Blue shift of optical band-gap in LiNbO3 thin films deposited by sol-gel technique

Lithium niobate (LN) thin films were deposited on quartz substrates by sol-gel technique. The measured absorption and photoluminescence spectra show that the band structure of LN thin films is direct unlike indirect band-gap in bulk LN and the optical band-gap of these LN thin films was measured to be 4.7 eV which is ~ 1 eV greater than that for stoichiometric bulk LN. The dependence of the blue shift of band-gap on several parameters like quantum confinement, composition (Li:Nb ratios of LN thin films) and strain was also investigated. The results obtained show that the large blue shift in band-gap of LN thin films is primarily due to strain in the film.     

Study of diamond-like carbon films on LiNbO3

Diamond-like carbon (DLC) films have been successfully deposited on Y-cut LiNbO₃ substrates using the plasma enhanced CVD technique. A thin interlayer of SiC between the DLC films and the LiNbO₃ is necessary to ensure a good adhesion of the DLC films to the LiNbO₃ substrate. The physical properties and structural network of the DLC films have been investigated in detail. It is observed that the film hardness is increased with increasing the film thickness, as is the adhesion of the DLC films to the LiNbO₃ substrates. The effect of accelerating surface acoustic wave by the DLC films has been confirmed.     

Optical properties change in amorphous (As2S3)0.87Sb0.13 thin films by photo and thermal induced process

Exposure with above band gap light and thermal annealing at a temperature near to glass transition temperature, of thermally evaporated amorphous (As₂S₃)_0.87Sb_0.13 thin films of 1 μm thickness, were found to be accompanied by structural effects, which in turn, lead to changes in the optical properties. The optical properties of thin films induced by illumination and annealing were studied by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Photo darkening or photo bleaching was observed in the film depending upon the conditions of the light exposure or annealing. These changes of the optical properties are assigned to the change of homopolar bond densities.     

Optical properties of the c-axis oriented LiNbO3 thin film

C-axis oriented Lithium Niobate (LiNbO₃) thin films have been deposited onto epitaxially matched (001) sapphire substrate using pulsed laser deposition technique. Structural and optical properties of the thin films have been studied using the X-ray diffraction (XRD) and UV-Visible spectroscopy respectively. Raman spectroscopy has been used to study the optical phonon modes and defects in the c-axis oriented LiNbO₃ thin films. XRD analysis indicates the presence of stress in the as-grown LiNbO₃ thin films and is attributed to the small lattice mismatch between LiNbO₃ and sapphire. Refractive index (n = 2.13 at 640 nm) of the (006) LiNbO₃ thin films was found to be slightly lower from the corresponding bulk value (n = 2.28). Various factors responsible for the deviation in the refractive index of (006) LiNbO₃ thin films from the corresponding bulk value are discussed and the deviation is mainly attributed to the lattice contraction due to the presence of stress in deposited film.     

Optical properties of HfO2 thin films deposited by magnetron sputtering: From the visible to the far-infrared

Hafnium oxide (HfO₂ or hafnia) holds promise as a high-index dielectric in optical devices and thermal barrier coatings, because of its transparency over a broad spectrum (from the ultraviolet to the mid-infrared) and chemical and thermal stability at high temperatures. In the present work, thin hafnia films of thicknesses from about 180 to 500 nm are deposited on Si substrates using reactive magnetron sputtering. The crystalline structure and surface topography are characterized by X-ray diffraction and atomic force microscopy, respectively. The optical and radiative properties of the film-substrate composites are measured at room temperature using spectroellipsometry and Fourier-transform infrared spectrometry. The optical constants are obtained from about 0.37 to 500 μm by fitting suitable models to the experimental results. Optical properties and dielectric function modeling are discussed with correlation to both film thickness and surface roughness. It is found that a single-oscillator dielectric-function model can describe radiative properties from about 1 to 20 μm. By combining Cauchy's formula (for the visible and near-infrared regions) with a multiple-oscillator Lorentz model (for the far-infrared region), a dielectric function is obtained for the HfO₂ films that is applicable from the visible to the far-infrared.     

