Electronic Impurity States Research Articles

Influence of crystal soft mode on phase transitions in impurity systems

The role of the soft mode of the host crystal in a phase transition in the system of reorienting impurity centers is discussed. The energies of the vibronically active crystalline vibrations in the para-phases of the impurity subsystem and in the ordered one are different due to their coupling with the impurity electronic states. This energy difference together with the interimpurity interaction determines the temperature of the phase transition. The soft vibration promotes the transition of the impurity subsystem to a new phase, but the lowering of its frequency below a certain value can result in reduction of the transition temperature.

Electronic states of Fe, Co and Ni impurities in liquid aluminium

The results of an experimental study of the residual resistivity and thermopower of Fe, Co and Ni dilute in liquid Al are presented. Combined with the magnetic susceptibility, the authors, results are found to be consistent with the predictions of the Friedel-Anderson model and are used to determine the parameters of the virtual bound state as well as the exchange-correlation energy between two electrons localized on the site of the impurity.

Isotope structure in optical spectra of LiYF4:Ho3+

We report the first observation of isotope shifts in optical spectra of an impurity in a solid associated with isotopes in the impurity surrounding. Static and dynamic couplings of impurity electronic states with the crystal lattice are discussed as a possible cause of the observed isotope structure in optical spectra of ${\mathrm{LiYF}}_{4}$:Ho

Impurity distribution and electronic states in doped conjugated polymers in the coherent-potential approximation.

The electronic level structure and the dimerization order parameter are studied in doped conjugated polymers when randomly distributed site-type impurities occupy different positions from those in previous investigations. The generalized Takayama-Lin-Liu-Maki model is analyzed with the help of the coherent-potential approximation using the assumption that the order parameter is uniform with one electron per site. The order parameter, the electronic level structure, and its phase diagram are obtained numerically

On the Anisotropy of Scattering in Doped n-Ge at Low Temperatures

On Ge : Sb crystals with dopant concentrations, N, ranging between 8 × 1016 and 1.5 × 1018 cm−3, the temperature and concentration dependences are found of the mobility anisotropy coefficient, K, over the temperature interval 4.2 K ≦ T ≦ 180 K and at T ≦ 4.2 K. At temperatures Tc close to the Hall-coefficient-maximum temperature, the K versus T curve exhibits a minimum at T = Tmin. The temperature Tmin corresponds to the boundary that separates the regions of weak and strong electron scattering. At sufficiently low temperatures, where the scattering is strong (the impurity and band states of electrons are mixed), neither the formulae for classical conductivity nor the formulae of the theory of quantum corrections permit a reliable determination of the value of mobility anisotropy K. [Russian Text Ignored].

Stark effect on confined impurities in semiconductor multi-quantum-well structures

We report the first observation of electric field effects on the electronic states of shallow impurities confined in semiconductor quantum wells. Low temperature, far infrared magnetospectroscopy of the ground state to first excited state transitions of hydrogenic donors in fields up to 9T has been carried out on a p-“i”-n diode with the “i” region consisting of 20 wide (500Å) well-center-doped GaAs quantum wells separated by 200Å Al 0.3Ga 0.7As barriers. The transition energy decreases dramatically with increasing electric field strength; a shift of ∼ 28 cm −1 in an electric field of 2.3 × 10 4 V/cm is observed, corresponding to a tunability of 75% of the zero magnetic field transition energy. Results are in good agreement with a variational calculation.

Phase transitions in systems with reorienting impurity centres

Phase transitions induced by the quenching of reorientations in a system of impurity centres with adiabatic potentials having several equivalent minima are discussed. In addition to inter-centre coupling the transition temperature depends on the interaction between the impurity and the crystal. The interaction of the impurity electronic states with the crystalline vibration causing the formation of the minima of the adiabatic potential also modifies the density of the crystal vibrations. The latter depends on the impurity reorientation which causes the essential change of the total energy of the crystalline vibrations in the phase transition. The system of Li+-KTaO3 and Jahn-Teller centres in twofold-degenerate electronic states are considered. The possibility of the formation of a Jahn-Teller glass phase is also discussed.

