D.c. properties of ZnO thin films prepared by r.f. magnetron sputtering

In the development of ZnO-based varistors the electrical properties of ZnO/Bi₂O₃ junctions and of the two individual oxides are being investigated. Following our recent work on a.c. conductivity in Al---ZnO---Al sandwich structures we currently report d.c. measurements. The structures were prepared by r.f. magnetron sputtering in an argon/oxygen mixture in the ratio 4:1. Capacitance-voltage data confirm that the Al/ZnO interface does not form a Schottky barrier and measurements of the dependence of capacitance on film thickness indicate that the relative permittivity of the films is approximately 9.7. With increasing voltage the current density changed from an ohmic to a power-law dependence with exponent n≈3. Furthermore measurements of current density as a function of reciprocal temperature showed a linear dependence above about 240 K, with a very low activation energy below this temperature consistent with a hopping process. The higher temperature results may be explained assuming a room-temperature electron concentration n₀ and space-charge-limited conductivity, dominated by traps exponentially distributed with energy E below the conduction band edge according to N = N₀exp(−E/kT_t), where k is Boltzmann's constant. Typical derived values of these parameters are: n₀ = 7.2 × 10¹⁶ m⁻³, N₀ = 1.31 × 10⁴⁵ J⁻¹ m⁻³ and T_t = 623 K. The total trap concentration and the electron mobility were estimated to be 1.13 × 10²⁵ m⁻³ and (5.7−13.1) ×10⁻³m²V⁻¹s⁻¹ respectively.     

Preparation and properties of r.f.-sputtered polymer-metal thin films

The ability to produce polymer-metal thin films by co-sputtering from a composite target was investigated using polytetrafluoroethylene (PTFE) (Teflon) and Ekonol polymers. The preparation of PTFE-metal thin films by sputtering was found to be limited by target cross-contamination possibly caused by negative ion re-sputtering processes. The Ekonol-metal films were found to have microstructure and conductivity properties similar to other metal-insulator composites.     

Characterization of complex anodic alumina films by electrochemical impedance spectroscopy

Complex anodic alumina films are formed by pore-filling (reanodization) of porous anodic films (matrices). These films are characterized by electrochemical impedance spectroscopy. It is determined that the recorded impedance spectra are well described by an equivalent circuit, containing a Constant Phase Element. The fitted parameters of the complex films are calculated, and information about certain dielectric characteristics of these films in contact with an electrolyte is obtained. The paper also discusses changes in the surface non-homogeneity of the complex films in comparison with those of the porous matrix.     

Thermally stable, highly conductive, and transparent Ga-doped ZnO thin films

Highly conductive and transparent films of Ga-doped ZnO (GZO) have been prepared by pulsed laser deposition using a ZnO target with Ga₂O₃ dopant of 3 wt.% in content added. Films with resistivity as low as 3.3 × 10^− 4 Ω cm and transmittance above 80% at the wavelength between 400 and 800 nm can be produced on glass substrate at room temperature. It is shown that a stable resistivity for use in oxidation ambient at high temperature can be attained for the films. The electrical and optical properties, as well as the thermal stability of resistivity, of GZO films were comparable to those of undoped ZnO films.     

Characterization of indium zinc oxide thin films prepared by pulsed laser deposition using a Zn3In2O6 target

Using a Zn₃In₂O₆ target, indium-zinc oxide films were prepared by pulsed laser deposition. The influence of the substrate deposition temperature and the oxygen pressure on the structure, optical and electrical properties were studied. Crystalline films are obtained for substrate temperatures above 200°C. At the optimum substrate deposition temperature of 500°C and the optimum oxygen pressure of 10⁻³ mbar, both conditions that indeed lead to the highest conductivity, Zn₃In₂O₆ films exhibit a transparency of 85% in the visible region and a conductivity of 1000 S/cm. Depositions carried out in oxygen and reducing gas, 93% Ar/7% H₂, result in large discrepancies between the target stoichiometry and the film composition. The Zn/In (at.%) ratio of 1.5 is only preserved for oxygen pressures of 10⁻²–10⁻³ mbar and a 93% Ar/7% H₂ pressure of 10⁻² mbar. The optical properties are basically not affected by the type of atmosphere used during the film deposition, unlike the conductivity which significantly increases from 80 to 1400 S/cm for a film deposited in 10⁻² mbar of O₂ and in 93% Ar/7% H₂, respectively.     

