Microwave Dielectric Properties of BiCu<Subscript>2</Subscript>PO<Subscript>6</Subscript> Ceramics with Low Sintering Temperature

A BiCu₂PO₆ microwave dielectric ceramic was prepared using a solid-state reaction method. As the sintering temperature increased from 800°C to 880°C, the bulk density of BiCu₂PO₆ ceramic increased from 6.299 g/cm³ to 6.366 g/cm³; the optimal temperature was 860°C. The best microwave dielectric properties [permittivity (? _r ) = ～16, a quality factor (Q × f) = ～39,110 GHz and a temperature coefficient of resonant frequency (τ _f ) = ～?59 ppm/°C] were obtained in the ceramic sintered at 860°C for 2 h. Then, TiO₂ with a positive τ _f (～+400 ppm/°C) was added to compensate the τ _f value. The composite material was found to have a near-zero τ _f (+2.7 ppm/°C) and desirable microwave properties (? _r  = 19.9, Q × f = 24,885 GHz) when synthesized at a sintering temperature of 880°C. This system could potentially be used for low-temperature co-fired ceramics technology applications.     

Charge transport in silver chalcogenides in the region of phase transition

Data on the σ(T), R(T), and U(T) dependences in Ag₂Te, Ag₂Se, and Ag₂S in the region of the phase transition are analyzed. It is found that the phase transition in Ag₂Te is accompanied by a decrease in the electron concentration and this transition in Ag₂Se is accompanied by an increase in this concentration. The concentration of intrinsic charge carriers in Ag₂Te decreases by a factor of 4 as a result of the phase transition and increases by a factor of 2 in Ag₂Se. The effect of variation in the energy-band parameters in the region of phase transition on the electron mobility is considered. It is established that, in Ag₂Te and Ag₂S, electrons are scattered by optical phonons in the region of the phase transition, while electrons are scattered by acoustic phonons in the α and β phases. It is assumed that the anomalously large increase in σ and U in Ag₂S as a result of the phase transition is caused by an increase in the concentration n and a simultaneous decrease in σ _g and m _n ^* by a factor of about 2.     

Effect of annealing temperature and rate of sputtering on the magnetic properties and microstructure of the polycrystalline nickel films with (200) texture

The dependences of the magnetic properties and morphology of polycrystalline nickel (Ni) films with the (200) texture that are fabricated using the dc magnetron sputtering on the SiO₂/Si(100) substrates on sputtering rate annealing temperature T, and film thickness d are analyzed. It is demonstrated that an increase in the sputtering rate from 17 to 35 nm/min does not affect the saturation magnetization 4πM and ferromagnetic resonance line width ΔH but leads to a significant increase in the coercivity H _c for the films whose thickness d is greater than critical thickness d _cr (d > d _cr). It is also demonstrated that d _cr depends on both sputtering rate and annealing temperature. The films with the thickness d > d _cr exhibit the stripe domain structure whose period increases with increasing d and rate v. The annealing of the films with d ≥ 40 nm at T ≈ 200–400°C results in an increase in ΔH and H _c by a factor of 2–4, an increase in 4πM by 25%, an increase in grain size ξ by a factor of 20–30, and the formation of the stripe domain structure in the films that do not exhibit such structure prior to annealing and substantial strengthening of the (200) texture.     

Features of the band structure and conduction mechanisms of <Emphasis Type="Italic">n</Emphasis>-HfNiSn heavily doped with Y

The crystalline and electronic structures, energy, kinetic, and magnetic characteristics of n-HfNiSn semiconductor heavily doped with Y acceptor impurity are studied in the ranges: T = 80–400 K, N _A ^Y ≈ 1.9 × 10²⁰–5.7 × 10²¹ cm^–3 (x = 0.01–0.30), and H ≤ 10 kG. The nature of the mechanism of structural defect generation is determined, which leads to a change in the band gap and the degree of semiconductor compensation, the essence of which is the simultaneous reduction and elimination of structural donor-type defects as a result of the displacement of ~1% of Ni atoms from the Hf (4a) site, and the generation of structural acceptor-type defects by substituting Hf atoms with Y atoms at the 4a site. The results of calculations of the electronic structure of Hf_1–x Y _x NiSn are in agreement with the experimental data. The discussion is performed within the Shklovskii–Efros model of a heavily doped and compensated semiconductor.     