Phase relaxation of electronic impurity states by quasi-local modes in glasses

Optical line shapes and widths of an impurity interacting linearly with quasi-local modes (QLMs) in an amorphous matrix are investigated. Dynamical shifts of the electronic energy levels, caused by the interaction with the QLMs, are the reason for the broadening mechanism considered in this paper. The theoretical line shapes are discussed in the light of experimental results.

Characteristics of Fractons: from Specific Realizations to Ensemble Averages

Specific realizations due to computer experiments show that fracton excitations possess a core in which the vibrational amplitudes fall off sharply at their edges. When constructing the ensemble average of the fracton wave functions, we find \(\langle\phi_{\text{fr}}\rangle{\sim}\exp[-(r/\varLambda(\omega))^{d_{\phi}}]\), where \(\varLambda(\omega){\sim}\omega^{-0.71}\). The exponent, -0.71, is in close agreement with the prediction of the fracton dispersion law for a two-dimensional percolating network, -0.705, and the geometrical exponent d φ is much larger than current theoretical limits for impurity electronic states. This suggests that the two problems are different.

Impurity optical absorption of SbSI: Ni single crystals

Single crystals of SbSI and SbSI: Ni were grown by the Bridgman technique. The optical absorption spectra of these single crystals were investigated, and also the electronic states of nickel impurity in the SbSI: Ni single crystal were identified. We observed the impurity absorption peaks at 4502, 7770, 8904, 9090, 12 180 and 14 598 cm −1 due to the electron transitions between the energy levels of Ni 2+ ions sited at T d symmetry of SbSI host lattice. The crystal field parameter and the Racah parameter are given by Dq = 327 and B = 663 cm −1, respectively.

The influence of structural defects on the electronic properties of interstitial alloys—IV. The electronic structure of 3d-, 4d-metallic impurities in ScC, TiC, VC and ZrC

Using the self-consistent LMTO-Green's function method the electronic and magnetic properties of isolated 3 d-, 4 d-metal impurities in the cubic monocarbides of Sc, Ti, V and Zr were studied. On the basis of the calculations carried out a general model of the electronic states of metallic impurities in cubic interstitial alloys has been proposed.

ELECTRONIC STATES OF SUBSTITUTIONAL AND INTERSTITIAL GROUP-IB IMPURITIES IN SILICON

The single-particle electronic states of group-IB impurities, Cu, Ag and Au, in silicon are calculated by use of the self-consistent field scattered-wave Xα method. The clusters XSi4Si＇12 and XSi10Si＇16, which have been used in some pioneer works and in our earlier successfully treatments of the 4d transition-metal impurities in silicon, are taken to simulate the silicon crystals locally perturbed by substitutional and tetrahedral interstitial group-IB impurities X, respectively. We find that the substitutional impurities introduce a t2 type level in the band gap. This state is by no means (d-lik, but rather mostly dangling bond-like on the silicon neighbors. The interstitial impurities introduce an a1 type level in the gap. In the case of Si:Au, this state is close to the bottom of conduction band and is rather delocalized. In addition, a hyperdeep s-like level just below the bottom of the valence band is obtained. The trends in the electronic properties of group-IB impurities in silicon have been discussed also.

Theory of electronic impurity states associated with primed subbands of inversion layers in silicon MOSFETs

Binding energies and transition energies for electric dipole transitions have been calculated for electronic impurity states associated with the lowest primed subbands of inversion layers of silicon MOSFETs. The results are compared with those of similar calculations for the unprimed subbands, and experimental possibilities are examined.

Impurity perturbed domains: Resonant enhancement of bulk mode CARS by coupling to the electronic states of dilute impurities

A new experimental method for the direct observation of electronic excitation–phonon coupling is presented. The CARS signal from the 69 cm−1 optical phonon mode of naphthalene crystals is resonantly enhanced by coupling to the electronic states of dilute pentacene impurities. The resonantly enhanced signal decays over three times faster than the nonresonant decay in the same crystal. This is explained in terms of impurity perturbed domains.

