negative-U Centers Research Articles

Parameters of the model of negative-U centers according to the Hall effect data for YBa2Cu3O x in the normal state

A qualitatively and quantitatively correct description of the temperature dependence of the Hall constant in the normal phase of YBa2Cu3O x has been obtained within the framework of the model of negative-U centers.

Dependence of the Sn0/2+ charge state on the Fermi level in semi-insulating CdTe

We explored the growth and characteristics of CdTe doped with Sn to heighten our understanding of the role of deep levels on electrical compensation and trapping. We demonstrated, for the first time, the strong dependence of the SnCd charge state on the Fermi-level variation (2–3kT) in high-resistivity CdTe. The concentration of deep traps for electrons was determined by the number of doubly positively charged Sn2+ atoms. Thermoelectric-effect spectroscopy and photovoltage measurements revealed the conversion of the SnCd defect from the electron SnCd2+ trap to the hole SnCd0 trap. The results agree well with the existence of a negative U-center in the SnCd0/2+ defect. We also showed that the neutral Sn defect is responsible for the near midgap C-band → bound hole radiative transitions band with a maximum at 0.76 eV.

Influence of negative-U centers on the temperature dependence of the carrier concentration in the normal HTSC phase

The temperature dependence of the hole concentration in high-temperature superconductors is considered in terms of the model of negative-U centers. It is shown that this dependence can be nonmonotonic due to the interaction of electron subsystems from the valence band with negative-U centers and the conditions necessary for this have been established. The results obtained are compared with experimental data on the Hall coefficient in cuprates.

Application of DLTS and Laplace-DLTS to defect characterization in high-resistivity semiconductors

This paper shows that the use of conventional analytical procedures to determine defect parameters from DLTS spectra may lead to erroneous results for high-resistivity semiconductors. The effect is observed when the temperature range of a DLTS peak encompasses the temperature at which the equilibrium Fermi level intersects the energy level of defect under study. Based on this Fermi level effect, a procedure is proposed to determine the occupancy levels for defects with a strong temperature dependence of the carrier capture cross-section. It has been found that the procedure can be useful for characterization of positive-U states through to negative-U centers in semiconductors.

Energetically deep defect centers in vapor-phase grown zinc oxide

Vapor-phase grown ZnO crystals were investigated by means of DLTS measurements. The generation of defect center E4 subsequent to annealing in different ambients was monitored. By conducting electron irradiations with energies, where either both the Zn- and O-sublattice are damaged or according to [1] only the Zn-lattice, a chemical assignment to the defect centers E4 and E3 could be accomplished. DLTS investigations of ZnO samples under illumination give evidence that E4 is a negative-U center.

Microstructural model of La 2− xSr xCuO 4 and the nature of “1/8”-anomaly

The microstructural model of La 2− x Sr x CuO 4 based on coexistence of microdomains with distinctive types of long-range doped hole ordering is proposed. It is shown that the regions of bulk superconductivity coincide with the existence areas of two types of domain in which the percolation over negative-U center (NUCs) takes place and the deep of T c at x ∼ 1/8 falls on the narrow area (0.11 < x < 0.12) where the percolation over NUCs is impossible.

The origin of hole carriers in HTSC

The mechanism of hole carrier generation is considered in the framework of a model assuming the formation of negative-U centers (NUCs) in HTSC materials under doping. The calculated dependences of carrier concentration on the doping level and temperature are in quantitative agreement with experiment.

The common origin of the pseudogap- and 60 K-phases in YBCO

Here we assume that the pseudogap observed in various experiments is nothing but the superconducting (SC) gap, formed at a temperature T > T c as a result of large fluctuations of the negative-U center (NUC) occupation due to electron transitions between oxygen sub-band and NUCs. The mechanism leading to suppression of the gap in our case is filling of NUCs with electrons. Fluctuation-induced variations of NUC occupation will amplify (reduce) the pairing interaction and can result in fluctuation-induced turning on (off) superconductivity at T ∗ > T > T c∞ ( T c < T < T c∞). Here, T c∞ is equilibrium value of T c for infinite cluster of NUCs. With doping decreasing, the average size of NUC clusters decreases and relative fluctuations in NUC occupation in these clusters increase (i.e., T ∗ increases and T c decreases). In the frameworks of this model, we have obtained the dependences T c( δ) and T ∗( δ) for YBa 2Cu 3O 6+ δ as two solutions of one quadratic equation. It is found that both calculated dependences are in a good agreement with experimental results.

