Electrical properties of Ga₂O₃:Sn/CIGS hetero-junction

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 n-type 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.