Toru Okino

High sensitivity image sensor overlaid with thin-film crystalline-selenium-based heterojunction photodiode

We have developed a stacked image sensor based on a thin-film crystalline selenium (c-Se) heterojunction photodiode. Uniform c-Se-based photodiodes laminated on complementary metal-oxide-semiconductor circuits were fabricated by tellurium-diffused crystallization. We herein present the first high-resolution images obtained with such a device. Furthermore, the dark current was significantly decreased by using an n-type wide-band-gap gallium oxide (Ga2O3) layer with a high hole-injection barrier.

High-Sensitivity Image Sensors Overlaid With Thin-Film Gallium Oxide/Crystalline Selenium Heterojunction Photodiodes

We developed a stacked CMOS image sensor overlaid with a thin-film gallium oxide (Ga2O3)/crystalline selenium (c-Se) heterojunction photodiode. Uniform c-Se films laminated on the CMOS circuits were fabricated through a low-temperature (200 °C) tellurium-diffused crystallization process. This stacking structure has several advantages, such as a high aperture ratio, low crosstalk, and high sensitivity. We successfully controlled the size of the polycrystalline particles to much smaller than the pixel size of the image sensors by adjusting the thickness of the c-Se films. Therefore, we herein present the first high-resolution images obtained with such a device. Furthermore, the dark current, which is induced by the carrier injection from an external electrode, was significantly reduced by employing a wide bandgap Ga2O3 film as a high hole-injection barrier.

A Real-Time Ultraviolet Radiation Imaging System Using an Organic Photoconductive Image Sensor

We have developed a real time ultraviolet (UV) imaging system that can visualize both invisible UV light and a visible (VIS) background scene in an outdoor environment. As a UV/VIS image sensor, an organic photoconductive film (OPF) imager is employed. The OPF has an intrinsically higher sensitivity in the UV wavelength region than those of conventional consumer Complementary Metal Oxide Semiconductor (CMOS) image sensors (CIS) or Charge Coupled Devices (CCD). As particular examples, imaging of hydrogen flame and of corona discharge is demonstrated. UV images overlapped on background scenes are simply made by on-board background subtraction. The system is capable of imaging weaker UV signals by four orders of magnitude than that of VIS background. It is applicable not only to future hydrogen supply stations but also to other UV/VIS monitor systems requiring UV sensitivity under strong visible radiation environment such as power supply substations.

Stacked image sensor using chlorine-doped crystalline selenium photoconversion layer composed of size-controlled polycrystalline particles

We demonstrate a stacked complementary metal-oxide semiconductor (CMOS) image sensor overlaid with a chlorine (Cl)-doped crystalline selenium (c-Se) photoconversion layer. The size of the polycrystalline particles (grains) of c-Se, which is strongly related to dark current pattern noise, is controlled by Cl doping to c-Se; hence, the resulting device provides clear images. Furthermore, avalanche multiplication was successfully observed in a Cl-doped c-Se film fabricated on a glass substrate at a relatively low applied voltage.