The possible origin of incommensurate spin textures in HTSC

An model of La2-xSrxCuO4 explaining the features of incommensurate spin textures without any assumption of stripe formation is proposed. The foundations of this model are the mechanism of negative-U center formation proposed earlier and the concept of specific ordering of doped ions. It is shown that within the framework of the proposed model the features of “stripe” textures of La2-xSrxCuO4 reflect exclusively the geometrical relations existing in a square lattice and the competition between different types of doped hole ordering.

Superconductivity in silicon nanostructures

We present the findings of single-hole and pair-hole tunneling into the negative-U centres at the quantum well—δ-barrier interfaces that seems to give rise to the superconductivity observed in the silicon nanostructures prepared by short time diffusion of boron on the n-type Si(100) surface. These Si-based nanostructures represent the p-type ultra-narrow self-assembled silicon quantum wells, 2nm, confined by the δ-barriers heavily doped with boron, 3nm. The EPR and the thermo-emf studies show that the δ-barriers appear to consist of the trigonal dipole centres, B+–B−, which are caused by the negative-U reconstruction of the shallow boron acceptors, 2B0→B++B−. Using the CV and thermo-emf techniques, the transport of two-dimensional holes inside SQW is demonstrated to be accompanied by single-hole tunneling through these negative-U centres that results in the superconductivity of the δ-barriers, which seems to be in frameworks of the mechanism suggested by E. Simanek and C.S. Ting (Sol. St. Commun. 32 (1979) 731; Phys. Rev. Lett. 45 (1980) 1213). The values of the correlation gaps obtained from these measurements are in a good agreement with the data derived from the temperature and magnetic field dependencies of the magnetic susceptibility, which reveal a strong diamagnetism and additionally identify the superconductor gap value.

Charge ordering, superconductivity, and stripes in doped La2CuO4

The special features of the phase diagrams of La2−xSrxCuO4 are considered in terms of the high-temperature superconductivity model according to which the mechanism responsible for the anomalous properties of these compounds is the interaction of electrons with diatomic negative U-centers. A microstructural model that assumes the coexistence of domains with different types of dopant ion ordering is suggested for La2−xSrxCuO4. According to this model, the main characteristics of the experimental phase diagrams of La2−xSrxCuO4 only reflect square lattice geometric relations and competition between different dopant ordering types. Close agreement between the calculated and experimental “superconducting” and “magnetic” phase diagrams is an important argument in favor of the suggested high-temperature superconductivity model.

Physical model for frequency-dependent dynamic charge trapping in metal-oxide-semiconductor field effect transistors with HfO2 gate dielectric

In this letter, we report on a physical model to explain the frequency dependence of dynamic charge trapping in metal-oxide-semiconductor (MOS) transistors with ultrathin HfO2 gate dielectrics. For transistors operating in a complementary MOS inverter circuit with a given gate voltage amplitude, we observed a reduction of charge trapping when the stress frequency is increased. This can be explained by the traps in the high-k HfO2 dielectric have the property of negative-U centers. One trap can capture two electrons sequentially, and the trap energy is reduced as a result of lattice relaxation. Results of calculation using the model show excellent agreement with all experiment data.

Phase diagram features and “stripe” structure characteristics of La 2− xSr xCuO 4 as consequences of specific dopant ordering

The model of La 2− x Sr x CuO 4 that explains all details of superconducting phase diagram and spin textures with no any assumption of electronic phase separation or stripe formation is proposed. This model has its backgrounds in the mechanism of negative-U center (NUC) formation proposed by us earlier [K.V. Mitsen, O.M. Ivanenko, Zh. Eksp. Teor. Fiz. 118 (2000) 666; JETP 91 (2000) 579] and in the notions of specific ordering of doped ion. It is shown that in the framework of proposed model the features of the superconducting phase diagram and the characteristics of “stripe” textures of La 2− x Sr x CuO 4 only reflect the geometrical relations existing in a square lattice and the competition of different types of dopant ordering. The complete agreement between the calculated and experimental phase diagrams of the La 2− x Sr x CuO 4 may serve as an important argument in favor of the proposed model of HTSC.

