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1.	Hansen, B, et al. (author) ANALYSIS OF LONG-TERM TREATMENT EFFECTS OF ODEVIXIBAT ON CLINICAL OUTCOMES IN CHILDREN WITH PROGRESSIVE FAMILIAL INTRAHEPATIC CHOLESTASIS IN ODEVIXIBAT CLINICAL STUDIES VS EXTERNAL CONTROLS FROM THE NAPPED DATABASE 2023 In: GUT. - 0017-5749. ; 78, s. S969-S970 Conference paper (other academic/artistic)
2.	Raiteri, C. M., et al. (author) WEBT and XMM-Newton observations of 3C 454.3 during the post-outburst phase - Detection of the little and big blue bumps 2007 In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 473:3, s. 819-827 Journal article (peer-reviewed)abstract Context. The quasar-type blazar 3C 454.3 was observed to undergo an unprecedented optical outburst in spring 2005, affecting the source brightness from the near-IR to the X-ray frequencies. This was first followed by a millimetric and then by a radio outburst, which peaked in February 2006. Aims. In this paper we report on follow-up observations to study the multiwavelength emission in the post-outburst phase. Methods. Radio, near-infrared, and optical monitoring was performed by the Whole Earth Blazar Telescope (WEBT) collaboration in the 2006-2007 observing season. XMM-Newton observations on July 2-3 and December 18-19, 2006 added information on the X-ray and UV states of the source. Results. The source was in a faint state. The radio flux at the higher frequencies showed a fast decreasing trend, which represents the tail of the big radio outburst. It was followed by a quiescent state, common at all radio frequencies. In contrast, moderate activity characterized the near-IR and optical light curves, with a progressive increase of the variability amplitude with increasing wavelength. We ascribe this redder-when-brighter behaviour to the presence of a ""little blue bump"" due to line emission from the broad line region, which is clearly visible in the source spectral energy distribution (SED) during faint states. Moreover, the data from the XMM- Newton Optical Monitor reveal a rise of the SED in the ultraviolet, suggesting the existence of a "" big blue bump"" due to thermal emission from the accretion disc. The X-ray spectra are well fitted with a power- law model with photoelectric absorption, possibly larger than the Galactic one. However, the comparison with previous X-ray observations would imply that the amount of absorbing matter is variable. Alternatively, the intrinsic X-ray spectrum presents a curvature, which may depend on the X-ray brightness. In this case, two scenarios are possible. i) There is no extra absorption, and the X-ray spectrum hardens at low energies, the hardening being more evident in bright states; ii) there is a constant amount of extra absorption, likely in the quasar environment, and the X-ray spectrum softens at low energies, at least in faint X-ray states. This softening might be the result of a flux contribution by the high-frequency tail of the big blue bump.
3.	Paskova, Tanja, 1961-, et al. (author) Characterization of mass-transport grown GaN by hydride vapour-phase epitaxy 2004 In: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 273:1-2, s. 118-128 Journal article (peer-reviewed)abstract A comprehensive study of the morphological, optical and microstructural properties of mass-transport (MT) and conventionally grown GaN by hydride vapour-phase epitaxy is presented. Spatially resolved techniques have been utilized to reveal in a comparative way, the characteristics of the material grown either in predominant vertical or lateral growth modes. A strong donor-acceptor pair (DAP) emission is observed from the MT regions with a distinctive intensity contrast between the exciton and DAP emission bands from MT and nontransport regions. Secondary ion mass spectroscopy and imaging were employed to investigate the impurity incorporation into different regions. An increase of residual oxygen and aluminium impurity concentrations was found in the MT areas. In addition, positron annihilation spectroscopy showed a strong signal of Ga vacancy clusters in the MT grown material. The increase of the point defect concentrations of both Ga vacancy and oxygen impurity, most likely forming defect complexes, is related to the enhancement of the DAP emission.
