Structural-optical study of high-dielectric-constant oxide films

High-k polycrystalline Pr₂O₃ and amorphous LaAlO₃ oxide thin films deposited on Si(0 0 1) are studied. The microstructure is investigated using X-ray diffraction and scanning electron microscopy. Optical properties are determined in the 0.75-6.5 eV photon energy range using spectroscopic ellipsometry. The polycrystalline Pr₂O₃ films have an optical gap of 3.86 eV and a dielectric constant of 16-26, which increases with film thickness. Similarly, very thin amorphous LaAlO₃ films have the optical gap of 5.8 eV, and a dielectric constant below 14 which also increases with film thickness. The lower dielectric constant compared to crystalline material is an intrinsic characteristic of amorphous films.     

Spectroscopic ellipsometry modeling of ZnO thin films with various O2 partial pressures

ZnO films were deposited on thermally oxidized SiO₂/p-type Si (100) substrates and glass substrates by DC magnetron sputtering using a metal Zn target. Three types of samples were prepared with various O₂/(Ar + O₂) ratios (O₂ partial pressure) of 20%, 50%, and 80%. The properties of these ZnO thin films were investigated using X-ray diffraction (XRD), optical transmittance, atomic force microscopy (AFM), and spectroscopic ellipsometry in the spectral region of 1.7–3.1 eV. The structural and optical properties of ZnO thin films were affected by O₂ partial pressure. Relationships between crystallinity, the ZnO surface roughness layer, and the refractive index (n) were investigated with varying O₂ partial pressure. It was shown that the spectroscopic ellipsometry extracted parameters well represented the ZnO thin film characteristics for different O₂ partial pressures.     

Morphological and optical characterization of thermally evaporated copper sulphide thin films

Thin films of Cu₂S on opaque gold layers and quartz substrates at the temperature of 393 K were deposited by a thermal evaporation technique. The surface morphology of the Cu₂S thin films at different thicknesses is investigated by AFM. It is seen that all the films are composed of highly coordinated spherical nano-sized particles well adhered to the substrate. The transmittance and reflectance spectra of Cu₂S thin films on the quartz substrate were recorded by a UV–visible spectrophotometer. The results show that the thermally evaporated Cu₂S thin films have the characteristic transmittance and reflectance suitable for optoelectronic applications. The stoichiometry and surface morphology of a grown Cu₂S thin film were confirmed by energy-dispersive X-ray spectroscopy (EDAX) and scanning electron microscopy (SEM), respectively. The dependence of the refractive index and the extinction coefficient on the photon energy for both the surface film and the opaque gold layer have been determined by ellipsometry. From the spectral behaviour of the absorption coefficient at two distinct absorption regions, a dual-band scheme of optical absorption for a Cu₂S thin film is described. The indirect and direct edges of Cu₂S are found to be about at 0.91 eV and 2.68 eV, respectively.     

Dehydrogenation at the Fe/Lu2O3 interface upon rapid thermal annealing

The interfaces between ferromagnetic electrodes and tunnel oxides play a crucial role in determining the performances of spin-based electronic devices, such as magnetic tunnel junctions. Therefore, a deep knowledge of the structural, chemical, and magnetic properties of the buried interfaces is required. We study the influence of rapid thermal annealing treatments up to 500 °C on the interfacial properties of the Fe/Lu₂O₃ system. As-grown stacks reveal the presence of hydrogenated Fe-Lu-H intermetallic phases at the Fe/Lu₂O₃ interface most likely due to the H absorption on the Lu₂O₃ surface upon exposure to air and/or to the oxide growth. The annealing treatments induce remarkable changes of the structural, chemical, and magnetic properties at the interface, as evidenced at the atomic scale by the sub-monolayer sensitivity of conversion electron Mössbauer spectroscopy. The use of complementary techniques such as X-ray diffraction, time-of-flight secondary ion mass spectrometry, and in situ X-ray photoelectron spectroscopy, confirms that the main effect of the annealing is to gradually promote the dehydrogenation at the Fe/Lu₂O₃ interface.     

