Electrical and optical properties of MoSe2 films prepared by r.f. magnetron sputtering

The electrical resistivity of MoSe₂ films prepared by r.f. magnetron sputtering was measured between 300 and 10 K. The main sputtering parameter governing the physical properties of the films was found to be the substrate temperature T_sub. The room temperature resistivity of the as-sputtered films increased from $\text{1.7 × 10}{}^{-1}\text{Ω}\text{cm}$(T_sub = -70 °C) to $\text{1.4 × 10}{}^1\text{Ω}\text{cm}\text{(T}{}_{\mathrm{<mtext>sub</mtext>}}\text{= 150 °}\text{C}\text{)}$. A check of the thermo-electrical response showed that the majority charge carriers are holes except for films deposited at T_sub = 150 °C which are n type. Hall effect measurements indicated very low Hall mobilities (3–5 cm² V^-1 s^-1). Thermal annealing increased the room temperature resistivities by more than one order of magnitude for the specimens sputtered at a low substrate temperature. The optical properties were weakly influenced by the process conditions. The optical gap was determined to be 1.06 eV.     

Pyroelectric and electrooptic effects in LiInS2 and LiInSe2

The electrooptic (r_ij and pyroelectric ($\text{dP}{}_{\mathrm{s}}\text{dT}$) coefficients of two new chalcogenides LiInS₂ and LiInSe₂ have been determined. The same linear relationship between $\text{dP}{}_{\mathrm{s}}\text{dT}$ and r₃₃, based on the anharmonic oscillator model, which has been found to hold for a wide variety of oxides also holds for these materials.     

Control of the growth-induced magnetic anisotropy in ferrimagnetic garnet films grown by liquid-phase epitaxy

The influence of melt composition and growth temperature T_g on the growth-induced magnetic anisotropy constant K^g_u in Bi substituted LPE ferrimagnetic garnet films has been investigated. In films grown from Bi₂O₃ based and from $\text{PbO}\text{B}{}_2\text{O}{}_3$ based melts the value of K^g_u increased upon decreasing T_g, associated with an increasing incorporation of B1. However, at given Bi content the films grown from Bi₂O₃ based fluxes generally exhibited smaller values of K^g_u than those from $\text{PbO}\text{B}{}_2\text{O}{}_3$ based melts. The addition of small amounts of Ca²⁺ to the Bi₂O₃ based melts resulted in (Y,Bi)-garnet films changing from n- to p-type electrical conductivity at a minimum value of K^g_u. Additions of Si⁴⁺ to a $\text{PbO}\text{B}{}_2\text{O}{}_3$ based melt reduced the uniaxial magnetic anisotropy of La-YIG-Ga films accompanied by a change from p- to n-type conductivity. Similarly, (Gd,Bi)-garnet films from $\text{PbO}\text{B}{}_2\text{O}{}_3$ based melts changed from n- to p-type conductivity upon decreasing T_g where K^g_u changes sign from negative to positive. From the temperature dependence of the electrical conductivity the electron and hole concentrations in some of these films were estimated suggesting that the sign change of K^g_u in the investigated (Gd,Bi)-garnet films and the minimum of K^g_u in the investigated (Y,Bi)-garnet films and (La,Y)-garnet films occur at donor-acceptor compensation. This experimental evidence led us to conclude that the growth induced magnetic anisotropy may be correlated with the presence of donor and acceptor centers in the garnet structure.     

