A Novel High-Dynamic a-Si:H Multicolor pin-Detector with ZnO:Al Front and Back Contacts
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A Novel High-Dynamic a-Si:H Multicolor pin-Detector with ZnO:Al Front and Back Contacts Andreas Bablich1, Krystian Watty1, Christian Merfort1, Konstantin Seibel1 and Markus Boehm1 1 Institute for Microsystem Technologies, Siegen University, Hoelderlinstrasse 3, 57076 Siegen, Germany.
ABSTRACT Conventional silicon sensors provide insufficient capability for color reproduction by producing three color signals (RGB) to fit their pixel sensitivities to that of the human eye. Photo detector devices based on a-SiC:H and a-SiGe:H offer the opportunity to shift spectral sensitivity continuously by varying external bias voltages. Based on the measurement results of Rieve et al. [2] we determined the absorption coefficients of a group of a-SiC:H and a-SiGe:H layers to simulate the penetration depth of photons at different energies into the device structure [3]. Knowing the indices of absorption, refraction and extinction, it is possible to engineer diodes in such a way that accumulations of charge carriers are generated at varying device depths. The use of common chromium (Cr) cathodes avoids a sharp falling edge of the sensitivity towards longer wavelengths. The use of low reflective and highly conductive aluminum-doped zinc oxide (ZnO:Al) as a material for the back contact of the diodes suppresses Fresnel reflections and has been verified experimentally. A dynamic range of more than 100 dB at 1000 lux could be obtained by I-/V- measurements of a-Si:H pin diodes fabricated between two ZnO:Al contacts. Optically the detectors show maxima between 440 nm and 630 nm with reduced Fresnel reflections and a very low leakage current of 10-8 A/cm² at -5 V bias voltage. Present research efforts concentrate on the verification of the optical properties and exact thicknesses of the chosen layers as well as on the development of a front and back illumination device structure which ensures a continuous spectral shift of the photosensitivity. This work requires extensive bandgap engineering and offers good prospects to improve security imaging tools as well as chemical analysis systems. INTRODUCTION Common security imaging systems working with CCD or CMOS detectors are not able to distinguish colorimetrically between a large number of dangerous chemical substances, for example whitish powders [1]. Due to the excellent photo electric properties of hydrogenated amorphous silicon (a-Si:H) and its carbon-/germanium alloys, this material has become one of the leading semiconductors for many electro-optical devices, for example photo detectors or solar cells. a-Si:H alloys exhibit a higher absorption coefficient in the spectral range between 400 nm up to 800 nm than crystalline silicon (x-Si). For this reason, the related devices can be used for spectroscopic reflectance measurements, where the detection spectra correspond to the visible spectral range. The low temperature PECVD fabrication of a-Si:H photo diodes ensures compatibility to organic or other crystalline processes. The spectral sensitivity of common x-Si photo diodes is limited to j
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