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.     

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.     

A comparative study of the properties of thermally evaporated CuIn2n + 1S3n + 2 (n = 0, 1, 2 and 3) thin films

CuInS₂, CuIn₃S₅, CuIn₅S₈ and CuIn₇S₁₁ compounds were synthesized by the horizontal Bridgman method using high-purity copper, indium and sulphur elements. Crushed powders of these ingots were used as raw materials for the vacuum thermal evaporation. So, CuIn_2n + 1S_3n + 2 (n = 0, 1, 2, and 3) thin films were deposited by single source vacuum thermal evaporation onto glass substrates heated at 150 °C. The structural, compositional, morphological, electrical and optical properties of the deposited films were studied using X-ray diffraction (XRD), energy dispersive X-ray, atomic force microscopy and optical measurement techniques. XRD results revealed that all the films are polycrystalline. However, CuInS₂ and CuIn₃S₅ films had a chalcopyrite structure with preferred orientation along 112 while CuIn₅S₈ and CuIn₇S₁₁ films exhibit a spinel structure with preferred orientation along 311. The absorption coefficients of the all CuIn_2n + 1S_3n + 2 films are in the range of 10⁻⁴ and 10⁻⁵ cm⁻¹. The direct optical band gaps of CuIn_2n + 1S_3n + 2 layers are found to be 1.56, 1.78, 1.75 and 1.30 eV for n = 0, 1, 2, and 3, respectively. CuIn₃S₅ and CuIn₅S₈ films are p type with electrical resistivities of 4 and 12 Ω cm whereas CuInS₂ and CuIn₇S₁₁ are highly compensated with resistivities of 1470 and 1176 Ω cm, respectively.     

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.     

Epitaxial growth of NiTiO3 with a distorted ilmenite structure

MTiO₃ (M = Fe, Mn, Ni) compounds have received recent attention as possible candidates for multiferroic materials capable of magnetization switching by application of an electric field. In an attempt to stabilize NiTiO₃ in the rhombohedral R3 structure, epitaxial Ni_1 − xTi_1 − yO₃ films of different thickness and composition were deposited on Al₂O₃(0001) by pulsed laser deposition, and characterized using several techniques. Structural parameters for ilmenite-type NiTiO₃ and the metastable LiNbO₃-type NiTiO₃ structure with the space group R3c were predicted using density functional theory calculations, and compared with the experimental results. Our structural data from X-ray diffraction and X-ray absorption spectroscopy indicate that epitaxial ilmenite-type NiTiO₃ films were grown. Furthermore, lattice strain exerted by the sapphire substrate results in a distorted ilmenite structure similar to the LiNbO₃-type one. While R3c NiTiO₃, the desired structure based on recent theory, cannot be claimed at this point, our results demonstrate the potential of oxide heteroepitaxy to stabilize metastable multiferroic phases that may be difficult to prepare or are inaccessible in the bulk.     

Synthesis of SrSnO3 thin films by pulsed laser deposition: Influence of substrate and deposition temperature

SrSnO₃ thin films were prepared by pulsed laser deposition on amorphous silica and single crystal substrates of R-sapphire, (100)LaAlO₃ and (100)SrTiO₃. High quality epitaxial (100) oriented films were obtained on LaAlO₃ and SrTiO₃ while a texture was revealed for films on sapphire deposited at the same deposition temperature of 700 °C. Amorphous films were obtained on silica but a post annealing at 800 °C induced crystallization with a random orientation. The screening of deposition temperature showed epitaxial features on SrTiO₃ from 650 °C while no crystallization was observed at 600 °C. The influence of substrate and deposition temperature was confirmed by Scanning Electron Microscopy and Atomic Force Microscopy observations.     

Structure and dielectric properties of sputtered bismuth magnesium niobate thin films

The Bi_1.5MgNb_1.5O₇ (BMN) thin films were prepared on platinum coated sapphire by rf magnetron sputter deposition. Effects of substrate temperature, sputter pressure and O₂/(O₂ + Ar) mixing ratio on phase structures and dielectric properties of thin films were investigated. The results indicated that sufficiently high substrate temperature and low sputter pressure would facilitate the formation of cubic pyrochlore in BMN thin films. Meanwhile, the appropriate O₂/(O₂ + Ar) mixing ratio of sputter atmosphere was required. The deposited Bi_1.5MgNb_1.5O₇ cubic pyrochlore thin films with (222) oriented texture exhibited large tunability of ~ 50% at a maximum applied bias field of 1.5 MV/cm, with low dielectric loss of ~ 0.007. The temperature and frequency dependent dielectric measurements indicated that no noticeable dielectric dispersion was detected in BMN cubic pyrochlore thin films.     

