Wide Bandgap Fluorinated Silicon-Carbide-Nitride Films Using NF 3

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WIDE BANDGAP FLUORINATED

SILICON-CARBIDE-NITRIDE FILMS USING NF3

H.C. GOH, S.M. TAN, H.A. NASEEM, S.S. ANG, AND W.D. BROWN Department of Electrical Engineering, University of Arkansas, Fayetteville, AR 72701 ABSTRACT Amorphous hydrogenated silicon carbide has been studied extensively because of its properties as a wide bandgap material. However, a large amount of methane is needed to deposit the material. Also, the high carbon content of these films poses some problems. The addition of NF 3 to the gas stream results in wide bandgap films with a substantial reduction in the required CH4 flow for deposition. Amorphous SiC1 NY:H:F films were prepared using rf glow discharge decomposition of silane, methane, and nitrogen trifluoride in a parallel-plate stainless steel reactor. Gas flow rate and power density were varied. For a gas mixture containing 6% NF 3 and 78% CH 4, FTIR measurements reveal a reduction in C-H peak heights at 2960 cm-1 and 2880 cm-1 with respect to the Si-H peak at 2080 cm-W indicating a smaller carbon content in the film. The C-H peaks shift to higher wavenumbers with increasing NF 3 . The use of NF 3 increases the bandgap from 2.6 to 3.14 eV while reducing the refractive index from 2.12. to 1.87. A maximum deposition rate of 625 A/min was achieved. This should be compared to the very low deposition rate of 18 A/min for comparable bandgap Si-C films deposited using 97% methane in silane. Increasing the deposition power density resulted in a larger bandgap and a smaller refractive index. INTRODUCTION P-type silicon carbide has been widely employed as a window layer for high efficiency PIN solar cells. Its wide bandgap was found to increase both the open-circuit voltage and the shortcircuit current. However, the carbon incorporation is very poor. To deposit a-SiC films with bandgaps of 2.5 eV or larger, as much as 60-70 % methane, in the silane-methane mixture, must be used. This large percentage of methane reduces the deposition rate substantially. A recent study [1] directly correlates the formation of microvoids in SiC films to the amount of CH 4 used in the gas phase. Microvoids result in films with poor optoelectronic quality. The CH 3 species in the film are thought to be linked to microvoid formations. Also, in poor quality films, the SiH peak at 2080 cm-' in the FTIR spectra, is related to microstructure [3]. Wide bandgap a-Si films deposited from the addition of NF 3 to silane, with improved thermal stability, have been reported previously [2]. In that study, the NF 3 was shown to increase the deposition rate by enhancing the formation of Si-Si bonds. The role of fluorine is believed to be that of an etchant in the gas phase; it is known to etch weak Si-Si and Si-N bonds. Also, fluorine may passivate the midgap dangling bonds. NF 3 was added to the silane-methane mixture in this work in an attempt to create SiC type bonds and increase the deposition rate.

Mat. Res. Soc. Symp. Proc. Vol. 219. @1991 Materials Research Society

764

EXPERIMENTAL PROCEDURES The a-SiC1 N,:H:F alloy films wer