Effect of Deposition Temperature on the Photoresponse of Crystallized Hydrogenated Amorphous Silicon Films
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Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
resulting polysilicon film obtained after crystallization of a-Si:H. In our previous work [13], we observed that the a-Si:H film deposition temperature plays a major role on the photoresponse of the polysilicon film obtained by the crystallization of dc glow discharge a-Si:H film by furnace annealing. We found that the increase in grain size and crystallinity with decreasing aSi:H deposition temperature improved the photoresponse. (There was no effect of hydrogen passivation in these films, since crystallization of a-Si:H by furnace annealing results in the evolution of hydrogen from the film.) In this work, we employed an alternate crystallization technique, namely pulsed laser annealing, whereby the out-diffusion of hydrogen is minimized [14, 15] and crystallization occurs in the liquid phase, as opposed to the solid phase crystallization occurring in furnace annealing. The investigation on photoresponse was conducted as a function of a-Si:H deposition temperature through dark and photoconductivity measurements, Fourier transform infrared spectroscopy (FTIR) and evolution gas analysis (EGA), and supplemented by Raman scattering and x-ray diffraction (XRD). These results were then compared to those obtained for furnace annealing and a correlation was made between the structural properties and the photoresponse of the laser annealed polysilicon films.
EXPERIMENTAL Intrinsic a-Si:H films were deposited at Plasma Physics Corp. on molybdenum substrates by plasma enhanced chemical vapor deposition (or dc glow discharge) in a gradient field with cathodic deposition from silane, with deposition rates up to 1 Pim min"1. The deposition temperatures were 150, 225, 275, 300 and 350 OC; the pressure was 250-300 mtorr; the Sil 4 flow was 10 cm 3 min"1. The film thickness was 7 jim. All the samples had a thin n+ amorphous silicon layer (formed by the addition of 1 % PH 3 to H2 ) deposited on molybdenum prior to the deposition of the intrinsic film to provide ohmic contacts between silicon and molybdenum. The thickness of the n+ layer was about 500 A. The a-Si:H films were annealed by a Nd:glass pulsed laser having an output emission wavelength of 1060 nm. The maximum output energy of the laser was 60 Joules at I ms. The laser energy density was altered by employing beam splitters and changing the beam size (by means of a convex lens) and the supply voltage. The beam size was altered by employing a convex lens. The pulse width of the laser was 0.2 ms and the pulse frequency was 1 pulse/s. During annealing, the sample stage was translated in a direction perpendicular to the laser beam by a DC step motor in order to obtain a quasicontinuous laser scan. The experimental details for solid phase crystallization by isothermal furnace annealing had been previously reported [13]. After the annealing treatment, the films were characterized for crystallization, hydrogen detection and conductivity measurements. Crystallization measurements were done by employing Raman
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