Defects in ion-implanted hcp-titanium: A first-principles study of electronic structures

The electronic structures of hexagonal closed-packed (h.c.p) titanium containing a vacancy and krypton impurity atoms at various insertion sites are calculated by first-principles methods in the framework of the density-functional theory (DFT). The density of states (DOS) for titanium containing a vacancy defect shows resonance-like features. Also, the bulk electron density decreases from ∼0.15/ų to ∼0.05/ų at the vacancy centre. Electronic structure calculations have been performed to investigate what underlies the krypton site preference in titanium. The DOS of the nearest-neighbour (NN) titanium atoms to the octahedral krypton appears to be less distorted (relative to pure titanium) when compared to the NN titanium atoms to the tetrahedral krypton. The electronic density deformation maps show that polarization of the titanium atoms is stronger when the krypton atom is located at the tetrahedral site. Since krypton is a closed-shell atom, thus precluding any bonding with the titanium atoms, we may conclude that the polarization of the electrons in the vicinity of the inserted krypton atoms and the distortion of the DOS of the NN titanium atoms to the krypton serve to indicate which defect site is preferred when a krypton atom is inserted into titanium. Based on these considerations, we conclude that the substitutional site is the most favourable one, and the octahedral is the preferred interstitial site, in agreement with recent DFT calculations of the energetics of krypton impurity sites.     

Persistent Photoconductivity in electron-irradiated ZnO bulk single crystals: Evaluation of the metastable conductive state by the dual light illumination

Persistent Photoconductivity (PPC) in 30 MeV electron-irradiated n-type ZnO single crystals is studied under the dual light illumination (DLI: the infrared light excitation after the blue illumination). Below 160 K, the remarkable reduction in PPC is observed by the subsequent infrared illumination, suggesting the release of electrons from the perturbed-host state (PHS) as a metastable conductive state to the nonconductive state ( and/or ) via the higher unoccupied states inside the conduction band. Above 160 K, the slight increase in photocurrent is observed by the subsequent illumination, suggesting the photoexcitation of electrons not relevant to the PHS. These results depend on the electron concentration in the PHS.     

Density of states of a two-dimensional electron gas measured by high-resolution photoelectron spectroscopy

We present high energy-resolution photoemission measurements of the spectral density at the discrete quantized electronic levels of a two-dimensional (2D) electron gas. The dynamical 2D electron gas has been obtained by generating a strong accumulation layer at the (110) surface of narrow-gap III–V semiconductors. Exploitation of a number of cases generating band bending (metallic chains or clusters, atomic structure, defects) demonstrates the generality of 2D electron gas formation at charge-accumulated semiconductor surfaces. A self-consistent solution of the Poisson and Schrödinger equations gives the potential well shape, the sub-band energy level position and the accumulated charge density, in excellent agreement with the present experimental data.     

Room temperature photoreflectance of MOCVD-grown InAs/GaAs quantum dots

Photoreflectance spectroscopy results are reported for InAs/GaAs self-organised quantum dots grown by low-pressure MOCVD. Quantum dot-related optical transitions have been observed for the first time at room temperature. Good agreement between experiment and theory based on a recent 8-band k·p theory has been obtained.     

Electronic and magnetic properties of silicon adsorption on graphene

Graphene has proved to be extremely sensitive to its surrounding environment, such as the supporting substrate and guest adatoms. In this work, the structural stabilities, and electronic and magnetic properties of graphene with low-coverage adsorption of Si atoms and dimers are studied using a first-principles method. Our results show that graphene with Si adatoms is metallic and magnetic with a tiny structural change in the graphene, while graphene with Si addimers is semi-metallic and nonmagnetic with a visible deformation of the graphene. The spin-polarized density of states is calculated in order to identify the electronic origin of the magnetic and nonmagnetic states. The present results suggest that the electronic and magnetic behaviors of graphene can be tuned simply via Si adsorptions.     

