Surface Electronic Density Of States Research Articles

Strain Effects in Twisted Spiral Antimonene.

Van der Waals (vdW) layered materials exhibit fruitful novel physical properties. The energy band of such materials depends strongly on their structures, and a tremendous variation in their physical properties can be deduced from a tiny change in inter-layer spacing, twist angle, or in-plane strain. In this work, a kind of vdW layered material of spiral antimonene is constructed, and the strain effects in the material are studied. The spiral antimonene is grown on a germanium (Ge) substrate and is induced by a helical dislocation penetrating through few atomic-layers of antimonene (β-phase). The as-grown spiral is intrinsically strained, and the lattice distortion is found to be pinned around the dislocation. Both spontaneous inter-layer twist and in-plane anisotropic strain are observed in scanning tunneling microscope (STM) measurements. The strain in the spiral antimonene can be significantly modified by STM tip interaction, leading to a variation in the surface electronic density of states (DOS) and a large modification in the work function of up to a few hundreds of millielectron-volts (meV). Those strain effects are expected to have potential applications in building up novel piezoelectric devices.

Passivation of Mid-Gap Electronic States at Calcium Aluminosilicate Glass Surfaces upon Water Exposure: An Ab Initio Study.

When a clean glass surface is exposed to humid air, a thin water layer forms on the hydrophilic surface. Using ab initio molecular dynamics, we simulate the changes in the electronic structure of a CaO-Al2O3-SiO2 glass model upon vacuum fracture and subsequent exposure to H2O. When the glass is fractured, dangling bonds form, which lower the band gap of the surface by ∼1.8 eV compared to the bulk value due to mid-gap surface states. When H2O adsorbs onto the vacuum-fractured surface, the band gap increases to a value closer to that of the bulk band gap. Using two different hydroxylation methods, we find that the calculated band gap of the glass surface depends on the hydroxylation state. Surfaces with ∼4.5 OH/nm2 have smaller band gaps due to unfilled surface states, and surfaces with ∼2.5 OH/nm2 have larger band gaps with no apparent unfilled surface states. The resulting changes in the electronic structure, quantified by electron affinity and work function values, are hypothesized to play an important role in the electrostatic charge transfer based on the principles of surface state theory, which posit that the density of electronic surface states determines the amount of electronic charge transfer to or from material surfaces.

Interpretable Deep Learning Model for Analyzing the Relationship between the Electronic Structure and Chemisorption Property.

The use of machine learning (ML) is exploding in materials science as a result of its high predictive performance of material properties. Tremendous trainable parameters are required to build an outperforming predictive model, which makes it impossible to retrace how the model predicts well. However, it is necessary to develop a ML model that can extract human-understandable knowledge while maintaining performance for a universal application to materials science. In this study, we developed a deep learning model that can interpret the correlation between surface electronic density of states (DOSs) of materials and their chemisorption property using the attention mechanism that provides which part of DOS is important to predict adsorption energies. The developed model constructs the well-known d-band center theory without any prior knowledge. This work shows that human-interpretable knowledge can be extracted from complex ML models.

Development of methods for studying intergranular surface features in semiconductor heterogeneous polycrystals of bismuth-antimony tellurides with the imposition of electric and deformation fields

The article discusses the technique and tools for studying the role of the effective density of electronic surface states innanocrystalline semiconductor films when cyclic deformation is applied directly from the analysis of experimental data. The surface electron states play the role of recombination and trapping centers depending on the number of carriers, the electron capture section and hole, the concentration of surface states of their type and energy position. To determine the effective density of surface states, we found both a change in the Fermi level and a change in the density of the effective surface charge. The effective density of electronic surface states was determined from the measured variations in the active resistance and capacity of nanocrystalline films of bismuth-antimony tellurides upon the application of irreversible deformation, and its strain dependence was found. From data analysis, one can judge the irreversibility of heterogeneous structures during deformation, that is, the electronic structure of a nanocrystalline semiconductor film changes greatly when cyclic deformation is applied.

Annealing Effect on the Composition and Electrophysical Properties of N-Type Silicon Surface

During low-temperature annealing, the segregation of the alloying element leads to a strong enrichment of the surface layer, causing a rearrangement of the surface electron structure. This change in the electron structure is manifested in the characteristic energy loss spectra. Annealing of single crystals at 400–500 K leads to an increase in the density of surface electron states. As shown by the calculations of the surface potential using experimental data on the temperature dependence of the surface concentration, dopant segregation causes a linear increase in the surface potential.