4.	Paskova, Tanja, 1961-, et al. (author) Defect and emission distributions in bulk GaN grown in polar and nonpolar directions : a comparative analysis 2008 In: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. ; , s. 68940D1- Conference paper (peer-reviewed)abstract We have investigated bulk GaN material grown by HVPE either in the conventional polar [0001] direction and subsequently sliced with nonpolar surfaces or grown in the nonpolar [11-20] direction. Spatially resolved techniques such as cathodoluminescence imaging and transmission electron microscopy, as well as profile measuring techniques such as positron annihilation spectroscopy and secondary ion mass spectroscopy were employed to directly visualize the extended structural defects, and point defect (impurity and vacancy) distributions along the growth axes. A comparative analysis of the results shows a distinctive difference in the distribution of all kind of defects along the growth axes. A significant decrease in the defect density in material grown along the polar direction, in contrast to the constant behavior of the high defect density in material grown along the nonpolar direction points out the low-defect superior quality of the former material and indicates the preferable way of producing high-quality GaN substrates with nonpolar surfaces.
5.	Valcheva, E., et al. (author) Elimination of nonuniformities in thick GaN films using metalorganic chemical vapor deposited GaN templates 2001 In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 90:12, s. 6011-6016 Journal article (peer-reviewed)abstract Thick hydride vapor phase epitaxial GaN layers are grown on metalorganic chemical vapor deposited GaN template layers as well as directly on sapphire, with the aim of investigating the effect of the template on the strain relaxation and spatial distribution of free carriers in the overgrown GaN films. Spatially resolved cross-sectional micro-Raman spectroscopy, cathodoluminescence, and transmission electron microscopy show improved crystalline quality for growth on metalorganic chemical vapor deposited GaN templates. The highly doped and highly defective near-substrate layer composed of columnar domains, typically present in hydride vapor phase epitaxial GaN films grown directly on sapphire, is absent in the layers grown on templates. Consequently, this results in elimination of the nonuniformities of free electron distribution, a lower residual free carrier concentration (<1017 cm-3), and improved strain relaxation. © 2001 American Institute of Physics.
6.	Valcheva, E., et al. (author) Erratum : Elimination of nonuniformities in thick GaN films using metalorganic chemical vapor deposited GaN templates (Journal of Applied Physics (2001) 90 (6011)) 2002 In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 91:10 I Journal article (other academic/artistic)
7.	Valcheva, E., et al. (author) Impact of MOCVD-GaN 'templates' on the spatial non-uniformities of strain and doping distribution in hydride vapour phase epitaxial GaN 2001 In: Materials Science & Engineering. - 0921-5107 .- 1873-4944. ; 82:1-3, s. 35-38 Journal article (peer-reviewed)abstract Thick HVPE-GaN layers are grown on Si-doped and undoped MOCVD-GaN 'template' layers as well as directly on sapphire, with the aim to investigate the effect of the MOCVD template on the strain relaxation and spatial distribution of free carriers in the overgrown HVPE films. Spatially resolved cross-sectional micro-Raman measurements, cathodoluminescence and transmission electron microscopy show improved crystalline quality resulting in elimination of the non-uniformities of electron distribution, a low free carrier concentration (< 1017 cm-3) as well as a significant strain relaxation effect. © 2001 Elsevier Science B.V.
8.	Arnaudov, B, et al. (author) Energy position of near-band-edge emission spectra of InN epitaxial layers with different doping levels 2004 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 69:11 Journal article (peer-reviewed)abstract We studied the shape and energy position of near-band-edge photoluminescence spectra of InN epitaxial layers with different doping levels. We found that the experimental spectra of InN layers with moderate doping level can be nicely interpreted in the frames of the "free-to-bound" recombination model in degenerate semiconductors. For carrier concentrations above n>5x10(18) cm(-3) the emission spectra can also be modeled satisfactorily, but a contribution due to a pushing up of nonequilibrium holes over the thermal delocalization level in the valence band tails should be considered in the model. The emission spectra of samples with low doping level were instead explained as a recombination from the bottom of the conduction band to a shallow acceptor assuming the same value of the acceptor binding energy estimated from the spectra of highly doped samples. Analyzing the shape and energy position of the free-electron recombination spectra we determined the carrier concentrations responsible for the emissions and found that the fundamental band gap energy of InN is E-g=692+/-2 meV for an effective mass at the conduction-band minimum m(n0)=0.042m(0).
