The present work demonstrates the possibility to use liquid phase epitaxy to incorporate nitrogen in epitaxial GaAsN/GaAs and GaInAsN/GaAs heterostructures, including nanoscaled ones. The structures are grown from Ga - and GaIn - melts containing polycrystalline GaN as a nitrogen source. The red shift of the absorption spectra corresponds to nitrogen content in the epitaxial layers near or less than 0.2 at %. Photoluminescence spectra of dilute nitride GaAsN and GaInAsN show emission from localized nitrogen states - N-nanoclusters of more than two N atoms. These studies show that the melt grown dilute GaAsN and GaInAsN nanostructures can be used for solar cells with extended long wavelength edge.

This paper presents investigation of electronic transport properties of GaAsN and InGaAsN epitaxial layers with low nitrogen content, the so called dilute nitrides, grown by liquid-phase epitaxy (LPE). The layers up to 2 microns thick have been grown from Ga- and In-rich melt at different initial epitaxy temperatures in the range 660-620 degrees C. Polycrystalline GaN has been used as a source for nitrogen. As grown, unintentionally doped GaAsN and InGaNAs are n-type with free carrier concentrations one order of magnitude higher than those for the reference nitrogen free undoped GaAs and InGaAs layers. Lattice matched to GaAs substrate InGaAsN layers exhibit Hall mobility values higher than 2000 V/cm(2).s

Zirconium nitrides (ZrN) coatings have shown better quality in comparison to titanium nitrides (TiN) ones regarding the application in the mechanical processing of aluminum and titanium alloys. This work presents the results from investigation on properties of ZrN-based coatings intended for industrial application. The ZrN and ZrTiN hard coatings in a thickness of (3 5) m were obtained on stainless steel substrates by cathodic arc evaporation method. The coating hardness in the range of 25-32 GPa was evaluated using the Vickers measurement technique. The coating properties were studied in relation to the surface morphology by Atomic force microscopy (AFM) and Scanning electron microscopy (SEM). The analyses showed that the number and size of the macroparticles decrease when N2 pressure increases in the deposition chamber. X-ray diffraction analysis (XRD) was performed to identify the crystallographic structure, preferred orientation and stress of the ZrN coatings.

We monitored the full sequence of electrochemical doping of a single-walled carbon nanotube-bundle sample (mean tube diameter 1.35nm) in a NH4Cl solution from double-layer charging to intercalation and functionalization with Raman spectroscopy, scanning electron microscopy, and transport measurements. The different doping dependences of the Raman intensity of metallic and semiconducting tubes are explained with the excitonic nature of the Raman effect in SWNTs. Temperature-dependent conductivity measurements indicate a decreasing metallicity of the sample in the non-covalent functionalization region where the conductivity still increases with doping level. We attribute this phenomenon to changes in the density of states of metallic SWNTs. (c) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We investigate light emission spectra at different excitation levels of nanoscale thin InGaN film participating in an InGaN/GaN quantum well (QW) with heavily doped barriers for green and blue light emitting diodes (LEDs). We model the spectral shape and energy position in frames of the free electron recombination model created first for highly doped 3D direct gap III-V semiconductor films and applied for QWs at low excitation. The model accounts for the influence on the potential width of the QW of the random impurity potential of the doped barriers which penetrates into the QW. The blue shift at high excitation is supposed to be due to the filling of the conduction band with degenerate 2D nonequilibrium electrons. A structure in the emission bands is observed and it is assumed to be a result from step-like 2D density-of-states (DOS) in the QW. A good agreement is obtained between the calculated and experimental spectra assuming that the barriers are graded.

Group III-V-nitrides, the so called dilute nitrides, are promising materials for multi-junction solar cells, telecommunications, heterojunction bipolar transistors and high frequency (THz) applications. The introduction of a tiny fraction (similar to 1 %) of nitrogen in GaAs or GaP induces a decrease of the band gap energy and a deformation of the conduction band structure, which extends the range of its applicability. In the present work, we demonstrate the possibility of incorporating nitrogen in melt-grown GaAs layers. The dilute gallium arsenide-nitride layers were grown on GaAs substrates by liquid-phase epitaxy, using polycrystalline GaN as a source of nitrogen in the melt. Characterisation of the structural and electrical properties was done by XRD, XPS analysis, FTIR spectra and Hall effect measurements in the range 80-300K.

