Electrical and gas-sensitive properties of nanostructured SnO<Subscript>2</Subscript>:ZrO<Subscript>2</Subscript> semiconductor films

The results of a study of the effect of atomic composition of metal-oxide semiconductor composites SnO₂:ZrO₂ on the surface morphology, polycrystal grain size, resistivity, concentration, free-carrier mobility, and gas-sensitive properties of thin films (0.5–2.5 μm) are presented. Films SnO₂ with ZrO₂ additives (the Zr content was varied from 0.5 to 4.6 at %) are grown by reactive ion-beam sputtering of metal targets of different compositions in a controlled Ar + O₂ atmosphere. Using transmission electron microscopy, atomic-force microscopy, and high-resolution transmission electron microscopy, it is experimentally shown that, as the Zr content increases in the SnO₂:ZrO₂ film composition, the polycrystal grain size decreases from 45 to 10 nm, the free-carrier concentration decreases almost by four orders of magnitude and the mobility increases by approximately nine times. As the Zr content increases in the SnO₂:ZrO₂ film from 0.5 to 4.6 at %, the temperature of maximum gas sensitivity of films to such gases as ethanol, isopropyl alcohol, and acetone decreases by 100–190°C.     

Relaxation of optically stimulated resistance of thin SnO<Subscript>2</Subscript> films

The results of investigation of the effect of irradiation with photons from an L5013VC violet light-emitting diode (LED) with the wavelength of 400 nm and power of 76 mW on the resistance of the sensitive layer of SnO₂-based test structures of gas sensors in air before and after the high-temperature stabilizing annealing are presented. The features in the variation of the SnO₂-layer resistance in time are established when the LED is switched on and off.     

Photovoltaic properties of interfaces of organic films of substituted perylene with TiO<Subscript>2</Subscript> and SnO<Subscript>2</Subscript> surfaces

The photovoltaic effect has been detected and studied in structures based on ultrathin vacuum-deposited organic films of perylene-3,4,9,10-tetracarboxylic acid dianhydride on the titanium and tin dioxide surfaces. The interfacial potential barrier shape in these structures is studied by low-energy electron total current spectroscopy. Changes in the surface potential upon exposure to visible light are recorded in situ using an electron-beam probe with energies from 0 to 25 eV. The photovoltage is detected at incident photon energies of 1.5–2.5 eV, which corresponds to the organic film absorption range and simultaneously to the transmission band of titanium and tin dioxides. An analysis of the spectral distributions and transient responses shows that two components of the observed photovoltage can be distinguished. The relation of one of the components to the excitation of interband transitions in the organic film and another component to electronic transitions involving interfacial energy states are discussed.     

Impedance Response and Dielectric Relaxation in Liquid-Phase Sintered Zn<Subscript>2</Subscript>SnO<Subscript>4</Subscript>-SnO<Subscript>2</Subscript> Ceramics

Impedance/admittance and dielectric spectroscopy were used to investigate the effect of temperature on the electrical response of liquid-phase sintered Zn₂SnO₄-SnO₂ ceramics. The measurements were performed over a wide frequency range (100 Hz to 10 MHz) at different temperatures. The real and the imaginary part of the complex impedance traced semicircles in the complex plane. The resistance and the capacitance of bulk and grain-boundary regions were determined by modeling the experimental results using several equivalent circuits taking into account bulk deep trap states. Admittance complex diagrams were also determined in order to understand better the conduction mechanisms occurring in the polycrystalline Zn₂SnO₄-SnO₂ system.     

Absorption Properties of Hybrid SnO<Subscript>2</Subscript> Nanocrystal-Carbon Nanotube Structures

Tin oxide (SnO₂) nanocrystals of a few nanometers are of great interest for electronic applications. Here we present a mini-arc plasma method to produce aerosol tin oxide nanocrystals at atmospheric pressure. The product SnO₂ nanocrystals are then assembled onto the external surface of carbon nanotubes (CNTs) to form hybrid SnO₂–CNT structures. The absorption properties of both the SnO₂ nanocrystals and the SnO₂–CNT hybrid structures have been characterized. Quantum size effects have been observed for as-produced SnO₂ nanocrystals. The intrinsic nanoparticle size selection during the assembly process results in a blue shift of the absorption spectrum for hybrid nanostructures.     

Electron–Electron Interactions in Sb-Doped SnO<Subscript>2</Subscript> Thin Films

Electrical conductivity and Hall-effect measurements on undoped and Sb-doped SnO₂ thin films prepared by the sol–gel technique were carried out as a function of temperature (55 K to 300 K). Structural characterizations of the films were performed by atomic force microscopy (AFM) and x-ray diffraction (XRD). A doping-induced metal–insulator transition (MIT) was observed. On the metallic side of the transition, the experimental data were interpreted in terms of electron–electron interactions (EEI). The existence of EEI was confirmed by excellent agreement between theoretical and experimental data. The experimental data on the insulator side of the transition were analyzed in terms of variable-range hopping (VRH) conduction. A complete set of parameters describing the properties of the localized electrons, including hopping energy, hopping distance, and the value of the density of states at the Fermi level, was determined.     

