Galvanomagnetic study of the quantum-well valence band of germanium in the Ge<Subscript>1−x</Subscript>Si<Subscript>x</Subscript>/Ge/Ge<Subscript>1−x</Subscript>Si<Subscript>x</Subscript> potential well

The structure of the quantum-well valence band in a Ge(111) two-dimensional layer is calculated by the self-consistent method. It is shown that the effective mass characterizing the motion of holes along the germanium layer is almost one order of magnitude smaller than the mass for the motion of heavy holes along the [111] direction in a bulk material (this mass is responsible for the formation of quantum-well levels). This creates a unique situation in which a large number of subbands appear to be populated at moderate values of the layer thickness d _w and the hole concentration p _s . The depopulation of two or more upper subbands in a 38-nm-thick germanium layer at a hole concentration p _s = 5 × 10¹⁵ m^?2 is revealed from the results of measuring the magnetoresistance in a strong magnetic field aligned parallel to the germanium layers. The destruction of the quantum Hall state at a filling factor ν = 1 indicates that the two lower subbands merge together in a self-formed potential profile of the double quantum well. It is demonstrated that, in a quasi-two-dimensional hole gas, the latter effect should be sensitive to the layer strain.     

First principles study on electronic and optical properties of Al<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>N and In<Subscript>y</Subscript>Ga<Subscript>1−y</Subscript>N

The band structure, density of states of Al_xGa_1?xN and In_yGa_1?yN was performed by the first-principles method within the local density approximation. The calculated energy gaps of the AlN, Al_0.5Ga_0.5N, GaN, In_0.5Ga_0.5N and InN were 5.48, 4.23, 3.137, 1.274 and 0.504 eV, which were in agreement with the experimental result. The dielectric functions, absorption coefficient and loss function were calculated based on Kramers–Kronig relations. Further more, the relationships between electronic structure and optical properties were investigated theoretically. For Al_xGa_1?xN and In_yGa_1?yN materials, the micromechanism of the optical properties were explained.     

Field-induced valence transition in EuPtP<Subscript>1−x</Subscript>As<Subscript>x</Subscript>

The ternary compound EuPtP exhibits two valence transitions at T ₁ = 235 K and T ₂ = 190 K. In order to examine a field-induced valence transition of Eu, we synthesized EuPtP_1−x As _x compounds with 0.05 ≤ x ≤ 0.5 and studied the magnetic and valence behavior. The substitution of As for P increases the lattice volume linearly and decreases both valence transition temperatures, T ₁ and T ₂, in contrast to the behavior under external pressures. The magnetization process in a pulsed magnetic field revealed that EuPtP_0.5As_0.5 exhibits an onset of metamagnetic transition above 50 T with a large hysteresis, which evidences a first-order field-induced valence transition. The analysis of the magnetization curves of x = 0.5 at various temperatures has demonstrated that the field-induced transition is essentially the same as the transition induced by temperature at T ₁.     

Photoacoustic method of determination of quantum efficiency of luminescence in Mn<Superscript>2+</Superscript> ions in Zn<Subscript>1−x−y</Subscript>Be<Subscript>x</Subscript>Mn<Subscript>y</Subscript>Se crystals

This paper presents step by step the procedure of determination of the quantum efficiency of luminescence of Mn²⁺ ions in the Zn_1−x−yBe_xMn_ySe crystals. The method is based on the photoacoustic spectroscopy approach. In the paper, the experimental spectra of absorbance, transmission, absorption and photoacoustic spectra of the samples are presented and analyzed from the point of view of the possibility of determination of the quantum efficiency of Mn²⁺ ion luminescence at room temperature. It was determined experimentally that in the investigated crystals the quantum efficiency of luminescence in the Mn²⁺ ions is about 35%, 40%, 32% for the absorption peaks at 430 nm, 470 nm, and 510 nm, respectively, for Zn_0.75Be_0.2Mn_0.05Se crystal.     