Radiative recombination in Ge<Superscript>+</Superscript>-implanted SiO<Subscript>2</Subscript> films annealed under hydrostatic pressure

Photoluminescence (PL) spectra and PL excitation spectra were recorded at room temperature from SiO₂ films implanted with Ge⁺ ions and annealed at temperature T _a =450–1100°C under hydrostatic pressure P=12 kbar. The emergence of features in the violet and green bands of the PL and PL excitation spectra correlates with the formation of hydrostatically strained Ge nanocrystals. The shift of the PL bands to higher energies, which occurs as the annealing temperature is raised to T _a ≥800°C, can be attributed to a shift of the energy levels related to the radiative recombination centers, which is caused by the increasing deformation potential. The observed PL is accounted for by the enhanced probability of direct radiative transitions in Ge nanocrystals with an X-like conduction band.     

Formation of silicon nanocrystals in multilayer nanoperiodic <Emphasis Type="Italic">a</Emphasis>-SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/insulator structures from the results of synchrotron investigations

The problem of the efficiency of the controllable formation of arrays of silicon nanoparticles is studied on the basis of detailed investigations of the electronic structure of multilayer nanoperiodic a-SiO _x /SiO₂, a-SiO _x /Аl₂О₃, and a-SiO _x /ZrO₂ compounds. Using synchrotron radiation and the X-ray absorption near edge structure (XANES) spectroscopy technique, a modification is revealed for the investigated structures under the effect of high-temperature annealing at the highest temperature of 1100°C; this modification is attributed to the formation of silicon nanocrystals in the layers of photoluminescent multilayer structures.     

Energy spectrum of charge carriers in Ag<Subscript>2</Subscript>Te

On the basis of investigations of the temperature and concentration dependences of kinetic coefficients (the Hall coefficientR, the electrical conductivity σ, and thermopower α₀) in n-type Ag₂Te, it is established that Ag atoms in Ag₂Te create the shallow donor levels located at a distance of (0.002?7 × 10^?5 T) eV from the bottom of the conduction band. It is shown that silver telluride has n-type conductivity starting with the deficiency of Ag ≥ 0.01 at % in the stoichiometric composition, and it is practically impossible to achieve the stoichiometric composition in Ag₂Te.     

Effect of the magnetic phase transition on the charge transport in layered semiconductor ferromagnets TlCrS<Subscript>2</Subscript> and TlCrSe<Subscript>2</Subscript>

TlCrS₂ and TlCrSe₂ crystals were synthesized by solid-state reaction. X-ray diffraction analysis showed that TlCrS₂ and TlCrSe₂ compounds crystallize in the hexagonal crystal system with lattice parameters a = 3.538 Å, c = 21.962 Å, c/a ≈ 6.207, z = 3; a = 3.6999 Å, c = 22.6901 Å, c/a ≈ 6.133, z = 3; and X-ray densities ρ _x = 6.705 and 6.209 g/cm³, respectively. Magnetic and electric studies in a temperature range of 77–400 K showed that TlCrS₂ and TlCrSe₂ are semiconductor ferromagnets. Rather large deviations of the experimental effective magnetic moment of TlCrS₂ (3.26 μ_B) and TlCrSe₂ (3.05 μ_B) from the theoretical one (3.85 μ_B) are attributed to two-dimensional magnetic ordering in the paramagnetic region of strongly layered ferromagnets TlCrS₂ and TlCrSe₂. The effect of the magnetic phase’s transition on the charge transport in TlCrS₂ and TlCrSe₂ is detected.     

A vacancy model of the heteropolytype epitaxy of SiC

A model for the transformation of SiC polytypes occurring during the growth of an epitaxial layer is suggested that is based on the variation over time of the concentration of carbon vacancies in a transition layer. Experimental data are analyzed in terms of this model. It is shown that the parameter η=Gτ/L _T (L _T is the thickness of the transition layer, G is the film growth rate, and τ is the lifetime of a vacancy in the transition layer) is invariant with respect to the method and temperature of the growth of the epitaxial layer. This parameter is determined only by the concentration of carbon vacancies in the substrate and in the film.     

Deep Radiation-Induced Defect Centers Created by a Fast Neutron Flux in CdZnTe Single Crystals

The effect of a fast neutron flux (Φ = 10¹⁴–10¹⁵ cm^–2) on the electrical and photoluminescence properties of p-CdZnTe single crystals is studied. Isothermal annealing is performed (T = 400–500 K), and the activation energy of the dissociation of radiation-induced defects is determined at E_D ≈ 0.75 eV.     

