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.     

Photoluminescence spectra of thin films of ZnTPP–C<Subscript>60</Subscript> and CuTPP–C<Subscript>60</Subscript> molecular complexes

The results of studies of thin composite films of zinc and copper tetraphenylporphyrins with different fractions of fullerene C₆₀ are reported. The photoluminescence spectra are recorded, and the composition and surface morphology are analyzed by means of scanning electron microscopy. The results show a difference in the structure of films with two types of metals (Zn, Cu) entering into the complex of the porphyrin macrocycle. An additional long-wavelength photoluminescence band at 1.4 eV is detected for the first time, which is evidence of the formation of ZnTPP–C₆₀ molecular complexes from a gas-dynamic vapor flow upon condensation. In CuTPP thin films, the processes of self-assembly into nanowires 20 nm in diameter and up to 50 µm in length and the formation of nanoheterojunctions upon the addition of fullerene C₆₀ are observed. Quantum-chemical calculations in the context of density-functional theory are carried out to interpret the experimental data.     

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.     

Preparation of Bi<Subscript>0.85</Subscript>Nd<Subscript>0.15</Subscript>FeO<Subscript>3</Subscript> Nanotube Arrays by a Sol–Gel Template Method

Bi_0.85Nd_0.15FeO₃ (BNF) nanotube arrays were prepared by a sol–gel template method. The microstructure and phase were observed by scanning electron microscopy and transmission electron microscopy. Highly ordered nanotube arrays with BNF nanotubes of 200 nm diameter, 60 μm length, and 20 nm wall thickness were obtained. The BNF nanotube array capacitor showed ferroelectricity with a remanent polarization (2P _r) of 68.7 μC/cm² and a coercive electric field (2E _c) of 141.9 kV/cm at frequency of 1000 Hz. The leakage current behavior of the BNF nanotube array was dominated by an ohmic conduction mechanism at E < 30 kV/cm and a space-charge-limited current mechanism at E = 30 kV/cm to 200 kV/cm.     

Diffusion of Selenium in Liquid-Phase Epitaxy–Grown Hg<Subscript>0.78</Subscript>Cd<Subscript>0.22</Subscript>Te

We present preliminary results on Se diffusion in liquid-phase epitaxy (LPE)–grown HgCdTe epilayers. The LPE Hg_0.78Cd_0.22Te samples were implanted with Se of 2.0 × 10¹⁴/cm² at 100 keV and annealed at 350–450°C in mercury saturated vapor. Secondary ion mass spectrometry (SIMS) profiles were obtained for each sample. From a Gaussian fit, we find that the Se diffusion coefficient D _Se is about 1–2 orders of magnitude smaller than that of arsenic. The as-implanted Se distribution is taken into account in case of small diffusion length in Gaussian fitting. The D _Se was found to satisfy the Arrhenius relationship .     

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.     

Thermal Conductivity Reduction in CoSb<Subscript>3</Subscript>–CeO<Subscript>2</Subscript> Nanocomposites

CoSb₃ + x% CeO₂ nanocomposites (x = 1, 3, 5) were synthesized by ball-milling and spark plasma sintering. Scanning electron microscopy showed that some CeO₂ nano-inclusions sit at the boundaries of CoSb₃ grains. These inclusions also reduce the sizes of the CoSb₃ grains and crystallites by inhibiting their growth during sintering. Hall-effect measurements show that the CeO₂ inclusions modify the charge-carrier concentration in CoSb₃. The variations of the electrical resistivity for the 1% and 3% CeO₂ samples can at least partially be attributed to these modifications of the carrier concentration. Nonetheless, the resistivity increase in the 5% CeO₂ sample can unambiguously be ascribed to the presence of the CeO₂ inclusions. Thermal conductivity is systematically reduced (by more than 15% at 300 K) upon CeO₂ addition. Phonon diffusion by the increased number of CoSb₃ grain boundaries is one of the mechanisms involved in this reduction.     

Preparation and Thermoelectric Properties of Ag<Subscript>0.5</Subscript>In<Subscript>0.5−<Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript>5</Subscript>Sn<Subscript>4</Subscript>Te<Subscript>10</Subscript>

A series of compounds with composition Ag_0.5In_0.5−x Pb₅Sn₄Te₁₀ (x = 0.05 to 0.20) were prepared by slowly cooling the melts of the corresponding elements, and the effect of In content on the thermoelectric transport properties of these compounds has been investigated. Results indicate that the compounds’ electronic structure is sensitive to In content, and that the carrier concentration of these compounds at room temperature increases from 4.86 × 10¹⁸ cm⁻³ to 3.85 × 10²¹ cm⁻³ as x increases from 0.05 to 0.20. For these compounds, electrical conductivity decreases and Seebeck coefficient increases with increasing In content. Ag_0.05In_0.03Pb_0.5Sn_0.4Te₁₀ shows very low lattice thermal conductivity, and has a maximum dimensionless figure of merit ZT of 1.2 at 800 K.     

