Volume 70 (2019): Issue 7 (December 2019)
Special Issue

Dispersion models are necessary for precise determination of the dielectric response of materials used in optical and microelectronics industry. Although the study of the dielectric response is often limited only to the dependence of the optical constants on frequency, it is also important to consider its dependence on other quantities characterizing the state of the system. One of the most important quantities determining the state of the condensed matter in equilibrium is temperature. Introducing temperature dependence into dispersion models is quite challenging. A physically correct model of dielectric response must respect three fundamental and one supplementary conditions imposed on the dielectric function. The three fundamental conditions are the time-reversal symmetry, Kramers-Kronig consistency and sum rule. These three fundamental conditions are valid for any material in any state. For systems in equilibrium there is also a supplementary dissipative condition. In this contribution it will be shown how these conditions can be applied in the construction of temperature dependent dispersion models. Practical results will be demonstrated on the temperature dependent dispersion model of crystalline silicon.

In this paper the overview of the most important approximate methods for the optical characterization of inhomogeneous thin films is presented. The following approximate methods are introduced: Wentzel–Kramers–Brillouin–Jeffreys approximation, method based on substituting inhomogeneous thin films by multilayer systems, method based on modifying recursive approach and method utilizing multiple-beam interference model. Principles and mathematical formulations of these methods are described. A comparison of these methods is carried out from the practical point of view, ie advantages and disadvantages of individual methods are discussed. Examples of the optical characterization of three inhomogeneous thin films consisting of non-stoichiometric silicon nitride are introduced in order to illustrate efficiency and practical meaning of the presented approximate methods.

This article discusses the importance of analytical and experimental approaches in Deep level transient Fourier spectroscopy in terms of reliability, to support the current research and the utilization of this technique for complex investigations. An alternative evaluation approach is proposed and validated by relevant experiments. Attention is focused on a GaAs p-i-n structure, the undoped layer induced defect conduction type statement difficulty, accurate evaluation of a dual type majority-minority carrier defect complex and possible limitations of the DLTS experimental technique. Comprehensive evaluation is carried out and the method is discussed in detail. In comparison with reference data, higher precision of calculated activation energies, differences even lower as 10⁻³ order of magnitude, were achieved.

Atomic layer deposited polycrystalline Cr₂O₃ films grown from chromyl chloride and methanol were analysed using spectrophotometry, spectral ellipsometry and atomic force microscopy. The films possessed polycrystalline eskolaite structure with rough sublayer in contact with air. Using the positions and peak widths of the two visible absorption bands as fixed from absorption measurements, we could determine the optical dispersion of the film material in 1.3 – 6 eV energy region. A direct band gap of chromia film grown in these conditions was 3.2 eV, the other also direct absorption band with a gap of 5.15 eV was found situated in UV.

The goal of the nanoparticle synthesis is, first of all, the production of nanoparticles that will be more similar in size and shape. This is very important for the possibility of studying and applying nanomaterials because of their characteristics that are very sensitive to size and shape such as, for example, magnetic properties. In this paper, we propose the shape analysis of the nanoparticles using three shape descriptors – elongation, convexity and circularity. Experimental results were obtained by using TEM images of hematite nanoparticles that were, first of all, subjected to segmentation in order to obtain isolated nanoparticles, and then the values of elongation, convexity and circularity were measured. Convexity C_x(S) is regarded as the ratio between shape’s area and area of the its convex hull. The convexity measure defines the degree to which a shape differs from a convex shape while the circularity measure defines the degree to which a shape differs from an ideal circle. The range of convexity and circularity values is (0, 1], while the range of elongation values is [1, ∞). The circle has lowest elongation (ε = 1), while it has biggest convexity and circularity values (C_x = 1; C = 1). The measures ε(S), C_x(S), C(S) proposed and used in the experiment have the few desirable properties and give intuitively expected results. None of the measures is good enough to describe all the shapes, and therefore it is suggested to use a variety of measures so that the shapes can be described better and then classify and control during the synthesis process.

