Electrical Properties of Photoconductor Using Ga 2 O 3 /CuGaSe 2 Heterojunction
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Electrical Properties of Photoconductor Using Ga2O3/CuGaSe2 Heterojunction Kenji Kikuchi1,2, Shigeyuki Imura1, Kazunori Miyakawa1, Misao Kubota1, and Eiji Ohta2 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 1
ABSTRACT The feasibility of using a photoconductor with a Ga2O3/CuGaSe2 heterojunction for visible light sensors was investigated. CIGS chalcopyrite semiconductors 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. Experimental results showed that the dark current was drastically reduced, and an avalanche multiplication phenomenon was observed at an applied voltage of over 6 V. However, this structure had sensitivity only in the ultraviolet light region because its depletion region was almost completely spread in the Ga2O3 layer since the carrier density of the Ga2O3 layer was much lower than that of the CIGS layer. These results indicate that the Ga2O3/CuGaSe2 heterojunction has potential for use in visible light sensors but that we also need to increase the carrier density of the Ga2O3 layer to shift the depletion region to the CIGS film. INTRODUCTION The use of CuIn1-xGaxSe2 (CIGS) chalcopyrite thin films as the absorber material for thin film solar cells has received much attention because of the band gap, high absorption coefficient, great stability, and high quantum efficiency of this type of film [1–3]. In this study, we examined the feasibility of using a photoconductor with CIGS chalcopyrite semiconductors for visible light sensors. CIGS thin film solar cells using CdS as a buffer layer have reached conversion efficiencies of 20%. However, CdS has a Cd toxicity classification and an absorption of blue light due to having a band gap of 2.4 eV. These are unfavorable for both solar cells and visible light sensors. Moreover, the dark current of CIGS thin film is too high for sensors [4] because of the low resistivity of CIGS. We applied a gallium oxide (Ga2O3) thin film as an n-type semiconductor layer to solve these problems. We assumed that the Ga2O3 thin film would function as a hole-blocking layer for the CIGS films and suppress the injection of holes from electrode [5]. The Ga2O3 thin films have a wide band gap of 4.7–4.9 eV and high transmittance in visible light [6]. We used CuGaSe2 for absorber layers because it has a suitable band gap of 1.64–1.67 eV [7, 8], which is sufficient for visible light sensors. Here, we report on the electrical characterization of the Ga2O3/CuGaSe2 heterojunctions.
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EXPERIMENT Alkali-free glass (Corning, Eagle-XG) with an Au layer served as the substrate and back contact. The CuGaSe2 layer was deposited onto the substrate by one-step radio frequency (RF) magnetron sputtering from a CuGaSe2 alloy targ
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