Influence of oxygen partial pressure on the properties of undoped InOx films deposited at room temperature by rf-PERTE

Transparent and conductive/semiconductive undoped indium oxide (InO_x) thin films were deposited at room temperature. The deposition technique used is the radio frequency (rf) plasma enhanced reactive thermal evaporation (rf-PERTE) of indium (In) in the presence of oxygen. The influence of oxygen partial pressure on the properties of these films is presented. The oxygen partial pressure varied between 3 × 10^?2 and 1.3 × 10^?1 Pa. Undoped InO_x films, 100 nm thick, deposited at the oxygen partial pressure of 6 × 10^?2 Pa show a conductive behaviour, exhibit an average visible transmittance of 81%, a band gap around 2.7 eV and an electrical conductivity of about 1100 (Ω cm)^?1. For oxygen pressures greater than 6 × 10^?2 Pa, semiconductive films are obtained, maintaining the visible transmittance. Films deposited at lower pressures are conductive but dark. From XPS data, films deposited at an oxygen partial pressure of 6 × 10^?2 Pa show the highest amount of oxygen in the film surface and the lowest ratio between oxygen in the oxide crystalline and amorphous phases.     

Conductivity percolation transition of Agx(Ge0.25Se0.75)100−x glasses

The ionic conductivity of several chalcogenide glasses increases abruptly with mobile ion addition from values typical of insulating materials (10⁻¹⁶–10⁻¹⁴ Ω⁻¹ cm⁻¹) to values of fast ionic conductors (10⁻⁷–10⁻¹ Ω⁻¹ cm⁻¹). This change is produced in a limited concentration range pointing to a percolation process. In a previous work [M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259] the transition from semiconductor to fast ionic conductor of Ag_x(Ge_0.25Se_0.75)_100−x glasses was detected at x¹ ≅ 10 at.% in the form of a steep change in the conductivity. Ag_x(Ge_0.25Se_0.75)_100−x glasses with x ⩽ 25 at.%, prepared by a melt quenching method, are investigated by impedance spectroscopy in the frequency range 5 Hz–2 MHz at different temperatures, T, from room temperature to 363 K and by DC measurements at room temperature. The conductivity of the glasses, σ, was obtained as a function of silver concentration and temperature. For x ⩾ 10 at.% our results are in agreement with those reported by Kawasaki et al. [M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259]. The percolation transition was observed in the range 7 ⩽ x ⩽ 8. The temperature dependence of the ionic conductivity follows an Arrhenius type equation σ = (σ_o/T) · exp(−E_σ/kT). The activation energy of the ionic conductivity, E_σ, and the pre-exponential term, σ_o, are calculated. The results are discussed in connection with other chalcogenide and chalcohalide systems and linked with the glass structures.     

Influence of hydrogen on the thermomechanical properties of a-CNx:H and a-CNx films deposited by glow discharge and ion beam assisted deposition

The coefficient of thermal expansion (CTE), Young’s modulus, Poisson’s ratio, stress and hardness of a-CN_x and a-CN_x:H were investigated as a function of nitrogen concentration. Hydrogenated films were prepared by glow discharge, GD, and unhydrogenated films were prepared by ion beam assisted deposition, IBAD. Using nanohardness measurements and the thermally induced bending technique, it was possible to extract separately, Young’s modulus and Poisson’s ratio. A strong influence of hydrogen, in a-CN_x:H films, was observed on the CTE, which reaches about ∼9 × 10⁻⁶ C⁻¹, close to that of graphite (∼8 × 10⁻⁶ C⁻¹) for nitrogen concentration as low as 5 at.%. On the other hand, the CTE of unhydrogenated films increases with nitrogen concentration at a much lower rate, reaching 5.5 × 10⁻⁶ C⁻¹ for 33 at.% nitrogen.     

