Infrared optical properties of Bi2Ti2O7 thin films by spectroscopic ellipsometry

Bi₂Ti₂O₇ thin films have been grown directly on n-type GaAs (1 0 0) by the chemical solution decomposition technique. X-ray diffraction analysis shows that the Bi₂Ti₂O₇ thin films are polycrystalline. The optical properties of the thin films are investigated using infrared spectroscopic ellipsometry (3.0–12.5 μm). By fitting the measured ellipsometric parameter (Ψ and Δ) data with a three-phase model (air/Bi₂Ti₂O₇/GaAs), and Lorentz–Drude dispersion relation, the optical constants and thickness of the thin films have been obtained simultaneously. The refractive index and extinction coefficient increase with increasing wavelength. The fitted plasma frequency ω_p is 1.64×10¹⁴ Hz, and the electron collision frequency γ is 1.05×10¹⁴ Hz, and it states that the electron average scattering time is 0.95×10⁻¹⁴ s. The absorption coefficient variation with respect to increasing wavelength has been obtained.     

Composition and substrate temperature dependence of structural and optical studies on ZnSexCdS1−x alloy films

Thin (t−0.60 μm) films of ZnSe_xCdS_1−x were formed by vacuum evaporation on glass substrates held at 350 and 470 K. XRD studies showed that all the films were polycrystalline in nature. Films with x0.70 were hexagonal whereas films with x0.80 were cubic in structure. The structural transition was in the range 0.70<x<0.80. The lattice parameter was higher in films formed at a higher temperature. The lattice parameter followed Vegard's law. Grain size increased with substrate temperature. From the optical transmission spectra recorded in the wavelength range 300–2500 nm, the extinction coefficient, refractive index and band gaps were obtained. Band gap values showed a downward bowing with ‘x' with a bowing parameter of 0.40 eV.     

High quality ZnO layers with adjustable refractive indices for integrated optics applications

Thin ( 1 μm) crystalline ZnO films with a good optical quality and good (0002) texture are grown under two considerably different process parameter sets using a r.f. planar magnetron sputtering unit. The optical parameters of the two corresponding ZnO layers are distinctly different: high refractive index ( 2.0 at λ = 632.8 nm) ZnO films resembling the single crystal form, and ZnO films with considerably lower (typical difference 0.05) refractive indices. The refractive index of the latter ZnO layers is adjustable ( 1.93–1.96 at λ = 632.8 nm) through the process deposition parameters. It is shown that the difference in refractive index between the two ZnO types most probably results from a difference in package density of the crystal columns. The optical waveguide losses of both ZnO types are typically 1–3 dB/cm at λ = 632.8 nm, however the low refractive index ZnO layers need a post-deposition anneal step to obtain these values. The two ZnO types are used to fabricate optical channel-and slab waveguides with small refractive index differences.     

不同厚度四面体非晶碳薄膜的高通量制备及表征

材料基因组工程能大幅度提高材料研发速度, 降低材料研发成本, 近年来受到广泛关注。本研究采用高通量制备工艺, 结合碳等离子体束流和基片位置的调控, 利用自主设计研制的45°双弯曲磁过滤阴极真空电弧设备, 沉积了厚度为4.7~183 nm的系列四面体非晶碳(ta-C)薄膜, 使用椭偏仪、原子力显微镜、拉曼光谱仪和X射线光电子能谱仪(XPS)表征了厚度对ta-C薄膜表面粗糙度、微结构和原子键态的影响。结果表明：通过碳等离子体束流和基片位置的调控, 实现了不同厚度ta-C薄膜的高通量制备。尽管膜厚不同, 所制备的ta-C薄膜均具有几乎不变的光滑表面(R_a=(0.38±0.02) nm)和色散值(Disp(G)), 说明不同厚度ta-C薄膜的sp³含量、sp²团簇尺寸保持相对稳定。XPS结果进一步证实ta-C薄膜的sp³相对含量均维持在(55±5)%。此外, 不同厚度ta-C薄膜的光学带隙E_opt均保持在(1.02±0.08)eV。相关结果为设计制备结构和光学性能可控的不同厚度ta-C薄膜提供了一种新思路。     