A Structural and Ellipsometric Investigation of Thin Gallium Oxide Layers Deposited on Si by Means of Laser Deposition

The morphology of Ga₂O₃ layers deposited on a silicon substrate by pulse laser deposition is studied using scanning electron microscopy in the thickness range of 30–200 nm. Using spectroscopic ellipsometry in the wavelength range from 250 to 900 nm, the thickness of layers and dispersion of their optical parameters (complex pseudodielectric function ε, refractive and absorptive indices) have been determined and the fraction of voids has been estimated in the scope of the Bruggeman effective medium approximation. In the spectrum of the imaginary part ε₂, there is observed an increase in values in the region of energy E = 4.8 eV corresponding to the direct interband transition in the Ga₂O₃ structure. X-ray structure analysis showed the presence of peaks typical for the β-Ga₂O₃ structure. The results of the investigations can be used for the formation of wide-bandgap gallium oxide layers in developing devices of power electronics and in creating optoelectronic devices for the UV range.     

The influence of Pr co-doping on the luminescent properties of Lu2O3:5 mol% Eu films

Uniform and crack free polycrystalline lutetium oxide (Lu₂O₃:(Eu,Pr)) films were fabricated by Pechini sol-gel method combined with the spin-coating technique. X-ray diffraction (XRD) and atomic force microscope (AFM) characterizations indicated that the obtained film was composed of polycrystalline cubic Lu₂O₃ phase with an average grain size around 30 nm. The photoluminescence(PL) spectra and decay performances of the Lu₂O₃:5 mol% Eu films co-doped by 0-0.5 mol% Pr³⁺ with different concentrations were characterized. It was found that the afterglow was reduced obviously due to the introduction of 0-0.5 mol% Pr³⁺ in the Lu₂O₃:5 mol% Eu films coupled by decrease in the emission intensity at 612 nm. The mechanism of afterglow diminishing was discussed based on the thermoluminescence measurements.     

The properties of the Y2O3 films exposed at elevated temperature

Yttrium trioxide (Y₂O₃) thin films have been deposited on silicon (1 1 1) substrates by RF magnetron sputtering. The influences of thermal exposure at high temperature in air on the structure, the surface morphology, roughness, and the refractive index of the Y₂O₃ thin film were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscopy (AFM), and spectroscopic ellipsometry (SE). The results indicate that chemical composition of the as-deposited Y₂O₃ film is apparently close to the stoichiometric ratio, and it has a cubic polycrystalline structure but the crystallinity is poor. The monoclinic and cubic phases can coexist in the Y₂O₃ film after thermal exposure to 900 °C, and the monoclinic phase disappears completely after 300 s exposure to 950 °C. The changes of the surface morphology, roughness, and the refractive index of the Y₂O₃ film are closely related to the crystal structure, the internal stress, and various defects influenced by thermal exposure temperature and time.     

Optical characterization of CuIn5S8 crystals by ellipsometry measurements

Optical properties of CuIn₅S₈ crystals grown by Bridgman method were investigated by ellipsometry measurements. Spectral dependence of optical parameters; real and imaginary parts of the pseudodielectric function, pseudorefractive index, pseudoextinction coefficient, reflectivity and absorption coefficients were obtained from the analysis of ellipsometry experiments performed in the 1.2–6.2 eV spectral region. Analysis of spectral dependence of the absorption coefficient revealed the existence of direct band gap transitions with energy 1.53 eV. Wemple–DiDomenico and Spitzer–Fan models were used to find the oscillator energy, dispersion energy, zero-frequency refractive index and high-frequency dielectric constant values. Structural properties of the CuIn₅S₈ crystals were investigated using X-ray diffraction and energy dispersive spectroscopy analysis.     

Annealing temperature dependent on structural,optical and electrical properties of indium oxide thin films deposited by electron beam evaporation method

In the present work the influence of annealing temperature on the structural and optical properties of the In₂O₃ films deposited by electron beam evaporation technique in the presence of oxygen was studied. The deposited films were annealed from 350 to 550 °C in air. The chemical compositions of In₂O₃ films were carried out by X-ray photoelectron spectroscopy (XPS). The film structure and surface morphologies were investigated as a function of annealing temperature by X-ray diffraction (XRD) and atomic force microscopy (AFM). The structural studies by XRD reveal that films exhibit preferential orientation along (2 2 2) plane. The refractive index (n), packing density and porosity (%) of films were arrived from transmittance spectral data obtained in the range 250–1000 nm by UV–vis-spectrometer. The optical band gap of In₂O₃ film was observed and found to be varying from 3.67 to 3.85 eV with the annealing temperature.     

