Evolution of Si (and SiC) nanocrystal precipitation in SiC matrix

Si_1−xC_x films with varying ratio of carbon to silicon (C/Si) were fabricated by magnetron co-sputtering from a combined C and Si target. The composition in films was changed by adjusting the ratio of sputtered target's area between C and Si. Analysis of X-ray photoelectron spectroscopy for as-deposited films shows that C/Si atomic ratios of our films have ranges of 0.33-1.02. Thermal annealing of as-deposited films was carried out at various temperatures from 800 to 1100 °C in a conventional furnace. Fourier transform infrared spectra show a shift of Si-C stretching peak towards higher wavenumbers from ∼ 737 cm^− 1 to ∼ 800 cm^− 1 with increasing annealing temperature. From the results of Raman spectroscopy, X-ray diffraction and transmission electron microscopy, it was found that the dominant type of nanocrystals changes from Si to SiC in the films annealed at 1100 °C when the C/Si atomic ratio increases from 0.33 to 1.02.     

Substrate bias voltage influenced structural, electrical and optical properties of dc magnetron sputtered Ta2O5 films

Tantalum oxide (Ta₂O₅) films were formed on silicon (111) and quartz substrates by dc reactive magnetron sputtering of tantalum target in the presence of oxygen and argon gases mixture. The influence of substrate bias voltage on the chemical binding configuration, structural, electrical and optical properties was investigated. The unbiased films were amorphous in nature. As the substrate bias voltage increased to −50 V the films were transformed into polycrystalline. Further increase of substrate bias voltage to −200 V the crystallinity of the films increased. Electrical characteristics of Al/Ta₂O₅/Si structured films deposited at different substrate bias voltages in the range from 0 to −200 V were studied. The substrate bias voltage reduced the leakage current density and increased the dielectric constant. The optical transmittance of the films increased with the increase of substrate bias voltage. The unbiased films showed an optical band gap of 4.44 eV and the refractive index of 1.89. When the substrate bias voltage increased to −200 V the optical band gap and refractive index increased to 4.50 eV and 2.14, respectively due to the improvement in the crystallinity and packing density of the films. The crystallization due to the applied voltage was attributed to the interaction of the positive ions in plasma with the growing film.     

Third-order optical nonlinearities in anatase and rutile TiO2 thin films

Titanium dioxide (TiO₂) films have been fabricated on fused quartz and Si(001) substrates by pulsed laser deposition technique and the single-phase anatase and rutile films were obtained under the optimal conditions. The surface images and optical transmission spectra were investigated by scanning electron microscopy and double beam spectrophotometer, respectively. The values of optical band-gap and linear refractive index of the anatase and rutile films were determined. The optical nonlinearities of the films were measured by Z-scan method using a femtosecond laser (50 fs) at the wavelength of 800 nm. Through the open-aperture and closed-aperture Z-scan measurements, the real and imaginary parts of the third-order nonlinear optical susceptibility were calculated and the results show that the anatase phase TiO₂ films exhibit larger nonlinear refractive effects compared with rutile phase. The figure of merit, T, defined by T = βλ/n₂, was calculated to be 0.8 for anatase films, meeting the requirement of T < 1 and showing potential applications in all-optical switching devices.     

Comparisons on the properties of (Zn1-xCox)2-W type barium hexaferrite thin films prepared on the Si (100) and (111) substrates

The (Zn_1-xCo_x)₂-W type barium hexaferrite thin films have been prepared by a radio frequency magnetron sputtering method on the Si (100) and the Si (111) substrates respectively. With increasing the annealing temperatures (800, 850, 900, 950, and 1000 °C), the Ba(CoZn)₂Fe₁₆O₂₇ phases emerge from the amorphous matrix. The hexaferrite thin films on Si (111) substrates have a larger saturation magnetic field (636.6 kA/m) than those on Si (100) substrates (159.1 kA/m). The magnetic hysteresis measurements show that they exhibit an isotropic behavior for thin films deposited on both substrates. Films on the Si (111) substrates are magnetically harder than those on the Si (100) substrates.     

Effect of cobalt doping on the structural and optical properties of TiO2 films prepared by sol-gel process

Pure and cobalt doped titanium dioxide thin films have been prepared on glass and Si (100) substrates by sol-gel spin coating method. The structural and optical properties of the films as a function of cobalt concentration (up to 15 wt.%) have been systematically studied by Rutherford backscattering spectroscopy, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, optical spectroscopy and spectroscopic ellipsometry methods. Rutherford backscattering studies show the presence of cobalt dopant in the films is almost equal to the precursor stoichiometry. Grazing incidence X-ray diffraction and Raman spectroscopy studies confirm the amorphous phase of the as-deposited films and crystalline anatase phase for the films annealed at 600 °C. The optical spectroscopy measurements show that the films are fully transparent in the visible region and there is a band gap narrowing upon increasing cobalt dopant concentration. The refractive index, band gap (E_g) and thickness of the films were determined from spectroscopic ellipsometry measurements. The refractive index of the films was found to increase from 2.2 to 2.7 with the increase in cobalt concentration with a simultaneous decrease of band gap from 3.1 to 2.8, which is favorable for photocatalytic applications. The packing density of the films was calculated by Clausius-Mossotti relation and is found to increase with cobalt concentration.     

