Formation Of Impurity Band Research Articles

Abnormal Magnetoresistance Effects at Low Temperature in Silver Telluride Thin Films

We report the low temperature abnormal longitudinal magnetoresistance (MR) in self-doped silver telluride films. We observe the large negative MR and multi-peaks of magnetic resistivity rho and MR in the Ag2.1Te films, which strongly depends on doping concentration and temperature. The origin of the abnormal MR behavior is tentatively ascribed to the complex structure and texture of the films and is discussed by means of the formation of impurity bands in the films.

Longitudinal Magnetoresistance Effect at Low Temperature in Silver Telluride Thin Films

Large longitudinal magnetoresistance (LMR) was observed in nonmagnetic silver telluride films. The magnitude of the LMR is not simply dependent on T and H, for example, multi-peaks of ρ at low field and low temperature appear in the Ag-Te film. Because both of -Δρ/ρ and a transition from -Δρ/ρ to +Δρ/ρ exist in Ag-Te films, the magnetoresistance (MR) behavior of the Ag-Te film does not like the bulk Ag-Te, but likes the doped semiconductors. About -27% of LMR was observed at low field in the Ag-Te films, while it could be negligible in the doped semiconductors. The MR behavior in Ag-Te films is discussed by means of a formation of impurity bands in the films.

Impurity scattering by guest 3 d metals and their impurity band formation in M xTiS 2[formula omitted

The electrical resistivities of 3 d metal intercalates M x TiS 2 (M=Mn, Fe, Co and Ni; x=0.05) have been measured at 4.2 K in pulsed magnetic fields up to 31 T. The magnetoresistances (except for Ni intercalate) tend to saturate at higher magnetic fields and the residual resistivity is found to depend on the guest species. On the basis of these results and the possible band model, we have calculated the ionized impurity scattering by guest M 2+ and host Ti 3+ ions formed through a charge transfer from the guest to the host conduction band, in satisfactory agreement with the experiments.

Monte Carlo study of electron transport in SiC

Temperature- and electric field-dependent electron transport in 3C–, 4H–, and 6H–SiC has been calculated by the Monte Carlo technique. Due to the freezeout of deep donor levels the role of ionized impurity scattering in 6H–SiC is suppressed and the role of phonon scattering is enhanced, compared to 3C– and 4H–SiC. There are indications of impurity band formation for impurity concentrations exceeding 1019 cm−3. It is found that ionized impurity scattering along with the deep donor ionization is responsible for the temperature dependence of mobility anisotropy ratio. Electron effective masses and electron-phonon coupling constants have been deduced from the comparison of Monte Carlo simulation results with available experimental data on low-field electron mobility. The extracted model parameters are used for high-field electron transport simulations. The calculated velocity-field dependencies agree with experimental results. The saturation velocities in all three polytypes are close, but the transient velocity overshoot at high electric field steps is much more pronounced in 3C–SiC.

Activation energy of hopping conductivity in low-doped and low-compensated semiconductor films

The influence of quantum size effects and the role of the environment on impurity band formation in thin semiconductor films is studied under low-doping, low-compensation conditions. An explicit expression for activation energy (Fermi level) is obtained and the form of the impurity band is studied.

The electronic structure of simple models of amorphous solids: dangling bonds, hybridization and binary alloys

We report a numerical study of the electronic structure of a simple model for amorphous solids. The geometry of the model is described by continuous random networks based on the honeycomb lattice; the electronic structure is computed from simple tight-binding Hamiltonians. The density of states and the localization properties of eigenstates have been computed for an ideal random network and systems containing dangling bonds. Models including hybridization and alloy effects have been studied, both leading to the formation of impurity bands. The origin and the physical implications of these impurity bands are discussed.

Supercell calculations for transition metal impurities in palladium

The electronic and magnetic behaviour of transition metal impurities is simulated by performing ab initio band structure calculations assuming ordered supercells with the composition Pd31TM (TM=Cr, Mn, Fe, Co, Ni). The results show the formation of narrow spin-split impurity bands originating from the TM with magnetic moments MTM deviating from the Slater-Pauling curve. Due to the high susceptibility of the Pd host the TM atom polarizes the surrounding Pd atoms (with MPd) causing the 'giant moments' when all the magnetic moment is attributed to the TM impurity. This polarization is described by a covalent polarization (CP) model, which the authors use for explaining both the change in sign of the Pd polarization in the TM series between Cr and Mn, and the linear dependence of the ratio MPd/MTM on the number of valence electrons of the TM. In addition, they estimate the Curie temperature by treating the localized TM moments in terms of a Weiss mean-field model and the itinerant electrons of the Pd host having spin fluctuations.

Formation of impurity bands in doped semiconductors.