9.	Arnaudov, B., et al. (author) Free-to-bound radiative recombination in highly conducting InN epitaxial layers 2004 In: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 36:4-6, s. 563-571 Journal article (peer-reviewed)abstract We present a theoretical simulation of near-band-edge emission spectra of highly conducting n-InN assuming the model of 'free-to-bound' radiative recombination (FBRR) of degenerate electrons from the conduction band with nonequilibrium holes located in the valence band tails. We also study experimental photoluminescence (PL) spectra of highly conducting InN epitaxial layers grown by MBE and MOVPE with electron concentrations in the range (7.7 × 1017-6 × 1018) cm-3 and find that the energy positions and shape of the spectra depend on the impurity concentration. By modeling the experimental PL spectra of the InN layers we show that spectra can be nicely interpreted in the framework of the FBRR model with specific peculiarities for different doping levels. Analyzing simultaneously the shape and energy position of the InN emission spectra we determine the fundamental bandgap energy of InN to vary between Eg = 692 meV for effective mass mn0 = 0.042m0 and Eg =710 meV for mn0 = 0.1m0. © 2004 Elsevier Ltd. All rights reserved.
10.	Arnaudov, B, et al. (author) Multilayer model for Hall effect data analysis of semiconductor structures with step-changed conductivity 2003 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:4 Journal article (peer-reviewed)abstract 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.
11.	Darakchieva, Vanya, 1971-, et al. (author) Deformation potentials of the E1 (TO) and E2 modes of InN 2004 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 84:18, s. 3636-3638 Journal article (peer-reviewed)abstract The determination of deformation potentials of E1(TO) and E 2 modes of InN were discussed. The deformation potentials were evaluated for two sets of stiffness constants using x-ray diffraction, IR spectroscopic ellipsometry (IRSE), Raman scattering, and Grüneisen parameter values. The InN layer were grown on GaN buffer layers on (0001) sapphire by molecular beam epitaxy. It was found that the strain-free values of the InN E1(TO) mode was 477.9 cm-1 and 491.9 cm -1 for the E2 modes.
12.	Darakchieva, Vanya, et al. (author) Infrared ellipsometry and Raman studies of hexagonal InN films : Correlation between strain and vibrational properties 2004 In: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 36:4-6, s. 573-580 Journal article (peer-reviewed)abstract The vibrational properties of InN films with different strain have been studied using Infrared ellipsometry and Raman scattering spectroscopy. We have established a correlation between the phonon mode parameters and the strain, which allows the determination of the deformation potentials and the strain-free frequencies of the InN E1(TO) and E2 modes. The LO phonons and their coupling to the free-carrier plasmon excitations are also discussed in relation to the carrier concentration in the films. © 2004 Elsevier Ltd. All rights reserved.
13.	Darakchieva, Vanya, et al. (author) Lattice parameters of GaN layers grown on a-plane sapphire : Effect of in-plane strain anisotropy 2003 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 82:5, s. 703-705 Journal article (peer-reviewed)abstract The lattice parameters of GaN layers grown on a-plane sapphire were investigated. The hydride vapor phase epitaxy and metalorganic vapor phase epitaxy were used for the determination of parameters. The strain anisotropy was found to have different values in the films and obtained values of parameters were grouped around two values.
14.	Darakchieva, Vanya, 1971-, et al. (author) Phonon mode behavior in strained wurtzite AlN/GaN superlattices 2005 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:11, s. 115329- Journal article (peer-reviewed)abstract We have studied phonons in AlN/GaN superlattices with different periods but a constant well-to-barrier ratio using a combination of infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The strain evolution in the superlattice structures is assessed by high-resolution x-ray diffraction and reciprocal space mapping. We have identified E1(TO), A 1(LO) and E2 localized, and E1(LO) and A 1(TO) delocalized superlattice modes. The dependencies of their frequencies on in-plane strain are analyzed and discussed, and the strain-free frequencies of the superlattice modes are estimated. A good agreement between theory and experiment is found in the case of GaN localized modes, while large deviations between theoretically estimated and experimentally determined frequency shifts are observed for the AlN localized modes. The delocalization effect on the A1(TO) and E1(LO) phonons, as well as the free-carrier effect on the E1(LO) phonon are also discussed. ©2005 The American Physical Society.