Optical and structural characterization of ultrananocrystalline diamond/hydrogenated amorphous carbon composite films deposited via coaxial arc plasma

Ultrananocrystalline diamond/hydrogenated amorphous carbon composite films were deposited in the ambient of hydrogen by coaxial arc plasma deposition. The film compositions and chemical bonding structures were investigated by X-ray diffraction, X-ray photoemission and hydrogen forward scattering spectroscopies. The sp³/(sp²+sp³) ratio and hydrogen content in the film were estimated to be 64% and 35?at.%, respectively. The optical parameters and the optical dispersion profile were determined by using a variable angle spectroscopic ellipsometer at 55°, 65° and 75° angle of incidence in the photon energy range of 0.9–5?eV. Combinations of multiple Gaussian, and Tauc-Lorentz or Cody-Lorentz dispersion functions are used to reproduce the experimental data. Results of ellipsometry showed a refractive index of approximately 2.05 (at 2eV) and optical band gap of 1.63?eV. The imaginary part of dielectric function exhibited a peak at 3.8?eV, which has assigned to π-π* electron transitions. Furthermore, Electron spin resonance measurements implied the existence of dangling bonds, which might have a partial contribution to the optical absorption properties of the deposited films. A correlation between optical parameters and structural profile of the deposited films is discussed.     

Infrared optical properties of Mn1.56Co0.96Ni0.48O4 thin films prepared by chemical solution deposition

～2 μm thick Mn_1.56Co_0.96Ni_0.48O₄ (MCN) films have been prepared on a Al₂O₃ substrate by the chemical solution deposition method. X-ray diffraction and microstructure analyses show good crystallization and the thickness of the films is 2.12 μm. Mid-infrared optical properties of MCN films have been investigated using transmission spectra and infrared spectroscopic ellipsometry. The optical band gap of the MCN film has been derived to be 0.64 eV by assuming a direct transition between valence and conduction bands. The optical constants and thickness of the thin films have been obtained by fitting the measured ellipsometric parameter data with the classical infrared model. The refractive index n of the MCN films decreases as the wavelength increases, but the extinction coefficient k monotonously increases in the wavelength range of 2–7 μm. The maximal n value is 2.63, and the maximal k value is only 0.024. The above results are instructive for the applications of MCN films in infrared detecting.     

Dielectric matrix imposed stress–strain effect on photoluminescence of Ge nanocrystals

Ge nanocrystals embedded in SiO₂ and Lu₂O₃ matrices were fabricated using the pulsed laser deposition method and investigated using high-resolution transmission electron microscopy, X-ray diffractometry and photoluminescence spectroscopy. X-ray diffractometry and Fullprof computer program clearly revealed the bond lengths of Ge nanocrystals embedded in Lu₂O₃ matrix is smaller than that in SiO₂ matrix, which can be attributed to the greater compressive stress exerted on Ge nanocrystals by the Lu₂O₃ matrix. The greater compressive stress will lead to much more defects induced at the interface of Ge nanocrystals and thus enhance the intensity of photoluminescence. The findings presented here indicate that the matrix environment of the nanocrystals plays a significant role in the photoluminescence property.     