Negative annealing in silicon after the implantation of high-energy sodium ions

The implantation of sodium ions with an energy of 300 keV is carried out into high-resistivity p-Si. The annealing of defects at T _ann = 350–450°C and related activation of atoms (the latter occurs at the “tail” of atom distribution) are described by a first-order reaction. At T _ann = 450–525°C and irrespective of the ion dose, negative annealing is observed; this annealing is accompanied by an appreciable increase in the surface resistance ρ _s . According to estimations, the activation energy of this process amounts to ~2 eV. It is our opinion that the annealing is related to the precipitation of sodium donor atoms, which occurs at a depth exceeding by two–three times the projected range R _p of ions. The annealing of defects at T _ann = 525–700°C, which leads to a further decrease in ρ _s , features an activation energy of ~2.1 eV. The hypothesis that the “tail” in the profiles of sodium atoms measured by secondary-ion mass spectroscopy is due to the diffusion of these atoms from the walls of the crater to its center is verified. It is shown that this process is not implemented since the profiles of sodium atoms measured at room temperature do not differ from those measured at–140°C.     

Ferromagnetism and anomalous transport in GaAs doped by implantation of Mn and Mg ions

GaAs layers doped by implantation of Mn and Mg ions to increase the hole concentration were synthesized and studied. Measurements using a SQUID magnetometer showed that there is ferromagnetism at temperatures as high as 400 K, which is related to the formation of the MnAs and Mn _y Ga_{1 ? y} clusters as a result of high-temperature annealing, in addition to the formation of the Ga_{1 ? x} Mn _x As alloy. The anomalous Hall effect was observed at temperatures in the range from 4.2 to 200 K. As temperature was increased starting with 4.2 K, the negative magnetoresistance with extremely large magnitude transformed into a giant positive magnetoresistance at T ≈ 35 K.     

Theoretical Study of Electronic Structure and Thermoelectric Properties of Doped CuAlO<Subscript>2</Subscript>

The doping level dependence of thermoelectric properties of delafossite CuAlO₂ has been investigated in the constant scattering time (τ) approximation, starting from the first principles of electronic structure. In particular, the lattice parameters and the energy band structure were calculated using the total energy plane-wave pseudopotential method. It was found that the lattice parameters of CuAlO₂ are a = 2.802 Å and c = 16.704 Å, and the internal parameter is u = 0.1097. CuAlO₂ has an indirect band gap of 2.17 eV and a direct gap of 3.31 eV. The calculated energy band structures were then used to calculate the electrical transport coefficients of CuAlO₂. By considering the effects of doping level and temperature, it was found that the Seebeck coefficient S(T) increases with increasing acceptor doping (A _d) level. The values of S(T) in our experiments correspond to an A _d level at 0.262 eV, which is identified as the Fermi level of CuAlO₂. Based on our experimental Seebeck coefficient and the electrical conductivity, the constant relaxation time is estimated to be 1 × 10^?16 s. The power factor is large for a low A _d level and increases with temperature. It is suggested that delafossite CuAlO₂ can be considered as a promising thermoelectric oxide material at high doping and high temperature.     

Structural transformations and optical properties of As<Subscript>2</Subscript>S<Subscript>3</Subscript> chalcogenide glasses

The effects of melt temperature T _i and quenching rate V _i on the structure and optical properties of As₂S₃ glasses is studied. It is found that the glass band gap increases with T _i and V _i , whereas a decrease is observed in the glass density, refractive index (from 2.71 to 2.48), and two-photon absorption coefficient (from 0.37 to 0.15 cm/MW), which is accompanied by an increase in the optical-breakdown damage threshold.     

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.     

A study of recombination centers in irradiated <Emphasis Type="Italic">p</Emphasis>-Si crystals

p-Si samples irradiated with 8-Mev electrons are studied. It is suggested that the multicomponent V₃+O or V₂+O₂ complexes are not recombination centers on the basis of an analysis of the dependences of the minority-carrier lifetime τ, the resistivity ρ, the concentration p, and the Hall mobility μ_H on the temperature of isochronous annealing T_ann. Deep donors with energy levels at ΔE_i=E_v+0.40 eV and the V₃+O₃ and the V₃+O₂ complexes affect the values of μ_H and τ. The curves of isochronous annealing are used to determine the annealing-activation energies E_ann for defects such as K centers, interstitial carbon atoms C_i, the V+B and V₂+O₂ complexes, divacancies V₂, and defects with a level at ΔE_i=E_v+0.20 eV. These energies were found to be equal to E_ann=0.9, 0.25, 1.6, 2, 1.54, and 2.33 eV, respectively.     

