Amorphous Silicon Based p-i-i-n Structure for Color Sensor
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Amorphous Silicon Based p-i-i-n Structure for Color Sensor S. Zhang, L. Raniero, E. Fortunato, L.Pereira, H. Águas, I. Ferreira, R. Martins Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa and CEMOP/UNINOVA, Campus da Caparica, 2829-516 Caparica, Portugal Phone: 351-212948524, Fax: 351-212941365, e-mail: [email protected]
This work deals with the study of the role of the film thickness and composition on the color selectivity of the collecting spectrum of glass/ZnO:Ga/p-a-Si1-xCx:H/ a-Si1xCx:H /a-Si:H/n-a-Si:H/Al photoelectronic detectors produced in a single chamber plasma enhanced chemical vapor deposition (PECVD) system. The cross contaminations were minimized by a rotate-cover substrate holder system. The devices can detect the blue illumination at small reverse bias and detect red illumination at large reverse bias. The role of the process parameters, especially the thickness of the p-type and intrinsic a-Si1xCx:H, and the intrinsic a-Si:H layers on the device performances were studied in detail aiming to achieve a better detectivity. 1. Introduction The optical band gap of hydrogenated amorphous silicon (a-Si:H) can be modified by adding controlled amounts of carbon or germanium during the deposition. Besides that, the a-Si:H based photo detectors can be obtained with sensitivities ranging from the ultraviolet to the infrared, in small or large areas, taking profit of the excellent electrooptical characteristics presented by these films when produced by chemical vapor deposition (PECVD) technique that make then quite suitable for a wide range of optoelectronic device applications such as solar cells, position sensors, and image devices [1-3]. In the recent years, bias controlled two- and three-color detectors on stacked a-Si1xCx:H /a-Si:H heterostructures have been extensively investigated due to their advantages [excellent photosensitivity from the ultraviolet A (UVA) to the near infrared (IR) part of the spectrum, low cost fabrication, the ability to be deposited at low temperatures and homogeneously over a large area and Eop controllability] [4-7]. The particular features of these devices allow the modulation of maximum spectral response by the applied bias voltages [8-10]. Bipolar color detectors require a sign reversal of the applied bias to switch the main collection region between the two diodes. All these devices suffer from a slow transient response due to long-term trap recharging currents caused by the back-toback configuration of two diodes [11]. The p-i-i-n layer structure (consist of two high band gap hydrogenated amorphous silicon carbide (a-Si1-xCx:H) layers in the front part of the device followed by two amorphous silicon layers) was first designed by Neidlinger, Schubert, Schmid, and Brummack to overcome the speed limitation of bipolar color detectors (such as n-i-p-i-n and p-i-n-i-p structures) [12]. In the unipolar p-i-i-n structure, the electric field profile allows the collection region of charge carriers to extend from the surfac
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