The influence of CH4 addition on composition, structure and optical characteristics of SiCN thin films deposited in a CH4/N2/Ar/hexamethyldisilazane microwave plasma

Amorphous silicon carbonitride (a-SiCN) thin films were synthesized in a microwave plasma assisted chemical vapor deposition system using N₂, Ar, CH₄ and hexamethyldisilazane vapor (HMDSN). Composition, morphology and optical constants of the layers have been studied as a function of CH₄ rate in the range 0 to 9%. It was found that films are mainly composed of silicon nitride like compound whatever the CH₄ rate. However, CH₄ addition leads to less hydrogenated and denser films. In addition, a refractive index augmentation from 1.7 to 2.0 and a Tauc gap decrease from 5.2 eV to 4.8 eV is measured with CH₄ rate increase. It is believed that the refractive index augmentation is due to higher thin film density whereas hydrogen bonds decrease is assumed to contribute to the band gap narrowing. Besides, CH₄ addition to the gaseous mixture increases thin film oxidation resistance. These results show the ability of varying composition, structure and optical constants of a-SiCN films by modifying CH₄ rate in a N₂/Ar/HMDSN plasma.     

SiO2/TiO2 thin films with variable refractive index prepared by ion beam induced and plasma enhanced chemical vapor deposition

SiO₂/TiO₂ optical thin films with variable compositions have been prepared by ion beam induced and plasma enhanced chemical vapour deposition (IBICVD and PECVD). While the films obtained by IBICVD were very compact, the PECVD ones with a high content of Ti presented a columnar microstructure. The formation of Si–O–Ti bonds and a change in the environment around titanium from four- to six-coordinated has been proved by vibrational and X-ray absorption spectroscopies. The refractive index increased with the titanium content from 1.45 to 2.46 or 2.09 for, respectively, the IBICVD and PECVD films. Meanwhile, the band gap decreased, first sharply and then more smoothly up to the value of pure TiO₂. It is concluded that the optical properties of SiO₂/TiO₂ thin films can be properly tailored by using these two procedures.     

Dielectric and structural characterization of KNbO3 ferroelectric thin films epitaxially grown by pulsed laser deposition on Nb doped SrTiO3

KNbO₃ thin films were deposited on SrTiO₃ substrates by pulsed laser deposition. The X-ray diffraction patterns highlight an epitaxial growth according to the (011) orientation. This epitaxial growth was then confirmed by Electron Channeling Pattern. In agreement with the structural characteristics the dense microstructure consists in regular and ordered grains. Dielectric measurements were performed in the 20 Hz to 1 MHz frequency range on a KNbO₃ thin film grown on 2 at.% Nb doped (100)SrTiO₃ substrate in a large range of temperature in order to investigate the paraelectric-ferroelectric transition. Measurements at room temperature revealed a dielectric constant of 450 at 10 kHz and a minimum value of the loss tangent of 0.075 at 100 kHz. Dielectric study in the 20-600 °C temperature range showed a maximum of permittivity at the Curie temperature T_c = 410 °C and evidenced a “progressive” first-order phase transition, different from the classical “diffuse” transition.     

Nanocrystallization of ferroelectric lithium niobate in LiNbO3-SiO2 glasses

Transparent glasses in the (100-x)LiNbO₃-xSiO₂ system where 20 ≤ × ≤ 35, were produced by conventional melt-quenching technique. The quenched samples were amorphous as proved by X-ray diffraction (XRD) technique. Annealing of the quenched samples at temperatures ranging from 580 to 975 °C resulted in the precipitation of lithium niobate nano-crystals. Scanning electron microscopy (SEM) showed the presence of randomly oriented LiNbO₃ nano-crystals dispersed in a continuous glass matrix. The relative dielectric constant (ε_r) was in the range of 80 to 180 and increased with increasing LiNbO₃ concentration. The glass ceramic samples annealed at temperatures up to 600 °C are fully transparent.     

