Study of density of localized states in Cd4Se96−xSx (x=0, 4, 8, 12) chalcogenide semiconductor

Thin films of Cd₄Se_96−xS_x (x=0, 4, 8, 12) chalcogenide semiconductor were deposited by the thermal evaporation technique on glass substrates. XRD pattern of CdSeS alloys show that the grain size decreases with the concentration of Sulfur (S). The surface morphology changes due to the addition of sulfur content. The effect of sulfur on the DC conductivity has been investigated, which show that the DC conductivity is a thermally activated process. The Mott parameter shows that dominate conduction is in the localized states, also the addition of sulfur in Cd–Se results an increase in electrical conductivity, which may be due to shift of Fermi level. Current–voltage (I–V) measurements at different fixed temperatures show two regions; Ohmic conduction at low bias having a unit slope, and non ohmic conduction at high bias. Observation of the data shows that conduction is dominated by trap limited space charge limited conduction (SCLC), from where the density of state has been calculated using SCLC measurement data. The increase in the density of states with sulfur concentration may be due to the increase in the defect states.     

Intense laser effects on nonlinear optical absorption and optical rectification in single quantum wells under applied electric and magnetic field

In this work the effects of intense laser on the electron-related nonlinear optical absorption and nonlinear optical rectification in GaAs-Ga_1−xAl_xAs quantum wells are studied under, applied electric and magnetic field. The electric field is applied along the growth direction of the quantum well whereas the magnetic field has been considered to be in-plane. The calculations were performed within the density matrix formalism with the use of the effective mass and parabolic band approximations. The intense laser effects are included through the Floquet method, by modifying the confining potential associated to the heterostructure. Results are presented for the nonlinear optical absorption, the nonlinear optical rectification and the resonant peak of these two optical processes. Several configurations of the dimensions of the quantum well, the applied electric and magnetic fields, and the incident intense laser radiation have been considered. The outcome of the calculation suggests that the nonlinear optical absorption and optical rectification are non-monotonic functions of the dimensions of the heterostructure and of the external perturbations considered in this work.     

Built-in-polarization and thermal conductivity of InxGa1-xN/GaN heterostructures

The effect of built-in-polarization (BIP) field on thermal properties of In_xGa_1−xN/GaN heterostructure has been investigated. The thermal conductivity k of In_xGa_1−xN alloy has been estimated using Callaway's formula including the BIP field for In content x = 0, 0.1, 0.3, 0.5 and 0.9. This study reports that irrespective of In content, the room temperature k of In_xGa_1−xN/GaN heterostructure is enhanced by BIP field. The result predicts the existence of a characteristic temperature T_p at which both thermal conductivities (including and excluding BIP field) show a crossover. This gives signature of pyroelectric nature of In_xGa_1−xN alloy which arises due to variation of polarization with temperature indicating that thermal conductivity measurement can reveal pyroelectric nature. The pyroelectric transition temperature of In_xGa_1−xN alloy has been predicted for various x. The composition dependent nature of room temperature k for x = 0.1 and 0.5 are in line with prior experimental studies. The result can be used to minimize the self heating effect in In_xGa_1−xN/GaN heterostructures.     

Linear and nonlinear optical properties in a semiconductor quantum well under intense laser radiation: Effects of applied electromagnetic fields

In this work we are studying the intense laser effects on the electron-related linear and nonlinear optical properties in GaAs–Ga_1?xAl_xAs quantum wells under applied electric and magnetic fields. The calculated quantities include linear optical absorption coefficient and relative change of the refractive index, as well as their corresponding third-order nonlinear corrections. The nonlinear optical rectification and the second and third harmonic generation coefficients are also reported. The DC applied electric field is oriented along the hererostructure growth direction whereas the magnetic field is taken in-plane. The calculations make use of the density matrix formalism to express the different orders of the dielectric susceptibility. Additionally, the model includes the effective mass and parabolic band approximations. The intense laser effects upon the system enter through the Floquet method that modifies the confinement potential associated to the heterostructure. The results correspond to several configurations of the dimensions of the quantum well, the applied electric and magnetic fields, and the incident intense laser radiation. They suggest that the nonlinear optical absorption and optical rectification are nonmonotone functions of the dimensions of the heterostructure and of the external perturbations considered in this work.     