In this work black silicon (b-Si) samples were prepared by anodic (electrochemical) etching of p-type silicon substrate in solution of hydrofluoric acid (HF). We studied influence of anodic etching conditions (etching time, electrical potential and current) on the spectral reflectance and Raman scattering spectra. Optical properties of b-Si structures were experimentally studied by UV-VIS (AvaSpec-2048) and Raman (Thermo DXR Raman) spectrometers. B-Si layer thickness of formed substrate were determined by using SCOUT software. Effective medium approximation theory (Looyenga) was used in construction of the reflectance model. Influence of the deformation of crystal lattice introduced during the substrate etching was studied by Raman scattering method. Teoretical model of the 1^st order Raman scattering profile was constructed by using pseudo-Voigt function and the profile parameters were extracted. The values of biaxial tensile stress were estimated by using optimized Raman profile parameters.

Black silicon layers were formed on silicon substrate by the surface structure chemical transfer method and by anodic etching method. Properties of microstructure of formed layers were experimentally studied by the electron microscopy methods (TEM) and characterized by statistical, Fourier and multifractal methods. Theoretical structures with defined fractal properties and surface roughness were generated and their microstructure properties were evaluated. Obtained results were used for the explanation of the real structure development during the forming procedure. By using of this approach, we study the correlation of roughness and fractality with optical properties. Black silicon layers were also investigated by using of Raman scattering method. Optimized theoretical model describing the 1^st order of black Si Raman scattering profile was constructed and used for evaluation of the biaxial tensile stress introduced during etching procedure.

Meyer-Neldel behaviour of the conductivity of phase separated La_1−xCa_xMnO₃ manganite system in the low Ca-doping range has been investigated. Evolution of the isokinetic temperature of the conductivity, modified by Ca-doping, hydrostatic pressure and current bias has been determined. In addition, the evolution of the isokinetic temperature with ageing has also been studied. It is found that the Meyer-Neldel behaviour of the manganite system stems from multi-excitation entropy mechanism. The isokinetic temperatures estimated from pressure and doping effects coincide but differ from those determined using current and ageing controlled conductivity changes. It is concluded that in the presence of a detailed theoretical model of the excitations coupling in manganites, the investigations of the Meyer-Neldel effect may became a powerful tool for characterization and investigation of transport mechanisms in phase separated manganites.

Using the sol-gel method we synthesized hematite (α − Fe₂O₃) nanoparticles in a silica matrix with 60 wt % of hematite. X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectra of the sample demonstrate the formation of the α − Fe₂O₃ phase and amorphous silica. A transmission electron microscopy (TEM) measurements show that the sample consists of two particle size distributions of the hematite nanoparticles with average sizes around 10 nm and 20 nm, respectively. Magnetic properties of hematite nanoparticles were measured using a superconducting quantum interference device (SQUID). Investigation of the magnetic properties of hematite nanoparticles showed a divergence between field-cooled (FC) and zero-field-cooled (ZFC) magnetization curves and two maxima. The ZFC magnetization curves displayed a maximum at around T_B = 50 K (blocking temperature) and at T_M = 83 K (the Morin transition). The hysteresis loop measured at 5 K was symmetric around the origin, with the values of coercivity, remanent and mass saturation magnetization H_C10K ≈ 646 A/cm, (810 Oe), M_r10K = 1.34 emu/g and M_S10K = 6.1 emu/g respectively. The absence of both coercivity (HC300K = 0) and remanent magnetization (Mr300K = 0) in M(H) curve at 300 K reveals super-paramagnetic behavior, which is desirable for application in biomedicine. The bimodal particle size distributions were used to describe observed magnetic properties of hematite nanoparticles. The size distribution directly influences the magnetic properties of the sample.

In this paper we report results from optical transmittance spectroscopy complemented with data from Raman scattering measurements to determine optical properties of two series of silicon based bilayers deposited by PECVD on glass substrate (intrinsic a-Si:H/p-type a-SiC:H and n-type mc-Si:H/intrinsic a-Si:H). These samples represent segments of common p-i-n thin film amorphous silicon solar cells with intrinsic hydrogenated silicon (a-Si:H) as the solar absorber. The members of the series differ by the KCN treatment conditions. Dispersive and absorptive optical properties – refractive indices, absorption coefficients and optical band gaps were determined from transmittance spectra. Each bilayer was considered as one effective thin film the optical properties of which can be regarded as effective optical properties of the bilayer structure. After KCN treatments refractive indices were modified probably due to the structural changes of bilayers. Moreover the effect of the solvent used in KCN solutions was recognized. Optical band gaps calculated either by the Tauc procedure or determined as iso-absorption levels were found to be only slightly KCN treatment dependent.

