Determination of the critical indium composition corresponding to the metal–insulator transition in InxGa1−xN (0.06 ⩽ x ⩽ 0.135) layers

The low-temperature conductivity of In_xGa_1−xN alloys (0.06 ⩽ x ⩽ 0.135) is analyzed as a function of indium composition (x). Although our In_xGa_1−xN alloys were on the metallic side of the metal–insulator transition, neither the Kubo-Greenwood nor Born approach were able to describe the transport properties of the In_xGa_1−xN alloys. In addition, all of the In_xGa_1−xN alloys took place below the Ioeffe–Regel regime with their low conductivities. The observed behavior is discussed in the framework of the scaling theory. With decreasing indium composition, a decrease in thermal activation energy is observed. For the metal–insulator transition, the critical indium composition is obtained as x_c = 0.0543 for In_xGa_1−xN alloys.     

Specifics of Heat Transfer in AlxGa1 – xN/GaN Heterostructures on Sapphire

Physics of the Solid State - The thermal conductivity of AlxGa1 – xN/GaN heterostructures (0.05 ≤ x ≤ 1) fabricated on sapphire by molecular beam epitaxy is...     

Surface effect on the reverse current of Al x Ga1−x Sbp−n structures

The effect of an invertedp-region along the free surface ofn-Al _x Ga_1−x Sb on the reverse current ofp−n structures from the given solid solution is analyzed. Expressions which describe “collection” of the inverted layer current on the cylindrical surface of ann-region are discussed. The contribution of the near-surface and bulk components to the reverse current ofp−n structures with a semi-infiniten-region is estimated. For structures with a two-layern-region of finite thickness we have calculated the dependence of the near-surface current on the voltage across thep−n structure, the thickness of then-region, and its composition and doping level. We have compared the calculated current-voltage characteristics with experiment using a Al_0.15Ga_0.85Sbp−n structure as an example. Tomsk State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 1, pp. 34–40, January, 1999.     

Finite confining effect on electron–phonon interaction in GaAs/Ga1−xAlxAs single heterostructures

The form factors of the electron–phonon interaction for GaAs/Ga_1−xAl_xAs single heterostructures have been evaluated using a finite height barrier. The calculations are performed within the extreme quantum limit approximation, assuming for the envelope electronic wavefunction a modified Fang–Howard wavefunction. Both types of long-wave phonons, longitudinal optical and interface phonons, are considered. It is found that the effect of the finite height is to reduce the strength of the electron–phonon interaction.     

Ferromagnetism in Zn1−xCrxTe (x = 0.05, 0.15) films grown on GaAs(1 0 0) substrate

Zn_1−xCr_xTe (x = 0.05, 0.15) films were grown on GaAs(1 0 0) substrate by thermal evaporation method. X-ray diffraction analysis showed the presence of ZnCrTe phase without any secondary phase. The surface was analyzed by high resolution magnetic force microscope and profile measurements showed orientation of magnetic domains in the range of 0.5–2 nm with increase of Cr content. Magnetic moment–magnetic field measurements showed a characteristic hysteresis loop even at room temperature. The Curie temperature was estimated to be greater than 300 K. From the electron spin resonance spectra, the valence state of Cr in ZnTe was found to be +2 with d² electronic configuration. Hall effect study was done at room temperature and the result showed the presence of p-type charge carriers and hole concentration was found to increase from 5.95 × 10¹² to 6.7 × 10¹² m⁻³ when Cr content increases. We deduce the origin of ferromagnetic behavior based on the observed experimental results.     

Ternary effects on the optical interface phonons in onion-like GaN/AlxGa1 − xN quantum dots

The interface optical phonons and its ternary effects in onion-like quantum dots are studied by using dielectric continuum model and the modified random-element isodisplacement model. The dispersion relations, the electron–phonon interactions and ternary effects on the interface optical phonons are calculated in the GaN/Al_xGa_1 − xN onion-like quantum dots. The results show that aluminium concentration has important influence on the interface optical phonons and electron–phonon interactions in GaN/Al_xGa_1 − xN onion-like quantum dots. The frequencies of interface optical phonons and electron–phonon coupling strengths change linearly with increase of aluminium concentration in high frequency range, and do not change linearly with increasing aluminium concentration in low frequency range.     

The effect of In/N ratio on the optical quality and lasing threshold in GaxIn1 − xAs1 − yNy/GaAs laser structures

We present a review of published work concerning the effect of In and N compositions on the operation wavelength, optical quality and lasing threshold in Ga_xIn_1 − xAs_1 − yN_y/GaAs QW and double heterostructure lasers. We show that the emission wavelength in the range between 1.0 and 1.4 μ m can be obtained for a wide range of In and/or N concentrations. However, in most Fabry–Perot lasers and vertical cavity surface emitting lasers (VCSELs) reported in the literature, the threshold current density plotted as a function of the relative In/N composition (R =  (1  − x) / y) indicate a broad minima for 40  < R <  70, suggesting an optimum relative composition. We also present the results of our studies concerning the optical quality of Ga_xIn_1 − xAs_1 − yN_y/GaAs single quantum wells for R =  15. We show that the optical quality of GaInAsN can be improved while achieving a red shift in the PL spectra. This is unlike the results obtained by rapid thermal annealing or conventional annealing, which are widely employed as post-growth treatment techniques, where any increase in the PL intensity is almost always accompanied by an undesired blue shift.     