Studying the luminescence efficiency of Lu<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu nanophosphor material for digital X-ray imaging applications

Scintillator materials are widely used in X-ray medical imaging detector applications, coupled with available photoreceptors like radiographic film or photoreceptors suitable for digital imaging like a-Si, charge-coupled devises (CCD), complementary metal-oxide-semiconductors (CMOS) and GaAs). In addition, scintillators can be utilized in non-medical imaging detectors such as industrial detectors for non-destructive testing (NDT) and detectors used for security purposes (i.e. airport luggage control). Image quality and dose burden in the above applications is associated with the amount of optical photons escaping the scintillator as well as the amount of optical photons captured by the photoreceptor. The former is characterized by the scintillator efficiency and the latter by the spectral matching between the emission spectrum of the scintillator and the spectral response of the photoreceptor. Recently, a scintillator material, europium-activated lutetium oxide (Lu₂O₃:Eu), has shown improved scintillating properties. Lu₂O₃:Eu samples of compact nanocrystalline non-agglomerated powder were developed in our laboratory using homogeneous precipitation from a water-toluene solution in the presence of polyvinyl alcohol as a surfactant. In order to test their light-emission properties, experimental measurements under the excitation of X-ray spectra with X-ray tube voltages between 50 kVp and 140 kVp were performed. This range of applied voltages is appropriate for X-ray radiology, NDT and security applications. Lu₂O₃:Eu was evaluated with respect to output yield and spectral compatibility of digital imaging photoreceptors (CCD-based, CMOS-based, amorphous silicon a:Si flat panels, ES20 and GaAs). High light yield and spectral compatibility increase the performance of the medical detector and reduce the dose burden to the personnel involved. In addition a theoretical model was used to determine the values for the Lu₂O₃:Eu optical photon light propagation parameters. The inverse diffusion length was found to be equal to 33 cm²/g. In addition Lu₂O₃:Eu was found to match well with several photoreceptors capable of digital imaging (i.e. GaAs).     

Epitaxial gallium oxide on a SiC/Si substrate

Well-textured gallium oxide β-Ga₂O₃ layers with a thickness of ~1 μm and a close to epitaxial layer structure were grown by the method of chloride vapor phase epitaxy on Si(111) wafers with a nano-SiC buffer layer. In order to improve the growth, a high-quality silicon carbide buffer layer ~100 nm thick was preliminarily synthesized by the substitution of atoms on the silicon surface. The β-Ga₂O₃ films were thoroughly investigated using reflection high-energy electron diffraction, ellipsometry, X-ray diffraction, scanning electron microscopy, and micro-Raman spectroscopy. The investigations revealed that the films are textured with a close to epitaxial structure and consist of a pure β-phase Ga₂O₃ with the (\(\overline 2 01\)) orientation. The dependence of the dielectric constant of epitaxial β-Ga₂O₃ on the photon energy ranging from 0.7 to 6.5 eV in the isotropic approximation was measured.     

射频磁控反应溅射制备的Ag2O薄膜的椭圆偏振光谱研究

Ag₂O薄膜在新型超高存储密度光盘和磁光盘方面具有潜在的应用前景.利用射频磁控反应溅射技术, 通过调节衬底温度在沉积气压为0.2 Pa、氧氩比为2:3的条件下制备了一系列Ag₂O 薄膜.利用通用振子模型(包括1个Tauc-Lorentz振子和2个Lorentz 振子)拟合了薄膜的椭圆偏振光谱.在1.5-3.5 eV能量区间,薄膜的折射率在2.2-2.7之间, 消光系数在0.3-0.9之间. 在3.5-4.5 eV能量区间,薄膜呈现了明显的反常色散,揭示Ag₂O薄膜的等离子体振荡频率在 3.5-4.5 eV之间. 随着衬底温度的升高,薄膜的光学吸收边总体上发生了红移, 该红移归结于薄膜晶格微观应变随衬底温度的升高而增大. Ag₂O薄膜的光学常数表现出典型的介质材料特性.     

慢电子速度成像法结合低温冷离子阱测量镥的电子亲和势

本文采用低温冷离子阱囚禁的手段累积镥的负离子束流,使得对其电子亲和势的测量变得实际可行.运用慢电子速度成像法获得具有高分辨率的镥负离子的光电子能谱,测得镥的电子亲和势为1926.2(50) cm~(-1)或0.23882(62) eV.此外,还观察到镥负离子的两个激发态.     

