Influence of strain on hydrogenic impurity states in a GaN/AlxGa1?xN quantum dot

Within the effective-mass approximation, we calculated the influence of strain on the binding energy of a hydrogenic donor impurity by a variational approach in a cylindrical wurtzite GaN/Al _x Ga_1−x N strained quantum dot, including the strong builtin electric field effect due to the spontaneous and piezoelectric polarization. The results show that the binding energy of impurity decreases when the strain is considered. Then the built-in electric field becomes bigger with the Al content increasing and the binding energy of hydrogenic donor impurity decreases when the Al content is increasing. For dot height L < 2 nm, the change of the binding energy is very small with the Al content variety. This work has been supported by the National Natural Science Foundation of China (No. 10564003) and the Key Project of the Science and Technology Research of the Educational Ministry of China (No. 208025)     

Drift velocity of electrons in quantum wells of selectively doped In0.5Ga0.5As/AlxIn1 ? xAs and In0.2Ga0.8As/AlxGa1 ? xAs heterostructures in high electric fields

The field dependence of drift velocity of electrons in quantum wells of selectively doped In_0.5Ga_0.5As/Al _x In_{1 − x} As and In_0.2Ga_0.8As/Al _x Ga_{1 − x} As heterostructures is calculated by the Monte Carlo method. The influence of varying the molar fraction of Al in the composition of the Al _x Ga_{1 − x} As and Al _x In_{1 − x} As barriers of the quantum well on the mobility and drift velocity of electrons in high electric fields is studied. It is shown that the electron mobility rises as the fraction x of Al in the barrier composition is decreased. The maximum mobility in the In_0.5Ga_0.5As/In_0.8Al_0.2As quantum wells exceeds the mobility in a bulk material by a factor of 3. An increase in fraction x of Al in the barrier leads to an increase in the threshold field E _th of intervalley transfer (the Gunn effect). The threshold field is E _th = 16 kV/cm in the In_0.5Ga_0.5As/Al_0.5In_0.5As heterostructures and E _th = 10 kV/cm in the In_0.2Ga_0.8As/Al_0.3Ga_0.7As heterostructures. In the heterostructures with the lowest electron mobility, E _th = 2–3 kV/cm, which is lower than E _th = 4 kV/cm in bulk InGaAs.     

Impurity states in a spherical GaAs–Ga1-x AlxAs quantum dots: Effects of hydrostatic pressure

We calculated the binding energies of shallow donors and acceptors in a spherical GaAs–Ga${}_{1-x}$Al${}_x$As quantum dot under isotropic hydrostatic pressure for both a finite and an infinitely high barrier. We use a variational approach within the effective mass approximation. The binding energy is computed as a function of hydrostatic pressure, the dot sizes and the impurity position. The results show that the impurity binding energy increases with the pressure for any position of the impurity. We have also found that the binding energy depends on the location of the impurity and the pressure effects are less pronounced for impurities on the edge.     

Design of GaAs/AlxGa1?xAs asymmetric quantum wells for THz-wave by difference frequency generation

The energy levels, wave functions and the second-order nonlinear susceptibilities are calculated in GaAs/Al0.2Ga0.8As/Al0.5Ga0.5As asymmetric quantum well (AQW) by using an asymmetric model based on the parabolic and non-parabolic band. The influence of non-parabolicity can not be neglected when analyzing the phenomena in narrow quantum wells and in higher lying subband edges in wider wells. The numerical results show that under double resonance (DR) conditions, the second-order difference frequency generation (DFG) and optical rectification (OR) generation susceptibilities in the AQW reach 2.5019 mm/V and 13.208 mm/V, respectively, which are much larger than those of the bulk GaAs. Besides, we calculate the absorption coefficient of AQW and find out the two pump wavelengths correspond to the maximum absorption, so appropriate pump beams must be selected to generate terahertz (THz) radiation by DFG.     

Simulation of relaxation times and energy spectra of the CdTe/Hg1 ? xCdxTe/CdTe quantum well for variable valence band offset, well width, and composition x

The dependences of relaxation times and energy spectrum of the CdTe/Hg_{1 − x} Cd _x Te/CdTe quantum well (QW) on its parameters were simulated in the cadmium molar fraction range 0 < x < 0.16. It was found that the x increase from 0 to 0.16 changes electron wave function localization in the QW. A criterion for determining the number of interface levels of localized electrons depending on QW parameters was obtained. The effect of a sharp (by two orders of magnitude) increase in the relaxation time of localized electrons was detected at small QW widths and x close to 0.16.     

