Development of Stacked Image Sensor With Avalanche Multiplication in Surface-Enhanced Crystalline-Selenium-Based Photoconversion Layer

We have developed a high-sensitivity complementary metal–oxide–semiconductor (CMOS) image sensor overlaid with a surface-enhanced crystalline selenium (c-Se) based photoconversion layer that uses avalanche multiplication. We used a thin cap layer of gallium oxide (Ga
₂
O
₃
) to restrict the vertical growth of the Se crystal grains, and we suppressed the strain caused by the large differences in thermal expansion coefficients of c-Se and Ga
₂
O
₃
by adjusting the thickness of the cap layer, which resulted in a flattened Ga
₂
O
₃
/c-Se interface. Therefore, the local electric field concentration generating in the interface that destroy the hole blocking effect of the Ga
₂
O
₃
was reduced, allowing higher applied electric fields and produced larger signal amplification. The amplified signal also increased sharply because the variation in electric field strength from place to place is mitigated by the uniform film thickness. We fabricated such a surface-enhanced c-Se-based stacked CMOS image sensor and demonstrated that it captured a brighter image with a signal multiplication factor of 1.6.