negative-U Centers Research Articles

The effect of vacancy defects on the electronic properties of β-Ga2O3

In this paper, the characteristics of oxygen vacancies (VO) and gallium vacancies (VGa) in β-Ga2O3 are studied using density functional theory. The performed relaxation analysis indicates that the relaxation of VGaⅡ increases its octahedral volume. Compared with the tetrahedron, the Ga atom of the octahedron surrounding VO is more stable. The formation energy results suggest also that VGa0 is expected to be most favorable in p-type β-Ga2O3, while VGa-3 is preferred in the n-type. The −3 charge state acts as a compensating acceptor, which reduces electron conductivity. VO+2, which acts as the compensation donor center, can easily form in p-type β-Ga2O3, while the VO0 forms readily in n-type β-Ga2O3. VGa at high density can exist in β-Ga2O3 in an oxygen-rich environment, while VO can exist under oxygen-poor conditions. The defect charge-state transition level reveals that VGa is a deep acceptor and VO is a deep donor. For VGa, the transition levels ε (0/-1), ε (-2/-1), and ε (-3/-2) can appear in the thermodynamic equilibrium system. However, for VOⅠ and VOIII, which are used as negative-U centers, only the transition level ε (2+/0) can appear in thermodynamic equilibrium. Furthermore, the calculated bandgap properties indicate that VGa is capable of introducing ferromagnetism in β-Ga2O3, while VO cannot. VGa introduces both acceptor levels and donor levels within the bandgap, which compensate each other, and they, thus, determine the acceptor characteristics of the system. The appearance of VO-introduced donor levels into the bandgap increases the overall electron concentration of the system. A Bader charge analysis shows that the vacancy defect can have a strong effect on the charge distribution of the crystal and alter the electronic characteristics of the crystal. The research results in this paper show that the electronic and magnetic properties of β-Ga2O3 can be changed significantly by introducing different types of vacancy defects with different charge states during the preparation process of the device. This can be done to meet specific performance requirements of a commercial electronic component.

Current Filamentation and Switching Effect in Chalcogenide Glassy Semiconductors: A Review

A review on current filamentation and switching effect in chalcogenide glassy semiconductors (CGSs), which are promising materials for the development of phase change memory devices, is presented. First, a history of the research on CGSs and their properties is considered. Next, formation of a current filament in CGSs is discussed and the scale of heat release in the material as well as the geometric shape of the filament is analyzed. Finally, various hypotheses developed for the explanation of the switching effect in CGSs are reviewed. It is shown that the most relevant model of the switching effect in CGS is the model of multi-phonon tunneling ionization of the so-called ‘negative-U centers’. This model is based on the assumption that an avalanche-like increase in current at a certain point in time is associated with mass tunneling of electrons located on atoms, occuring due to thermal vibrations of atoms.

Effect of temperature on the energy gap in a superconductivity model using U-centers.

According to the theory of U-centers, a strong electron-lattice interaction can lead to the fact that the binding energy of two electrons at certain temperatures is higher than the energy of their Coulomb repulsion and, therefore, the possibility of the formation of Cooper pairs appears. If these pairs move coherently without falling apart, then superconductivity occurs. In this work using the Hubbard Hamiltonian and applying the secondary quantization method, we calculate the temperature dependence of the superconducting gap Δ = Δ(T). A temperature dependence of the HTSC resistance in the normal state demonstrates the features of both semiconductor and metallic behavior. It is known that high-temperature superconductors exhibit such properties only in the normal state. In the work presented now, using mathematical calculations, we discuss the presence of four regions of the HTSC phase diagram T = T(ν), where ν is the relative concentration of U-centers. The formation mechanisms of negative U-centers are not considered here. We only assume that they are present in HTSC.

Prospects For THz Therapy: Effective Treatment Of Affections Caused By COVID-19

The characteristics of Terahertz (THz) irradiation generated by a Silicon Nanosandwich Structure (SNS) under the conditions of a stabilized drain - source current are demonstrated. The frequency of irradiation arising from the quantum Faraday Effect is determined by the parameters of micro cavities embedded in the edge channels of a silicon nanosandwich structure confined by the negative-U centers.

