Carrier Transport and Photogeneration in Amorphous Silicon / Crystalline Silicon Heterojunctions with i/n and p/n Interf
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CARRIER TRANSPORT AND PHOTOGENERATION IN AMORPHOUS SILICON / CRYSTALLINE SILICON HETEROJUNCTIONS WITH i/n AND p/n INTERFACES 1
Yu. Vygranenko, 1M. Fernandes, 2C. Nunes Carvalho, 2G. Lavareda, 1P. Louro, 2A. Amaral, 1,3 R. Schwarz and 1M. Vieira. 1 Electronics and Communications Dept., ISEL, R. Conselheiro EmÃdio Navarro, P 1949-014 Lisboa, Portugal, Tel: +351 1 8317181, Fax: +351 1 8317114, e-mail: [email protected]. 2 CFM, IST, Av. Rovisco Pais, 1 P-1096, Lisboa, Portugal. 3 Dep. Phys. IST, Av. Rovisco Pais, P-1096, Lisbon, Portugal. ABSTRACT Amorphous hydrogenated silicon films deposited by Plasma Enhanced Chemical Vapour Deposition (PE-CVD) using standard rf-glow discharge at 13.56 MHz were used to produce amorphous silicon heterostructures. Junction properties were studied from current-voltage (IV), capacitance-voltage (C-V) and spectral response measurements. The photosensitivity of these structures was investigated for different amorphous film thicknesses and different applied bias voltages. It was shown that the output device characteristics could be improved by plasma hydrogen treatment before the deposition of the amorphous layer. The results show that ITO/a-Si:H/c-Si structures present high internal gain in the visible infra-red region and high collection efficiency in the blue range. They can be used as visible/near-IR photodiodes or for current amplifications proposes. INTRODUCTION Devices with hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) interfaces are very attractive since the formation of the heterojunction is a low temperature process. Compared to conventional silicon homojunction technology, the number of processing steps is substantially reduced. Apart from solar cells the possible applications of those structures lie in the field of optical sensors and X-ray detectors [1, 2]. The reproducible manufacturing of those devices implies a specific physical and technological knowledge for the growth of an amorphous layer onto a monocrystalline surface. However, the large interface state density is one of the major problems. Interface defect states can be formed due to contamination and oxidation of the substrate, to ion bombardment of the interface during the plasma processing steps or to the existing dangling bonds at the crystalline surface [3]. The insertion of a thin intrinsic amorphous layer between the crystalline substrate and the emitter layer reduces the interface state density and improves the conversion efficiency of heterojunction solar cells [4, 5]. In our opinion, the chemical preparation of the crystalline surface, the pre-deposition plasma treatment and the deposition parameters need to be investigated for further technology optimisation. This study focuses on ITO/(i- or p-type) a-Si:H/(n-type) c-Si heterostructures in order to develop the processing techniques and to investigate carrier transport and photogeneration in such structures. EXPERIMENT Heterojunctions having ITO/(i)a-Si:H/(n)c-Si/metal and ITO/(p)a-Si:H/(n)c-Si/metal configurations were fabricated and characterise
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