An ab initio investigation of the low-lying electronic states of BeH

Potential energy curves(PECs) for the ground state(X 2 Σ +) and the four excited electronic states(A 2 Π,B 2 Π,C 2 Σ +,4 Π) of a BeH molecule are calculated using the multi-configuration reference single and double excited configuration interaction(MRCI) approach in combination with the aug-cc-pVTZ basis sets.The calculation covers the internuclear distance ranging from 0.07 nm to 0.70 nm,and the equilibrium bond length R e and the vertical excited energy T e are determined directly.It is evident that the X2Σ+,A2Π,B2Π,C2Σ+ states are bound and 4Π is a repulsive excited state.With the potentials,all of the vibrational levels and inertial rotation constants are predicted when the rotational quantum number J is set to be equal to zero(J = 0) by numerically solving the radial Schr¨odinger equation of nuclear motion.Then the spectroscopic data are obtained including the rotation coupling constant ω e,the anharmonic constant ωexe,the equilibrium rotation constant Be,and the vibration-rotation coupling constant αe.These values are compared with the theoretical and experimental results currently available,showing that they are in agreement with each other.     

Thermoelectric properties and electronic structure of Te-doped CoSb3 compounds

Co₄Sb_12−xTe_x compounds were prepared by mechanical alloying combined with cold isostatic pressing, and the effects of Te doping on the thermoelectric properties were studied. The electronic structure of Te-doped and undoped CoSb₃ compounds has been calculated using the first-principles plane-wave pseudo-potential based on density functional theory. The experimental and calculated results show that the value of the solution limit x of Te in Co₄Sb_12−xTe_x compounds is between 0.5 and 0.7. The Fermi surface of CoSb₃ is located between the conduction band and the valence band, and its electrical resistivity decreases with increasing temperature. The density of states is mainly composed of Co 3d and Sb 5p electrons for intrinsic CoSb₃.The Fermi surface of Te-doped compounds moves to the conduction band and its electrical resistivity increases with increasing temperature, exhibiting n-type degenerated semiconductor character. Under the conditions of the experiment, the maximum value 2.67 mW/m K² of the power factor for Co₄Sb_11.7Te_0.3 is obtained at 600 K; this is about 14 times higher than that of CoSb₃.     

Ab initio electronic band structure calculation of InP in the wurtzite phase

We present ab initio calculations of the InP band structure in the wurtzite phase and compare it with that of the zincblende phase. In both calculations, we use the full potential linearized augmented plane wave method as implemented in the WIEN2k code and the modified Becke-Johnson exchange potential, which provides an improved value of the bandgap. The structural optimization of the wurtizte InP gives , , and an internal parameter u=0.371, showing the existence of a spontaneous polarization along the growth axis. As compared to the ideal wurtzite structure (that with the lattice parameter derived from the zincblende structure calculations), the actual wurtzite structure is compressed (−1.3%) in plane and expanded (0.7%) along the c-direction. The value of the calculated band gaps agrees well with recent optical experiments. The calculations are also consistent with the optical transitions found using polarized light.     

The effects of O deficiency on the electronic structure of rutile TiO2

Ab initio density functional calculations (plane wave GGA, CASTEP) were performed to determine the effect of O deficiency on the electronic structure of rutile, TiO₂. O deficiency was introduced through either the removal of O or the insertion of interstitial Ti atoms. At physically realistic concentrations of O vacancies in the rutile lattice (i.e. 25% and less) O deficiency results in the population of the bottom of the conduction band, the location of the Ti 3d orbitals in the pure structure, increasingly with increasing vacancy concentration. We propose that this could be confused with the formation and population of gap states especially where O vacancies occur in isolated positions in the lattice. In contrast, Ti interstitials introduce a defect state into the energy gap, without an overall reduction in the size of the energy gap. O vacancies result in a spin polarized solution, whereas Ti interstitials do not.     

Study on the doping stability and electronic structure of wurtzite Zn1−xCdxO alloys by first-principle calculations

An ab initio calculation based on density functional theory is applied to study the doping stability and electronic structure of wurtzite Zn_1−xCd_xO alloys. It is found that the different alloy configurations of Zn_1−xCd_xO with a given Cd content are possible thermodynamically, but having different band gaps. With increasing Cd content, the formation enthalpy of Zn_1−xCd_xO alloy increases sharply. The Cd-content dependence of the band-gap values can be fitted with a second-order polynomial. The reduction of band gap can be attributed to the contributions of the hybridization of Zn-4s and Cd-5s, the enhancement of p-d repulsion, and the tensile strain due to Cd-doping.     

