Influence of oxygen pressure on the structural, electrical and optical properties of VO2 thin films deposited on ZnO/glass substrates by pulsed laser deposition

The influence of oxygen pressure on the structural and electrical properties of vanadium oxide thin films deposited on glass substrates by pulsed laser deposition, via a 5-nm thick ZnO buffer, was investigated. For the purposes of comparison, VO₂ thin films were also deposited on c-cut sapphire and glass substrates. During laser ablation of the V metal target, the oxygen pressure was varied between 1.33 and 6.67 Pa at 500 °C, and the interaction and reaction of the VO₂ and the ZnO buffer were studied. X-ray diffraction studies showed that the VO₂ thin film deposited on a c-axis oriented ZnO buffer layer under 1.33 Pa oxygen had (020) preferential orientation. However, VO₂ thin films deposited under 5.33 and 6.67 Pa were randomly oriented and showed (011) peaks. Crystalline orientation controlled VO₂ thin films were prepared without such expensive single crystal substrates as c-cut sapphire. The metal-insulator transition properties of the VO₂/ZnO/glass samples were investigated in terms of electrical conductivity and infrared reflectance with varying temperatures, and the surface composition was investigated by X-ray photoelectron spectroscopy.     

Optical properties of thin films of amorphous vanadium oxides

The results of an experimental investigation of the optical properties of anodic vanadium oxide films are presented. It is shown that films of different phase composition (VO₂, V₂O₅, or a mixture of two phases) can be obtained, depending on the oxidation regime, and that the absorption and transmission spectra are modified significantly in accordance. The optical properties of the oxides, whose composition is close to stoichiometric vanadium dioxide, demonstrate the occurrence of a metal-semiconductor phase transition in the amorphous films. The results presented are important both from the standpoint of technical applications of thin film systems based on anodic vanadium oxides and for more detailed understanding of the physical mechanism of the metal-semiconductor phase transition and the influence of structural disorder on the transition. Pis’ma Zh. Tekh. Fiz. 25, 81–87 (April 26, 1999)     

Study on optical and electrical switching properties and phase transition mechanism of Mo6+-doped vanadium dioxide thin films

In the present work using V₂O₅ and MoO₃ powders as precursors, a novel method, the inorganic sol-gel method, was developed to synthesize Mo⁶⁺ doped vanadium dioxide (VO₂) thin films. The structure, valence state, phase transition temperature, magnitude of resistivity change and change in optical transmittance below and above the phase transition of these films are determined by XRD, XPS, four-point probe equipment and spectrophotometer. The results showed that the main chemical composition of the films was VO₂, the structure of MoO₃ in the films didn't change, and the phase transition temperature of the VO₂ was obviously lowered with increasing MoO₃ doped concentration. The magnitude of resistivity change and change in optical transmittance below and above phase transition were also decreased, of which the magnitude of resistivity change was more distinct. However, when the MoO₃ concentration was 5 wt%, the magnitude of resistivity change of doped thin films still reached more than 2 orders, and the change in optical transmittance below and above phase transition was maintained. Analysis showed that the VO₂ doped films formed local energy level, and then reduced the forbidden band gap of VO₂ as the donor defect changing its optical and electrical properties and lowering the phase transition temperature.     

Molybdenum-doped vanadium dioxide coatings on glass produced by the aqueous sol-gel method

Thin films of vanadium dioxide (VO₂) on glass substrates were produced by the aqueous sol-gel method. Various levels of doping were achieved by adding small quantities of a water-soluble molybdenum compound to the sol. After dip coating, the substrates were reduced by heat treatment in a low-pressure carbon monoxide/carbon dioxide (CO/CO₂) atmosphere. The change in electrical conductivity with temperature, and optical reflectance in the semiconductor and metallic phases were measured and compared to undoped VO₂ films. Doping the VO₂ films with molybdenum lowered the transition temperature of the semiconductor-to-metal phase change; at a doping level of 7 at.% the transition temperature was measured at 24 °C, as indicated by the electrical conductivity. All the films showed a substantial change in reflectance upon heating through the transition. The optical reflectance in the semiconductor state increased slightly with additional dopant, while the reflectance in the metallic state remained constant.     