Structural, electrical and optical characteristics of SnO2:Sb thin films by ultrasonic spray pyrolysis

Antimony-doped tin oxide (SnO₂:Sb) thin films were fabricated by an ultrasonic spray pyrolysis method. The effect of antimony doping on the structural, electrical and optical properties of tin oxide thin films were investigated. Tin(II) chloride dehydrate (SnCl₂·2H₂O) and antimony(III) chloride (SbCl₃) were used as a host and a dopant precursor. X-ray diffraction analysis showed that the non-doped SnO₂ thin film had a preferred (211) orientation, but as the Sb-doping concentration increased, a preferred (200) orientation was observed. Scanning electron microscopy studies indicated that the polyhedron-like grains observed for the non-doped SnO₂ thin films became rounder and decreased in size with the Sb-doping concentration. The lowest resistivity (about 8.4 × 10^− 4 Ω·cm) was obtained for the 3 at.% Sb-doped films. Antimony-doping led to an increase in the carrier concentration and a decrease in Hall mobility. The transmittance level in the near infrared region was lowered with the Sb-doping concentration.     

Electrical and optical switching properties of ion implanted VO2 thin films

The metal-insulator transition in vanadium dioxide thin films implanted with O⁺ ions was studied. Ion implantation lowered the metal-insulator transition temperature of the VO₂ films by 12 °C compared to the unimplanted ones, as measured both optically and electrically. The lowering of the transition temperature was accomplished without significantly reducing the mid-wave infrared optical transmission in the insulating state for wavelengths > 4.3 μm. Raman spectroscopy was used to examine changes to the crystalline structure of the implanted films. The Raman spectra indicate that ion implantation effects are not annealed out for temperatures up to 120 °C.     

Structural, morphological and optical properties of thermal annealed TiO thin films

Structural, morphological and optical properties of TiO thin films grown by single source thermal evaporation method were studied. The films were annealed from 300 to 520 °C in air after evaporation. Qualitative film analysis was performed with X-ray diffraction, atomic force microscopy and optical transmittance and reflectance spectra. A correlation was established between the optical properties, surface roughness and growth morphology of the evaporated TiO thin films. The X-ray diffraction spectra indicated the presence of the TiO₂ phase for the annealing temperature above 400 °C.     

Structural and electrical properties of sputtered indium-zinc oxide thin films

In this study, indium-zinc oxide (IZO) thin films have been prepared at a room temperature, 200 and 300 °C by radio frequency magnetron sputtering from a In₂O₃-12 wt.% ZnO sintered ceramic target, and their dependence of electrical and structural properties on the oxygen content in sputter gas, the substrate temperature and the post-heat treatment was investigated. X-ray diffraction measurements showed that amorphous IZO films were formed at room temperature (RT) regardless of oxygen content in sputter gas, and micro-crystalline and In₂O₃-oriented crystalline films were obtained at 200 and 300 °C, respectively. From the analysis on the electrical and the structural properties of annealed IZO films under Ar atmosphere at 200, 300, 400 and 500 °C, it was shown that oxygen content in sputter gas is a critical parameter that determines the local structure of amorphous IZO film, stability of amorphous phase as well as its eventual crystalline structure, which again decide the electrical properties of the IZO films. As-prepared amorphous IZO film deposited at RT gave specific resistivity as low as 4.48 × 10^− 4 Ω cm, and the highest mobility value amounting to 47 cm²/V s was obtained from amorphous IZO film which was deposited in 0.5% oxygen content in sputter gas and subsequently annealed at 400 °C in Ar atmosphere.     

Preparation and electrical/optical bistable property of potassium tetracyanoquinodimethane thin films

Potassium tetracyanoquinodimethane (K(TCNQ)) thin films were prepared using physical vapor deposition combined with solid state chemical replacement reaction. Reversible electrical bistable behavior at or even above room temperature was observed; and, the optical bistable property of K(TCNQ) film was observed.     

Highly conducting indium tin oxide (ITO) thin films deposited by pulsed laser ablation

Highly conducting and transparent indium tin oxide (ITO) thin films were prepared on SiO₂ glass and silicon substrates by pulsed laser ablation (PLA) from a 90 wt.% In₂O₃-10 wt.% SnO₂ sintered ceramic target. The growths of ITO films under different oxygen pressures (P_O2) ranging from 1×10⁻⁴–5×10⁻² Torr at low substrate temperatures (T_s) between room temperature (RT) and 200°C were investigated. The opto-electrical properties of the films were found to be strongly dependent on the P_O2 during the film deposition. Under a P_O2 of 1×10⁻² Torr, ITO films with low resistivity of 5.35×10⁻⁴ and 1.75×10⁻⁴ Ω cm were obtained at RT (25°C) and 200°C, respectively. The films exhibited high carrier density and reasonably high Hall mobility at the optimal P_O2 region of 1×10⁻² to 1.5×10⁻² Torr. Optical transmittance in excess of 87% in the visible region of the solar spectrum was displayed by the films deposited at Po₂≥1×10⁻² Torr and it was significantly reduced as the P_O2 decreases.     

