Ján Greguš

Inhomogeneous Broadening of Mn2+ Photoluminescence in CdMnTe.

Photoluminescence spectra of Mn 2+ in CdMnTe are investigated using tunable laser in a temperature range from 15 K to 80 K. The shift of the photoluminescence peak energy with the excitation energy is observed. From analysis using the shift and the photoluminescence width, we obtain the temperature dependencies of homogeneous and inhomogeneous broadening widths of the d-d transition. In addition, the effects of high hydrostatic pressure on the electron-phonon coupling strength and the homogeneous and inhomogeneous widths are investigated.

Effect of Post-Deposition Annealing Treatment on the Structural, Optical and Gas Sensing Properties of TiO2 Thin Films

One of the potential applications of TiO2 is its use in gas sensor technology. The aim of this work was to study the gas sensing properties of TiO2 thin films in combination with the effect of post-deposition annealing treatment. Titanium dioxide thin films with thickness 100 nm were prepared by the reactive dc magnetron sputtering. The thin films were deposited on sapphire substrate from a titanium target in an oxygen atmosphere. The samples were then post-annealed in air in the temperature range 600 °C 1000 °C. Crystal structure, surface topography and absorption edge of the thin films have been studied by X-ray Diffraction technique, Atomic Force Microscopy and UV-VIS Spectroscopy. It was found that the phase gradually changed from anatase to rutile, the grain size and roughness tended to increase with increasing post-annealing temperature. The effect of these factors on gas sensing properties was discussed. For electrical measurements comb-like Pt electrodes were prepared by standard photolithography and the films were exposed to different concentrations of H2 gas up to 10000 ppm in synthetic air at various operating temperatures from 200 °C to 350 °C.

Temperature Dependence of Inhomogeneous Broadening and Mn2+ Photoluminescence Energy Shift in CdMnTe

Energy shift in Mn2+ photoluminescence relating with an excitation energy was observed in CdMnTe at low temperature. The shift is considered as the selective excitation of inhomogeneously broadened states. Using this shift we estimated the inhomogeneous broadening of the d-d transition in Mn2+ ions. The temperature dependence of the inhomogeneous broadening is discussed.

Structure of Hydrogen Gas Sensing TiO2 Thin Films Prepared by Sol-Gel Method and their Comparison with Magnetron Sputtered Films

TiO2 thin films with a thickness of about 150 nm were deposited by spin coating method on sapphire substrate from a sol-gel system. The hydrogen sensing properties of TiO2 films annealed at various temperatures were studied and correlated with their structure, optical and electrical properties. The annealing temperatures in the range of 600 800 °C lead to anatase films with a roughness in the range of 0.6 0.9 nm. Their sensitivity towards hydrogen is low. The thin films annealed at temperatures in the range 900 1000 °C consist of rutile phase and their roughness increased to 11.7 13.5 nm. They showed good hydrogen sensitivity with optimal operating temperature 200 250 °C. The structure and sensing properties of the prepared films are compared with those synthesized with magnetron sputtering. The maximum of sensitivity was measured on the thin films with diameter of the grains about 100 nm in both cases, i.e. on thin films prepared by sol-gel method as well as on thin films prepared by magnetron sputtering. The maximum sensitivity correlates with the diameter of the grains and dont depend on the allotropy of the titanium dioxide anatase or rutile.

Oxidized Al Film as an Insulation Layer in AlGaN/GaN Metal–Oxide–Semiconductor Heterostructure Field Effect Transistors

We report on the technology of a very thin oxidized Al sputtered film used for gate insulation and passivation in Al2O3/AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistors (MOSHFETs). Their transport properties are presented. The MOSHFET with the Al2O3 layer had improved static output and transfer characteristics compared with the reference heterostructure field-effect transistors (HFETs): (1) their saturation drain current IDS was ~600 mA mm-1 at gate voltage VG=1 V (HFETs with 2.5 µm gates had ~430 mA mm-1); (2) their transconductance was 116–140 mS mm-1 (HFETs had ~70 mS mm-1).

Regarding the Optical Properties of Porous Layers Prepared on Si Substrates

The paper deals with the complex refractive index and photoluminescence in the IR-VIS light region of two sample types (i) black p-type silicon (BSi) produced by the surface structure chemical transfer method using Pt catalytic mesh, and (ii) porous p-type silicon prepared by standard electrochemical etching. We present, compare, and discuss the values of the IR-VIS complex refractive index obtained by calculation using the Kramers-Kronig transformation and the photoluminescence properties thereof. The results indicate that differences between the optical properties of the BSi and the porous Si are given by (a) the oxidation procedure of BSi, (b) the thickness of the formed black and porous Si layer, and by (c) the porosity of both layer types. We assume that the photoluminescence signal generated by oxidized BSi structures can be mainly related to the quantum confinement effect, while the photoluminescence of the porous p-type Si is caused by the optical activity of the SiOxHy compounds covering its surface region.

Morphology and FT IR spectra of porous silicon

Abstract 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 SiOxHy complexes in both p-and n-type samples. From the conditions leading to porous silicon layer formation (the presence of holes) we suggest both SiOxHy and SiFxHy complexes in the layer.

Temperature dependence of photoluminescence peaks of porous silicon structures

Abstract 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.

Physical Properties and Light-Related Applications of Black Silicon Structures

This contribution deals with the black silicon (BS) nanocrystalline specimens produced using the surface structure chemical transfer method (SSCT). This method can produce a nanocrystalline Si black color layer on c-Si with a thickness range of ~50 nm to ~300 nm via the contact of c-Si immersed in the chemical solution HF + H2O2 with a catalytic mesh. The photoluminescence properties are related to the formation of nanocrystals, the structural properties of which are similar to those formed on the back of a sawn Si wafer and the resulting splitting of large Si crystals. X-ray diffraction of the Si front and back sides confirms the dominant reflection of the 311 Si crystalline planes. We suppose that the formation of the black silicon over-layer is pre-determined by the crystalline defects induced by the applied sawing procedure, even though saw damage (defects introduced by the applied sawing procedure) is not necessary for the SSCT method. The formation of the pn type Si solar cell is presented, including black silicon over-layer and without antireflection coating, with efficiency of ~19.1%.