Thickness of diamond-like carbon coatings quantified with Raman spectroscopy

A model is developed for quantifying the thickness of thin coatings and wear scars using Raman spectroscopy. The model, which assumes that both incident and Raman light obey Beer's law, was applied to Raman spectra from a diamond-like carbon (DLC) coating containing Si and O, known as DLN (diamond-like nanocomposite). The coatings ranged in thickness from 10 nm to 2 μm, according to stylus profilometry. Systematic variations in the Raman carbon (G band) and Si (1st order) peak intensities vs. thickness were found. Fits to the model gave an optical mean free path of λ250 nm for DLN. This value is in good agreement with optical absorption coefficient values of other DLC films. Thickness profiles of wear tracks in the coatings determined by the model compared well with depths determined by profilometry.     

Preparation and properties of TiN and AlN films from alkoxide solution by thermal plasma CVD method

Single phase TiN and AlN films were prepared on a Si wafer from titanium tetra-etoxide and aluminum tri-butoxide solutions dissolved in ethanol and toluene, respectively, using an Ar/N₂/H₂ radio-frequency (r.f.) inductive thermal plasma chemical vapor deposition (CVD) method. The films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, measurement of electrical resistivity and Vickers microhardness. Factors affecting the formation of the films (lattice parameter, chemical composition, oxygen/carbon content, and deposition rate of the films) were examined in terms of the N₂ flow rate (2.5–4.5 slm), substrate temperature (300–700°C), feed rate of the solution (0.025–0.3 ml/min), and the mole ratio of the alkoxide solution (1:1–1:3). The optimum conditions for preparation of TiN films produced a film 0.2–3 μm thick with an oxygen content of 8 at.% and a free carbon content of 4 at.%, showing an electrical resistivity of 370 μΩ cm. The optimum conditions for AlN films produced a film 0.3 μm thick containing 14 at.% oxygen and 8 wt.% carbon. The deposition rate of the TiN film was determined to be 30–35 nm/min. The Vickers microhardness of the TiN and AlN films was found to be 10±1 and 13±3 GPa, respectively.     

Optical characterization of sputtered semitransparent zirconium nitride films

Thin semi-transparent ZrN films have been prepared using reactive dc magnetron sputtering. The films had thickness from 11 to 43 nm and were grown on heated and room temperature glass substrates. The optical constants, N=n+ik, of the thin films have been determined with an RT inversion method in the wavelength interval 0.40 to 2.0 μm. The thickness of the films was determined from the photometric measurements. The optical properties of the thin films on glass were compared to opaque and thin ZrN films grown on single crystalline Si. The Drude parameters were calculated from the measured optical constants in the relaxation region of the thin films. The relaxation time, τ, of the thin films was found to increase with film thickness, substrate temperature and substrate crystallinity. The relaxation time is the mean free time for the electrons between collisions and a long relaxation time corresponds to a film with high optical quality. The observed decrease of τ with decreasing film thickness can be explained by the higher statistical probability of the electrons in a thin film to collide with the two surfaces of the film. Another explanation to the decrease of τ with film thickness is scattering from grain boundaries and lattice impurities. The higher optical quality of films grown on heated substrates is probably due to an increased grain size. The measured optical constants were compared with calculated optical constants, using the Drude model, and the optical behaviour of thin ZrN films was found to be well described by the screened free-electron model.     

Photoluminescence in amorphous carbon thin films and its relation to the microscopic properties

The photoluminescence properties of amorphous hydrogenated carbon (a-C:H) films grown using a r.f. plasma have been investigated. It is found that the photoluminescence (PL) spectra exhibit a red shift, accompanied by a sharp decrease in PL efficiency and an increase in electron spin density as the applied r.f. power increases. The results can be interpreted by a model in which the a-C:H films consist of two phases with π-bonded clusters embedded in an sp³-bonded amorphous matrix. The excitation and the recombination of electron hole pairs are believed to take place in the π-bonded clusters. As a result of the confinement effect of electron hole pairs in these clusters, the cluster size becomes an important parameter in understanding the PL properties of a-C:H films.     

