High‐responsivity AlGaN–GaN multi‐quantum well UV photodetector

In this paper, we propose a set of AlGaN–GaN multi‐quantum well (MQW) photodetectors based on p‐i‐n heterostructures with 14 AlGaN–GaN MQW structures in i‐region, where GaN quantum well has 6 nm thickness and Al_xGa_1−xN barrier thickness is 3 nm. In this structure, the peak responsivity of 0.19 A/W at 246 nm is reported. In addition, we investigate effects of various parameters on responsivity, and we show that responsivity of MQW‐based photodetectors strongly depends on proper device design, that is, number of quantum wells, well thickness, barrier thickness, and mole fraction. We also show that increasing number of quantum wells, thickness of wells, and mole fraction as well as decreasing thickness of barriers, increase the responsivity. Using obtained results, we proposed optimal structure. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of band parameters on interband optical absorption in quantum wire structure of low band gap III–V semiconductors

A generalized theory is presented to study the effect of band parameters on inter band optical absorption in quantum wire structure of III–V compound semiconductors considering the wave-vector ( $\vec{k}$ ) dependence of the optical transition matrix element (OME). The band structures of these low band gap semiconducting materials with sufficiently separated split-off valance band are frequently described by the three energy band model of Kane. This has been adopted to calculate the inter band optical absorption coefficient (IOAC) for a wide range of III–V compound semiconductors like, InAs, InSb, Hg_1?x Cd _x Te and In_1?x Ga _x As _y P_1?y lattice matched to InP, having varied split-off energy band compared to their energy band gap. It has been found that IOAC for quantum wires (QWRs) increases in oscillatory manner with increasing incident photon energy and the positions of peaks of oscillation of the coefficient are more closely spaced in the three band model of Kane than those with parabolic energy band approximations reflecting the direct the influence of band energy constants. This effect of band parameters is better revealed from the study of light polarization dependence of the absorption coefficient.     

Dielectric anomalies of Pb0.7La0.2TiO3-based perovskite

Dielectric transitions from normal ferroelectric to relaxor-like and then to quantum paraelectric-like behaviour were observed by substituting 10 to 60% of Ca²⁺ for Pb²⁺ into A-site of Pb_0.7La_0.2TiO₃ with the required stoichiometry of Pb_0.7(1−x)Ca_0.7x La_0.2TiO₃ (PCLT). The quantum ferroelectric relation that commonly applies to SrTiO₃-based perovskites, T _maxα (X − X _c )^1/2, fails to describe the observed dielectric anomalies in PCLT, whereby the transition temperature vanishes with a finite slope. Quantum ferroelectric behaviour that exhibits a sharp dielectric peak but violates the Curie-Weiss law was not observed throughout the composition range studied. Unlike the typical quantum paraelectric SrTiO₃, the quantum paraelectric-like behaviour observed for PCLT with x = 0.55 and 0.60 exhibit negative transition temperatures, as shown by the fittings to both the Curie-Weiss and Barrett’s relations. Thermal hysteresis was surprisingly observed in substituted PCLT with x = 0.30 and 0.40 that exhibit frequency dispersive relaxation. To establish a correlation between the dielectric anomalies and structural parameters, analyses on global and local perovskite lattices were carried out using X-ray diffraction technique and micro-Raman spectroscopy.     

Longitudinal piezoresistance and conductance of the p-type silicon nanostructures in ballistic transport regime: from bulk to nanowires

Longitudinal piezoresistance coefficient π _l and conductance in the p-type silicon nanostructures with different dimensionality of holes D such as quantum wires D=1, quantum films D=2 and bulk silicon D=3 are investigated in the ballistic transport regime. Energy spectrum of holes for the low dimensional structures was accepted in the quasi-classical approximation. Piezoresistance coefficient π _l and conductance of nanostructures oriented along the [100], [110] and [111] crystallographic directions are calculated as function of temperature, concentration of holes, confining dimensions and elastic stress. The origin of the giant pezoresistance effect in the p-type silicon nanostructures at room temperature is explained by the concentration of stress in regions of the nanostructure depleted by holes. Using the only one parameter (the stress concentration factor) both qualitative and quantitative agreement was obtained between computational results and known experimental data.     

