Fabrication of Stacked CMOS Image Sensor With NiO/Ga2O3 UV Photoconversion Films Using Low-Oxygen RF-Sputtered NiO for Transparent Image Sensor

This letter demonstrated a stacked complementary metal-oxide-semiconductor (CMOS) image sensor utilizing nickel oxide (NiO)/gallium oxide (Ga
₂
O
₃
) photoconversion films. The combination of the ultrawide bandgap
p
-type NiO and
n
-type Ga
₂
O
₃
enables a photodiode sensitive to UV but not to visible light, thereby realizing a transparent image sensor in combination with transparent circuits in the future. To fabricate a highly efficient image sensor, one of the most crucial challenges is to reduce the dark current in the photodiodes, which deteriorates the image quality. NiO and Ga
₂
O
₃
are materials with immense potential to significantly suppress the dark current owing to their heteroepitaxial growth and the ability to control the carrier concentration through impurity doping while suppressing defects. This letter fabricated a CMOS image sensor overlaid with NiO/Ga
₂
O
₃
photoconversion films to reduce the dark current to 2 nA/cm
²
with an applied voltage of 20 V by optimizing the deposition conditions during sputtering and successfully confirmed the photo response under UV irradiation.