Giant Thermopower at Low Temperatures in Novel Clathrates Ba<Subscript>8</Subscript>{Cu,Zn}<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>46−<Emphasis Type="Italic">x</Emphasis></Subscript>

In the present work we focus on cubic type I Ba-Ge-based clathrates, where Ge in Ba₈Ge₄₃ \square₃\rm Ba_8Ge_{43} \square_3 (where □ is a vacancy) is substituted by Cu, Zn, Pd, and Si. Structural investigations in all cases confirm cubic primitive symmetry consistent with the space group type Pm[`3]nPm\bar{3}n of a typical type I clathrate structure with lattice parameter a ≈ 1.06 nm. Electronic transport has been studied in a broad temperature range from 4.2 K to about 800 K, demonstrating that substitution allows fine-tuning of the charge carrier density, shifting the materials into the proximity of a metal-to-insulator transition. This is evidenced from giant thermopower reaching values of 400 μV/K in the case of Ba₈Cu_5.2Zn_0.8Ge_40.0 at temperatures well below room temperature (T _max ^S  ≈ 150 K).     

Radiative recombination channels in Si/Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanostructures

Using the solution of the 2D Schrödinger equation, systematic features of distribution of charge carriers in the Si/Si_{1 ? x} Ge _x nanostructures and variations in the efficiency of radiative recombination when pyramidal 2D clusters are transformed into 3D dome clusters with increasing thickness of nanolayers are established. The effect of the composition of the layers on the efficiency of the elastic stress in the structure and, as a consequence, the variation in conduction bands and valence band of the Si_{1 ? x} Ge _x nanostructures is taken into account. On realization of the suggested kinetics model, which describes recombination processes in crystalline structures, saturation of radiation intensity with increasing the pump intensity caused by an increase in the contribution of the Auger recombination is observed. A decrease in the contribution of the nonradiative Auger recombination is attained by decreasing the injection rate of carriers into the clusters, and more precisely, by an increase in the cluster concentration and an increase in the rate of radiative recombination.     

Metal-insulator transition in chromium-doped Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te alloys

The electrical properties of chromium-doped n-Pb_1?x Ge _x Te alloys (x = 0.02–0.13) have been studied. A decrease in the free-electron concentration and a metal-insulator transition are observed as the germanium content of alloys increases. This is due to the Fermi level pinning by the chromium impurity level and to the flow of electrons from the conduction band to the impurity level. The experimental data obtained are used to calculate, in terms of the two-band Kane dispersion law, the dependences of the electron concentration and Fermi energy on the germanium content in the alloy. The motion rate of the chromium-related level with respect to the conduction band bottom is determined and a model of variation of the electronic structure with the matrix composition is suggested.     

Compositional and Strain Characterization of Ion-Beam-Synthesized Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> Thin Films

A thin film of Ge-rich Ge _x Si_1−x on a (100) Si substrate was synthesized by ion implantation followed by thermal oxidation. Proper oxidation conditions were maintained to produce a film with Ge atomic content of more than 95%, confirmed by both high-resolution Rutherford backscattering spectrometry (RBS) and Raman spectroscopy. The strain state of the Ge-rich thin film is a function of its thickness, as determined by the implantation fluence. The use of Raman spectroscopy to monitor the composition and strain state of the Ge thin film formed is discussed.     

Highly Efficient Ge-Rich Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te Thermoelectric Alloys

The search for alternative energy sources is presently at the forefront of applied research. In this context, thermoelectricity for direct energy conversion from thermal to electrical energy plays an important role. This paper is concerned with the development of highly efficient p-type Ge _x Pb_1−x Te alloys for thermoelectric applications, using spark plasma sintering. The carrier concentration of GeTe was varied by alloying of PbTe and/or by Bi₂Te₃ doping. Very high ZT values up to ~1.8 at 500°C were obtained by doping Pb_0.13Ge_0.87Te with 3 mol% Bi₂Te₃.     