Theory of photostructural changes in glasses with negative-U centers: dependencies of steady-state photodarkening on gap-light intensity and frequency, pressure and temperature

A theory is presented of a mechanism for the photostructural changes observed in semiconducting glasses. The theory takes into account that in these materials negative-U centers are the basic charge carriers, and gap-light generated excited states of such centers are accompanied by metastable “defects” in the original structure, which can be identified as the photostructural changes. The theory gives rise to a consistent interpretation, and some predictions, of dependencies of the steady-state photodarkening on gap-light intensity and frequency, pressure and temperature.

Theory of photostructural changes in glasses with negative-U centers: dependencies of steady-state photodarkening on gap-light intensity and frequency, pressure and temperature

A theory is presented of a mechanism for the photostructural changes observed in semiconducting glasses. The theory takes into account that in these materials negative-U centers are the basic charge carriers, and gap-light generated excited states of such centers are accompanied by metastable “defects” in the original structure, which can be identified as the photostructural changes. The theory gives rise to a consistent interpretation, and some predictions, of dependencies of the steady-state photodarkening on gap-light intensity and frequency, pressure and temperature.

Fluctuation-induced pseudogap in high- Tc cuprates

The pseudogap in HTSC is treated in the framework of the model assuming that the interaction of band electrons with negative-U centers (NUC) is responsible for anomalous properties of HTSC compounds. It is shown that the pseudogap is the superconducting gap arising in under- and optimum-doped HTSC at T> T c due to the large fluctuations of NUC occupation in small clusters. The temperature of pseudogap opening T ∗ is calculated for YBa 2Cu 3O 6+ δ as a function of doping. The results are in a good agreement with experiment.

Electrical signature for configurational bistability of self-interstitial clusters in ion-damaged silicon

Self-interstitial clusters introduced by high-energy Ar ion implantation in silicon are studied by capacitance transient spectroscopic techniques. Trapping kinetics studies at low temperature over a wide range of trap-filling time reveal the presence of three trap levels, two of which are interrelated. A detailed analysis of the trap occupancy features using numerical simulation show that the two interrelated traps are two configurations of the same defect. From model simulation, it is also shown that exact occupancy features observed experimentally could be fitted by assuming the metastable defect to be a negative-U (Hubbard correlation energy) center.

Doping of chalcopyrites by hydrogen

First-principles total-energy calculations for hydrogen impurities in CuInSe2 (CIS) and CuGaSe2 (CGS) show that H+ takes up the Cu–Se bond center position, whereas H0 and H− take up tetrahedral interstitial site next to In (in CIS) or Ga (in CGS). Hydrogen creates a negative-U center (i.e., H0 is never stable), with a (+/−) transition level at Ec−0.39 eV in CIS, and Ec−0.57 eV in CGS. However, once combined with the 2VCu−+IIICu2+ complex, hydrogen forms shallower centers with transition levels at Ec−0.15 eV in CIS, and Ec−0.39 eV in CGS. We conclude that hydrogen could convert CIS to n type, but not CGS.

Defect levels and charge carrier photogeneration in amorphous selenium layers

Time-of-flight post-transit data that suggested positions in the band gap at ∼Ev+0.4 eV and ∼Ec−0.55 eV for the thermally accessible levels of the intrinsic negative-U centers of a-Se have been re-examined. The introduction into the Onsager formalism of a distribution of initial carrier separations in the photogeneration process, together with random internal field fluctuations, and of the ensuing processes of bimolecular recombination and deep trapping lead to a good description of experimentally observed photogeneration efficiency in a-Se.

La214 phase diagram features as a consequence of percolation over negative-U centers

The features of La214 phase diagram (including superconducting and stripe phases) are explained in terms of formation of negative-U center (NUC), ordering of localized doped carriers and percolation over NUC.