Improved Electrical Properties of Ga 2 O 3 :Sn/CIGS Hetero-Junction Photoconductor
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Improved Electrical Properties of Ga2O3:Sn/CIGS Hetero-Junction Photoconductor Kenji Kikuchi1,2, Shigeyuki Imura1, Kazunori Miyakawa1, Hiroshi Ohtake1, Misao Kubota1, and Eiji Ohta2 1 NHK Science and Technology Research Laboratories, 1-10-11, Kinuta, Setagaya-ku, Tokyo, 157–8510, Japan 2 Graduate School of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kouhoku-ku, Yokohama, 223–8522, Japan ABSTRACT We examined the potential application of CuIn1-xGaxSe1-ySy (CIGS) film for visible light image sensors. CIGS chalcopyrite semiconductors, which are representative of high efficiency thin film solar cells, have both a high absorption coefficient and high quantum efficiency. However, their dark current is too high for image sensors. In this study, we applied gallium oxide (Ga2O3) as a hole-blocking layer for CIGS thin film to reduce the dark current. The dark current of this hetero-junction was 10-9 A/cm2 at less than 7 V. Moreover, an avalanche multiplication phenomenon was observed at an applied voltage of over 8 V. However, this structure had sensitivity only in the ultraviolet light region due to the much lower carrier density of the Ga2O3 layer. We therefore used a tin-doped Ga2O3 (Ga2O3:Sn) layer deposited by pulsed laser deposition (PLD) for the n-type layer to increase the carrier density. The sensitivity of the visible region was observed in the Ga2O3:Sn/CIGS hetero-junction. We also investigated the influence of the laser frequency of the PLD on the transmittance of Ga2O3:Sn and the quantum efficiency of this hetero-junction. Ga2O3:Sn film deposited at a 0.1-Hz laser repetition rate had higher transmittance than at a 10-Hz repetition rate. The Ga2O3:Sn/CIGS hetero-junction also had a higher quantum efficiency with the lower rate (50%) than with the higher rate (30%). INTRODUCTION CuIn1-xGaxSe1-ySy (CIGS) chalcopyrite thin films are p-type semiconductors and promising candidates for highly efficient devices. They have an adjustable band gap (1.0–2.4 eV), high absorption coefficient (105 cm-1), great stability, and high quantum efficiency (over 90 %) [1–4]. In this study, we investigated the potential application of CIGS chalcopyrite semiconductors for visible light image sensors. Conventional CIGS thin film solar cells use a cadmium sulfide (CdS) buffer layer and have reached conversion efficiencies of 20% [3, 4]. However, the band gap of CdS is 2.4 eV, so sensitivity of blue light is low due to the absorption of blue light. And, the toxicity of Cd is undesirable from the viewpoint of environmental safety. Moreover, the dark current of conventional photoconductors using CIGS thin film is high (about 10-8 A/cm2) for visible light sensors [5, 6]. To solve these problems, we applied a gallium oxide (Ga2O3) thin film as an n-type semiconductor layer. Ga2O3 thin films have high transmittance in visible light, a wide band gap of 4.7–4.9 eV [7, 8], can function as a hole-blocking layer for CIGS films, and can suppress the injection of holes from electrodes [9]. We used CuIn0.48Ga0.52S2 for the absorber
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