Electrical conductivity and single oscillator model properties of amorphous CuSe semiconductor thin film

The structural, optical dispersion and electrical conductivity properties of the CuSe thin film have been investigated using X-ray diffraction, electrical and optical characterization methods. X-ray diffraction results indicate that CuSe thin film has an amorphous structure. The electrical conductivity of the CuSe film increases with increasing temperature. The activation energy and room temperature conductivity values of the film were found to be 1.32 meV and 3.89 × 10⁻³ S/cm, respectively. The refractive index dispersion of the thin film obeys the single oscillator model and single oscillator parameters were determined. The E_o, n_∞, and S_o values of the CuSe thin film were found to be 5.08 eV, 3.55 and 1.92 × 10¹⁴ m⁻², respectively. The obtained results suggest that CuSe film is an amorphous semiconductor.     

Optical and electrical properties of as-prepared and annealed (Se80Te20)100 − xAgx (0 ≤ x ≤ 4) ultra-thin films

Optical and electrical properties of the (Se₈₀Te₂₀)_100 ? xAg_x (0 ≤ x ≤ 4) ultra-thin films have been studied. The ultra-thin films were prepared by thermal evaporation of the bulk samples. Thin films were annealed below glass transition temperature (328 K) and in between glass transition temperature and crystallization temperature (343 K). Thin films annealed at 343 K showed crystallization peaks for Se–Te–Ag phases in the XRD spectra. The transmission and reflection of as-prepared and annealed ultra-thin films were obtained in the 300–1100 nm spectral region. The optical band gap has been calculated from the transmission and reflection data. The refractive index has been calculated by the measured reflection data. It has been found that the optical band gap increases, but the refractive index, extinction coefficient, real and imaginary dielectric constant decrease with increase in Ag content. The optical band gap and refractive index show the variation in their values with increase in the annealing temperature. The extinction coefficient increases with increasing annealing temperature. The surface morphology of ultra-thin films has been determined using a scanning electron microscope (SEM). The measured dc conductivity, under a vacuum of 10^? 5 mbar, showed thermally activated conduction with single activation energy in the measured temperature range (288–358 K) and it followed Meyer–Neldel rule. The dc activation energy decreases with increase in Ag content in pristine and annealed films. The results have been analyzed on the bases of thermal annealing effects in the chalcogenide thin films.     

Ion transport studies in CuI-doped silver borovanadate glassy system

In the present report, ionic transport properties and microstructural investigations of superionic materials in a cost-effective glassy system xCuI–(100 ? x)[2Ag₂O–0.7V₂O₅–0.3B₂O₃], where x = 30, 40, 45, 50 and 60, have been described. The temperature dependent electrical conductivity studies were carried out by ac impedance analysis. The microstructure of the materials studied by SEM indicated the presence of dispersed CuO and AgI micro-crystals in the silver oxysalt glass matrix. High room temperature electrical conductivity of 3.58 × 10^?3 S cm^?1 and low activation energy of 0.24 eV were obtained for the best conducting composition. The ac impedance data were analyzed using impedance and modulus formalisms. These materials show extremely high t_i value of 0.999 and the ionic conductivity is apparently due to Ag⁺ ions only. The use of two glass formers helped to form materials with higher T_g, higher thermal stability and better ionic transport properties.     

Electrical conductivity and dielectric relaxation in non-crystalline films of tungsten trioxide

Amorphous tungsten trioxide (a-WO₃) thin films were prepared by thermal evaporation technique. The electrical conductivity and dielectric properties of the prepared films have been investigated in the frequency range from 100 Hz to 100 kHz and in the temperature range 293–393 K. In spite of the absence of the dielectric loss peaks, application of the dielectric modulus formulism gives a simple method for evaluating the activation energy of the dielectric relaxation. The frequency dependence of σ(ω) follows the Jonscher’s universal dynamic law with the relation σ(ω) = σ_dc + Aω^s, where s is the frequency exponent. The conductivity in the direct regime, σ_dc, is described by the small polaron model. The electrical conductivity and dielectric properties show that Hunt’s model is well adapted to a-WO₃ films.     