Synthesis of c-axis oriented AlN thin films on different substrates: A review

Highly c-axis oriented AlN thin films have been deposited by reactive sputtering on different substrates. The crystallographic properties of layered film structures consisting of a piezoelectric layer, aluminum nitride (AlN), synthesized on a variety of substrates, have been examined. Aluminum nitride thin films have been deposited by reactive pulsed-DC magnetron sputtering using an aluminum target in an Ar/N₂ gas mixture. The influence of the most critical deposition parameters on the AlN thin film crystallography has been investigated by means of X-ray diffraction (XRD) analysis of the rocking curve Full-Width at Half Maximum (FWHM) of the AlN-(0 0 0 2) peak. The relationship between the substrate, the synthesis parameters and the crystallographic orientation of the AlN thin films is discussed. A guide is provided showing how to optimize these conditions to obtain highly c-axis oriented AlN thin films on substrates of different nature.     

Investigation on Mn doped ZnO thin films grown by RF magnetron sputtering

In this paper, we have investigated the Zn_1 − x Mn_xO (x = 0.05, 0.10 and 0.15) thin films grown by RF magnetron sputtering. The grown films on sapphire [Al₂O₃(0001)] substrates have been characterized using X-ray Diffraction (XRD), Photoluminescence (PL) and Vibrating Sample Magnetometer (VSM) in order to investigate the structural, optical and magnetic properties of the films respectively. It is observed from XRD that all the films are single crystalline with (002) preferential orientation along c-axis. PL spectra reveal that the addition of Mn marginally shifts the Near Band Edge (NBE) position towards the higher energy side. The magnetic measurements of the films using VSM clearly indicate the ferromagnetic nature.     

Room temperature deposition of ZnSe thin films by successive ionic layer adsorption and reaction (SILAR) method

The zinc selenide (ZnSe) thin films are deposited onto glass substrate using relatively simple and inexpensive successive ionic layer adsorption and reaction (SILAR) method. The films are deposited using zinc acetate sodium selenosulphate precursors. The concentration, pH, immersion and rinsing times and number of immersion cycles have been optimized to obtain good quality ZnSe thin films. The X-ray diffraction (XRD) study and scanning electron microscopy (SEM) studies reveals nanocrystalline nature alongwith some amorphous phase present in ZnSe thin films. Energy dispersive X-ray (EDAX) analysis shows that the films are Se deficient. From optical absorption data, the optical band gap ‘E_g’ for as-deposited thin film was found to be 2.8 eV and electrical resistivity in the order of 10⁷ Ω cm.     

Surface acoustic wave properties of aluminum oxide films on lithium niobate

Aluminum oxide (Al₂O₃) films have been deposited on lithium niobate (LiNbO₃) substrates by electron beam evaporation without any interlayer between them to ensure a good adhesion of the Al₂O₃ films to LiNbO₃ substrates. As Al₂O₃ thin films can sufficiently increase the surface acoustic wave (SAW) velocity, they can be used to improve the performance of the SAW device. The SAW phase velocity in the Al₂O₃/LiNbO₃ structure was found to increase with the insertion of an Al₂O₃ film, which can be attributed to the stiffening effect of the Al₂O₃ layer. The velocity change ratio of SAW was about 4.39% (at 304 MHz) for the Al₂O₃ (9.7 μm)/LiNbO₃ sample. A comparison with other findings in literature reveals that this result is better than what is available from diamond-like carbon/SiC buffer layer/LiNbO₃ structure, whose the velocity change ratio is 2%.     

