| Summary: | 碩士 === 國立交通大學 === 材料科學與工程學系所 === 106 === We fabricated crystalline selenium (c-Se) based photodetectors with a lateral metal-insulator-semiconductor-insulator-metal (MISIM) structure and studied their photoconducting properties under visible light illumination. Compared with traditional photoconductor materials, such as silicon and amorphous selenium, c-Se has a high absorption coefficient over the visible spectrum; therefore, it has a better photoelectric conversion efficiency.
The c-Se photoconductor of the photodetector was transformed from amorphous Se (a-Se) by thermal annealing. The c-Se photodetector was fabricated on 2 μm-thick wet SiO2 layer on the Si substrate. Comb-shaped Al/Ti bilayer electrodes were prepared by photolithography and dry etch. A dielectric carrier blocking layer (CBL) and a Te adhesion layer were sequentially deposited on the electrodes, followed by evaporation-deposition of a 500 nm-thick a-Se thin film, which was then thermally annealed at 200oC and below in atmosphere.
According to photoelectric measurements, the c-Se photodetector has a much higher quantum efficiency (QE) than the a-Se photodetector over the visible light region due to its higher absorption coefficient. Moreover, it has a lower operation voltage because of its lower resistivity. The c-Se thin film with a larger grain size distribution exhibits avalanche multiplication at a lower electric field. The c-Se layer with a larger grain size has a better crystallinity and less grain boundaries, leading to a larger drift mobility of photogenerated carriers in the layer. As a results, avalanche multiplication in the c-Se layer can be triggered at a small applied electric field.
We have selected three wide band gap dielectric, including Ga2O3, HfO2 and Al2O3, as the CBL of the photodetector that has the c-Se photoconductor thermally transformed from a-Se at 200oC for 1 min. We found that photodetector with a higher junction barrier between the CBL and the Al electrode has a lower dark current. The 10 nm-thick Ga2O3 CBL has the lowest electrical bias (15 V) triggering avalanche multiplication. The QE can reach a value of 500 at 28 V; the excellent photoelectric conversion efficiency reduces the risk of high voltage operation.
Keywords: crystalline selenium, visible light photodetector, dark current, photocurrent, blocking layer, avalanche, multiplication, gain
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