Conductivity of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>–GaAs Heterojunctions

The effect of annealing in argon at temperatures of T_an = 700–900°C on the I–V characteristics of metal–Ga₂O₃–GaAs structures is investigated. Samples are prepared by the thermal deposition of Ga₂O₃ powder onto GaAs wafers with a donor concentration of N _d = 2 × 10¹⁶ cm^–3. To measure theI–V characteristics, V/Ni metal electrodes are deposited: the upper electrode (gate) is formed on the Ga₂O₃ film through masks with an area of S _k = 1.04 × 10^–2 cm² and the lower electrode in the form of a continuous metallic film is deposited onto GaAs. After annealing in argon at T_an ≥ 700°C, the Ga₂O₃-n-GaAs structures acquire the properties of isotype n-heterojunctions. It is demonstrated that the conductivity of the structures at positive gate potentials is determined by the thermionic emission from GaAs to Ga₂O₃. Under negative biases, current growth with an increase in the voltage and temperature is caused by field-assisted thermal emission in gallium arsenide. In the range of high electric fields, electron phonon-assisted tunneling through the top of the potential barrier is dominant. High-temperature annealing does not change the electron density in the oxide film, but affects the energy density of surface states at the GaAs–Ga₂O₃ interface.     

Structure and Properties of Nanostructured YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7–δ</Subscript>, BiFeO<Subscript>3</Subscript>, and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>

The development of new nanostructured materials based on YBa₂Cu₃O_7–δ, BiFeO₃, and Fe₃O₄ compounds is considered. The structure, morphology, and properties of these materials are studied. The possibilities of fabricating YBa₂Cu₃O_7–δ ceramics with given densities from nanopowders in a single stage by an energy efficient method and growing superconducting films of the same composition on a silicon substrate (on a SiO₂ layer) are demonstrated. The technique for fabricating BiFeO₃ nanopowder, making it possible to obtain nanostructured ceramics without additional accompanied phases commonly forming during BiFeO₃ synthesis is developed. Two methods of the single-stage synthesis of Fe₃O₄ nanopowder are presented: burning of nitrate-organic precursors and the electrochemical three-electrode method in which one of the electrodes, i.e., an anode containing scrap iron and slurry, is used as an expendable material.     

Free-Standing β-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> Thin Diaphragms

Free-standing, very thin, single-crystal β-gallium oxide (β-Ga₂O₃) diaphragms have been constructed and their dynamical mechanical properties characterized by noncontact, noninvasive optical measurements harnessing the multimode nanomechanical resonances of these suspended nanostructures. We synthesized single-crystal β-Ga₂O₃ using low-pressure chemical vapor deposition (LPCVD) on a 3C-SiC epilayer grown on Si substrate at temperature of 950°C for 1.5 h. The synthesized single-crystal nanoflakes had widths of ～ 2 μm to 30 μm and thicknesses of ～ 20 nm to 140 nm, from which we fabricated free-standing circular drumhead β-Ga₂O₃ diaphragms with thicknesses of ～ 23 nm to 73 nm and diameters of ～ 3.2 μm and ～ 5.2 μm using a dry stamp-transfer technique. Based on measurements of multiple flexural-mode mechanical resonances using ultrasensitive laser interferometric detection and performing thermal annealing at 250°C for 1.5 h, we quantified the effects of annealing and adsorption of atmospheric gas molecules on the resonant characteristics of the diaphragms. Furthermore, we studied the effects of structural nonidealities on these free-standing β-Ga₂O₃ nanoscale diaphragms. We present extensive characterization of the mechanical and optical properties of free-standing β-Ga₂O₃ diaphragms, paving the way for realization of resonant transducers using such nanomechanical structures for use in applications including gas sensing and ultraviolet radiation detection.     

Photoluminescence and excitation features of Nd<Superscript>3+</Superscript> ions in (La<Subscript>0.97</Subscript>Nd<Subscript>0.03</Subscript>)<Subscript>2</Subscript>S<Subscript>3</Subscript> · 2Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

A combined study of the spectral photoluminescence distribution and excitation spectra of photoluminescence in La₂S₃ · 2Ga₂O₃ and (La_0.97Nd_0.03)₂S₃ · 2Ga₂O₃ glasses, along with the study of the transmission spectra of these glasses, was carried out. The radiative channel was ascertained to be the main channel for the energy transfer from the host matrix to the Nd³⁺ ions upon excitation of the glasses with light at a wavelength of the fundamental absorption band. Oxygen centers with the level E _c -2.0 eV act as sensitizing agents. The structural disordering of the glass host increases the variance in the magnitude of splitting of the multiplet levels from the 4f electronic states of the Nd³⁺ ion. This promotes nonradiative relaxation of the electrons from excited states to the laser ⁴F_3/2 level. The (La_0.97Nd_0.03)₂S₃ · 2Ga₂O₃ glasses can be considered as promising laser materials for obtaining the stimulated emission of radiation of Nd³⁺ ions under an optical pump in the range of the fundamental absorption band of the glass.     