Doping of rf plasma deposited diamond-like carbon films

Rf plasma deposited diamond-like carbon (DLC) films have been doped n-type with the addition of nitrogen as a feed gas to a magnetically confined rf plasma. Controlled amounts of nitrogen are added to the CH₄/He plasma and the films are characterised. The electronic properties together with the microstructure of the deposited films are examined. Activation energy studies show the Fermi level can be moved from 0.5 eV away from the valence band for the undoped DLC films, through a maximum activation energy of 0.9 eV corresponding to the midgap and to 0.45 eV away from the conduction band with maximum N incorporation. The optical band gap first increases, indicative of a reduction in the band-edge tail states, and then tends to a steady value of ˜2 eV. Activation energy studies together with the optical band gap data are used to analyse the density of states for the deposited films. The preferential doping configuration of the atomic nitrogen and the importance of the π-π* states for electronic conduction for DLC:N films is discussed in the light of the findings.     

Ellipsometric and X-ray specular reflection studies on naturally grown overlayers on aluminium thin films

The results of detailed investigations on the natural surface layer formed at room temperature on aluminium films exposed to air are presented. Aluminium films of high perfection, deposited onto very smooth glass substrates, have been studied over a 2 year period using ellipsometry. Soft X-ray specular reflection analysis revealed a composite surface layer structure composed by a thin (d₂ = 0.8 nm) very compact alumina layer in contact with the aluminium substrate and by a thick (d₃ = 3 nm) hydrated oxide layer. A new computer procedure was applied for this composite layer system, which evaluated 72 ellipsometric experimental data and achieved a best fit of the measured and calculated Ψ and Δ values. The resultant optical constants of the aluminium substrate were n = 1.09, 0.95, 0.535 and 0.370 for λ = 579 nm, 546 nm, 436 nm and 365 nm respectively, whereas k = 6.72, 6.40, 4.96 and 4.23 respectively for the same mercury lines. Among widely scattered data from the literature, these are in good agreement with results of Hass on the assumption of a similar surface layer structure, using n₂ = 1.77 (alumina) and n₃ = 1.58 (hydrated oxide).

Our optical constants for aluminium were applied for evaluating ellipsometric experimental data obtained during the 2 year period. A slight systematic change in the d₂ and d₃ values of the samples was found, owing to hydration.     

Preparation and characterization of chemically deposited PbSnS3 thin films

High-quality and well-reproducible PbSnS₃ thin films have been prepared by a simple and inexpensive chemical-bath deposition method from an aqueous medium, using thioacetamide as a sulphide ion source. X-ray diffraction analysis of the deposited films revealed that the as-deposited films were amorphous, however, an amorphous-to-crystalline phase transition was observed as the result of thermal annealing at 425 K for 1 h. The X-ray structure analysis of the collected powder from the bath annealed at 425 K for 1.5 h revealed an orthorhombic phase.

Analysis of the optical absorption data of crystalline PbSnS₃ films revealed that both direct and indirect optical transitions exist in the photon energy range 1.24–2.48 eV with optical band gaps of 1.68 and 1.42 eV, respectively. However, a forbidden direct optical transition with a band gap value of 1.038 eV dominates at low energy (<1.24 eV). The refractive index changes from 3.38 to 2.16 in the range 500–1300 nm. The high frequency dielectric constant and the carrier concentration to the effective mass ratio calculated from the refractive index analysis were found to be 4.79 and 2.3×10²⁰ cm⁻³, respectively. The temperature dependence of the electrical resistivity of the deposited films follows the semiconductor behaviour with extrinsic and intrinsic conduction. The determined activation energies range are 0.35–0.42 and 0.76–85 eV, respectively.     

Properties of SnO2 films fabricated using a rectangular filtered vacuum arc plasma source

Transparent and conducting SnO₂ films of 57–200nm thickness were deposited on microscope glass slide substrates, using a rectangular filtered vacuum arc deposition system. The 40 glass slides were equally distributed on a 400 × 420mm substrate carriage, and were exposed to a Sn plasma beam, produced by a rectangular vacuum arc plasma gun with a Sn cathode, and passed through a rectangular magnetic macroparticle filter towards the substrates. The carriage with the substrates was transported past the 94 × 494mm filter outlet. The SnO₂ films were fabricated on the glass substrates at room temperature by maintaining the chamber oxygen background pressure at 0.52Pa. The film composition, and electrical and optical properties were studied as a function of the film thickness. The films were stored under ambient air conditions, and their electrical resistance was measured as a function of storage time over a period of several months.

