The electronic subband structure of periodically n-type b-doped silicon is calculated selfconsistently within the local-density approximation. Two types of energy levels are distinguished, one due to valleys transverse to the superlattice axis, and the other due to longitudinal valleys.Minibands, potential pro61es, miniband occupancies, and Fermi-level positions are studied and their dependence on the spacing d between b layers and the doping concentration ND is obtained.Pronounced changes with increasing ND and decreasing d are observed. For d ) 150 A, and 10 & N~& 10 cm, the system behaves as a set of practically independent isolated 8-doped wells. Significant dispersion of the higher subbands takes place for d lower than 150 A. . The transition from a multiple-b-doped-well behavior to a superlattice regime is observed for doping concentrations ) 2.0 x 10 cm and periods d ( 50 L. The twofold degeneracy of longitudinal levels and the fourfold degeneracy of tranverse levels are removed by the many-valley coupling. The corresponding splitting energies are calculated.

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An old controversy concerning the origin of the short-period oscillations in the photoreflectance (PR) spectra of GaAs high electron mobility transistor (HEMT) structures is solved. Combining experimental and theoretical PR studies, it is demonstrated that these oscillations are not generated in the channel region, but are due to an ionized acceptor plane at the buffer/substrate interface which causes a weak homogeneous electric field in the buffer layer. The interface sheet charge density is obtained from the PR oscillation period. The utilization of PR spectroscopy for measuring charge densities at interfaces is of interest beyond the scope of this work.

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