Ab initio study of Pr1-xSrxCrO3-δ cubic perovskites: Solid oxide fuel cells applications

Perovskite-type materials have been proposed to produce solid oxide fuel cells (SOFCs). The cathode is one layer that makes up the SOFCs, and figure as important player in order to catalyze the reduction of O2. As of the Density Functional Theory, this work presents a theorical study of oxygen vacancy (VO) effects on perovskites Pr1-xSrxCrO3-δ (x ​= ​0, 0.25 and 1; δ ​= ​0.042), in oxygen-poor and oxygen-rich growth atmospheres, for several charge states (VOq). Oxygen vacancy has a fundamental role to optimize the catalysis process in SOFCs cathode. From the formation energy (Ef) calculation, VO+1 is the most stable state in metallic SrCrO2.958, which indicates its donor character. For PrCrO2.958, oxygen vacancy is a negative-U center, with transition energy between charge states (VO2+/VO0) at 0.38 ​eV above valence band top. Among the three distinct structures of Pr0.75Sr0.25CrO2.958, namely, C1, C2 and C3, VO1− is energetically most favorable in C3, which gives its acceptor character. Relative to other systems, SrCrO2.958 has the lowest VO formation energy. Through magnetic moment study, oxidation states of chromium are Cr4+ (d2) in SrCrO3-δ, and Cr3+ (d3) in PrCrO3-δ and Pr0.75Sr0.25CrO3-δ. Furthermore, praseodymium Pr3+ (f2) is present in PrCrO3-δ and Pr0.75Sr0.25CrO3-δ.

The Quest for High(er) Tc in BaBiO3-Based Heterostructures

Almost all known superconductors with high Tc have layered structure and are quasi two-dimensional (2D) electronically. Among those that are not, hole-doped BaBiO3 has the highest Tc (and also the highest among all oxides other than cuprates). Coincidentally, Bi is a valence-skipping element and a natural candidate for a negative-U center, with the potential of enhancing superconductivity, as often pointed out by Ted Geballe. With this motivation, we have been trying to synthesize quasi-2D embodiments of BaBiO3. The technique we use is atomic-layer-by-layer molecular beam epitaxy (ALL-MBE), which allows for “digital” synthesis of atomically precise heterostructures and superlattices. We have succeeded in synthesizing some novel, artificial, metastable, layered compounds such as Ba3Bi2LaO9. So far, neither has shown superconductivity, but we will persist in trying.

Low-Threshold Field Emission from Carbon Structures

The surface of graphene-like structures placed in an electric field with a strength of 1−10 V/μm is found to undergo considerable deformation. This effect is observed in both a direct field (emission current is present) and a reverse field (emission current is absent). The deformation of the surface of cathodes facilitates low-threshold electron emission. The qualitative model proposed here to explain the effect of low-energy electron emission is based on the idea of surface reconstruction and the formation of centers with negative correlation energies (negative-U centers).

Influence of negative-U centers related carrier dynamics on donor-acceptor-pair emission in fluorescent SiC

E 1/E2 defects are the typical negative-U centers in n-type 6H silicon carbide (SiC). They are the main contributors to non-radiative recombination, which limits the carrier lifetime. In this study, two fluorescent 6H silicon carbide (f-SiC) samples and one bulk substrate were characterized via time-resolved photoluminescence (TRPL) and static photoluminescence (PL) measurements, where all the samples were nitrogen-boron co-doped 6H n-type. The existence of E1/E2 defects, which caused the diminution of the internal quantum efficiency (IQE) and luminescence intensity of each sample, was confirmed by applying a carrier dynamics model based on negative-U centers. The carrier dynamics simulation reveals that the density of the E1/E2 defects in bulk 6H SiC is two orders of magnitude higher than that of the f-SiC sample, causing much lower PL intensity in the bulk substrate compared to the two f-SiC samples. The IQE of the two f-SiC samples was extracted from the corresponding TRPL results, where the contrast between their IQE was further confirmed by the related PL measurement results. The slight difference in IQE between the two f-SiC samples was attributed to slightly different E1/E2 defect concentrations. On the other hand, by implementing a steady-state donor-acceptor-pair (DAP) recombination calculation, it was found that the f-SiC sample with lower IQE had a higher DAP transition probability due to the higher doping level. This prompted further optimizations in the f-SiC crystal growth conditions in order to decrease the E1/E2 defects while maintaining the correct doping parameters.