4.6 Some optical properties of thin Cd3As2 films

The transmission of vacuum deposited Cd₃As₂ films has been measured in the spectral range 0.6–15 ωm. The absorption coefficient α has been calculated from the data for crystalline films obtained at substrate temperature T_s = 440 K and also for amorphous films obtained on substrates at T_s = 300?393 K. The position of the low-energy absorption edge in the spectral range 0.1–0.3 eV is discussed and the value Δ_g^opt = 0.06 eV for indirect transion Γ → Γ₁₅ is obtained for crystalline films. For amorphous films, a shift of the minimum absorption edge to $\text{Δ}\text{E}{}_{\mathrm{g}}{}^{\mathrm{opt}}\text{? 0.4?0.5}\text{eV}$ was observed. For higher energies two direct transitions have been established: the first at E₁ = 0.9 eV in the photon energy range 0.9–1.2 eV.and the second at E₂ = 1.2?1.3 eV in the photon energy range 1.3?1.6 eV. The last value is in good agreement with the value calculated by Lin Chung and with reflectivity data; it corresponds to the transition Γ₁₅ → X₁ in the hypothetical band model of Lin Chung. Differences in the reflective index values for crystalline and amorphous Cd₃ As₂ are also discussed.     

Nonstoichiometry and phase transition in NdMnO3

The nonstoichiometry of neodymium manganite NdMnO₃ phase has been studied by a gravimetric method at 1273 K. The oxygen partial pressures were controlled by using flowing $\text{O}{}_2\text{Ar}$ or $\text{H}{}_2\text{CO}{}_2$ mixtures. The existence of hyperstoichiometric phase and the homogeneity range of the phase NdMnO_3+x from x = 0 to 0.065 at 1273 K under various oxygen partial pressures has been revealed. And also the effect of oxygen nonstoichiometry on the phase transition at high temperature has been studied by electrical conductivity and DTA measurements. The transition temperature decreases with increasing oxygen content.     

Optimization of growth conditions for ZnTe films

An optimum substrate temperature T₀ of 573 K was found for the preparation of ZnTe thin films by vacuum deposition. Films prepared at this temperature are stoichiometric and exhibit a maximum electrical resistivity. The observations are explained on the basis of the Vincett-Barlow-Roberts theory which predicts a value of nearly 0.33 for $\text{T}{}_0\text{T}{}_{\mathrm{<mtext>b</mtext>}}$, where T_b is the boiling point.     

Some optical and electrical properties of the As2Se3−xTex system

Some optical and electrical properties of the As₂Se_3?xTe_x system (0 ≤ x ≤ 0.15) have been studied. The value of the optical gap decreases from 1.74 eV to 1.59 eV. The slope of the absorption edge changes, so that value of E_o from the relation α ～ exp ($\text{h}\text{?}\text{ω}\text{E}{}_{\mathrm{o}}$) exhibits a maximum value in neighbourhood of composition As₂Se_2.9Te_0.1. The d.c. and a.c. conductivities have been studied. The value of the activation energy E_el ～ 1.8 eV is almost unchanged up to x = 0.15. The a.c. conductivity fulfils the relation $\text{б(ω) ～ ω}{}^{\mathrm{<mtext>s</mtext>}}$ where s ? 1 and 1.1. at frequencies f < 1 kHz and f > 1 kHz respectively.     

Deposition of silica films by the oxidation of silane in oxygen: I: The kinetics and physicochemical model of the process

The kinetics of growth of silica films were studied in a resistance-heated quartz tube reactor over the temperature range 250–600°C under atmospheric pressure in the presence of argon. The concentration of silane was varied from 0.01 to 0.1 mol.% and the ratio [O₂]/[SiH₄] ranged from 5 to 300. The composition of the gaseous mixture was analysed at the outlet of the reactor by infrared spectroscopy.It has been shown that the oxidation of silane starts in the gaseous phase and follows a branched chain reaction. For our ignition limits the following expression was obtained. $\text{[O}{}_2\text{]}\text{[SiH}{}_4\text{]}$= 2.6×10⁵ exp (-$\text{9700}\text{RT}$)Film growth is only observed when silane combustion occurs in the gaseos phase. A physicochemical model of the process is suggested which assumes that at least two intermediate products containing silicon are formed during the chain reaction in the gaseous phase. One of these, P₁, diffuses to the surface and the SiO₂ film results from a heterogeneous reaction. The second product, P₂, is transformed into a white deposit of SiO₂ at the outlet of the reactor.     

