Resonant Defect States Research Articles

Electroluminescence from pure resonant states in hBN-based vertical tunneling junctions

Defect centers in wide-band-gap crystals have garnered interest for their potential in applications among optoelectronic and sensor technologies. However, defects embedded in highly insulating crystals, like diamond, silicon carbide, or aluminum oxide, have been notoriously difficult to excite electrically due to their large internal resistance. To address this challenge, we realized a new paradigm of exciting defects in vertical tunneling junctions based on carbon centers in hexagonal boron nitride (hBN). The rational design of the devices via van der Waals technology enabled us to raise and control optical processes related to defect-to-band and intradefect electroluminescence. The fundamental understanding of the tunneling events was based on the transfer of the electronic wave function amplitude between resonant defect states in hBN to the metallic state in graphene, which leads to dramatic changes in the characteristics of electrons due to different band structures of constituent materials. In our devices, the decay of electrons via tunneling pathways competed with radiative recombination, resulting in an unprecedented degree of tuneability of carrier dynamics due to the significant sensitivity of the characteristic tunneling times on the thickness and structure of the barrier. This enabled us to achieve a high-efficiency electrical excitation of intradefect transitions, exceeding by several orders of magnitude the efficiency of optical excitation in the sub-band-gap regime. This work represents a significant advancement towards a universal and scalable platform for electrically driven devices utilizing defect centers in wide-band-gap crystals with properties modulated via activation of different tunneling mechanisms at a level of device engineering.

Defect-Induced Modification of Low-Lying Excitons and Valley Selectivity in Monolayer Transition Metal Dichalcogenides.

We study the effect of point-defect chalcogen vacancies on the optical properties of monolayer transition metal dichalcogenides using abinitio GW and Bethe-Salpeter equation calculations. We find that chalcogen vacancies introduce unoccupied in-gap states and occupied resonant defect states within the quasiparticle continuum of the valence band. These defect states give rise to a number of strongly bound defect excitons and hybridize with excitons of the pristine system, reducing the valley-selective circular dichroism. Our results suggest a pathway to tune spin-valley polarization and other optical properties through defect engineering.

Magnetic states of linear defects in graphene monolayers: Effects of strain and interaction

The combined effects of defect-defect interaction and of uniaxial or biaxial strains of up to 10\% on the development of magnetic states on the defect-core-localized quasi-one-dimensional electronic states generated by the so-called 558 linear extended defect in graphene monolayers are investigated by means of {\it ab initio} calculations. Results are analyzed on the basis of the heuristics of the Stoner criterion. We find that conditions for the emergence of magnetic states on the 558 defect can be tuned by uniaxial tensile parallel strains (along the defect direction) at both limits of isolated and interacting 558 defects. Parallel strains are shown to lead to two cooperative effects that favor the emergence of itinerant magnetism: enhancement of the DOS of the resonant defect states in the region of the Fermi level and tuning of the Fermi level to the maximum of the related DOS peak. A perpendicular strain is likewise shown to enhance the DOS of the defect states, but it also effects a detunig of the Fermi level that shifts away from the maximum of the DOS of the defect states, which inhibts the emergence of magnetic states. As a result, under biaxial strains the stabilization of a magnetic state depends on the relative magnitudes of the two components of strain.

First-principles density functional theory study of native point defects in Bi2Te3

We present a first-principles study of the native point defects in the thermoelectric material Bi${}_{2}$Te${}_{3}$. Calculated formation energies of defects and electronic densities of states were analyzed in detail. The most prominent native point defects considered are vacancies and antisite defects on the Bi, Te1, and Te2 sublattices of the Bi${}_{2}$Te${}_{3}$ structure. Vacancies on all three sublattices are found to have much higher formation energies than antisite defects. The most dominant antisite defects are found to be Bi${}_{\mathrm{Te}1}$ at Bi-rich conditions, and Te${}_{\mathrm{Bi}}$ at Te-rich conditions. These lead to the formation of resonant defect states at the top of the valence band and bottom of the conduction band, respectively. Hence they are expected to impact charge and energy transport in a profound way. Furthermore antisite defect pairs tend to form at nearest-neighbor distances, and lead to substantial changes in the electronic structure and hence in the thermoelectric properties of Bi${}_{2}$Te${}_{3}$.

Assisted tunneling in ferromagnetic junctions and half-metallic oxides

Different mechanisms of spin-dependent tunneling are analyzed with respect to their role in tunnel magnetoresistance (TMR). Microscopic calculation within a realistic model shows that direct tunneling in iron group systems leads to about a 30% change in resistance, which is close but lower than experimentally observed values. The larger observed values of the TMR might be a result of tunneling involving surface polarized states. It is found that tunneling via resonant defect states in the barrier radically decreases the TMR by order of magnitude. One-magnon emission is shown to reduce the TMR, whereas phonons increase the effect. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR. The model presented here is applied qualitatively to half metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000%. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2.

Magneto-transport properties of semiconductors from flatband magnetocapacitance spectroscopy

We introduce the flatband magnetocapacitance measurement to study magnetotransport properties in bulk semiconductors. This method is better, in some cases, than the Hall measurement because it can avoid some problems inherent to the latter method, such as the influence of the surface conducting layer, the carrier mobility, and the sample geometry correction factor. By using this experimental method some interesting phenomena concerning magnetotransport properties in an n-type InSb sample were observed, including Shubnikov–de Haas oscillations, resonant defect states, conduction process transitions, and the metal–insulator phase transition. The experimentally determined magnetotransport properties for the InSb sample are compared with those reported in literature.