Blue luminescence at room temperature in defective MgO films

Luminescence spectroscopy has been used to characterize MgO films prepared by rf-sputtering. A clear correlation is found between the appearance of an emission peak centered at approximately 460 nm and the detection of ferromagnetic ordering in the samples. We suggest that cationic vacancies are responsible for the blue-light emission by introducing p states into the electronic band-gap. In accordance with this, our results strongly indicate that cationic vacancies are at the heart of the appearance of long-range magnetic ordering in MgO films.     

Crystal structure, electronic and transport properties of AgSbSe2 and AgSbTe2

Undoped and p- and n-doped AgSbX₂ (X=Se and Te) materials were synthesized by direct fusion technique. The structural properties were investigated by X-ray diffraction and SEM microscopy. The electrical conductivity, thermal conductivity and Seebeck coefficient have been measured as a function of temperature in the range from 300 to 600 K.To enlighten electron transport behaviours observed in AgSbSe₂ and AgSbTe₂ compounds, electronic structure calculations have been performed by the Korringa-Kohn-Rostoker method as well as KKR with coherent potential approximation (KKR-CPA) for ordered (hypothetical AgX and SbX as well as AgSbX₂ approximates) and disordered systems (Ag_1−xSb_xX), respectively. The calculated density of states in the considered structural cases shows apparent tendencies to opening the energy gap near the Fermi level for the stoichiometric AgSbX₂ compositions, but a small overlap between valence and conduction bands is still present. Such electronic structure behaviour well agrees with the semimetallic properties of the analyzed samples.     

First-principles studies of the electronic and elastic properties of Ti2GeC

We have performed first-principles studies on electronic structure and elastic properties of Ti₂GeC. The calculated band structure shows that this compound is electrical conductor. From the pressure dependence of elastic constants, we find that Ti₂GeC is most stable in the pressure range from 0 to 100 GPa. The strong Ti 3d, Ge 4p and C 2p hybridization may stabilize the structure of Ti₂GeC. By analyzing the ratio between the bulk and shear moduli, we conclude that Ti₂GeC is brittle in nature, and the brittleness of Ti₂GeC originated from the large value of Ti atom occupying the internal parameter z.     

Structural, electronic and optical calculations of CaxZn1−xO alloys: A first principles study

First principles density functional calculations, using full potential linearized augmented plane wave (FP-LAPW) method, have been performed in order to investigate the structural, electronic and optical properties of Ca_xZn_1−xO alloy in B1 (NaCl) phase. Dependence of structural parameters as well as the band gap values on the composition x have been analyzed in the range 0?x?1. Calculated electronic structure and the density of states of these alloys are discussed in terms of the contribution of Zn d, O p and Ca p and d states. Furthermore, optical properties such as complex dielectric constants ε(ω), refractive index including extinction coefficient k(ω), normal-incidence reflectivity R(ω), absorption coefficient α(ω) and optical conductivity σ(ω) are calculated and discussed in the incident photon energy range 0-45 eV.     

Analysis of electronic structures of 3d transition metal-doped TiO2 based on band calculations

The electronic structures of titanium dioxide (TiO₂) doped with 3d transition metals (V, Cr, Mn, Fe, Co and Ni) have been analyzed by ab initio band calculations based on the density functional theory with the full-potential linearized-augmented-plane-wave method. When TiO₂ is doped with V, Cr, Mn, Fe, or Co, an electron occupied level occurs and the electrons are localized around each dopant. As the atomic number of the dopant increases the localized level shifts to lower energy. The energy of the localized level due to Co is sufficiently low to lie at the top of the valence band while the other metals produce midgap states. In contrast, the electrons from the Ni dopant are somewhat delocalized, thus significantly contributing to the formation of the valence band with the O p and Ti 3d electrons. Based on a comparison with the absorption and photoconductivity data previously reported, we show that the t_2g state of the dopant plays a significant role in the photoresponse of TiO₂ under visible light irradiation.     

