Effects of precursor additives on the stability of plasma enhanced chemical vapor deposited a-GeC(O):H films
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Germanium- and carbon-based films were deposited by plasma-enhanced chemical vapor deposition from tetramethylgermane (TMGe) with additions of oxygen, hydrogen, or argon. The index of refraction, extinction coefficient, and optical gap and Fourier transform infrared spectra of the films were measured as well as their stability in regular ambiance. It was found that the films deposited from pure TMGe were stable in time only if deposited at a negative bias above −250 V direct current. Films deposited at a bias of −150 V direct current could be stabilized by significant additions of oxygen to the plasma and complete stabilization was achieved at O2 /TMGe ratios larger than 3 in the gas feed when GeOx films containing small amounts of C and H were obtained. Additions of hydrogen or argon to TMGe had only slight effects in improving the stability off the films.
I. INTRODUCTION
Amorphous films based on group IV elements have been studied extensively, and have shown potential for a variety of electronic, photovoltaic, optical, and mechanical applications. This is true for amorphous hydrogenated silicon (a-Si:H), amorphous hydrogenated carbon (aC:H), and their alloys (a-SiC:H). However, amorphous hydrogenated germanium carbon (a-GeC:H) films have not been investigated as extensively. Characteristics of such films prepared by sputtering have been reported in several studies1,2 while properties of a-GeC:H films prepared by plasma-enhanced chemical vapor deposition (PECVD) have been reported in others.3–7 The studies of films prepared by radio-frequency (rf) PECVD from tetraethylgermane or tetramethylgermane (TMGe) have shown that the film properties are strongly dependent on the deposition conditions, especially on the power input to the plasma used for their preparation.5–7 The films prepared at low energy input to the plasma were soft and semitransparent, and had a relatively low density (around 2.0 g/cm3) and an electrical conductivity below 10−10 S/m. At high rf power the films became stiff and non-transparent, their density rose to about 3.5 g/cm3 and their electrical conductivity to about 10−4 S/m.6 Furthermore, the films prepared under low power input underwent an aging process in the ambient atmosphere, and their properties changed with time. This behavior was attributed to the existence of substantial amounts of
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Also affiliated with Ecole Polytechnique, De´partement de Physique, 91128 Palaiseau Cedex, France J. Mater. Res., Vol. 17, No. 2, Feb 2002
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dangling bonds in the as-deposited a-GeC:H films, which, upon exposure to the atmosphere, became saturated with atmospheric oxygen with the formation of Ge–O bonds. The aging caused a reduction of the density of the localized states in the bulk of the film as well as a drastic decrease in the surface states.6,7 Any practical application of a-GeC:H films will require film stability. It was therefore the scope of the present study to investigate the potential of stabilizing these films by several approaches ba
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