Effects of Zn/Mg Ratio on the Microstructure and Microwave Dielectric Properties of (Zn1?xMgx)2SiO4 Ceramics

Ceramics of nominal composition (Zn_1−x Mg _x )₂SiO₄ were synthesized by the solid-state method. The phase evolution, microstructure, and microwave dielectric characteristics of the (Zn_1−x Mg _x )₂SiO₄ (0 < x < 1.0) ceramics were investigated systematically. The sintering range was widened and the densification temperatures of the present ceramics were much lower compared with the Zn₂SiO₄ and Mg₂SiO₄ end-members. Coexistence of Mg₂SiO₄ and Zn₂SiO₄ phases was observed in the (Zn_1−x Mg _x )₂SiO₄ ceramics with x = 0.4 and 0.6. The MgSiO₃ secondary phase was also observed due to Mg substitution. Changes in grain shapes from equiaxed to rectangular were observed in sintered samples as x varied from 0.7 to 1.0. The microwave characteristics of (Zn_1−x Mg _x )₂SiO₄ ceramics were significantly improved by the suppression of the MgSiO₃ phase, where an enhanced quality factor (Qf) value was obtained. The best microwave characteristics were achieved in the (Zn_1−x Mg _x )₂SiO₄ ceramic with Zn/Mg ratio of 1.5 sintered at 1250°C: ε _r = 6.2, Qf = 148,740 GHz, τ _f = −54.2 ppm/°C.     

Effect of the number of pairs of the layers on the quality of the superlattices of the InxGa1 ? xAs/GaAs/.../GaAs(001) type grown by molecular beam epitaxy under computer control

By the specified program of computer control over the growth process, superlattices of the In _x Ga_{1 − x} As/GaAs/.../GaAs(001) type with the number of alternating pairs N ₁ = 12, N ₂ = 6, and N ₃ = 3 were grown. Using the X-ray methods of quantitative analysis, the values of periods T ₁ = 22 nm, T ₂ = 22.5 nm, and T ₃ = 22.3 nm and the In content x ₁ = 0.09, x ₂ = 0.091, and x ₃ = 0.092 in these superlattices are measured. In the superlattice with N ₁ = 12, the plastic deformation is detected. In superlattices with N ₂ = 6 and N ₃ = 3, no plastic deformation during the growth was observed. The obtained magnitudes of the periods turned out smaller by a factor of approximately 2 and the alloy concentration turned out smaller by 1/3 than it was specified by the computer program. The measured and calculated values of elastic stresses turned out equal to σ_2N = 2.43 × 10⁶ Pa and σ^{2N − 1} = −0.88 × 10⁹ Pa, at which the plastic deformation took place in the layers of the superlattice with N ₁ = 12. In superlattices with N ₂ = 6 and N ₃ = 3, the elastic stresses were σ_2N = 1.96 × 10 6 Pa, σ_{2N − 1} = −1.45 × 10⁹ Pa and σ_2N = 0.99 × 10⁶ Pa, σ_{2N − 1} = −1.88 × 10⁹ Pa, respectively. These stresses turned out insufficient to initiate the process of dislocation generation. Original Russian Text ? G.F. Kuznetsov, 2009, published in Fizika i Tekhnika Poluprovodnikov, 2009, Vol. 43, No. 4, pp. 493–500.     

The Influence of Sintering Temperature on the Microstructure and Thermoelectric Properties of n-Type Bi2Te3?xSex Nanomaterials

Pure Bi₂Te₃ and Bi₂Se₃ nanopowders were hydrothermally synthesized, and n-type Bi₂Te_3−x Se _x bulk samples were prepared by hot pressing a mixture of Bi₂Te₃ and Bi₂Se₃ nanopowders at 623 K, 648 K or 673 K and 80 MPa in vacuum. The phase composition of the powders and bulk samples were characterized by x-ray diffraction. The morphology of the powders was examined by transmission electron microscopy. The microstructure and composition of the bulk samples were characterized by field-emission scanning electron microscopy and energy-dispersive x-ray spectroscopy, respectively. The density of the samples increased with sintering temperature. The samples were somewhat oxidized, and the amount of oxide (Bi₂TeO₅) present increased with sintering temperature. The samples consisted of sheet-like grains with a thickness less than 100 nm. Seebeck coefficient, electrical conductivity, and thermal conductivity of the samples were measured from room temperature up to 573 K. Throughout the temperature range investigated, the sample sintered at 623 K had a higher power factor than the samples sintered at 648 K or 673 K.     

Dielectric confinement on exciton binding energy and nonlinear optical properties in a strained Zn1-xinMgx（in）Se/Zn1-x（out）Mgx（out）Se quantum well

正The band offsets for a Zn_(1-Xin)Mg_(Xin)Se/Zn_(1-Xout)Mg_(1-Xout)Se quantum well heterostructure are determined using the model solid theory.The heavy hole exciton binding energies are investigated with various Mg alloy contents.The effect of mismatch between the dielectric constants between the well and the barrier is taken into account.The dependence of the excitonic transition energies on the geometrical confinement and the Mg alloy is discussed.Non-linear optical properties are determined using the compact density matrix approach.The linear,third order non-linear optical absorption coefficient values and the refractive index changes of the exciton are calculated for different concentrations of magnesium.The results show that the occurred blue shifts of the resonant peak due to the Mg incorporation give the information about the variation of two energy levels in the quantum well width.     