Nitrogen-doped Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> films for nonvolatile memory

Nitrogen-doped Ge₂Sb₂Te₅ (GST) films for nonvolatile memories were prepared by reactive sputtering with a GST alloy target. Doped nitrogen content was determined by using x-ray photoelectron spectroscopy (XPS). The crystallization behavior of the films was investigated by analyzing x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Results show that nitrogen doping increases crystallization temperature, crystallization-activation energy, and phase transformation temperature from fcc to hexagonal (hex) structure. Doped nitrogen probably exists in the grain vacancies or grain boundaries and suppresses grain growth. The electrical properties of the films were studied by analyzing the optical band gap and the dependence of the resistivity on the annealing temperature. The optical band gap of the nitrogen-doped GST film is slightly larger than that of the pure GST film. Energy band theory is used to analyze the effect of doped nitrogen on electrical properties of GST films. Studies reveal that nitrogen doping increases resistivity and produces three relatively stable resistivity states in the plot of resistivity versus annealing temperature, which makes GST-based multilevel storage possible. Current-voltage (I-V) characteristics of the devices show that nitrogen doping increases the memory’s dynamic resistance, which reduces writing current from milliampere to microampere.     

Switching effects in the films based on Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript>

Experimental results on the switching effects related to the phase transitions in Ge₂Sb₂Te₅ in the presence of external voltage or laser irradiation are presented. An electron model of the reversible switching is discussed.     

Thermoelectric Properties of SnO<Subscript>2</Subscript> Ceramics Doped with Sb and Zn

Polycrystalline SnO₂-based samples (Sn_0.97−x Sb_0.03Zn _x O₂, x = 0, 0.01, 0.03) were prepared by solid-state reactions. The thermoelectric properties of SnO₂ doped with Sb and Zn were investigated from 300 K to 1100 K. X-ray diffraction (XRD) analysis revealed all XRD peaks of all the samples as identical to the rutile structure, except for the x = 0.03 sample, which had a small amount of Zn₂SbO₄ as a secondary phase. We found that the power factor of the x = 0.03 sample was significantly improved due to the simultaneous increase in the electrical conductivity and the Seebeck coefficient. A power factor value of ∼2 × 10⁻⁴ W m⁻¹ K⁻² was obtained for the x = 0.03 sample at 1060 K, 126% higher than that for the undoped sample.     

Optical properties of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin films

In₂Se₃ films are produced by ion-beam evaporation at substrate temperatures of 313 and 623 K. As the target, In₂Se₃ single crystals grown by the vertical Bridgman method are used. The composition and structure of the crystals and films are determined by the X-ray spectral analysis and X-ray diffraction techniques, respectively. It is established that the crystals and films crystallize with the formation of a hexagonal structure. The band gap and refractive index of the In₂Se₃ films are determined from the transmittance and reflectance spectra. It is found that, as the substrate temperature is increased, the band gap increases.     

Phase transitions in thin Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> chalcogenide films according to Raman spectroscopy data

Data on the Raman spectra of thin Ge₂Sb₂Te₅ chalcogenide semiconductor films are reported. The study is performed with the purpose of determining the temperatures of phase transitions initiated by laser radiation.     

Current-voltage characteristics of MnGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> single crystals

The current-voltage characteristics of MnGa₂Se₄ single crystals have been investigated. The measurements were performed in the range of electric fields from the level at which the Ohm law is satisfied to 10 V/cm, and in the temperature range 300–400 K. The data obtained are interpreted within the theory of injection-contact phenomena and the theory of field ionization of traps due to the Poole-Frenkel effect.     

Current-voltage characteristics of ZnGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> compound polycrystals

Current-voltage characteristics of the In-ZnGa₂Se₄-In structure have been studied in the temperature range of 90–335 K. Based on the data calculated for the concentration of three trap types in ZnGa₂Se₄, the values N _t = 1.4 × 10¹³, 8.2 × 10¹², and 2.6 × 10¹² cm⁻³ are obtained. The contact region transparency D _k ^*= 10⁻⁵, surface recombination velocity S _k = 0.65 m/s, and carrier lifetime τ = 1.5 × 10⁻⁴ s were determined. It was found that the current transmission mechanism in electric fields weaker than 10³ V/cm is caused by monopolar carrier injection.     

Optical and structural properties of Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> thin films obtained by pulsed laser deposition in a H<Subscript>2</Subscript>S atmosphere with subsequent annealing in a N<Subscript>2</Subscript> atmosphere

The structural and optical properties of Cu₂ZnSnS₄ thin-film layers formed by reactive pulsed laser deposition in a H₂S atmosphere at room temperature with the use of a Cu metal target and a Zn–Sn alloy target are studied in relation to the parameters of annealing in a N₂ atmosphere.     