Oscillator strengths of the intersubband electronic transitions in the multi-layered nano-antidots with hydrogenic impurity

In this study, we have obtained the exact solutions of the Schr?dinger equation for a multi-layered quantum antidot (MLQAD) within the effective mass approximation and dielectric continuum model for the spherical symmetry. The MLQAD is nano-structured semiconductor system that consists of a spherical core (e.g. Ga_1?x Al _x As) and a coated spherical shell (e.g. Ga_1?y Al _y As) as the whole anti-dot is embedded inside a bulk material (e.g. GaAs). The dependence of the electron energy spectrum and its radial probability density on nano-system radius are studied. The numeric calculations and analysis of oscillator strength of intersubband quantum transition from the ground state into two first allowed excited states at the varying radius, for both the finite and infinite confining potential (CP) as well as constant shell thickness, are performed. It is shown that, in particular, the binding energy and the oscillator strength of the hydrogenic impurity of a MLQAD behave differently from that of a single-layered quantum antidot (SLQAD). For a MLQAD with finite core and shell CPs, the state energies and the oscillator strengths of the impurity are found to be dependent on the shell thickness. At the large core radius and very small shell thickness, our results are closer to respective values for a SLQAD that previously reported.     

Quantized conductance without reservoirs: Method of the nonequilibrium statistical operator

We introduce a generalized non-equilibrium statistical operator (NSO) to study a current-carrying system. The NSO is used to derive a set of quantum kinetic equations based on quantum mechanical balance equations. The quantum kinetic equations are solved self-consistently together with Poisson’s equation to solve a general transport problem. We show that these kinetic equations can be used to rederive the Landauer formula for the conductance of a quantum point contact, without any reference to reservoirs at different chemical potentials. Instead, energy dissipation is taken into account explicitly through the electron-phonon interaction. We find that both elastic and inelastic scattering are necessary to obtain the Landauer conductance.     

Majorana bound states in a quantum dot device coupled with a superconductor zigzag chain

Research in condensed matter physics on topological insulators and superconductors has contributed greatly to the characterization of the surface properties and zero modes of nanowires. In this work we investigated theoretically, using the recursive Green’s function approach, electron transport through a T-shaped single-level spinless quantum dot, connected to a zigzag chain and coupled to a p-wave superconductor. This model is an extension of the Kitaev chain for a triangular network of finite size with three, four, and five sites. We found that the Majorana zero modes can be tuned through the coupling parameters of the device and that the linear conductance shows Majorana bound states (MBS) in the topological phase, being maximally robust in the general topological phase. This more realistic model permits the detection of MBS via control of the parameters governing the electronic tunneling and could be helpful for relevant experiments.     

Higher harmonics and ac transport from time dependent density functional theory

We report on dynamical quantum transport simulations for realistic molecular devices based on an approximate formulation of time-dependent Density Functional Theory with open boundary conditions. The method allows for the computation of various properties of junctions that are driven by alternating bias voltages. Besides the ac conductance for hexene connected to gold leads via thiol anchoring groups, we also investigate higher harmonics in the current for a benzenedithiol device. Comparison to a classical quasi-static model reveals that quantum effects may become important already for small ac bias and that the full dynamical simulations exhibit a much lower number of higher harmonics. Current rectification is also briefly discussed.     

Transport and magnetic properties of Sr doped Nd0.55Ca0.45MnO3

The effect of Sr doping on the transport and magnetism properties of charge/orbital-ordered Nd_0.55Ca_0.45MnO₃ is investigated. A low temperature phase transition from a charge/orbital ordered insulator to a ferromagnetic metal was founded in the Nd_0.55(Ca_1?x Sr _x )_0.45MnO₃ samples. The ground state is the charge/orbital ordered insulator at x?<?0.2, while for x?>?0.3 it changes to a ferromagnetic metal. The resistivities of the samples exhibit almost ideal percolation behavior with a percolation threshold of 0.3. Therefore, this system shows an intricate competition between metallic ferromagnetism and charge/orbital ordered insulator and the percolation behavior demonstrates that the relative volumes of ferromagnetic phase and charge/orbital phases changes with the doping content x and temperature.     