Copper Interconnects: Surface State Engineering to Facilitate Specular Electron Scattering

In situ electron transport measurements on epitaxial 10-nm-thick Cu(001) with Al and AlO x cap layers indicate that the electron surface scattering specularity increases when the density of electronic surface states decreases. The Cu layers were sputter deposited on MgO(001) substrates, and their resistance is measured in situ as a function of the Al cap thickness ${d}_{\text {Al}} = {0}\!-\!{1.4}$ nm as well as oxygen exposure from $10^{-{1}}$ to $10^{{5}}$ Pa $\cdot \text{s}$ . The resistance increases with increasing ${d}_{\text {Al}}$ , indicating a decrease in the surface scattering specularity that is well described by an exponential decay, attributed to the linearly increasing number of localized surface states available for electron scattering. In contrast, exposure to 0.04–50 Pa O2 results in a decrease in the measured resistance, as the Al cap is oxidized to form AlO x , leading to a decreasing surface density of states and increasing scattering specularity with increasing ${x}$ . The conductance enhancement is most pronounced for ${d}_{\text {Al}} = {0.4}$ nm since a larger ${d}_{\text {Al}}$ leads to incomplete Al oxidation, while a smaller ${d}_{\text {Al}}$ insufficiently suppresses copper surface oxidation, causing a transition to diffuse surface scattering. The overall results indicate that insulating liner layers provide a conductance advantage for narrow interconnect lines.

Enhancing the Circular Polarization of Spin Light-Emitting Diodes by Processing in Selenium Vapor

Spin light-emitting diodes based on InGaAs/GaAs heterostructures with a CoPt ferromagnetic injector were fabricated. It was demonstrated that the processing of these structures in selenium vapor prior to the deposition of a CoPt contact provides an opportunity to enhance the circular polarization degree of diode emission. The observed increase in the polarization degree is attributed to the suppression of spin relaxation at the metal/semiconductor interface due to surface passivation and a reduction in the density of surface electron states as a result of processing in selenium vapor.

Повышение степени циркулярной поляризации спиновых светоизлучающих диодов путем обработки в парах селена

AbstractSpin light-emitting diodes based on InGaAs/GaAs heterostructures with a CoPt ferromagnetic injector were fabricated. It was demonstrated that the processing of these structures in selenium vapor prior to the deposition of a CoPt contact provides an opportunity to enhance the circular polarization degree of diode emission. The observed increase in the polarization degree is attributed to the suppression of spin relaxation at the metal/semiconductor interface due to surface passivation and a reduction in the density of surface electron states as a result of processing in selenium vapor.

Math modeling of electronic processes and deep level ionization kinetic

Mathematical model of kinetics of charge deep levels in the forbidden band of the semiconductor, which takes into account the processes of carriers charge exchange between deep levels and both allowed bands, which adequately describes the nature of the non-exponential relaxation capacity, is proposed. A method for determining the spectrum of deep level transient spectroscopy having greater accuracy and resolution in comparison with traditional methods using a relaxation time approximation. The results of numerical experiments using the kinetics charge deep levels model in the frameworks of proposed approximations are presented. Account of generational and recombination components of charge exchange processes of all deep levels in the forbidden band of the semiconductor leads to the conclusion that the kinetics of ionization of these centers, in general, does not obey the Boltzmann statistics. Account of charge exchange processes between the deep levels a significantly effects on their recharge kinetics. Numerical analysis results show that the processes of deep levels ionization are more complicated than the staged-type kinetics. It is shown that in most cases stagedtype kinetics at deep level transient spectroscopy leads to significant methodological error in the parameters determination. From the results of numerical analysis follows, that the density of surface electronic states has a significant impact on the overall recharged kinetics of deep levels. Donor deep levels recharge analysis revealed not only the features of the deep levels ionization in semiconductors, but also allowed to answer some questions that are typical to all deep-level transient spectroscopy in general.

Surface states in a 3D topological insulator: The role of hexagonal warping and curvature

We explore a combined effect of hexagonal warping and a finite effective mass on both the tunneling density of electronic surface states and the structure of Landau levels of 3D topological insulators. We find the increasing warping to transform the square-root van Hove singularity into a logarithmic one. For moderate warping, an additional logarithmic singularity and a jump in the tunneling density of surface states appear. By combining the perturbation theory and the WKB approximation, we calculate the Landau levels in the presence of hexagonal warping. We predict that due to the degeneracy removal, the evolution of Landau levels in the magnetic field is drastically modified.

Electrical passivation of silicon surface by a polyelectrolyte coating

The influence of a cationic polyelectrolyte (PE) monolayer on the electrical properties of single crystals of p-Si with different doping levels was studied. It is demonstrated that photostimulated PE adsorption on p-Si provides an additional tenfold decrease in effective density of surface electronic states as compared to passivation by PE adsorption in the dark. Maximum effect is observed for the characteristic times of “fast” states in p-Si with higher doping levels.

