Charge Carrier Transport in a-Si:H/a-SiC:H Heterojunction with Blocking Layer
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Charge carrier transport in a-Si:H/a-SiC:H heterojunction with blocking layer Yu. Vygranenko, M. Fernandes, P. Louro, A. Maçarico and M. Vieira Electronics and Communications Dept., ISEL, R. Conselheiro Emídio Navarro, P 1949-014 Lisboa, Portugal. ABSTRACT This paper presents a one-dimensional numerical simulation of the charge carrier transport and photogeneration within a p-i-n (a-Si:H) homojunction and a p (a-SiC:H)/i (a-Si:H)/n (a-SiC:H) heterojunction with weakly-doped n-layers. A good matching between the simulated J-V characteristics and the corresponding experimental curves has been achieved for both configurations. By analysing the simulated band diagrams, electric field distributions, the electron and hole current densities, and the free carrier population profiles we conclude that in short-circuit mode the carrier transport is different in the homojunction and heterojunction due to band offsets. Our results show that in the heterostructure, as the light intensity increases, the potential drop across the a-SiC:H n-layer increases leading to a significant change in the drift-diffusion balance across the i-layer. In the homojunction, at the same incident fluxes, the transport process remains drift dominated. INTRODUCTION Besides a wide variety of conventional devices such as photovoltaic cells, photodiodes or Light Emitting Diode the a-Si-based structures can also be designed to be used as single sensing element image sensors. In this type of sensor a light pattern projected onto the photosensitive area leads to a potential barrier modulation across the junction that can be read out by scanning with a low-power modulated laser beam [1]. Based on this analogue readout method, microcrystalline and amorphous silicon TCO/p-i-n/metal image transducers were developed [2]. In contrast to conventional photodiodes and solar cells, the improvement of the main image sensor parameters such as spatial resolution, threshold image brightness as well as signal-to-noise ratio can be achieved by decreasing the conductivity of doped layers. The best output characteristics have been achieved by applying a wide band gap a-Si1-xCx:H alloy as contact material [3]. The aim of this paper is to shed light on this finding and describe charge carrier transport and photogeneration in the a-Si:H-based p-i-n structures with weakly-doped a-SiC:H contact layers. EXPERIMENTAL Large area (4×4 cm2) amorphous single layers and p-i-n structures in the assembly glass/ZnO:Al/p (SiC:H)/i (Si:H)/n (SiC:H)/Al were produced. All the layers were deposited by PECVD, at 13.56 MHz radio frequency [4]. Electrical and optical characterization of the films included measuring the electrical conductivity and the absorption spectra from transmission and reflection measurements. The main electro-optical properties of the single layers are described elsewhere [5]. The conductivities of the n-layer varied between 10-7 and 10-12 Ω-1⋅cm-1. Junction properties were investigated from dark and illuminated current-voltage (J-V) measurements. Series resistances up to 106
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