Jaroslav Kováč

Investigation of trap effects in AlGaN∕GaN field-effect transistors by temperature dependent threshold voltage analysis

We report on a temperature dependent threshold voltage analysis of the AlGaN∕GaN heterostructure field-effect transistors (HFETs) and Al2O3∕AlGaN∕GaN metal-oxide-semiconductor HFETs (MOSHFETs) in order to investigate the trap effects in these devices. The threshold voltage of both types of devices decreases with increased ambient temperature up to 450°C. This indicates on donor traps to be present. The temperature induced threshold voltage shift is −1.6 and −8.5mV∕°C for the HFETs and MOSHFETs, respectively. A thermally activated energy level of ∼0.2eV is evaluated and attributed to the nitrogen vacancy in the AlGaN near surface. The trap density for the MOSHFETs is about two times higher than that for the HFETs. This might be due to the high-temperature treatment (∼600°C) of the MOSHFET structure during the gate insulator deposition.

Light emission from InGaN quantum wells grown on the facets of closely spaced GaN nano-pyramids formed by nano-imprinting

InxGa1-xN/GaN quantum wells have been grown on the {1011} facets of dense arrays of self-assembled GaN nano-pyramids formed by selective area growth and characterised by high spatial resolution cathodoluminescence. The pyramids are shown to have significantly reduced defect (green-yellow) band emission and the quantum well luminescence is correspondingly intense. The peak energy of this luminescence is shown to blue-shift as the sampled region is moved up the pyramid facets, revealing that InN incorporation in such closely spaced epitaxial nanostructures differs from that in widely spaced micron-size pyramidal structures decreasing rather than increasing towards the nano-pyramid tips.

ZnO-nanorod-array/p-GaN high-performance ultra-violet light emitting devices prepared by simple solution synthesis

Pure ultra-violet (UV) (378 nm) electroluminescence (EL) from zinc oxide (ZnO)-nanorod-array/p-gallium nitride (GaN) light emitting devices (LEDs) is demonstrated at low bias-voltages (∼4.3 V). Devices were prepared merely by solution-synthesis, without any involvement of sophisticated material growth techniques or preparation methods. Three different luminescence characterization techniques, i.e., photo-luminescence, cathodo-luminescence, and EL, provided insight into the nature of the UV emission mechanism in solution-synthesized LEDs. Bias dependent EL behaviour revealed blue-shift of EL peaks and increased peak sharpness, with increasing the operating voltage. Accelerated bias stress tests showed very stable and repeatable electrical and EL performance of the solution-synthesized nanorod LEDs.

Characterization of mechanochemically synthesized lead selenide

Mechanochemical synthesis of lead selenide PbSe nanocrystals was performed by high-energy milling of lead and selenium powders in a planetary ball mill. The structure properties of synthesized lead selenide were characterized by XRD analysis that confirmed crystalline nature of PbSe nanocrystals. Calculated average size of PbSe crystallites was 37 nm. The methods of particle size distribution analysis, specific surface area measurement, SEM and TEM were used for the characterization of surface and morphology of PbSe nanocrystals. SEM analysis revealed agglomerates of PbSe particles. However, HRTEM analysis confirmed perfect stoichiometric PbSe cubes with NaCl structure as well. UV-VIS-NIR spectrophotometry was used to confirm the blue shift of the small particles occurring in the powder product obtained by the mechanochemical synthesis.

PbS nanostructures synthesized via surfactant assisted mechanochemical route

PbS nanocrystals using surfactant assisted mechanochemical route has been successfully prepared. The methods of XRD, SEM, surface area and particle size measurements were used for nanocrystals characterization. The XRD patterns confirmed the presence of galena PbS (JCPDS 5–592) whatever treatment conditions were applied. The strong observable peaks indicate the highly crystalline nature in formation of PbS nanostructures where preferential crystal growth in the (200) direction after chelating agent (EDTANa2•2H2O) addition has been observed. The mean volume weighted crystallite size 4.9 nm and 35 nm has been calculated from XRD data using Williamson-Hall method for PbS synthesized without and/or with chelating agent, respectively corresponding with surface weighted crystallites sizes of 2.9 and 18.8 nm. The sample prepared without surfactant yields the smaller crystallites and the higher microstrain compared with surfactant assisted synthesis. The obtained results illustrate a possibility to manipulate crystal morphology by combining effect of milling and surfactant application.