Structure and electrical properties of nitrided NbN–TiN sol–gel derived films

The results of investigations of electrical conductivity and the structure of NbN–TiN thin films in a different NbN/TiN molar ratio are presented in this work. Sol–gel derived xNb₂O₅?(100?x)TiO₂ coatings (where x = 100, 90, 80, 70, 60, 50, 40, 0 mol%) were nitrided at 1200 °C to obtain NbN–TiN films. The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrical conductivity was measured with a conventional four-terminal method in the temperature range of 5–280 K. The NbN–TiN samples exhibited a negative temperature coefficient of resistivity. The positive temperature coefficient of resistivity was observed only for the x = 0 sample. The results of conductivity versus temperature may be described on the grounds of a model proposed for a weakly disordered system. The film thickness effect on the superconducting properties was studied for x = 80 and x = 100 samples. The superconducting transition was not observed in all samples, the exception was x = 80 sample, 1050 nm in thickness. It is not clear, why all x = 100 samples do not exhibit superconducting transition in resistivity measurements. It seems to be possible, that the Josephson junction formation between NbN grains could be blocked by non-superconducting phases present in these samples.     

Molecular dynamics simulation of ternary glasses Li2S–P2S5–LiI

The structure of superionic glasses in ternary systems x(0.4Li₂S–0.6P₂S₅)–(1 − x)LiI and x(0.5Li₂S–0.5P₂S₅)–(1 − x)LiI (x = 0.9, 0.75) has been studied by molecular dynamics (MD) simulations. The configurations obtained by MD were analyzed by graph theory. Phosphorus is surrounded by sulfur and iodine atoms. Li is surrounded by sulfurs alone and LiI clusters are not present as speculated by earlier spectroscopic reports. The equilibrium configuration is made up of (Li, S) and (P, S, I) rich regions which creates wide channels for Li⁺ movement. Reported variations of glass transition temperature (T_g) and ionic conductivity (σ) with LiI addition are explained based on the simulation results.     

Preparation and ionic conductivity of transparent glass−ceramics containing a large quantity of lithium−mica

In order to crystallize a large quantity of the lithium?mica in glass?ceramics, 5.1 mass% MgF₂ was added to the starting materials of the parent glasses having chemical compositions of Li_(1+x)Mg₃AlSi_3(1+x)O_10+6.5xF₂ (x = 0.5 and 1.0). Transparent glass?ceramics, in which a large quantity of lithium?mica with particle size of <50 nm was separated, could be prepared from the MgF₂-added parent glass with x = 0.5. While the parent glass, which had a binodal phase separation structure, did not exhibit electrical conductivity, the transparent glass–ceramic was given conductivity by the formation of an interlocking structure of mica. As the separated mica formed a tighter interlocking structure, the conductivity increased and reached a value of 2.0 × 10^?3 S/cm at 600 °C. The MgF₂-added parent glass with x = 1.0 was not transparent because of coarse spinodal phase separation. The conductivity was 4.3 × 10^?4 S/cm at 600 °C but was significantly decreased by the separation of mica.     

Fabrication and characterization of transparent conductive ZnO:Al thin films prepared by direct current magnetron sputtering with highly conductive ZnO(ZnAl2O4) ceramic target

Using a highly conductive ZnO(ZnAl₂O₄) ceramic target, c-axis-oriented transparent conductive ZnO:Al₂O₃ (ZAO) thin films were prepared on glass sheet substrates by direct current planar magnetron sputtering. The structural, electrical and optical properties of the films (deposited at different temperatures and annealed at 400°C in vacuum) were characterized with several techniques. The experimental results show that the electrical resistivity of films deposited at 320°C is 2.67×10⁻⁴ Ω cm and can be further reduced to as low as 1.5×10⁻⁴ Ω cm by annealing at 400°C for 2 h in a vacuum pressure of 10⁻⁵ Torr. ZAO thin films deposited at room temperature have flaky crystallites with an average grain size of ∼100 nm; however those deposited at 320°C have tetrahedron grains with an average grain size of ∼150 nm. By increasing the deposition temperature or the post-deposition vacuum annealing, the carrier concentration of ZAO thin films increases, and the absorption edge in the transmission spectra shifts toward the shorter wavelength side (blue shift).     