Optical,electrical and mechanical properties of Ga-doped ZnO thin films under different sputtering powers

We present the optical, electrical and mechanical properties of Ga-doped zinc oxide (GZO) thin films prepared by radio-frequency (RF) magnetron sputtering at room temperature under different RF powers (80–180 W). The thickness, electron concentration, and electron mobility of the GZO thin film were determined by fitting the visible-to-near-infrared transmittance spectrum of GZO film/glass using the transfer matrix method. The bending force per unit width was measured by a home-made Twyman–Green interferometer with the fast Fourier transform method. The obtained results show that the optical, electrical and mechanical properties of GZO thin film are subject to the RF power. At an RF power of 140 W, the local minimum of bending force per unit width corresponds to the highest electron mobility in GZO thin film. This study demonstrates that the optical, electrical and mechanical properties of GZO thin film can be fully resolved by non-contact optical methods.     

Drying temperature effects on microstructure, electrical properties and electro-optic coefficients of sol-gel derived PZT thin films

Transparent lead zirconium titanate (PZT) thin film is suitable for a variety of electro-optic application, and the increasing of the electro-optic coefficient of PZT film is one of the important factors for this application. In this study, the main processing variable for improving an electro-optic coefficient was the drying temperature: 300, 350, 450 and 500°C in sol-gel derived PZT thin films. The highest linear electro-optic coefficient (1.65×10⁻¹⁰ (m/V)) was observed in PZT film dried at 450°C. The PZT film showed the highest perovskite content, polarization (P_max=49.58 μC/cm², P_r=24.8 μC/cm²) and dielectric constant (532). A new two-beam polarization (TBP) interferometer with a reflection configuration was used for electro-optic testing of PZT thin films which allows measurement of the linear electro-optic coefficient of thin film with strong Fabry–Perot (FP) effect usually present in PZT thin film.     

Electrical sensing properties of silica aerogel thin films to humidity

Mesoporous silica aerogel thin films have been fabricated by dip coating of sol-gel derived silica colloid on gold electrode-patterned alumina substrates followed by supercritical drying. They were evaluated as the sensor elements at relative humidity 20-90% and temperature 15-35 °C under an electrical field of frequency 1-100 kHz. Film thickness and pore structure were two main parameters that determined the sensor performance. The film with a greater thickness showed a stronger dielectric characteristic when moisture abounded, and presented a smaller hysteresis loop and a higher recovery rate, due to the large size of pore throats. As the film thickness decreased, at low humidity the surface conductivity enhanced and the response rate increased. The silica aerogel based humidity sensor can be modeled as an equivalent electrical circuit composed of a resistor and a capacitor in parallel, and is driven by ionic conduction with charged proton carriers.     

Effects of the annealing treatment on electrical and optical properties of ZnO transparent conduction films by ultrasonic spraying pyrolysis

Effects of the annealing treatment on properties of ZnO thin films prepared on silica glass substrates by the ultrasonic spraying pyrolysis process were studied. Zinc acetate dihydrate and methanol were used as a starting material and a solvent, respectively. For ZnO thin films untreated with annealing, the preferred grain growth along the (0 0 2) plane was observed. The electrical resistivity and the direct band gap values of these films decreased with increasing the deposit temperature. By applying the annealing treatment in a reducing atmosphere, while the degree of the preferred (0 0 2) orientation of films decreased, the electrical conductivity of films was improved. When compared with the resistivity values of films without the annealing treatment, the values of films annealed in the reducing atmosphere were decreased by about two orders of magnitude. The lowest resistivity value was 1.62×10⁻¹ Ω cm, which was obtained in the film annealed at 500 °C in nitrogen with 5% hydrogen. The optical transmittances of the films were higher than 80% regardless of the application of the annealing treatment in a reducing atmosphere. The direct band gap values of films annealed in a reducing atmosphere were approximately 3.27 eV.     