Periodic structures prepared by two-beam interference method

We present an optical interference method as the efficient tool for high-quality one- (1D) and two-dimensional (2D) periodic structure fabrication. Different types of 2D periodic structures are prepared using two-beam interference technique by double exposure. Also, the patterning of Au layer is provided using two-beam interference method. Quite homogeneous 1D and 2D structures with small periodicity were prepared in photoresist and Au layer. The experimentally prepared structures are well in agreement with the theoretically designed ones. According to experimental results, we favor two-beam interference technique using multi-exposure process as a very useful method for fabrication of 2D periodic structures and devices for integrated optoelectronics.

CdS/ZnS nanocomposites: from mechanochemical synthesis to cytotoxicity issues.

CdS/ZnS nanocomposites have been prepared by a two-step solid-state mechanochemical synthesis. CdS has been prepared from cadmium acetate and sodium sulfide precursors in the first step. The obtained cubic CdS (hawleyite, JCPDS 00-010-0454) was then mixed in the second step with the cubic ZnS (sphalerite, JCPDS 00-005-0566) synthesized mechanochemically from the analogous precursors. The crystallite sizes of the new type CdS/ZnS nanocomposite, calculated based on the XRD data, were 3-4 nm for both phases. The synthesized nanoparticles have been further characterized by high-resolution transmission electron microscopy (HRTEM) and micro-photoluminescence (μPL) spectroscopy. The PL emission peaks in the PL spectra are attributed to the recombination of holes/electrons in the nanocomposites occurring in depth associated with Cd, Zn vacancies and S interstitials. Their photocatalytic activity was also measured. In the photocatalytic activity tests to decolorize Methyl Orange dye aqueous solution, the process is faster and its effectivity is higher when using CdS/ZnS nanocomposite, compared to single phase CdS. Very low cytotoxic activity (high viability) of the cancer cell lines (selected as models of living cells) has been evidenced for CdS/ZnS in comparison with CdS alone. This fact is in a close relationship with Cd(II) ions dissolution tested in a physiological solution. The concentration of cadmium dissolved from CdS/ZnS nanocomposites with variable Cd:Zn ratio was 2.5-5.0 μg.mL(-1), whereas the concentration for pure CdS was much higher - 53 μg.ml(-1). The presence of ZnS in the nanocrystalline composite strongly reduced the release of cadmium into the physiological solution, which simulated the environment in the human body. The obtained CdS/ZnS quantum dots can serve as labeling media and co-agents in future anti-cancer drugs, because of their potential in theranostic applications.

Investigation of deep energy levels in heterostructures based on GaN by DLTS

In this paper we report our results of DLTS investigations of deep-level defects in Schottky-gate AlGaN/GaN LP-MOVPE structures grown on sapphire substrate. The exact location of heterostructures interface below the surface (20 nm) was determined from the concentration profile to depletion region width dependence. The free charge carrier density was calculated (n<inf>2D</inf> = 4.75÷5.09×10¹⁶ m⁻²). Four deep energy levels have been identified from selected DLTFS spectra (activation energies: E1=E<inf>C</inf>−0.545 eV, E2=E<inf>C</inf>−0.599 eV, E3=E<inf>C</inf>−0.642 eV, and E4=EC-1,118 eV).

Measurement Systemwith Hall and a Four Point Probes for Characterization of Semiconductors

One of the biggest challenges of communication networks is the video transmission in real time. It requires high demands on the available network capacity and transport mechanisms. Availability of smart mobile devices with batteries, which keep the terminal working for several hours, caused an increased interest in the research of the deployment of video transmission in wireless transmission systems. The presented paper deals with the transmission of video encoded with H.264/AVC (Advanced Video Coding) video coding standard through wireless local area network (WLAN) using the programming environment OPNET Modeller (OM). The test network studied in this work was prepared by combining real and simulated networks, which allows interesting possibilities when working with the OM tools. Such an approach to working with OM allows a detailed video streaming analysis, because the video output was noticeably not only in the form of statistics, but we can see the real impact of transmission failures. Using the OM simulation environment allows to design the transmission systems, which would be difficult to establish in laboratory conditions.

Enhanced permanent degradation of organic TFT under electrical stress and visible light exposure

Abstract We subjected OTFTs to electrical stress under various illumination conditions, in the visible-light range. Negligible intrinsic degradation is observed without stress and light permanent effects of light without stress on OTFT properties are very modest. When light and stress are applied simultaneously, the degradation is much stronger, than stress alone. We found that the acceleration factor is over 2 if blue light is shed on the devices.