Relaxation processes in conductivity of Cd1 − x Mn x Te crystals (0.02 < x < 0.55)

The results of studying the temperature dependences of resistivity and the Hall coefficient in crystals of Cd_{1 ? x} Mn _x Te alloys (0.02 < x < 0.55) are used to gain insight into the conductivity relaxation processes observed in the temperature range 200–420 K in Cd_{1 ? x} Mn _x Te crystals with the manganese content x > 0.06. An anomalous isothermal variation in the hole concentration by an amount from half to three orders of magnitude occurs in these crystals. It is shown that relaxation of resistivity in the crystals under consideration can be provided by quasi-chemical reactions with involvement of uncontrolled Cu impurity and defects of CdTe crystal structure.     

Deposition of heteroepitaxial layers of topological insulator Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> in the trimethylbismuth–isopropylselenide–hydrogen system on the (0001) Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and (100) GaAs substrates

Thin solid layers that are formed upon heating of the gaseous trimethylbismuth–isopropylselenide–hydrogen system on the (0001) Al₂O₃ and singular and vicinal (100) GaAs surfaces are studied. The conditions for deposition of metal Bi and phases of Bi₄Se₃, BiSe, and topological insulator Bi₂Se₃ using the MOCVD method are determined. Pure metastable phase BiSe is obtained for the first time. Bi₂Se₃ films with a thickness of no less than 200 nm, a relatively low volume concentration of 3 ×10¹⁸ cm^–3, and a high mobility of carriers at 300 K (1000 cm² V^–1 s^–1) are fabricated.     

Possibility of obtaining the (GaSb)1 − x (Si2) x films on silicon substrates by the method of liquid-phase epitaxy

Semiconductors - It is shown that it is possible to grow a continuous series of (GaSb)1 ? x (Si2) x (x = 0?1) alloys on silicon substrates by the method of liquid-phase epitaxy from a...     

Spectral shift of photoluminescence bands of the (SiC)1−x (AlN)x epitaxial films due to laser annealing

The effect of laser annealing on the photoluminescence properties of (SiC)_1?x (AlN)_x epitaxial films was studied. It was proposed that annealing causes the displacement of the Al and N atoms from their lattice sites and the formation of Al_Si-N_C donor-acceptor pairs acting as the luminescence centers. According to this model, the increase in the annealing time is accompanied by the formation of donor-acceptor pairs with the shortest interatomic distances at the expense of associations of the distant defects and by a shift of the respective photoluminescence band to the high-energy spectral region.     

Effect of implantation of Ba<Superscript>+</Superscript> and Nb<Superscript>+</Superscript> ions on the morphology,composition, and emission properties of Мо (111)

The effect of implantation of Ba⁺ and Nb⁺ ions on the composition and emission properties of Мо (111) single crystals is investigated. It is shown that implantation of Ba⁺ ions leads to the complete purification of the Мо surface from С and О atoms, rapid decrease in the work function, and intensification of the secondary and photoelectron emission coefficients, while implantation of Nb⁺ ions leads to rapid (up to 30 at %) growth of the surface concentration of С. It is established that, after heating at Т = 1400 K, the surface is completely purified from carbon and Nb + Мо nanocrystalline phases are formed.     

Effect of Al Substitution on the Thermoelectric Properties of the Type VIII Clathrate Ba<Subscript>8</Subscript>Ga<Subscript>16</Subscript>Sn<Subscript>30</Subscript>

Single-crystal samples of the type VIII clathrate Ba₈Ga_16−x Al _x Sn₃₀ (0 ≤ x ≤ 12) were prepared by the Sn-flux method, and the structural and electrical properties were studied from 300 K to 600 K. The lattice parameter increases linearly as x is increased from 0 to 10.5, which is the solubility limit of Al. For all samples, the electrical conductivity σ decreases monotonically as the temperature is increased. σ(T = 300 K) increases from 1.88 × 10⁴ S/m for x = 0 to 3.03 × 10⁴ S/m for x = 2, and then gradually decreases to 2.4 × 10⁴ S/m with further increase of x to 8. The increase of σ for Al-substituted samples is attributed to enhancement of carrier mobility. The Seebeck coefficients of samples with 0 ≤ x ≤ 8 are negative with large values, and the absolute values increase from 240 μV/K to 320 μV/K as the temperature increases from 300 K to 600 K. At 300 K, the effective mass m*/m ₀ is in the range from 0.53 to 0.67, and the samples with x = 6 and x = 8 have a rather low thermal conductivity of 0.72 W/mK and 0.78 W/mK, respectively. ZT reaches 1.2 at 500 K for x = 6.     