Optical constants and band gap determination of Pb0.95La0.05Zr0.54Ti0.46O3 thin films using spectroscopic ellipsometry and UV–visible spectroscopy

We report the structural evolution and optical properties of lanthanum doped lead zirconate titanate (PLZT) thin films prepared on Pt/TiO₂/SiO₂/Si substrates by chemical solution deposition. X-ray diffraction demonstrates the post-deposition annealing induced crystallization for PLZT films annealed in a temperature (T_a) range of 550–750 °C. PLZT films annealed at higher temperature exhibit polycrystalline structure along with larger grain size. Optical band gap (E_g) values determined from UV–visible spectroscopy and spectroscopic ellipsometry (SE) for PLZT films were found to be in the range of 3.5–3.8 eV. E_g decreases with increasing T_a. The optical constants and their dispersion profiles for PLZT films were also determined from SE analyses. PLZT films show an index of refraction in the range of 2.46–2.50 (λ = 632.8 nm) with increase in T_a. The increase in refractive index at higher T_a is attributed to the improved packing density and crystallinity with the temperature.     

Properties of AgInSe<Subscript>2</Subscript> films grown by magnetron sputtering

n-Type AgInSe₂ films 0.5 to 0.9 μm thick were grown by dc magnetron sputtering. As targets, we used AgInSe₂ crystals grown by a modified Bridgman process using high-purity precursors. The crystal structure, morphology, electrical conductivity, and Hall coefficient of the films were studied at various temperatures. We determined the optimal growth and annealing temperatures of the films (500 and 250°C, respectively). Using structures based on the films, we obtained the spectral dependences of their photoresponse, established the nature of interband transitions in the films, and evaluated their band gap. The ability to vary electrical and optical properties with no changes in stoichiometry is of interest for concentrated solar power applications.     

Influence of target to substrate distance on the sputtered CuCl film properties

CuCl is a potential candidate for UV optoelectronic devices due to its superior optical properties and lattice matching with Si. Stoichiometric CuCl thin films of polycrystalline nature were grown by RF magnetron sputtering technique. The effect of varying the target to substrate distance on the compositional, structural and optoelectronic properties of the sputtered films was analysed. A critical target to substrate distance (d_ts) was observed and the film properties were clearly different above and below this distance. Based on the film properties, the optimum spacing of d_ts = 6 cm was found to yield stoichiometric and high optical quality films. The existence of more than one chemical bonding state was identified in nonstoichiometric, chlorine rich, films by analysing the Cu 2p_3/2 core level XPS spectra. Chlorine rich samples were found to show a noticeable emission from deep levels at ∼ 515 nm in cathodoluminescence (CL) spectroscopy. An exciton mediated sharp UV luminescence (385 nm) emission was realized at room temperature in the stoichiometric CuCl thin films.     

Structural and optical properties of In doped Se–Te phase-change thin films: A material for optical data storage

Se_75−xTe₂₅In_x (x = 0, 3, 6, & 9) bulk glasses were obtained by melt quench technique. Thin films of thickness 400 nm were prepared by thermal evaporation technique at a base pressure of 10⁻⁶ Torr onto well cleaned glass substrate. a-Se_75−xTe₂₅In_x thin films were annealed at different temperatures for 2 h. As prepared and annealed films were characterized by X-ray diffraction and UV–Vis spectroscopy. The X-ray diffraction results show that the as-prepared films are of amorphous nature while it shows some poly-crystalline structure in amorphous phases after annealing. The optical absorption spectra of these films were measured in the wavelength range 400–1100 nm in order to derive the extinction and absorption coefficient of these films. It was found that the mechanism of optical absorption follows the rule of allowed non-direct transition. The optical band gap of as prepared and annealed films as a function of photon energy has been studied. The optical band gap is found to decrease with increase in annealing temperature in the present glassy system. It happens due to crystallization of amorphous films. The decrease in optical band gap due to annealing is an interesting behavior for a material to be used in optical storage. The optical band gap has been observed to decrease with the increase of In content in Se–Te glassy system.     

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.     

