, was always found p-type and experiments for n-type doping failed, although the material is generally recognized as electronically compensated. Charge carrier densities could be adjusted by adequate doping from 10 12 ‐1 0 19 cm 23 , while effective mobilities in single crystals were determined to range to about 80 cm 2 V 21 s 21 at room temperature. 2 These optical and electronic properties qualify CuGaSe2 for absorber layer applications in thin-film solar cells and energy conversion efficiencies of 9.7% and 9.3% have already been achieved for single-crystal 3 and thin-film cells, 4 respectively. The knowledge of intrinsic defects like stoichiometry deviations and extrinsic defects like unintentional doping are important prerequisites to understand the electronic properties of such a material and to identify strategies for device optimization. Electron paramagnetic resonance ~EPR! is a powerful tool for this purpose and some dozen EPR investigations of chalcopyrite semiconductors have already been performed. These investigations mainly concentrated upon sulfur-containing wide-band-gap materials and lead to the identification of paramagnetic transition-metal impurities and stoichiometry deviations. 5‐11 Later, selenium-based compounds were studied 12‐16 that are also under investigation for