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.     

Vanadium dioxide thin film with low phase transition temperature deposited on borosilicate glass substrate

A nanostructured vanadium dioxide (VO₂) thin film showing a low metal-insulator transition temperature of 30 °C has been fabricated through reactive ion beam sputtering followed by thermal annealing. The thin film was grown on borosilicate glass substrate at the temperature of 280 °C with a Si₃N₄ buffer layer. Both scanning electron microscopy and atomic force microscopy images have been taken to investigate the configuration of VO₂ thin film. The average height of the crystallite is 20 nm and the grain size ranges from 40 nm to 100 nm. The transmittance measured from low to high temperatures also reveals that the film possesses excellent switching property in infrared light at critical transition temperature, with switching efficiency of 52% at 2600 nm. This experiment paves the way of VO₂ thin film's application in smart windows.     

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).     

Structure and optical properties of Zr1−xSixN thin films on sapphire

A series of zirconium silicon nitride (Zr_1−xSi_xN) thin films were grown on r-plane sapphire substrates using reactive RF magnetron co-sputtering of Zr and Si targets in a N₂/Ar plasma. X-ray diffraction pole figure analysis, X-ray reflectivity, X-ray photoelectron spectroscopy (XPS), optical microscopy, and optical absorption spectroscopy were used to characterize the film stoichiometries and structures after growth at 200 °C and post-deposition annealing up to 1000 °C in ultra-high vacuum. The atomically clean r-plane sapphire substrates induce high quality (100) heteroepitaxy of ZrN films rather than the (111) orientation observed on steel and silicon substrates, but the addition of Si yields amorphous films at the 200 °C growth temperature. After the annealing treatment, films with Si content x < 0.15 have compressive stress and crystallize into a polycrystalline structure with (100) fiber texture. For x > 0.15, the films are amorphous and remain so even after ultra-high vacuum annealing at 1000 °C. XPS spectra indicate that the bonding changes from covalent to more ionic in character as Si―N bonds form instead of Zr―N bonds. X-ray reflectivity, atomic force microscopy (AFM) and optical microscopy data reveal that after post-deposition annealing the 100 nm thick films have an average roughness < 2 nm, except for Si content near x = 0.15 corresponding to where the film becomes amorphous rather than being polycrystalline. At this stoichiometry, evidence was found for regions of film delamination and hillock formation, which is presumably driven by strain at the interface between the film and sapphire substrate. UV-visible absorption spectra also were found to depend on the film stoichiometry. For the amorphous Si-rich films (x > 0.15), the optical band gap increases with Si content, whereas for Zr-rich films (x < 0.15), there is no band gap and the films are highly conductive.     

Substrate temperature effect on the SiC passivation layer synthesized by an RF magnetron sputtering method

This paper describes amorphous silicon carbide (a-SiC) film as an alternative material to silicon nitride (SiN) and silicon oxide (SiO₂) for the passivation layer of solar cells. We deposited the film on p-type silicon (100) wafers and glass substrates by RF magnetron sputtering using a SiC (99%) target. Structural and optical properties of the films were investigated according to the process temperature (room temperature, 300 °C, 400 °C, 500 °C and 600 °C). The structural properties were analyzed by Raman microscopy and XPS (X-ray Photoelectron Spectroscopy). The XPS showed that the content of SiC in the film is increased when the substrate temperature is higher. The optical properties of the films were examined by UV-visible spectroscopy and Ellipsometer. The optical characteristic measurement showed that the lowest refractive index of the film is 2.65. Also, using carrier lifetime measurement, we investigated the performance of SiC as the passivation layer. At the substrate temperature of 600 °C, we obtained a highest carrier lifetime of 7.5 μs.     

