Properties of LiNbO3 based heterostructures grown by pulsed-laser deposition for optical waveguiding application

Epitaxial LiNbO₃ (LN) thin films have been grown onto (00.1) Al₂O₃ substrates and onto sapphire covered with a conductive ZnO buffer layer. For the two systems, the LN thin films are well crystallised and highly (00.1) oriented. Epitaxial relationships between the different layers are evidenced both on the LN/sapphire film and the LN/ZnO/sapphire heterostructure. The optical waveguiding propagation losses of the LN/sapphire films are very low (1 ± 0.5 dB/cm) while the LN/ZnO/sapphire heterostructure does not exhibit satisfying waveguiding properties mainly due to the high conductivity (600 S m^− 1) of the ZnO buffer layer.     

Fabrication of KTa0.65Nb0.35O3 film by pulsed laser deposition on glass substrate with various buffer layers

KTa_0.65Nb_0.35O₃ (KTN) thin films were deposited on amorphous glass substrates using a range of single buffer layers such as indium tin oxide (ITO), zinc oxide (ZnO), 3 at% Al-doped ZnO (AZO), and 3 at% Ga-doped ZnO (GZO), as well as a variety of multi-buffer layers such as SrTiO₃ (STO)/ITO, STO/ZnO, STO/AZO, and STO/GZO using a pulsed laser deposition system. All films showed a polycrystalline perovskite phase with the exception of all single buffer layers and STO/ITO multi-buffer layers. The STO buffer layer is important for crystallizing KTN films due to the similar lattice constant and same crystal structure. The optical transmittance of all films exhibited a transmittance ?90% in the wavelength range.     

Effect of sputtering pressure and post-annealing on hydrophilicity of TiO2 thin films deposited by reactive magnetron sputtering

A series of TiO₂ thin films was deposited onto glass substrates without intentional heating or biasing by magnetron sputtering of a titanium target using Ar/O₂ reactive mixtures over a broad range of total sputtering pressures from 0.12 Pa to 2.24 Pa. Each of the film types was deposited by the threshold poisoned mode at a specific given oxygen flow rate monitored in-situ by optical emission spectroscopy. Both the sputtering pressure and thermal annealing are the key factors for the TiO₂ films to yield fast-response superhydrophilicity with a water contact angle of 5°. The mechanism of superhydrophilicity for the TiO₂ films deposited by high-pressure sputtering will be discussed based on empirical studies of X-ray diffractometry, high-resolution scanning microscopy and atomic force spectroscopy.     

Optical and structural properties of ZnO/TiO2/ZnO multi-layers prepared via electron beam evaporation

We prepared a ZnO/TiO₂/ZnO multi-layer on quartz glass substrate via electron beam evaporation. Optical and structural properties of the ZnO/TiO₂/ZnO multi-layer were investigated. The TiO₂ buffer layer is found to improve the crystallinity of the ZnO thin film. A green emission of the ZnO thin film deposited on the TiO₂ buffer layer was significantly enhanced due to the increased defect concentration of oxygen vacancy. Photoluminesence spectra measured at 9 K revealed that a violet luminescence at 409 nm was attributed to the draft of the donor's defect levels in the ZnO thin 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.     

Annealing effect on crystallization behavior of cubic MgxZn1−xO films on A-plane and M-plane sapphire

Cubic Mg_xZn_1−xO thin films with Mg composition around 70% were deposited on A-plane and M-plane sapphire substrates by rf-reactive magnetron sputtering. Measured structural and optical properties of these thin films indicated an optimal annealing temperature of 700 °C which produced high quality cubic MgZnO thin films on both substrates. Moreover, when the annealing temperature exceeded 750 °C, a much rougher surface resulted, and several large mosaic particles on the surface of the annealed films appeared. From EDX results, the Mg composition was lower than that found in other sections of the annealed films. We attributed this to thermally induced reconstruction of the crystallites. This phenomenon was more obvious for annealed MgZnO films on A-plane sapphire than that on M-plane sapphire. Thermal expansion mismatch with the substrate is the principal reason.     

