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.     

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.     

Optical properties of single-wall carbon nanotube films deposited on Si/SiO2 wafers

The paper describes a set of simple experiments performed to develop an optical model to describe Si/SiO₂ substrates coated with two transparent films of carbon nanotubes. The final goal is to use such optical model to investigate the interaction of proteins with carbon nanotubes. Experiments were performed to assess light reflection as a function of the wavelength or angle of incidence using two substrates (same material, different amounts) composed of oxidized carbon nanotubes. The experimental results indicate that the selected carbon nanotubes layers are anisotropic and significantly different from each other. Experiments performed by spectroscopic ellipsometry (as a function of the wavelength and incident angle) enabled the development of an effective medium approximation model consisting in a two-fraction phase (arc-evaporated carbon and void space). Furthermore, the model enabled calculating the amount of protein adsorbed on the surface of the carbon nanotube film.     

Growth of epitaxial YSZ films on LiNbO3 substrates

We report on the epitaxial growth and characterization of yttria-stabilized zirconia (YSZ) films on X-cut LiNbO₃ single crystals. Epitaxial (100) YSZ films were synthesized by the on-axis RF magnetron sputtering technique. Extensive analyses of the structure and microstructure of films with different thickness reveals the existence of an Li-deficient phase, LiNb₃O₈, at the interface between the substrate and the film. The origin of the presence of this interface is discussed, as well as its consequences on the structural and morphologic properties of the epitaxial YSZ film.     

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.     

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.     

Surface and bulk properties of Cu nanocluster composites in LiNbO3

Implantation of 60 keV Cu⁻ ions into LiNbO₃ resulted in formation of complex nanocomposites consisting of metal Cu nanoparticles distributed among nanodomains of the host medium. The nanodomains of the host medium correspond to regions of a few hundred naometers in diameter with a constant refractive index. Distributions of the Cu nanoparticles and nanodomains in the implanted region do not correlate with each other. Variations of linear and non-linear optical absorption of the nanocomposites are mainly determined by the change of chemical composition and structure of implanted regions of LiNbO₃.     

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.     

Photoluminescence characteristics of Li-doped CaTiO3:Pr thin films grown on Si (100) substrate by PLD

The effects of Li-doped CaTiO₃:Pr³⁺ thin films have been investigated by varying the lithium ion concentrations from 0 to 5 wt.%. The films have been deposited on Si (100) substrate using a pulsed laser deposition technique. Structural properties of these films have been studied by the measurement of their XRD, SEM, and AFM. The variation of Li⁺ concentration influences the crystallinity and surface morphology of the CaTiO₃:Pr³⁺ thin films. As Li⁺ content increases from 0 to 1 wt.%, the crystallinity and intensity of emission increases. The dominant emission is from ¹D₂ → ³H₄ transition at 613 nm. The ¹D₂ emission quenching has also been observed in highly doped sample and is related to the cross-relaxation process between Pr³⁺ ions.     

Dielectric properties and thickness metrology of strain engineered GaN/AlN/Si (111) thin films grown by MOCVD

Maturity of silicon nanoelectronics and the high quality of 300 mm epi-Si wafers make these substrates an ideal choice for the growth of high quality III-Nitride devices. The results of our substrate engineering technique, which involves implantation of nitrogen into Si through an AlN thin film, have shown to simultaneously and significantly reduce the dislocation density and macro-cracks in epitaxially grown 2 μm GaN films. In this study, high quality strain engineered GaN films were grown by metalorganic chemical vapor deposition (MOCVD) and spectroscopic ellipsometry was used to characterize the dielectric properties, thickness, and stress of the complex structure. The uniaxial, anisotropic dielectric functions of wurtzite GaN and AlN were determined for the processes used in this study, and using this information, the thickness of each layer was determined in the completed film stack. IR spectroscopic ellipsometry (IRSE) was used as the non-destructive characterization technique to identify the IR sensitive phonon modes in AlN. The stress evolution in the films was investigated as a function of the phonon frequency shift and the broadening of the phonon modes. The results obtained by IRSE were further complemented by high resolution X-ray diffraction (HRXRD) and Raman scattering measurements.     

Investigations of the structural, optical, and electrical properties of Pb0.8Sn0.2Te layers grown on Si(100) using BaF2/CaF2 buffer

Pb_0.8Sn_0.2Te thin films were grown on Si(100) substrates using intermediate BaF₂/CaF₂ buffer layers in a molecular beam epitaxy system. Atomic force microscopy showed that the PbSnTe films become more flat as deposition rate increases, and substrate temperature decreases. The optimal substrate temperature was found to be about 350 °C. Fourier-transform infrared spectroscopy and variable angle spectroscopic ellipsometry were used for the optical characterization of the PbSnTe films in the infrared and visible to ultraviolet range of the light, respectively. It was found that the PbSnTe film grown at about 350 °C with higher deposition rate has the most pronounced optical characteristics as well as smoothest surface. The charge carrier concentrations and mobilities of the Pb_0.8Sn_0.2Te films were determined by means of infrared spectroscopy and also using conventional Hall-effect measurements, these results were then compared and analyzed. The different values obtained using these two techniques are explained by the grain structure of the surface and by occurrence of the microcracks in the PbSnTe films. The thicker PbSnTe films have the higher values of carrier mobility.     

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.     