Computation of the electronic structure and direct-gap absorption spectra in Ge-rich Si<Subscript>1−x</Subscript> Ge<Subscript>x</Subscript>/Ge/Si<Subscript>1−x</Subscript>Ge<Subscript>x</Subscript> type-I quantum wells

This paper presents the results of conduction band discontinuities calculation for strained/relaxed Si_1?x Ge _x /Si_1?y Ge _y heterointerfaces in Γ _15C , Γ _2′C and L upper bands minima, as well as the room-temperature strained (vs. relaxed) band gaps deduced from the classical model-solid theory. Based upon the obtained data, we propose a type-I W-like Si_1?y Ge _y /Si_1?x Ge _x /Ge/Si_1?x Ge _x /Si_1?y Ge _y quantum wells heterostructure optimized in terms of compositions and thicknesses. Electronic states and wave functions are found by solving Schrödinger equation without and under applied bias voltage. An accurate investigation of the optical properties of this heterostructure is done by calculating the energies of the interband transitions and their oscillator strengths. Moreover, a detailed computation of the bias-voltage evolution of the absorption spectra is presented. These calculations prove the existence of type-I band alignment at Γ _2′C point in compressively strained Ge quantum wells grown on relaxed Ge-rich Si_1?y Ge _y buffers. The strong absorption coefficient (> 8 × 10³ cm^-1) and the large Stark effect (0.1 eV @ 2 V) of the Γ _2′C transitions thresholds open up perspectives for application of these heterostructures for near-infrared optical modulators.     

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.     

Rotatable anisotropy of epitaxial Fe<Subscript>1−x</Subscript>Ga<Subscript>x</Subscript> thin films

We show by a combined magnetic force microscopy and synchrotron radiation spectroscopy study that stripe-like patterned magnetic domains are present in Fe_1?x Ga _x thin films. These stripes, whose origin is attributed to an out-of-plane magnetic component, can be rotated by an external magnetic field.     

Hall effect in the Fe<Subscript>x</Subscript>Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S magnetic semiconductors

The electrical properties and the Hall effect in the Fe_xMn_1?xS magnetic semiconductors (0<x≤0.5) have been experimentally studied in the range 77–300 K in magnetic fields of up to 15 kOe. The cation-substituted sulfides with 0.25≤x≤0.3 possessing colossal magnetoresistance (CMR) were established to be narrow-gap semiconductors with carrier concentrations n ～ 10¹¹–10¹⁵ cm^?3 and high carrier mobilities μ ～ 10²–10⁴ cm² V^?1 s^?1. It is believed that the CMR effect in these sulfides can be explained in terms of the model of magnetic and electron phase separation, which is analogous to the percolation theory in the case of heavily doped semiconductors.     

Spin-wave resonance in Ge<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films exhibiting percolation ferromagnetism

Magnetic properties of Ge_{1 ? x} Mn _x (x = 0.02, 0.04, 0.08) thin films obtained by ion-implanting Mn⁺ ions into single-crystal Ge plates are investigated. The contributions of the subsystem of dispersed Mn²⁺ ions, Ge³Mn⁵ ferromagnetic clusters, and Mn-enriched ferromagnetic domains of Ge to the magnetic moment of Ge_{1 ? x} Mn _x films are distinguished. In the subsystem of dispersed Mn²⁺ ions in Ge_{1 ? x} Mn _x films at temperatures below 10 K, a spin-wave resonance is observed in the magnetically ordered state due to percolation ferromagnetism. It is established that, in the films with percolation ferromagnetism, the exchange integrals determined from static measurements correspond to those determined by dynamic measurements.     

Transport properties and ferromagnetism of Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>S sulfides

We have studied the resistivity and thermoelectromotive force (thermo emf) in a temperature range of T = 80–1000 K, the magnetic susceptibility and magnetization in a temperature range of T = 4.2–300 K at an external magnetic field of up to 70 kOe, and the structural characteristics of Co _x Mn_1?x S sulfides (0 ≤ x ≤ 0.4). Anomalies in the transport properties of these compounds have been found in the temperature intervals ΔT ₁ = 200–270 K and ΔT ₂ = 530–670 K and at T ₃ ～ T _N. The temperature dependences of the magnetic susceptibility, magnetization, and resistivity, as well as the current-voltage characteristics, exhibit hysteresis. In the domain of magnetic ordering at temperatures below the Néel temperature (T _N), the antiferromagnetic Co _x Mn_1?x S sulfides possess a spontaneous magnetic moment that is explained using a model of the orbital ordering of electrons in the t _2g bands. The influence of the cobalt-ion-induced charge ordering on the transport and magnetic properties of sulfides has been studied. The calculated values of the temperatures corresponding to the maxima of charge susceptibility, which are related to a competition between the on-site Coulomb interaction of holes in various subbands and their weak hybridization, agree well with the experimental data.     

