Photoelectron spectroscopy study of amorphous silicon-carbon alloys deposited by plasma-enhanced chemical vapor depositi
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Photoelectron spectroscopy study of amorphous silicon-carbon alloys deposited by plasma-enhanced chemical vapor deposition G. Cicala, G. Bruno, P. Capezzuto, and P. Favia Centro di Studio per la Chimica dei Plasmi CNR, Dipartimento di Chimica Universit`a di Bari, Via Orabona, 4-70126 Bari, Italy (Received 13 March 1996; accepted 8 August 1996)
X-ray photoelectron spectroscopy (XPS) coupled with Fourier transform infrared (FTIR) and optical transmission spectroscopy (OTS) has been used for the characterization of silicon-carbon alloys (a-Si1–x Cx : H, F) deposited via plasma, by varying the CH4 amount in SiF4 –CH4 –H2 feeding mixture. XPS measurements have shown that carbon-rich a-Si1–x Cx : H, F alloys include large amounts of fluorine (.11 at. %), which make the films susceptible to the air oxidation. In addition, the effect of the alloying partner carbon on the valence band (VB) and on the VB edge position of amorphous silicon is also described.
I. INTRODUCTION
Hydrogenated amorphous silicon-carbon alloys (aSi1–x Cx : H) have been extensively studied for their importance as active layers in thin film and Si bipolar transistors, image sensors, photoreceptors, light-emitting diodes (LED’s), photodiodes, and photovoltaic cells.1 Numerous studies on a-SiC : H alloys obtained by plasma-enhanced chemical vapor deposition (PECVD) from SiH4 –CH4 –H2 mixtures have been reported.1–3 In contrast, few investigations on alternative mixtures, such as SiF4 –CH4 –H2 and SiH4 –CF4 –H2 , to obtain hydrogenated and fluorinated alloys (a-Si1–x Cx : H, F) have been done.4,5 The characterization of PECVD silicon-based materials is usually carried out with various spectroscopic techniques. Among these, x-ray photoelectron spectroscopy (XPS), mainly utilized to obtain the surface composition of the deposited material, is also suitable to analyze the structure of its valence band (VB). The photoemission of valence electrons, with binding energy (BE) in the range 0–40 eV, gives additional information with respect to that coming from the analysis of the core-level signals. In fact, the VB analysis allows one to distinguish materials having the same chemistry, but different organization in their structure (e.g., degree of crystallinity, morphology, etc.). For example, despite the identical spectra provided by the core-level electrons of crystalline and amorphous silicon, the analysis of VB electrons reveals the structure difference in these materials, as will be shown in Sec. III. B. In addition, some recent studies utilize this type of analysis to measure the VB offsets of lattice-matched materials, like in the heterojunction of III –V compounds.6 The interpretation of VB spectral features is sometimes complex, and a rigorous analysis often requires a comparison of experimental spectra with those obtained J. Mater. Res., Vol. 11, No. 12, Dec 1996
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theoretically.7,8 Presently, few examples of VB analysis with XPS technique are reported in
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