Preparation and Characterization of BaTiO<Subscript>3</Subscript>–PbZrTiO<Subscript>3</Subscript> Coating for Pyroelectric Energy Harvesting

Harvesting energy from waste heat is a promising field of research as there are significant energy recovery opportunities from various waste thermal energy sources. The present study reports pyroelectric energy harvesting using thick film prepared from a (x)BaTiO₃–(1 ? x)PbZr_0.52Ti_0.48O₃ (BT–PZT) solid solution. The developed BT–PZT system is engineered to tune the ferro to paraelectric phase transition temperature of it in-between the phase transition temperature of BaTiO₃ (393 K) and PbZrTiO₃ (573 K) with higher pyroelectric figure-of-merit (FOM). The temperature-dependent dielectric behavior of the material has revealed the ferro- to paraelectric phase transition at 427 K with a maximum dielectric constant of 755. The room-temperature (298 K) pyroelectric coefficient (Pi) of the material was obtained as 738.63 μC/m²K which has yielded a significantly high FOM of 1745.8 J m^?3 K^?2. The enhancement in pyroelectric property is attributed to the morphotopic phase transition between tetragonal and rhombohedral PZT phases in the BT–PZT system. The developed BT–PZT system is capable of generating a power output of 1.3 mW/m² near the Curie temperature with a constant rate (0.11 K/s) of heating. A signal conditioning circuit has been developed to rectify the time-varying current and voltage signals obtained from the harvester during heating cycles. The output voltage generated by the pyroelectric harvester has been stored in a capacitor for powering wearable electronics.     

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.     

Preparation and study of the thermoelectric properties of Fe<Subscript>2</Subscript>TiSn<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Heusler alloys

Polycrystalline alloys Fe₂TiSn_1–x Si _x (0 ≤ x ≤ 1) theoretically predicted as highly efficient thermoelectric materials are experimentally studied. Structural studies show that the partial substitution of Sn with Si results in the formation of a multiphase state in samples with x > 0. Impurity phases in general lead to a significant decrease in the Seebeck coefficient and an increase in the thermal conductivity of Fe₂TiSn_1–x Si _x samples, which does not allow consideration of these materials as promising thermoelectrics.     

Plasmon–phonon coupling in the infrared reflectance spectra of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> films

The results of studies of optical reflection in the far- and mid-infrared spectral regions are reported. The reflectance of five Bi₂Se₃ topological insulator films grown by molecular-beam epitaxy on Si(111) substrates is measured. The characteristic parameters of phonons and plasmons are determined by means of dispersion analysis for multilayer structures. It is found that the plasma frequency in a layer close to the Si–film interface is noticeably higher than that in the film bulk. Calculations of the loss function show that plasmon–phonon coupling plays an important role in Bi₂Se₃ films. The attenuated total internal reflection method is used to determine the frequency of the surface plasmon–phonon mode.     

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.     

Preparation and Ferroelectric Properties of Ho<Superscript>3+</Superscript>/Mo<Superscript>6+</Superscript> Cosubstituted Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> Thin Films by Sol–Gel Method

We have prepared Ho³⁺/Mo⁶⁺ cosubstituted bismuth titanate [(Bi_3.6Ho_0.4)_3.99- Ti_2.985Mo_0.015O₁₂, BHTM] thin films on Pt/Ti/SiO₂/Si substrates using the sol–gel method. The crystal structure and morphology of the BHTM films were characterized. The BHTM samples show a single phase of Bi-layered Aurivillius structure and a dense microstructure. The dielectric constant and dielectric loss of the BHTM films were about 359 and 0.043, respectively, at a frequency of 1 MHz. The films exhibit 2P _r of 57.7 μC/cm² and 2E _c of 290.0 kV/cm at an applied field of 470 kV/cm, and have fatigue-free characteristics. They also showed good insulating behavior according to leakage current testing.     

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.     

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.     

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.     

Catalyst-Free Direct Vapor-Phase Growth of Hexagonal ZnO Nanowires on <Emphasis>α</Emphasis>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The evolution of ZnO nanowires has been studied under supersaturation of Zn metal species with and without a ZnO thin-film buffer layer on α-Al₂O₃ deposited by the pulsed laser ablation technique. The nanowires had diameters in the range of 30 nm to 50 nm and lengths in the range of 5 μm to 10 μm with clear hexagonal shape and [000[`1]] [000\bar{1}] , [10[`1]1] [10\bar{1}1] , and [10[`1]0] [10\bar{1}0] facets. X-ray diffraction (XRD) measurements indicated crystalline properties for the ZnO nanostructures grown on pulsed laser deposition (PLD) ZnO nucleation layers. The optical properties were analyzed by photoluminescence (PL) and cathodoluminescence (CL) measurements. The ZnO nanowires were found to emit strong ultraviolet (UV) light at 386 nm and weak green emission as observed by PL measurements. The stoichiometry of Zn and O was found to be close to 1 by x-ray photoelectron spectroscopy (XPS) measurements. The process-dependent growth properties of ZnO nanostructures can be harnessed for future development of nanoelectronic components including optically pumped lasers, optical modulators, detectors, electron emitters, and gas sensors.     

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.