A new method of sulfur vapor pressure control and its application to the V-S system

A new apparatus for obtaining partial pressure of sulfur was devised by using liquid sulfur and carrier gas of nitrogen. This successfully works in the range of ～10^?4 to ～1 atm of Ps₂ at any temperature higher than about 400°C up to about 1000°C. As an application of it, the equilibrium study of V-S system was done at 800°C. Two phases V₃S₄ and V₅S₈, each having homogeneity range, were found while no existence of the V₂S₃ phase could be detected thermodynamically and X-ray crystallogrpahically at this temperature. The standard free energy of following reaction, $\text{VS}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{+}\text{2}\text{15}$$\text{S}{}_2\text{=}\text{VS}\text{8}\text{5}$, was calculated by considering the transition-Ps₂ and the activities and · ΔG° (1073K) = ?1150 cal·mole^?1was obtained with an estimated uncertainty of ± 100 cal to the above reaction.     

Studies on the formation of a solid solution compound in Li2O impregnated CuAl2O4 catalyst

Li₂O impregnated CuAl₂O₄ samples were calcined at two temperatures (673 K and 1173 K). X-ray analysis showed that the lattice parameter of the spinel phase decreased with increasing calcining temperature. X-ray studies on the solid solution series Cu_1?x$\text{Li}{}_{\mathrm{<mtext>x</mtext><mtext>2</mtext>}}$$\text{Al}{}_{\mathrm{<mtext>2+</mtext><mtext>x</mtext><mtext>2</mtext>}}$ O₄ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) showed that the lattice parameter decreased with increasing concentration of lithium and from the comparison of lattice parameters it appears that Cu_0.84 Li_0.08 Al_2.08 O₄ is formed in the impregnated sample calcined at 673 K. Electrical resistivity measurements are consistent with the formation of a solid solution compound.     

The precipitation and characterization of cobalt (II) oxalate tetrahydrate

Mixing of solutions of Co²⁺ and C₂O₄^2? ions can lead to crystallization of cobalt (II) oxalate both as a tetrahydrate and a dihydrate. Up to about 60°C acicular crystallites of a greyish pink tetrahydrate initially result. In a period depending mainly on the temperature and to a lesser extent on the degree of supersaturation and the $\text{[}\text{C}{}_2\text{O}{}_4{}^{\mathrm{2?}}\text{]}\text{[}\text{Co}{}^{\mathrm{2+}}\text{]}$ ratio, the precipitated tetrahydrate recrystallizes to β-cobalt (II) oxalate, a bright pink dihydrate. The two cobalt (II) oxalates have been distinguished by means of X-ray diffraction (XRD), (electron)microscopy and thermal analysis. The d-values and relative peak heights of the most pronounced maxima of the XRD-pattern of the tetrahydrate are presented, as these are not available in standard reference works.     

Optical absorption and photoconductovity in amorphous indium selenide thin films

Optical absorption coefficient and photoconductivity measurements were performed on amorphous indium selenide thin films produced by vacuum evaporation. The dependence of the absorption coefficient α on the photon energy ħω at the edge of the absorption band is well described by the relation $\text{α}\text{h}\text{̵}\text{ω = B(}\text{h}\text{̵}\text{ω - E}{}_{\mathrm{<mtext>opt</mtext>}}\text{)}{}^2$ where B is a quality factor and E_opt is the optical band gap. The steady state photoconductivity as a function of the temperature and light intensity is tentatively interpreted in terms of a simple kinetic model proposed by Weiser et al.     