Crystal growth and the electronic structure of Tl3PbCl5

Total and partial densities of states of constituent atoms of two tetragonal phases of Tl₃PbCl₅ (space groups P4₁2₁2 and P4₁) have been calculated using the full potential linearized augmented plane wave (FP-LAPW) method and Korringa-Kohn-Rostoker method within coherent potential approximation (KKR-CPA). The results obtained reveal the similarity of occupations of the valence band and the conduction band in the both tetragonal phases of Tl₃PbCl₅. The FP-LAPW and KKR-CPA data indicate that the valence band of Tl₃PbCl₅ is dominated by contributions of the Cl 3p-like states, which contribute mainly to the top and the central portion of the valence band with also significant contributions throughout the whole valence-band region. Further, the bottom of the valence band of Tl₃PbCl₅ is composed mainly of the Tl 6s-like states, while the bottom of the conduction band is dominated by contributions of the empty Pb 6p-like states. The KKR-CPA results allow to assume that the width of the valence band increases somewhat while band gap, E_g, decreases when changing the crystal structure from P4₁2₁2 to P4₁. The X-ray photoelectron core-level and valence-band spectra for pristine and Ar⁺-ion-irradiated surfaces of a Tl₃PbCl₅ monocrystal grown by the Bridgman-Stockbarger method have been measured.     

Resistance switching in transparent magnetic MgO films

We have studied the abrupt and hysteretic changes of resistance in MgO-based capacitor devices. The switching behavior is discussed in terms of the formation and rupture of conduction filaments due to the migration of structural defects in the electric field, together with the redox events which affects the mobile carriers. The results presented in this paper suggest that MgO transparent films combining ferromagnetism and multilevel switching characteristics might pave the way for a new method for spintronic multibit data storage.     

Effects of S incorporation on Ag substitutional acceptors in ZnO:(Ag, S) thin films

Ag–S codoped ZnO thin films have been fabricated on Si substrates by radio frequency (RF) magnetron sputtering using a thermal oxidation method. XRD and SEM measurements showed that the sample has hexagonal wurtzite structure with a preferential (002) orientation and the surface is composed of compact and uniform grains. Ag_Zn–nS_O defect complexes were observed in the Ag–S codoped ZnO films by XPS analysis. Low temperature PL spectra showed neutral acceptor bound exciton emission related to Ag_Zn–nS_O. The corresponding acceptor ionization energy of 150 meV is much lower than that of monodoped Ag (246 meV), which is favorable for p-type doping of ZnO.     

A first-principles theoretical simulation on the electronic structures and optical absorption properties for O vacancy and Ni impurity in TiO2 photocatalysts

The band structures and optical absorption spectra of O vacancy and Ni ion doped anatase TiO₂ were successfully calculated and simulated by a plane wave pseudopotential method based on density functional theory (DFT). From the calculated results, a phenomenon of “impurity compensation” was found: the lower formation energy for O vacancy than Ni impurity indicated that introducing the intrinsic defect of O vacancy into Ni ion doped TiO₂ sample was very possible; the positive binding energy for the combination of O vacancy and Ni impurity indicated that two defects were apt to bind to each other; While Ni impurity produced the donor levels in the forbidden band of TiO₂, Ni impurity with O vacancy produced the acceptor levels upon which the excitation led to the photogenerated electrons with high energy and transferability. The combination of absorption spectra for O vacancy and Ni impurity with O vacancy models could reproduce the experimental measurement very well.     

In situ scanning electron microscope electrical characterization of GaN nanowire nanodiodes using tungsten and tungsten/gallium nanoprobes

A lithography-free technique for measuring the electrical properties of n-type GaN nanowires has been investigated using nanoprobes mounted in a scanning electron microscope (SEM). Schottky contacts were made to the nanowires using tungsten nanoprobes, while gallium droplets placed in situ at the end of tungsten nanoprobes were found to be capable of providing Ohmic contacts to GaN nanowires. Schottky nanodiodes were fabricated based on single n-type nanowires, and measured current–voltage (I–V) results suggest that the Schottky nanodiodes deviate from ideal diodes mainly due to their nanoscopic contact area. Additionally, the effect of the SEM electron beam on the I–V characteristics was investigated and was found to impact the transport properties of the Schottky nanodiodes, possibly due to an increase in carrier density in the nanodiodes.