The average resistivity of films was 10–17mΩ cm for films with thickness (t) less than 100nm, but that of t > 100nm it was 5–9mΩ cm. The resistivity of the films with t > 100nm did not change significantly after 8months of storage in ambient air. The optical transmittance of the films in the visible spectrum was in the range of 75–90%. The optical constants, i.e., the refractive index and the extinction coefficient of the films at wavelength λ = 550nm were in the range of 2.02–2.09 and 0.013–0.023, respectively, and the optical band gap energy was 4.15–4.21eV. Unlike the electrical resistivity, the optical parameters weakly depended on t.     

The optical and magneto-optical properties of MnSbBi films

The optical and magneto-optical properties of Mn(Sb_1−xBi_x) films were investigated for incident photons within the energy range from 1.3 to 4.0 eV. The diagonal and off-diagonal optical conductivities were determined from the results of spectroscopic ellipsometry and the polar Kerr effect measurements. A large Kerr rotation angle of about 0.57° was observed at photon energy ω≈2.65 eV for the Mn₅₅Sb₃₆Bi₉ film with an average grain size of 30 nm. The changes of the exchange splitting and the spin–orbit interaction strength are responsible for the strong spectral dependence of the Kerr effect in Mn(Sb_1−xBi_x) nanocrystalline films.     

Preparation, electrical and optical properties of (Pb,Ca)TiO3 thin films using a modified sol-gel technique

Calcium modified lead titanate sol was synthesized using lead acetate trihydrate, calcium nitrate tetrahydrate and titanium tetra-n-butoxide as starting materials, methanol and ethanolamine were selected as solvent and stabilizing or complexing agent, respectively. (Pb_0.76Ca_0.24)TiO₃ thin films were prepared on platinum-coated silicon and fused silica substrates with the solution using the spinning method. The surface morphology and crystal structure, surface compositions and chemical states, electrical and optical properties of the thin films were investigated. The films have good composition homogeneity and thickness uniformity. The dielectric constant and dissipation factor of 1 kHz at room temperature were found to be 280 and 0.027, respectively, for thin films with 0.5 μm thickness annealed at 600°C for 1 h. The remanent polarization and coervive field were 15 μC/cm² and 64 kV/cm, respectively. The thin films exhibited good optical transmissitivity, and had optical direct transitions. The dispersion relation of refractive index and wavelength followed the single electron oscillation model. The band gap of the film which annealed at 650°C was 3.68 eV. The results also confirmed that ethanolamine was very effective in preparing uniform and dense oxide films, owing to the superior stability of the sols during hydrolytic polycondensation.     

Fractal structure and optical properties of semicontinuous silver films

Semicontinuous silver films, prepared by vacuum evaporation on substrate of KBr crystals, have been made with area coverage in the range 0.3–0.8. The morphology of the films were found to change with coverage p. At small or large coverage range, homogeneous films were observed, but when the coverage approaches a critical value p_c (p_c ≈ 0.65), the film was inhomogeneous. The topologic parameters of films such as mean size of cluster S_AV, correlation length ξ and fractal dimension D_f were measured. When the area coverage was close to p_c, S_AV and ξ rapidly diverged and a knee point appeared in the curve of D_f vs. p. The transmittance of films in mid-infrared wave-band (2.5–12.5 m) was measured. For homogeneous films, as wavelength increased, transmittance increased when p < p_c but decreased when p < p_c. On the other hand, transmittance was wavelength independent for inhomogeneous films. The optical percolation phenomenon is observed at a region where the fractal dimension D_f of the film was kept approximately constant while correlation length diverges. These experimental results were compared with existing theory and we interpret the effect of deposition and coalescence on the percolation parameters by considering the growth mechanism of a nano-structured metal film.     

