Transparent P-type Conducting LaCuOS Layered Oxysulfide
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Transparent P-type Conducting LaCuOS Layered Oxysulfide Kazushige Ueda, Shin-ichiro Inoue, Sakyo Hirose, Hiroshi Kawazoe1 and Hideo Hosono Materials and Structures Laboratory, Tokyo Institute of Technology 4259 Nagatsuta, Midori, Yokohama 226-8503, JAPAN 1 R&D Center, HOYA Corporation 3-3-1 Musashino, Akishima 196-8510, JAPAN ABSTRACT Materials design for transparent p-type conducting oxides was extended to oxysulfide system. LaCuOS was selected as a candidate for a transparent p-type semiconductor. It was found that the electrical conductivity of LaCuOS was p-type and controllable from semiconducting to semi-metallic states by substituting Sr2+ for La3+. LaCuOS films showed high transparency in the visible region, and the bandgap estimated was approximately 3.1 eV. Moreover, it was revealed that LaCuOS showed sharp excitonic absorption and emission at the bandgap edge, which is advantageous for optical applications. A layered oxysulfide, LaCuOS, was proposed to be a promising material for optoelectronic devices. INTRODUCTION Transparent conducting oxides (TCOs) like In2O3:Sn have been widely used as transparent electrodes in various flat panel devices such as displays and touch panels. Since the conventional TCOs show only n-type electrical conduction, several research groups are actively developing p-type TCOs to extend the application of TCOs to transparent semiconductor devices [1-4]. Taking into account the advantage of transparent character, it is thought that optoelectronic devices like blue- or UV-light emitting diodes are attractive applications of TCOs. We have been developing novel p-type TCOs on the basis of the materials design that chemical modulation of the valence band by Cu1+ ions increases positive-hole mobility [1, 5]. Some p-type TCOs, CuMO2 (M=Al, Ga, In) and SrCu2O2, were proposed [1,5-10], and optoelectronic devices based on these materials were fabricated [11-13]. In further search for new p-type TCOs, the new materials are required to have the following properties: (1) transparent character in the visible region, (2) p-type electrical conduction, (3) electrical conductivity controllable by intentional doping, and (4) direct bandgap. In the present study, the materials design for p-type TCOs was attempted to apply to oxysulfides.
La Cu O S
Figure 1. Crystal structure of LaCuOS. F2.7.1
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log σ (Scm )
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La0.95Sr0.05CuOS
-4 -6
LaCuOS
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20 3
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10 /T (K ) Figure 3. Electrical conductivities as a function of inverse temperature. RESULTS AND DISCUSSION Single phase of powder or thin film samples was confirmed by the XRD measurement. No diffraction peak from secondary phases was seen in the XRD patterns of the samples as shown in figure 2. The thickness of the thin films measured by a stylus was about 200 nm. Electrical and optical measurements hereafter were carried out on the thin films of LaCuOS single phase. Temperature dependence of electrical conductivities of La1-xSrxCuOS (x=0, 0.05) was shown in figure 3. Non-doped sample showed semiconducting beha
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