Large area single and stacked p-i-n photodiodes as a color image sensors
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Large area single and stacked p-i-n photodiodes as a color image sensors 1
P. Louro, 1M. Fernandes, 1A. Fantoni, 1A. Maçarico, 2C.Nunes de Carvalho, 2G. Lavareda, 1 M. Vieira 1 Electronics Telecommunications and Computer Dept., ISEL, Lisbon, Portugal. 2 Centro de Física Molecular, IST, Lisbon, Portugal. ABSTRACT Single and stacked p-i-n sensing elements for image recognition and color extraction applications are presented. The aim of this work is to optimize the performance of the a-SiC:H thin films layers in order to enhance its performance when making part of the structure of large area image and color sensors. The efforts are focused mainly on doped n- and p-type layers at high and low doping levels with and without carbon. The structural and optoelectronic properties of the single layers were determined through infrared and visible spectroscopy, temperature-dependent conductivity, and were complemented by CPM measurements. Junction properties, carrier transport, photogeneration and collection efficiency are investigated from dark and illuminated current-voltage characteristics and spectral response measurements, with and without additional background illumination and under different light bias conditions. The spectral response dependence on the applied voltage and on optical bias was also studied. Results show that the spectral sensitivity is strongly dependent on the applied voltage, namely the maximum spectral sensitivity shifts with the voltage, and at certain wavelengths the spectral response goes down to zero, which allows a different selectivity, and enables color recognition. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) exhibits excellent photosensitive properties. It is very sensitive to visible light and its spectral response matches the sensitivity of the human eye much better than crystalline silicon does. Therefore, a-Si:H has been largely employed in imaging devices. Thin films of aSi:H and related alloys have also been intensively used in devices for colour recognition, as the applied bias voltage can modulate its spectral response. Various structures and sequences have been suggested [1, 2]. In our group efforts have been devoted towards the development of a new kind of color sensor, the Color Laser Scanned Photodiode Sensor (CLSP) [3, 4, 5]. Simultaneous image and color detection are achieved by combining the wavelength filtering property of silicon with the sensor responsivity dependence on the applied electrical and optical bias. The optimization of this trade-off demands a full understanding of the transport mechanism. The aim of this work is to describe the results of experiments as well as to discuss the usefulness of the a-SiC:H doped layers in the improvement of the sensor performance (image and color sensing). DEVICE STRUCTURE Amorphous silicon and silicon carbide pin and stacked p-i-n-p-i-n structures were produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) in a three chamber load-lock UHV-system [6] at a
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substrate temperature of 110 ºC, a pressure of
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