Structural and dielectric properties of La- and Ti-modified K0.5Na0.5NbO3 ceramics

We report the results of studies of rectifying behavior, positive magneoresistance (MR), the charge-transport mechanism and the effect of an electric field on a ZnO (n)/La_0.5Pr_0.2Sr_0.3MnO₃ (LPSMO) (p)/SrNb_0.002Ti_0.998O₃ (SNTO) (n) heterostructure comprising two p–n junctions fabricated using the pulsed laser deposition technique. The heterostructure exhibits rectifying behavior over a wide temperature and field range having hysteresis in I–V behavior (forward bias) due to the tunneling of charge carriers. It is also observed that, depending on the nature of the electric field bias to n-type ZnO and SNTO, the junction resistance becomes modified, which has been explained on the basis of spin injection in the heterostructure. The observation of unconventional positive MR at room temperature has been justified on the basis of interface effects and the reduction in barrier height obtained using fitting of the I–V data by a thermionic emission model.     

Exchange bias and training effect in Ni/Ag-doped NiO bilayers

A series of polycrystalline Ag-doped Ni_1−xAg_xO/Ni bilayers with x up to 0.2 were prepared by magnetron sputtering. X-ray diffraction, atomic force microscopy and transmission electron microscopy analyses reveal that Ag doping significantly reduces the mean NiO grain size and leads to the appearance of Ag nanoparticles on the surface of the Ag-doped NiO films. As x increases, the exchange bias field and coercivity at room temperature decrease as a consequence of the reduced thermal stability of smaller NiO grains and the screening effect resulting from the interfacial Ag nanoparticles. At lower temperatures, a slight enhancement of the exchange bias field is observed in the Ag-doped sample, indicating that the Ag doping increases the uncompensated NiO spin density. In addition, our studies find that the training effect of the Ag-doped sample can be well described by a spin configurational relaxation model, regardless of the presence of Ag nanopartiles at the interface.     

Nonlinear optical absorption and optical rectification in near-surface double quantum wells: combined effects of electric, magnetic fields and hydrostatic pressure

The theoretical study of the combined effects of electric and magnetic fields and hydrostatic pressure on the nonlinear optical absorption and rectification is presented for electrons confined within an asymmetrical GaAs?Ga_1-x Alx As double quantum well. The effective mass, parabolic band, and envelope function approaches are used as tools for the investigation. The electric field is taken to be oriented along the growth direction of the heterostructure and the magnetic field is applied parallel to the interfaces of the quantum wells. The pressure-induced mixing between the two lowest conduction bands is considered both in the low and high pressure regimes. According to the results obtained it can be concluded that the nonlinear optical absorption and rectification coefficients depend in a non-trivial way on some internal and external parameters such as the size of the quantum wells, the direction of applied electric field, the magnitude of hydrostatic pressure, the stoichiometry of the wells and barriers, and the intensity of the applied magnetic field.     

Characterization of cubic phase MgZnO/Si(1 0 0) interfaces

The microstructural properties of the Mg_xZn_1−xO/Si(1 0 0) interface were investigated using transmission electron microscopy (TEM) and chemical states of the heterostructure were studied by high resolution X-ray photoelectron spectroscopy (XPS). By analyzing the valence band spectra of thin Mg_xZn_1−xO/Si(1 0 0) heterostructures, the valence band offset between such Mg_0.55Zn_0.45O and Si(1 0 0) was obtained to be 2.3 eV. Using the cubic ternary thin films as insulators, metal-insulator-semiconductor (MIS) capacitors have been fabricated. Leakage current density lower than 3 × 10⁻⁷ A/cm² is obtained under the electrical field of 600 kV/cm by current-voltage (I-V) measurement. Frenkel-Poole conduction mechanism is the main cause of current leakage under high electrical field.     

