Design and Modeling of Optical Sensors in Multi-Channel Technology
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DESIGN AND MODELING OF OPTICAL SENSORS IN MULTI-CHANNEL TECHNOLOGY D. KNIPPa,c, H. STIEBIGa, P.G. HERZOGb a) Research Center Jülich, Institute of Photovoltaic, 52425 Jülich, Germany b) Aachen University of Technology, Technical Electronics Institute, 52056 Aachen, Germany c) now with Xerox Corporation, Palo Alto Research Center, Palo Alto, CA 94304, USA ABSTRACT We have realized color sensors based on vertically integrated thin film structures to overcome color moiré or color aliasing effects. The complete color information of the color aliasing free sensors can be detected at the same position of a sensor array without optical filters. The color separation is realized by the wavelength dependent absorption of amorphous silicon and its alloys. The sensors consist of three amorphous silicon pin diodes. The spectral sensitivity of the sensors can be controlled by the optical and optoelectronic properties of the materials on one hand and the design of the devices on the other hand. In order to investigate the optical wave propagation within the device and to optimize the color separation we have developed an analytical optical model, which takes the optical properties of the materials and the device design into account. The optical model has been combined with a colorimetrical model, which facilitates the benchmarking of the color sensors and the reduction of the color error of the sensors. The analytical colorimetric/optical model and an improved device design will be presented. INTRODUCTION Color image processing is usually performed with the aid of CFA (color filter array) coated CCD or CMOS sensor arrays. However, color detection with a CFA leads to the color moiré or color aliasing effect, which is observed when structures with high spatial frequencies are captured. Furthermore, traditional sensor systems exhibit a rather limited resolution and low area fill factors, because one color pixel is split into several chromatic sub pixels. In order to overcome the color moiré effect, vertically integrated sensor structures have been proposed, which detect the color information in the depth of the sensor structure. Therefore, we have realized thin film devices based on three vertically arranged diodes of amorphous silicon and its alloys. Due to the device design blue light is mainly absorbed in the top diode and green light in the second diode, whereas red light mainly determines the photocurrent of the bottom diode. In order to match the demands of different applications the optical band gap as well as the transport properties of the material can be controlled over a wide range by the deposition conditions. Despite the advantages of amorphous silicon and its alloys, the color recognition of the multi-channel sensors is limited by the mismatch between the spectral sensitivity of the sensor and the sensitivity of the human eye. Particularly, on one hand there is a lack of absorption materials with a high optical band gap (>2.3eV) and good electrical properties, which limit the blue sensitivity of the top diode. On the oth
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