X-RAY Photoemission Spectroscopic Study of Light-Induced Structural Changes in Amorphous Silicon
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X-RAY PHOTOEMISSION SPECTROSCOPIC STUDY OF LIGHT-INDUCED STRUCTURAL CHANGES IN AMORPHOUS SILICON *
Shuran Sheng , Edward Sacher and Arthur Yelon Groupe de Recherche en Physique et Technologie des Couches Minces & Département de Génie Physique et de Génie des Matériaux, École Polytechnique de Montréal, Montréal (Québec) H3C 3A7 Canada ABSTRACT Light- and annealing-induced structural changes in undoped hydrogenated amorphous silicon (a-Si:H), pure amorphous silicon (a-Si) and crystalline silicon (c-Si) have been investigated in detail by X-ray photoemission spectroscopy (XPS). Both the Si2s and Si2p peaks in a-Si:H films were found to shift simultaneously to lower binding energies by the same amount with illumination time, and nearly reach saturation at about 0.06 eV after one hour of lightsoaking at the intensity used. In contrast to the metastable changes in electronic properties [Staebler-Wronski effect (SWE)], the light-induced shifts in both peaks are unstable even at room temperature and can be reversed by annealing with a lower activation energy than that for the SWE. The absence of metastable XPS changes in pure a-Si and c-Si suggests that hydrogen is actively involved in the light-induced structural changes. Furthermore, visible light exposure produces XPS changes in a-Si:H less effectively than X-ray irradiation, despite its much higher incident intensity, indicating a high-energy photon irradiation effect. Our present results suggest that essentially the whole Si network structure is affected by light-soaking or X-ray irradiation, and becomes more stable after repeated irradiation-annealing training. These structural changes may be an independent metastable phenomenon or a precursor process of the SWE. INTRODUCTION In the last few years, several new experimental measurements have demonstrated metastable structural changes over the amorphous silicon network [1-7], indicating that defect creation [Staebler-Wronski effect (SWE)] [8] is not the only light-induced metastable change in a-Si:H. We recently reported [5,7] an X-ray photoemission (XPS) study of X-ray irradiation-induced structural changes in undoped a-Si:H. Such changes, and others in the microstructure and mechanical properties of a-Si:H [1-7], are of scientific interest. The possibility that they are related to the degradation of electronic properties (SWE) gives them technological interest, as well. In our study [5,7], we observed reversible saturated shifts of about 0.1 eV in both the Si2s and Si2p peaks, to lower bonding energies, following about one hour of X-ray irradiation at the intensity used (~6×1012 photons/cm2·s, estimated by the manufacturer). Those simultaneous, identical changes in both peak positions, induced by X-ray irradiation, could be described by a stretched exponential rule, similar to that observed for the SWE [9]. The XPS changes were *
Present address: Department of Physics and Astronomy, University of California at Los Angeles, LOS ANGELES, CA 90024 USA. Electronic mail: [email protected]
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