Redistribution of oxygen ions in single crystal YBa2Cu3O7-x owing to external hydrostatic pressure

The effect of high hydrostatic pressure on the temperature dependences of the electrical resistance in the basal plane of single crystal YBa2Cu3O7-x with an oxygen deficit is studied. It is found that an external hydrostatic pressure P ≈ 7 kbar substantially intensifies the diffusive coalescence of oxygen clusters, i.e., causes an increase in their average size. This, in turn, produces an increased number of negative U-centers whose presence leads to the appearance of a phase capable of generating paired carriers of electrical charge and is, therefore, characterized by a higher transition temperature Tc. Changes in the form of the temperature and time dependences of the electrical resistivity under external hydrostatic pressure are discussed in terms of this same hypothesis regarding the mechanism of diffusive coalescence of oxygen clusters.

Adsorption of H2, O2, H2O, OH and H on monolayer MoS2

Hydrogen and hydrogen-containing gases are commonly used as reductants in chemical vapor deposition growth of MoS2. Here, we consider the defects resulting from the presence of hydrogen during growth and the resulting electronically active defects. In particular, we find that the interstitial hydrogen defect is a negative-U center with amphoteric donor and acceptor properties. Additionally, we consider the effects of interaction with water and oxygen. The defects are analysed using density functional theory calculations.

Diffusion coalescence in НоBa2Cu3O7−x single crystals under the application of hydrostatic pressure

Experimental results on the effect of external hydrostatic pressure up to 5 kbar on the ρ(T) dependence in the ab plane of HoBa2Cu3O7−x single crystals (x ≈ 0.35) in the temperature range from 300 K to the superconducting transition temperature Tc are presented and discussed. It was established that the application of external hydrostatic pressure P = 5 kbar significantly intensified the process of diffusion coalescence of oxygen clusters, causing the growth of their average size. This leads to an increase in the number of negative U-centers, the presence of which results to the appearance of a phase capable of generating paired carriers of electric charge and, correspondingly, characterized by a higher transition temperature Tc. Employing this hypothesis that concerns the mechanism of the diffusion coalescence of oxygen clusters, the change in the form of the temperature and time dependences of the electrical resistivity under the application of external hydrostatic pressure is discussed.

<i>Ab Initio</i> Study of MgTe Self-Interstitial (Mg<sub>i</sub> and Te<sub>i</sub>): A Wide Band Gap Semiconductor

We present results of defect formation energies and charge state thermodynamic transition levels of Mg and Te interstitials in MgTe wurzite structure. We use the generalized gradient approximation and local density approximation functionals in the framework of density functional theory for all calculations. The formation energies of the Mg and Te interstitials in MgTe for both the tetrahedral and hexagonal configurations were obtained. The Mg and Te interstitials in MgTe depending on the functional, introduced transition state levels that are either donor or acceptor within the band gap of the MgTe. The Te interstitial exhibit charge states controlled metastability, negative-U and DX centre properties. The Mg interstitial acts as deep or shallow donor and there is no evidence of acceptor levels found for the Mg interstitial.

Electronic properties of the residual donor in unintentionally doped β-Ga2O3

Electron paramagnetic resonance was used to study the donor that is responsible for the n-type conductivity in unintentionally doped (UID) β-Ga2O3 substrates. We show that in as-grown materials, the donor requires high temperature annealing to be activated. In partly activated materials with the donor concentration in the 1016 cm−3 range or lower, the donor is found to behave as a negative-U center (often called a DX center) with the negative charge state DX− lying ∼16–20 meV below the neutral charge state d0 (or Ed), which is estimated to be ∼28–29 meV below the conduction band minimum. This corresponds to a donor activation energy of Ea∼44–49 meV. In fully activated materials with the donor spin density close to ∼1 × 1018 cm−3, donor electrons become delocalized, leading to the formation of impurity bands, which reduces the donor activation energy to Ea∼15–17 meV. The results clarify the electronic structure of the dominant donor in UID β-Ga2O3 and explain the large variation in the previously reported donor activation energy.

Dynamics of the current filament formation and its steady-state characteristics in chalcogenide based PCM

In the phase-change memory (PCM) crystallization occurs in the high-current filament which forms during switching to the conductive state. In the present paper we conduct a numerical modeling of the current filament formation dynamics in thin chalcogenide films using an electronic-thermal model based on negative-U centers tunnel ionization and Joule heating. The key role of inhomogeneities in the filament formation process is shown. Steady-state filament parameters were obtained from the analysis of the stationary heat conduction equation. The filament formation dynamics and the steady-state filament radius and temperature could be controlled by material parameters and contact resistance. Consequently it is possible to control the size of the region wherein crystallization occurs. A good agreement with numerous experimental data leads to the conclusion that thermal effects play a significant role in CGS conduction and high-current filament formation while switching.

