Semiconducting and transport properties of mono- and polycrystalline nickel oxide

On the basis of high-temperature studies of electrical conductivity of poly- and monocrystalline nickel oxide and making use of the results of studies on chemical diffusion coefficients obtained by several authors and in the present work, the structure of point defects in nickel oxide has been considered. It has been shown that in the temperature range 900 to 1300° C and at the oxygen pressure from 10^–4 to 1 atm there occur in nickel oxide singly- and doubly-ionized cationic vacancies in comparable quantities.Assuming such to be the model of defect structure in Ni_1–yO, the equilibrium concentration of cationic vacancies as a function of temperature has been calculated for the oxygen pressure of 1 atm. It has been shown that the results obtained are in good agreement with the results of direct determinations of concentration of cationic vacancies in NiO.     

Conductivity measurements on cobalt and nickel oxides in highly-enriched oxygen atmospheres

The pressure dependencies of the conductivities of nickel and cobalt monoxides have been investigated in the range 10^–2 to 7.5 × 10² atmospheres at temperatures in the region of 1000° C. Conductivity saturation occurs in CoO due to the formation of Co₃O₄, and the experimental data correlates well with a model for the separation of the higher phase. NiO exhibits no saturation effect in the pressure range investigated and the p^1/5 dependence of the conductivity at 950° C with this oxide above 0.1 atm is interpreted in terms of the formation of singly ionised vacancies.     

Electrical property and defect structure of lanthanum-doped polycrystalline barium titanate

Electrical conductivity of two types of lanthanum-doped barium titanote ceramics with different dopant levels was measured at temperatures between 900 and 1250° C andP _{O 2}from 10^–5 to 1 atm. The activation energies of the conduction for the two are interpreted in terms of the formation energy of ionized oxygen vacancies even in such a highP _{O 2}region. This fact is in contrast with a well -known controlled -valency model proposed for rare- earth -doped semi-conducting perovskites. In a lightly lanthanum-doped specimen, semiconduction achieved at elevated temperatures is retained on cooling the specimen to room temperature, whereas in a heavily doped specimen, the resultant high-temperature semiconduction changed to insulation on cooling. The former behaviour on cooling is successfully explained by a metastabilization of oxygen vacancies accompanied by electrons formed at elevated temperatures.     

Single-crystal growth of La2−2x Sr1+2x Mn2O7 under pressure

Large single crystals of La_2–2x Sr_1+2x Mn₂O₇ (x=0.3 to 0.525) have been prepared under controlled atmospheric conditions. The crystals were grown by the floating-zone technique in an image furnace under a mixed oxygen/argon atmosphere pressurized to 6–8×10⁵ Pa. Rectangular single crystals with sizes up to 50×9×4 mm³ have been obtained. The phase-purity, composition, and quality of the crystals were analyzed by X-ray diffraction and electron probe microanalysis. The magnetic behavior is found to be sensitive to the composition of the atmosphere during growth.     

Sol-gel synthesis of nickel oxide thin films and their characterization

Sol-gel method has been employed for the synthesis of nanocrystalline nickel oxide (NiO). The NiO powders were sintered at 400-700 °C for 1 h in an air. Thin films of sintered powders were prepared on glass substrate using spin coating technique and changes in the structural, morphological, electrical and optical properties were studied. The structural and microstructural properties of nickel oxide films were studied by means of X-ray diffraction and field emission scanning electron microscopy. Structural analysis shows that all the films are crystallized in the cubic phase and present a random orientation. Surface morphology of the nickel oxide film consists of nanocrystalline grains with uniform coverage of the substrate surface with randomly oriented morphology. The electrical conductivity showed the semiconducting nature with room temperature electrical conductivity increased from 10^− 4 to 10^− 2 (Ω cm) ^− 1 after sintering. The electron carrier concentration (n) and mobility (μ) of NiO films annealed at 400-700 °C were estimated to be of the order of 1.30 to 3.75 × 10¹⁹ cm^− 3 and 1.98 to 4.20 × 10^− 5 cm² V^− 1 s^− 1.The decrease in the band gap energy from 3.86 to 3.47 eV was observed for NiO sintered between 400 and 700 °C. These mean that the optical quality of NiO films is improved by sintering.     

