Infrared Spectroscopy of Deuterated a-Si,Ge:D,F Alloys Prepared by DC Glow Discharge Deposition
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INFRARED SPECTROSCOPY OF DEUTERATED a-Si,Ge:D,F ALLOYS PREPARED BY DC GLOW DISCHARGE DEPOSITION*
Y. OKADA"), D. SLOBODIN, S. F. CHOU, R. SCHWARZ and S. WAGNER Department of Electrical Engineering, Princeton University Princeton, New Jersey 08544
ABSTRACT Deuterated and fluorinated amorphous silicon-germanium alloys, a-Si,Ge:D,F, were studied by Fourier transform infrared (IR) spectroscopy. No Ge.-F modes are observed. The intensity of the Si-F and Si-F 2 modes increases with Ge concentration. So does thae intensity of SiF 4 which is trapped as isolated molecules. No DF (IR) or F2 (Raman) is observed. The IR spectra of alloys annealed at 300, 400, 500 and 600' C show that the fluorine in the Si-F and Si-F 2 groups and in the SiF 4 molecules is in thermochemical equilibrium. INTRODUCTION The structure of amorphous hydrogenated silicon (a-Si:H) has been the subject of extensive study. Of particular interest are bond angle and length distortion [1] and compositional and structural inhomogeneity [2-4]. These structural features give rise to the finite density of states in the mobility gap. Together with the dangling-bond defects and impurities, the structure controls the subgap optical absorption and the electronic transport properties. The density of dangling-bond defects in a-Si:H recently has been shown to be a thermal equilibrium property [5,6]. In this paper we report the first observation of thermodynamic equilibrium for a structural modifier, fluorine. The fluorine bound to the Si network in the form of Si-F and Si-F 2 groups and the trapped SiF 4 molecules is in equilibrium. The bonding of fluorine in a-Si,Ge:H,F alloys is of great interest as it may shed light on the improved electronic properties reported for this semiconductor, by comparison to its nonfluorinated counterpart [7,8]. The most powerful tool for studying F-bonding is IR absorption spectrometry, which indeed has been applied to a-Si:F [9] and a-Si:H,F [10-12]. Stretching modes associated with the Si-F (828 cm- 1) and Si-F 2 (930 cm- 1) configurations and the SiF 4 molecules (1010 cm- 1 ) have been observed [11,12]. The observation of the corresponding Ge-F mode predicted for 630 cm- 1 [13] requires deuterated material, for the purpose of shifting the Si-H and Ge-H wagging modes from -630 cm- 1 to lower wavenumbers. With the purpose of understanding the bonding of fluorine, we complemented the IR spectrometry with step-wise annealing to study changes in IR spectra and to measure gas evolution. EXPERIMENTAL TECHNIQUES
The a-Si,Ge:D,F films were grown from mixtures of SiF 4, GeF 4 and D2 gases in a DC triode system. The base pressure of the turbomolecular-pumped reactor was < 10-5 Pa. The gas flow rates were 28 sccm SiF 4, 7 sccm D2 and 0 to 1.5 sccm GeF 4 . The pressure during film growth was 0.3 Torr (40 Pa), the substrate temperature between 240 and 300* C, and the power density 200 to 500 mWcm-2. The alloys grew at rates between 0.2 and 0.4 *) Funded by the Electric Power Res.
Mat. Res. Soc. Symp, Proc. Vol. 70. - 1986 Materials Research Soci
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