[Invited Paper] High Sensitivity Crystalline Selenium-based CMOS Image Sensor Using Avalanche Multiplication

4 K and 8 K broadcasting began in 2018, providing ultra-high image quality far superior to that of current high-definition television. The ultra-high-definition image will soon become a familiar part of people's lives. We have been conducting research and development for the realization of the next-generation ultra-realistic broadcasting system "8 K Super Hi-Vision". Even after the start of this broadcast, we are continuing our research into improving the performance of 8K cameras so that they can adapt to more diverse shooting environments. In particular, with regard to the miniaturization of pixels in imaging devices accompanying high-definition imaging, pixel sizes below 1 μm have already been reported1, 2). Such a trend results in a decrease in the amount of incident light per pixel, in other words, a decrease in the imaging device' sensitivity, which is regarded a serious issue as a factor limiting the shooting conditions. We propose a highsensitivity imaging device in which a photoconversion layer capable of avalanche multiplication of signal carrier is stacked on a complementary metal oxide semiconductor (CMOS) circuit (Fig. 1). Various methods for increasing sensitivity in conventional imaging devices, such as reducing readout noise and improving optical aperture ratio, have been tried, but these methods have yet to significantly solve the decrease in sensitivity caused by pixel miniaturization. The use of signal amplification by avalanche Abstract We present our work on a complementary metal oxide semiconductor (CMOS) image sensor that uses crystalline selenium as the photoconversion layer and enables avalanche multiplication at low voltage, with the goal of realizing a high-definition, high-sensitivity camera. Gallium oxide, used as a hole blocking layer, and nickel oxide used as an electron blocking layer effectively prevents the increase of external dark current caused by carrier injection from an external electrode. In addition, a new crystallization method was developed to improve the crystallinity of selenium for the fabrication of crystalline selenium films. We were able to capture high-quality images in a crystalline selenium-based CMOS image sensor and confirm signal amplification by a factor of approximately 1.4 at a reverse bias voltage of 22.6 V by using these film structures and deposition conditions.