The determination of the width and energetic positions of impurity bands in doped semiconductors has been performed in the past by band-structure calculations of hypothetical impurity superlattices or by applying multiple-scattering theories (e.g., the Klauder-V approximation) with essentially different results. The related problems of ionization energies in dense plasmas and of exciton ionization energies in highly excited semiconductors have additionally been treated using two-particle Green's-function techniques with the result of vanishing bandwidths with vanishing temperature for all densities below the Mott value

Role of oxygen vacancies in anodic TiO 2 thin films

Defects play an important role in the electronic and optical properties of amorphous solids in general. Here we present both experimental and theoretical investigations on the nature and origin of defect states in anodic rutile TiO 2 thin films (thickness 5–20 nm). There is experimental evidence that the observed gap state at 0.7 eV below the edge of the conduction band is due to an oxygen vacancy. For this reason oxygen vacancies are used in our model. A comparison of the calculated bulk-photoconductivity to photospectroscopy experiment reveals that the films have bulk-like transport properties. On the other hand, a fit of the surface density of states to the scanning tunneling microscopy (STM) on the (001) surfaces has suggested a surface defect density of 5% of oxygen vacancies. To resolve this discrepancy, we calculated the DC-conductivity where localization effects are included. Our results show an impurity band formation at about p c=9% of oxygen vacancies. We concluded that the gap states seen in STM are localized and the oxygen vacancies are playing the role of trapping centers (deep levels) in the studied films.

INVARIANCE OF THE MOBILITY EDGE IN ANODIC TITANIUM OXIDES

We present a theoretical investigation to explain the electronic and optical properties of anodic rutile TiO2 thin films of different thicknesses (ranging from 5nm to 20nm). There is experimental evidence that the observed gap state at 0.7eV below the edge of conduction-band is due to an oxygen vacancy. For this reason, oxygen vacancies are used as defects in our model. A comparison of the calculated bulk-photoconductivity to photospectroscopy experiment reveals that the films have bulk-like transport properties with a bandgap Eg = 3.0eV. On the other hand, a fit of the surface density of states to the scanning tunneling microscopy (STM) experiment on the (001) surfaces has suggested a surface defect density of 5% of oxygen vacancies. To resolve this discrepancy, we calculated the dc-conductivity where localization effects are included. Our results show an impurity band formation at about pc = 9% of oxygen vacancies. We concluded that the studied films have defect densities below the threshold of impurity band formation. As a consequence the gap states seen in STM are localized (i.e: the oxygen vacancies are playing the role of trapping centers, deep levels) and the mobility edge is invariant.

The influence of oxygen on the electrical and optical properties of GaN crystals grown by metalorganic vapor phase epitaxy

Oxygen was observed to influence the electrical and optical properties of GaN layers grown by metalorganic vapor phase epitaxy. The carrier concentrations obtained from Van der Pauw–Hall measurements increased an order of magnitude when oxygen was incorporated into the grown layers. Additionally, the presence of oxygen in the GaN layers also changed the compensation behavior of Zn. Anomalous behavior of optical transitions in the oxygen-doped GaN layers was observed by optical absorption spectroscopy and low-temperature (4.2 K) photoluminescence measurements. These properties were studied as a function of growth parameters including growth temperature, amount of doping, etc. A model based on impurity band formation is proposed to explain these experimental results, and it is concluded that oxygen is a ‘‘shallow’’ deep donor in GaN.

Single-crystal growth and magnetic properties of La2CuO4 and related compounds (T, T*, and T′ phases) (invited) (abstract)

We have performed magnetic, transport, and optical measurements on well-characterized crystals of (un)doped La2CuO4-related compounds, so-called T, T* and T′ phases, in order to understand the physical properties of quasi-2D S=1/2 antiferromagnet with/without added carriers. Single crystals of the (un)doped single-layer cuprates with large in-plane dimension and optically flat surface have been grown by the flux technique with CuO excess. Ways to control and measure dopant level in the crystals will be discussed. Our various measurements of magnetism in the pristine compounds indicate that magnetic phenomena in the insulating layer systems are qualitatively understandable in terms of classical spin models of 2D S=1/2 antiferromagnet, that is, quantum fluctuation effects, expected to be large in S=1/2 systems, are only perturbative. In this context, we discuss results of various investigations such as studies of magnetic dilution effects, field-induced transition, two-magnon scattering, and inelastic neutron scattering. Optical spectra have been measured over a wide range of energies and carrier densities. In undoped crystals, one simple excitation is the electron-hole pair at the edge of a charge transfer gap, whose energy scales with the Cu-O spacing. With a few added carriers, additional excitations appear due to the formation of impurity bands. We discuss a possible relationship between the excitations and spin dynamics.

Doping disorder and level structures in conjugated polymers

Electronic states in conducting polymers containing impurities are analyzed. Assuming an ideal random distribution of the impurities, we use the coherent potential approximation to study the influence of backward- and forward-scattering (bond and site) impurities in the TLM model. The electronic states are half-filled and a uniform dimerization is represented by a constant order parameter. The order parameter, electronic level structure, and density of states are obtained as functions of the impurity concentration and strengths of impurity potentials. At low concentrations, the formation of impurity bands in the gap is suppressed if the strength of the bond-impurity is larger than that of the site-impurity, because of the one-dimensional nature of the system where the scattering processes on the Fermi surface are limited. When the site-impurity is stronger, an isolated impurity band arises. It broadens, as the concentration increases, and is finally absorbed by either the valence or the conduction band, thereby reducing the magnitude of the gap. At higher impurity concentrations, the order parameter and the gap vanish when the site-impurity is strong enough. Phase diagrams are extensively given, the concentration and the two impurity strengths being used as variables. They give boundaries among five phases, each of which is characterized by the number of impurity bands and the order parameter. Consequences for the interpretation of experiments are discussed.