15.	Donchev, V., et al. (author) Optical properties of thick GaInAs(Sb)N layers grown by liquid-phase epitaxy 2017 In: 19TH INTERNATIONAL SCHOOL ON CONDENSED MATTER PHYSICS (ISCMP): ADVANCES IN NANOSTRUCTURED CONDENSED MATTER: RESEARCH AND INNOVATIONS. - : IOP PUBLISHING LTD. Conference paper (peer-reviewed)abstract We present an experimental and theoretical study of GaInAs(Sb)N layers with thickness around 2 mu m, grown by liquid-phase epitaxy (LPE) on n-type GaAs substrates. The samples are studied by surface photovoltage (SPV) spectroscopy and by photoluminescence spectroscopy. A theoretical model for the band structure of Sb-containing dilute nitrides is developed within the semi-empirical tight-binding approach in the sp(3)d(5)s*s(N) parameterisation and is used to calculate the electronic structure for different alloy compositions. The SPV spectra measured at room temperature clearly show a red shift of the absorption edge with respect to the absorption of the GaAs substrate. The shifts are in agreement with theoretical calculations results obtained for In, Sb and N concentrations corresponding to the experimentally determined ones. Photoluminescence measurements performed at 300K and 2 K show a smaller red shift of the emission energy with respect to GaAs as compared to the SPV results. The differences are explained by a tail of slow defect states below the conduction band edge, which are probed by SPV, but are less active in the PL experiment.
16.	Harati Zadeh, Hamid, 1964-, et al. (author) Optical observation of discrete well width fluctuations in wide band gap III-nitride quantum wells 2007 In: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 244:5, s. 1727-1734 Journal article (peer-reviewed)abstract A detailed observation of discrete well width fluctuations via localized excitons in the photoluminescence (PL) spectra of MOCVD-grown undoped GaN/Al0.07Ga0.93 N multiple quantum wells (MQWs) has been reported. Doublet excitonic features with a distance varying between 10 and 25 meV for different well widths (1.5 to 4.5 nm) are observed in the PL spectra. They are explained in terms of discrete well width variations by one c-lattice parameter, i.e. two GaN monolayers. By mapping the PL measurements across the samples with different excitation spot size, it is shown that the extension of areas with a constant well width is less than 1 μm2. TEM pictures give evidence of interface roughness, although the contrast is weak at this low Al composition. In addition we observe a long-range variation of the PL peak position across the sample, interpreted as a fluctuation in Al composition in the barriers. The residual broadening of an excitonic peak (apart from the splitting related to well width fluctuations) is about 10 meV, somewhat larger for larger well widths, and is mainly ascribed to hole localisation potentials in the QWs. Additional broadening occurs in the MQWs due to inequivalent properties of each QW within the excitation spot. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.
17.	Milanova, M., et al. (author) Experimental study of the effect of local atomic ordering on the energy band gap of melt grown InGaAsN alloys 2017 In: Semiconductor Science and Technology. - : IOP PUBLISHING LTD. - 0268-1242 .- 1361-6641. ; 32:8 Journal article (peer-reviewed)abstract We present a study of melt grown dilute nitride InGaAsN layers by x-ray photoelectron spectroscopy (XPS), Raman and photoluminescence (PL) spectroscopy. The purpose of the study is to determine the degree of atomic ordering in the quaternary alloy during the epitaxial growth at near thermodynamic equilibrium conditions and its influence on band gap formation. Despite the low In concentration (similar to 3%) the XPS data show a strong preference toward In-N bonding configuration in the InGaAsN samples. Raman spectra reveal that most of the N atoms are bonded to In instead of Ga atoms and the formation of N-centred In3Ga1 clusters. PL measurements reveal smaller optical band gap bowing as compared to the theoretical predictions for random alloy and localised tail states near the conduction band minimum.
18.	Milanova, M., et al. (author) Investigation of LPE grown dilute nitride InGaAs(Sb)N layers for photovoltaic applications 2019 In: 10TH JUBILEE CONFERENCE OF THE BALKAN PHYSICAL UNION. - : AMER INST PHYSICS. - 9780735418035 Conference paper (peer-reviewed)abstract We report on LPE growth and characterization of dilute nitride InGaAs(Sb)N layers nearly lattice matched to GaAs. In order to obtain high quality epitaxial layers without phase separation low-temperature variant of LPE method has been used. The composition and crystalline quality of the grown InGaAs(Sb)N layers have been determined by energy dispersive X-ray microanalysis and X-ray diffraction methods. SEM and AFM measurements on grown samples revealed flat interfaces and surface roughness in the range 0.2 - 0.3 nm. In order to identify the N-bonding mechanism in the alloys and the nature of nitrogen related clusters IR absorption and Raman scattering spectroscopy have been applied. The optical band gap of the samples is studied by photoluminescence (PL) spectroscopy at low and room temperatures and by surface photovoltage (SPV) spectroscopy at room temperature. The SPV and PL spectra reveal a red shift of the absorption edge and PL peak position as compared to GaAs, as well as localized states near the conduction band minimum. However, the optical band gap bowing of the samples appears smaller with respect to the random alloy, which is explained by the short-range ordering favored by LPE growth at near-equilibrium conditions. Variable angle ellipsometry is applied to determine the spectral behavior of the complex refractive index and estimate the band gap energy of the samples.
