Hydrogenated Amorphous Silicon / ZnO Shottky Heterojunction for Position Sensitive Detectors
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Hydrogenated Amorphous Silicon / ZnO Shottky Heterojunction for Position Sensitive Detectors H. Águas, P. Nunes, E. Fortunato, R. Silva, V. Silva, J. Figueiredo, F. Soares, R. Martins Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2825-114 Caparica – PORTUGAL ABSTRACT In this work a new structure is proposed for position sensitive detectors consisting of glass/Cr/aSi:H(n+)/a-Si:H(i)/ZnO, where the ZnO forms an heterojunction with the a-Si:H(i). The results show that this structure works with success in the fabrication of linear position sensitive detectors. The devices present a good nonlinearity of ≈ 2% and a good sensitivity to the light intensity. The main advantages of this structure over the classical p-i-n are an easier to built topology and a higher yield due to a better immunity to the a-Si:H pinholes, since the ZnO does not diffuse so easily into a-Si:H as the metal does, which are the cause of frequent failure in the p-i-n devices due to short-circuits caused by the deposition of the metal over the a-Si:H. In this structure the illumination is made directly on the ZnO, so a transparent substrate is not needed and a larger range of substrates can be used. INTRODUCTION The use of position sensors based on amorphous silicon structures (a-Si:H) for components alignments or to control the positioning of articulates parts of platforms is growing up [1]. Up to now the most used structures are either the p-i-n structure [2,3] or alternatively the MIS (metal insulator semiconductor) structure [4]. Nevertheless, for microelectronic detectivity applications, it is required the use of high response devices [1] or to produce these devices in flexible substrates to control micro motors, for instance [5]. Therefore, to produce these devices using Schottky structures is quite attractive if response times have to be improved. In this paper we present a new structure to produce high response time position sensors based on an heterojunction between a-Si:H and ZnO:Al films. Up to now, (ZnO) thin films have been widely applied as transparent ohmic contact in a-Si:H solar cells due to its high transparency, relatively high electrical conductivity, and textured surface [6]. Apart from that, ZnO has the advantage of not diffusing inside the a-Si:H and so, does not give rise to micro-shunts and can also form efficient Schottky type heterojunction with a-Si:H. In this work we studied a structure using a-Si:H/ZnO junction, where ZnO has to be enough resistive to avoid the formation of a device equipotential and the a-Si:H enough thick to absorb the radiation light to be used [1]. In this structure ZnO forms a Schottky barrier with intrinsic aSi:H. This ZnO/a-Si:H heterojuntion is presented here by the first time as an alternative to the classical p-i-n structure in linear position sensitive detectors (PSD) devices. The PSD devices made with this new structure present as the first advantage to have one-step process less in their production than the p-i-n, besides presenting fas
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