Surprising stability of neutral interstitial hydrogen in diamond and cubic BN

In virtually all semiconductors and insulators, hydrogen interstitials () act as negative-U centers, implying that hydrogen is never stable in the neutral charge state. Using hybrid density functional calculations, we find a different behavior for in diamond and cubic BN. In diamond, is a very strong positive-U center, and the charge state is stable over a Fermi-level range of more than 2 eV. In cubic BN, a III–V compound similar to diamond, we also find positive-U behavior, though over a much smaller Fermi-level range. These results highlight the unique behavior of in these covalent wide-band-gap semiconductors.

A Light-induced Threshold Voltage Instability Based on a Negative-U Center in a-IGZO TFTs with Different Oxygen Flow Rates

A Light-induced Threshold Voltage Instability Based on a Negative-U Center in a-IGZO TFTs with Different Oxygen Flow Rates

Stable and metastable Si negative-U centers in AlGaN and AlN

Electron paramagnetic resonance studies of Si-doped AlxGa1−xN (0.79 ≤ x ≤ 1.0) reveal two Si negative-U (or DX) centers, which can be separately observed for x ≥ 0.84. We found that for the stable DX center, the energy |EDX| of the negatively charged state DX−, which is also considered as the donor activation energy, abruptly increases with Al content for x ∼ 0.83–1.0 approaching ∼240 meV in AlN, whereas EDX remains to be close to the neutral charge state Ed for the metastable DX center (∼11 meV below Ed in AlN).

Negative-Usuperconductivity on the surface of topological insulators

We study the effects of a finite density of negative-$U$ centers (NUCs) on the surface of a three-dimensional topological insulator. The surface Dirac fermions mediate a power-law interaction among the local Cooper pairs at the NUCs, and the interaction remains long-ranged for weak disorder. Superconductivity can be generated in the presence of a random distribution of NUCs. The NUCs play dual roles as both pair creator and pair breaker, and the competition of the two effects results in non-monotonic dependence of the mean field superconducting transition temperature on the density of NUCs. Global phase coherence is established through coupling the locally superconducting puddles via Josephson coupling. Rare fluctuations play important roles, and a globally superconducting phase can only be achieved at large enough concentration of NUCs. The p-wave component of the superconducting order parameter gives rise to frustration among the superconducting grains, which is captured by a Potts-XY type model. New phases with chiral order, glass order, and possibly topological order can then emerge in the system of superconducting grains.

Generation of adjustable pure spin currents in negative-U systems

Single-particle sequential tunneling is studied through a negative-U center hybridized with a superconducting, a ferromagnetic, and a normal metal electrodes. In stark contrast to the case of positive U, the single-particle tunneling in attractive charging energy is usually prohibited by ground states with electrons in pairs. We find a microscopic mechanism to induce single-particle sates from pair states. As a consequence, in the nonpolarized metal terminal a remarkable pure spin current with no charge currents survives over a wide range of gate- and bias-voltages, which is rather crucial for experimental observation and design of spintronic devices. In addition, a significant spin-filter effect is presented in certain bias regime.

Photoinduced changes in amorphous selenium

Photoinduced phenomena in thin films of amorphous selenium (a-Se) have been a subject of intensive researches so far. Thin films of a-Se were deposited on corning glass by thermal evaporation, with several thicknesses. The influence of light exposure, with different colors (blue or red or natural white light), on the optical and structural properties of a-Se thin films was carried out by using ultraviolet–visible transmittance spectroscopy. It was found that the gap of the samples illuminated with blue and natural white light at room temperature shifts to lower energies. This photodarkening is stable at room temperature and is irreversible even after several days. On the other hand the photodarkening in the samples illuminated with red light is much smaller than that for samples illuminated with blue and natural white light. The photodarkening is accompanied by an increase in the refractive index. These results are discussed with results, previously obtained, about the photoinduced changes of the negative-U centers in, T– and T+, studied using time-of-flight (TOF) traces recorded on samples prepared in the same conditions. These TOF measurements show that the defect level T– at 0.4 eV above the valence band edge shifts to 0.5 eV under illumination with white light at room temperature. This effect is not seen if the illumination is done at 35 °C. These phenomena are attributed to nanocrystallization in the a-Se films.