Characteristics of CeOx–VO2 composite thin films synthesized by sol–gel process

Pure vanadium dioxide (VO₂) and CeOx–VO₂ (1.5 < x < 2) composite thin films were grown on muscovite substrate by inorganic sol–gel process using vanadium pentaoxide and cerium(III) nitrate hexahydrate powder as precursor. The crystalline structure, morphology and phase transition properties of the thin films were systematically investigated by X-ray diffraction, Raman, X-ray photoelectron spectroscopy, FE-SEM and optical transmission measurements. High quality of the VO₂ and CeOx–VO₂ composite films were obtained, in which the relative fractions of +4 valence state vanadium were above 70 % though the concentrations of cerium reached 9.77 at %. However, much of cerium compounds were formed at the edge of grains and the addition of cerium resulted in more clearly defined grain boundaries as shown in SEM images. Meanwhile, the composite films exhibited excellent phase transition properties and the infrared transmittance decreased from about 70 to 10 % at λ = 4 μm bellow and above the metal–insulator phase transition temperature. The metal–insulator phase transition temperatures were quite similar with about 66 °C of the pure VO₂ and CeOx–VO₂ composite thin films. But hysteresis widths increased with more addition of cerium, due to the limiting effect of grain boundaries on the propagation of the phase transition. Particularly, the CeOx–VO₂ composite film with an addition of 7.82 at % Ce showed a largest hysteresis width with about 20.6 °C. In addition, the thermochromic performance of visible transmittance did not change obviously with more addition of cerium.     

Compositional and metal-insulator transition characteristics of sputtered vanadium oxide thin films on yttria-stabilized zirconia

Vanadium dioxide (VO₂) thin films have been shown to undergo a rapid electronic phase transition near 70 °C from a semiconductor to a metal, making it an interesting candidate for exploring potential application in high speed electronic devices such as optical switches, tunable capacitors, and field effect transistors. A critical aspect of lithographic fabrication in devices utilizing electric field effects in VO₂ is the ability to grow VO₂ over thin dielectric films. In this article, we study the properties of VO₂ grown on thin films of Yttria-Stabilized Zirconia (YSZ). Near room temperature, YSZ is a good insulator with a high dielectric constant ($\epsilon _{\rm r} > 25$\epsilon _{\rm r} > 25). We demonstrate the sputter growth of polycrystalline VO₂ on YSZ thin films, showing a three order resistivity transition near 70 °C with transition and hysteresis widths of approximately 7 °C each. We examine the relationship between chemical composition and transition characteristics of mixed phase vanadium oxide films. We investigate changes in composition induced by low temperature post-deposition annealing in oxidizing and reducing atmospheres, and report their effects on electronic properties.     

Correlation between optical, electrical and structural properties of vanadium dioxide thin films

VO₂ films have been prepared on normal microscope glass slides by reactive rf magnetron sputtering of vanadium target in a mixture of argon and oxygen. Optical properties of the films were investigated by the UV/Vis/NIR Perkin–Elmer Lamda 9. Transmission electron microscope and atomic force microscope were used to investigate the structure of the films. Correlation between structural and optical properties of VO₂ thin films is investigated with respect to the dependence of both to substrate temperature.     

High temperature coefficients of resistance of VO2 films grown by excimer-laser-assisted metal organic deposition process for bolometer application

With bolometer application in mind, we prepared VO₂ films on TiO₂ (001) substrates by an excimer-laser-assisted metal organic deposition process at 300 °C or less. A metal-to-insulator transition of VO₂ is expected to induce high temperature coefficient of electrical resistance (TCR) useful for high-performance infrared sensors, but the practical use of crystalline VO₂ films has been prevented due to the accompanied wide hysteresis. In this study, by forming the epitaxial phase only near the substrate interface, the transition of the film was successfully broadened and the hysteresis disappeared. The maximum TCR of the film was more than -10%/°C near room temperature, and the temperature range in which TCR was higher than -4%/°C was very wide (280-320 K).     

Optical properties of thermochromic VO2 thin films on stainless steel: Experimental and theoretical studies

Thermochromic films of VO₂ were deposited by DC reactive magnetron sputtering on stainless steel substrate. Complex refractive indexes of VO₂ were determined by ellipsometric spectroscopy (0.35-16.5 μm) for different film thicknesses. Optical simulations were performed to model the spectral reflectance of the film/substrate system for a film thickness of 100 nm and 200 nm and to monitor the optical contrast of the thermochromic layers by comparing the spectral reflectance at 25 °C and 100 °C. The good agreement observed between experimental and theoretical spectra demonstrates the adequacy of the model for predicting the optical properties of the samples.     