The structure and properties of SnS thin films prepared by pulse electro-deposition

SnS films were prepared onto the ITO-coated glass substrates by pulse-form electro-deposition. The potential applied to the substrates was of pulse-form and its “on” potential, V_on was − 0.75 V (vs. SCE )and “off ” potential, V_off was varied in the range of − 0.1-0.5 V. The SnS films deposited at different V_off values were characterized by XRD, EDX, SEM and optical measurements. It shows that all the films are polycrystalline orthorhombic SnS with grain sizes of 21.54-26.93 nm and lattice dimensions of a = 0.4426-0.4431 nm, b = 1.1124-1.1134 nm and c = 0.3970-0.3973 nm, though the V_off has some influence on the surface morphology of the films and Sn/S ratio. When V_off = 0.1-0.3 V, the SnS films have the best uniformity, density and adhesion, and the Sn/S ratio is close to 1/1. The direct band gap of the films was estimated to be between 1.23 and 1.33 eV with standard deviation within ± 0.03 eV, which is close to the theoretical value. The SnS films exhibit p-type or n-type conductivity and their resistivity was measured to be 16.8-43.1 Ω cm.     

FTIR and ellipsometry characterization of ultra-thin ALD TaN films

In this paper, Fourier-transform infrared (FTIR) spectroscopy and ellipsometric spectroscopy were used to characterize the optical properties of atomic layer-deposited (ALD) ultra-thin TaN films on a Si(1 0 0) single crystal. The analysis of FTIR spectra indicates that the incorporated impurities are in the form of radicals of NH_x, CH_x and OH_x. SiH_x is also detected due to interfacial reactions between NH_x and the Si substrate native oxide. These H-containing radicals can be removed by post-annealing the samples. The vibration of Ta–N bonding is at the wavenumber of 1190 cm⁻¹, which is independent of the film thickness and post-annealing temperature. The results of ellipsometric spectra show that the band gaps are 3.28 eV, 2.65 eV and 2.50 eV as the films thicknesses are 1 nm, 5 nm and 10 nm, respectively. A slight red-shift of the band gap takes place after annealing the ultra-thin films. The mechanisms of the film optical properties were analyzed in the paper.     

Capacitance�Cvoltage characteristics of Pt/Bi2VO5.5/p-Si structures

Ferroelectric Bi₂VO_5.5 thin films were fabricated on p-type (100) Si substrates by sol?Cgel method and then annealed at different temperatures. The microstructures and surface morphologies of the Bi₂VO_5.5 thin films were examined by X-ray diffraction and atomic force microscope, respectively. The results indicate that the Bi₂VO_5.5 thin films show high c-axis preferred orientation and are compatible well with p-type Si substrates. The capacitance?Cvoltage characteristics of Pt/Bi₂VO_5.5/Si capacitors measured at 1 MHz shows a clockwise hysteresis loop. The memory window of the hysteresis loop is 0.42 V with the gate voltage from ?4 to 4 V. It is found that the memory window may be determined by the competition between ferroelectric polarization and charge injection.     

Ce1−xNdxO2−δ/Si thin films obtained by pulsed laser deposition: Microstructure and conduction properties

Substituted Ce_1−xNd_xO_2−δ cerium dioxide thin films are obtained by pulsed laser deposition technique. The films are deposited for various deposition times and at.% Nd, on [100] Si substrates, covered by a thin native SiO₂ layer. The evolution of the cell parameters with Nd content shows that a solid solution is formed, up to x = 0.27. The thin films are homogenous in composition at a nanometer scale. The morphology of the grains does not change significantly with Nd content. The microstructure is columnar, with a preferential [100] growth direction. The width of the grains varies from 20 to 30 nm. The conductivities of the thin films are determined from impedance spectroscopy analyses, in the temperature range 200 °C to 600 °C. The experimental data are explained in the frame of the space charge layer model.     

Effect of substrate temperature on the optical parameters of thermally evaporated Ge-Se-Te thin films

Thin films of Ge₁₀Se_90 − xTe_x (x = 0, 10, 20, 30, 40, 50) glassy alloys were deposited at three substrate temperatures (303 K, 363 K and 423 K) using conventional thermal evaporation technique at base pressure of ~ 10^− 4 Pa. X-ray diffraction results show that films deposited at 303 K are of amorphous nature while films deposited at 363 K and 423 K are of polycrystalline nature. The optical parameters, refractive index and optical gap have been derived from the transmission spectra (using UV-Vis-NIR spectrophotometer) of the thin films in the spectral region 400-1500 nm. This has been observed that refractive index values remain almost constant while the optical gap is found to decrease considerably with the increase of substrate temperature. The decrease in optical gap is explained on the basis of change in nature of films, from amorphous to polycrystalline state, with the increase of substrate temperature. The optical gap has also been observed to decrease with the increase of Te content.     