Temperature dependence of the growth-induced anisotropy of Ge-substituted garnet films by resonance techniques

Ferromagnetic resonance techniques have been used to measure the growth-and stress-induced uniaxial magnetic anisotropy of LPE grown garnet films of the nominal composition Y_1.64Eu_0.10Lu_0.30Ca_0.96Ge_0.96Fe_4.04O₁₂ from 130°K to near the Curie temperature, 455°K. The longitudinal and transverse resonance fields were measured prior and subsequent to an anneal at 1453°K. The growth-induced anisotropy, K^G_u ～?0.03 × 10⁴ ergs/cm³ at 300°K, was found to be approximately 10% of the stress-induced anisotropy, and of opposite sign for these tensile films. The associated anisotropy field, $\text{2}\text{K}{}^{\mathrm{G}}{}_{\mathrm{u}}\text{M}$ exhibited a reversal of slope, with a maximum occurring at 310°K. A model which assumes the existence of a surface anisotropy is proposed to explain these observations. The temperature dependence of the cubic anisotropy, the magnetization, the linewidth, and the exchange constant have been measured also.     

Thermodynamic stability of barium zirconates

The standard free energy of formation of BaO·ZrO₂ prepared by solid state reaction at elevated temperatures was determined in the temperature range 937 K to 1150 K using barium fluoride as solid electrolyte and barium hexa ferrite as the reference electrode. The data obtained are represented by the equation

${}^{\mathrm{\Delta G}}\text{BaO·ZrO}{}_2\text{= ?105164 + 49.37T (±340J)}$

where ΔG_BaO·ZrO₂ is the free energy of formation of BaO·ZrO₂ from BaO and ZrO₂.     

Morphology of ion-plated titanium and aluminum films deposited at various substrate temperatures

Titanium and aluminum films were deposited in an argon atmosphere of 2.7×10^-2 mbar onto a circular metal sheet. A radial a.c. current flowing through the substrate generated a temperature ranging from about 1000°C at the center to about 100°C at the edge of the sheet. With the two metals evaporated by means of an electron beam gun it was thus possible to cover a $\text{T}\text{T}{}_{\mathrm{<mtext>m</mtext>}}$ range from 0.19 to 1.36 (T_m is the melting point of the deposited metal in kelvins). Simultaneously a negative bias potential of up tp 11 kV was applied to the substrate.The resulting deposits of 6–18 μm thickness were examined by scanning electron microscopy. With no bias potential applied the microstructures of titanium described in the literature were essentially confirmed. There were differences, however: (a) below $\text{T}\text{T}{}_{\mathrm{<mtext>m</mtext>}}\text{=0.20}$ we observed a zone of fine fibrous grains extending through the entire deposit; (2) the columnar structure of zone 2 changed from tapered crystallites with a scaled surface at lower temperatures to smooth fibrous crystallites with faceted ends at $\text{T}\text{T}{}_{\mathrm{<mtext>m</mtext>}}\text{>0.4}$; (3) the equiaxed grains of zone 3 existed only on the top half or less of the cross section and tended to have rounded edges at $\text{T}\text{T}{}_{\mathrm{<mtext>m</mtext>}}\text{>0.55.}$The application of a bias potential did not appear to alter the morphology of the deposits, in spite of the expected competition between particel removal and deposition in the ion plating process. The typical microstructures and the corresponding transition zones were observed at increasingly lower temperatures with increasing bias potential however. The high temperature onset $\text{T}{}_2\text{T}{}_{\mathrm{<mtext>m</mtext>}}$ of columnar grains was thus shifted from 0.49 with no bias to 0.38 and 0.33 with a 5 kV and a 10 kV bias potential, respectively.The corresponding experiments with aluminum permitted a closer examination of the morphology of films which were formed by ion plating at substrate temperatures above the melting point of the deposited metal.     

High temperature standard gibbs free energy determinations for Co-o systems by e.m.f. measurements. A statistical approach to evaluate the reliability of the current methods

The standard Gibbs free energies of the following reactions: $\text{Co +}\text{1}\text{2}\text{O}{}_2\text{? CoO}$ (1) and $\text{3CoO +}\text{1}\text{2}\text{O}{}_2\text{? Co}{}_3\text{O}{}_4$ (2) have been calculated from the e.m.f. measurements carried out using a simple compartment solid state galvanic cell, as well for the reaction: 3Co + 20₂ ? Co₃O₄ (3) The temperature-equilibrium oxygen partial pressure relationship has been evaluated. A phase diagram for Co, CoO and Co₃O₄ is proposed in the temperature range 600–900 °C. A statistical analysis on reported ΔG_f⁰ values at 1100 K for reaction (1) has been performed and the results are discussed.     