Effect of pressure on the coking yields of coal tar pitches

Pyrolysis of five coal tar pitches with wide ranging characteristics, made from the same coal tar precursor, has been studied under nitrogen pressures of 10⁵, 50×10⁵, 90×10⁵ and 160×10⁵ Pa, at a temperature of 550 °C. The residues were further heat-treated to 900 °C to obtain the ultimate normal (10⁵ Pa) and pressure coking yields of these pitches. The literature states that for pitches with relatively lower softening points the carbonization pressure not only increases the coking yield but also lowers the temperature at which the pyrolysis is complete. This is seen to hold true for the present set of pitches, having a much wider range of softening points. Further, one of the pitches, earlier reported by us to be a good preforming pitch for carbon-carbon composites, gave an ultimate coking yield of 88% on subjection to a nitrogen pressure of 160×10⁵ Pa at 550 °C followed by ambient pressure carbonization to 900 °C. It thus appears that a carbonization pressure of 160×10⁵ Pa for a suitable preforming pitch can act as a reasonably good alternative to the expensive hot isostatic pressure impregnation carbonization technique employed in the production of carbon-carbon composites.     

Mechanisms of carriers transport in Ni/n-SiC, Ti/n-SiC ohmic contacts

A mechanism of carriers transport through metal-semiconductor interface created by nickel or titanium-based ohmic contacts on Si-face n-type 4H-SiC is presented herein. The mechanism was observed within the temperature range of 20 °C ÷ 300 °C which are typical for devices operating at high current density and at poor cooling conditions. It was found that carriers transport depends strongly on concentration of dopants in the epitaxial layer. The carriers transport has thermionic emission nature for low dopant concentration of 5×10¹⁶ cm^?3. The thermionic emission was identified for moderate dopant concentration of 5×10¹⁷ cm^?3 at temperatures higher than 200 °C. Below 200 °C, the field emission dominates (for the same doping level of 5×10¹⁷ cm^?3). High dopant concentration of 5×10¹⁸ cm^?3 leads to almost pure field emission transport within the whole investigated temperature range.     

Oxygen partial pressure dependence of the properties of MgZnO thin films during annealing

Undoped n-type MgZnO films were deposited on c-plane sapphire substrates by molecular-beam epitaxy and subsequently annealed in O₂ at different pressures. After annealing at 3.03 × 10⁵ Pa, oxygen content in the annealed films show increases and the films transform into p-type conduction. However, the decreases of oxygen content and the increases of electron concentration were obtained while the films annealed at 1.01 × 10⁵ Pa or 2.05 × 10⁻³ Pa. The changes in intensity of the emission peak located at 2.270 eV are similar to the changes of the oxygen content in the films annealed at different pressures. According to the defect levels and the relationship between photoluminescence spectra and annealing condition, it was suggested that this emission peak was related to interstitial oxygen (O_i). The obtained p-type conduction is attributed to that the O_i acceptor can compensate oxygen vacancy and interstitial zinc donor.     

Microstructure tuning of epitaxial BaTiO3 − x thin films grown using laser molecular-beam epitaxy by varying the oxygen pressure

Microstructural properties are found to be variant in the BaTiO_3 − x films grown on SrTiO₃(001) substrate under various oxygen pressures from 2 × 10^− 2 Pa to 2 × 10^− 5 Pa by laser molecular-beam epitaxy. Transmission electron microscopic studies reveal that the predominant defects in the films change from threading dislocations into (111) planar defects (i.e. stacking faults and nanotwins) by lowering the oxygen pressure. High density of these defects was observed in the BaTiO_3 − x film prepared at the oxygen pressure of 2 × 10^− 5 Pa, which shows metallic behavior. The relationships between oxygen pressure, microstructure, and electrical properties are established on the basis of oxygen deficiency. The formation of nanotwins in highly oxygen-deficient BaTiO_3 − x epitaxial thin films results from accommodating excess oxygen vacancies induced by lowering oxygen pressure.     

Effect of oxygen partial pressure on the electrical and optical properties of highly (200) oriented p-type Ni<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O films by DC sputtering

Thin films of NiO (bunsenite) with (200) preferential orientation were synthesized on glass substrates by direct current sputtering technique in Ar+O₂ atmosphere. Nanostructural properties of the NiO films were investigated by X-ray diffraction and also by atomic force microscopic (AFM) studies. Electrical and optical properties of the deposited films were investigated as a function of different partial pressure of oxygen in the sputtering gas mixture during deposition. The films showed p-type electrical conduction and the conductivity depends on the partial pressure of oxygen. The electrical conductivity (σ_RT) was found to be .0615 S cm⁻¹ for films deposited with 100% O₂ and its value sharply decreased with the decrease the partial pressure of O₂; for example σ_RT for 50% O₂ was 6.139 × 10⁻⁵ S cm^-1. The mechanism of the origin of p-type electrical conductivity in the NiO film is discussed from the viewpoint of nickel or oxygen vacancies, which generate holes and electrons respectively. X-ray photoelectron spectroscopic studies supported the above argument. Corresponding optical properties showed that the transparency decreases with increasing oxygen partial pressure and the bandgap also decreases.     