Annealing effects on the crystal structure and physical properties of FeSe1−xTex (0.6 ≦ x ≦ 1) single crystals

Annealing effects of FeSe_1?xTe_x (0.6 ≦ x ≦ 1) single crystals have been investigated from measurements of the powder X-ray diffraction and specific heat. Through the annealing, several peaks of powder X-ray diffraction have become sharp and a clean jump of the specific-heat at the superconducting (SC) transition temperature, T_c, has been observed for x = 0.6–0.9, indicating bulk superconductivity. For annealed single-crystals of x = 0.6–0.8, the SC condensation energy, U₀, and the SC gap, Δ₀, at 0 K have been estimated as ～1.8 J/mol and 2.3–2.5 meV, respectively. The value of 2Δ₀/k_BT_c is 3.9–4.5, indicating a little strong-coupling superconductivity. Both the electronic specific-heat coefficient in the normal state, γ_n, and the residual electronic specific-heat coefficient in the SC state, γ₀, have been found to show significant x dependence. The values of γ_n are much larger than those estimated from the band calculation.     

Pressure-induced non-linear optical properties in a wurtzite GaN/Al x Ga1− x N strained quantum dot

The effects of hydrostatic pressure on the exciton ground-state binding energy and the interband emission energy in a GaN/Al _x Ga_{1??? x} N quantum dot are investigated. The effects of strain and the internal field due to spontaneous and piezo-electric polarizations are included in the Hamiltonian. Numerical calculations are performed using variational procedure within the framework of single-band effective-mass approximation. The dependence of non-linear optical processes on the dot sizes is brought out in the influence of pressure. Pressure-induced optical properties are obtained using the compact density matrix approach. The effects of hydrostatic pressure on the linear, third-order non-linear optical absorption coefficients and the refractive index changes of the exciton as a function of photon energy are calculated. Our results show that the effects of pressure and the geometrical confinement have great influence on the optical properties of GaN/Al _x Ga_{1??? x} N dot.     

On the interface states and series resistance profiles of (Ni/Au)–Al0.22Ga0.78N/AlN/GaN heterostructures before and after 60Co (γ-ray) irradiation

The values of interface states (N _SS) and series resistance (R _S) of (Ni/Au)–Al_0.22Ga_0.78N/AlN/GaN heterostructures were obtained from admittance and current–voltage measurements before and after 250 kGy ⁶⁰Co irradiation. The analyses of these data indicate that the values of capacitance and conductance decrease, as the R _S increases with increasing dose rate due to the generation of N _SS. The increase in R _S with increasing dose rate was attributed to two main models. According to the first model, it has been attributed to a direct decrease in the donor concentration in semiconductor material as a result of the elimination of shallow donor states. According to the second model, it is a result of irradiation because of the formation of deep acceptor centers in the semiconductor bulk, and electrons from the shallow donor centers are captured by these acceptors.     

n doping effect modeling in 1.3 μm GaN0.58yAs1−1.58yBiy/GaAs quantum wells

The effect of n doping on the band structure of lattice-matched GaNAsBi/GaAs quantum wells was investigated using a self-consistent calculation combined with the 16-band anti-crossing model. Bi/N incorporation and doping effects can offer a huge potential to engineer the electronic band structure of such materials suitable for the design of photodetectors and emitters operating at 1.3 µm. The increase of the doping density induces a blue-shift of the fundamental transition energy in the doping range between 6×10¹⁷ and 5×10¹⁸ cm⁻³. The absorption spectra dependence on the well width are discussed. To maintain the fundamental transition fixed at the wavelength 1.3 µm, we have adjusted the Bi composition for the well width range between 4.5 and 10 nm with respect of the confinement conditions.     

Electric field effect on the binding energy of a hydrogenic impurity in coaxial GaAs/Al xGa1 − x As quantum well-wires

The ground state binding energy of a hydrogenic impurity in a coaxial cylindirical quantum well wire system subjected to an external electric field applied perpendicular to the symmetry axis of the wire system is studied within a variational scheme. Binding energy calculations were performed as functions of the inner barrier thickness and the electric field for two different impurity positions. The main result is that a sharp decrease in the binding energy, which may be important in device applications, occurs in certain conditions.     

Effect of Fe doping on the room temperature ferromagnetism in chemically synthesized (In1−xFex)2O3 (0 ⩽ x ⩽ 0.07) magnetic semiconductors

Observation of room temperature ferromagnetism in Fe doped In₂O₃ samples (In_1−xFe_x)₂O₃ (0 ⩽ x ⩽ 0.07) prepared by co-precipitation technique is reported. Lattice parameter obtained from powder X software shows distinct shrinkage of the lattice constant indicating an actual incorporation of Fe ions into the In₂O₃ lattice. X-ray diffraction data measurements show that the entire sample exhibits single phase polycrystalline behavior. SEM micrographs showed the prepared powder was in the range 25–36 nm. SEM EDS mapping showed the presence of Fe and In ions in the Fe doped In₂O₃ sample. The highest remanence magnetization moment (6.624 × 10⁻⁴ emu/g) is reached in the sample with x = 0.03.     