Film thickness dependence of microstructures and magnetic properties in single-layered FePt films by in-situ annealing

The FePt films with various thicknesses (t) of 5 to 50 nm are deposited on Si(100) substrate without any underlayer by in-situ annealing at substrate temperature (Ts) of 620 °C. A strong (001) texture of L1₀ FePt film is obtained and presents high perpendicular magnetic anisotropy as the film thickness increases to 30 nm. By further increasing the thickness to exceed 30 nm, the (111) orientation of L1₀ FePt is enhanced greatly, indicating that the quality of perpendicular magnetic anisotropy degrades when the thickness of the FePt film is greater than 30 nm. The single-layered FePt film with thickness of 30 nm by in-situ depositing at 620 °C shows good perpendicular magnetic properties (perpendicular coercivity of 1033 kA/m (13 kOe), saturation magnetization of 1.08 webers/m² and perpendicular squareness of 0.91, respectively), which reveal its significant potential for perpendicular magnetic recording media.     

Structural, electrical and optical properties of ITO films with a thin TiO2 seed layer prepared by RF magnetron sputtering

Tin-doped indium oxide (ITO) films were deposited by RF magnetron sputtering on TiO₂-coated glass substrates (the TiO₂ layer is usually called seed layer). The properties of ITO films prepared at a substrate temperature of 300 °C on bare and TiO₂-coated glass substrates have been analyzed by using X-ray diffraction, atomic force microscope, optical and electrical measurements. Comparing with single layer ITO film, the ITO film with a TiO₂ seed layer of 2 nm has a remarkable 41.2% decrease in resistivity and similar optical transmittance. The glass/TiO₂ (2 nm)/ITO film achieved shows a resistivity of 3.37 × 10⁻⁴ Ω cm and an average transmittance of 93.1% in the visible range. The glass/TiO₂ may be a better substrate compared with bare glass for depositing high quality ITO films.     

Plastic substrate with gas barrier layer and transparent conductive oxide thin film for flexible displays

A novel plastic substrate for flexible displays was developed. The substrate consisted of a polycarbonate (PC) base film coated with a gas barrier layer and a transparent conductive thin film. PC with ultra-low intrinsic birefringence and high temperature dimensional stability was developed for the base film. The retardation of the PC base film was less than 1 nm at a wavelength of 550 nm (film thickness, 120 µm). Even at 180 °C, the elastic modulus was 2 GPa, and thermal shrinkage was less than 0.01%. The surface roughness of the PC base film was less than 0.5 nm. A silicon oxide (SiO_x) gas barrier layer was deposited on the PC base film by a roll-to-roll DC magnetron reactive sputtering method. The water vapor transmission rate of the SiO_x film was less than 0.05 g/m²/day at 40 °C and 100% relative humidity (RH), and the permeation of oxygen was less than 0.5 cc/m² day atm at 40 °C and 90% RH. As the transparent conductive thin film, amorphous indium zinc oxide was deposited on the SiO_x by sputtering. The transmittance was 87% and the resistivity was 3.5 × 10^− 4 ohm cm.     

Preparation and thermochromic property of VO2/mica pigments

Thermochromic VO₂/mica pigments were fabricated by an aqueous sol-gel and spray-drying method. XRD and SEM were used to investigate the structure and morphology of pigments, and the results show that the VO₂ layer was composed of randomly oriented worm-like particles less than 300 nm in width. A thermochromic composite was prepared with the pigments and UV curing resin. The infrared transmittance change in the composite was measured from 24 °C to 100 °C by FTIR, showing very good thermochromic performance. The composite exhibits a transmittance of 50-55% in the visible range.     

Effects of post-deposition surface treatment on the optical, structural and hydrogen sensing properties of TiO2 thin films

TiO₂ thin films were prepared by DC reactive magnetron sputtering in a mixture of oxygen and argon on glass and oxidized silicon substrates. The effect of post-deposition annealing (300 °C, 500 °C and 700 °C for 8 h in air) on the structural and morphological properties of TiO₂ thin films is presented. In addition, the effect of Pt surface modification (1, 3 and 5 nm) on hydrogen sensing was studied. XRD patterns have shown that in the range of annealing temperatures from 300 °C to 500 °C crystallization starts and the thin film structure changes from amorphous to polycrystalline (anatase phase). In the case of samples on glass substrate, optical transmittance spectra were recorded. TiO₂ thin films were tested as sensors of hydrogen at concentrations 10,000-1000 ppm and operating temperatures within the 180-200 °C range. The samples with 1 nm and in particular with 3 nm of Pt on the surface responded to hydrogen fast and with high sensitivity.     