Preparation and characterization of C60S16 and C70S48 thin films

In this study, we have investigated different methods for preparation of thin films of C₆₀ and C₇₀-sulfur compounds. Films of good quality were obtained by reaction of amorphous C₆₀ and C₇₀ films with a saturated sulfur solution in toluene at 40°C or with saturated sulfur vapour at a temperature of 140°C for several hours. The quality of the fullerene-sulfur films were strongly dependent on the microstructure of the initially deposited fullerene film and the synthesis temperature. X-ray diffraction analyses showed that both methods lead to the formation of films consisting of C₆₀S₁₆ and C₇₀S₄₈ (space groups C 2/c and Amm2, respectively). C₆₀S₁₆ films synthesised on Al₂O₃(012) and Si(100) substrates were texture-free while C₇₀S₄₈ films typically exhibited a preferential (100) orientation. The films were also characterised by Raman and IR- spectroscopy, which confirmed that the interactions between the fullerene molecules and the S₈ rings are weak. The fullerene-sulfur compounds were found to be unstable at high vacuum conditions. Both materials C₆₀S₁₆ and C₇₀S₄₈ are non-conductive at room temperature with conductivities less then 10⁻⁵ (Ω/cm).     

Pulsed laser deposition of La0.67Ca0.33MnO3 thin films on LiNbO3 and surface acoustic wave studies

Surface Acoustic Waves on piezoelectric substrates can be used to investigate the dynamic conductivity of thin films in a non-contact and very sensitive way, especially at low conductivities. Here, we report on such surface acoustic wave studies to characterize thin manganite film like La_0.67Ca_0.33MnO₃, exhibiting a Jan Teller effect with a strong electron phonon interaction and a metal insulator transition at high temperatures.

We report on the deposition of La_0.67Ca_0.33MnO₃ on piezoelectric substrates (LiNbO₃ in different crystal cuts, employing a pulsed laser deposition technique. The structural quality of the thin films are examined by X-Ray Diffraction, Scanning Electron Microscope and Energy Dispersive X-ray spectroscopy. For the electrical characterization, we employ the surface acoustic wave technique, accompanied by conventional direct current resistance measurements for comparison.     

Rib waveguides based on Zn-substituted LiNbO3 films grown by liquid phase epitaxy

The fabrication of epitaxially grown Zn-substituted LiNbO₃ (Zn:LiNbO₃) waveguide films and rib waveguides is reported and detailed investigations about microstructure, morphology and optical waveguide properties are provided. Zn:LiNbO₃ films were grown on congruent X-cut LiNbO₃ substrates by a modified liquid phase epitaxy in solid–liquid coexisting solutions. The homogeneously Zn-substituted films exhibit high crystalline perfection and extremely flat surfaces with averaged surface roughness of rms = 0.2–0.3 nm. At the film/substrate interface a Zn-containing transient layer has been observed, which allows the growth of elastically strained Zn:LiNbO₃ film lattices. X-ray diffraction reciprocal-space measurements prove the pseudomorphic film growth. The refractive index difference between substrate and film depends on the zinc substitution content, which increase with rising growth temperatures. For films with 5.3 mol% Zn (Δn_o ≈ +5 × 10⁻³) only ordinary ray propagation was observed, while for films with 7.5 mol% Zn (Δn_o ≈ +8 × 10⁻³, Δn_e ≈ +5 × 10⁻³) both modes, TM and TE propagate. Stress-induced refractive index changes are in the order of Δn ≈ 10⁻⁴. In rib waveguide microstructures singlemode propagation with nearly symmetrical field distribution has been observed. To demonstrate the potential of the proton exchange-assisted dry-etching technique interferometer microstructures were fabricated.     

Characteristics of highly orientated BiFeO3 thin films on a LaNiO3-coated Si substrate by RF sputtering

BiFeO₃ (BFO) films were grown on LaNiO₃-coated Si substrate by a RF magnetron sputtering system at temperatures in the range of 300-700 °C. X-ray reflectivity and high-resolution diffraction measurements were employed to characterize the microstructure of these films. For a substrate temperature below 300 °C and at 700 °C only partially crystalline films and completely randomly polycrystalline films were grown, whereas highly (001)-orientated BFO film was obtained for a substrate temperature in the range of 400-600 °C. The crystalline quality of BFO thin films increase as the deposition temperature increase except for the film deposited at 700 °C. The fitted result from X-ray reflectivity curves show that the densities of the BFO films are slightly less than their bulk values. For the BFO films deposited at 300-600 °C, the higher the deposition temperature, the larger the remnant polarization and surface roughness of the films present.     