Characteristics of heteroepitaxial Cu2−xMnxO/Nb–SrTiO3 p–n junction

Mn-doped cuprous oxide Cu_2?xMn_xO (CMO), where x = 0.03, is a p-type diluted magnetic semiconductor (DMS) with Curie temperature above room temperature [M. Wei, N. Braddon, et al., Appl. Phys. Lett. 86 (2005) 0725141; Y.L. Liu, S. Harrington, et al., Appl. Phys. Lett. 87 (2005) 222108]. We have grown CMO (x = 0.03) thin films of about 200 nm thick on n-type semiconducting (0 0 1)Nb–SrTiO₃(NSTO) single crystal substrates by pulsed laser deposition. Cubic crystalline phases of CMO layers were obtained in a narrow deposition pressure window of about 20 mTorr at growth temperature of 650 °C. X-ray diffraction and TEM studies of these heterostructures reveal a cube-on-cube epitaxial relationship of [CMO]₀₀₁/[NSTO]₀₀₁. All the oxide p–n junctions with the size of 500 × 500 μm were fabricated by the shadow masking technique. These junctions show highly asymmetric I–V characteristics. The rectification ratio at room temperature is about 10³ at ±2 V. Leakage current density of 10^?4 A cm^?2 at ?1 V is observed. No apparent junction breakdown is recorded at reverse bias voltages down to ?5 V. From the 1/C²–V plots, the forward bias turn on voltage is ～1.4 V. Clear junction current rectifying property is maintained at up to 200 °C. Our results have demonstrated that epitaxial CMO films can be fabricated on lattice matched cubic substrates. They are suitable DMS for above room temperature spintronic junction applications.     

Mixed transition-ion effect in the glass system: Fe2O3-MnO-TeO2

In earlier studies on phosphate and tellurite glasses containing vanadium and iron oxides, non-linear variation of physical properties as functions of the ratios of the transition ions (V/V + Fe) were observed. The most striking effect was observed with electrical conductivity, where a 3 orders of magnitude reduction in conductivity was observed at a V/V + Fe ratio of ~ 0.4. The effect was termed Mixed Transition-ion Effect or MTE. In phosphate glasses, however, MTE was not observed when one of the transition ions was manganese. It was concluded that Mn does not contribute to conduction in these glasses. In the present study, we demonstrate a mixed transition ion effect in tellurite glasses containing MnO and Fe₂O₃ (xFe₂O₃(0.2 ? x) MnO0.8TeO₂ with x varying from 0 to 0.2). A maximum in the property at an intermediate composition (x = 8.5 mol%), was observed in DC resistivity, activation energy, molar volume etc. Mossbauer and optical absorption (UV–VIS–NIR) measurements were performed on these glasses and the transport mechanism has been identified to be hopping of small polarons between Fe^3 + (Mn^3 +) and Fe^2 + (Mn^2 +) sites.     