Study on the optical property and crystallinity of Ge90Te10 films deposited by electron beam evaporation

Optical property and crystallinity of Ge₉₀Te₁₀ films prepared by electron beam evaporation have been studied. The films grown at different substrate temperatures (Ts) and deposition rates (R) have been characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and spectroscopic ellipsometry. The polycrystalline film was obtained at Ts = 300 °C, while the amorphous film was obtained when Ts ≦ 200 °C. However, the film showed the columnar structure when Ts ≦ 100 °C. It was found that Ts had the stronger effect on the crystallinity of the film rather than R. The optical constant in the infrared region was determined. All the film exhibited no absorption, but the refractive index was varied with the change of Ts and R. The relationship between optical constant, the film structure and the deposition parameters were also discussed. In addition, the optimum deposition condition of Ge₉₀Te₁₀ film was found.     

Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

Influence of substrate temperature and bias voltage on the optical transmittance of TiN films

Titanium nitride (TiN) thin films were prepared by means of reactive DC sputtering on quartz and sapphire substrates. Structural, electrical and optical effects of deposition parameters such as thickness, substrate temperature, substrate bias voltage were studied. The effect of substrate temperature variations in the 100-300°C range and substrate bias voltage variations in the 0-200 V DC range for 45-180 nm thick TiN films were investigated. Temperature-dependent electrical resistivity in the 100-350 K range and optical transmission in the 300-1500 nm range were measured for the samples. In addition, structural and morphological properties were studied by means of XRD and STM techniques.The smoothest surface and the lowest electrical resistivity was recorded for the optimal samples that were biased at about V_s=−120 V DC. Unbiased films exhibited a narrow optical transmission window between 300 and 600 nm. However, the transmission became much greater with increasing bias voltage for the same substrate temperature. Furthermore, it was found that lower substrate temperatures produced optically more transparent films.Application of single layers of MgF₂ antireflecting coating on optimally prepared TiN films helped increase the optical transmission in the visible region to more than 40% for 45 nm thick samples.     

Effect of annealing on the electrical, optical and structural properties of cadmium stannate thin films prepared by spray pyrolysis technique

Polycrystalline thin films of cadmium stannate (Cd₂SnO₄) were deposited by spray pyrolysis method on the Corning substrates at substrate temperature of 525 °C. Further, the films were annealed at 600 °C in vacuum for 30 min. These films were characterized for their structural, electrical and optical properties. The experimental results showed that the post-deposition annealing in vacuum has a significant influence on the properties of the films. The average grain size of the film was increased from 27.3 to 35.0 nm on heat treatment. The average optical transmittance in the visible region (500-850 nm) is decreased from 81.4% to 73.4% after annealing in vacuum. The minimum resistivity achieved in the present study for the vacuum annealed films is the lowest among the reported values for the Cd₂SnO₄ thin films prepared by spray pyrolysis method.     

Low-Loss Optical Waveguides for the Near Ultra-Violet and Visible Spectral Regions with Al(2)O(3) Thin Films from Atomic Layer Deposition

In this work, we report low-loss single-mode integrated optical waveguides in the near ultra-violet and visible spectral regions with aluminum oxide (Al₂O₃) films using an atomic layer deposition (ALD) process. Alumina films were deposited on glass and fused silica substrates by the ALD process at substrate/chamber temperatures of 200 °C and 300 °C. Transmission spectra and waveguide measurements were performed in our alumina films with thicknesses in the range of 210-380 nm for the optical characterization. Those measurements allowed us to determine the optical constants (n_w and k_w), propagation loss, and thickness of the alumina films. The experimental results from the applied techniques show good agreement and demonstrate a low-loss optical waveguide. Our alumina thin-film waveguides are well transparent in the whole visible spectral region and also in an important region of the UV; the measured propagation loss is below 4 dB/cm down to a wavelength as short as 250 nm. The low propagation loss of these alumina guiding films, in particular in the near ultra-violet region which lacks materials with high optical performance, is extremely useful for several integrated optic applications.     