Highly-ordered mesoporous titania thin films prepared via surfactant assembly on conductive indium-tin-oxide/glass substrate and its optical properties

Highly ordered mesoporous titanium dioxide (titania, TiO₂) thin films on indium-tin-oxide (ITO) coated glass were prepared via a Pluronic (P123) block copolymer template and a hydrophilic TiO₂ buffer layer. The contraction of the 3D hexagonal array of P123 micelles upon calcination merges the titania domains on the TiO₂ buffer layer to form mesoporous films with a mesochannel diameter of approximately 10 nm and a pore-to-pore distance of 10 nm. The mesoporous titania films on TiO₂-buffered ITO/glass featured an inverse mesospace with a hexagonally-ordered structure, whereas the films formed without a TiO₂ buffer layer had a disordered microstructure with submicron cracks because of non-uniform water condensation on the hydrophobic ITO/glass surface. The density of the mesoporous film was 83% that of a bulk TiO₂ film. The optical band gap of the mesoporous titania thin film was approximately 3.4 eV, larger than that for nonporous anatase TiO₂ (~ 3.2 eV), suggesting that the nanoscopic grain size leads to an increase in the band gap due to weak quantum confinement effects. The ability to form highly-ordered mesoporous titania films on electrically conductive and transparent substrates offers the potential for facile fabrication of high surface area semiconductive films with small diffusion lengths for optoelectronics applications.     

Influence of thickness and growth temperature on the optical and electrical properties of ZnO thin films

Zinc oxide transparent conductive thin films were prepared using the pulsed laser deposition technique onto Corning glass substrates and the dependences of their optical and electrical properties on the thickness and the growth temperature were investigated. As shown, the films present 90% average transmittance, their energy gap position depending on the film thickness and the growth temperature. An additional absorption band was also observed near 3.44 eV, the position of its maximum also depending on the growth parameters. Finally, the electrical properties of the films were found to be affected mainly by the growth temperature and less by the thickness.     

Annealing effect on properties of transparent and conducting ZnO thin films

This work presents the effect of postdeposition annealing on the structural, electrical and optical properties of undoped ZnO (zinc oxide) thin films, prepared by radio-frequency sputtering method. Two samples, 0.17 and 0.32 µm-thick, were annealed in vacuum from room temperature to 350 °C while another 0.32 µm-thick sample was annealed in air at 300 °C for 1 h. X-ray diffraction analysis revealed that all the films had a c-axis orientation of the wurtzite structure normal to the substrate. Electrical measurements showed that the resistivity of samples annealed in vacuum decreased gradually with the increase of annealing temperature. For the 0.32 µm-thick sample, the gradual decrease of the resistivity was essentially due to a gradual increase in the mobility. On the other hand, the resistivity of the sample annealed in air increased strongly. The average transmission within the visible wavelength region for all films was higher than 80%. The band gap of samples annealed in vacuum increased whereas the band gap of the one annealed in air decreased. The main changes observed in all samples of this study were explained in terms of the effect of oxygen chemisorption and microstructural properties.     

Ion bombardment-induced enhancement of the properties of indium tin oxide films prepared by plasma-assisted reactive magnetron sputtering

Research on tin doped indium oxide (ITO) has for many years been stimulated by the need to simultaneously optimize the electrical, optical and mechanical properties, and by new challenges related to the deposition of transparent conducting oxides on flexible plastic substrates. In the present work, we investigate the growth and optical, electrical, and mechanical (hardness, elastic modulus and stress) properties of ITO films deposited by plasma assisted reactive magnetron sputtering (PARMS) from an indium-tin alloy target. PARMS achieves an effective control of bombardment by reactive species (e.g., O₂⁺, O⁺) on the surface of the growing film by varying the bias voltage, V_B, induced by a radiofrequency power applied to the substrate. Stress-free films possessing high transparency (> 80% — film on glass) and low resistivity (4 × 10^− 4 Ω cm) can be deposited by PARMS under conditions of intense ion bombardment (≤ 600 eV).     

Sol–gel derived N-doped ZnO thin films

N-doped ZnO (NZO) thin films have been prepared by a sol–gel method and their electrical and optical properties have been investigated. The prepared NZO films were p-type, and had excellent electrical properties. They had an optical transparency above 85% in the visible range. The UV absorption edge was red-shifted with increasing N-doping concentration. Two emission bands were observed in the photoluminescence (PL) spectra, with one band located in the UV range and the other band consisting of green luminescence. Both UV and green emissions were enhanced with increasing N-doping concentration.