Quality education is a key priority for democratic societies. Investment in youth is a token for success with high returns. Bulgarian students achieve considerable success at various international forums such as Olympiads, competitions, and contests and frequently win medals and prestigious rewards. One of the principal reasons for the success of our students is precisely the approach in preparation and selection of students chosen to compete at such forums.The National Biology Competition for Students grades 7 and 8 is a considerable platform for future achievements in the field of biology. The idea for conducting a large scale display of students' capabilities is directly related to pupils' strong interest, enveloping student's creative skills and love for the science of biology. The original style and formulation of the problems at the National Biology Competition stimulate creativity and thinking, encourage curiosity and serve as a trampoline for deciphering, learning and solving scientific problems.In the current report, we analyze the theoretical knowledge and practical skills of students in solving the problems in three sub-areas of biology: anatomy, physiology, and human hygiene, suggested by the National Biology Competition conducted in 2007 in the city of Gabrovo.

Crafting a Curriculum in Ecology for the Study of the Local Environment is dictated by the alarming concerns for preserving life and nature. The opportunities for applying such a Curriculum lay in the experimental activities offered in its modules as well as in the partnerships and inter-institutional cooperation it engenders.The Curriculum is adapted to the age group/grade of students as well as their knowledge about nature. The rare biodiversity of Ostritza Reserve Park offers a unique opportunity for students' investigative and research work. The answers to questions such as "Are we ready to meet the challenges of nature?", "How acute is the threat of global warming?", "How to preserve the treasures of nature, studying our local environment?" emerge from various parts of the Curriculum -- debates in Discussion Club, poster sessions, round tables, eco-projects, etc.The ultimate "destination" of the Curriculum is the human behavior, its "vehicle" the civic duty and responsibility of every person. The Curriculum meets the national educational requirements and standards in ecology in a non-conventional, but attractive for students, approach. By sharing experience in the process of studying the local environment, the students advance their knowledge in nature preservation and ecology, at par with the European norms for education in these areas.

We have studied depth distributions of the electrical parameters in MBE grown InN films with two types of AlN and GaN buffers. Using independently determined Hall effect electron concentration and mobility profiles, as well as electron concentration profile by photoluminescence measurements, we model the real depth profile of carrier mobility, assuming graded inhomogeneity of the sample. The obtained profiles follow power dependences of the same order for layers grown on the two buffers with a small difference in the function coefficients attributed to a contribution of the interface charge in layers grown on AlN buffers. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The paper presents a microhardness study of thick crack-free hydride vapor phase epitaxial GaN layers (not intentionally doped), and of thin metal-organic vapor phase epitaxial (MOVPE) GaN layers (undoped and Si-doped), grown on sapphire. A Vickers indentation method was used to determine the microhardness under applied loads up to 2 N. An increase in the microhardness was observed with decreasing carrier concentration and increasing mobility. A dip at an indentation depth of about 0.75 μm is observed in the microhardness profile in the MOVPE films, and is correlated with peculiarities in the spatially resolved cathodoluminescence spectra. The relationship between the mechanical and electrophysical parameters is discussed.

We study longitudinal electron transport in InGaN/GaN multiple quantum wells (MQWs) at moderate magnetic fields. We observe a stepwise behavior of both the Hall coefficient and magnetoresistivity. The peculiarities are explained by a magnetic-field-induced localization of electrons in a two-dimensional (2D) potential relief of the InGaN MQW due to composition fluctuations. We extend the model for a magnetic localization of electrons, treating every QW like a quasi-2D system with a cylindrical potential relief. The calculated values of the decrease of the sheet electron concentrations in a magnetic field based on such an assumption for 2D density of states in a InGaN MQW system are in good accordance with the experimentally obtained values.