Passivation of the GaP(111) surface by treatment in selenium vapors

Results of examination of structural phase transitions on the GaP(111) surface after heat treatment in selenium vapors in a vacuum chamber with a quasi-closed volume are described. The electrophysical characteristics of Schottky-barrier diodes on GaP(111) are studied before and after treatment in selenium vapors by measuring the current-voltage characteristics and by deep-level transient spectroscopy. It is found that, after treatment in selenium vapors, the Schottky-barrier height becomes dependent on the work function of the metal in accordance with the Schottky-Mott rule for an ideal diode. It is shown that a decrease in the density of surface electronic states in GaP(111) results from the formation of a Ga2Se3(111)(√3 × √3)-R30° surface phase with ordered stoichiometric gallium vacancies.

Structure and chemical reactivity of the polar three-fold surfaces of GaPd: A density-functional study

The polar threefold surfaces of the GaPd compound crystallizing in the B20 (FeSi-type) structure (space group P2(1)3) have been investigated using density-functional methods. Because of the lack of inversion symmetry the B20 structure exists in two enantiomorphic forms denoted as A and B. The threefold {111} surfaces have polar character. In both nonequivalent (111) and (111) directions several surface terminations differing in structure and chemical composition are possible. The formation of the threefold surfaces has been studied by simulated cleavage experiments and by calculations of the surface energies. Because of the polar character of the threefold surfaces calculations for stoichiometric slabs permit only the determination of the average energy of the surfaces exposed on both sides of the slab. Calculations for nonstoichiometric slabs performed in the grand canonical ensemble yield differences of the surface energies for the possible terminations as a function of the chemical potential in the reactive atmosphere above the surface and predict a transition between Ga- and Pd-terminated surfaces as a function of the chemical potential. The {100} surfaces are stoichiometric and uniquely defined. The calculated surface energies are identical to the average energies of the {100} surfaces of the pure metals. The {210} surfaces are also stoichiometric, with an energy very close to that of the {100} surfaces. Assuming that for the {111} surfaces the energies of different possible terminations are in a proportion equal to that of the concentration-weighted energies of the {111} surfaces of the pure metals, surface energies for all possible {111} terminations may be calculated. The preferable termination perpendicular to the A<111> direction consists of a bilayer with three Ga atoms in the upper and three Pd atoms in the lower part. The surface energy of this termination further decreases if the Pd triplet is covered by additional Ga atom. Perpendicular to the A<111> direction the lowest energy has been found for a bilayer with three Ga atoms per surface cell in the upper layer and one Ga and one Pd in the lower part. The calculated surface energies are in agreement with a simulated cleavage experiment. However, cleavage does not result in the formation of the lowest-energy surfaces, because all possible {111} cleavage planes expose a low-energy surface on one, and a high-energy surface on the other side. The prediction of Ga-terminated surfaces has been tested against the available experimental information. The calculated surface electronic density of states is in very good agreement with photo-emission spectroscopy. Calculated STM images of the most stable surfaces agree with all details of the available experimental images. The chemical reactivity of the most stable surfaces has been studied by the adsorption of CO molecules. The adsorption energies and maximum coverages calculated for the Ga-terminated surfaces permit a reasonable interpretation of the observed thermal desorption spectra, whereas for the Pd-terminated surfaces the calculated adsorption energies are far too high.

Density of Surface States at CdSe Quantum Dots by Fitting of Temperature-Dependent Surface Photovoltage Transients with Random Walk Simulations

Surface photovoltage (SPV) transients were measured for a monolayer of CdSe quantum dots (QDs) on planar indium tin oxide (ITO) substrate at temperatures between 200 and 40 °C. By fitting with a random walk simulation, the energy distribution and density of electronic surface states at the QDs were determined. SPV transients show the relaxation of separated charge carriers. This relaxation was described by a model based on an isolated QD approximation. In this model Miller–Abrahams hopping of holes between surface states and the first excitonic state of CdSe QDs and recombination of holes with electrons separated at the ITO surface were considered as mechanisms leading to charge relaxation. Parameters of the model were obtained by a statistic multiparameter fitting procedure of measured SPV transients. All SPV transients could be excellently fitted by using the same Couchy energy distribution of hole traps at the CdSe QD surface. The number of hole traps at CdSe QD surfaces determined by the simulation wa...