Diffusion of interstitial magnesium in dislocation-free silicon

The diffusion of magnesium impurity in the temperature range T = 600–800°C in dislocation-free single-crystal silicon wafers of p-type conductivity is studied. The surface layer of the wafer doped with magnesium by the ion implantation technique serves as the diffusion source. Implantation is carried out at an ion energy of 150 keV at doses of 5 × 10¹⁴ and 2 × 10¹⁵ cm^–2. The diffusion coefficient of interstitial magnesium donor centers (D _i ) is determined by measuring the depth of the p–n junction, which is formed in the sample due to annealing during the time t at a given T. As a result of the study, the dependence D _i (T) is found for the first time. The data show that the diffusion process occurs mainly by the interstitial mechanism.     

Role of electrostatic fluctuations in doped semiconductors upon the transition from band to hopping conduction (by the example of <Emphasis Type="Italic">p</Emphasis>-Ge:Ga)

The electrostatic model of ionization equilibrium between hydrogen-like acceptors and v-band holes in crystalline covalent p-type semiconductors is developed. The range of applicability of the model is the entire insulator side of the insulator–metal (Mott) phase transition. The density of the spatial distribution of acceptor- and donor-impurity atoms and holes over a crystal was assumed to be Poissonian and the fluctuations of their electrostatic potential energy, to be Gaussian. The model takes into account the effect of a decrease in the energy of affinity of an ionized acceptor to a v-band hole due to Debye–Hückel ion screening by both free v-band holes and localized holes hopping over charge states (0) and (–1) of acceptors in the acceptor band. All donors are in charge state (+1) and are not directly involved in the screening, but ensure the total electroneutrality of a sample. In the quasiclassical approximation, analytical expressions for the root-mean-square fluctuation of the v-band hole energy W_p and effective acceptor bandwidth W_a are obtained. In calculating W_a, only fluctuations caused by the Coulomb interaction between two nearest point charges (impurity ions and holes) are taken into account. It is shown that W_p is lower than W_a, since electrostatic fluctuations do not manifest themselves on scales smaller than the average de Broglie wavelength of a free hole. The delocalization threshold for v-band holes is determined as the sum of the diffusive-percolation threshold and exchange energy of holes. The concentration of free v-band holes is calculated at the temperature T_j of the transition from dc band conductivity to conductivity implemented via hopping over acceptor states, which is determined from the virial theorem. The dependence of the differential energy of the thermal ionization of acceptors at the temperature 3T_j/2 on their concentration N and degree of compensation K (the ratio between the donor and acceptor concentrations) is determined. Good quantitative agreement between the results of the calculation and data on the series of neutron transmutation doped p-Ge samples is obtained up to the Mott transition without using any fitting parameters.     

Special features of annealing of radiation defects in irradiated <Emphasis Type="Italic">p</Emphasis>-Si crystals

p-Si single crystals grown by the Czochralski method were studied; the hole concentration in these crystals was p = 6 × 10¹³ cm^?3. The samples were irradiated with 8-MeV electrons at 300 K and were then annealed isochronously in the temperature range T _ann = 100–500°C. The studies were carried out using the Hall method in the temperature range of 77–300 K. It is shown that annealing of divacancies occurs via their transformation into the B _s V ₂ complexes. This complex introduces the energy level located at E _v + 0.22 eV into the band gap and is annealed out in the temperature range of 360–440°C. It is assumed that defects with the level E _v + 0.2 eV that anneal out in the temperature range T _ann = 340–450°C are multicomponent complexes and contain the atoms of the doping and background impurities.     

Electrical and thermoelectric properties of <Emphasis Type="Italic">p</Emphasis>-Ag<Subscript>2</Subscript>Te

Temperature dependences of the Hall coefficient R, electrical conductivity σ, and thermopower α₀ are investigated in the range of 4–300 K. The specific features observed in temperature dependences R(T), σ(T), and α₀(T) are interpreted in the context of a model with two types of charge carriers.