Growth of VO2 films with metal-insulator transition on silicon substrates in inductively coupled plasma-assisted sputtering

Single-phase monoclinic vanadium dioxide (VO₂) films were grown on a Si(100) substrate using inductively coupled plasma (ICP)-assisted sputtering with an internal coil. The VO₂ film exhibited metal-insulator (M-I) transition at around 65 °C with three orders of change in resistivity, with a minimum hysteresis width of 2.2 °C. X-ray diffraction showed structural phase transition (SPT) from monoclinic to tetragonal rutile VO₂. For conventional reactive magnetron sputtering, vanadium oxides with excess oxygen (V₂O₅ and V₃O₇) could not be eliminated from stoichiometric VO₂. Single-phase monoclinic VO₂ growths that are densely filled with smaller crystal grains are important for achieving M-I transition with abrupt resistivity change.     

Porous nano-structured VO2 films with different surfactants: synthesis mechanisms, characterization, and applications

Porous nano-structured vanadium dioxide (VO₂) thin films have been prepared on mica substrates via sol–gel process using surfactant cetyltrimethyl ammonium bromide, nonionic surfactant polyethylene glycol, and anionic surfactant sodium dodecyl sulfate as nano-structure directing agents. Models concerning the structure forming were proposed to explain the synthesis mechanisms between V₂O₅ colloid and different surfactants. Porous nano-structured VO₂ films with sphere-shaped, island-shaped and strip-shaped nanocrystals are synthesized in the experiments, and the optical properties and thermochromic properties of these films are compared. The porous nano-structured VO₂ films showed excellent infrared transmittance (nearly 70 %), low transition temperature (59.7 °C without doping), wide hysteresis width (37.8 °C), and different optical transmittance difference before and after the phase transition (39–67 %). The results suggest that these porous nano-structured VO₂ films have significant importance in practical application in VO₂-based optical and electronic devices.     

Low temperature amorphous growth of semiconducting Y-Ba-Cu-O oxide thin films in view of infrared bolometric detection

YBa₂Cu₃O_6 + x (YBCO) compounds are well known to exhibit superconducting properties for x > 0.5 and semiconducting properties for lower oxygen content. In this work, YBCO oxide thin films of the semiconducting phase were deposited by direct-current (DC) hollow cathode sputtering at low temperature in the 100 to 400 °C range. Structural, electrical and optical properties are investigated and discussed in relation with the envisaged bolometric detection application. Structural characterizations show that films are amorphous, with a granular structure of low roughness (3 nm rms). DC electrical measurements both reveal that films grown at 100 °C exhibit a high temperature coefficient of resistance (TCR ~ − 3% K^− 1 to − 4% K^− 1 at 300 K) and an optimized low resistivity value of 345 Ω·cm at 300 K. Consequently, this material is suitable for uncooled infrared bolometer application and can be deposited at 100 °C in a complementary metal-oxide-semiconductor compatible technological process for co-integration with readout circuitry. In addition, optical measurements performed in the 0.5 to 2.2 μm wavelength range on films grown at 100 °C highlight optical conductivity values in line with those expected for YBCO material, as well as the presence of two optical band gaps that are discussed with respect to the film nanostructure.     

VO2 thin films deposited on silicon substrates from V2O5 target: Limits in optical switching properties and modeling

Thermochromic VO₂ thin films presenting a phase change at T_c = 68 °C and having variable thickness were deposited on silicon substrates (Si-001) by radio-frequency sputtering. These thin films were obtained from optimized reduction of low cost V₂O₅ targets. Depending on deposition conditions, a non-thermochromic metastable VO₂ phase might also be obtained. The thermochromic thin films were characterized by X-ray diffraction, atomic force microscopy, ellipsometry techniques, Fourier transform infrared spectrometry and optical emissivity analyses. In the wavelength range 0.3 to 25 μm, the optical transmittance of the thermochromic films exhibited a large variation between 25 and 100 °C due to the phase transition at T_c: the contrast in transmittance (difference between the transmittance values to 25 °C and 100 °C) first increased with film thickness, then reached a maximum value. A model taking into account the optical properties of both types of VO₂ film fully justified such a maximum value. The n and k optical indexes were calculated from transmittance and reflectance spectra. A significant contrast in emissivity due to the phase transition was also observed between 25 and 100 °C.     

Phase transition and switching in pyrolytic VO2 films

VO₂ films about 3000 Å thick have been prepared by the pyrolysis of vanadile acetylacetonate (C₅H₇O₂)₂ VO. The changes in the electrical, optical and structural properties of the films during the phase transition have been investigated. Various switching effects in VO₂ films have also been studied.     