Phase composition of films in a Bi-S system and formation of Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> films with different substructures

Electron diffraction was used to study the formation of phases in a Bi-S system. It is shown that the phase with composition Bi₂S₃ appears as result of consecutive deposition of Bi and S irrespective of the order of deposition; this phase also appears in the case of simultaneous deposition of components. The films formed at room temperature are amorphous. Amorphous thin Bi₂S₃ films are stable at room temperature and crystallize at temperatures in the region of ～423 K. The conditions for formation of Bi₂S₃ films with different substructures are established.     

Magnetic susceptibility of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> alloys

The magnetic susceptibility of Czochralski-grown single crystals of Bi₂Te₃-Sb₂Te₃ alloys containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, or 100 mol % Sb₂Te₃ has been investigated. The magnetic susceptibility of these crystals was determined at the temperature T = 291 K and the magnetic field H oriented parallel (χ_‖) and perpendicularly (χ_⊥) to the trigonal crystallographic axis C ₃. A complicated concentration dependence of the anisotropy of magnetic susceptibility χ_‖/χ_⊥ has been revealed. The crystals with the free carrier concentration p ≈ 5 × 10¹⁹ cm^?3 do not exhibit anisotropy of magnetic susceptibility. The transition to the isotropic magnetic state occurs for the compositions characterized by a significantly increased (from 200 to 300 meV) optical bandgap.     

Thermoelectric power of Bi<Subscript>92</Subscript>Sb<Subscript>8</Subscript> and Bi<Subscript>85</Subscript>Sb<Subscript>15</Subscript> thin films

The results of studying the galvanomagnetic and thermoelectric properties of thin block Bi₉₂Sb₈ and Bi₈₅Sb₁₅ films on mica and polyimide substrates are presented. The method used for measuring the thermoelectric power allowed us to study the temperature dependence the thermoelectric power, without introducing additional deformations into the substrate–film system. A significant difference in the temperature dependences of the galvanomagnetic and thermoelectric properties of films on mica and polyimide is found. The free charge-carrier concentrations and mobilities in the films on mica and polyimide and levels of the chemical potential for electrons and holes are calculated within the two-band approximation. The difference in the charge-carrier parameters for films on mica and polyimide is associated with strains in the film–substrate system.     

Optical Properties of K<Subscript>2</Subscript>O-Li<Subscript>2</Subscript>O-WO<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> Glasses: Evidence of Mixed Alkali Effect

Glass with compositions xK₂O-(30 ? x)Li₂O-10WO₃-60B₂O₃ for 0 ≤ x ≤ 30 mol.% have been prepared using the normal melt quenching technique. The optical reflection and absorption spectra were recorded at room temperature in the wavelength range 300–800 nm. From the absorption edge studies, the values of the optical band gap (E _opt) and Urbach energy (ΔE) have been evaluated. The values of E _opt and ΔE vary non-linearly with composition parameter, showing the mixed alkali effect. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple Di-Domenico model.     

Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Superlattices Grown by Nanoalloying

In this work, Bi₂Te₃-Sb₂Te₃ superlattices were prepared by the nanoalloying approach. Very thin layers of Bi, Sb, and Te were deposited on cold substrates, rebuilding the crystal structure of V₂VI₃ compounds. Nanoalloyed super- lattices consisting of alternating Bi₂Te₃ and Sb₂Te₃ layers were grown with a thickness of 9 nm for the individual layers. The as-grown layers were annealed under different conditions to optimize the thermoelectric parameters. The obtained layers were investigated in their as-grown and annealed states using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy, transmission electron microscopy (TEM), and electrical measurements. A lower limit of the elemental layer thickness was found to have c-orientation. Pure nanoalloyed Sb₂Te₃ layers were p-type as expected; however, it was impossible to synthesize p-type Bi₂Te₃ layers. Hence the Bi₂Te₃-Sb₂Te₃ superlattices consisting of alternating n- and p-type layers showed poor thermoelectric properties.     

Microwave Dielectric Properties of ZnO-2TiO<Subscript>2</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> Ceramics with BaCu (B<Subscript>2</Subscript>O<Subscript>5</Subscript>) Addition

The influence of BaCu(B₂O₅) (BCB) addition on the sintering temperature and microwave dielectric properties of ZnO-2TiO₂-Nb₂O₅ (ZTN) ceramic has been investigated using dilatometry, x-ray diffraction, scanning electron microscopy, and microwave dielectric measurements. A small amount of BCB addition to ZTN can lower the sintering temperature from 1100°C to 900°C. The reduced sintering temperature was attributed to the formation of the BCB liquid phase. The ZTN ceramics containing 3.0 wt.% BCB sintered at 900°C for 2 h have good microwave dielectric properties of Q × f = 19,002 GHz (at 6.48 GHz), ε _r = 45.8 and τ _f  = 23.2 ppm/°C, which suggests that the ceramics can be applied in multilayer microwave devices, provided that Ag compatibility exists.