Properties and structure of (1 − x)Li2O–xNa2O–Al2O3–4SiO2 glass systems

(1 − x)Li₂O–xNa₂O–Al₂O₃–4SiO₂ glasses were studied for the progressive percentage substitution of Na₂O for Li₂O at the constant mole of Al₂O₃ and SiO₂. The crystallization temperature at the exothermic peak increased from 898 to 939 °C when the Na₂O content increases from 0 to 0.6 mol. The coefficient of thermal expansion and density of these as-quenched glasses increase from 6.54 × 10⁻⁶ °C⁻¹ to 10.1 × 10⁻⁶ °C⁻¹ and 2.378 g cm⁻³ to 2.533 g cm⁻³ when the Na₂O content increases from 0 to 0.4 mol, respectively. The electrical resistivity has a maximum value at Na₂O · (Li₂O + Na₂O)⁻¹ = 0.4. The activation energy of crystallization decreases from 444 to 284 kJ mol⁻¹ when the Na₂O content increased from 0 to 0.4 mol. Moreover, the activation energy increases from 284 kJ mol⁻¹ to 446 kJ mol⁻¹ when the Na₂O content increased from 0.4 to 0.6 mol. The FT-IR spectra show that the symmetric stretching mode of the SiO₄ tetrahedra (1035–1054 cm⁻¹) and AlO₄ octahedra (713–763 cm⁻¹) exhibiting that the network structure is built by SiO₄ tetrahedra and AlO₄.     

Electrical properties of A2.6+xTi1.4−xCd(PO4)3.4−x (A = Li,K; x = 0.0–1.0) phosphate glasses

Electrical properties of A_2.6+xTi_1.4−xCd(PO₄)_3.4−x (A = Li, K; x = 0.0–1.0) phosphate glasses are investigated over a frequency range from 42 Hz to 1 MHz at different temperatures. Impedance spectroscopy is used to separate the bulk conductivity from electrode effect of electrical conductivity data. The bulk dc conductivity is Arrhenius activated, with activation energies and pre-exponential factors following the Meyer–Neldel rule. The real part of ac conductivity shows universal power law feature. The variation of dielectric constant with frequency is attributed to ion diffusion and polarization occurring in the phosphate glasses. The frequency dependent imaginary part of electric modulus M″(ω) plot shows non-Debye feature in conductivity relaxation. The Kohlrausch–Williams–Watts stretched exponential function was used to describe the modulus spectra and the stretching exponent β is found to be temperature independent. Scaling in M″(ω) shows that the electrical relaxation mechanisms are independent of temperature for given composition at different temperatures.     

Electrical properties of a-Se85−xTe15Snx thin films

Se–Te alloys are an important system of chalcogenide glasses from application point of view. The incorporation of Sn additive alters the electrical properties of these alloys. The conductivity measurements have been done on the thin films of a-Se_85−xTe₁₅Sn_x (x = 0, 2, 4, 6 and 10 at.%) deposited using vacuum evaporation technique. Both dark (σ_d) and photoconductivity (σ_ph) show a maximum for x = 6 at.% of Sn, which, decreases on further Sn addition to the binary Se–Te alloy. The dark activation energy (ΔE_d) shows a minimum for x = 2 at.% of Sn, but increases on further Sn addition. There is a sharp decrease in photosensitivity (σ_ph/σ_d) on Sn addition to Se₈₅Te₁₅ alloy. The charge carrier concentration (n_σ) calculated with the help of dc conductivity measurements also show a maximum at x = 6 at.% of Sn. The results are explained on the basis of increase in the density of localized states present in the mobility gap on Sn incorporation.     

Preparation and characterization of telluride glasses

Chalcogenide bulk glasses Ge₂₀Se_80?xTe_x for x ∈ (0, 10) have been prepared by systematic replacement of Se by Te. Selected glasses have been doped with Er and Pr, and all systems have been characterized by transmission spectroscopy, measurements of dc electrical conductivity and low-temperature photoluminescence. Absorption coefficient has been derived from measured transmittance and estimated reflectance. Both absorption and low-temperature photoluminescence spectra reveal shifts of absorption edge and/or dominant luminescence band to longer wavelength due to Te → Se substitution. Arrhenius plots of dc electrical conductivity, in the temperature range 300–450 K, are characterized by activation energies roughly equal to the half of the optical gap. Arrhenius plots for temperatures below 300 K yield much lower activation energies. The dominant low-temperature luminescence band centered at about half the band gap energy starts to quench above 200 K and a new band appears at 900 nm. The band at 900 nm, due to band to band transitions, overwhelms the spectra at room temperature. Systems doped with Er exhibit a strong luminescence due to ⁴I_13/2 → I_15/2 transition of Er³⁺ ion at 1539 nm, and Pr doped samples exhibit a relatively weak luminescence peak at 1590 nm, which we tentatively assign to ³F₃ → ³H₄ transition of Pr³⁺ ion.     

