Structural Changes in a-Si:H Studied by X-Ray Photoemission Spectroscopy
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creation is not the only manifestation of the SWE. Indeed, there is experimental evidence of metastable structural changes over the whole or, at least, a major part of the Si network [2-6]. Among these experimental observations, Masson et al. [4] used X-ray photoemission spectroscopy (XPS) to study the structural changes in undoped a-Si:H and observed a reversible shift of about 0.1 eV in the Si2p peak to lower bonding energy following 24 hour exposure to white light of 0.1 W/cm 2 intensity, but without a corresponding shift in the Si2s peak. Density functional calculations indicated, however, that both the Si2p and the Si2s core levels should be displaced simultaneously. The absence of the corresponding shift in the Si2s peak is difficult to understand because both peaks would have been expected to shift simultaneously in the same direction and by the same amount. In almost all XPS measurements of a-Si:H, including those in which light-induced changes in the XPS spectra were investigated, the possible effect of the X-rays on the experimental results has not been considered. In their review of XPS analysis of the Si0 2/Si system, Iwata and Ishizaka [7] pointed out that, in XPS, in which a surface is irradiated by Xrays, this irradiation may affect the electronic state of the sample. Thus, what one measures may not be the state without X-ray irradiation. Indeed, shortly after the discovery of the SWE, it was found that such irradiation could also produce metastable changes in both the optoelectronic and structural properties of a-Si:H [8]. Consequently, the observed shift in the Si2p peak [4] may be attributed to both X-ray irradiation-induced and light-induced metastable effects, as well as 359 Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
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Fig. 1 X-ray irradiation time dependences of the 2s and 2p peak positions of a c-Si wafer.
Fig.2 X-ray irradiation time dependences of 2 the 2s and p peak positions of an a-Si :1 film. The curves are a fit of the data to eq. (1).
charging effect. The difference in shifts between the Si2s and Si2p peaks might therefore be due to differences in measurement conditions. The present work reports detailed investigations of the X-ray irradiation-induced structural changes in undoped a-Si:H as measured by XPS. A further study of light-induced structural changes in a-Si:H will be reported elsewhere [9]. EXPERIMENTAL Undoped device-quality a-Si:H films were deposited at NREL on crystalline silicon (cSi) substrates at 230'C from pure silane by the rf plasma-enhanced chemical vapor deposition (PECVD) technique. The rf power density was 60 mW/cm 2
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