Dynamic magnetic susceptibility of compounds in Gd<Subscript>5</Subscript>Si<Subscript>2−x</Subscript>Ge<Subscript>2−x</Subscript>Sn<Subscript>2x</Subscript> (2x = 0 − 0.1) system

The dynamic magnetic susceptibility (χ_ac) of magnetically ordered Gd₅Si_2?xGe_2?xSn_2x compounds with the partial substitution for silicon and germanium atoms by isovalent tin atoms (2x = 0 ? 0.1) has been investigated experimentally. From the temperature dependence of χ_ac the Curie temperatures of these alloys are determined. It is established that tin-doped alloys have higher Curie temperatures as compared to Gd₅Si₂Ge₂ (ΔT _C ≈ 15 K).     

Raman scattering and electrical studies of the phase stability in the Hg<Subscript>1−x</Subscript>Cd<Subscript>x</Subscript>Te

Raman scattering is performed to access phase stability in the boron-implanted Hg_0.7Cd_0.3Te with fluences ranging from 1 × 10¹² to 1 × 10¹⁵ cm^?2. Threshold fluence for the formation of an amorphous phase is invoked here using Thomas–Fermi statistical model. Asymmetric broadening and red shift of the Raman active HgTe-like LO phonon mode are observed with varying fluencies. Electrical properties such as sheet carrier concentration and mobility are also changed with the fluence and reach their saturated values beyond threshold fluence of 5 × 10¹³ cm^?2. Threshold fluence for the formation of amorphous phase is also validated by the Raman measurements and electrical transport properties in the implanted layers. The excess free energy of 6.8 kJ/mole is found corresponding to the threshold fluence for phase transition.     

Negative magnetoresistance in binary distorted perovskites Ca(Cu<Subscript>x</Subscript>Mn<Subscript>3−<Emphasis Type="Italic">x</Emphasis></Subscript>)Mn<Subscript>4</Subscript>O<Subscript>12</Subscript>

The substitution of copper for manganese in one position of the double distorted perovskite Ca(Cu_xMn_3?x)Mn₄O₁₂ leads to a change in the valence of manganese atoms in the other position. This is accompanied by a sharp increase in the temperature of magnetic ordering and by a change in the conductivity type from semiconductor to metallic. These perovskites also exhibit a negative magnetoresistance in a broad temperature range, with the [ρ(H)?ρ(0)]× 100%/ρ(0) ratio (at liquid nitrogen temperature) reaching 10% in a field of 1 T.     

Photoluminescence of TlCu<Subscript>1−x</Subscript>GaSe<Subscript>2</Subscript> single crystals

The results of the study of photoluminescence and its excitation spectra in Tl _x Cu_1−x GaSe₂ single crystals are presented. The crystals under study are layered and characterized by anisotropic optical properties. In this respect, it is important to investigate optical properties of the crystals under study.     

Kinetic properties of Ni<Subscript>3</Subscript>Al<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys in the range of transition from band to spin-localized ferromagnetism

The kinetic properties of ferromagnetic Ni₃Al_{1 − x} Mn _x alloys with _x ≤ 0.6 are studied at T ≤ 800 K and H ≤ 7 MA/m. The behavior of the electrical resistivity, the thermopower, the magnetoresistance, the Hall effect, and the spontaneous Hall effect are analyzed in the range of transition from band (Ni₃Al) to spinlocalized (Ni₃Mn) ferromagnetism at x ∼ 0.15.     