The relationship between refractive index-energy gap and the film thickness effect on the characteristic parameters of CdSe thin films

CdSe thin films were deposited on glass substrates using Successive Ionic Layer Adsorption and Reaction (SILAR) method at room temperature and ambient pressure. The relationship between refractive index and energy bandgap was investigated. The film thickness effect on the structural, morphological, optical and electrical properties of CdSe thin films was investigated. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies showed that all the films exhibit polycrystalline nature with hexagonal structure and are covered well with glass substrates. The crystalline and surface properties of the films improved with increasing film thickness. The optical absorption studies revealed that the films are found to be a direct allowed transition. The energy bandgap values were changed from 1.93 to 1.87 eV depending on the film thickness. The electron effective mass (m_e^?/m_o), refractive index (n), optical static and high frequency dielectric constant (ε_o, ε_∞) values were calculated by using the energy bandgap values as a function of the film thickness. The resistivity of the films changed between 10⁶ and 10² Ω-cm with increasing film thickness at room temperature.     

Luminescence spectroscopy of excitons and antisite defects in Lu3Al5O12 single crystals and single-crystal films

The nature of the intrinsic luminescence of the lutetium aluminum garnet Lu₃Al₅O₁₂ (LuAG) has been analyzed on the basis of time-resolved spectral kinetic investigations upon excitation of two model objects, LuAG single crystals and single-crystal films, by pulsed X-ray and synchrotron radiations. Due to the differences in the mechanisms and methods of crystallization, these objects are characterized by significantly different concentrations of Lu_Al antisite defects. The energy structure of luminescence centers in LuAG single crystals (self-trapped excitons (STEs), excitons localized near antisite defects, and Lu_Al antisite defects) has been established. For single-crystal LuAG films, grown by liquid-phase epitaxy from a Pb-containing flux, the energy parameters of the following luminescence centers have been determined: STEs in regular (unperturbed by the presence of antisite defects) sites of the garnet lattice and excitons localized near Pb²⁺ ions. The structure of the luminescence centers, related to the background emission of impurity Pb²⁺ ions, has also been established in the UV and visible ranges. It is suggested that, in contrast to the two-halide hole self-trapping, a self-trapped state similar to STEs in simple oxides (Al₂O₃, Y₂O₃) is formed in LuAG; this state is formed by self-trapped holes in the form of singly charged O^? ions and electrons localized at excited levels of Lu³⁺ cations.     

Synthesis, characterization, and luminescent properties of Lu2O3:Eu phosphors

Eu-doped lutetia (Lu₂O₃:Eu) nano-phosphors were synthesized by the sol-gel combustion process from a mixed aqueous solution of europium and lutetium nitrates, using organic glycine as the fuel. Powder X-ray diffraction shows that cubic Lu₂O₃:Eu crystallites are directly obtained by the sol-gel combustion process without further calcination. Electron microscopy reveals that the as-prepared phosphors are agglomerated and have a fluffy, fine, and porous morphology, consisting of primary particle size of 8-10 nm. The excitation spectrum is characterized by three dominant bands centered at 395, 466, and 534 nm, respectively. Both the photoluminescent and radioluminescent spectra are very similar and exhibit intense emission peaks centered at 612 nm due to ⁵D₀→⁷F₂ transition of Eu³⁺ ions. The energy transfer from Lu₂O₃ host to Eu³⁺ activator is more efficient in the case of calcined phosphors than for the as-prepared phosphors due to their improved lattice perfection.     

Ellipsometric analysis and optical absorption characterization of gallium phosphide nanoparticulate thin film

Gallium phosphide (GaP) nanoparticulate thin films were easily fabricated by colloidal suspension deposition via GaP nanoparticles dispersed in N,N-dimethylformamide. The microstructure of the film was performed by x-ray diffraction, high resolution transmission electron microscopy and field emission scanning electron microscopy. The film was further investigated by spectroscopic ellipsometry. After the model GaP+void|SiO₂ was built and an effective medium approximation was adopted, the values of the refractive index n and the extinction coefficient k were calculated for the energy range of 0.75 eV-4.0 eV using the dispersion formula in DeltaPsi2 software. The absorption coefficient of the film was calculated from its k and its energy gaps were further estimated according to the Tauc equation, which were further verified by its fluorescence spectrum measurement. The structure and optical absorption properties of the nanoparticulate films are promising for their potential applications in hybrid solar cells.