Single-source chemical vapor deposition of clean oriented Al2O3 thin films

Clean oriented Al₂O₃ thin film with a dominant Al₂O₃ <1 1 3> plane was deposited on Si <1 0 0> substrate at 550 °C, by single-source chemical vapor deposition (CVD) using aluminium(III) diisopropylcarbamate, Al₂(O₂CNⁱPr₂)₆. This process represents a substantial reduction in typical CVD film growth temperatures which are typically > 1000 °C. Through the studies of thermal stability of this precursor, we propose a specific β-elimination decomposition pathway to account for the low temperature of the precursor decomposition at the substrate, and for the lack of carbon impurity byproducts in the resulting alumina films that are characterized using X-ray photoelectron spectroscopy and depth profiling.     

Normally-off characteristics of LiNbO3/AlGaN/GaN ferroelectric field-effect transistor

With the help of MgO mask layer, LiNbO₃ (LN) ferroelectric films were etched effectively using wet etching method and LN/AlGaN/GaN ferroelectric field-effect transistors (FFETs) were fabricated. The electrical properties of the FFETs were studied. Due to the ferroelectric polarization nature of LN films, normally-off characteristics with a turn-on voltage of about + 1.0 V were exhibited in the device. The operation mechanisms of the LN/AlGaN/GaN FFET devices were proposed by the numerical calculations of the electronic band structure and charge distribution.     

Electronic structure of amorphous InGaO3(ZnO)0.5 thin films

The electronic structure of amorphous semiconductor InGaO₃(ZnO)_0.5 thin films, which were deposited by radio-frequency magnetron sputtering process, was investigated using X-ray photoelectron spectroscopy and O K-edge X-ray absorption spectroscopy. The overall features of the valence and conduction bands were analyzed by comparing with the spectra of Ga₂O₃, In₂O₃, and ZnO films. The valence and conduction band edges are mainly composed of O 2p and In 5sp states, respectively. The bandgap of the films determined by spectroscopic ellipsometry was approximately 3.2 eV. Further, it is found that the introduction of oxygen gas during the sputter-deposition does not induce significant variations in the chemical states and band structure.     

TiO2 thin films using organic liquid materials prepared by Hot-Wire CVD method

TiO₂ thin films prepared by Hot-Wire CVD method have been studied as a protecting material of transparent conducting oxide (TCO) against atomic hydrogen exposures for the fabrications of Si thin film solar cells. It was found that electrical conductivity of the films at room temperature reached a value of 0.4 S/cm. This value is 2-3 orders of magnitude higher than that of TiO₂ films prepared by RF magnetron sputtering and electron-beam evaporation methods in our previous works. The conductivity improvement seems to be partly due to the enlargement of TiO₂ crystallites.     

Growth of SrTiO3 thin epitaxial films by aerosol MOCVD

SrTiO₃ thin films were prepared by aerosol metal-organic chemical vapour deposition on (001) MgO, R-plane Al₂O₃ and (001) Si single-crystal substrates. Strontium tetramethyl heptadionate and titanium n-butoxide dissolved in diethyleneglycol dimethyl ether were used as precursors. The structure of the films was investigated by X-ray diffraction and transmission electron microscopy. Epitaxial films with [001] and [111] orientation perpendicular to the substrate surface were obtained on MgO and Al₂O₃, respectively. The epitaxial films on the MgO substrate were found to be in a relaxed state with lattice parameters corresponding to the bulk values. SrTiO₃ films on the Si substrate were grown as highly textured in the [011]direction and randomly oriented in the plane parrallel to the substrate surface.     

Pulsed laser deposition of Bi2Te3 thin films

A feasibility study of thin stoichiometric Bi₂Te₃ films by pulsed laser deposition (PLD) was performed, the difficulty arising from the differences of vapour pressure between Te and Bi. The films were elaborated using a pulsed Nd:YAG laser under various experimental conditions and were characterized by electron microsprobe, scanning electron microscopy, X-ray diffraction and secondary ion mass spectroscopy analyses. Congruent transfer of stoichiometry occurs from the target to the substrate on several cm² and a good crystallinity can be achieved, even on glass substrates at room temperature, by combining convenient target to substrate facing and distance, respectively. Depletion in Te observed in some films may result from a laser beam-plume interaction that was put forward during elaboration of films on large scale substrates.