Stacked n-i-p-n-i-p Heterojunctions for Image Recognition
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Stacked n-i-p-n-i-p heterojunctions for image recognition M. Vieira, A. Fantoni, M. Fernandes, P. Louro, I. Rodrigues Electronics Telecommunication and Computer Dept. ISEL, Rua Conselheiro Emídio Navarro, P 1949-014 Lisboa, Portugal Tel: +351 21 8317181, Fax: +351 21 8317114, E-mail: [email protected]. ABSTRACT This work aims to clarify possible improvements and physical limits of the Color Laser Scanned Photodiode image sensor when used as high sensitive non-pixel image reader. A new design based on a stacked n-i-p-n-i-p heterojunction is proposed and compared with the old single n-i-p sensing structure. Results show that a B-W image is acquired with an improved resolution. The readout frequency is optimized showing that scans speeds up to 104 lines per second can be achieved without degradation in the resolution. A physical model is presented and supported by an electrical and a numerical simulation of the output characteristics of the sensor. INTRODUCTION Amorphous silicon-carbon (a-SiC:H) is a material that exhibits excellent photosensitive properties. The possibility to modify the optical gap enables the detection from the UV to the IR part of the spectrum. This feature has been intensively used in the development of image sensors [1, 2]. In our group efforts have been devoted towards the development of a new kind of image sensor, the Color Laser Scanned Photodiode sensors (CLSP) [3, 4, 5]. The CLSP consists on one large cell detector and the image is scanned by sequentially detecting scene information at discrete XY coordinates. The advantages of this approach are quite obvious: the feasibility of large area deposition on different substrate materials (e.g. glass, polymer foil, etc.), the simplicity of the device and associated electronics, the high resolution, the uniformity of measurement along the sensor and the cost/simplicity of the detector. The design allows a continuous sensor without the need for pixel-level patterning, and so can take advantage of the amorphous silicon technology. It can also be integrated vertically, i. e. on top of a read-out electronic, which facilitates low cost large area detection systems where the signal processing can be performed by an ASIC chip underneath. This work aims to clarify possible improvements in the CLSP image sensor. The image capture device and the scanning reader are optimized and the effects of the sensor structure on the output characteristics is discussed. EXPERIMENTAL Sample preparation and characterization Single and tandem a-SiC:H p-i-n diodes were fabricated in a three-chamber load-lock UHVsystem by Plasma Enhanced Chemical Vapor Deposition at 13.56 MHz on ITO and Cr coated glass substrates, respectively. The back metal contacts define the active area of the sensor (4×4 cm2). The single structure is a p-i-n diode and the tandem one is composed by two stacked n-i-p diodes (see Figure 1). The p-layers of both single diode and of the second diode of the stacked structure were fabricated introducing methane during the deposition process, in order to decr
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