Thin Film Transistors Made from Hydrogenated Microcrystalline Silicon

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For convenience, one-mask structure was adopted to fabricate TFT, and it is shown in Fig. 4. In order to confirm the effect of structure change of the materials studied above, thin film transistors based on hydrogenated silicon were fabricated. The Si:H materials made with hydrogen dilution and atomic hydrogen treatment were used as the channel layer of the TFT's. Inverted staggered structures of TFT's with PECVD deposited silicon nitride (SiNx:H) as the gate insulator and n+ a-Si:H for ohmic contact with aluminum source/drain metal were fabricated. The W/L was 2500 gm/ 50 jim. The deposition conditions and characterization of the layers used for the TFT's are summarized in Table I. RESULTS AND DISSCUSION Fig. 1 shows the results of NMR spectra. Fig. 1 (a) shows the NMR spectrum for the Si film deposited by the convetional method (H 2/(SiH 4 +H2 ) : 40%). The spectrum includes a narrow Lorentzian-shape spectrum with full-width half-maximum (FWHM) of about 3.4 KHz and a broad spectrum with FWHM of about 23.5 KHz. The narrow line-shape corresponds to the randomly distributed Si-H structure in the a-Si:H network, and the broad spectrum corresponds to the region associated with clustered hydrogen which may contain internal surface, poly-hydride groups, and poly-silane chains. Fig. 1 (b) shows the NMR spectrum of the Si film deposited by Dilu. method in which both the Lorentzian-shape spectrum and the Gaussian-shape spectrum were narrower than those of Fig. 1 (a). Fig. 1 (c) shows the NMR spectrum of the Si film deposited by Htr. method. Sharp line-shape spectrum were obtained from the samples shown in Figs. I (c) and (d) .The sharp line-shape should come from molecular hydrogen. The Raman spectra were investigated to study the microstructure of Si films deposited by the methods mentioned above. The typical Raman shift is peaked at 480 cm-1 for amorphous Si and peaked at 520 cm"1 for crystalline Si. Fig. 2 shows the Raman spectra of the Si films deposited by (a) Cony. method, (b) 90% Dilu. method, (c) 98% Dilu. method, and (d) 90% Htr. method. Figs. 2 (a) and (b) show the broad spectra with a peak at around 480 cm-1 and having a FWHM of 60 cm-1. The result means that the silicon films deposited by Cony. method and 90% Dilu. method were mostly amorphous phase and possibly with some small fraction of microcrystalline phase. Figs.2 (c) and (d) both show the sharp spectrum with a peak centered at 520 cm-1 having a FWHM close to 15 cm- 1. The fraction of microcrystalline phase is much enhanced in the silicon film deposited by 98% Dilu. and 90% Htr. method. The room temperature optical (Tauc's) bandgap of amorphous silicon is about 1.7 eV, and the optical bandgap of crystalline silicon is 1.1 eV. Fig. 3 shows the optical bandgap of silicon films deposited by different methods. The optical bandgap of sample made by the Cony. method (40% Dilu.) is 1.69 eV, and this value increased a little for the sample made by 90% Dilu. method. As the H 2/(SiH 4 +H 2) fraction was further increased, the optical bandgap descreased. 1.64 eV ban