The influence of annealing temperatures on the properties of Bi2VO5.5/LaNiO3/Si thin films

Bi₂VO_5.5 ferroelectric thin films were fabricated on LaNiO₃/Si(100) substrate via chemical solution deposition. Ferroelectric and dielectric properties of the thin films annealed at 500-700 °C were studied. The thin film annealed at 700 °C exhibited more favorable ferroelectric and dielectric properties than those annealed at lower temperatures. The values of remnant polarization 2P_r and coercive field E_c for the film annealed at 700 °C are 10.62 µC/cm² and 106.3 kV/cm, respectively. The leakage current of the film is about 1.92 × 10^− 8 A/cm² at 6 V. The possible mechanism of the dependence of electrical properties of the films on the annealing temperature was discussed.     

Optical properties of silver sulphide thin films formed on evaporated Ag by a simple sulphurization method

Silver sulphide (Ag₂S) thin films were grown on the surface of silver films (Ag) deposited on glass substrate by using a simple chemical sulphurization method. According to X-ray diffraction analysis, the Ag₂S thin films display low intensity peaks at 34.48°, 36.56°, and 44.28°, corresponding to diffraction from (100), (112) and (103) planes of the acanthite phase (monoclinic). A model of the type Ag₂S/Ag/glass was deduced from spectroscopic ellipsometric measurements. Also, the optical constants (n, k) of the system were determined. Furthermore, the optical properties as solar selective absorber for collector applications were assessed. The optical reflectance of the Ag₂S/Ag thin film systems exhibits the expected behavior for an ideal selective absorber, showing a low reflectance in the wavelength range below 2 µm and a high reflectance for wavelengths higher than that value. An absorptance about 70% and an emittance about 3% or less were calculated for several samples.     

Hydrogen gas-sensing with bilayer structures of WO3 and Pd in SAW and electric systems

A bi-layer sensor structure of WO₃ (~ 100 nm) with a very thin film of palladium (Pd~ 18 nm) on the top, has been studied for hydrogen gas-sensing application at ~ 80 °C and ~ 120 °C and low hydrogen concentrations (0.025-1%). The structures were obtained by vacuum deposition (first the WO₃ and then the Pd film) onto a LiNbO₃ Y-cut Z-propagating substrate making use of the Surface Acoustic Wave method and additionally (in this same technological processes) onto a glass substrate with a planar microelectrode array for simultaneous monitoring of planar resistance of the structure. A very good correlation has been observed between these two methods — frequency changes in SAW method correlate very well with decreases in the bi-layer structure resistance. The SAW method is faster at the lower interaction temperature such as 80 °C, whereas at an elevated temperature of 120 °C, the electrical planar method is also fast and has a lower limit of detection.     

The effect of annealing on Al-doped ZnO films deposited by RF magnetron sputtering method for transparent electrodes

In this study, transparent conducting Al-doped zinc oxide (AZO) films with a thickness of 150 nm were prepared on Corning glass substrates by the RF magnetron sputtering with using a ZnO:Al (Al₂O₃: 2 wt.%) target at room temperature. This study investigated the effects of the post-annealing temperature and the annealing ambient on the structural, electrical and optical properties of the AZO films. The films were annealed at temperatures ranging from 300 to 500 °C in steps of 100 °C by using rapid thermal annealing equipment in oxygen. The thicknesses of the films were observed by field emission scanning electron microscopy (FE-SEM); their grain size was calculated from the X-ray diffraction (XRD) spectra using the Scherrer equation. XRD measurements showed the AZO films to be crystallized with strong (002) orientation as substrate temperature increases over 300 °C. Their electrical properties were investigated by using the Hall measurement and their transmittance was measured by UV-vis spectrometry. The AZO film annealed at the 500 °C in oxygen showed an electrical resistivity of 2.24 × 10^− 3 Ω cm and a very high transmittance of 93.5% which were decreased about one order and increased about 9.4%, respectively, compared with as-deposited AZO film.     

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.