negative-U Centers Research Articles

Theory of negative - U “effects” in amorphous (glassy) semiconductors

A review is presented of the theory of negative-U centres in glassy semiconductors, which is based on the “soft-configuration model” of glasses. The negative-U “defects” appear to be singlet electron (or hole) pairs self-trapped in soft configurations. Some basic features of the mobility-gap spectrum and of related properties of glassy semiconductors are discussed.

Experimental evidence for a negative-U center in gallium arsenide related to oxygen.

Fourier-transform infrared spectroscopy on the charge-state-dependent local modes of the off-center substitutional oxygen impurity in GaAs has revealed an inverted ordering of the two gap levels. The metastable one-electron state shows the characteristic disproportionation into the zero- and the two-electron state. From the thermally activated decay of the local mode lines and the observed threshold energies for the photoionization cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathit{p}}^{0}$(1) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{n}}^{0}$(2), binding energies of 0.15 and 0.62 eV for the first and the second electron, respectively, are deduced.

Reply to "Comment on 'Negative-U property of the DX center in AlxGa1-xAs:Si' "

Dmochowski (preceding Comment) has suggested that the shallow donor level we proposed in our earlier paper [Phys. Rev. B 40, 1430 (1989)] could be a negative-U center. While this proposal is possible, it will require a number of modifications in our calculations also.

Statistical mechanics of the local-pair superconductor: phase diagrams and specific heats

The quantum-lattice-gas model, originally introduced by Matsubara and Matsuda to describe the superfluid properties of 4He, relates the hamiltonian of a system of hard-core bosons hopping on a lattice with nearest-neighbour transfer integral t and interaction v, to that of a spin S = 1 2 , uniaxially anisotropic Heisenberg antiferromagnet in a magnetic field. Interest in the charged-boson version of this model has recently been revived since it describes the superconductivity (superfluidity) of local pairs of carriers (negative-U centers). Here we investigate its application to the phase diagrams of T c versus carrier density, and the ordering anomalies in the specific heat of the superconducting oxides.

Properties of a negative-U superconductor with magnetic impurities

Effects of magnetic impurities are theoretically studied for a superconductor induced by negative-U centers. The order parameter, the energy gap and the transition temperature are calculated as a function of the impurity concentration. It is shown that the depression of these parameters due to the magnetic depairing is much softened as compared with the case of a BCS superconductor. The results are consistent with experimental data for high-Tc oxides.

Possible negative-U properties of the cation vacancy in HgCdTe

The cation vacancy in HgCdTe is commonly used for p-type doping, but the energy levels of the vacancy acceptor are not known. We propose that the vacancy is a negative-U center, with inverted energy levels. Comparison with the vacancy in silicon,which is a well-characterized negative-U defect, indicates that the HgCdTe vacancy is likely to be a negative-U center also. The minority carrier recombination rate involving gap levels of the vacancy will show a quadratic dependence on the vacancy concentration, and the deep levels of a negative-U vacancy may be particularly suitable as recombination centers due to their location near the middle of the gap.

Self-trapped single-particle excitations in equilibrium and photoconductivity in glassy semiconductors.

A consistent theory of photoconductivity in glassy semiconductors is presented, which is related to negative-U centers associated with soft atomic configurations. Properties of localized single-particle excitations related to negative-U centers and their contributions to both equilibrium phenomena (e.g., thermally induced paramagnetism) and weakly nonequilibrium ones (photoconductivity, etc.) are studied. An analysis of the energy dependence of cross sections of carrier trapping and related effects is given. A new recombination channel associated with the single-particle excitations is described. Manifestations of both continuum properties and effective discrete energy-level features of the mobility-gap spectrum in glassy semiconductors are considered.

Negative-U property of the DX center in AlxGa1-xAs:Si.

We present two major points in this paper: (1) Statistics derived from the negative-U property of the DX center are not consistent with existing Hall experiments of Si-doped ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As. (2) The discrepancy between the negative-U model and Hall experiments can be improved if there exist two different donors SD and DX with comparable concentrations ${N}_{\mathrm{SD}}$ and ${N}_{\mathrm{DX}}$, ${N}_{\mathrm{SD}}$/${N}_{\mathrm{DX}}$ increases with increasing ${N}_{\mathrm{Si}}$. DX is a negative-U center binding two electrons and SD is a shallow donor. SD centers provide electrons and the electrons are captured by DX centers, and therefore the Fermi energy is no longer pinned to the DX energy level. Many experiments are reinterpreted in this view.

Superconducting Tc enhancement due to negative-U impurities: Monte Carlo study of a local exciton model.

The enhancement of superconductivity by a small concentration of excitonic negative-U centers is studied by a new method which combines diagrammatic perturbation theory and Monte Carlo simulation. Results are given which show how the impurity parameters should be selected in order to obtain the maximum increase in the superconducting transition temperature ${T}_{c}$. We also discuss the underlying physics of these results.

Cu4O4 as a negative-U centre in high-Tc superconductors

Exact calculations of the ground-state energy of a Hubbard model for a planar Cu4O4 cluster show that, under certain conditions, two holes introduced into the Cu2+O2- system may attract each other, even if both the copper and the oxygen atoms have on-site Coulomb repulsion U>0. This result can be obtained without complicating the model by introducing terms such as nearest-neighbour repulsion. Clusters which contain only atoms with U>0 but which behave as effective U<0 centres may have relevance to high-Tc superconductivity. The calculations have been extended to larger Cu4O8 clusters in a few cases. The results suggest that the pairing is not merely an artefact of the extreme boundary conditions.