Inductively coupled RF oxygen plasma characterization by optical emission spectroscopy

Optical emission spectroscopy was applied for the characterization of inductively coupled RF oxygen plasma at pressures between 10 and 300 Pa. The plasma was generated with an RF generator at a frequency of 27.12 MHz and output power of 300 W. Spectra were measured in the range 200-1100 nm by an optical spectrometer. At high pressure, the main spectral features observed were the wavelengths of the atomic oxygen transitions at 777.2 and 844.6 nm. Molecular oxygen band at 762 nm was observed as well. The atomic emission intensity showed a maximum when the pressure was about 75 Pa, while molecular band intensity increased monotonically as the total pressure increased. On decreasing the oxygen pressure, other atomic and molecular features appeared in spectra, such as H atomic lines, molecular OH band, and O₂⁺ band. The behavior of spectral features was explained by collision phenomena in the ionized gas.     

Radiation-stimulated point defects in Li2B4O7 single crystals

Using thermally stimulated luminescence, optical absorption, and x-ray crystallographic analysis methods, it is established that oxygen vacancies appear near Li ions in lithium tetraborate single crystals after the crystals are irradiated with (Sr:Y)-90 β particles with dose 7×10⁵ Gy and that these vacancies are capable of trapping one or two electrons, which are freed at temperatures 565 and 630 K, respectively. It is shown that hole localization on oxygen ions, joining triply and quadruply coordinated boron ions near Li vacancies, and freeing of the holes at 414 K are possible. Pis’ma Zh. Tekh. Fiz. 25, 78–83 (September 12, 1999)     

High-temperature electrical conductivity of aluminium nitride

The electrical conductivity of hot-pressed polycrystalline aluminium nitride doped with oxygen and beryllium was measured as a function of temperature from 800 to 1200° C and over a range of nitrogen partial pressure from 10² to 10⁵ Pa. The effect of beryllium dopant, the independence of conductivity from nitrogen partial pressure, and the observed activation energy suggested extrinsic electronic species or aluminium vacancies as charge carriers. Polarization measurements made with one electrode blocking to ionic species indicated that the aluminium nitride with oxygen impurity was an extrinsic electronic conductor.     

Electrical conductivity and defect structure of polycrystalline tin dioxide doped with antimony oxide

Electrical conductivity () of tin dioxide doped with antimony has been measured as functions of temperature and oxygen partial pressure (p0_2> ). Variation of electrical conductivity is explained by assuming that the antimony oxide forms a substitutional solid solution and doubly ionized oxygen vacancies are predominant defects. Above –10^–5 atm oxygen partial pressure antimony ions are present predominantly in the pentavalent state in tin dioxide lattice. However, it is converted to the trivalent state below this oxygen partial pressure accompanied by a sudden rise in conductivity.     

Effects of annealing atmosphere on thermoelectric signals from ZnO films

Zinc oxide epilayer films were grown on vicinal cut sapphire substrates by pulsed laser deposition with in situ annealing oxygen pressures varied from 0 to 10 × 10³ Pa. The best crystalline quality was obtained for ZnO layer with annealing oxygen pressure of 6 × 10³ Pa. Laser induced thermoelectric voltage (LITV) were observed along the tilting angle orientation of the substrate when the pulsed KrF excimer laser of 248 nm were irradiated on the films. The largest LITV signal was measured for the film grown at 6 × 10³ Pa annealing oxygen pressure. According to the measured LITV signals, Seebeck anisotropy was evaluated and was found to range from 3 to 12 μV/K for ZnO films annealed at different oxygen pressures from 2 to 10 × 10³ Pa. It is suggested that oxygen ambient plays an important role in the electronic properties of the ZnO films.     