Photoluminescence behaviors in ZnGa2O4 thin film phosphors deposited by a pulsed laser ablation

ZnGa₂O₄ thin film phosphors have been deposited using a pulsed laser deposition technique on Si (1 0 0) and Al₂O₃ (0 0 0 1) substrates at a substrate temperature of 550 °C with various oxygen pressures 100, 200 and 300 mTorr, and various substrate temperatures of 450, 550 and 650 °C with a fixed oxygen pressure of 100 mTorr. The films grown under different deposition conditions have been characterized using microstructural and luminescent measurements. Under the different substrate temperatures, ZnGa₂O₄ thin films show the different crystallinity and luminescent intensity. The crystallinity and photoluminescence (PL) of the ZnGa₂O₄ films are highly dependent on the deposition conditions, in particular, oxygen pressure, substrate temperature, a kind of substrates. The luminescent spectra show a broad band extending from 350 to 600 nm peaking at 460 nm. The PL brightness data obtained from the ZnGa₂O₄ films grown under optimized conditions have indicated that the sapphire is one of the most promised substrates for the growth of high quality ZnGa₂O₄ thin film phosphor.     

The properties of transparent semiconductor Zn1 − xTixO thin films prepared by the sol-gel method

Transparent semiconductor thin films of Zn_1 − xTi_xO (0 ≦ x ≦ 0.12) were deposited on alkali-free glass substrates by the sol-gel method. The effects of Ti addition on the crystallization, microstructure, optical properties and resistivity of ZnO thin films were investigated. The as-coated films were preheated at 300 °C, and then annealed at 500 °C in air ambiance. X-ray diffraction results showed all polycrystalline Zn_1 − xTi_xO thin films with preferred orientation along the (002) plane. Ti incorporated within the ZnO thin films not only decreased surface roughness but also increased optical transmittance and electrical resistivity. In the present study, the Zn_0.88Ti_0.12O film exhibited the best properties, namely an average transmittance of 91.0% (an increase of ~ 12% over the pure ZnO film) and an RMS roughness value of 1.04 nm.     

Optimization of L10-FePt/MgO/CrRu thin films for next-generation magnetic recording media

L1₀-FePt thin films were deposited on silicon substrates with the structure of Si/CrRu/MgO/FePt. The magnetic and microstructural properties were optimized by varying the FePt sputter pressure and temperature, as well as the thicknesses of all three layers. High coercivity films greater than 1.8 T were grown when the FePt sputter pressure was at 1.33 Pa with a thickness of only 4 nm, on CrRu and MgO underlayers as thin as 10 nm and 2 nm, respectively.     

Effects of substrate on the structural, electrical and optical properties of Al-doped ZnO films prepared by radio frequency magnetron sputtering

Transparent and conductive Al-doped ZnO (AZO) thin films were deposited on substrates including alkali-free glass, quartz glass, Si, and SiO₂ buffer layer on alkali-free glass by using radio frequency magnetron sputtering. The effects of different substrates on the structural, electrical and optical properties of the AZO films were investigated. It was found that the crystal structures were remarkably influenced by the type of the substrates due to their different thermal expansion coefficients, lattice mismatch and flatness. The AZO film (100 nm in thickness) deposited on the quartz glass exhibited the best crystallinity, followed sequentially by those deposited on the Si, the SiO₂ buffer layer, and the alkali-free glass. The film deposited on the quartz glass showed the lowest resistivity of 5.14 × 10^− 4 Ω cm among all the films, a carrier concentration of 1.97 × 10²¹ cm^− 3 and a Hall mobility of 6.14 cm²/v·s. The average transmittance of this film was above 90% in the visible light spectrum range. Investigation into the thickness-dependence of the AZO films revealed that the crystallinity was improved with increasing thickness and decreasing surface roughness, accompanied with a decrease in the film resistivity.     

Effect of copper-oxide segregation on the dielectric properties of CaCu3Ti4O12 thin films fabricated by pulsed-laser deposition

We investigated the effects of post-deposition cooling conditions on the surface morphologies and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) thin films grown by pulsed-laser deposition on Pt/TiO₂/SiO₂/Si substrates. CCTO thin films cooled under the typical cooling parameters, i.e., slow cooling (3 °C/min) at high oxygen pressure (66 kPa) showed a severe segregation of nanoparticles near the grain boundaries, which was identified to be copper oxide from electron probe micro analyzer mapping. On the other hand, we could not observe any segregation on the film surface when the samples were cooled fast (∼ 20 °C/min) at relatively low oxygen pressure (100 Pa). The dielectric constant, ε_r, of CCTO thin films deposited at 750 °C with severe surface segregation (ε_r ∼ 750 at 10 kHz) was found to be much lower than that (ε_r ∼ 2000 at 10 kHz) of CCTO thin films with smooth surface. As the copper-oxide segregation becomes more serious, which preferentially occurs at relatively high ambient oxygen pressure and temperature, the degradation in the dielectric properties of CCTO films becomes larger. The variation of dielectric constant of CCTO films with no copper-oxide segregation could be related to the presence of an impurity phase at grain boundaries.     