EPR and optical studies of Mn2+ ions in Li2O–Na2O–B2O3 glasses – An evidence of mixed alkali effect

EPR and optical absorption spectra of 0.5 mol% MnO₂ doped xLi₂O–(30 − x)Na₂O–69.5B₂O₃ (5 ⩽ x ⩽ 25) glasses have been studied. The EPR spectra exhibit resonance signals characteristic of Mn²⁺ ions. The resonance signal at g ≅ 2.0 is due to Mn²⁺ ions in an environment close to octahedral symmetry, whereas the resonances at g ≅ 4.3 and g ≅ 3.3 are attributed to the rhombic surroundings of the Mn²⁺ ions. The ionic character (A), the number of spins participating in resonance (N), optical band gap energies (E_opt) and Urbach energies (ΔE) show the mixed alkali effect (MAE) with composition. The present study gives an indication that the size of alkalis we choose, is also an important contributing factor in showing the MAE. The variation of N with temperature obeys the Boltzmann law. The optical absorption spectra show a single broad band at ∼21 000 cm⁻¹ corresponding to the transition ⁶A_1g(S) → ⁴T_1g(G) which exhibits a blue shift with x. The theoretical values of optical basicity (Λ_th) have also been evaluated.     

Electrical characterizations of silver chalcogenide glasses

We present a study of the electrical properties of silver chalcogenide glasses ‘40AgI’–30Ag₂S–30GeS₂, 45AgI–27.5Ag₂S–27.5GeS₂ and 50AgI–25Ag₂S–25GeS₂ in the 77–400 K temperature and the 20 Hz to 1 MHz frequency ranges. In our temperature range, a large variation of the real permittivity is observed, in relation with an electrodes polarization effect. As the amount of silver iodide increases in the Ag₂S–GeS₂ matrix, the glass transition temperature and the activation energies decrease, the electrical conductivity increases and reaches 4 Ω⁻¹ m⁻¹ at room temperature for the glass with 50% AgI. The study of the conductivity shows a behavior due to a high ionic conductivity, thermally activated with E_dc = 0.21 eV, E₁ = 0.075 eV (40AgI–30Ag₂S–30GeS₂, 45AgI–27.5Ag₂S–27.5GeS₂), E_dc = 0.17 eV, E₁ = 0.055 eV for 50AgI–25Ag₂S–25GeS₂. For these glasses, we have seen three conductivity regimes. The first two terms are thermally activated. The third term cannot be actually clearly identified because either it is thermally activated with a very low activation energy and frequency dependent, or it is almost non-thermally activated and frequency dependent.     

Experimental characterization of amorphous As–Se–Sb alloys

Thin films were thermally evaporated from ingot pieces of the As₃₀Se_70−xSb_x (with 2.5 ⩽ x ⩽ 17.5 at.%) glasses under vacuum of ∼10⁻⁵ Torr. Increasing Sb content was found to affect the thermal and optical properties of these films. Non-direct electronic transition was found to be responsible for the photon absorption inside the investigated films. The chemical bond approach has been applied successfully to interpret the decrease of the glass optical gap with increasing Sb content. Decreasing the thermal stability of the As₃₀Se_70−xSb_x specimen by increasing Sb content is responsible for occurring the amorphous–crystalline process at lower temperatures. Binary As₂Se₃ and Sb₂Se₃ phases are the main components of the stoichiometric As₃₀Se₆₀Sb₁₀ composition.