Two-Dimensional Electronic Structures in Layered Oxychalcogenide Semiconductors, LaCuOCh (Ch=S, Se, Te) and La 2 CdO 2 S
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Two-Dimensional Electronic Structures in Layered Oxychalcogenide Semiconductors, LaCuOCh (Ch=S, Se, Te) and La2CdO2Se2 Toshio Kamiya1,2, Kazushige Ueda1, Hidenori Hiramatsu2, Hiromichi Ohta2, Masahiro Hirano2 and Hideo Hosono1,2 1 Materials and Structures Laboratory, Tokyo Institute of Technology, Japan 2 Hosono Transparent Electro-Active Materials Project, ERATO, JST, Japan ABSTRACT Electronic structures of layered oxychalcogenides, LaCuOCh (Ch=S, Se, Te) and La2CdO2Se2, were studied using ab-initio band calculations in relation to their optical and electronic properties. It was found that the dispersions of the top valence bands are much smaller in Γ-Z direction than in Γ-X direction, indicating that the electronic structure is highly two-dimensional, and that holes are confined in the CuCh or CdSe layers. The two-dimensional electronic structure is supported experimentally by staircase-like structure observed in optical absorption spectra at 10 K associated with two excitonic absorption peaks split by spin-orbit interaction of Ch ions. La2CdO2Se2 has the largest bandgap due to the two-dimensional network structure of CdSe tetrahedra. INTRODUCTION Low-dimensional systems have been studied intensively because novel functions are expected originating from quantum confinement or size effects [1]. Such systems have been fabricated artificially so far, e.g. by alternate stacking of thin layers using molecular-beam epitaxy (artificial superlattices). However, as such processes are time consuming and expensive, alternative techniques have been sought. An idea is to use nanostructures naturally formed in materials. Oxide semiconductor crystals have advantages over conventional semiconductors such as Si and GaAs in this aspect. Some oxides have natural nanostructures aligned one- to three-dimensionally as a consequence of long-range Coulomb interaction and the coexistence of mixed-valence ions [2], both of which originate from features inherent to ionic crystals. We have studied especially transparent oxide semiconductors (TOSs) because they are interesting for science and future opto-electronic device technology due to coexistence of optical transparency and good controllability of carrier concentration. More interestingly, we have found TOSs having natural nanostructures, which include 12CaO·7Al2O3 [3], homologous series layered compounds [4] and layered oxychalcogenides LnCuOCh (Ln=lanthanides, Ch=chalcogens) [5]. We discovered LnCuOCh when we surveyed new p-type TOSs according to our material design concept: that is, to employ extended chalcogen p orbitals instead of O 2p to increase dispersions of valence band maximum (VBM) and decrease hole effective masses. For example, LaCuOS shows good p-type conduction and optical transparency associated with a large bandgap of 3.1 eV [5,6]. In addition, we have found that LnCuOCh exhibited interesting properties, such as room-temperature stable exciton [7] and p-type degenerate conduction with keeping moderately large hole mobility and strong light emission [6]. We spec
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