AhsbacL The p d a l A I and 0 valence spectral densities of a-and y-alumina in bulk and in the superficial zone of the samples are investigated using x-ray emission spectroscopy induced by electrons. These valence states are mixed over the whole width of the band. We show that changes in the atomic environment affect the hybndizalion of states io a narrow energy range. For the y-phase and superficial zones of the two phases. increased hybridization is'abserved. This is correhted with an increase in the covalent character of A I 4 bonds at the surface and the y-phase. Defect states are observed both in the gap between 0 2s and 0 2p stat= and in the optical gap; for the a-phase, a structure is seen at about 1 eV above fhe top of the valence band which we interpret as due to an oxygen-vacancy state.
AbshciThe Al 3p omupied density of states in GaAs(30 A)-Gao.7Al@.,kr(31 A) hetemstNetum is studied and compared with that of bulk Gao,7AIo.~As thmugh x-ray emission spen"py induced by eleclmns The radiative recombination of the Al Is mn: &ton is also analysed. Partial dehybridization of the AI p and s valence and conduction slates is o k m d for the first Lime in h e barriers of a superlattice and of a single quantum well. These changes suggest that the Al electron distributions of neighbouring barrien are not coupled in hetemstructures of 30 A period and a x governed bj their quasi-2D character.
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