The general concepts are analyzed regarding the approach for the formation of paramagnetic species of noble metals, with a non-rigid (labile) molecule being used as a supporting matrix. The formation of the metal nanospecies follows three stages: (i) the metal ions are captured by the matrix, (ii) the reducing agent causes formation of individual atoms separated by the matrix fragments, (iii) the individual atoms agglomerate due to conformational transformations of the labile molecule-matrix. This algorithm is realized in two distinct systems: Ag-containing nanospecies embedded within the system of polyacrylic acid (PAA) chains grafted to the film of fluorinated ethylene propylene copolymer (FEP) and Au-containing nanospecies in the free matrix of tannin-citrate- oxo-hydroxo aluminate. The evolution of the electron paramagnetic resonance (EPR) spectra while cooling down demonstrates the appearance of the exchange interaction which is suppressed at higher temperatures by the vibrational modes of the molecule-matrix. The role of the oxo-hydroxo aluminate form is one of a molecular motor sorting the individual nanospecies by their size and charge state.

In this work, we study the effect of the various substrates on the growth and superconducting properties of NbN thin films grown by using pulsed laser ablation in a N₂ + 1%H₂ atmosphere on MgO, Al₂O₃ and Si substrates. Structural and superconducting analyses of the films demonstrate that using MgO and Al₂O₃ substrates can significantly improve the film properties compared to Si substrate. The X-ray diffraction data indicate that MgO and Al₂O₃ substrates produce highly oriented superconducting NbN films with large coherent domain size in the out-of plane direction on the order of layer thickness and with a superconducting transition temperature of 13.1 K and 15.2 K, respectively. On the other hand, the NbN film grown on the Si substrate exhibits random polycrystalline orientation. Together with the smallest coherent domain size it leads to the lower critical temperature of 8.3 K. Finally, by using a passivation surface layer we are able to improve superconducting properties of NbN thin film and we observe superconducting transition temperature 16.6 K, the one of the highest value reported so far for 50 nm thick NbN film on sapphire.

Nuclear magnetic resonance spectroscopy (NMR) ¹H, ³⁵Cl, ²⁷Al and ¹³C was applied to study underlying processes at the various stages of the synthesis of Au/Al nanoparticles. ³⁵Cl spectrum was downfield shifted by 2.6 ppm as to the reference signal of the hydrated Cl⁻ ion in NaCl solution. The evolution of the NMR spectra points to the formation of the stabilized shell around the gold containing nucleus. The shell restricts the supply of the reducing agents, which is the condition for the formation of Au²⁺ state at the concentration range in question. The electron paramagnetic resonance (EPR) spectra reveal formation of both Au²⁺ (g = 2.17) and Au⁺ (g < 2) intermediates incompletely reduced as well as Au⁰ clusters (g = 2.062) with odd number of atoms. The latter is coupled in many cases by the narrow signal with g = 2.0048 attributed to the radical in the supporting surrounding (tannin containing matrix in our case).

Reduced graphene oxide/bismuth oxide (rGO/Bi₂O₃) composites were prepared at various weight ratios of raw materials, GO and bismuth nitrate at 1:0.1, 1:0.3, 1:0.6, 1:0.9 and 1:1.2, respectively, by the improved Hummer’s method. During the process, the mixed solutions were prepared, and then rGO was obtained by hydrothermal method. Finally, the complex of rGO/Bi₂O₃ was prepared by calcination after hydrothermal treatment. The results show that the removal of oxygen-containing functional groups in rGO are increased with the increase of graphene agglomeration, and the Raman shifts of G band tending to the lowest wave-number. The electrochemical characteristics of the as-prepared rGO/Bi₂O₃ materials were also examined in 1 M KOH electrolyte. The dominating charge storage mechanisms are attributed to the electric double layer behaviors. As the content of bismuth nitrate increased, the rGO/Bi₂O₃ had a higher capacitance. The rGO/Bi₂O₃ obtained from the weight ratio of GO and bismuth nitrate of 1:1.2 as raw materials exhibit a maximum specific capacitance of 216 Fg⁻¹, revealing that rGO/Bi₂O₃ obtained by hydrothermal synthesis method can be used for the carbon-electrode of a super capacitor.

