Density Of Impurity States Research Articles

Monte Carlo simulations of spatial correlation effects of charged centres in delta -doping layers

This paper presents results of Monte Carlo simulations of a system of charged donors in delta -doping layers, aiming at elucidating the effects related to a spatial correlation of donor charges in a 2D configuration. The simulations yielded several quantities describing the system, among them the pair correlation functions, gain in the energy of the system due to correlation of charge locations and density of impurity states, all as functions of temperature. The calculations have direct application in analysis of the behaviour of the DX centres in GaAs planarly doped with Si.

Electric-field effects on shallow impurity states in GaAs-(Ga,Al)As quantum wells.

The influence of an applied electric field on shallow donor and acceptor states in GaAs-(Ga,Al)As quantum wells is studied. We work within the effective-mass approximation and adopt a trial envelope wave function for the impurity carrier, which leads to the exact results for vanishing applied electric fields and limiting values of the quantum-well thickness. Results for the binding energies and density of impurity states as functions of the impurity position, well thicknesses, and applied electric field are reported. Some results for the effects of electric field on the donor-related optical properties are also presented. As a general feature, the density of impurity states and impurity-related optical absorption for finite electric fields exhibit three van Hove--like singularities corresponding to the binding energies associated with impurities at the two edges of the quantum well and at the position at which the binding energy has a maximum.

Conductivity of n-type GaAs near the Mott transition.

Low-T transport data on n-type GaAs in several regimes from insulating to metallic is presented. On the insulating side of the Mott transition, it is shown that the power s in the variable-range-hopping law, ${\ensuremath{\sigma}}_{v}$=${\ensuremath{\sigma}}_{0}$exp[-(${T}_{0}$/T${)}^{s}$], which is found to be (1/4 for pure enough samples, can reach (1/2 for a narrow range of higher impurity concentrations. A small metal-like contribution to the conductivity can explain the observed shift in s and is supported by Hall-mobility data. Consequently, the corresponding behavior can hardly be interpreted as due to a Coulomb gap in the density of impurity states. Evidence is presented for weak localization and then normal metallic behavior for higher impurity concentrations.

Density of impurity states associated with inversion layers.

Density of impurity states associated with inversion layers.

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.

Concentration-fluctuation model of a doped semiconductor in the nonmetallic regime: Pseudocluster investigation

We have performed an improved unrestricted Hartree-Fock pseudocluster calculation with a correlated two-electron wave function for the ${D}^{\ensuremath{-}}$ state to investigate the microscopic structure of impurity bands in doped semiconductors. Though the impurity density of states and the estimated specific heat support the Mott-Hubbard-Anderson model, in the nonmetallic regime the impurity states are cluster states and a doped semiconductor can be described as statistically distributed clusters of various sizes. The distribution function of the cluster states agrees with the recent conclusion obtained from analyzing the optical, magnetic, dielectric, and transport data. A new picture of thermally activated hopping is provided which is relevant to the observed non-Ohmic conductivity and large characteristic electronic length.

Virtual bound states in dilute noble metal based disordered alloys according to the resonant model potential theory

The virtual bound state (VBS) d density of states arising from small concentration of a transitional impurity in noble metals based disordered alloys is calculated from the resonant model potential theory. A simple formula is derived for the impurity density of states. The effect of the noble metal d bands is taken into account by a perturbative method valid for energies outside the host metal d band. The VBS parameters are expressed in terms of the resonant potential parameters and of the number of resonant d electrons Zd (which controls the VBS position with respect to the Fermi level and is assumed to be a constant for each impurity). The theory is applied to NiCu, NiAg, NiAu, TiAu, PdAg and PdAu. The resonant potential parameters are determined from the corresponding perfect metal bandstructures and Zd determined from the observed gold-based alloys resonance position.

Study of the yellow emission of natural α-HgS

First we present an experimental study of the emission of α-HgS (function of temperature, excitation, lifetime measurements) and give the first conclusions. We show in particular that the emission, which is attibuted to bound—bound transitions at very low temperature and to conduction band-acceptor transitions for T77 K, cannot be described by the recombination models generally used. Secondly, we interpret the yellow luminescence in terms of heavily-doped semiconductors, in which the discrete impurity levels are replaced by bands of states. We present a conduction band-to-acceptor band of states transition model in which the impurity density of states is described by the expression p ∞ exp − ( E−Eλ) 2 2γ 2 We show that the application of this model to the yellow emission reveals an acceptor band of states with the characteristics: E A ≅ 85 meV and γ ≅ 25 meV, and two “discrete” donor levels with ∽5 and ∽40 meV binding energy.

Spectrum of Magnetic Excitations of a Ferromagnet with Weakly Bound Antiferromagnetic Interstitial Impurities

AbstractThe spectrum of impurity excitations of a Heisenberg cubic ferromagnet with randomly distributed weakly‐bound antiferromagnetic interstitials is investigated. It is shown that if the impurity concentration c is less than some characteristic concentration c0, the density of impurity states has an exponentially narrow peak at the localized‐level frequency ω0. If, however, c ≫ c0, an impurity band appears between the frequencies ω0 and ω1 = ω0, (1 ‐ c(S1/S)). Impurity excitations in this band are characterized with wave vectork with wave vector k(k ⪅ c1/3a−1), their inverse lifetime being small compared with the band width c(S′/S) ω0. Inelastic scattering of neutrons as well as light absorption are studied in the resonant region near ω0 When c > c0, the spectrum of light absorption due to impurities involves two strongly asymmetrical peaks (not one, as in the case of c < c0). The main peak width (that at ω1)is of order (c/co)2 c(S′/S) ω0 and falls with growth of the impurity concentration.

Impurity density of states of dilute CuNi alloys

The experimental optical absorption of the impurity state formed by Ni impurities in Cu cannot be explained consistently as due to a single Lorentzian density of states. The CuNi optical absorption can however be accounted for by an impurity density of states given by the non-magnetic regime of the localized spin fluctuation model of Zlatic, Gruner and Rivier (see Solid St. Commun., vol.14, p.639 (1974)).