An Amorphous Silicon Photoconductor for UV Detection
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A18.15.1
AN AMORPHOUS SILICON PHOTOCONDUCTOR FOR UV DETECTION Matthias Hillebrand, Frank Blecher1, Jürgen Sterzel2, Markus Böhm Institute for Microsystem Technologies (IMT), Universität Siegen, D-57068 Siegen, Germany 1 now with LambdaLab, Kohlbettstraße 20, D-57072 Siegen, Germany 2 now with Jena Optronik GmbH, Prüssingstraße 41, D-07745 Jena, Germany ABSTRACT An amorphous silicon photoconductor to detect wavelengths between 180 nm to 550 nm without scintillator is presented. The photoconductor is based on a coplanar configuration of the electrodes, similar to measurement structures to determine material characteristics of amorphous layers, e.g. for the Constant Photocurrent Method (CPM). After passing through a thin transparent passivation layer, the incident radiation is directly absorbed in the intrinsic a-Si:H material. The carrier collecting electrical field is applied perpendicular to the incoming light. Test structures have been fabricated with 80 nm thick sputtered chromium contacts on top of a 60 nm carbonized hydrogenated i-layer and a SixNx passivation layer with a thickness of about 36 nm. The spacing between the Schottky contacts is varied between 3 µm and 100 µm. They are deposited on top or below the a-SiC:H layer. First experiments with this simple coplanar design show that with an increasing voltage a shift towards UV wavelengths can be observed. The new UV detector is applicable in the field of TFA image sensors (Thin Film on ASIC) and in the new Lab-on-a-Chip concept presently under development at the institute for microsystem technologies. INTRODUCTION Conventional UV detectors are made of semiconductor materials such as SiC and GaN. The advantage is the solar blindness. UV-sensitive CCDs usually use scintillators, which results in a high responsivity for visible wavelengths. Especially for flame detection less cross responsivity for visible light is needed. Amorphous silicon UV detectors are mostly based on ordinary pin-structures [1, 2, 3]. For this detector a TCO (Transparent Conductive Oxide) contact made of ZnOx or SnOx is applied as front electrode. Palma et al. [4] achieved solar blindness by using an aluminum grid as front contact, while the UV imager presented in [5] uses a transparent aluminum contact, supplemented with additional filters to shield the visible light. All of the above radiation detectors use structures with a collecting electrical field parallel to the incoming light and therefore suffer from losses in the first doped layer. The concept presented here is based on a coplanar structure (COS), which has not yet been used for UV detection. These structures are more often used for the qualification of a-Si:H layers, for example for CPM measurements, resistance measurements, SSPG measurements and noise measurements [6, 7]. Other coplanar structures are used in two dimensional position sensitive detectors (PSD) [8] and in thin film transistors (TFT) [9]. The UV detector presented here may be used in low cost TFA imagers [10] and offers a wide range of applications for securit
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