NEGF simulation of the RTD bistability

The simulation of I-V characteristics of Al_0.3Ga_0.7As-GaAs and AlAs-GaAs resonant tunneling diodes (RTD) is presented. The nonequilibrium Green function (NEGF) based 1D quantum transport simulator Wingreen is used in our case. The plateau region on the IV characteristics usually present only by the Wigner function equation (WFE) based simulation appeared now by the NEGF simulation of our AlAs-GaAs RTD and its shape is comparable with our experimental measurements. Analysis of our results from point of view of the scattering and geometrical parameters of the RTD structure is presented.     

The Electrical Conductance of SrFeO2.5+x Thin Films

Thin films of the non-stoichiometric perovskite SrFeO_2.5+x have been grown by the pulsed excimer laser deposition technique onto sapphire substrates. The electrical conductance properties of the thin films have been determined in a series of experiments done both isothermally and with programmed temperature changes from ambient to 490°C and under O₂/N₂ atmospheres with oxygen concentrations in the range from 100 ppm to 100%. Over these ranges of temperature and oxygen partial pressure a wide range of oxygen stoichiometry in SrFeO_2.5+x occurs (approximately 0 < x < 0.5), which includes all four known phases in the SrFeO_{2.5 + x} + O₂ system. The experimentally measured values for the activation energy of conduction, _A, for SrFeO_2.5+x films at temperatures 100 < T < 200°C are in the range 0.30 < _A < 0.47 eV under oxygen at partial pressures 0.001 O ₂)< 0.05 atm and 0.18 < _A < 0.28 eV for 0.2 O ₂)< 1 atm. These values for _A are typical for compositions of SrFeO_2.5+x with stoichiometries in the range 0.25 < x < 0.45. For T < 300°C and for P(_{O 2})< 0.001 atm the films were essentially insulators. For T > 250^°C and P(_{O 2})> 0.001 atm, the oxygen stoichiometries of the films change during the programmed temperature ramps. For these conditions, the values _A/ T exhibit minima/maxima in the temperature range 250 < T < 320^°C which are interpreted as being due to the onset of the order-disorder phase transition from the cubic to the tetragonal and orthorhombic ordered phases of SrFeO_2.5+x with oxygen stoichiometry in the range 0.08 < x < 0.38. The SrFeO_2.5+x thin films have application as oxygen sensing materials, and a relationship between conductance and oxygen sensitivity, S _ox , has been derived. The values of S _ox for SrFeO_2.5+x thin films increases by more than an order of magnitude for compositions close to the lower stoichiometric limit where the principal phase conversion is between the cubic perovskite and the brownmillerite forms.     

Non-equilibrium quantum transport theory: current and gain in quantum cascade lasers

We have developed a self-consistent non-equilibrium Green’s function theory (NEGF) for charge transport and optical gain in THz quantum cascade lasers (QCL) and present quantitative results for the I-V characteristics, optical gain, as well as the temperature dependence of the current density for a concrete GaAs/Al_.15Ga_.85As QCL structure. Phonon scattering, impurity, Hartree electron-electron and interface roughness scattering within the self-consistent Born approximation are taken into account. We show that the characteristic QCL device properties can be successfully modeled by taking into account a single period of the structure, provided the system is consistently treated as open quantum system. In order to support this finding, we have developed two different numerically efficient contact models and compare single-period results with a quasi-periodic NEGF calculation. Both approaches show good agreement with experiment as well as with one another.     

