Volume 68 (2017): Issue 7 (December 2017)

Nanocrystalline diamond films with bright photoluminescence of silicon-vacancy colour centres have been grown using a microwave plasma enhanced CVD technique. The influence of substrate material (quartz, Al₂O₃, Mo and Si) on a reproducible fabrication of diamond thin films with Si-V optical centres is presented. Film quality and morphology are characterized by Raman spectroscopy and SEM technique. SEM shows well faceted diamond grains with sizes from 170 to 300 nm. The diamond peak is confirmed in Raman spectra for all samples. In the case of the quartz substrate, a redshift of the diamond peak is observed (≈3.5 cm⁻¹) due to tension in the diamond film. The steady-state photoluminescence intensity was measured in the temperature range from 11 K to 300 K. All spectra consist of a broad emission band with a maximum near 600 nm and of a sharp zero phonon line in the vicinity of 738 nm corresponding to Si-V centres that is accompanied with a phonon sideband peaking at 757 nm. Activation energies for the thermal quenching of Si-V centre photoluminescence were determined and the effect of the substrate on photoluminescence properties is discussed too.

In this paper we report results from optical transmittance spectroscopy complemented with data on structure from XRD measurements to determine optical properties of a series of ZnTiO₃ perovskite thin films deposited on glass by reactive magnetron co-sputtering. The members of the series differ by the titanium content that was revealed as an origin of the changes not only in structure but also in dispersive optical properties. Low porosity has been discovered and calculated using the Bruggeman effective medium approximation. An apparent blue-shift of the optical band gap energies with increasing titanium content was observed. The observed band gap engineering is a good prospective for eg optoelectronic and photocatalytic applications of ZnTiO₃.

Si nanopowder is fabricated using the simple beads milling method. Fabricated Si nanopowder reacts with water in the neutral pH region between 7 and 9 to generate hydrogen. The hydrogen generation rate greatly increases with pH, while pH does not change after the hydrogen generation reaction. In the case of the reactions of Si nanopowder with strong alkaline solutions (eg pH13.9), 1600 mL hydrogen is generated from 1 g Si nanopowder in a short time (eg 15 min). When Si nanopowder is etched with HF solutions and immersed in ethanol, green photoluminescence (PL) is observed, and it is attributed to band-to-band transition of Si nanopowder. The Si nanopowder without HF etching in hexane shows blue PL. The PL spectra possess peaked structure, and it is attributed to vibronic bands of 9,10-dimethylantracene (DMA) in hexane solutions. The PL intensity is increased by more than 3,000 times by adsorption of DMA on Si nanopowder.

Titanium oxide (TiO₂) films have been synthesized on quartz, silicon and textured silicon substrates by chemical ultrasonic spray deposition. The textured silicon substrate was carried out using Na₂CO₃ solution. The sample surface exhibits uniform pyramids with an average height of 5 µm. In this paper, particular attention is given to the TiO₂ films prepared by spray ultrasonic system using Tetra iso-Propoxide Orthotitanate Titanium (TPOT) as a precursor. The solutions were sprayed onto substrates heated at various temperatures 350 - 550 °C. The properties of films as a function of temperature parameter were investigated using structural and optical analysis. According to XRD, FTIR and Micro-Raman spectroscopies, the anatase phase was found and exhibits nanograins of 9 to 15 nm in size. The indirect and direct bad gap were found to increase by increasing substrate temperature due to the decreasing of nanograins size and were estimated to be around 3.28 and 3.38 eV. A transmittance higher than 80% was found. This paper reports on anti-reflection coating application of TiO₂ layers due to its good transparency and appropriate refractive index varies between 2.19 - 2.40 at λ = 632.8 nm as a function of temperature determined by UVVisNIR spectrophotometer and Ellipsometry. To achieve optimum anti-reflection characteristics different anti-reflection designs were experimentally examined with polished and textured substrates. The average reflectance of the polished silicon used in this study is 39%, with TiO₂ it decreases to 9%. The textured surface reduces the average reflectance of silicon to be around 14% and it decreases dramatically to 5% after deposition of a single layer of TiO₂ as an anti-reflection coating. The gain in density of the short-circuit photocurrent assigned to the reduction of reflection losses up to 44% and 58% were predicted with TiO₂ single-coating in polished and textured silicon substrates respectively.