A theory for the formation of tetrahedral amorphous carbon including deposition rate effects

A theoretical model is presented to describe the effect of ion beam bombardment rate on the formation of tetrahedral amorphous carbon (ta-C) films. The critical ion energy, E_c, corresponding to a 50% sp³ content in the films, is found to be dependent on both the effective thermal resistance and the ion beam bombardment rate. In the model, the ‘window’ width in the ion energy scale for the formation of ta-C material increases with decreasing deposition rate and with a reduction in the effective thermal resistance, until limited by lower and upper boundary thresholds. Experimental data are reproduced by the model. The plasmon energy, which correlates with sp³ fraction, is found experimentally of be higher for lower deposition rate and smaller effective thermal resistance. Data points for high sp³ content ta-C films deposited on silicon substrates at room temperature occupy a region in the ion energy-deposition rate (E-r) diagram similar to that predicated from the theory.     

Characterization of ion-beam-deposited diamond-like carbon films

Diamond-like carbon (DLC) films are excellent prospects for a wide range of high-technology applications but their precise structure and properties are not well understood. The purpose of the present work was to use several complementary techniques to characterize the nature, structure and microstructure of DLC films. Thin DLC films were deposited on various substrates in the presence of a Si interlayer (500 Å thick) using CH₄ ion-beam deposition at an acceleration energy of 750 eV and a current density of about 2.5 mA cm⁻². The Si interlayer was deposited by either e-beam evaporation or Si evaporation enhanced by Ar⁺ beam bombardment (1 keV). The produced DLC films were featureless, very smooth and of high hardness (2900–3300 kg mm⁻²). Auger electron spectroscopy and electron diffraction showed that the films were mainly amorphous. Their microstracture was characterized by a three-dimensional network structure with a medium-range order of about 25 nm. Fourier transform infrared and Raman spectroscopies showed that the films were mainly composed of sp³ and sp² carbon-bonded hydrogen. The sp³/sp² ratio varied from 3.2 to 4.1 and was found to depend on the nature of the Si bond layer. The results showed that the nucleation of the diamondlike structure was promoted on the Si interlayer that was deposited under Ar⁺ beam bombardment. This effect can be explained by the higher surface roughness produced in this interlayer as suggested by the reflectivity measurements. Spectroscopic ellipsometry revealed that the films had an optical band gap between 1.56–1.64 eV. The present results are consistent with previous proposals suggesting that the DLC structure is composed of small graphitelike clusters (involving fused six-fold rings) that are interconnected by sp³-bonded carbon.     

Optical and structural characterizations of Cu-doped KCl films

The production of highly Cu⁺-doped KCl films and the properties of their 266 nm absorption band, which has an off-center property in doped single crystals, open the possibility of application of these films as ultra-violet optical filters. The investigated films, of approximately 1 μm thickness, were prepared by resistive co-evaporation of KCl and CuCl powders on different substrates of CaF₂, Al₂O₃, SiO₂, KCl and Si. The Cu⁺ concentration, as determined by energy-dispersive X-ray, ranges from 10²⁰ to 10²¹ cm⁻³, for 1–15% CuCl nominal mole percent concentration. Structural and optical properties were investigated through scanning electron microscopy, X-ray diffraction, ellipsometry, optical absorption and transmission. The films are polycrystalline, and the gain size decreases with increasing Cu⁺ impurity concentration, yielding an increase of visible transmission to a limited CuCl concentration. These films show a 6.295 Å lattice parameter with a f.c.c. structure and an index of refraction of 1.53 at 266 nm. When the Cu⁺ concentration is increased, the UV band position remains the same and no clusters are evidenced even to the high 15% CuCl concentration investigated, which differs very much from single crystals samples grown by the Kyropoulos-Czochralski method. For a Cu⁺ concentration of 8×10²⁰ cm⁻³ the film shows a transmission better than 88% at 350 nm wavelength.     

Optical transitions in laser-evaporated thin CuInSe2 films

The optical absorption coefficients of highly oriented laser-evaporated thin films were determined from the measured reflectance R(λ) and transmittance T(λ) in the wavelength range 400–1700 nm. The optical absorption spectrum of CuInSe₂ thin films shows three energy gaps, which are associated with the fundamental edge and valence band splitting by the tetragonal crystal-field and spin-orbit effects, and four optical transitions from the copper d levels to the conduction bands.