Image Acquisition Using Non-Pixeled Amorphous Silicon Based Sensors
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Image Acquisition Using Non-Pixeled Amorphous Silicon Based Sensors
M. Fernandes, Yu. Vygranenko, J. Martins and M. Vieira Electronics and Communication Dept., ISEL, R. Conselheiro Emidio Navarro, 1949-014 Lisboa, Portugal. ABSTRACT We suggest to enhance the performance of image acquisition systems based on large area amorphous silicon based sensors by optimizing the readout parameters such as the intensity and cross-section of scanner beam, acquisition time and bias conditions. The main output device characteristics as image responsivity, signal to noise ratio and spatial resolution were analyzed in open circuit, short circuit and photodiode modes. The result show that the highest signal to noise ratio and best dark to bright ratio can be achieved in short circuit mode. It was shown that the sensor resolution is related to the basic device parameters and, in practice, limited by the acquisition time and scanning beam properties. The scanning beam spot size limits the resolution due to the overlapping of dark and illuminated zones leading to a blurring effect on the final image and a consequent degradation in the resolution. INTRODUCTION Large area hydrogenated amorphous silicon p-i-n structures with low conductivity doped layers were proposed as single element image sensors [1]. The Laser Scanned Photodiode is fundamentally different from the other electrically scanned image systems since it is based on one single sensing element and an opto-mecanical acquisition system. The image to acquire is optically mapped on the photosensitive surface and a low-power chopped laser scans the sensor in the raster mode. The physical process of the laser scanning image acquisition is explained by the modulation of the electrical field across the junction under non-uniform steady-state illumination [2,3]. If, in addition, a weak light spot is scanning the device, in the dark regions the carriers generated by the probe beam are separated by the junction electric field and collected (high ac value of the photocurrent). Those generated inside the illuminated regions can diffuse or drift in the lateral direction. A large number of those carriers will, however, recombine inside the amorphous bulk (low ac value of the photocurrent). Several enhancements on the sensor structure were proposed [4] resulting in an optimization of the sensor structure and composition, but maintaining the same readout technique and conditions. Further optimization of the sensor performance can be obtained by a correct choice of the readout parameters, like scanner beam spot size, image acquisition time and sensor bias. This work aims to clarify the possible improvements, physical limits and performance of this type of device. SENSOR STRUCTURE For this work large area (4×4 cm2) amorphous p-i-n structures in the assembly glass/ZnO:Al/p (Si:H)/i (Si:H)/n (SiC:H)/Al were used. All the layers were deposited by Plasma D5.17.1
Enhanced Chemical Vapor Deposition, at 13.56 MHz radio frequency [5]. The deposition pressure was 200 mTorr, the substrate temperature was
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