We study longitudinal electron transport in InGaN/GaN multiple quantum well (MQW) structures at moderate magnetic field. A step-wise behaviour in magnetic field dependences of both the Hall coefficient and the magnetoresistivity is explained by a magnetic induced localization of electrons in a two-dimensional (2D) potential relief due to the composition fluctuations in the wells. To describe the results we extend the model for a magnetic localization of electrons by treating every quantum well like a quasi-2D system with a cylindrical potential relief. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present a multilayer model for analysis of Hall effect data of semiconductor structures composed of sublayers with different thicknesses and contacts placed on the top surface. Based on the circuit theory we analyze the contributions of the conductivity of every sublayer and derive general expressions for the conductivity and carrier mobility of a multilayer planar sample. The circuit analysis is performed taking into account the fact that the sample sublayers are partially connected in parallel to each other by series resistances formed in areas lying below the contacts from each upper layer. In order to solve the inverse problem of determining the electrical parameters of one of the sublayers, a procedure for analysis of the Hall effect data is proposed. The model is simplified for a structure composed of two layers with the same type of conductivity, and is used to determine the electrical parameters of GaN films grown on relatively thick GaN buffers.

We develop a model for analysis of Hall effect data of GaN structures composed of sublayers with different thicknesses and contacts placed on the top surface, We analysed the contributions of the conductivity of every sublayer of a planar sample taking into account the fact that the sample sublayers are partially connected in parallel to each other by series resistances formed in areas lying below the contacts from the upper layer. Correction factors, which reduce the contribution of the underlying layers to the measured whole sample conductivity, are obtained from the equations relevant to the respective equivalent circuit.

The influence of high temperature buffer layers on the structural characteristics of GaN grown by hydride vapour phase epitaxy on sapphire was investigated. Strain relaxation as well as mismatch-induced defect reduction in thick GaN layers grown on AlN buffer was microscopically identified using cathodoluminescence and micro-Raman spectroscopy in cross-section of the films. The results were correlated with photoluminescence and Hall-effect data of layers with different thicknesses. These relaxation processes were suggested to account for the specific defect distribution in the buffers revealed by high-resolution X-ray diffraction and transmission electron microscopy.

We simulate the spectral distribution of the free-electron recombination band in optical emission spectra of GaN with a free-carrier concentration in the range of 5 x 10(17)-1 x 10(20) cm(-3). The influence of several factors, such as nonparabolicity, electron-electron interaction. and electron-impurity interaction on both the spectral and energy position and the effective gap narrowing are taken into account. The calculated properties of the free-electron-related emission bands are used to interpret the experimental photoluminescence and cathodoluminescence spectra of GaN epitaxial layers.

We report on an improved quality of thick HVPE-GaN grown on MOCVD-GaN 'template' layers compared to the material grown directly on sapphire. The film-substrate interface revealed by cathodoluminescence measurements shows an absence of highly doped columnar structures which are typically present in thick HVPE-GaN films grown directly on sapphire. This improved structure results in a reduction of two orders of magnitude of the free carrier concentration from Hall measurements. It was found that the structure, morphology, electrical and optical properties of homoepitaxial thick GaN layers grown by HVPE were strongly influenced by the properties of the MOCVD-GaN 'template'. Additionally the effect of Si doping of the GaN buffer layers on the HVPE-GaN properties was analysed.

Emission spectra of thick unintentionally doped GaN layers grown on sapphire by hydride vapor phase epitaxy have been studied. In addition to sharp exciton emission lines, a broad asymmetric band extending to energies above the band gap is observed in the near band-gap regions of the photoluminescence and cathodoluminescence spectra. We explain this feature as a free-electron recombination band. Using spatially resolved cathodoluminescence we were able to separately record the spectra of lightly doped regions which contain only sharp bound exciton, the spectra of highly doped regions which show the broadband only, as well as the spectra of the areas containing surface defects which show both spectral features simultaneously. Using a model in which the electrons are degenerate and the holes are localized together with the results of Hall-effect measurements we can simulate the spectral shape of the broadband. Raman scattering spectroscopy confirms the coexistence of lightly doped (∼1×1017 cm−3) and heavily doped (∼1019 cm−3) regions in our films.