Point Defect Changes in CuGaSe 2 Induced by Gas Annealing
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Point Defect Changes in CuGaSe2 Induced by Gas Annealing Akimasa Yamada, Akihiko Nishio*, Paul. Fons, Hajime Shibata, Koji Matsubara, Shigeru Niki and Hisayuki Nakanishi*, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan, *Tokyo University of Science, Noda, Chiba, Japan ABSTRACT Epitaxial CuGaSe2 films were grown on GaAs substrates under Cu-excess conditions to obtain stoichiometric compositions. The films were annealed in Ar, Sex or O2 ambients with or without a Cu or Cu-Se cap layer with the intention of changing the intrinsic defect concentrations. Samples were evaluated using low-temperature photoluminescence (PL) measurements. Annealing of the samples dramatically changed the PL spectra indicating that not only interdiffusion had occurred, but defect species and populations were changed. Comprehensive consideration of the changes led to the conclusion that the emissions at 1.62 eV, 1.66 eV and in the range from 1.2 to 1.4 eV are related to specific defects of Se vacancies, Cu vacancy-Se vacancy complexes and interstitial Cu, respectively. INTRODUCTION The ternary Cu-III-VI2 chalcopyrites are promising materials for high-efficiency thin film solar cell absorber layers [1]. CuGaSe2 (CGS) is being investigated for high-efficiency solar cell applications not only as an endpoint of Cu(InGa)Se2 system but also as a promising material in itself. The conversion efficiency of CGS solar cells studied up to now, however, has been limited to less than 10% [2,3]. Careful investigation of CGS is essential and many studies have been carried out on CGS defect characterization to date, but the identification of defect origins is still uncertain [4-12]. In this work, samples were prepared by molecular beam epitaxy (MBE) to avoid the effects of grain boundaries and to avoid the effects of unintentional impurities. Photoluminescence (PL) was adopted as a characterization technique because of its high sensitivity to low concentration of point defects. Identification of the defect species was undertaken with the aid of PL spectrum transformations caused by annealing in various gas ambients which are expected to either lead to or suppress dissociation of volatile components. EXPERIMENTAL Epitaxial CuGaSe2 films ~0.6 µm thick were grown on (001)-oriented GaAs substrates at 490°C by using a conventional MBE from elemental sources under Cu-excess conditions to obtain stoichiometric compositions [13]. The chemical composition of the films was adjusted by changing the molecular beam flux of the Cu and Ga sources under a Se over pressure. All
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compositional measurement reported in this paper are based upon measurements using electron probe micro-analysis (EPMA) with a 7 keV probe beam. The compositional [Cu]/[Ga] ratio of the as-grown samp
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