Optical properties and ferromagnetism of ternary Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te nanocrystals

Cd_1−x Mn _x Te (x = 0, 0.1, 0.2) nanocrystals have been synthesized by mechanical alloying (MA) Cd, Mn, and Se elemental powders. XRD patterns and HRTEM images confirmed the formation of cubic Cd_1−x Mn _x Te nanocrystals. All the diffraction peaks from elemental Cd, Mn, and Te powders disappeared completely in those XRD patterns of as-milled Cd_1−x Mn _x Te nanocrystals for more than 20 h. When the MA process was carried out for 40 h, typical zinc blende structure diffraction mode was exhibited in the XRD pattern. Subsequently, capping the surface of as-milled Cd_1−x Mn _x Te nanocrystals with long chain trioctylphosphine/trioctylphosphine oxide/nitric acid (TOP/TOPO/NA) molecules has achieved colorful dispersion solution, which shows similar optical properties to those CdTe nanocrystals prepared by wet chemical process. The grain size is within the range of 2–8 nm for the capped Cd_1−x Mn _x Te nanocrystals being ball milled for 40 h. The PL excitation peak red shifts to longer wavelength side with increasing Mn concentration. Pure CdTe nanocrystals show ferromagnetism behavior at room temperature, the saturation magnetization value and magnetic hysteresis loop increase with the content of substituting Mn ions within the Cd_1−x Mn _x Te nanocrystals.     

Photosensitivity of surface-barrier and point structures on Cd<Subscript>1 − x</Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te single crystals

Two new types of photosensitive structures are proposed and fabricated for the first time on solid-solution single crystals of diamondlike Cd_{1 − x} Mn _x Te (x = 0−0.7) magnetic semiconductors. The photoelectric properties of surface-barrier (In/Cd_{1 − x} Mn _x Te) and welded (weld/Cd_{1 − x} Mn _x Te) structures are studied at T=300 K. The photosensitivity spectra of these structures are analyzed and compared; as a result, the character of the interband optical transitions and the energy gaps of the Cd_{1 − x} Mn _x Te crystals are determined. These structures can be applied in magnetic photoelectronic devices.     

The method for calculation of carrier concentration in narrow-gap n-type doped Hg<Subscript>1−x</Subscript>Cd<Subscript>x</Subscript>Te structures

A simple method for the computation of carrier concentration in n-type doped Hg_1?xCd_xTe (MCT) structures is proposed. The method is based on the postulate of the existence of donor bands. In our model the donor bands are postulated to have a Gaussian distribution of density of states characterized by two parameters only (mean energy for this distribution and standard deviation). These parameters could be obtained with experimental data, which were comprised of a wide range of doping levels for various kinds of dopants.     

Correlation between raman spectra and structural properties of Zn<Subscript>2 − 2<Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>x</Subscript>In<Subscript>x</Subscript>Se<Subscript>2</Subscript> films

The Raman spectra of Zn_{2 ? 2x} Cu_xIn_xSe₂ (ZCIS) semiconductor films designed for use as optically active layers in thin-film solar cells have been investigated. The Raman spectra of ZCIS films are characterized by the presence of the dominant mode A ₁, which is observed in A^IB^IIIC ₂ ^VI compounds with chalcopyrite structure. The spectra of CuInSe₂ films (x = 1) obtained at low temperatures (T ≤ 400°C) contain and additional mode at 258 cm^?1, which is due to the presence of the impurity Cu_xSe phase. All modes observed in the spectra of ZCIS films with a Zn concentration ≤20 at % obtained under optimal conditions (520–540°C) correspond to the symmetry of vibrations in the chalcopyrite structure. The broadening and blue shift of the A ₁ mode occurring with an increase in the Zn concentration are indicative of degradation of the chalcopyrite crystal structure and the chalcopyrite → sphalerite phase transition at Zn concentrations exceeding 20 at %.     

Magnetic susceptibility and superconductivity of (Pb<Subscript>0.2</Subscript>Sn<Subscript>0.8</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>x</Subscript>Te alloys as a function of in content

Experimental data on the superconductivity of In-doped Pb_zSn_1?zTe alloys (z=0.2) are discussed. The superconducting transition was detected from simultaneous measurements of the resistivity and magnetic susceptibility of a series of samples with different indium contents (2–12 mol % InTe). The superconducting transition detected by the magnetic susceptibility was observed at a temperature which was, on the average, 0.1 K below that determined from the resistivity. The increase in the superconducting transition temperature T _c with increasing indium content is of a threshold character, with T _c being proportional to the inverse electronic density of states at the Fermi level. The observed features in the experimental data are accounted for in terms of indium impurity resonance states in the material.