Chloride epitaxy of β-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> layers grown on <Emphasis Type="Italic">c</Emphasis>-sapphire substrates

The method of chloride epitaxy is employed to grow β-Ga₂O₃ epitaxial layers on a c-sapphire substrate. Purified dry air is used as the source of oxygen. The layers are studied using the methods of X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and micro-Raman spectroscopy. It is found that the growth plane of the layers is (\(\bar 2\)01), parallel to the substrate surface. The layers consist of separate large crystalline grains of three different in-plane orientations rotated relative to each other in the growth plane by 60°. Misorientation can be caused by the different symmetry of the substrate and the epitaxial layer.     

Enhanced Photocatalytic Activity of Two-Pot-Synthesized BiFeO<Subscript>3</Subscript>–ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Heterojunction Nanocomposite

BiFeO₃–ZnFe₂O₄ heterojunction nanocomposites have been produced by a chemical synthesis method using one- and two-pot approaches. X-ray diffraction patterns of as-calcined samples indicated formation of pure zinc ferrite (ZnFe₂O₄) and bismuth ferrite (BiFeO₃) phases, each retaining its crystal structure. Diffuse reflectance spectrometry was applied to calculate the optical bandgap of the photocatalysts, revealing values in the range from 2.03 eV to 2.17 eV, respectively. The maximum photodegradation of methylene blue of about 97% was achieved using two-pot-synthesized photocatalyst after 120 min of visible-light irradiation due to the higher probability of charge separation of photogenerated electron–hole pairs in the heterojunction structure. Photoluminescence spectra showed lower emission intensity of two-pot-synthesized photocatalyst, due to its lower recombination rate originating from greater charge separation.     

Superconducting quantum interference devices based on YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7–<Emphasis Type="Italic">x</Emphasis> </Subscript> films for nondestructive testing

Original superconducting quantum interference devices (SQUIDs) with a working temperature of 77 K based on high-temperature superconducting (HTSC) YBa₂Cu₃O_7–x films can be used for the measurement systems of nondestructive testing using magnetic and eddy-current methods. A dynamic range of 120 dB with respect to the amplitude of the measured signal and a spatial resolution of about 10 µm are reached for the measurement system with the HTSC SQUID in which a receiving loop with a size of 50 µm is placed at a distance of about 0.3 mm from the room-temperature object under study. The sensitivity with respect to magnetic field (4 fT/ \(\sqrt {Hz} \) at a temperature of 77 K) of the HTSC SQUID magnetometer with multilayer superconducting flux transformer is sufficient for applications in biomagnetic measurements in magnetocardiography and magnetoencephalography. HTSC SQUID gradiometers with multilayer superconducting flux transformers exhibit stable operation in magnetically unshielded space at a sensitivity of 15 fT/cm \(\sqrt {Hz} \) with respect to the gradient of magnetic field at 77 K. Such a sensitivity is sufficient for the detection of single magnetic particles with a size of about 10 µm at a distance of about 15 mm.     

Single-Crystal NdBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7–<Emphasis Type="Italic">x</Emphasis></Subscript> Films for Microwave Electronics

The growth of single-crystal films of high-temperature superconductors of the NdBa₂Cu₃O_7–x composition with a thickness of 1–1.5 μm on Al₂O₃ + CeO₂ substrates during the laser spraying is investigated. Technological conditions of the epitaxial growth of the films with a temperature of superconducting transition of 95 K and a critical current of more than 10⁶ A/cm² at the temperature of liquid nitrogen are determined. It is shown that the structure of fabricated NdBa₂Cu₃O_7–x films is more perfect and homogeneous than the structure of YBa₂Cu₃O_7–x films widely used in microwave electronics.     

A ferroelectric field effect transistor based on a Pb(Zr<Subscript>x</Subscript>Ti<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>)O<Subscript>3</Subscript>/SnO<Subscript>2</Subscript> heterostructure

The possibility of fabricating a ferroelectric FET based on a Pb(Zr_xTi_1?x)O₃/SnO₂ (PZT/SnO₂) heterostructure is investigated. Sb-doped epitaxial SnO₂/Al₂O₃ thin film deposited by YAG laser ablation from a metal target is used as the FET channel. The highest obtained electron mobility in the channel is 25 cm²/(V s) at an electron density of 8 × 10¹⁹ cm^?3. The possibility of growing PZT film directly on SnO₂ film using two different techniques, laser ablation and magnetron sputtering, is demonstrated. Both methods have been used in the fabrication of Au/PZT/SnO₂ capacitor heterostructures, whose top Au electrode is 250×250 μm² in size. These cells demonstrate a capacitance of 1000 pF at a 10-V bias and remnant polarization up to 16 μC/cm². A Au/PZT/SnO₂/Al₂O₃ transistor structure with 94% modulation of the channel current is fabricated. The difference in the channel current under the effect of positive and negative remnant polarization of the undergate ferroelectric is 37%.     

