Transition Metal Impurities In Semiconductors Research Articles

Impurity levels of transition metal ions in the crystal field of semiconductors

A theory of electron states due to transition metal impurities in semiconductors which includes the real symmetry of the crystal has been developed. The resonance potential, which describes the scattering of valence electrons by excitation of the atomic shell and has the symmetry of the crystal point group, results in the splitting of impurity d-levels and does not affect shallow s-levels. The proposed scheme unites the resonance model of deep levels with conventional ligand field theory which is usually used to explain the optical spectra of wide-gap semiconductors. The possibility of virtual Friedel states in the valence or conduction band and their influence on shallow hydrogen-like impurity levels is also considered.

On the theory of the deep levels of transition metal impurities in semiconductors

The scattering of band electrons accompanied by the excitation of an unfilled d shell of an impurity atom is considered to be the origin of the deep levels associated with transition-metal impurities in semiconductors. In order to describe the energy states of the system, the Anderson model, generalized for the case of the electron spectrum with the gap, is used. The activation-energy dependence of the parameters of the model is found. It is shown that this mechanism is able to explain the existence of deep levels with symmetries which coincide with that of an unfilled impurity shell.

Simple model of multiple charge states of transition-metal impurities in semiconductors

The Anderson model for magnetic impurities in metals is extended to semiconductors. It is shown how self-consistent Hartree-Fock solutions can exist in the gap for many different charge states of the impurity, providing the matrix elements coupling the impurity and substrate are large enough.

Chemical Bonding of a Molecular Transition-Metal Ion in a Crystalline Environment

A theoretical description of the chemical bonding of transition metals and other complex metal atoms in molecules and solids has been formulated in terms of a spin-unrestricted self-consistent-field cluster model. This model permits the accurate calculation, from first principles, of electronic energy levels and wave functions for the cluster, but requires relatively little computer time. The cluster, which may be a free polyatomic molecule, part of a macromolecule, or a polyatomic complex in an ordered or disordered solid, is geometrically partitioned into contiguous atomic, interatomic, and extramolecular regions. The one-electron Schr\"odinger equation is numerically integrated within each region in the partial-wave representation for spherically averaged and volume-averaged potentials which include Slater's $X\ensuremath{\alpha}$ statistical approximation to exchange correlation. The wave functions and their first derivatives are joined continuously throughout the cluster via multiple-scattered-wave theory similar to that developed originally by Korringa. The effects of a particular environment on the cluster are described by boundary conditions, e.g., the matching of the solutions of Schr\"odinger's equation in the extramolecular region to those within the atomic regions at an artificial spherical boundary surrounding the entire cluster. This numerical procedure is repeated, using the wave functions obtained at each iteration to generate a charge density and new potential, until self-consistency is attained. The model is illustrated for the tetrahedrally coordinated permanganate ion (Mn${\mathrm{O}}_{4}^{\ensuremath{-}}$) in the stabilizing field of a typical crystalline environment KMn${\mathrm{O}}_{4}$. The spin-unrestricted cluster calculation leads to a ground-state electronic structure of Mn${\mathrm{O}}_{4}^{\ensuremath{-}}$ which is consistent with the observed Van Vleck paramagnetism of permanganate crystals. Computer-generated contour maps of the cluster wave functions and charge densities are presented, showing the formation of directed chemical bonds. Both $\ensuremath{\pi}"$ and "$\ensuremath{\sigma}$ bonding" between oxygen $2p$ electrons and manganese $3d$ electrons are shown to be important. In conjunction with Slater's transition-state theory, which accurately describes the effects of orbital relaxation in optical transitions, the cluster model is used to interpret the measured optical properties of permanganate crystals. The characteristic purple color of KMn${\mathrm{O}}_{4}$ is shown to originate from the induced transfer of electrons between the oxygen ligands and central manganese atom in the Mn${\mathrm{O}}_{4}^{\ensuremath{-}}$ cluster. Finally, with the use of larger clusters, the theoretical model is extended to the chemical bonding of transition-metal impurities in semiconductors and transition metals which are the biologically active centers of certain enzymes and proteins.

Ground states and photoionization of iron group transition metal impurities in semiconductors

Approximate ground state wavefunctions and potentials for iron group transition metal impurity centres in semiconductors, for instance Cr, Fe, Cu, Ni, Co, Mn in GaAs are calculated using a model in which both the screened long range Coulomb potential and the short range core potential are taken into consideration. The magnitude and the wavelength dependence of the photoionization cross section are obtained via the wavefunction, in favourable agreement with experiment.

Comments on the theory of photoionization of transition metal impurities in semiconductors

AbstractThe shape of photoionization spectrum connected with removal of an electron from the 3dn shell of a transition metal impurity into the conduction band has been calculated. The influence of configurational mixing between 4pn and 3dn states on the shape of the spectrum is discussed. The absorption coefficient for uniaxial crystals (α for E ‖ c and E ⊥ c, c crystal axis) is given.