CW Argon-ion Laser Crystallization of a-Si:H Thin Films
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CW Argon-ion Laser Crystallization of a-Si:H Thin Films A. Sunda-Meya*, D. Gracin*†, J. Dutta*, B. Vlahovic*, R.J. Nemanich**, *Department of Physics, North Carolina Central University, Durham, NC **Department of Physics, North Carolina State University, Raleigh, NC †On leave from Materials Science Division, Rudjer Boskovic Institute, Zagreb, Croatia ABSTRACT Thin a-Si:H films, with a thickness of 1 µm, with different hydrogen concentrations, prepared by hot wire deposition were crystallized by 514.5 nm cw Ar ion laser radiation, with a power density between 150 and 270 kW/cm2. The crystallization was continuously monitored by Raman spectroscopy for exposures up to hours. The analysis of crystallization process using Johnson-Mehl phenomenological equations showed an apparent crystallization energy of around 0.5 eV and low dimensional crystal growth. The mean value of the crystal size decreases with increasing irradiation energy and initial hydrogen content and varies between 3 and 6 nm. INTRODUCTION The crystallization process of amorphous silicon films on glass substrates is important from a technological point of view for its potential use in the production of hybrid, amorphous/polycrystalline silicon solar cells (1-5). Laser crystallization appears to be more promising compared to thermal crystallization because it overcomes the conflicting requirements of high temperature for crystallization and low temperature for not destroying the glass substrate, by depositing almost all of the laser energy directly into the silicon film (6). From the scientific point of view, while there has been extensive work in the field, there remains significant details that are not well understood (7). EXPERIMENTAL The 1 µm thick amorphous silicon films, prepared by hot wire deposition plasma enhanced chemical vapour deposition (HW PECVD) on a glass and monocrystalline silicon substrate, were illuminated with 514.5 nm radiation from an Ar ion laser. The laser beam was focused through the optical system on a spot with diameter of about 5 µm. The laser irradiation power density was varied between 150 and 270 kW/cm2 and irradiation times were between 0.85 h and 2 h. During the laser exposure, the Raman spectra were measured by using an Innova Ar ion laser and a U1000 ISA spectrometer, equipped with an Olympus microscope and photomultiplier detector. The hydrogen concentration was estimated from FTIR spectroscopic data using the method described in ref (10). The average temperature in the irradiated volume was estimated from the ratio of Stokes and anti-Stokes intensities using the procedure described in ref (8), (9). This approach typically yields absolute values, which are too low by about 100K, but the relative temperatures are more accurate.
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RESULTS AND DISCUSSION At the beginning of the annealing procedure, the Raman spectra are typical of amorphous silicon, with a pronounced, broad TO peak at about 480 cm-1 and a broad TA peak at 150 cm-1. Shortly after initiating the laser exposure, the peaks corresponding to amorphous ph
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