Charge transport in boron-doped diamond thin films

Abstract Detailed studies are reported for the temperature dependence of nominally undoped and boron-doped diamond thin films over the temperature range 400 to 150K; boron doping concentrations in the films range from 20 to 968 parts per million. In all samples, no single-valued activation energy is observed although the dependence on temperature systematically decreases with increasing boron con-centration. This suggests that the acceptor states introduced by the boron are themselves energetically situated in a relatively high density of impurity or defect states extending from the valence band edge. It is believed that charge transport in these films involves a combination of extended-state conduction and hopping within these distributions of localized states and evidence of impurity band formation is observed.

Absence of impurity bands in conjugated polymers.

Electronic states in conducting polymers containing impurities are analyzed. The influence of backward and forward scattering (bond and site) impurities is studied using the coherent potential approximation. Because of the 1D nature of the problem, the formation of impurity bands in the gap, as known from different circumstances such as semiconductors and superconductors, is suppressed if strength of the bond impurity is larger than that of the site impurity. Consequences for the interpretation of experiments are discussed.

Electronic properties of micro-n-i-p-istructures in silicon

The electronic properties of ultrathin, heavily doped n-i-p-i structures in silicon have been studied with use of the ab initio pseudopotential method. Three doping configurations are used to model n-i-p-i structures by replacing silicon atoms with aluminum and arsenic atoms. The doped silicon samples are found to retain an indirect energy band gap. The heavy doping levels result in the formation of impurity bands. The top of the valence band and the bottom of the conduction band are impurity-related bands. The electron-hole charge separation even occurs when the n- and p-type impurities are nearest neighbors. The heavy doping significantly increases the electron-hole recombination matrix elements as compared to the light-doping case in which the bulk silicon band structure can be used to determine the matrix elements. However, the matrix elements for the three n-i-p-i structures in Si which were investigated are still 5 orders of magnitude smaller than those for the direct-band-gap transition in GaAs.

Cathodoluminescence spectroscopy studies of laser-annealed metal–semiconductor interfaces

We have extended cathodoluminescence spectroscopy (CLS) to the study of new compound and defect formation at metal–semiconductor interfaces. CLS provides evidence for Cu2S and/or impurity band formation after laser annealing Cu on UHV-cleaved CdS. Al deposition on UHV-cleaved CdS followed by laser annealing leads to formation of at least two deep levels distributed at different depths from the initial interface. The energies, densities, and spatial distribution of these levels depend sensitively on the laser intensity and the presence or absence of particular metal overlayers. These results demonstrate the utility of CLS in revealing electronic features of the buried metal–semiconductor interface while still maintaining depth resolution on the order of hundreds of Å or less.

Effects of Doping Induced Disorder in Polyacetylene

Abstract In this paper, the effects of disorder on the electronic structure of doped trans-polyacetylene are evaluated. In effect, at the high doping levels currently obtained on (CH)x, it is not realistic to consider the dopant molecules as isolated impurities. Therefore we have examinated in this work how a random distribution of dopant molecules broadens the impurity levels lying within the band gap. We consider a model where the molecules form covalent bonds with the neighbouring n orbitals. Making use of the SSH model for (CH)X chain, the electronic density of states is obtained by averaging the Green's function over the random distribution of dopants along the lines of a cluster CPA. From our results, we discuss the formation of impurity bands in the gap and their dependence on both the doping concentration and the chemical nature of the doping molecules.

Characteristics of tin and cadmium doping in liquid-phase epitaxial grown InGaAsP

The characteristics of tin and cadmium doping of liquid-phase epitaxial grown InGaAsP layers have been investigated. A change in the solid composition of the quaternary is introduced as a result of the perturbation of the liquid solution composition by the dopants. This leads to a change in the lattice constant and a shift of the photoluminescence peak wavelength of the doped layers. Impurity band formation and band filling enhance the shift of the peak wavelength towards shorter wavelength in the case of tin doping while it substantially counteracts this shift in the case of cadmium. The critical carrier concentration for obtaining maximum luminescence efficiency was determined for the two dopants and the results are consistent with previously published results on InGaAsP/InP double heterostructure lasers. Unintentionally doped InGaAsP layers have also been examined.

From band tailing to impurity-band formation and discussion of localization in doped semiconductors: A multiple-scattering approach

Klauder's best multiple-scattering approximation which allows the use of a realistic interaction potential and in which electron-electron interactions may be incorporated is shown to constitute a sound basis for the study of the electronic structure of doped semiconductors. The implementation of this formalism requires the solution of a self-consistent set of nonlinear integral equations. This has been done numerically over a large impurity-concentration range. We have thus shown that as the concentration decreases, the band tail gradually splits off from the main band, giving an impurity band. Spectral-density analysis allows one to distinguish between localized and extended states. Compensation effects have also been analyzed. Finally, our results are discussed and compared with various experiments.