Effects of thermal annealing on the morphology of the AlxGa(1−x)N films

Effects of thermal annealing on the morphology of the Al_xGa_(1−x)N films with two different high Al-contents (x=0.43 and 0.52) have been investigated by atomic force microscopy (AFM). The annealing treatments were performed in a nitrogen (N₂) gas ambient as short-time (4 min) and long-time (30 min). Firstly, the films were annealed as short-time in the range of 800–950 °C in steps of 50–100 °C. The surface root-mean-square (rms) roughness of the films reduced with increasing temperature at short-time annealing (up to 900 °C), while their surface morphologies were not changed. At the same time, the degradation appeared on the surface of the film with lower Al-content after 950 °C. Secondly, the Al_0.43Ga_0.57N film was annealed as long-time in the range of 1000–1200 °C in steps of 50 °C. The surface morphology and rms roughness of the film with increasing temperature up to 1150 °C did not significantly change. Above those temperatures, the surface morphology changed from step-flow to grain-like and the rms roughness significantly increased.     

Specific features of the temperature dependence of the conduction electron concentration in the narrow-gap and zero-gap states of Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te

Results of studies of the conductivity σ and the Hall coefficient R in the Cd _x Hg_{1 − x} Te crystals with x = 0.1, 0.12, 0.14, and 0.15 are analyzed in the temperature range T = 4.2–300 K and the magnetic field range B = 0.005–2.22 T. Using data on the R(B) in low and high magnetic fields and the data on σ(T), electron and hole concentrations and mobilities are determined. It is shown that the electron concentration n in the studied samples is almost independent of T in the range 4.2–15 K, while as T increases, it increases according to the law n ∝ T ^r (r > 3/2), where r = f(n, T, x). It is found that r varies from 1.7 at x = 0.1 to 3.1 at compositions with x = 0.14 and 0.15. The results for n(T) are compared with theory, taking into account nonparabolicity of the variance law for ⃛(T), and with the theory of impurity states in narrow-gap and zero-gap semiconductors. It is shown that the constancy of n(T) up to ∼15 K and the strong dependence n(T) (r > 3/2) at higher temperatures are caused by the intense ionization of electrons localized at acceptor states.     

Analysis of the magnetic properties of (La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>0.93</Subscript>MnO<Subscript>3</Subscript> manganites at high temperatures

For the first time, the Faraday method is used to measure the temperature dependence of paramagnetic susceptibility χ(T) of (La_{1 ? x} Sr _x )_0.93MnO₃ (x = 0.2, 0.25, or 0.3) manganites in the temperature interval 60–850°C. It is demonstrated that the dependences have two kinks and three linear sections. The kink of curve χ^?1(T) is related to polymorphic transformations (Q′ → Q* and Q* → R) that take place in the crystal lattices of the samples. The main magnetic characteristics of the samples are determined with the least-squares processing of curve χ^?1. Is is demonstrated that dependence χ^?1(T) obeys the Curie-Weiss law. The energy state of the magnetoactive manganese atom in the Q′-and Q*-phase samples is close to the energy state of a free Mn²⁺ ion. In the R phase, this state is close to the state of a free Mn³⁺ ion.     

