Low Defect Density Microcrystalline-Si Deposited by the Hot Wire Technique
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825 Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society
obtaining state of the art Ac-Si material by the HW technique at lower temperatures and with higher deposition rates. EXPERIMENTAL DETAILS The HW samples used in this study were deposited concurrently in a single chamber reactor on Coming 7059 glass and crystal Si (c-Si) substrates using a deposition geometry described previously [8]. While the H 2 to SiH 4 dilution ratio remained fixed at 10/1, deposition rates and film quality were examined as a function of substrate temperature (Ts), chamber pressure (Pch), total gas flow, and filament current. The filament temperatures were determined using a two-wavelength optical pyrometer, and were found to agree very well with temperatures predicted from tables of current vs. wire diameter for pure tungsten filaments [9]. On all samples, infrared (IR) and X-ray diffraction measurements were done on the samples deposited on c-Si to examine the H bonding, confirm sample microcrystallinity, and determine crystallite orientation. On selected samples, X-ray diffraction measurements were made on samples deposited on both the glass and c-Si substrates; the data in all cases showed similar features, indicating identical structural growth on both the amorphous and crystalline substrates. A dual beam Perkin-Elmer 580-B Spectrometer was used for the IR measurements, which probed the 1 1 total H content (CH) as well as the type of H bonding (2000 cm- vs. 2090 cm- ), while a Braggwas monochrometer, crystal a graphite with selected X-rays with Cu-Ka geometry, Brentano used for the X-ray analyses. For the crystallite orientation, the relative heights of the various diffraction peaks were compared to that of a powder pattern for c-Si. On selected samples, Raman and CPM (constant photocurent method) measurements were performed to explore the film microcrystalline fraction and the defect density. The Raman measurements were made at room temperature in the backscattering geometry using the 514.5 nm line of an Ar laser. For the CPM measurements, Ni-Cr electrodes were evaporated, using a direct mask, with spacings of 2 mm, 200 pim, and 80 gm, and the CPM absorption spectra were measured in the range 2.2-0.8 eV using methods described elsewhere [10]. Because the present samples look 'mirror-like', the CPM measurements with a narrow interelectrode spacing are not influenced by light scattering, and therefore yield the true values of the optical absorption coefficients. Film thicknesses ranged from 0.7-2.0 4m. RESULTS AND DISCUSSION In the following discussion we show the CPM absorption spectra (Fig. 1), representative IR spectra (Fig. 2) illustrating the type of H bonding, and the X-ray diffraction patterns (Fig. 3) for a series of HW films deposited at two different T. as a function of Pch. Also shown in Fig. 1 are comparison curves representing c-Si and the state-of-the-art VHF-GD deposition technology [11]. The high Ts HW films were deposited using a flow rate ratio of 80/9 (in sccm), a filament current of 14A (which c
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