Band Gap Engineering and Electrical Field Tailoring for Voltage Controlled Spectral Sensitivity
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0910-A17-02
Band Gap Engineering and Electrical Field Tailoring for Voltage Controlled Spectral Sensitivity M. Vieira1, P. Louro1, A. Fantoni1, M. Fernandes1, G. Lavareda2, and C.N. Carvalho2 1 DEETC, ISEL, R. Conselheiro Emidio Navarro, 1, Lisbon, 1949-014, Portugal 2 FCT-UNL, Caparica, 2829-516, Portugal ABSTRACT Stacked ITO/(a-SiC:H)pinpi /(a-Si:H)i’n’/ITO color sensitive detectors are analyzed using a laser scanned photodiode technique. Results show that band gap engineering, together with the laser scanned technique allows a voltage controlled shift of the collection regions and color discrimination in the readout voltage that cancels the self-bias effect induced by the steady-state illumination, across the back diode. The threshold voltage, between the green and red discrimination depends on the thickness ratio between a-Si:H (-i’)/a-SiC:H (-i) layers. As this ratio increases, the self-reverse effect due to the front absorption will be balanced by the decrease of the self-forward effect due to the back absorption shifting the threshold voltage to lower reverse bias. The various design parameters, and the optical readout process trade-offs are discussed and supported by a 2D numerical simulation. A self-bias model is proposed to explain the voltage controlled spectral sensitivity. INTRODUCTION Amorphous silicon carbon (a-SiC:H) based color detectors with controllable sensitivity are considered to be a promising technology [1, 2]. Light filtering, employing distinct wavelength optimized structures in an array containing many integrated detectors, is rather complex and is an expensive solution. Therefore, the possibility to obtain RGB signals directly from one single large area detector is attractive. Optical filters may be eliminated by using a-SiC:H multilayer stacked devices, in which the detector structure itself behaves as a filter. The complexity of the interconnection is reduced, while the sequence of test voltages applied to the device provides color information. By sampling the absorption region under different bias voltages, and extracting separately the integrated information about the radiation absorbed in each region is possible to identify two or all the RGB components of the visible spectrum [3, 4]. In this paper color pinpii’n’ sensitive detectors, with different layers thickness are tested using the laser scanned photodiode technique (LSP) [5, 6]. This technique allows the complete color analysis to be performed with a single two terminal detector element and an optically addressed readout. The effect of the read-out voltage on the color selectivity is discussed and supported by a selfbias model. The physics behind the device functioning is explained by recurring to a numerical simulation of the device in the dark and under different wavelength irradiations. EXPERIMENTAL DETAILS Fabrication and characterization of the double-junction sensing structures. The sensor element consists of a 4x4 cm2 glass/ITO/p-i-n a-SiC:H multilayer structure which faces the incident illumination (front diode) follow
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