Linear-scaling density functional theory simulations of polar semiconductor nanorods

Binary polar semiconductors in the wurtzite structure can be grown in the form of nanorods aligned along the wurtzite [0001] direction. In such structures, very large dipole moments have been observed experimentally. We have studied the distribution of charge in GaAs nanorods, so as to elucidate the origin of the dipole moments. To make contact with the realistic experimental regime, we need to model systems of several thousand atoms, necessitating the use of a Linear-Scaling formulation of DFT. We use the ONETEP code, which combines the benefits of O(N) computational effort and plane-wave accuracy. We show that both the direction and magnitude of the dipole moment of a nanorod, and its electric field, depend sensitively on how its surfaces are terminated and do not depend strongly on the spontaneous polarization of the underlying lattice. Furthermore, we observe that the Fermi level for an isolated nanorod always coincides with a significant density of electronic surface states at the polar surfaces, which are either mid-gap states or band-edge states. These states pin the Fermi level, and therefore fix the potential difference across the rod. We provide evidence that this effect can have a determining influence on the polarity of nanorods, and has consequences for the way a rod responds to changes in its surface chemistry.

Fermi-level pinning can determine polarity in semiconductor nanorods

First-principles calculations of polar semiconductor nanorods reveal that their dipole moments are strongly influenced by Fermi level pinning. The Fermi level for an isolated nanorod is found to coincide with a significant density of electronic surface states at the end surfaces, which are either mid-gap states or band-edge states. These states pin the Fermi level, and therefore fix the potential difference across the rod. We provide evidence that this effect can have a determining influence on the polarity of nanorods, and has consequences for the way a rod responds to changes in its surface chemistry, the scaling of its dipole moment with its size, and the dependence of polarity on its composition.

Surface analysis of single-crystalline [formula omitted]-[formula omitted

Clean surfaces of the single crystalline β-FeSi2 have been prepared and investigated by means of surface analysis techniques. X-ray photoelectron spectroscopy of the surface oxide reveals that the surface Si is mainly oxidized while Fe isn’t. After removing the surface oxide, clean surface can be obtained showing well-defined atomic features in low-energy electron diffraction and scanning tunneling microscopy. No drastic surface reconstruction is found possibly reflecting strong Fe–Si bond. Density functional theory calculation suggests the spin polarized surface electronic density of states when Fe comes at the surface, although such a situation seems to be unlikely.

Evolution of CdS:Mn nanoparticle properties caused by pH of colloid solution and ultrasound irradiation

AbstractThe effects of pH value and precursors concentrations in the growth solution as well as of ultrasound irradiation on the properties of CdS:Mn nanoparticles in PVA matrix are studied. Analysis of the photoluminescence spectra indicates that the increase of the pH value in the growth solution leads to the increase of the density of surface electron states that participate in radiative transitions and to the enlargement of CdS:Mn nanoparticles.We discuss a stabilization of the sizes of CdS:Mn nanoparticles at pH=6 and change of nanoparticle size with decreasing pH value at elevated concentrations of ions HS‐. It is shown that in the ultrasound‐treated samples the size of the particles does not depend on pH value. The effects of ultrasound result primarily from acoustic cavitation. It is demonstrated that the action of two external factors (the value of the medium pH and ultrasound treatment) on the characteristics of nanoparticles is correlated. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The conductivity of Bi(111) investigated with nanoscale four point probes

The room temperature conductance of Bi(111) was measured using microscopic four point probes with a contact spacing down to 500 nm. The conductance is remarkably similar to that of the bulk, indicating that surface scattering is not a major mechanism for restricting the mobility at this length scale. Also, the high density of electronic surface states on Bi(111) does not appear to have a major influence on the measured conductance. The lower limit for the resistivity due to electronic surface states is found to be around 5 Ω. With such a value for the surface resistivity, surface conduction should not be a significant factor to inhibit the observation of the predicted semiconductor to semimetal transition for thin films of Bi.

SPECTRALLY SENSITIZED PHOTOCONDUCTIVITY OF ZINC OXIDE CRYSTALS*

Abstract— Spectral sensitization of photoconductivity was found after coating single crystals with an organic dye in a methanol solution. Similar experiments have been carried out under conditions designed to exclude the influence of solvent and adsorbed gases. The dye was deposited from vapor onto clean crystal surfaces in ultrahigh vacuum and the spectral distribution of the photoconductivity was observed without admission of air. With both methods of dyeing, the quantum yields, referred to the absorbed light, were comparable to the yield in the intrinsic region. Also the influence of a transverse electric field upon surface conductivity before and after dyeing was studied. From the ‘field effect’ it is concluded that the adsorbed dye causes a considerable increase of the density of electronic surface states near the Fermi level.