75 FLUORINATED SILICON NITRIDE (a-Si:N,F,H) FILMS USING NF 3 FOR AMORPHOUS SILICON BASED SOLAR CELLS FREDDY H.C. GOH, S.M. TAN, K. NG, H.A. NASEEM, W.D. BROWN, AND A.M. HERMANN Department of Electrical Engineering, University of Arkansas, Fayetteville, AR 72701 ABSTRACT The large amounts of methane commonly used to prepare wide-gap a-SiC:H films present a lot of problems. This study was aimed at investigating the properties of fluorinated silicon nitride as an alternative to silicon carbide and to study the thermal stability of the films. The RF glow discharge of NF 3 (an etchant gas) and pure SiH 4 produced a-Si:H,N,F films. Films were deposited with a gas ratio of NF 3 over (NF 3 + SiH 4 ) from 0% to 15%. The substrate temperature, chamber pressure, and power were kept constant; only flow rate was varied. Characterizations of the films included AES, FTIR, visible/UV spectroscopy, dark conductivity, and photoconductivity. Deposition rate increased about eight times with an addition of 6% NF 3 . The deposition rate increased from 1.22 A/s to 10.5 A/s. The Tauc optical gap increased from 1.72 eV to about 2.06 eV, suggesting excellent N incorporation. The dark conductivity decreased one order of magnitude while the photoconductivity decreased by as much as five orders of magnitude with ELH light. FTIR spectra showed the nitrogen incorporation. The Si-N bonds replaced the Si-Si bonds with increasing NF 3 concentration. Also with increasing NF 3 content in the gas phase, the hydrogen concentration in the material decreased and the films became more thermally stable. INTRODUCTION Optical gap of the window layer is an important parameter in the fabrication of high efficiency p-i-n based amorphous silicon solar cells. Heavy boron doping of the p-layer narrows its optical gap [1). This restricts the amount of light entering the i-layer. To alleviate this problem, carbon is often introduced in the p-layer to form heterostructure p- SiC/i-Si/n-Si solar cells. However, carbon incorporation efficiency is very small, and it introduces large density of defects in the film [2]. Some studies of a-SiN:H as a possible wide gap window layer have been reported in the literature (3). Fluorine has been reported to passivate the midgap dangling bond states more effectively than hydrogen [4). This is believed to be due to the higher electronegativity of fluorine as compared to hydrogen. Fluorine in the plasma is also a strong etchant of SiN and Si. It is expected to etch away weak Si-Si and Si-N bonds. Although hydrogen is needed to passivate the 1019 cm" 3 or so dangling bonds, excess hydrogen (6-10%) present in the films is known to be a source of degradation of a-Si solar cells [5). Lower hydrogen content in these films is expected to improve the light and Mat. Res. Soc. Symp. Proc. Vol. 192. 01990 Materials Research Society