ITO/a-SiN x :H/a-Si:H Photodiode with Enhanced Photosensitivity and Reduced Leakage Current Using Polycrystalline ITO De
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ITO/a-SiNx:H/a-Si:H Photodiode with Enhanced Photosensitivity and Reduced Leakage Current Using Polycrystalline ITO Deposited at Room Temperature S. Tao, Q. Ma*, D. Striakhilev, and A. Nathan Electrical and Computer Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 *currently at: PerkinElmer Optoelectronics, Vaudreuil, Quebec, Canada J7V 8P7 Abstract We report an ITO/a-SiNx:H/a-Si:H MIS photodiode structure based on room temperature deposition of optically transparent polycrystalline ITO for applications in large area optical and x-ray imaging. The photodiode structure exhibits device characteristics with reduced leakage current and enhanced photosensitivity giving rise to a hundred-fold improvement in dynamic range. This notable improvement in performance is believed to be due to the reduced diffusion of oxygen from the ITO to the a-Si:H layer, and thus reducing the density of defect states inside the a-Si:H layer. The behavior of photo and dark current is consistent with an elaborate transport model for the Schottky barrier. The model agrees reasonably well with measurement data for the dark current and provides a consistent picture in terms of the photo current behavior in the MIS structure, where the insulating layer serves to reduce the oxygen diffusion. Introduction ITO/a-Si:H Schottky diodes have been used in solar cells and optical imaging applications [1]. Coupled with an x-ray phosphor material to convert x-rays to visible light, these devices can also be used for x-ray imaging [2]. Here, and in particular for medical imaging applications, the requirement of low x-ray dose and high imaging resolution needs further improvement on leakage current and dynamic range of Schottky detectors. In a previous paper [3], we reported an ITO/a-Si:H Schottky photodiode with low leakage current based on room temperature polycrystalline ITO deposition. Secondary Ion Mass Spectroscopy (SIMS) measurements show that the diffusion of oxygen from the ITO into a-Si:H is inevitable even when the ITO is deposited at room temperature [3]. The oxygen inside the a-Si:H layer increases the density of ionized defect states, which consequently increase the leakage current and reduce photosensitivity. To reduce the diffusion of oxygen, and hence the density of ionized defect states in the aSi:H layer, a MIS structure with an a-SiNx:H insulator layer is proposed. I-V measurements show this structure can reduce the leakage current and dramatically increase the dynamic range due to an increase in the photocurrent. Because these diodes are commonly operated under reverse bias, we will first discuss the reverse current transport mechanisms across a metal/a-Si:H barrier. Mechanisms Underlying Reverse Current Three mechanisms underlying reverse current transport across a metal/a-Si:H barrier have been identified [4], as shown in Figure 1. Since the defect density is relatively low in the intrinsic a-Si:H layer, under medium bias we assume that the leakage current stems from thermionic emission and thermionic-field emission
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