Electroreflectance spectra from multiple InGaN/GaN quantum wells in the nonuniform electric field of a <Emphasis Type="Italic">p–n</Emphasis> junction

A line at E = 2.77 eV (with a width of Γ = 88 meV) related to interband transitions in the region of multiple quantum wells in the active region is detected in the electroreflectance spectra of the GaN/InGaN/AlGaN heterostructure. As the modulation bias is reduced from 2.9 to 0.4 V, the above line is split into two lines with energies of E ₁ = 2.55 eV and E ₂ = 2.75 eV and widths of Γ₁ = 66 meV and Γ₂ = 74 meV, respectively. The smaller widths of separate lines indicate that these lines are caused by interband transitions in particular quantum wells within the active region. The difference between the interband transition energies E ₁ and E ₂ in identical quantum wells in the active region is related to the fact that the quantum wells are in an inhomogeneous electric field of the p–n junction. The magnitudes of the electric-field strengths in particular quantum wells in the active region of the heterostructure are estimated to be 1.6 and 2.2 MV/cm.     

Effects of Ir Substitution and Processing Conditions on Thermoelectric Performance of p-Type Zr0.5Hf0.5Co1?xIrxSb0.99Sn0.01 Half-Heusler Alloys

A series of samples with the composition Zr_0.5Hf_0.5Co_1−x Ir _x Sb_0.99Sn_0.01 (x = 0.0 to 0.7) were synthesized by high-temperature solid-state reaction at 1173 K. High-density pellets of the powders were obtained using hot press (HP) and spark plasma sintering (SPS) techniques. The thermoelectric properties of the pellets were measured from 300 K to 750 K. Independently of the pressing conditions, all Ir-containing samples (x > 0) showed p-type semiconducting behavior. At 300 K, the electrical conductivity and thermopower of Zr_0.5Hf_0.5Co_1−x Ir _x Sb_0.99Sn_0.01 materials surprisingly increased with increasing Ir concentration. The largest electrical conductivity and thermopower values of 150 S/cm and 140 μV/K, respectively, were observed at 300 K for x = 0.7. The thermal conductivity of the synthesized materials decreased with increasing Ir content, went through a minimum value (x = 0.3), and increased thereafter with further addition of Ir. Pellets fabricated by SPS showed smaller thermal conductivity than pellets of the same composition obtained from uniaxial hot pressing. A thermal conductivity value of ∼2.0 W/m K was observed at 300 K for an SPS pellet with the com-position Zr_0.5Hf_0.5Co_0.5Ir_0.5Sb_0.99Sn_0.01. The thermal conductivity of Zr_0.5Hf_0.5-Co_1−x Ir _x Sb_0.99Sn_0.01 decreased with rising temperature, and the smallest value of ∼1.5 W/m K was observed at 750 K for the SPS specimen with x = 0.5.     

Growth of the (In2S3)x(FeIn2S4)1 ? x single crystals and properties of photoelectric structures on their basis

Complete mutual solubility in the (In₂S₃) _x (FeIn₂S₄)_{1 − x} system is established. The technology is developed, and single crystals of the continuous series of the (In₂S₃) _x (FeIn₂S₄)_{1 − x} solid solutions are grown for the first time. The linear dependence of the unit cell parameter of single crystals with a cubic spinel lattice on the solid solution composition is found. First, photosensitive Schottky barriers are fabricated, and then, based on studies of their photosensitivity, the character of band-to-band transition is discussed and the values of the band gap depending on the atomic composition are estimated. The possibility of using the obtained solid solutions as broadband photoconverters of optical radiation is revealed.     

Discovery of the (In2S3)x(MnIn2S4)1 ? x solid solutions and fabrication of photosensitive structures based on them

A technology of growing single crystals of (In₂S₃) _x (MnIn₂S₄)_{1 − x} solid solutions that provides control over their atomic composition in the entire concentration range 0 ≤ x ≤ 1 is developed. It is shown that, in the range x = 0–1, the single crystals have the cubic spinel structure and the unit cell parameter a follows the linear dependence on x. The exponential character of the temperature dependence of resistivity of solid solutions, on which the first photosensitive Cu/(In₂S₃)x(MnIn₂S₄)_{1 − x} structures are obtained, is revealed. The first photosensitivity spectra of these structures are obtained, and, based on these spectra, dependences of energy of the direct and indirect band-to-band transitions on the composition x are determined. The possibility of applying these structures in broad-band photoconverters of optical radiation is concluded.