Room temperature ferromagnetism in Zn0.99La0.01O and pure ZnO nanoparticles

Room temperature ferromagnetism (RTFM) was observed in both La-doped and pure ZnO nanoparticles synthesized by the sol–gel method. RTFM is intrinsic according to the results of X-ray diffraction and X-ray photoelectron spectroscopy. The saturation magnetization (M_S), the remnant magnetization at zero field and coercive field are 5 × 10⁻³, 7 × 10⁻⁴ emu g⁻¹, 100 Oe for Zn_0.99La_0.01O nanoparticles and 1.5 × 10⁻⁴, 1 × 10⁻⁵ emu g⁻¹, 50 Oe for pure ZnO nanoparticles, respectively. The magnetization is enhanced greatly by doping of La. Furthermore, the M_S of Zn_0.99La_0.01O nanoparticles decreases from 0.005 to 0.001 emu g⁻¹ as the annealing temperature increases from 500 to 700 °C. The doping of La introduces more oxygen vacancies into ZnO. The decrease of annealing temperature also produces more oxygen vacancies in La-doped ZnO. These results indicate that the origin of the RTFM is related to oxygen vacancies.     

Effects of argon-ion bombardment on the properties of the surface layer in spinel crystals of different composition

Ion-induced photon emission (IPE) during bombardment of magnesium aluminate spinel crystals MgO·nAl₂O₃ by 20 keV Ar⁺ ions was studied. The dependence of the yield of particles in specific excited states on the fluence of incident ions in the range of (0.1–1.6)×10¹⁷ ions/cm² was measured. It was shown that yield of magnesium and aluminum atoms and ions in most excited states do not depend (or slightly depend) on the fluence of ion bombardment. An exception was found for yields of Mg⁺ ions in the 4s ²S excited state and Al atoms in the 5p ²P⁰ excited state leading to emission lines at 292.8 and 669.6 nm, respectively. The yield of particles in these excited states drastically decreases at the start of ion bombardment. Analysis of these results and published data on the bombardment-induced surface modification of spinel crystals allows to elucidate the role of crystal structure and chemical bonding in the formation of some excited states. The dependence of excited state yield (except of Mg and Al indicated above) from spinel crystals of different composition MgO·nAl₂O₃ (n=1.0, 1.5, 2.0, and 2.5) does not reflect quantitatively the variation of the calculated bulk concentration of constituent atoms in these targets.     

Effect of process parameters on the growth and properties of impurity-doped zinc oxide transparent conducting thin films by RF magnetron sputtering

Zinc oxide transparent conducting thin films co-doped with aluminum and ruthenium were grown on polyethylene terephthalate substrates at room temperature using RF magnetron sputtering. The crystal growth and physical properties of the films were investigated with respect to the variation of discharge power density from 1.5 to 6.1 W/cm² and sputtering pressure from 0.13 to 2.0 Pa. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) showed that the films grown with 3.6 W/cm² power density and sputtering pressure of 0.4 Pa had the best crystallinity and larger pyramid-like grains. The optimized electrical resistivity had a lowest measured value of about 9 × 10⁻⁴ Ω cm. The low carrier mobilities of the films (3-8.9 cm² V⁻¹ s⁻¹) have been discussed in terms of what is believed to be the dominant effect of ionized impurity scattering, but in addition chemisorption of oxygen on the film surface and effect of grain boundaries are also thought to be significant. The transmittances of the films in the visible range are greater than 80%, while the optical band gaps are in the order of 3.337-3.382 eV.     

Indium tin oxide films deposited by thermionic-enhanced DC magnetron sputtering on unheated polyethylene terephthalate polymer substrate

Indium tin oxide thin films were deposited onto polyethylene terephthalate substrates via thermionic enhanced DC magnetron sputtering at low substrate temperatures. The structural, optical and electrical properties of these films are methodically investigated. The results show that compared with traditional sputtering, the films deposited with thermionic emission exhibit higher crystallinity, and their optical and electrical properties are also improved. Indium tin oxide films deposited by utilizing thermionic emission exhibit an average visible transmittance of 80% and an electrical resistivity of 4.5 × 10⁻⁴ Ω cm, while films made without thermionic emission present an average visible transmittance of 74% and an electrical resistivity of 1.7 × 10⁻³ Ω cm.     

Catalytic-free growth of ZnGa2O4 nanowires on amorphous carbon layers

Zinc gallate (ZnGa₂O₄) nanowires were directly grown on the amorphous carbon-coated silicon substrates using a facile chemical vapor deposition method without any metal catalysts. The growth mechanism can be attributed to a self-organization vapor-liquid-solid (VLS) process. The amorphous carbon layer plays an important role in the nucleation and growth process of the ZnGa₂O₄ nanowires. The photoluminescence (PL) of the nanowires shows a broad, strong green emission band centered at 532 nm and a weak UV emission band at 381 nm, which can be attributed to a large amount of ionized oxygen vacancies and the combination of Ga³⁺ ions with free electrons in coordinated oxygen vacancies, respectively.