Effect of Al and Ga interdiffusion on the electronic states in GaAs/Ga<Subscript>1−x</Subscript>Al<Subscript>x</Subscript>As semiconductor superlattice

The effect of interdiffusion of Al and Ga atoms on the confining potential and band structure of a three-dimensional superlattice, composed of initially spherical GaAs/Ga_1?xAl_xAs quantum dots, is investigated in the framework of the modified Wood-Saxon potential model. It is shown that the interdiffusion leads to the disappearance of the quantum dots’ spherical symmetry and to the broadening of the superlattice energy minibands.     

High-Density Effects on the Exciton Systems and Their Dynamical Behavior in Al x Ga 1− x As/AlAs Quantum Wires

In Al x Ga 1 m x As/AlAs quantum wire (QWR) structures, the lower lying indirect exciton (IE) photoluminescence (PL) peak shows remarkable blue-shift under intense light-excitation contrary to the higher lying direct exciton (DE) PL band with very small blue-shift, although the two kinds of exciton states consist of the common hole state. In time-resolved PL spectra in the type-II QWR of x = 0.4, the DE PL band appears at an earlier stage without peak-shift and the excitons relax to the IE state making the IE PL peak dominant with time evolution. The blue-shift of the IE peak in a quasi equilibrium after the relaxation seriously depends on the excitation density. The origin of the blue-shift is explained in terms of many-body effects including band-bending effect due to the electric field induced by spatially separated electrons and holes in the QWR structures.     

Theoretical investigation of electronic structure and optical response in relation to the transport properties of Ga1−xInxN (x = 0, 0.25, 0.50, 0.75)

The state-of-the-art all-electron FLPAW method and the BoltzTrap software package based on semi-classical theory were adopted to explore the electronic structure and the optical and thermoelectric properties of Ga_1−xIn_xN. Ga_1−xIn_xN is predicted to be a direct band gap material for all values of x. Moreover, the band gap varies between 2.99 eV and 1.95 eV as x changes. Optical parameters such as the dielectric constant, absorption coefficient, reflectivity and refractive index are calculated and discussed in detail. The doping of In plays an important role in the modulation of the optical constants. The static dielectric constant ɛ(0) of Ga_1−xIn_xN was calculated as 3.95, 3.99, 3.99 and 4.03 at x = 0.00, 0.25, 0.50 and 0.75, respectively. The static refractive index is 2.0 for pure Ga_1−xIn_xN at x = 0.00. The thermal properties varied greatly as x fluctuated. The ternary alloy has large values for the Seebeck coefficient and figure of merit at high temperatures and is thus suitable for thermoelectric applications. Pure Ga_1−xIn_xN at x = 0 exhibited ZT = 0.80 at room temperature, and at higher temperatures, the thermal conductivity decreased with increased In doping.     

Phonon and electron-hole plasma effects on binding energies of excitons in wurtzite GaN/In<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>N quantum wells

The binding energy of an exciton screened by the electron-hole plasma in a wurtzite GaN/In _x Ga_1−x N quantum well (in the case of 0.1 < x < 1 within which the interface phonon modes play a dominant role) is calculated including the exciton-phonon interaction by a variational method combined with a self-consistent procedure. The coupling between the exciton and various longitudinal-like optical phonon modes is considered to demonstrate the polaronic effect which strongly depends on the exciton wave function. All of the built-in electric field, the exciton-phonon interaction and the electron-hole plasma weaken the Coulomb coupling between an electron and a hole to reduce the binding energy since the former separates the wave functions of the electron and hole in the z direction and the later two enlarge the exciton Bohr radius. The electron-hole plasma not only restrains the built-in electric field, but also reduces the polaronic effect to the binding energy.     

The influence of disorder on magnetocaloric effect and the order of transition in ferromagnetic manganite (La1−x Nd x )2/3(Ca1−y Sr y )1/3MnO3

Magnetocaloric effect and order of transition in (La_1?x Nd _x )_2/3(Ca_1?y Sr _y )_1/3MnO₃, prepared by conventional solid-state reaction, have been investigated. Using Banerjee criterion, we demonstrate first-order transition for (J1) and (J2 ) as well as second-order transition for (J3 ), (J4 ), and (J5 ) samples. The ΔS _M ^max is ranging between 9.18 Jkg^?1 K^?1 and 4.87 when Nd and Sr content changes leading to relative cooling power (RCP) varying between 330 and 229.35 J/kg. Both ΔS _M ^max and the RCP are found sensitive to the disorder σ ². The universal behavior obtained from ΔS variation curves confirmed the first-order transition for (J1) and (J2 ) samples and second-order transition for (J3), (J4), and (J5 ) samples obtained by Banerjee criterion. All samples with second-order phase transition exhibit inhomogeneous character estimated from local exponent n.