The paper presents fabrication and characterization of amorphous silicon carbide (a-SiC:H) based structures for photo-electrochemical (PEC) water splitting. The increase of the photocurrent of PEC upon the decreased of CH₄ flow during the deposition is associated with the decrease of the band gap and increased absorption of light in a-SiC:H. Photocurrent of 50 µA/cm² is achieved for PEC structure prepared with the lowest CH₄ flow during the deposition. An ITO/a-SiC:H/Si silicon heterojunction structure forming a simple photovoltaic cell (PV) with efficiency of 9.66% was prepared to support additional voltage hereby forming a hybrid PEC-PV system. ASA simulation revealed that a photocurrent of 0.62 mA/cm² and solar to hydrogen efficiency of 0.76% can be achieved for hybrid a PEC-PV structure with 5 PVs connected in series behind the PEC cell. Further opportunities for increasing the performance are discussed and summarized.

In order to study the influence of the substrate bias on the properties of ZrN thin films deposited by radio-frequency magnetron sputtering for biomedical application. Films of ZrN were grown onto 316L stainless steel substrate using radio-frequency (rf) magnetron sputtering from a pure zirconium target in Ar - N₂ gas mixture. The substrate bias voltage was varied from 0 to −100 V, which produces a variation in the structural and electrochemical properties of the obtained films. The deposited films were characterized by X-Rays Diffraction, Atomic Force Microscopy, scanning force microscopy and potentiodynamic polarization.

ZrO₂ thin films were deposited on 316L stainless steel substrate by a radio-frequency magnetron sputtering system. The substrate bias voltage, the working gas rate and the reactive gas fraction in the gas mixture were varied. These variations produce a variation in the deferent properties of the obtained films. The deposited films were characterized by X-Rays Diffraction, Atomic Force Microscopy, nano-indentation and potentiodynamic polarization. The experimental results show that the film thickness and the roughness of the films are highly influenced by the plasma parameters. XRD results show that the monoclinic phase is predominant in unbiased deposited films. The best anti-corrosion performance and hardness were obtained for ZrO₂ deposited with a substrate bias voltage of −75 V, Ar rate of 6 sccm and oxygen fraction of 25%.

The study demonstrates that resistivity of an alumina wafer is highly sensitive to trace concentrations of acetone vapors at room temperature. Though, a thermal pretreatment is necessary to precede the room-temperature sensing of acetone vapors, whilst the sensitiveness increases with the pretreatment temperature. This advocates the alumina being suitable for an adequate acetone sensor in the ppm range. A plausible mechanism of the response is discussed.

Cu doped transparent ZnO thin films (CZO) were sputtered on soda lime glass substrates at three different distances between substrate and target. The effects of copper doping on the structural and optical properties were investigated by X-ray diffraction (XRD) and transmittance measurements. The XRD results indicated that CZO thin films have a preferential crystallographic orientation along the hexagonal wurtzite (002) axis. With increasing the distance between substrate-target, from 4 cm to 8 cm, the refractive index of the CZO films decreased. In the visible wavelength region, the average value of the transmittance was above 80%. Thus, significant changes in the structural and optical properties have occurred due to the decrease of the distance between the target-substrate and the residual compressive stress at the film-substrate interface arising during deposition.

We report on influence of the surface functionalization on the properties of highly oriented pyrolytic graphite. The samples were processed in nitric acid and characterized by XPS, Raman and EDX spectroscopies, AFM, SEM and optical microscopy. It is shown that interaction of nitric acid with the surface of HOPG leads to two types of reactions: oxidation of the graphite and intercalation of the nitrate ions at the blistered areas.