Compositional contrast in AlxGa1−xN/GaN heterostructures using scanning spreading resistance microscopy

Scanning spreading resistance microscopy has found extensive use as a dopant-profiling technique for silicon-based devices, and to a lesser extent for some III-V materials. Here we demonstrate its efficacy for wide bandgap nitrides and, in particular, show that it may be used to differentiate between layers of different Al-content in an Al_xGa_1−xN/GaN heterostructure. A monotonic increase in resistance signal with increasing Al-content is demonstrated, under optimal imaging conditions. The variation in measured resistance with applied bias is shown to be dependent on the aluminium content, and this is discussed, along with other issues, in the context of potential quantification of unknown samples. The procedure for forming an optimal image is different from that for silicon, in terms of contact forces and applied biases.     

Coexistence of type-I and type-II band alignment in Ga(Sb,P)/GaP heterostructures with pseudomorphic self-assembled quantum dots

Band alignment of heterostructures with pseudomorphic GaSb_{1 ? x} P _x /GaP self-assembled quantum dots (SAQDs) lying on a wetting layer was studied. Coexistence of type-I and type-II band alignment was found within the same heterostructure. Wetting layer has band alignment of type-I with the lowest electronic state belonging to the X _XY valley of GaSb_{1 ? x} P _x conduction band, in contrast to SAQDs, which have band alignment of type-II, independently of the ternary alloy composition x. It is shown that type-I-type-II transition is a result of GaP matrix deformation around the SAQD.     

Experimental observation of a striking similarity between quantum hall transport coefficients

The derivative of the Quantum Hall resistance, ρ_xy, with respect to the carrier density, n, has been measured for a two-dimensional electron gas in a GaAs-Al_xGa_1?xAs heterostructure, as a function of magnetic field. dρ_xy/dn exhibits a remarkable similarity to the diagonal resistivity, ρ_xy, to the extent that one is almost directly proportional to the other. Our result suggests the possibility of a fundamental connection between the two quantities.     

Study of DD-reaction yields from a Pd/PdO:Dx heterostructure at low energies using the GELIS setup

The results of measurements of the DD-reaction yields from the Pd/PdO:D_x heterostructure in the energy range of 10?C25 keV are presented. The neutron and proton fluxes are measured using a neutron detector based on He-3 counters and a CR-39 plastic track detector. Comparisons with calculations show significant effects of DD-reaction yield amplification. The screening potential for this heterostructure under these experimental conditions is estimated as U _e = 630?C980 eV.     

Zn1−xMgxO/ZnO heterostructures studied by Kelvin probe force microscopy conjunction with probe characterizer

The surface potential of Zn_1−xMg_xO/ZnO heterostructure grown by radical source molecular beam epitaxy was measured by Kelvin force microscopy (KFM). A clear correlation was observed between the topographic image and the surface potential of Zn_0.56Mg_0.44O/ZnO heterostructure. The potential area around the surface pits was about 60 mV lower than that of the surrounding region, which suggests the effects of the pits on the electrical properties of the potential layer. In order to guarantee the accuracy of measurement, the probe shape was analyzed by probe characterizer and using Au thin films as a potential standard.     

The effect of barrier strain on the reliability of Inx Al1–xN/AlN/GaN heterostructure field‐effect transistors

We report on the reliability of In_x Al_1–xN/AlN/GaN‐based heterostructure field‐effect transistors (HFETs) fabricated on five different wafers with varying indium compositions (0.12 ≤ x ≤ 0.20) encompassing the tensile/compressive strain fields. All of the tested devices underwent high field on‐state stress at 20 V DC drain bias and zero gate bias for five hours. We monitored the drain current and low‐frequency noise (LFN) a priori and a posteriori the stress treatment to quantify device degradation. HFETs suffering tensile strain showed remarkably large degradation which manifested itself with up to 25 dB increase in noise power and up to 72% loss of drain current after stress. On the other hand, devices fabricated on compressively strained structures remained intact after stress, but they had about 30 dB higher pre‐stress noise‐power levels and about 50% lower drain‐current densities to begin with. The results show that the nearly lattice‐matched In_0.17Al_0.83N barrier exhibited very low degradation along with current density remaining high compared with the devices having barriers with lower or higher indium content. Our results suggest that the nearly‐lattice‐matched InAlN can be a good candidate for devices due to its relatively better reliability while maintaining a high current density. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhancement of the upper critical magnetic field in La1.2−xEuxMo6S8 compounds

Measurements of the upper critical magnetic field (H_c2) of the pseudoternary system La_1.2?xEu_xMo₆S₈ were made at temperatures above 1.5 K. An enhancement of the value of H_c2 was found for 0 < x ? 0.6. The results are attributed to an increase of the orbital critical field with increasing x, compensation of the applied magnetic field by a negative exchange field due to an antiferromagnetic exchange interaction between the conduction electron spins and the Eu magnetic moments (Jaccarino-Peter effect), and exchange scattering of conduction electrons by the rare earth magnetic moments.     