First-principles investigation of the optical properties of CuIn(SxSe1–x)2

We optimized the lattice structure of sulfur-doped CuInSe₂ using first principles. The lattice constants for CuIn(S_xSe_1–x)₂ vary linearly with x according to a(x)=–0.02828x+0.58786 nm and c(x)=–0.05692x+1.1834 nm, which agree well with experimental data. The optical properties of CuIn(S_xSe_1–x)₂ were then systematically investigated using first-principles calculations with the HSE06 functional. We present data for the complex dielectric function, refractive index, extinction coefficient, reflectivity index, absorption coefficient, and optical bandgap for CuIn(S_xSe_1–x)₂. The optical bandgap E_g obtained from the absorption coefficient is 1.07 eV for CuInSe₂ and 1.384 eV for CuInS₂. These values are very close to experimental results, indicating that first-principles calculations can yield accurate bandgap values. The optical bandgap of CuIn(S_xSe_1–x)₂ increases linearly with the sulfur concentration according to E_g=0.3139x+1.0825 eV.     

Phonon spectra of (GaAs)<Subscript>n</Subscript>(Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript>x</Subscript>As)<Subscript>m</Subscript> superlattices according to the Keating model

Phonon spectra of (GaAs)_n(Ga_1?xAl_xAs)_m superlattices were calculated using the Keating model with allowance made for long-range forces. The calculated summed longitudinal acoustical branches are consistent with observed experimental Raman doublets. The tendency towards a shift to higher frequencies of longitudinal optical modes as the number of GaAs monolayers increases, which is observed both in calculations and in experiments, is caused by localization of these modes within the GaAs layer.     

Defects in the crystal structure of Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te layers grown on the Si (310) substrates

Microstructure of the CdTe (310) and CdHgTe (310) layers grown by molecular-beam epitaxy on Si substrates has been studied by the methods of transmission electron microscopy and selective etching. It is established that formation of antiphase domains in the CdHgTe/CdTe/ZnTe/Si(310) is determined by the conditions of formation of the ZnTe/Si interface. Monodomain layers can be obtained by providing conditions that enhance zinc adsorption. An increase in the growth temperature and in the pressure of Te₂ vapors gives rise to antiphase domains and induces an increase in their density to the extent of the growth of poly-crystals. It is found that stacking faults exist in a CdHgTe/Si(310) heterostructure; these defects are anisotropically distributed in the bulk of grown layers. The stacking faults are predominantly located in one (111) plane, which intersects the (310) surface at an angle of 68°. The stacking faults originate at the ZnTe/Si(310) interface. The causes of origination of stacking faults and of their anisotropic distribution are discussed.     

Optical properties of (CuInSe<Subscript>2</Subscript>)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>(2MnSe)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys

The transmittance spectra of (CuInSe₂)_{1 − x} (2MnSe) _x alloy crystals grown by the Bridgman method are studied in the temperature range from 10 to 300 K. For these materials, the band gap and its temperature dependence are determined. It is shown that the band gap decreases with increasing temperature. The dependences of the band gap of the (CuInSe₂)_{1 − x} (2MnSe) _x alloys on the composition parameter x are plotted.     

Role of Boron Element on the Electronic Properties of <Emphasis Type="Italic">α</Emphasis>-Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript>: A First-Principle Study

Transition metal silicides (TMSis) are attracting increasing interest from the microelectronics and nanoelectronic industries. In this paper, we use the first-principles method to investigate the B-doped mechanism and the influence of B on the electronic properties of α-Nb₅Si₃. The calculated results show that B-doped Nb₅Si₃ is thermodynamically stable at the ground state. The calculated electronic structure shows that the thermodynamically stable B-doped Nb₅Si₃ is attributed to the 3D-network B-Si bonds and B-Nb bond. In particular, B element prefers to occupy B _-IT4 site in comparison to other sites. Moreover, the calculated band structure indicates that Nb₅Si₃ exhibits metallic behavior at the ground state. We find that B-doping can improve charge overlap between conduction band and the valence band, which effectively improves the electronic properties of Nb₅Si₃.     