TiO₂ has an easily tunable bandgap and a great absorption dye ability being widely used in many fields and in a number of fascinating applications. In this study, a wet chemical route, particularly a sol gel method using spin-coating is adopted to deposit TiO₂ thin films onto soda lime glass and silicon substrates. TiO₂ films were prepared by using an alcoholic solution of analytical reagent grade TiCl₄ as titanium precursor at various experimental conditions. The accent was put on the conditions of preparation (spin time, spin speed, precursor concentration, number of coating layers etc), doping and on the post-deposit treatment namely the drying and the crystallization. The results showed a strong dependence on the drying temperature and on the temperature and duration of the crystallization. We found that the solution preparation and its color are important for getting a reproducible final product. The Raman spectra recorded at room temperature, showed the characteristic peaks of anatase which appear at 143 and around 396 cm⁻¹. These peaks confirm the presence of TiO₂.

The X-ray diffraction (XRD) was used to identify the crystalline characteristic of TiO₂ while the chemical states and relative amounts of the main elements existing in the samples were investigated by X-ray Photoelectron Spectroscopy (XPS). The morphology of the samples was visualized by AFM. We show by this work the feasibility to obtain different nanostructured TiO₂ by changing the concentration of the solution. Photocatalytic activity of TiO₂ films was evaluated. Rhodamine B is a recalcitrant dye and TiO₂ was successfully tested for its oxidation. An abatement of 60% was obtained under sunlight for an initial concentration of 10 mg/l.

We have developed a technology for fabrication of black − Si by use of the surface structure chemical transfer (SSCT) method. The ultralow reflectance below 3% results from formation of a graded porosity structure of a nanocrystalline Si layer formed by the SSCT method. The nanocrystalline Si layer with an extremely large surface area is effectively passivated by deposition of phosphosilicate glass (PSG) followed by heat treatment at 925 ◦C. After PSG passivation, the minority carrier lifetime greatly increases, and the internal quantum efficiency in the short wavelength region is also greatly increased. Using the SSCT method and the PSG passivation method, the high conversion efficiency of 19.7% is achieved.

A set of structures with nanocrystalline-Si/Si interfaces formed on p-type Si substrate appropriated for photovoltaic application was prepared. The Acoustic DLTS technique based on the acoustoelectric response signal produced by the structure when a longitudinal acoustic wave propagates through the structure was used together with electric characterization to determine deep defects and the role of both individual layers. Several kinds of interface deep centers with activation energies typical for dangling bonds, oxygen participated Si or point defects were observed as well as a particular influence of individual layers on the interface states. The obtained results are analyzed, discussed and subsequently compared.

Suppression of spectral reflectance from the surface of solar cell is necessary for achieving a high energy conversion efficiency. We developed a simple method for forming nanocrystalline layers with ultralow reflectance in a broad range of wavelengths. The method is based on metal assisted etching of the silicon surface. In this work, we prepared Si solar cell structures with embedded nanocrystalline layers. The microstructure of embedded layer depends on the etching conditions. We examined the microstructure of the etched layers by a transmission electron microscope and analysed the experimental images by statistical and Fourier methods. The obtained results provide information on the applied treatment operations and can be used to optimize the solar cell forming procedure.

The morphology and chemical bods of p-type and n-type porous Si was compared. The surface of n-type sample is smooth, homogenous without any features. The surface of p-type sample reveals micrometer-sized islands. FTIR investigation reveals various distribution of SiO_xH_y complexes in both p-and n-type samples. From the conditions leading to porous silicon layer formation (the presence of holes) we suggest both SiO_xH_y and SiF_xH_y complexes in the layer.

The present work reports the fabrication of p-Si/SiO₂ /TiO₂ and p-Si/SiO₂ /TiO₂ /ZnO heterostructures deposited by RF sputtering on p-Si substrate. The structural properties of the heterostructures were characterized by X-ray reflectivity and SIMS depth profiling. The electrical and optical properties of the heterostructures were investigated by I − V, C-V measurements and VIS spectroscopy, respectively. The measurements reveal that I − V characteristics in dark show semiconductor-insulator-semiconductor (SIS) structure properties. The I − V characteristics under illumination exhibit changes with significant increase of photocurrent due to photoassisted tunnelling and injection through SiO₂ /TiO₂ interlayer.