Cathodoluminescence from dilute GaN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> solutions (<Emphasis Type="Italic">x</Emphasis> ≤ 0.03)

Cathodoluminescence from GaN _x As_1?x layers (0 ≤ x ≤ 0.03) was measured at photon energies ranging from the intrinsic absorption edge to 3 eV at room temperature. An additional emission band was visible in the visible range of the cathodoluminescence spectra. The intensity of this band is two orders of magnitude lower than the edge-emission intensity. The photon energy corresponding to the peak of this band and its FWHM are virtually independent of x and equal to ～2.1 and 0.6–0.7 eV, respectively. This emission is related to indirect optical transitions of electrons from the L _6c and Δ conduction-band minimums to the Γ₁₅ valence-band maximum.     

Structure and Thermoelectric Properties of Te- and Ge-Doped Skutterudites CoSb<Subscript>2.875−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>0.125</Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

n-Type CoSb_2.875−x Ge_0.125Te _x (x = 0.125 to 0.275) compounds with different Te contents have been synthesized by a melt–quench–anneal–spark plasma sintering method, and the effects of Te content on the structure and thermoelectric properties have been investigated. The results show that all specimens exhibited n-type conduction characteristics. The solubility limit of Te in CoSb_2.875−x Ge_0.125Te _x is found to be x = 0.25. The solubility of Te in CoSb₃ is increased through charge compensation of the element Ge. The room-temperature carrier concentration N _p of CoSb_2.875−x Ge_0.125Te _x skutterudites increases with increasing Te content, and the compounds possess high power factors. The maximum power factor of 3.89 × 10⁻³ W m⁻¹ K⁻² was obtained at 720 K for the CoSb_2.625Ge_0.125Te_0.25 compound. The thermal conductivity decreases dramatically with increasing Te content due to strong point defect scattering. The maximum value of the thermoelectric figure of merit ZT = 1.03 was obtained at 800 K for CoSb_2.625Ge_0.125Te_0.25, benefiting from a lower thermal conductivity and a higher power factor. The figure of merit is competitive with values reported for single-filled skutterudites.     

Thermoelectric Properties and Electronic Structure of Bi- and Ag-Doped Mg<Subscript>2</Subscript>Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Compounds

Mg₂Si_1−x Ge _x compounds were prepared from pure elements by melting in tantalum crucibles. The reaction was conducted under an inert gas in a special laboratory setup. Samples for thermoelectric measurements were formed by hot pressing. Structure and phase composition of the obtained materials were investigated by x-ray diffraction (XRD). Morphology and chemical composition were examined by scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS), respectively. Thermoelectric properties, i.e., the Seebeck coefficient, the electrical conductivity, and the thermal conductivity, were measured in the temperature range of 500 K to 900 K. The effect of Bi and Ag doping on the thermoelectric performance of Mg-Si-Ge ternary compounds was investigated. The electronic structures of binary compounds were calculated using the Korringa–Kohn–Rostoker (KKR) method. The effects of disorder, including Ge substitution and Bi or Ag doping, were accounted for in the KKR method with coherent potential approximation calculations. The thermoelectric properties of doped Mg₂Si_1−x Ge _x are discussed with reference to computed density of states as well as the complex energy band structure.     

High-Temperature Growth of GaN and Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N via Ammonia-Based Metalorganic Molecular-Beam Epitaxy

The effect of high-temperature growth on the crystalline quality and surface morphology of GaN and Al _x Ga_1?x N grown by ammonia-based metalorganic molecular-beam epitaxy (NH₃-MOMBE) has been investigated as a means of producing atomically smooth films suitable for device structures. The effects of V/III ratio on the growth rate and surface morphology are described herein. The crystalline quality of both GaN and AlGaN was found to mimic that of the GaN templates, with (002) x-ray diffraction (XRD) full-widths at half- maximum (FWHMs) of ~350 arcsec. Nitrogen-rich growth conditions have been found to provide optimal surface morphologies with a root-mean-square (RMS) roughness of ~0.8 nm, yet excessive N-rich environments have been found to reduce the growth rate and result in the formation of faceted surface pitting. AlGaN exhibits a decreased growth rate, as compared with GaN, due to increased N recombination as a result of the increased pyrolysis of NH₃ in the presence of Al. AlGaN films grown directly on GaN templates exhibited Pendellösung x-ray fringes, indicating an abrupt interface and a planar AlGaN film. AlGaN films grown for this study resulted in an optimal RMS roughness of ~0.85 nm with visible atomic steps.     

Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te Alloys: Application Considerations

The search for alternative energy sources is at the forefront of applied research. In this context, thermoelectricity, i.e., direct conversion of thermal into electrical energy, plays an important role, particularly for exploitation of waste heat. Materials for such applications should exhibit thermoelectric potential and mechanical stability. PbTe-based compounds include well-known n-type and p-type compounds for thermoelectric applications in the 50°C to 600°C temperature range. This paper is concerned with the mechanical and transport properties of p-type Pb_0.5Sn_0.5Te:Te and PbTe<Na> samples, both of which have a hole concentration of ∼1 × 10²⁰ cm⁻³. The ZT values of PbTe<Na> were found to be higher than those of Pb_0.5Sn_0.5Te:Te, and they exhibited a maximal value of 0.8 compared with 0.5 for Pb_0.5Sn_0.5Te:Te at 450°C. However, the microhardness value of 49 H_V found for Pb_0.5Sn_0.5Te:Te was closer to that of the mechanically stable n-type PbTe (30 H_V) than to that of PbTe<Na> (71 H_V). Thus, although lower ZT values were obtained, from a mechanical point of view Pb_0.5Sn_0.5Te:Te is preferable over PbTe<Na> for practical applications.     

Electron spin resonance in Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>x</Subscript>Te and Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>x</Subscript>Te compounds

Electron spin resonance in semimagnetic Cd_1?xMn_xTe (0<x<0.7) and Zn_1?xMn_xTe (0<x<0.53) compounds was studied at temperatures of 77 and 300 K. It is found that two types of paramagnetic centers exist in Zn_1?xMn_xTe, one of which is related to Mn²⁺ ions and the other is attributed to structural defects in the crystals.     

Low-temperature microwave magnetoresistance of lightly doped <Emphasis Type="Italic">p</Emphasis>-Ge and <Emphasis Type="Italic">p</Emphasis>-Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>x</Subscript>

The magnetoresistance of a lightly doped p-Ge_1?xSi_x alloy is studied in the range of compositions x = 1–2 at %. The results are compared with the available data for lightly doped p-Ge. The studies have been carried out using ESR measurements at a frequency of 10 GHz in the temperature range 10–120 K. It is established that micrononuniformity in the distribution of Si in the Ge lattice (Si clusters) suppresses the interference part of the anomalous magnetoresistance and, in addition, results in an averaging of the effects of light and heavy holes. This observation suggests a sharp decrease in the inelastic scattering time in the case of a Ge_1?xSi_x solid solution as compared to that of Ge.     

Electroluminescence and phototrigger effect in single crystals of GaS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> alloys

The effects of switching and electroluminescence as well as the interrelation between these effects in single crystals of GaS _x Se_1?x alloys are detected and studied. It is established that the threshold voltage for switching depends on temperature, resistivity, and composition of alloys, and also on the intensity and spectrum of photoactive light. As a result, a phototrigger effect is observed; this effect arises under irradiation with light from the fundamental-absorption region. Electroluminescence is observed in the subthreshold region of the current-voltage characteristic; the electroluminescence intensity decreases drastically to zero as the sample is switched from a high-resistivity state to a low-resistivity state. Experimental data indicating that the electroluminescence and the switching effect are based on the injection mechanism (as it takes place in other layered crystals of the III-V type) are reported.     

Grain boundary related electrical transport in Al-rich Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>N layers grown by metal-organic chemical vapor deposition

Electrical transport data for Al-rich AlGaN layers grown by metal-organic chemical vapor deposition (MOCVD) are presented and analyzed in the temperature range 135–300 K. The temperature dependence of electrical conductivity indicated that conductivity in the films was controlled by potential barriers caused by carrier depletion at grain boundaries in the material. The Seto’s grain boundary model provided a complete framework for understanding of the conductivity behavior. Various electrical parameters of the present samples such as grain boundary potential, donor concentration, surface trap density, and Debye screening length were extracted.     