The large modification of phase transition characteristics of VO2 films on SiO2/Si substrates

The vanadium oxide (VO₂) films have been prepared on SiO₂/Si substrates by using a modified Ion Beam Enhanced Deposition (IBED) method. During the film deposition, high doses of Ar⁺ and H⁺ ions have been implanted into the deposited films from the implanted beam. The resistance change of the VO₂ films with temperature has been measured and the phase transition process has been observed by using the X-ray Diffraction technique. The phase transition of the IBED VO₂ films starts at a low temperature of 48 °C and ends at a high temperature of 78 °C. It is found that the phase transition characteristics can be adjusted by changing the annealing temperature or the time and the phase transition characteristics of the IBED VO₂ films depend on the quantity and location of argon atoms in the film matrix.     

Deposition of zirconium oxynitride films by reactive cathodic arc evaporation and investigation of physical properties

The area of metal oxynitrides is poorly explored, and understanding of the fundamental mechanism that explains structural, mechanical, electrical, and optical properties is still insufficient. Therefore, the purpose of the present investigation is to analyze structural, electrical, and optical properties of ZrN_xO_y films deposited by reactive cathodic arc evaporation.Depending on the oxygen flow, cubic ZrN:O, monoclinic ZrO₂:N, and tetragonal ZrO₂:N phases films were prepared. The sheet resistance and the optical transmittance very much depend on the oxygen flow. Optical transparent ZrN_xO_y films with transmittance of 86% at 650 nm, the sheet resistance 1.1 · 10³ Ω/sq, and the figure of merit 2 · 10^− 4 Ω^− 1 are deposited with the 60 sccm oxygen flow.     

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 of homogeneous VOX thin films by ion beam sputtering and annealing process

Polycrystalline VO_x thin films that were prepared for thermal-sensitive material of far infrared sensor had been deposited on Si substrates by ion beam sputtering deposition. Scanning electron microscopy images indicated that VO_x thin films (oxygen pressure of 1.5×10⁻³ Pa) were grown into compact and ultra-fine grains (?50 nm), the film surfaces seemed smooth and uniform. Four-point probe measurements showed that the homogeneity of the films was better than 98% in a size of 30×30 mm². The four-point probe measurement on hot plate presented the sheet resistance and the temperature coefficient of resistance of the VO_x thin film that were 50 kΩ/square and −0.021 K⁻¹ at 28°C, respectively. In addition, some samples annealed in Ar atmosphere had their resistance decreased. Thus, vanadium oxide films containing more amount VO₂ were obtained.     

Structural, optical and electrical properties of indium nitride polycrystalline films

The structural, optical and electrical properties of InN polycrystalline films on glass substrate are investigated by means of X-ray photoelectron spectroscopy, Raman scattering measurements, X-ray diffraction analysis, optical spectroscopy, and electrical measurements as a function of the inverse of temperature. The absorption edge for the films is most likely due to an impurity band formed by the presence of defects in the material. Such an impurity band, located at 1.6 eV extends itself to about 1.8 eV above the Fermi level, and it is attributed to nitrogen vacancies present in the material. The Raman scattering data also reveal the incorporation of oxygen in the InN films, leading to the formation of the In₂O₃ amorphous phase during the process of sputtering. Additionally, the X-ray photoelectron spectroscopy of the valence band, which is highly desirable to the determination of the Fermi level, confirms the optical gap energy. Furthermore, the X-ray diffraction patterns of the thinner films present broader peaks, indicating high values for the strain between the film lattice and the glass substrate. Finally, first principles calculations are used to investigate the optical properties of InN and also to support the experimental findings.     

Control of carrier concentration of p-type transparent conducting CuScO2(0001) epitaxial films

Excess oxygen and 1-at% Mg co-doped CuScO₂[3R](0001) epitaxial films were prepared on a-plane sapphire substrates by combining two-step deposition and post-annealing techniques. The optical and electrical transport properties of the co-doped epitaxial films were compared with those of the CuScO₂[3R](0001) epitaxial films. No significant increase in optical absorption was observed in the co-doped epitaxial films, and the energy gap for direct allowed transition was estimated at 3.7 eV. The carrier concentration of CuScO₂[3R](0001) epitaxial films was controlled from ~ 10¹⁶ cm^- 3 to ~ 10¹⁸ cm^- 3 at room temperature by adjusting the excess oxygen and Mg co-doping. The electrical conductivity, carrier concentration, and Hall mobility of the most conductive film were 3.6 × 10^- 2 Scm^- 1, 8.5 × 10¹⁷ cm^- 3 and 2.6 × 10^- 1 cm²V^- 1 s^- 1 at room temperature, respectively. The temperature dependence of the electrical transport properties of the film exhibited semiconducting characteristics, and the activation energy estimated from the temperature dependence of the carrier concentration was 0.50 eV.