Photoelectric-Yield Studies of c-Si/a-Si:H Interfaces
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low temperature (70 C) growth. An estimate of the thickness of the amorphous phase in the overlayer was obtained by fitting the Si 2p core level lineshape [6]. The ordered growth of the first few monolayers is in qualitative agreement with the recent report by Eaglesham et al. [7], who showed by using the MBE technique that the epitaxial growth of Si on smooth Si(100) surfaces occurs also at room temperature, albeit with a limiting epitaxial thickness of -10-30 A only. In the yield spectroscopy the exciting photons were in the 3.5-6.5 eV energy range. The samples were negatively biased in order to reduce the background. In these experimental conditions up to seven order of magnitude of signal intensity change as a function of energy could be followed. System resolution was 80 meV. The YS was operated in the constant final state (CFS) mode [8], i.e. the photoemitted electrons of a chosen kinetic energy were collected as a function of photon energy. This operating mode is at variance with the previous use [9,10,11] of the YS technique, where the total yield mode was employed. In the total yield mode all photoemitted electrons are collected, regardless their kinetic energy, and the current intensity is analysed as a function of photon energy. Typical electron kinetic energy chosen for the CFS-YS was 0.1-0.2 eV above the vacuum level. For such a low kinetic energy the conservation at the free surface of k parallel implies that the electrons which can escape into the vacuum are distributed in a narrow cone around the direction perpendicular to the surface. Hence, the final state for the transitions involved in the CFS-YS is fixed both in energy and in kdirection. The interpretation of the signal becomes straightforward, contrary to the total yield case where a variety of functional dependencies were found [12] related to the electronic structure in a very indirect way. In the case of transitions in an amorphous material or from localized states and surface states in a crystalline material, it is expected that the transitions are non-direct. With the usual assumption that matrix elements are slowly varying functions of energy, the CFS-YS spectrum is directly proportional to the density of initial states. In the case of transitions from extended states in a crystalline material, we must distinguish between two possibilities. If the transitions are direct and k is conserved in the optical transition, a narrow peak in the CFS spectrum is expected when the photon energy is such that the final state is in the chosen escape cone. The CFS-YS becomes, substantially, an angle-resolved photoemission spectroscopy. Quite to the contrary, if the transitions are indirect and the missing k is provided by, e.g. the phonons, the CFS-YS intensity is again proportional to the density of initial states. RESULTS AND DISCUSSION We discuss preliminarily the CFS-YS data taken on the surfaces of crystalline and amorphous silicon samples. In Fig. 1 the spectra of the hydrogenated c-Si(100) surface and that of an a-Si:H surface are compared. The a
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