Electrical Properties of Ga 2 O 3 :Sn/CIGS Hetero-junction
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Electrical Properties of Ga2O3:Sn/CIGS Hetero-junction 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 There is an increased need for highly sensitive imaging devices to develop high resolution and high speed image sensors. Incident light intensity per pixel of image sensors is getting lower because the pixel resolution and frame rate of image sensors are becoming higher. We investigated the feasibility of using a photoconductor with tin-doped gallium oxide (Ga2O3:Sn)/Cu(In,Ga)Se2 (CIGS) hetero-junction for visible light image sensors. CIGS chalcopyrite thin films have great potential for improving the sensitivity of image sensors and CIGS chalcopyrite semiconductors have both a high absorption coefficient and high quantum efficiency. Moreover, the band gap can be adjusted for visible light. We applied Ga2O3 as an ntype semiconductor layer and a hole-blocking layer to CIGS thin film to reduce the dark current. The experimental results revealed that dark current was drastically reduced due to the application of Ga2O3 thin film, and an avalanche multiplication phenomenon was observed at an applied voltage of over 6 V. However, non-doped Ga2O3/CIGS hetero-junction only had sensitivity in the ultraviolet light region because their depletion region was almost completely spread throughout the Ga2O3 layer due to the low carrier density of the Ga2O3 layer. Therefore, we used Ga2O3:Sn for the n-type layer to increase carrier density. As a result, the depletion region shifted to the CIGS film and the cells had sensitivity in all visible regions. These results indicate that Ga2O3:Sn/CIGS hetero-junction are feasible for visible light photoconductors. INTRODUCTION We have been researching and developing a “Super Hi-Vision” system for future ultra-high definition TVs [1]. It has 16 times the resolution and double the frame rate of the current HiVision system. However, the incident light intensity per pixel of image sensors is getting lower due to advances in the pixel resolution and frame rate. Therefore, there is an increased need for highly sensitive imaging devices to develop high resolution and high speed image sensors. The use of CuIn1-xGaxS1-ySey (CIGS) chalcopyrite thin films as absorber materials, for example in thin film solar cells, has attracted a great deal of attention because of their band gap, high absorption coefficient, great stability, and high quantum efficiency of these types of films [2–3]. We examined the feasibility of Ga2O3:Sn/CIGS hetero-junction for visible light sensors in this study. CIGS thin film solar cells are generally applied to cadmium sulfide (CdS) buffer layers. However, CdS has a Cd toxicity classification and absorbs blue light because it has a band gap of 2.4 eV. These are also unfavorable for visible light sensors. Moreover