Growth of crystalline HgCr2S4 thin films at mild reaction conditions

Thin films of crystalline HgCr₂S₄ have been deposited on glass substrates at low temperature as low as 65 °C using a chemical bath deposition method. Typical thickness of the deposited HgCr₂S₄ thin films was 264 nm.The films were composed of closely packed irregular grains of 165-175 nm in diameter. The X-ray diffraction analysis and the selected area electron diffraction analysis revealed the deposited thin films were polycrystalline with highly (2 2 0) preferential orientation. The films exhibit a pure faint black. Their direct band gap energy was 2.39 eV with room temperature electrical resistivity of the order of 10⁻³ Ω cm.     

The role of the Zn buffer layers in the structural and photoluminescence properties of ZnO films on Zn buffer layers deposited by RF magnetron sputtering

Highly c-axis oriented ZnO thin films were grown on Si (100) substrates with Zn buffer layers. Effects of the Zn buffer layer thickness on the structural and optical qualities of ZnO thin films were investigated for the ZnO films with the buffer layers 90, 110, and 130 nm thick using X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) analysis techniques. It was confirmed that the quality of a ZnO thin film deposited by RF magnetron sputtering was substantially improved by using a Zn buffer layer. The highest ZnO film quality was obtained with a Zn buffer layer 110 nm thick. The surface roughness of the ZnO thin film increases as the Zn buffer layer thickness increases.     

Dielectric properties of DC reactive magnetron sputtered Al2O3 thin films

Alumina (Al₂O₃) thin films were sputter deposited over well-cleaned glass and Si < 100 > substrates by DC reactive magnetron sputtering under various oxygen gas pressures and sputtering powers. The composition of the films was analyzed by X-ray photoelectron spectroscopy and an optimal O/Al atomic ratio of 1.59 was obtained at a reactive gas pressure of 0.03 Pa and sputtering power of 70 W. X-ray diffraction results revealed that the films were amorphous until 550 °C. The surface morphology of the films was studied using scanning electron microscopy and the as-deposited films were found to be smooth. The topography of the as-deposited and annealed films was analyzed by atomic force microscopy and a progressive increase in the rms roughness of the films from 3.2 nm to 4.53 nm was also observed with increase in the annealing temperature. Al-Al₂O₃-Al thin film capacitors were then fabricated on glass substrates to study the effect of temperature and frequency on the dielectric property of the films. Temperature coefficient of capacitance, AC conductivity and activation energy were determined and the results are discussed.     

Deposition of Al-doped ZnO films on polyethylene naphthalate substrate with radio frequency magnetron sputtering

100 nm Al-doped ZnO (AZO) thin films were deposited on polyethylene naphthalate (PEN) substrates with radio frequency magnetron sputtering using 2 wt.% Al-doped ZnO target at various deposition conditions including sputtering power, target to substrate distance, working pressure and substrate temperature. When the sputtering power, target to substrate distance and working pressure were decreased, the resistivity was decreased due to the improvement of crystallinity with larger grain size. As the substrate temperature was increased from 25 to 120 °C, AZO films showed lower electrical resistivity and better optical transmittance due to the significant improvement of the crystallinity. 2 wt.% Al-doped ZnO films deposited on glass and PEN substrates at sputtering power of 25 W, target to substrate distance of 6.8 cm, working pressure of 0.4 Pa and substrate temperature of 120 °C showed the lowest resistivity (5.12 × 10^− 3 Ω cm on PEN substrate, 3.85 × 10^− 3 Ω cm on glass substrate) and high average transmittance (> 90% in both substrates). AZO films deposited on PEN substrate showed similar electrical and optical properties like AZO films deposited on glass substrates.     

