Transition Metal Impurities In Semiconductors Research Articles

Multiple exchange interactions induced by Jahn-Teller distortions in dilute magnetic semiconductors

Transition metal impurities in semiconductors exhibit intricate magnetic interactions, usually rationalized via symmetry orbitals assuming an undistorted point symmetry of the host semiconductor. These interactions may then be described via simple rules based on the occupation of these symmetry orbitals. Here we show how the lowering of symmetry via Jahn-Teller-like distortions enriches the variety of possible magnetic interactions, and describe the emerging strong short-range ferromagnetic interactions as concerted multiple exchange interactions caused by an unusual Cr-Cr chemical bond formation in the AlN semiconductor host.

The electronic consequences of multivalent elements in inorganic solar absorbers: Multivalency of Sn in Cu2ZnSnS4

Multivalent transition metal impurities in semiconductors are known to create deep levels inside the band gap that are associated with changes in the oxidation state. Some emerging functional semiconductor materials now contain multivalent elements not just as impurities, but as part of their structural skeleton (“multivalent semiconductors”). This raises the possibility that the performance of such materials may be affected by those skeleton elements transitioning from one oxidation state to another, in response to charge-altering perturbations such as illumination or doping. Here we address the correlation between multivalency and the electronic properties of these new semiconductor materials.

Effects of magnetic ions on optical properties: the case of (Ga, Fe)N

Because of strong exchange interactions between localized spins and effective mass carriers,transition metal impurities in semiconductors lead to giant magneto-optical effects.Furthermore, band-gap levels derived from open d shells of magnetic impurities act asefficient recombination centers for photo-carriers. This paper reviews studies of excitonicmagneto-reflectivity performed on (Ga, Fe)N epilayers, and shows how hybridizationbetween d levels and band states, particularly strong in nitrides and oxides, renormalizesthe exchange splitting of the valence band states in these systems. Photoluminescencemeasurements on the same structures demonstrate an increase of infrared Fe-relatedemission at the expense of ultraviolet near band-gap luminescence. This sensitivity ofluminescence to the presence of Fe impurities is exploited to monitor the aggregation ofFexN nanocrystals that account for the room temperature ferromagnetism of (Ga, Fe)N, but donot act as inhibitors of excitonic luminescence.

Electronic structure of transition-metal impurities in semiconductors: Cu in GaP

A numerical method for calculation of the electronic structure of transition metal impurities in semiconductors based on the Green function technique is developed. The electronic structure of $3d$ impurity is calculated within the $\text{LDA}+\text{U}$ (local density approximation with interaction) version of density functional method, whereas the host electron Green function is calculated by using the linearized augmented plane wave expansion. The method is applied to the Cu impurity in GaP. The results of calculations are compared to those obtained within the supercell local density approximation procedure. It is shown that in the Green function approach Cu impurity has an unfilled $3d$ shell. This result paves a way to explanation of the magnetic order in dilute ${\text{Ga}}_{1\ensuremath{-}x}{\text{Cu}}_{x}\text{P}$ alloys.

Crystal-field theory ofCo2+in doped ZnO

We present a crystal field theory of transition metal impurities in semiconductors in a trigonally distorted tetrahedral coordination. We develop a perturbative scheme to treat covalency effects within the weak ligand field case (Coulomb interaction dominates over one-particle splitting) and apply it to ZnO:Co$^{2+}$ (3d$^7$). Using the large value of the charge transfer energy $\Delta_{pd}$ compared to the $p$-$d$ hoppings, we perform a canonical transformation which eliminates the coupling with ligands to first order. As a result, we obtain an effective single-ion Hamiltonian, where the influence of the ligands is reduced to the one-particle 'crystal field' acting on $d$-like-functions. This derivation allows to elucidate the microscopic origin of various 'crystal field' parameters and covalency reduction factors which are usually used empirically for the interpretation of optical and ESR experiments. The connection of these parameters with the geometry of the local environment becomes transparent. The experimentally known $g$-values and the zero-field splitting 2D are very well reproduced by the exact diagonalization of the effective single-ion Hamiltonian with only one adjustable parameter $\Delta _{pd}$. Alternatively to the numerical diagonalization we use perturbation theory in the weak field scheme (Coulomb interaction $\gg$ cubic splitting $\gg$ trigonal splitting and spin-orbit coupling) to derive compact analytical expressions for the spin-Hamiltonian parameters that reproduce the result of exact diagonalization within 20% of accuracy.