Electronic and vibrational states in InN and In<Subscript>x</Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N solid solutions

The review presents the results of optical studies of the fundamental physical characteristics of InN, the material which remains the least studied among nitrides of Group-III elements. The results of early optical studies of InN are analyzed and compared with recent data. New experimental facts reported in the review refer to hexagonal single-crystal epitaxial InN layers with an electron concentration of (1?2)×10¹⁸ to 6×10²⁰ cm^?3, which are grown by molecular beam epitaxy (MBE) and metal-organic vapor-phase epitaxy (MOVPE) on Al₂O₃ substrates. The aim of this review is to make a joint analysis of optical spectra (absorption, photoluminescence (PL), PL excitation, and photomodulated reflection) near the fundamental band gap. Furthermore, basic structural and electrical characteristics that have been obtained by a whole range of techniques are given for epitaxial layers of hexagonal InN. The principal result of recent studies is that the hexagonal InN crystal is a narrow-gap semiconductor with a band gap of 0.65–0.7 eV. Previously, the band gap of this material was considered to be 1.89 eV. It is shown that the Burstein-Moss effect accounts for the strong difference between the band gap and the optical absorption threshold in InN samples with a high concentration of electrons. The small value of the band gap of hexagonal InN is confirmed by optical studies of In_xGa_1?xN solid solutions at high concentrations of In. Theoretical calculations of the band structure of hexagonal InN crystals are briefly reviewed.     

Temperature-dependent excitonic absorption in long-period multiple In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As/GaAs quantum well structures

Temperature variations in the fundamental absorption edge of long-period In _x Ga_{1 − x} As/GaAs structures are studied for samples with different numbers of quantum wells and similar periods. The quantum wells were close in composition and width. Experimental data are interpreted in the model of exciton-polariton light transfer involving localized excitons in confined structures with a finite number of quantum wells. The experimentally observed low-temperature anomaly of the integrated absorption coefficient is attributed to reemission of resonance localized excitons along a finite chain of quantum wells, with no excitonic transfer. The radiative decay time of an exciton in a single quantum well is estimated from the experimental data. It is demonstrated that, at low temperatures, the major contribution to the width of the experimentally observed absorption line corresponding to the ground heavy-hole exciton state is made by inhomogeneous broadening of the line by the field of potential fluctuations associated with the compositional disorder of the alloy. At low temperatures, the inhomogeneous broadening is much more pronounced than the broadening governed by the true radiative and nonradiative dissipative decay.     

Pb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te epitaxial films for terahertz detectors

Liquid-phase epitaxy is used to fabricate Pb_0.8Sn_0.2Te films, undoped or doped with indium to different levels. The depth profiles of the carrier density and dopant concentration in the films are measured and examined. A uniform dopant concentration to a depth of 15 μm is obtained. Electrical-conduction inversion is observed at a temperature of 77.3 K as the doping level is varied. The liquid-phase epitaxial method is shown to be a more suitable technology for the reproducible manufacture of epitaxial films with a given carrier density, such as the ones used in terahertz detectors.     

The short-wavelength edge of intrinsic photoluminescence in diluted GaN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript> alloys

It is suggested that the analysis of the short-wavelength edge of intrinsic photoluminescence in diluted GaN _x As_{1 − x} alloys at room temperature be used to study the specific features of the energy dependence of the density of states in the conduction band. It is found that, in the GaN _x As_{1 − x} alloys with x ≥ 0.002, this dependence is inconsistent with the model of the anticrossing band and suggests that there are extra states. These states are thought to be the states formed in nitrogen clusters and interacting with the conduction band. The energy of these states is at least 1.45 eV above the top of the valence band.     

Electrodeposition of Bi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Thermoelectric Films from DMSO Solution

The electrochemical behavior of nonaqueous dimethyl sulfoxide solutions of Bi^III, Te^IV, and Sb^III was investigated using cyclic voltammetry. On this basis, Bi _x Sb_2−x Te _y thermoelectric films were prepared by the potentiodynamic electrodeposition technique in nonaqueous dimethyl sulfoxide solution, and the composition, structure, morphology, and thermoelectric properties of the films were analyzed. Bi _x Sb_2−x Te _y thermoelectric films prepared under different potential ranges all possessed a smooth morphology. After annealing treatment at 200°C under N₂ protection for 4 h, all deposited films showed p-type semiconductor properties, and their resistances all decreased to 0.04 Ω to 0.05 Ω. The Bi_0.49Sb_1.53Te_2.98 thermoelectric film, which most closely approaches the stoichiometry of Bi_0.5Sb_1.5Te₃, possessed the highest Seebeck coefficient (85 μV/K) and can be obtained under potentials of −200 mV to −400 mV.