Three oxide dispersion strengthened (ODS) steels with different chromium content (MA 956, MA 957 and ODM 751) were studied as candidate materials for new nuclear reactors in term of their radiation stability. The radiation damage was experimentally simulated by helium ion implantation with energy of ions up to 500 keV. The study was focused on surface and sub-surface structural change due to the ion implantation observed by mostly non-destructive techniques: positron annihilation lifetime spectroscopy and nanoindentation. The applied techniques demonstrated the best radiation stability of the steel ODM 751. Blistering effect occurred due to high implantation dose (mostly in MA 956) was studied in details.

The interest about zinc oxide is increasing thanks to its unique chemical and physical properties. Our attention has focused on preparation powder of 1D nanostructures of ZnO nanowires with precisely defined morphology include characterization size (length and diameter) and shape controlled in the scanning electron microscopy (SEM). We have compared results of SEM with dynamic light scattering (DLS) technique. We have found out that SEM method gives more accurate results. We have proposed transformation process from ZnO nanowires on substrates to ZnO nanowires powder by ultrasound peeling to colloid followed by lyophilization. This method of the mass production of the ZnO nanowires powder has some advantages: simplicity, cost effective, large-scale and environment friendly.

We have optimized the deposition of the highly electrically resistive undoped (intrinsic) polycrystalline ZnO thin layers on fused silica substrates by the DC reactive magnetron sputtering of metallic zinc target in argonne/oxide atmosphere and we introduced the post-deposition hydrogen plasma doping. The thickness of thin film was evaluated by reflectance interferometry using the metallographic optical microscope fiber coupled to the CCD spectrometer operating in 400-1000 nm spectral range. The optical absorption was measured by photothermal deflection spectroscopy operating in 300-1600 nm spectral range. The change of the optical absorption edge and the increase of the infrared optical absorption was detected in hydrogenated ZnO. The increase of the infrared optical absorption goes with the increase of the electrical conductivity. We conclude that the plasma hydrogenation of the intrinsic ZnO thin films is related to increase of the free carrier concentration.

When measuring optical parameters of a photovoltaic silicon cell, precise results bring good electrical parameters estimation, applying well-known physical-mathematical models. Nevertheless, considerable re-combination phenomena might occur in both surface and intrinsic thin layers within novel materials. Moreover, rear contact surface parameters may influence close-area re-combination phenomena, too. Therefore, the only precise electrical measurement approach is to prove assumed cell electrical parameters. Based on theoretical approach with respect to experiments, this paper analyses problems within measurement procedures and equipment used for electrical parameters acquisition within a photovoltaic silicon cell, as a case study. Statistical appraisal quality is contributed.

Evaluation of photoluminescence spectra of porous silicon (PS) samples prepared by electrochemical etching is presented. The samples were measured at temperatures 30, 70 and 150 K. Peak parameters (energy, intensity and width) were calculated. The PL spectrum was approximated by a set of Gaussian peaks. Their parameters were fixed using fitting a procedure in which the optimal number of peeks included into the model was estimated using the residuum of the approximation. The weak thermal dependence of the spectra indicates the strong influence of active defects.

The paper deals with the complex refractive index in the IR light region of two types of samples (i) as prepared black silicon, and (ii) thermally oxidized black silicon (BSi) nano-crystalline specimens produced both by the surface structure chemical transfer method using catalytic Ag evaporated spots (as prepared sample) and by the catalytic Pt catalytic mesh (thermally oxidized sample). We present, compare, and discuss the values of the IR complex refractive index obtained by calculation using the Kramers-Krönig transformation. Results indicate that small differences between optical properties of as prepared black Si and thermally oxidized BSi are given by: (i) – oxidation procedure, (ii) – thickness of the formed black Si layer, mainly, not by utilization of different catalytic metals, and by iii) the different thickness. Contamination of the surface by different catalytic metals contributes almost equally to the calculated values of the corresponding complex refractive index.