X-Ray Diffraction Analysis of Features of the Crystal Structure of GaN/Al<Subscript>0.32</Subscript>Ga<Subscript>0.68</Subscript>N HEMT-Heterostructures by the Williamson–Hall Method

The fitting of θ/2θ and ω peaks in X-ray diffraction curves is shown to be most accurate in the case of using an inverse fourth-degree polynomial or probability density function with Student’s distribution (Pearson type VII function). These functions describe well both the highest-intensity central part of the experimental peak and its low-intensity broadened base caused by X-ray diffuse scattering. The mean microdeformation ε and mean vertical domain size D are determined by the Williamson–Hall method for layers of GaN (ε ≈ 0.00006, D ≈ 200 nm) and Al_0.32Ga_0.68N (ε = 0.0032 ± 0.0005, D = 24 ± 7 nm). The D value obtained for the Al_0.32Ga_0.68N layer is most likely to result from the nominal thickness of this layer, which is 11 nm.     

On the band gap of Cu<Subscript>2</Subscript>ZnSn(S<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>4</Subscript> alloys

The transmittance spectra of single-crystal Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ compounds and Cu₂ZnSn(S _x Se_1–x )₄ alloys grown by chemical vapor-transport reactions are studied in the region of the fundamental absorption edge. From the experimental spectra, the band gap of the compounds and their alloys is determined. The dependences of the band gap on the composition parameter x of the alloy are constructed. It is established that the band gap nonlinearly varies with x and can be described as a quadratic dependence.     

Specific features of molecules’ pyrolysis on the epitaxial surface in the case of growth of the Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> layers from hydrides in vacuum

Specific features of molecules’ pyrolysis in the case of growth of Si_{1 ? x} Ge _x films from hydrides in vacuum are considered in the context of kinetic approximation on the basis of a growth experiment. The type of solution characteristic of the kinetic problem was studied in relation to the values of the system’s parameters for a scheme of decomposition of monohydrides of Si and Ge with a dominant role of SiH₂ and GeH₂ radicals in the pyrolysis process. It is shown for the first time that, in the system under consideration, there can exist at least two types of solutions that differ radically from each other in the rate of incorporations of atoms into the growing layer and in the degree of filling of the surface states with products of molecules’ pyrolysis. The type of solution characteristic of the actual experiment is determined from the condition for the segregation-related accumulation of Ge atoms at the growth surface of the Si_{1 ? x} Ge _x film. Numerical analysis performed in relation to the growth temperature made it possible to estimate the rate of the monohydrides’ decomposition and the values of the factor of incorporation of Si and Ge adatoms into the crystal. The estimate of the characteristic decomposition time for monosilane yields the value of 3–4 s at the growth temperatures of 450–700°C; for monogermane, this value is about 2 s.     

Formation of acceptor centers under the action of redox media on the surface of Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te films

The effect of redox media on the formation of acceptor centers in the Cd _x Hg_{1 − x} Te films grown by molecular beam epitaxy on the GaAs (301) substrates is studied. When tested for long-term stability, the untreated n-type films do not change their parameters, whereas the treated films exhibit a decrease in the conductivity and the mobility of charge carriers by nearly two orders of magnitude. It is shown that, on the treatments, a source of acceptors is formed at the surface, and the acceptors are most likely mercury vacancies.     

Thermoelectric Properties of (Na<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript>M<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>)<Subscript>1.4</Subscript>Co<Subscript>2</Subscript>O<Subscript>4</Subscript> (M = Sr,Li)

Oxide thermoelectric materials (Na_1−y M _y )_1.4Co₂O₄ (M = Sr, Li; y = 0 to 0.4) were prepared by a sol–gel method. The influence of doping on the thermoelectric properties was investigated, and the phase composition was characterized by x-ray diffraction. Experimental results showed that the main crystalline phase of the undoped and Sr/Li-doped samples was γ-Na_1.4Co₂O₄. The thermoelectric properties of Na_1.4Co₂O₄ can be improved slightly by doping with Sr. Doping with Li improves the thermoelectric properties of Na_1.4Co₂O₄. For a doping fraction of y = 0.1, the electrical conductivity of (Na_1−y Li _y )_1.4Co₂O₄ at 288 K achieves its maximum value of 301.19 (Ω mm)⁻¹. The Seebeck coefficient and power factor of (Na_1−y Li _y )_1.4Co₂O₄ at 288 K achieve their maximum values of 172.28 μV K⁻¹ and 7.44 mW m⁻¹ K⁻² at a doping fraction of y = 0.4.