On the Theory of “Deep” Electronic Impurity States in Many‐Valley Semiconductors

AbstractFor many‐valley semiconductors a theory of electron states localized on short‐range point defects, i.e. defects with action ranges of the order of the lattice constant, is developed. Resonance electron scattering near the bottom of conduction band and a small value of the bound energy of the localized state are assumed. The transition from the general multiband formulation of the problem to the zero‐range potential approximation is accomplished. For crystals placed in crossed electric and magnetic fields the general expressions are derived determining the energy spectrum and wave functions of localized states.

Infrared radiative recombination in Rh 3+doped AgBr

Infrared photoluminescence was used to characterize the low-temperature radiative recombination processes in Rh 3+doped silver bromide single crystals. A single structureless band was observed with a peak emission wavelength of 1.6 μm. The results of our experiments suggest that Rh 3+ impurity sites are responsible for this emission. The lifetime of the emission was determined as a function of temperature; the 6.5 K lifetime is 1.6 ± 0.1 ms. A configuration coordinate model for the electronic states of transition-metal impurities in silver halides is used to describe the results.

Theoretical study of the electronic states of substitutional transition-metal impurities in InP

The electronic structure of the transition-metal impurities Cr, Mn, Fe, Co, Ni and Cu and of a vacancy in InP have been studied theoretically using the multiple-scattering X alpha -SCF method. It is found that the dangling bond hybrid t2 level moves into the band gap. For Fe this is in the middle of the gap providing a clue to the semi-insulating effect of Fe in InP. The crystal-field resonance level e goes from the gap into the valence band as one goes from Cr to Cu. The corresponding t2 level goes successively deeper into the valence band from Cr to Cu. The population density study shows that the general trend is an increase of transfer of charge as one scans the latter part of the first transition-metal series.

Acceptor-bound phonons in cubic semiconductors.

The theory of impurity-electron--LO-phonon binding considered by Dean, Manchon, and Hopfield, among others, is reformulated and generalized. When there are several impurity electronic states having excitation energies comparable with the LO-phonon energy, the impurity-phonon binding energies can essentially be obtained by diagonalizing the exchange integrals among electronic ground and relevant excited states. The theory is then applied to the LO phonons bound to acceptors in cubic semiconductors, whereby the effect of valence-band degeneracy is taken into account in the spherical approximation of Baldereschi and Lipari. Compared with the coupling to the hydrogenlike impurity, binding becomes deeper because of the shrinkage of acceptor wave functions and the bound phonon states acquire fine structures which are classified according to the total momentum change in excitation. Detailed numerical calculations are carried out for ZnS, ZnSe, ZnTe, and CdTe, and compared with available experimental data on ZnTe.

Electronic states of transition metal impurities in II-VI and III-V semiconductors

The electronic structures of substitutional 3d transition metal (TM) impurities in II-VI (ZnO, ZnS) and III-V (GaP) semiconductors have been calculated by the cluster method within the local density functional formalism. The calculated level structures of TM impurity states reveal a quantitative difference between II-VI and III-V semiconductors. The energy shifts of impurity states in varying charged states are larger in ZnO than in GaP, and as a result the valency of charged states tends to change significantly with varying TM impurity species in ZnO, while multivalent charged states easily appear in GaP. This difference is ascribed to the large covalency effect in GaP. The calculated results show that in ZnO, Cu2+, Ni2+ and Co2+ are stable as doubly charged states while Co3+, Fe3+, Mn3+ and Cr3+ are stable as triply charged states. Besides, Cu1+ is also stable. As regards GaP, triply charged states are all stable from Ni3+ to Cr3+, and Ni2+ and Co2+ are also stable. The crystal field splitting of TM impurity states, the so-called 10 Dq, has been also calculated for the above TM impurities, and it has been shown that the covalency plays an important role in determining the value of 10 Dq and also in producing the multivalent charged state. The excitation energies of charge transfer transitions in the case of Cu in ZnO and ZnS calculated by the transition state method, and the values of 10 Dq are in good agreement with the observed ones.

Ionization probability of the bound impurity states of two-dimensional electrons under strong magnetic and finite electric fields

The ionization probability of the bound impurity states in a semiconductor inversion layer under strong magnetic fields H is calculated as a function of temperature and electric field value E. The conditions for relatively large ionization probability are determined. The possibility of the bound state ionization must be taken into account for the interpretation of the inversion layer properties at the Shubnikov-De Haas conductivity minima.