A real-time optical signal and image processing p-i-n/p-i-n device
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A real-time optical signal and image processing p-i-n/p-i-n device M. Vieira, P. Louro, M. Fernandes, A. Fantoni, C. Mendes, J. Martins Electronics Telecommunication and Computer Dept. ISEL, Rua Conselheiro Emídio Navarro, P 1949-014 Lisboa, Portugal Tel: +351 21 8317290, Fax: +351 21 8317114 , [email protected] ABSTRACT Large area (4x4 cm2) optical signal and image processing (OSIP) devices were produced at low temperatures (110 ºC) by PE-CVD. The OSIP device consists of two stacked sensing/switching diodes (p(SiC:H)/i(Si:H)/n(SiC:H)) with or without an internal blocking layer between them and two semitransparent contacts. An optical scanner is used for charge readout. In this work the main emphasis will be put on the analysis of the optical characteristics. The use of a metal grid (290x290 µm2 Cr pixels with 40 µm spacing) between the two diodes, working as light screening layer or as floating anode via an a-SiN insulator layer, is analyzed. Its influence on the transfer functions, resolution, responsivity and response time of the sensor is presented. The various design parameters trade-offs are discussed. The optical-to-electrical transfer characteristics show high quantum efficiency, broad spectral response, and reciprocity between the optical and the electrical images. When the light screening floating anode is present an effective optical decoupling from both photodiodes is achieved while maintaining a good electrical conductivity and an increased light-to-dark sensitivity. A trade-off is established between sensor design and light pattern and scanner wavelengths in order to minimize the cross talk between the write and the read beams and to improve the light to dark sensitivity. INTRODUCTION In our group large area hydrogenated amorphous silicon single and stacked p-i-n structures with low conductivity doped layers were proposed as image sensors [1, 2, 3]. These sensors are different from the other electrically scanned image sensors [4, 5] as they are based on only one sensing element with an opto-mechanical readout system. The image to acquire is optically mapped onto the photosensitive surface and a low-power light spot scans the device. The photocurrent generated by the moving spot is recorded as the image signal, and its magnitude depends on the light pattern localization and intensity. Advantages to this approach are large area imaging, high resolution, and uniformity of measurement along the sensor. Through the tailoring of the sensor configuration and readout parameters this work evaluates the possibility of using two stacked sensing/switching p-i-n photodides, with an optical screening layer in-between, as a high-sensitivity optically addressed read-write image-processing device (OSIP). Efforts are focused mainly on the optimization of the sensor performance (contrast and resolution). A trade-off is established between sensor design and light pattern and scanner wavelengths. EXPERIMENTAL OSIP configuration The optically addressed device (OSIP) consists of two stacked p(SiC:H)/i(Si:H)/n(SiC:H) se
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