Reduction of Dark Current Under Reverse Bias in a-Si:H p-i-n Photodetectors
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Reduction of dark current under reverse bias in a-Si:H p-i-n photodetectors S. Morrison1, P. Servati2, Y. Vygranenko2, A. Nathan2, and A. Madan1 1 MVSystems Inc., Golden, CO 80401, USA 2 Electrical and Computer Eng., University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
ABSTRACT This paper presents the development of low dark current amorphous silicon (a-Si:H) based heterojunction photodiodes. A series of p-i-n and n-i-p structures have been deposited by plasma-enhanced chemical vapor deposition (PECVD). Junction properties and carrier transport are investigated in terms of dark and light current-voltage characteristics, time dependence of the dark current, and spectral photoresponse measurements. It is demonstrated that a thin (~4 nm) undoped a-SiC:H buffer layer introduced between the p and i layers reduces the leakage current and improves the diode ideality factor. A dark current density of ~10 pA/cm2 at reverse bias of 1 V was achieved for the n-i-p structure. Optimization of device design for further improvement of dark current and photoresponse is discussed. INTRODUCTION A-Si:H technology is attractive for imaging applications due to its large area processing capability and its high absorption efficiency in the visible light range. Large area active matrix image sensor arrays have been demonstrated for both optical and x-ray imaging applications, in which the photosensitive element is a p-i-n photodiode [1]. The dark current of the p-i-n photodiode, however, is a critical parameter since it limits the resolution and dynamic range of the imaging system. It is known that the dark current of p-i-n diodes is affected by the quality and nature of the p/i interface rather than the thickness of the intrinsic a-Si:H layer [2-4]. For example, improvement in the efficiency of a-Si:H solar cells have been achieved by replacing the a-Si:H p-layer with the equivalent a-SiC:H window [5]. In addition, a graded buffer p/i interface has been demonstrated to have enhanced the performance of the cell in the blue region of the spectrum [6]. It has also been shown that insertion of 10-20 Å undoped a-SiC:H between the p-layer and i-layer in the solar cell leads to improved fill factor and open circuit voltage [7]. In this work, we present a-SiC:H/a-S:H p-i-n heterojunction photodiodes, with an undoped layer at the p/i interface for reduction of dark current, along with their electrical and optical properties for different layer configurations. EXPERIMENTAL Figure 1 shows cross sections of the different photodiode structures that were fabricated for this study. The samples were prepared using a multi-chamber PECVD system, where the different chambers were connected via a loadlock. The a-Si:H layer was deposited by decomposition of silane at ~260oC and rf power of 5W. The n+a-Si:H layer was deposited following decomposition of SiH4+(~1%)PH3 mixture. The a-SiC:H layer was deposited in a A7.4.1 Downloaded from https:/www.cambridge.org/core. Columbia University Libraries, on 28 May 2017 at 14:25:46, subject to the Cambridge Core te
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