The magnetic properties of Fe(OH)2

Ferrous hydroxide free from oxidation was prepared by precipitation from an aqueous solution in an inert gas atmosphere and measured for magnetic susceptibility and magnetization. The antiferromagnetic transition was observed at 34 K, where the susceptibility showed a sharp maximum. The magnetization at 4.2 K increased as the external field increased, and reached 3.6 $\text{μ}{}_{\mathrm{B}}$ per ion at 75 KOe, which is about 90 % of the saturation magnetization of Fe⁺⁺ ion. By analyzing the magnetization curve and the Mössbauer effect, the magnetic structure in an ordered state was deduced as follows: the moments within the layer are arrayed parallel, while those between the adjacent layers antiparallel. The spin easy axis lies in the c-plane. The quadrupole splitting was 3.00 mm/sec at 90 K and showed little temperature dependence between 4.2 K and room temperature. The energy state of Fe⁺⁺ ion in Fe(OH)₂ was specified by the orbital singlet 5A_1g derived from the sign and the magnitude of $\text{1}\text{2}\text{e}{}^2\text{qQ}$. The difference in the magnetic property between Fe(OH)₂ and FeCl₂ was discussed from the energy state of Fe⁺⁺ ion.     

Models for the effect of chemisorbed gases on the electrical resistance of metal films

Three hypotheses are considered for the proportionate change of resistance $\text{ΔR}\text{R}{}_1$ when a continuous metal film of resistance R₁ adsorbs gas: (a) a loss of metallic properties at the surface; (b) a change in the electron scattering parameter at the surface; and (c) a change in the electron concentration throughout the film. Two models of the film structure are used to compare the predictions of these hypothesis with our experimental data for the systems $\text{O}{}_2\text{Er}$, $\text{CO}\text{Er}$, $\text{CO}\text{Ti}$ and $\text{O}{}_2\text{Ti}$. The flat plate model accounts for neither R₁ nor $\text{ΔR}\text{R}{}_1$. A simple version of the columnar model accounts for $\text{ΔR}\text{R}{}_1$ by either (a) or (b) for the first three systems and by (c) for the fourth, but it does not explain all the properties of the clean films. The linear approximation given by Mola and Heras for the columnar model of Mayadas and Shatzkes accounts satisfactorily for R₁ and $\text{ΔR}\text{R}{}_1$ for $\text{O}{}_2\text{Er}$, $\text{CO}\text{Er}$ and $\text{CO}\text{Ti}$ by increases in the electron scattering coefficient at the grain boundaries. It does not account for $\text{ΔR}\text{R}{}_1$ with $\text{O}{}_2\text{Ti}$ because that system incorporates gas into the metal lattice.     

Electrical properties of ferromagnetic Gd3−xS4 near its Curie temperature

Nonstoichiometric $\text{Gd}{}_{\mathrm{<mtext>3?</mtext><mtext>x</mtext>}}\text{S}{}_4\text{(0 <}\text{x}\text{<}\text{1}\text{3}\text{)}$, which exhibits a metallic behavior, was obtained by heating an insulating Gd₂S₃ at various temperatures under a vacuum. Electrical and magnetic properties of the samples obtained have been investigated from 4.2 K to 300 K. A maximum in resistivity was observed in the curve of resistivity vs. temperature. The temperature T_P, at which the maximum emerged, was very close to the Curie temperature T_C for the sample. An increase in resistivity at T_P, Δ_?, is proportional to ?_P exp ($\text{E}\text{k}{}_{\mathrm{B}}\text{T}{}_{\mathrm{P}}$), where ?_P is the resistivity obtained by the extrapolation from the linear portion to T_P in the ? vs. T curve. Using the relationship obtained, a model based on the formation of magnetic polaron was proposed for this system.