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

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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


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