Isotope shift in semiconductors with transition-metal impurities: Experiment and theory applied toZnO:Cu

Isotope shifts for various lines associated with excitations of transition-metal impurities in semiconductors are considered. Special attention is paid to $\mathrm{Z}\mathrm{n}\mathrm{O}:\mathrm{C}\mathrm{u},$ for which experimental results are presented. Isotope shifts are measured for the so-called photoluminescence $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ zero-phonon lines associated with excitations of bound excitons, and of the zero-phonon line associated with the intracenter ${\mathrm{Cu}}^{2+}{(}^{2}{T}_{2}{\ensuremath{-}}^{2}E)$ transition. These shifts appear to be negative and nearly equal. A theoretical model explaining these results is proposed, which incorporates the mode softening mechanism and the covalent swelling of the impurity $d$ electron wave functions. It is shown that, contrary to transitions in simple neutral impurities, this mechanism works both for the excited and ground states of all processes in transition-metal impurities considered here. Using reasonable values of the parameters of the system, we are able to explain both the sign and value of the isotope shifts.

Configuration-interaction energy level diagrams for d 7 and d 8 charge-state impurities in II–VI and III–V compound semiconductors

The so-called configuration-interaction (CI) approach is considered to provide an alternative description of transition metal atoms in solids compared with the conventionally used ligand-field theory which represents a molecular-orbital type approach. To evaluate the potential of the CI approach to describe internal excitations of transition metal impurities in semiconductors, CI energy level diagrams are provided for the d 7 and d 8 charge states in tetrahedral coordination which reveal the explicit energy level dependence on the parameters of the CI approach. The perturbation calculation was restricted to interaction with single-hole configurations of the type |d N + 1 L 〉 where L is a ligand hole. The observed trends in excited-state level energies of cobalt and nickel impurity centres in II–VI and III–V compound semiconductors can well be understood within the CI approach. Though the CI approach does not represent a quantitatively better tool than the classical Tanabe-Sugano approach to describe these transitions occurring at below band-gap energies, it provides a bridge between this more empirical method and the understanding of the influence of covalency on the observed crystal-field splitting and nephelauxetic effect even linking acceptor-type charge-transfer level energies.

Isotope Shift of Local Vibrational Modes at Transition-Metal Impurities in Semiconductors*

Isotope Shift of Local Vibrational Modes at Transition-Metal Impurities in Semiconductors*

Isotope Shift of Local Vibrational Modes at Transition-Metal Impurities in Semiconductors*

Isotope Shift of Local Vibrational Modes at Transition-Metal Impurities in Semiconductors*

Electronic Structure of 3d Transition-Metal Impurities in Semiconductors

The d-d optical absorption spectra, photoemission spectra and donor and acceptor ionization energies of 3d transition-metal impurities in II-VI semiconductors have been investigated using the cluster and Anderson impurity models with configuration interaction. It is shown that both systematic chemical trends and multiplet effects are essential to explain the variations of the donor and acceptor levels with transition-metal elements.

Analytical Approach to the Theory of Transition-Metal Impurities in Semiconductors

Analytical Approach to the Theory of Transition-Metal Impurities in Semiconductors

Systematic Aspects of the Electronic Structure of 3d Transition-Metal Compounds

Systematic changes in the electronic structure of 3d transition-metal compounds with varying chemical compositions are studied by photoemission spectroscopy and configuration-interaction cluster-model analysis. The changes consist of (i) the smooth variation of model parameters such as the charge-transfer energy Δ and the d-d Coulomb repulsion energy U with atomic number and valence and (ii) the apparently irregular multiplet corrections to Δ and U, which are functions of the nominal electron number. These systematics are reflected upon the band gaps, covalency and character of doped carriers in transition-metal oxides and chalcogenides, and upon the optical absorption spectra and the ionization energies of substitutional transition-metal impurities in semiconductors.