Effect of substrate temperatures on the optical properties of evaporated Sc2O3 thin films

Scandium oxide (Sc₂O₃) films were deposited by electron beam evaporation with substrate temperatures varying from 50 to 350 °C. X-ray diffraction, scanning electron microscopy, spectrometer, and optical profilograph were employed to investigate the structural and optical properties of the films. The refractive index and extinction coefficient were calculated from the transmittance and reflectance spectra, and then the energy band gaps were deduced and discussed. Laser induced damage threshold of the films were also characterized. Optical and structural properties of Sc₂O₃ films were found to be sensitive to substrate temperature.     

New type of piezo-damping aluminum matrix composites with ZnO:Al-coated LiNbO3 particles

A new type of piezo-damping aluminum matrix composite containing ZnO:Al-coated LiNbO₃ particles was prepared. The dependence of the damping properties of composites on the resistivity of ZnO:Al coatings, altered by Al doping concentration, was investigated. Dynamic mechanical thermal analysis revealed that decreasing the resistivity of ZnO:Al coatings causes the loss factors of the composites to initially increase until the maximum value, before rapidly decreasing. Based on this piezo-damping material, the LiNbO₃ particles contribute to the transformation of mechanical vibration energy into electric energy, which is then converted into Joule's heat in the networks within the ZnO:Al coatings and metal matrix. An optimum formulation for piezo-damping metal matrix composites can be designed based on the results of this study.     

Influence of substrate absorption on accuracy of determination of refractive index and thickness of uniform thin chalcogenide Cu1[As2(S0.5Se0.5)3]99 film

The envelope method is a commonly used method for determination of some important optical constants, by using the envelopes of the transmittance T(λ) and/or reflectance R(λ) spectrum of the thin film deposited on transparent substrate. Two envelope methods were carried out in this paper: standard—method which assumes that substrate is absolutely transparent and modified—method which takes substrate absorption into account.The investigated sample is a uniform thin chalcogenide Cu₁[As₂(S_0.5Se_0.5)₃]₉₉ film, deposited onto two kinds of a weakly absorbing substrates that differ in thickness.It was shown that the degree of accuracy in determination of chosen optical parameters for both investigated samples is notably improved when the absorbance of the bare substrates is considered in the expressions for the envelopes.     

Characteristics of the precursors and their thermal decomposition during the preparation of LiNbO3 thin films by the Pechini method

Lithium niobate thin films have been deposited on Pt/Ti/SiO₂/Si(100) substrates by Pechini method. Characterization of the initial precursor solutions containing citric acid (CA), niobium and lithium ions has been performed by Fourier transform infrared spectroscopy, Raman spectroscopy and carbon nuclear magnetic resonance spectroscopy. The results indicate that citric acid coordinate to niobium ions to form a niobium-CA complex through one terminal carboxyl group, the hydroxyl group and the central carboxyl group as a tridentate ligand. The thermal decomposition of the Li-Nb precursors gel powder has been studied and the results show that LiNbO₃ phase is formed directly from the thermal decomposition of the precursor gel. By heat-treatment at 600 °C for 2 h, polycrystalline LiNbO₃ thin films with smooth and crack-free surface could be achieved.     

Fabrication of free standing LiNbO3 single crystal micro-platelets and their integration to Si-on-insulator platforms

Free standing single crystal LiNbO₃ micro-platelets (mm long and 1 μm thick) have been obtained from a z-cut LiNbO₃ wafer by ion implantation and thermal treatment. X-ray diffraction measurement has proved that they have a single crystal structure. Their surface roughness and film quality have been investigated by atomic force microscope and field emission scanning electron microscope. These micro-platelets are transferable and bondable to other materials which require material properties of LiNbO₃. They have been transferred, positioned and bonded to SiO₂/LiNbO₃, SiO₂/Si, and Si-on-insulator (SOI: Si/SiO₂/Si) by direct bonding method with optimum annealing conditions. For SiO₂/Si and SOI substrates, there were large thermal mismatch between LiNbO₃ and Si. They were, however, bonded at high temperature since these ultra thin micro-platelets were flexible and stretchable. Finally, to realize multifunctional SOI applications, a hybrid structure of LiNbO₃ film and Si waveguide has been demonstrated.     

Optical properties of BaTiO3/ZnO heterostructures under the effect of an applied bias

We report electro-optical measurements of BaTiO₃/ZnO heterostructures grown by pulsed laser deposition. The optical properties of the heterostructures were examined with and without an applied bias. A change in the heterostructure optical properties is found and attributed to a linear electro-optical effect causing a change in the band gap. Moreover the formation of an electric polarization in the BaTiO₃ layer causes a remanent change in the dielectric function if the bias is removed. The change could be estimated to be around 5 meV.     

Effect of precursor solution on the formation of perovskite phase of Pb(Mg1/3Nb2/3)O3 thin films

Precursor solutions for Pb(Mg_1/3Nb_2/3)O₃ (PMN) synthesis were obtained by Pechini's method. The influence of the concentration of organic materials on the phase formation has been studied. For this purpose, PMN solutions were prepared with different precursors and were characterized by thermogravimetric and differential thermal analysis. The obtained solutions were deposited onto a Si (100) substrate by dip coating and pre-treated in a hot plate at 300 °C for 1 h. The films were annealed at 600, 700, 800 and 900 °C for 1 h and characterized by X-ray diffraction. The perovskite phase was formed after annealing at 600 and 700 °C when the solution of PMN was prepared with a lower amount of organic material and starting with niobium oxide. By increasing the temperature to 800 or 900 °C, only the formation of pyrochlore phase was observed. With the solution prepared from niobium ethoxide, only the presence of pyrochlore phase was observed independently of the annealing temperature.