Enhancement of DD-Reaction yields from the Pd/PdO:Dx heterostructure by N+ and Ne+ ion beams using the GELIS setup

The results on DD-reaction yield enhancement from the Pd/PdO:D_x heterostructure by N⁺ and Ne⁺ ion beams in the energy range of 10–25 keV are presented. Neutron and proton fluxes were measured using a neutron detector based on He-3 counters and a CR-39 plastic track detector. Measurements showed significant DD-reaction yield enhancement effects. The screening potential for this heterostructure under these experimental conditions was determined as U _e = 897 eV.     

Enhancement of DD-reaction yields from the Ti/TiO2:Dx heterostructure by H+ and N+ ion beams using the GELIS setup

The results on DD-reaction yield enhancement from the Ti/TiO₂:D_x heterostructure by H⁺ and N⁺ ion beams in the energy range of 10–25 keV are presented. Neutron and proton fluxes weremeasured using a neutron detector based onHe-3 counters and a CR-39 plastic track detector. Measurements showed significant DD-reaction yield enhancement effects. The screening potential for this heterostructure under these experimental conditions was determined as U _e = 796 eV.     

Study of DD-reaction yields from the Ti/TiO2:Dx heterostructure at low energies using the GELIS setup

The results of measurements of the DD-reaction yields from the Ti/TiO₂:D_x heterostructure in the energy range of 10–25 keV are presented. Neutron and proton fluxes were measured using a neutron detector based on He-3 counters and a CR-39 plastic track detector. A comparison with calculations showed the existence of significant effects of DD-reaction yield amplification. The screening potential for this heterostructure under these experimental conditions was estimated to be in the range U _e = 160–750 eV.     

Carrier transport mechanisms in the ZnO based heterojunctions grown by MBE

This paper presents a review of models of the current transport in different kind of heterojunctions (HJs) and their characteristics. In order to effectively deduce the dominant electron transport for the HJs based on ZnO or Zn_1?xMg_xO layers grown on Si substrate by MBE a comparison is performed – which type of the HJ exhibits better electrical properties. The current–voltage characteristics for the studied HJs were measured within 280–300 K. The transport properties of the HJs are explained in terms of Anderson model with reference to aforementioned current transport models. It is found, that the mechanisms of current transport for all of the studied HJs are similar. At a low forward voltage bias the tunneling current dominates while at medium voltage bias (0.5–1 V) multitunneling capture-emission prevails with the electron trap located at 0.1–0.25 eV below the bottom of a ZnO (Zn_1?xMg_xO) conduction band. Beyond this voltage bias space charge limited current governs the current transport.     

Leakage current study of Si1−xCx embedded source/drain junctions

This work investigates the impact of various processing parameters on the leakage current of in situ P-doped Si_1−xC_x embedded source/drain (S/D) junctions, i.e., the carbon content x (%) and the thermal budget used either before or after the selective epitaxial deposition. It is shown that while the area leakage current density, generated by defects in the depletion region is not affected by the epitaxial process or the strain in the substrate, the perimeter leakage current density (J_P) increases with x. From the stronger reverse bias dependence of J_P, it is derived that a higher electric field exists along the junction periphery. This is confirmed by capacitance-voltage (C-V) measurements, demonstrating a higher p-well B doping density for increasing x. It is believed that this originates from the strain dependence of the B diffusivity in the p-well region. No evidence of electrically active extended defect formation was found, so it is expected that the off-state current of embedded Si_1−xC_x S/D nMOSFETs will not be adversely affected by the selective epitaxial deposition.