A. Ferreira da Silva

Bandwidth narrowing in n-type many-valley semiconductors

Abstract A simple method that takes into account the many-valley effects of the host conduction band is outlined for the calculation of the impurity density of states. N random impurity sites were simulated with a computer, within a diamond lattice as a host. A Kohn-Luttinger donor wave function was associated with each impurity. It was found that the low energy side (or the high energy side when the effect of non-orthogonality is taken into account) of the density of states is strongly reduced to a small tail when compared to the case of neglecting many-valley effects. At the Fermi energy, the density of states is considerably enhanced which leads to an enhancement of the extrinsic specific heat. The inverse participation ratio varies with the impurity concentration, presenting different states which are responsible for different physical properties. An analytical calculation taking into account these effects was carried out for the sake of comparison.

A mixed bonding band structure calculation for GaAs and AlAs using the APW-k · p method

Abstract The Augmented Plane Wave (APW) method and the k.p expansion are used to obtain the band structure of GaAs in a mixed covalent and ionic bonding. The energy levels at the Γ point are also obtained for AlAs. The ionicity is introduced as a parameter into the crystalline potential. We explore the dependence of the energy levels and the momentum matrix elements on the values of the ionicity. The value of the ionicity which gave the direct gap Eg = 1.52eV for GaAs (the accepted experimental value) was found to be 0.1, and for AlAs Eg = 2.50 eV was obtained for an ionicity of 0.2.

Bandwidth narrowing in n-type many valley semiconductors. II. Self-consistent many-body and unrestricted Hartree-Fock cluster approaches

Abstract A recent one-band model for impurity states in n -type many-valley semiconductor has been extended by self-consistent many-body and unrestricted Hartree-Fock approaches. The formulations are presented in order to discuss the shallow donor states in semiconductors. The impurity band splits into two Hubbard bands, presenting different aspects of the impurity states when the many-valley effects of the host conduction band are considered. These effects turn out to be essential for a better interpretation of the theoretical and experimental results. The results agree with some theoretical calculations and experimental findings.

Electric conduction in n-type germanium and cadmium sulfide

Abstract The impurity conduction of n-type Ge and CdS is calculated via a previously developed theory for impurity bands in doped semiconductors. Rough agreement with experimental data over a wide range of impurity concentration is found. The comparison with AMO-MT calculation shows a large enhancement due to a stronger electron correlation.

The impurity resistivity of In-doped CdS

The author has investigated further the previously developed theory of Matsubara and Toyozawa (see Prog. Theor. Phys., vol.26, p.739, 1961) for impurity bands in doped semiconductors by including the effect of correlation via an alternant molecular orbital (AMO) method in order to calculate the impurity resistivity of CdS:In. Good agreement with experimental data is found.

The impurity conductivities of Si:P, Ge:Sb and CdS:Cl

The previously developed theory, by incorporating the alternant-molecular-orbital method to the Matsubara-Toyozawa scheme for impurity conductivity in heavily doped semiconductors, has been applied to Si:P, Ge:Sb and CdS:Cl. The results agree well with experiments.

A cluster model for two-dimensional disordered systems

The authors have investigated the two-dimensional (2D) electron hopping energy integral in order to calculate the impurity density of states of doped semiconductors. A cluster model is outlined for the two-dimensional disordered system. It is shown that the hopping matrix is very sensitive to a change in the dimensionality of the system, i.e. from 3D to 2D system. The impurity band is symmetric and has a considerable bandwidth for high concentration, while for low concentration it is drastically reduced by the cut-off of the long-range hopping energy. The results of other models are discussed.