Raman scattering and Rutherford backscattering studies on InN films grown by plasma-assisted molecular beam epitaxy

A series of InN thin films was grown on sapphire substrates via plasma-assisted molecular beam epitaxy (PA-MBE) with different nitrogen plasma power. Various characterization techniques, including Hall, photoluminescence, Raman scattering and Rutherford backscattering, have been employed to study these InN films. Good crystalline wurtzite structures have been identified for all PA-MBE grown InN films on sapphire substrate, which have narrower XRD wurtzite (0002) peaks, showed c-axis Raman scattering allowed longitudinal optical (LO) modes of A₁ and E₁ plus E₂ symmetry, and very weak backscattering forbidden transverse optical (TO) modes. The lower plasma power can lead to the lower carrier concentration, to have the InN film close to intrinsic material with the PL emission below 0.70 eV. With increasing the plasma power, high carrier concentration beyond 1 × 10²⁰ cm^− 3 can be obtained, keeping good crystalline perfection. Rutherford backscattering confirmed most of InN films keeping stoichiometrical In/N ratios and only with higher plasma power of 400 W leaded to obvious surface effect and interdiffusion between the substrate and InN film.     

Morphology and growth of e-beam deposited YSZ thin films

Yttria-stabilized zirconium (YSZ) thin films were grown from the tetragonal phase of ZrO₂ stabilized by 8 wt% of Y₂O₃ (8% of YSZ) ceramic powders using e-beam deposition technique (EB-PVD). The influence of the type of substrate on the microstructure of deposited YSZ thin films was analysed. YSZ thin films (2-3 μm of thickness) were deposited on three different types of substrates: optical quartz (SiO₂), porous Ni-YSZ substrates and Alloy 600 (Fe-Ni-Cr). The dependence of the substrate temperature (from 20 to 600 °C) on the thin film structure and the surface morphology were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that (i) the substrate temperature has an influence on the crystallite size, which varied between 12 and 50 nm, (ii) the substrate type has an influence on the growth mechanism of YSZ thin films, and (iii) a bias voltage applied to the substrate during the deposition of thin films has an influence on the densification of YSZ layers.     

Low temperature annealing effects on the structure and optical properties of ZnO films grown by pulsed laser deposition

ZnO thin films were deposited on glass substrates at room temperature (RT) ∼500 °C by pulsed laser deposition (PLD) technique and then were annealed at 150-450 °C in air. The effects of annealing temperature on the microstructure and optical properties of the thin films deposited at each substrate temperature were investigated by XRD, SEM, transmittance spectra, and photoluminescence (PL). The results showed that the c-axis orientation of ZnO thin films was not destroyed by annealing treatments; the grain size increased and stress relaxed for the films deposited at 200-500 °C, and thin films densified for the films deposited at RT with increasing annealing temperature. The transmittance spectra indicated that E_g of thin films showed a decreased trend with annealing temperature. From the PL measurements, there was a general trend, that is UV emission enhanced with lower annealing temperature and disappeared at higher annealing temperature for the films deposited at 200-500 °C; no UV emission was observed for the films deposited at RT regardless of annealing treatment. Improvement of grain size and stoichiometric ratio with annealing temperature can be attributed to the enhancement of UV emission, but the adsorbed oxygen species on the surface and grain boundary of films are thought to contribute the annihilation of UV emission. It seems that annealing at lower temperature in air is an effective method to improve the UV emission for thin films deposited on glass substrate at substrate temperature above RT.     

Composition, structure and optical properties of sputtered thin films of CuInSe2

Thin films of copper indium selenide (CuInSe₂) were produced by radio frequency (RF) sputtering due to the ability of this technique to achieve stoichiometric layers and its scalability to large-area devices. Results of energy dispersive analysis of X-rays (EDAX) revealed that the sputtered films were near to stoichiometry for substrate temperatures T_Sub not exceeding 200 °C. X-ray diffraction (XRD) patterns indicate that the films exhibited some pattern similar to that of bulk crystals of tetragonal chalcopyrite, predominantly [112] oriented. Based on the XRD patterns, the lattice parameters and grain sizes were examined. The band gap E_g, estimated from optical absorption data, was between 0.6-1.08 eV, depending on sputtering conditions such as substrate temperature and bias voltage. High optical absorption coefficients (> 10⁴ cm^− 1) were found.     