Spin Tunneling in Conducting Oxides

ABSTRACTDifferent tunneling mechanisms in conventional and half-metallic ferromagnetic tunnel junctions are analyzed within the same general method. Direct tunneling is compared with impurity-assisted, surface state assisted, and inelastic contributions to a tunneling magnetoresistance (TMR). Theoretically calculated direct tunneling in iron group systems leads to about a 30% change in resistance, which is close to experimentally observed values. It is shown that the larger observed values of the TMR might be a result of tunneling involving surface polarized states. We find that tunneling via resonant defect states in the barrier radically decreases the TMR (down to 4% with Fe-based electrodes), and a resonant tunnel diode structure would give a TMR of about 8%. With regards to inelastic tunneling, magnons and phonons exhibit opposite effects: one-magnon emission generally results in spin mixing and, consequently, reduces the TMR, whereas phonons are shown to enhance the TMR. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR.The model presented here is applied qualitatively to half-metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin-mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2, and an account of their peculiar band structures is presented. The implications and relation of these systems to CMR materials, which are nearly half-metallic, are discussed.

Magnetism and Spin Tunneling in Nanostructures

ABSTRACTIn the present paper different tunneling mechanisms in conventional and half-metallic ferromagnetic tunnel junctions are analyzed within the same general method. Theoretically calculated direct tunneling in iron group systems leads to about a 30% change in resistance, which is close but lower than experimentally observed values. It is shown that the larger observed values of the TMR might be a result of tunneling involving surface polarized states. We find that tunneling via resonant defect states in the barrier radically decreases the TMR (down to 4% with Fe-based electrodes), and a resonant tunnel diode structure would give a TMR of about 8%. With regards to inelastic tunneling, magnons and phonons exhibit opposite effects: one-magnon emission generally results in spin mixing and, consequently, reduces the TMR, whereas phonons are shown to enhance the TMR. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR.The model presented here is applied qualitatively to half-metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin-mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2, and an account of their peculiar band structures is presented. The implications and relation of these systems to CMR materials, which are nearly half-metallic, are discussed.

MEASUREMENT OF RESONANT DEFECT STATES IN NARROW GAP SEMICONDUCTORS

By using a newly developed measuring technique,i.e. the quantum capacitance spectroscopy,resonant defect states were observed in the valence band and the conduction band of narrow gap semiconductor InSb and HgCdTe materials. On the basis of a developed experimental model,the properties of the resonant defect states are investigated.

Influence of resonant defect states on subband structures in Hg1−xCdxTe

A revised C–V fitting model is presented to determine the subband structures of n-type inversion layer in p-type Hg1−xCdxTe with higher native doping concentration by taking into account the influence of a resonant defect level, which is found from the capacitance measurements. By the model the subband structure and its dependence on surface electron concentrations Ns as well as the relationship between the subband structures and doping concentrations for Hg1−xCdxTe with x=0.21 and a wide doping range of NA = 3.6 × 1016–6.95 × 1017 cm−3 are obtained. By the C–V fitting procedure the resonant defect level, which is located at 45 meV above the conduction band edge for the samples of x=0.21, and its density are also obtained. The annealing and Au-doping experiments on the sample as well as the theoretical analysis reveal that the resonant defect states arise from mercury vacancies occupied by oxygen atoms. The way to eliminate the defect states is also discussed.

Investigation of the electronic structure of the Sigma 5(210) grain boundary in silicon by the linear muffin-tin orbitals/tight-binding method

The electronic structure of the Sigma 5(210) grain boundary in silicon was calculated by the first-principles, self-consistent, linear muffin-tin orbitals/tight-binding representation method on the basis of the atomic structure simulated by the bond orbital model. The calculated electronic structure is found to have localized and resonant defect states, which are caused by the distortions of the atomic structure at the grain boundary. Sharply localized states in the pseudo-gap were observed along with splitting of s states and dissipation of s-p mixed states. A rehybridization effect was revealed at the grain boundary on the atoms with strongly distorted bonds. However, there are no states inside the band gap. The increase of the s-state occupancy in comparison with the p-state occupancy appeared on the atoms with strongly distorted bonds at the grain boundary. The relationship between local electronic structure and local arrangement of the atoms is discussed.

Anomalous galvanomagnetic behavior of orthorhombic layered GeS single crystals.

Galvanomagnetic effects of orthorhombic layered GeS have been investigated in the temperature range 293--423 K. The results showed the existence of negative magnetoresistance and a kind of rapid oscillation of both the magnetoresistance and the Hall coefficient, depending on both the temperature and the magnetic field. To explain this peculiar behavior, the electronic structure of the material, which is a p-type semiconductor, was considered. The negative magnetoresistance was attributed to arise from localized moments located at impurity atoms, such as transition-metal atoms, which form antiferromagnetic clusters. Concerning the rapid oscillations of the magnetoresistance and the Hall coefficient, it was found that resonant defect states in the valence bands, probably from the same defects that give rise to the negative magnetoresistance, may be responsible for this behavior.

Resonant defect states in Pb 1−xSn xSe ( x=0.125) irradiated with electrons

The galvanomagnetic properties of electron irradiated n−Pb 1−xSn xSe ( x=0.125) under high hydrostatic pressure up to 18 kbar have been investigated. Has been shown that electron irradiation creates a band of resonant states probably associated with selenium vacancies in the energy spectrum of Pb 1−xSn xSe. Parameters of the resonant band have been received.