Effective strong-coupling Hamiltonians for bipolaron centers and magnetic impurities with on-site electron-phonon coupling.

We have studied a model of bipolaron (negative-U) centers and magnetic impurities which includes a repulsive on-site Coulomb interaction as well as a coupling of the electronic on-site energy to the displacement amplitude of a local phonon mode. By means of a generalized Schrieffer-Wolff transformation, we derive an effective strong-coupling (or weak-hybridization) Hamiltonian for the low-lying states of the system. In the case of a bipolaron center (where the phonon-mediated on-site attraction dominates over the Coulomb repulsion), the effective Hamiltonian describes conduction-electron scattering and electron-pair tunneling processes between the localized impurity orbital and the conduction band of the metallic host. In the limit of very strong electron-phonon coupling, the pair tunneling matrix elements become exponentially small, whereas conduction electron scattering is suppressed only algebraically. The impurity orbital occupation number is then almost ``frozen in'' and the bipolaron center becomes inefficient in enhancing the superconducting pairing correlations of the host. In the case of a magnetic impurity (with a predominant repulsive Coulomb interaction), the effective Hamiltonian reduces to the form of an ordinary Kondo spin-exchange model. The spin-exchange coupling constants are not affected by the presence of the electron-phonon interaction.

The role of negative-U centres in the photoconductivity of glassy semiconductors

Thermally induced paramagnetism and steady-state photoconductivity in glassy semiconductors are analyzed in accordance with modern theoretical concepts of mobility-gap structure-soft atomic configurations, negative-U centres. The localized states with continuous energy distribution density are shown to act as discrete temperature dependent levels.

Superconducting Tc enhancement due to excitonic negative-U centers: A Monte Carlo study.

The enhancement of superconductivity by a small concentration of excitonic negative-U centers is studied by a new method which combines diagrammatic perturbation theory and Monte Carlo simulation. Results are given which show how the impurity parameters should be selected in order to obtain the maximum increase in the superconducting transition temperature ${\mathrm{T}}_{\mathrm{c}}$.

Remarks on correlations of negative-U centre effects and low-temperature anomalies (structural changes) in glassy semiconductors

A general correlation of high-energy excitations of negative-U centres with metastable structural changes in glassy semiconductors is shown to follow from a recent theory identifying the negative-U centres with self-trapped electron and hole pairs. Some related effects characteristic of glassy semiconductors are considered. Correlations between the selftrapped pair excitations and low-temperature anomalies in the glasses are discussed.

Tunneling negative-U centers and photo-induced reactions in solids

A model of tunneling centers which acquire the negative-U value as a result of a dependence shown by the electron-vibration interaction constant upon the center charge state is proposed. We address the question whether the model is of a sufficiently general character to be applied to the investigation of the tunneling probabilities for centers due to the reversible rearrangements brought about in lattice defects by optical pumping, photo-induced reactions occurring in semiconductors and photochromic materials, and by adsorbate photodesorption. The tunneling systems of the type being considered have been observed in the experimentally investigated gold centers in silicon.

Charge carrier statistics of semiconductors containing defects with negative electronic correlation energy

The free charge carrier concentration as a function of the reciprocal temperature and the doping level [p(1/T)- andp(C)-characteristics] is calculated from the neutrality equation of a semiconductor containing positive or negativeU centers. The typical exponential laws and power laws of thep(1/T)- andp(C)-characteristics are given both for positive and negative correlation energy of the bound charge carrier pairs. Furthermore, the characteristics are evaluated differentially, in order to obtain criteria for the presence of negativeU centers in semiconductors.

Phase transition in narrow band metals: A model of “superconducting glass”

Abstract Superconducting phase transition in narrow band metal described by Hubbard Hamiltonian with negative U-centers is studied within the mean field approximation. Unlike in the BCS theory of superconductivity, the gap to critical temperature ratio, Δ o /T c , proves to be greater than 2 and is weakly dependent on the occupancy ν (the number of electrons per lattice site). The jump in the specific heat at T c equals ΔC s / C n =5.25/(1+4 ν ) at ν ⪡ 1. Critical temperature diminishes, at even value of ν, which may serve as an explanation of the behavior observed in transition-metal superconductors (Matthias rule).

Self-trapping of electron pairs in materials with tunneling states. Electrons in “perfect” covalent glasses

It is shown that in perfect covalent glasses (and some local structures) the self-trapping of negative-U electron (on hole) pairs is realized on selected “flexible” atomic configurations. The basic concepts of the theory of the phenomenon are presented; the criteria and some associated effects are analyzed. The selected configuration contains an appropriate atom (atomic group) in “critical local potential” with unusually small spring constant in respective well. The phenomenon (self-trapping of electron or hole pair) can generate both Anderson negative-U centres in “perfect” glass (i.e. not associated with defects in glasses) and, possibly, a class of deep-level impurity centres in some crystals.

Electronic Properties of Semiconductors with Negative-U Centers

Electronic Properties of Semiconductors with Negative-U Centers

Negative-U Properties for Point Defects in Silicon

ABSTRACTA defect has negative-U properties if it can trap two electrons (or holes) with the second bound more strongly than the first. It is as if there were a net attraction between the two carriers (negative Hubbard correlation energy U) at the defect, and the defect energy levels in the gap are therefore inverted from their normal order. Experimental evidence is presented that interstitial boron and the lattice vacancy, both common simple point defects produced by electron irradiation of silicon, have this unusual property. These defects represent the first and only concrete examples of negative-U centers in any material and serve as models for an understanding of the phenomenon.