Dark current suppression and thermal stability improvement for amorphous selenium based photosensors

• Main Authors: Yu, Tung-Yuan, 余東原
• Other Authors: Pan, Fu-Ming
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/61076514149271957328

博士 === 國立交通大學 === 材料科學與工程學系所 === 104 === This research fabricated amorphous photosensors using amorphous selenium (a-Se) as the photoconductive layer. For photosensor application, a-Se should have a high photoconversion gain with a very low dark current and high thermal stability so that a high image contrast can be obtained. We have proposed a new hole blocking layer for dark current reduction in single and multilayer structure a-Se based photoconductor for photosensor applications. ZnO thin films as a hole blocking layer (HBL) between a-Se and anode electrode were deposited by applying a reactive sputter deposition technique to suppress the dark current in the a-Se photosensor. And then the multilayer structure a-Se based film was fabricated using an alternating multilayer structure of a-Se and AsxSe1-x by applying a rotational thermal evaporation deposition technique to enhance the thermal stability in the a-Se photosensor. The ZnO HBL layers prepared at various oxygen flow rates were characterized using X-ray photoelectron spectroscopy, Raman scattering analysis, and photoluminescence spectroscopy. The results indicated that the oxygen flow rate considerably influenced the density of oxygen vacancies in the ZnO thin films. Deep-level transient spectroscopy measurement was conducted, reveals two hole trap levels in the ZnO thin films deposited using a reactive sputter deposition process; one of the levels was located at 0.94 eV and the other was located at 0.24 eV, above the valence band edge. The number of oxygen vacancies in the ZnO thin film considerably influenced the electrical performance of the a-Se photosensor. The a-Se photosensor containing the ZnO HBL that comprised the most oxygen vacancies exhibited the lowest dark current and highest breakdown field. The multilayer a-Se based photosensors with an alternating multilayer structure comprising a-Se and AsxSe1-x were fabricated using a rotational thermal evaporation deposition process. The atomic concentration of As in the amorphous AsxSe1-x layer and the thickness of each a-Se and AsxSe1-x layer depended on the evaporation temperature. The thermal stability of the multilayer thin film was examined through X-ray diffractometry, Raman spectroscopy analyses, and time-of-flight secondary ion mass spectrometry. During the deposition of the amorphous AsxSe1-x layers, As diffused into the underlying a-Se layers, improving the thermal stability of the multilayer photosensor. Although the As doping introduced carrier traps in the a-Se layers, the multilayer photosensors demonstrated an effective quantum efficiency comparable to that of the single layered a-Se sensor under blue light illumination but with a lower dark current density by two orders of magnitude. Moreover, the a-Se/AsxSe1-x sensor was robust to a higher breakdown field because of its high thermal stability. The improved thermal stability and low dark current density enabled using the a-Se/AsxSe1-x multilayer structure as a sensitive photosensor with a high image contrast.