Physical properties of very thin SnS films deposited by thermal evaporation

SnS films with thicknesses of 20-65 nm have been deposited on glass substrates by thermal evaporation. The physical properties of the films were investigated using X-ray diffraction (XRD), scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and ultraviolet-visible-near infrared spectroscopy at room temperature. The results from XRD, XPS and Raman spectroscopy analyses indicate that the deposited films mainly exhibit SnS phase, but they may contain a tiny amount of Sn₂S₃. The deposited SnS films are pinhole free, smooth and strongly adherent to the surfaces of the substrates. The color of the SnS films changes from pale yellow to brown with the increase of the film thickness from 20 nm to 65 nm. The very smooth surfaces of the thin films result in their high reflectance. The direct bandgap of the films is between 2.15 eV and 2.28 eV which is much larger than 1.3 eV of bulk SnS, this is deserving to be investigated further.     

Synthesis and characterization of Nb2AlC thin films

We report on the synthesis and characterization of epitaxial c-axis oriented Nb₂AlC thin films deposited on c-axis sapphire (Al₂O₃) substrates by magnetron sputtering. Selected area electron diffraction reveal that independent of substrate temperature or film stoichiometry, there is the growth of a secondary phase not found in bulk, Nb₅Al₃C_x with a- and c-axis lattice constants of 7.746 Å and 5.246 Å, respectively. Scanning electron micrographs reveal large surface features, many with hexagonal shape and faceted texture. Atomic force microscopy topographical measurements indicate a surface roughness of approximately 15% of the total film thickness. Electrical transport measurements show typical metal-like conduction with a room temperature resistivity of ≈ 0.9 μΩ-m and a residual resistivity ratio of 2.5. A superconducting transition was found at ≈ 440 mK.     

Epitaxial growth of LiNbO3 thin films in a microwave oven

Oriented LiNbO₃ thin films were prepared using a polymeric precursor solution deposited on (0001) sapphire substrate by spin coating and crystallized in a microwave oven. Crystallization of the films was carried out in a domestic microwave oven. The influence of this type of heat treatment on the film orientation was analyzed by X-ray diffraction and electron channeling patterns, which revealed epitaxial growth of films crystallized at 550 and 650 °C for 10 min. A microstructural study indicated that the films treated at temperatures below 600 °C were homogeneous and dense, and the optical properties confirmed the good quality of these films.     

Effect of dopant concentration on the structural, electrical and optical properties of Mn-doped ZnO films

The Mn-doped ZnO (Zn_1 − xMn_xO) thin films with manganese compositions in the range of 0-8 at.% were deposited by radio-frequency (RF) magnetron sputtering on quartz glass substrates at room temperature (RT). The influence of Mn concentration on the structural, electrical and optical properties of Zn_1 − xMn_xO films has been investigated. X-ray diffraction (XRD) measurements reveal that all the films are single phase and have wurtzite structure with (002) c-axis orientation. The chemical states of Mn have been identified as the divalent state of Mn²⁺ ions in ZnO lattice. As the content of Mn increases, the c-lattice constant and the optical band gap of the films increase while the crystalline quality deteriorates gradually. Hall-effect measurements reveal that all the films are n-type and the conductivity of the films has a severe degradation with Mn content. It is also found that the intensity of RT photoluminescence spectra (PL) is suppressed and saturates with Mn doping.     

Thermal stability of amorphous molybdenum trioxide films prepared at different oxygen partial pressures by reactive DC magnetron sputtering

Thin films of MoO₃ were prepared on quartz and Si (1 0 0) substrates by reactive dc magnetron sputtering of a Mo target in an oxygen and argon atmosphere. The structural and optical changes induced in the films due to post-growth annealing have been systematically studied by Rutherford backscattering (RBS), X-ray diffraction (XRD), X-ray reflectivity (XRR) and by optical methods. RBS studies reveal no change in composition of the films upon annealing at high temperatures. Grazing angle XRD studies show that the as-deposited films are amorphous and crystallize to β-MoO₃ phase with small contribution of α-MoO₃ upon annealing at 300 °C. The film prepared at 0.40 Pa transforms to α-MoO₃ upon annealing at 650 °C, while the film deposited at 0.19 Pa still has some β-MoO₃ phase contribution. XRR measurements reveal that the film thickness decreases upon annealing with simultaneous increase of film density. The surface roughness of the films strongly increases after crystallization. The contraction of the film deposited at 0.40 Pa is much greater than the contraction of the film prepared at 0.19 Pa. The mass variation of the film deposited at 0.19 Pa and that deposited at 0.40 Pa are completely different. The optical properties of MoO₃ films deposited at 0.19 and 0.40 Pa are changed strongly by annealing.