Ito/A-Si:H Schottky Photodiode With Low Leakage Current And High Stability
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Qinghua Ma, Arokia Nathan, and R.V.R. Murthy Department of Electrical and Computer Engineering University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
ABSTRACT We report the design, fabrication, and characterization of an indium tin oxide/hydrogenated amorphous silicon (ITO/a-Si:H) Schottky photodiode based on room temperature deposition of ITO. The optical transmittance of the ITO is larger than 80% in the visible light range and its resistivity is less than 6 x 10-' 2-cm. The fabricated photodiode exhibits low leakage current and stable I-V characteristics. The leakage current is 7x10-"' A/cm2 when biased at -2 V and the shift in leakage current stabilizes to a value less than 9% after 2 seconds of biasing at -2 V. The improvement in performance can be attributed to the high integrity ITO/a-Si:H interface achieved with the low temperature deposition. INTRODUCTION ITO/a-Si:H Schottky diodes have been used in solar cells and image sensors due to its fast optical response, reduced leakage current, and simplicity in process. In large area imaging applications, further improvement in device characteristics is expected; here, the (dark) leakage current and its stability (its shift with time) are two very important considerations. For example, the leakage current is a source of noise and its accumulation limits the integration time when used in large area arrays. The shift in leakage with time further degrades imaging performance due to tahe integration mode of photodiode operation. Contributions to the sensor leakage current stem from contact injection, bulk thermal generation, and edge effects [1]. Bulk thermal generation is a feature of the active layer and it is generally in the low end of the leakage current. Edge effects usually contribute when the area is very small. Contact injection depends on the interface structure and is the main contributor to the leakage current. The injection is determined not only by the ITO and a-Si:H workfunctions but also by the ITO/a-Si:H interface structure, including interface states, interface charges, and interface nonuniformity. In ITO/a-Si:H Schottky diodes, the high density of interface states increases tunnelling, which in turn increases the leakage current. This high density of interface states is generated by the diffusion of oxygen and indium from the ITO into the a-Si:H, which is significant when the ITO is deposited at an elevated temperature [2]. In addition, there is instability in the leakage current, which arises not only from the generation of deep defect states in the intrinsic a-Si:H layer but also from the interface states at the interface. The latter strongly depends on the intricate detail of the ITO preparation process. Therefore there is a clear need for good quality ITO that can be deposited at relatively low temperatures. Following the above considerations, we have developed a deposition process for polycrystalline ITO at room temperature and using this process we have fabricated an ITO/a-Si:H
231 Mat. Res. Soc. Symp. Proc. Vol. 558 ©2000 Materials Rese
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