Defect states in a representative amorphous oxide semiconductor, a-InGaZnO4, were studied by optical analyses and first-principle calculations. The optical analyses suggested that the as-deposited a-IGZO film have weak subgap absorptions around 0.6 and 2 eV. Local density approximation calculations showed that an oxygen defect works as an electron trap as well as a shallow donor depending on its local structure. It suggests that a large vacancy space remained in an oxygen deficient structure forms a deep levels in the band gap and traps electrons, while if such a large space is annihilated e.g. by a film growth process and post thermal annealing, oxygen deficiency may form a shallow donor level. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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