Study on the stability of charge-ordered state and rectifying properties of heteroepitaxial structure for manganites

We report the study on the stability of charge-ordered (CO) state and rectifying properties of heteroepitaxial structure for manganites R _1-x Ca _x MnO₃ (R = La, Pr, and Sm) by measuring magnetoresistance, magnetization, specific heat, I?V characteristics, and X-ray diffraction. For the La_1-x Ca _x MnO₃ system, near the Ca²⁺concentration of x?=?0.75 the CO state is mostly stabilized and insensitive to external magnetic fields, which is supported by the charge transport, magnetic, and specific heat properties. By analyzing powder X-ray diffraction patterns, we found that the Jahn–Teller (JT) distortion in the CO state is the largest at x?=?0.75, which confirms the close relationship between the JT lattice distortion and the stability of the CO state for La_1-x Ca _x MnO₃. The stability of the CO state is also related to the A-site average ionic radius, 〈r _A〉. With decreasing the radius of the doped rare-earth elements (La, Pr, and Sm), the most stable CO state for the each doped system shifts gradually toward lower Ca²⁺concentration. In addition, we have fabricated simple p–n junctions using a heteroepitaxial structure of La_1-x Ca _x MnO₃/Nb-doped SrTiO₃. These junctions show good rectifying properties for temperature from 85 to 305 K. The photovoltage as a function of light wavelength at room temperature has also been studied and the maximum photovoltage can reach 180 mV. The good rectifying behaviors and strong photovoltaic effect make these simple p–n junctions promising for applications.     

Protonic Conduction in Perovskite-type Oxide Ceramics Based on LnScO3 (Ln=La, Nd, Sm or Gd) at High Temperature

Ionic transport properties of perovskite-type oxides based on LnScO₃, Ln_{1- x}Ca_xScO_3- (Ln=La, Nd, Sm and Gd) and LaSc_1-xMg_xO_3- were studied using an electrochemical method at elevated temperatures. Conductivity in these oxide systems increased by more than three orders of magnitude upon doping with divalent atoms such as Ca or Mg. However, when x0.1, conductivities are almost independent of x in La_{1- x}Ca_{xScO_3-} and LaSc_{1- x}Mg_xO_3-. Gd_0.9Ca_0.1ScO_3- showed lower conductivity than Ln_1-xCa_xScO_3- (Ln=La, Nd and Sm). Protonic conduction in these oxides under hydrogen containing atmospheres was confirmed by emf measurements of hydrogen concentration cells and by electrochemical hydrogen pumping using these oxides as a diaphragm. In LaSc_1-xMg_xO_3- the transport number of protons under hydrogen containing atmosphere was unity, suggesting that electronic conduction never became dominant, even under strong reducing conditions. Under condition of high oxygen partial pressure, the transport number of ions was less than 0.1, suggesting that the majority conductive carriers under such conditions were holes.     

Evolution of Structure and Dielectric Properties on Bismuth-Based Pyrochlore with TiO2 Incorporation

The lattice constant and thermal expansion of the pyrochlore oxides (Bi_1.5Zn_0.5)(Zn_0.5–x/3Ti_x Sb_1.5–2x/3)O₇ (0 x 1.5) were studied by X-ray powder diffraction, SEM and IR spectra, respectively. The average grain size is enhanced as the Ti-doping level is increased. The XRD results indicate that all samples have a cubic structure (Fd3m space group) with the lattice parameter decreasing with rising Ti content. The variation of lattice parameters with composition was found to obey Vegard's law. The assignments for the absorption bands in the IR spectra of the series of samples have also been given. The results of diffraction patterns and IR spectra demonstrated that these compounds have the cubic pyrochlore structure. The dielectric properties of the samples were systematically studied. Dielectric constant and temperature coefficient of dielectric constant exhibit strong dependence on the Ti concentration. Dielectric constant varies sharply from 30 when x = 0 to 115 when x = 1.5.     

Conductance fluctuations in graphene nanoribbons

Over the past few years, the amazing properties of graphene have led to predictions for its use in a variety of areas, not the least of which is in semiconductor devices. But, the transport is an important aspect of any possible application. At low temperature, fluctuations are observed in the conductance through nanoribbons. These fluctuations arise from the presence of a random potential in the semiconductor, which arises from e.g. impurities present in the material structure. In this work, we examine the nature of these fluctuations in nanoribbons using an atomic basis quantum transport simulation.     