Fabrication and characterization of all-oxide heterojunction p-CuAlO2 + x/n-Zn1 − xAlxO transparent diode for potential application in “invisible electronics”

Transparent p-n heterojunction diodes have been fabricated by p-type copper aluminum oxide (p-CuAlO_2 + x) and n-type aluminum doped zinc oxide (n-Zn_1 − xAl_xO) thin films on glass substrates. The n-layers are deposited by sol-gel-dip-coating process from zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) and aluminum nitrate (Al(NO₃)₃·9H₂O). Al concentration in the nominal solution is taken as 1.62 at %. P-layers are deposited onto the ZnO:Al-coated glass substrates by direct current sputtering process from a prefabricated CuAlO₂ sintered target. The sputtering is performed in oxygen-diluted argon atmosphere with an elevated substrate temperature. Post-deposition oxygen annealing induces excess oxygen within the p-CuAlO_2 + x films, which in turn enhances p-type conductivity of the layers. The device characterization shows rectifying current-voltage characteristics, confirming the proper formation of the p-n junction. The turn-on voltage is obtained around 0.8 V, with a forward-to-reverse current ratio around 30 at ± 4 V. The diode structure has a total thickness of 1.1 μm and the optical transmission spectra of the diode show almost 60% transmittance in the visible region, indicating its potential application in ‘invisible electronics’. Also the cost-effective procedures enable the large-scale production of these transparent diodes for diverse device applications.     

Effects of annealing atmosphere on thermoelectric signals from ZnO films

Zinc oxide epilayer films were grown on vicinal cut sapphire substrates by pulsed laser deposition with in situ annealing oxygen pressures varied from 0 to 10 × 10³ Pa. The best crystalline quality was obtained for ZnO layer with annealing oxygen pressure of 6 × 10³ Pa. Laser induced thermoelectric voltage (LITV) were observed along the tilting angle orientation of the substrate when the pulsed KrF excimer laser of 248 nm were irradiated on the films. The largest LITV signal was measured for the film grown at 6 × 10³ Pa annealing oxygen pressure. According to the measured LITV signals, Seebeck anisotropy was evaluated and was found to range from 3 to 12 μV/K for ZnO films annealed at different oxygen pressures from 2 to 10 × 10³ Pa. It is suggested that oxygen ambient plays an important role in the electronic properties of the ZnO films.     

Effect of oxygen pressure of SiOx buffer layer on the electrical properties of GZO film deposited on PET substrate

The present work was made to investigate the effect of oxygen pressure of SiO_x layer on the electrical properties of Ga-doped ZnO (GZO) films deposited on poly-ethylene telephthalate (PET) substrate by utilizing the pulsed-laser deposition at ambient temperature. For this purpose, the SiO_x buffer layers were deposited at various oxygen pressures ranging from 13.3 to 46.7 Pa. With increasing oxygen pressure during the deposition of SiO_x layer as a buffer, the electrical resistivity of GZO/SiO_x/PET films gradually decreased from 7.6 × 10^− 3 to 6.8 × 10^− 4 Ω·cm, due to the enhanced mobility of GZO films. It was mainly due to the grain size of GZO films related to the roughened surface of the SiO_x buffer layers. In addition, the average optical transmittance of GZO/SiO_x/PET films in a visible regime was estimated to be ~ 90% comparable to that of GZO deposited onto a glass substrate.     

Structural and optical studies of nanocrystalline V2O5 thin films

We report the structural and optical properties of nanocrystalline thin films of vanadium oxide prepared via evaporation technique on amorphous glass substrates. The crystallinity of the films was studied using X-ray diffraction and surface morphology of the films was studied using scanning electron microscopy and atomic force microscopy. Deposition temperature was found to have a great impact on the optical and structural properties of these films. The films deposited at room temperature show homogeneous, uniform and smooth texture but were amorphous in nature. These films remain amorphous even after postannealing at 300 °C. On the other hand the films deposited at substrate temperature T_S > 200 °C were well textured and c-axis oriented with good crystalline properties. Moreover colour of the films changes from pale yellow to light brown to black corresponding to deposition at room temperature, 300 °C and 500 °C respectively. The investigation revealed that nanocrystalline V₂O₅ films with preferred 001 orientation and with crystalline size of 17.67 nm can be grown with a layered structure onto amorphous glass substrates at temperature as low as 300 °C. The photograph of V₂O₅ films deposited at room temperature taken by scanning electron microscopy shows regular dot like features of nm size.