Influence of Nanocrystallinity on Properties of Photodiode and TFT Image Sensor Structure
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detected by the a-Si:H photodiode and the photogenerated charge is stored as the diode capacitance Cp. After one frame time, the transistor is switched on and the signal charge is transferred to the readout amplifier. The dynamic performance of the image sensor is limited by the readout of the pixel capacitance Cp, through the transistor on-resistance Ron, and by thermal emission of trapped charge within the photodiode. With an optimized transistor technology, readout can be accomplished in about 10jts. The influence of inconvenient phenomena such as charge trapping can be lessened when using microcrystalline silicon instead of amorphous silicon. Throughout this paper we consider a few new technological experiments used during the process of creating integrated multilayer structures of a-Si:H image sensors consisting of switching TFT and pin photodiodes. Our proposal is to use the reactive magnetron sputtering technique to create aforementioned multilayer structures, particularly nc-Si:H layers [3]. The premises of creation of a photodiode based on a nc-Si:H layer lie in its ability to retain high optical absorption and photo-response, characteristic of the a-Si:H layer, and electronic stability as well as thermal trapped charge on the other hand. The grain size in these films is a relevant parameter closely related to the electronic properties. In case of the switching TFT, the needs of increased Ron and channel mobility ýt can be achieved by means of transistor channel recrystallization [4]. In this paper results are described of channel direct recrystallization using a YAG laser, mainly because of the simplicity and low cost of this method. Because of the degrading electronic features, influenced by the interface states between the nc-Si:H and SiNx films as well as multilayer diffusion 913 Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
between n+, p+ and i a-Si:H layers, additional nitridation by strong flux on N ions has been used. The aforementioned experimental results actually are mean values collected over 160 samples. EXPERIMENTAL DETAILS The TFT and diode structures in applications as LCDs, ISFETs and linear and 2D image sensors (visible range) have been studied over the last few years. For these applications we used 2 to make test samples of the form: two rows and 30 to 160 pixels per row with dimensions 0.1 mm to Imm 2. During our research, the DC reactive magnetron sputtering technique was used to obtain appropriate films and multilayer structures, and a considerable number of preparatory parameters had to be optimized to produce low defect n+, p+, i a-Si:H, a-SiNx structures as well as their in-
terfaces. It can be observed that the properties of the mentioned films are functions of substrate temperature, Ts, applied power, Pr, argon partial pressure, pAr, hydrogen partial pressure, PH2, ammonia partial pressure, PNH3, phosphine partial pressure PPH3, diborane partial pressure, PB2H6, total pressure, P2, shield potentials, surface quality and degree of consumption of silicon t
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