Growth of AgInSe2 thin films

Bulk AgInSe₂ was prepared by melting the constituent elements in stoichiometric pro proportions. Thin films were then grown from the bulk by a flash evaporation technique. X-ray diffraction studies were carried out on both the bulk material and the thin films. The data obtained were compared with data reported in the literature. The effect of the substrate temperature on the orientation of the films was also studied by electron diffraction. It was found that polycrystalline films of AgInSe₂ can be grown in the temperature range 150–250 °C.     

Effects of a Bi2Ti2O7 seeding layer on properties of Bi3.5Nd0.5Ti3O12 thin film

A thin-film bilayer structure consisting of polycrystalline Bi_0.5Nd_0.5Ti₃O₁₂ (BNdT) and preferential (111)-orientated Bi₂Ti₂O₇ (BTO) has been prepared on n-type Si (100) substrate by a chemical solution deposition and spin-coating technique. The crystallization of BNdT film can be enhanced and the surface consists of larger grains because of the intervention of BTO. We fabricated the Au/BNdT/BTO/Si and Au/BNdT/Si structures, respectively. Compared with BNdT, the BNdT/BTO film shows better insulating and memory properties. The room temperature resistivity is 10¹¹ Ω cm at a dc bias of 5 V. The C–V hysteresis curve is referred to as polarization-type switching and the memory window is about 3 V. All these results show that the BTO is an effective diffusion-barrier and plays a brilliant role as a seeding layer.     

The optical properties of Bi3.25La0.75Ti3O12 thin films with different thickness prepared by chemical solution deposition

Bi_3.25La_0.75Ti₃O₁₂(BiLT) thin films with different thickness were successfully deposited onto fused quartz by chemical solution deposition. X-ray diffraction analysis shows that BiLT thin films are polycrystalline with (0 0 2)-preferred orientation. The dispersion of refractive indices of the BiLT thin films was investigated by the optical transmittance spectrum. The optical band gap energy was estimated from the graph of (hνα)² versus hν. The results show that the refractive index and band-gap energy of the BiLT thin films decrease with the films thickness.     

Influence of thermal annealing on optical properties and surface morphology of NiOx thin films

NiO_x thin films were deposited by reactive DC-magnetron sputtering from a nickel metal target in Ar + O₂ with the relative O₂ content of 5%. Thermal annealing effects on optical properties and surface morphology of NiO_x films were investigated by X-ray photoelectron spectroscopy, thermogravimetric analysis, scanning electron microscope and optical measurement. The results showed that the changes in optical properties and surface morphology depended on the temperature. The surface morphology of the films changed obviously as the annealing temperature increased due to the reaction NiO_x → NiO + O₂ releasing O₂. The surface morphology change was responsible for the variation of the optical properties of the films. The optical contrast between the as-deposited films and 400 °C annealed films was about 52%. In addition, the relationship of the optical energy band gap with the variation of annealing temperature was studied.     

Structure, morphology and optical properties of nanocrystalline yttrium oxide (Y2O3) thin films

Yttrium oxide (Y₂O₃) thin films were grown onto Si(1 0 0) substrates using reactive magnetron sputter-deposition at temperatures ranging from room temperature (RT) to 500 °C. The effect of growth temperature (T_s) on the growth behavior, microstructure and optical properties of Y₂O₃ films was investigated. The structural studies employing reflection high-energy electron diffraction RHEED indicate that the films grown at room temperature (RT) are amorphous while the films grown at T_s = 300-500 °C are nanocrystalline and crystallize in cubic structure. Grain-size (L) increases from ∼15 to 40 nm with increasing T_s. Spectroscopic ellipsometry measurements indicate that the size-effects and ultra-microstructure were significant on the optical constants and their dispersion profiles of Y₂O₃ films. A significant enhancement in the index of refraction (n) (from 2.03 to 2.25) is observed in well-defined Y₂O₃ nanocrystalline films compared to that of amorphous Y₂O₃. The observed changes in the optical constants were explained on the basis of increased packing density and crystallinity of the films with increasing T_s. The spectrophotometry analysis indicates the direct nature of the band gap (E_g) in Y₂O₃ films. E_g values vary in the range of 5.91-6.15 eV for Y₂O₃ films grown in the range of RT-500 °C, where the lower E_g values for films grown at lower temperature is attributed to incomplete oxidation and formation of chemical defects. A direct, linear relationship between microstructure and optical parameters found for Y₂O₃ films suggest that tuning optical properties for desired applications can be achieved by controlling the size and structure at the nanoscale dimensions.