Growth of (InSb)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>(Sn<Subscript>2</Subscript>)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films on GaAs substrates by liquid-phase epitaxy

The possibility of growing single-crystal substitutional (InSb)_{1 − x} (Sn₂) _x alloy (0 ≤ x ≤ 0.05) on the GaAs substrate by liquid-phase epitaxy from the In solution melt is established. The X-ray diffraction patterns and spectral and current-voltage characteristics of obtained n-GaAs-p-(InSb)_{1 − x} (Sn₂) _x heterostructures are studied at different temperatures. The lattice parameters of the (InSb)_{1 − x} (Sn₂) _x alloy are determined. It is found that the forward portion of the current-voltage characteristic of such structures at low voltages (up to 0.7 V) is described by the exponential dependence I = I ₀exp(qV/ckT), and at high voltages (V > 0.9 V), there is a portion of sublinear increase in the current with the voltage V ≈ V ₀exp(Jad). The experimental results are interpreted based on the injection depletion theory. It is shown that the product of mobility of majority carriers by the concentration of deep-level impurities increases as the temperature increases.     

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.     

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.     

The modification of BaCe<Subscript>0.5</Subscript>Zr<Subscript>0.3</Subscript>Y<Subscript>0.2</Subscript>O<Subscript>3–δ</Subscript> with copper oxide: Effect on the structural and transport properties

The effect of the content of CuO additive on the sinterability, phase composition, microstructure, and electrical properties of BaCe_0.5Zr_0.3Y_0.2O_3–δ proton-conducting material is studied. Ceramic samples were produced by the citrate–nitrate synthesis method with the addition of 0, 0.25, 0.5, and 1% CuO. It is shown that the relative density of the samples containing 0.5 and 1% CuO is higher than 94% at a sintering temperature of 1450°C, whereas the relative density of the material is no higher than 85% at a lower content of the sintering additive. From the data of X-ray diffraction analysis and scanning electron microscopy, it is established that the introduction of a small CuO content (0.25%) is inadequate for single-phase and high-dense ceramics to be formed. The conductivity and scanning electron microscopy data show that the sample with BaCe_0.5Zr_0.3Y_0.2O_3–δ + 0.5% CuO composition possesses high total and ionic conductivities as well as a high degree of microstructural stability after hydrogen reduction of the ceramics. The citrate–nitrate method modified by the introduction of a small CuO content can be recommended for the production of single-phase, gas-tight, and high-conductivity electrolytes based on both BaCeO₃ and BaZrO₃.     

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.     

The Power Factor of Bi<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Tl<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>3</Subscript> Single Crystals

Single crystals of the ternary system Bi_2−x Tl _x Se₃ (nominally x = 0.0 to 0.1) were prepared using the Bridgman technique. Samples with varying content of Tl were characterized by measurement of lattice parameters, electrical conductivity σ _⊥c, Hall coefficient R _H(B║c), and Seebeck coefficient S(ΔT⊥c). The measurements indicate that incorporation of Tl into Bi₂Se₃ lowers the concentration of free electrons and enhances their mobility. This effect is explained within the framework of the point defects in the crystal lattice, with formation of substitutional defects of thallium in place of bismuth (Tl_Bi) and a decrease in the concentration of selenium vacancies (V_Se ^{+ 2} V_{\rm{Se}}^{ + 2} ). The temperature dependence of the power factor σS ² of the samples is also discussed. As a consequence of the thallium doping we observe a significant increase of the power factor compared with the parent Bi₂Se₃.     

Synthesis and Transport Properties of In<Subscript>4</Subscript>(Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>3</Subscript>

Polycrystalline samples of In₄(Se_1−x Te _x )₃ were synthesized by using a melting–quenching–annealing process. The thermoelectric performance of the samples was evaluated by measuring the transport properties from 290 K to 650 K after sintering using the spark plasma sintering (SPS) technique. The results indicate that Te substitution can effectively reduce the thermal conductivity while maintaining good electrical transport properties. In₄Te₃ shows the lowest thermal conductivity of all compositions tested.     

Concentration dependence of the band gap of Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>2</Subscript>S<Subscript>4</Subscript> alloys on their composition

The transmittance spectra of MnIn₂S₄ and FeIn₂S₄ ternary compounds and Mn _x Fe_{1 − x} In₂S₄ alloys in the fundamental absorption edge region are studied. The samples were grown by planar melt crystallization. From the experimental spectra, the band gaps of the compounds and their alloys are determined, and the concentration dependence of the alloy band gap on the component content is established. It is found that the band gap nonlinearly varies with the composition parameter x and can be described by a quadratic function.