Defect-molecule model calculations of 3d transition metal ions in II-VI semiconductors

The cation substitutional impurities of 3d transition metal ions (though V to Ni) in II-VI semiconductors ZnS(Se, Te) and CdSe(Te) have been studied by using the defect-molecule model with renormalized parameters of the host crystal atoms. It was found that more charge states can exist in the energy gap of semiconductors for V, Cr, and Mn than for Fe, Co, and Ni. The energy levels of transition metals are found to be aligned with respect to each other with a group of common anion semiconductors, which confirms the more recent observations of transition metal impurities in semiconductors, but a slight difference occurs with varying anoins of the semiconductors. With the spin-polarized Hartree–Fock approach, the binding energies of acceptors and donors are calculated and are in reasonable agreement with the experimental data. The polarization of the bond between impurity and host atoms is analyzed. Based on the calculated crystal-field splittings of 3d levels, the internal transition of 3d electrons of transition metal ions in CdTe are predicted.

New theoretical approach of transition-metal impurities in semiconductors.

New theoretical approach of transition-metal impurities in semiconductors.

Theory of Transition Metal Impurities in Semiconductors

The present understanding of transition metal impurities (T.M.) in semiconductors is reviewed with special emphasis on the trends in several physical properties. A simple molecular model is introduced whose validity is confirmed by a full selfconsistent tight binding Green's function calculation which itself is confronted to the results of local density calculations. An empirical law relating the crystal-field splitting and ionization energy is justified. The relation existing between T.M. gap level positions and band offsets at semi-conductor heterojunctions is qualitatively and quantitatively analyzed. The trends in the photoionization cross sections along the 3d series are determined. Finally the accuracy of the existing theories is discussed with a particular attention on estimates of multiplet splitting.

Transition-metal impurities in semiconductors and heterojunction band lineups.

The validity of a recent proposal that transition-metal impurity levels in semiconductors may serve as a reference in band alignment in semiconductor heterojunctions is positively verified by using the most recent data on band offsets in the following lattice-matched heterojunctions: ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As/GaAs, ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ga}}_{\mathrm{x}}$${\mathrm{As}}_{\mathrm{y}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathrm{y}}$/InP, ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ga}}_{\mathrm{x}}$P/GaAs, and ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Hg}}_{\mathrm{x}}$Te/CdTe. The alignment procedure is justified theoretically by showing that transition-metal energy levels are effectively pinned to the average dangling-bond energy level, which serves as the reference level for the heterojunction band alignment. Experimental and theoretical arguments showing that an increasingly popular notion on transition-metal energy-level pinning to the vacuum level is unjustified and must be abandoned in favor of the internal-reference rule proposed recently [J. M. Langer and H. Heinrich, Phys. Rev. Lett. 55, 1414 (1985)] are presented.

Transition-metal impurities in semiconductors-their connection with band lineups and Schottky barriers.

The dependence of cation-substitutional transition-metal impurity levels upon the host semiconductor is calculated self-consistently in a defect-molecule approach. Heterojunction band lineups and Schottky-barrier heights can be calculated within the same tight-binding model. In all three systems, the characteristic behavior is determined by an approximate local charge-neutrality condition imposed by electrostatic self-consistency (pinning), with a dangling-bond level playing the role of the neutrality level. In this way we explain the observed correlation between transition-metal impurity levels, heterojunction band lineups, and Schottky barriers.

On the possibility of negativeU systems for transition metals impurities in semiconductors

Using the Haldane–Anderson formalism we study the possibility of obtaining a negative U system for transition metal impurities in semiconductors. We propose, even without including electron-lattice interaction, the occurrence of negative-U as a consequence of the impurity-host hybridization to explain why some oxidation states would be missing in these systems.

Transition-metal impurities in semiconductors and the Haldane-Anderson model

Haldane and Anderson have extended the Anderson model to describe transition-metal impurities in semiconductors. They explain the confinement of multiple charged states within the band gap to be a consequence of the impurity-host hybridization. In this Comment we derive a simple and physically appealing expression for the change in total energy when the impurity occupancy is changed within the Haldane-Anderson framework. This, we believe, renders the hybridization mechanism transparent.

Calculation of the electronic structure of transition metal impurities in semiconductors

The electronic structure of ideal substitutional 3d impurities in germanium is calculated self-consistently within the local density formalism. Regulating the chemical trend of impurity levels is revealed in the case of the non-magnetic state. The magnetic state of the iron impurity is also presented.