Electronic signature of single-molecular device based on polyacetylene derivative

We reported for polyacetylene chains containing N = 6, 10, 20 carbon atoms making bridges with gold electrodes which exhibit linear and nonlinear current/voltage signatures. The non-equilibrium quantum transport calculations are obtained from (1) low-voltage regime of 0–0.1 V and (2) high-voltage regime of 0–1.0 V. The nonlinear current/voltage behavior for high-voltage regime 0–1.0 V suggests that the system can operate as follows: (1) at 0.27 V there is a resonance captured by the I–V plot; (2) from 0.42 to 0.65 V a plateau is reached presenting a field effect transistor behavior; (3) there is a decrease in conductance from 0.66 V, and a negative differential resonance emerges with minimum value of 0.72 V. The linear I–V behavior will be reviewed, and we discuss the destructive quantum interference status. The effect occurs when we analyze the (1) low-voltage regime, and our results show that the conductance in oscillations maximizes the impact of quantum interferences (QI) on the I–V curve. In the present work, we demonstrate that QI in nanowire molecules is intimately related to the topology of the molecule’s \(\uppi \) system and establish the existence of QI-induced transmission antiresonances when different voltage regimes are triggered.     

High quality lead zirconate titanate films grown by organometallic chemical vapour deposition

Abstract

Organometallic chemical vapour deposition is a suitable technique for the deposition of thin films of oxidic compounds such as lead zirconate titanate, PbZr_xTi_1?xO₃. Above a deposition temperature of about 600°C stoichiometric PbZr_xTi_1?xO₃ films can be grown on platinized silicon wafers within a large process window, independent of the precursor partial pressures and the deposition temperature. This is the result of a self-regulating mechanism. The PbZr_xTi_1?xO₃ films have excellent ferroelectric properties exhibiting high values, up to 60μC/cm², for the remanent polarisation. The value of the coercive field strength varies between 50 and 180 kV/cm, dependent on the composition. Layers with comparable properties can also be grown at lower temperatures, down to 500°C. In this case careful control of the gas-phase composition is required to obtain films with the correct stoichiometry.     

Properties of reactively sputtered IrOx for PZT electrode applications

Abstract

The use of IrO_x for electrodes in PbZr_xTi_1?xO₃ (PZT) capacitors for ferroelectric memory applications has proven to be advantageous in several respects. In comparison with Pt, IrO_x often exhibits improved fatigue and provides resistance to hydrogen induced degradation at elevated temperatures. Since IrO_x is often produced by sputtering in an oxygen containing environment, several forms of IrO_x can be produced depending on the process conditions. This work concentrates on an analysis of the DC reactive sputtering of IrO_x from an Ir metal target. As with other oxidizable metals, Ir exhibits a transition between metal and oxide mode sputtering when sputtered in oxygen containing atmospheres. Variations in the Ar/O₂ gas flow ratio were used to produce Ir and IrO_xfilms on both sides of the metal-to-oxide mode reactive sputtering transition. Changes in the IrO_x film properties were quantified by using a combination of metrics including X-ray diffraction, sheet resistance, and stress. It was found that the, IrO_x crystalline structure and other IrO_x properties could be directly related to the ferroelectric switching performance of PZT capacitors with IrO_xtop electrodes. A relationship between IrO_x deposition processes and resistance to etch induced damage of the ferroelectric properties was also observed.     

Study of structure and dielectrc properties of non-stoichiometric Bi2O3-ZnO-Nb2O5 ceramics

Bi-based ceramics with pyrochlore structure exhibit interesting dielectric properties such as high dielectric constant and small dielectric loss at high frequency and low firing temperature. Structure and dielectric properties of non-stoichiometric ceramics Bi_1.5?x ZnNb_1.5O_7?1.5x (with x?=??0.25～0.6) were investigated. The compounds departured from stoichiomeric compositions lead to structural parameters change. The second phase appeared when x?≥?0.4. The lattice parameters of cubic pyrochlore Bi_1.5?x ZnNb_1.5O₇ and grain size decreased as x increased. The experimental results show that dielectric constant decrease with increasing of the x value. The dielectric relaxation and defect resulting from non-stoichiometric ceramics Bi_1.5?x ZnNb_1.5O_7?1.5x were discussed.