19.	Monemar, Bo, et al. (author) Optical investigation of AlGaN/GaN quantum wells and superlattices 2004 In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:10, s. 2251-2258 Journal article (peer-reviewed)abstract We report a detailed optical study of several sets of multiple quantum wells (MQWs) in the AlGaN/GaN system, as well as AlN/GaN superlattice (SL) structures. In this study all materials were grown by MOCVD, as opposed to most previous studies where MBE was employed. In undoped MQWs discrete photoluminescence (PL) peaks related to discrete well width fluctuations by one full c lattice parameter are clearly observed. In doped samples this effect appears to be screened. While the recombination process in undoped samples is excitonic, in MQWs doped with Si above about 5 x 10(18) cm(-3) free electrons (a 2DEG) are dominant, and the PL process is a free electrons-localized hole transition at low temperatures. The hole localization prevails up to very high n-doping, as was previously observed in bulk GaN. The hole localization is demonstrated via several experiments, including results on PL transient decay times and LO phonon coupling. Near surface band bending, due mainly to dopant depletion in doped structures or interaction with surface states in case of higher Al content in barriers, influences the distribution of electron filling among the QWs, making a detailed modeling of the spectral shape somewhat ambiguous. It is found that AlN barriers promote a strong room temperature PL signal from the QWs, as opposed to the case with AlGaN barriers.
20.	Paskova, T., et al. (author) 6H-SiC crystallinity behaviour upon B implantation studied by Raman scattering 1998 In: Silicon carbide, III-nitrides and related materials : ICSCIII-N'97. - : Trans Tech Publications Inc.. - 0878497900 ; , s. 741-744 Conference paper (peer-reviewed)abstract In this study, B ion implantation was performed in n-type 6H-SiC single crystals at 500 degrees C. The implanted specimens were annealed at 1700 degrees C in SiH4 atmosphere. Lattice damage induced by implantation and its recovery was characterised by Raman scattering. Reduced damage compared with other ions implantation was observed. Recrystallization of the implanted material and absence of amorphous phases was detected after high temperature annealing.
21.	Paskova, Tanja, et al. (author) Defect and stress relaxation in HVPE-GaN films using high temperature reactively sputtered AlN buffer 2001 In: Journal of Crystal Growth. - 0022-0248 .- 1873-5002. ; 230:3-4, s. 381-386 Journal article (peer-reviewed)abstract 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. © 2001 Elsevier Science B.V. All rights reserved.
22.	Paskova, Tanja, et al. (author) Effect of Si doping of metalorganic chemical vapor deposition-GaN templates on the defect arrangement in hydride vapor phase epitaxy-GaN overgrown layers 2000 In: Journal of Applied Physics. - 0021-8979 .- 1089-7550. ; 88:10, s. 5729-5732 Journal article (peer-reviewed)abstract Two different types of dislocation arrangements have been observed in hydride vapor-phase epitaxial GaN films grown on sapphire substrates using both undoped and Si-doped GaN templates grown by metalorganic chemical vapor deposition: (i) predominantly straight threading dislocations parallel to the [0001] direction in the layer grown on an undoped template, and (ii) a network of interacting dislocations of edge, screw, and mixed character in the layer grown on a Si-doped template. The two types of defect distribution result in essentially different surface morphologies, respectively: (i) low-angle grain boundaries formed by pure edge dislocations around spiral grown hillocks, and (ii) smooth surface intersected by randomly distributed dislocations. The Si doping of the GaN templates was found to enhance defect interaction in the templates and to enable a reduction of the dislocation density in the overgrown thick GaN films, although it does not lead to an improvement of the overall structural properties of the material. (C) 2000 American Institute of Physics. [S0021-8979(00)08422-X].
23.	Paskova, Tanja, et al. (author) Growth of GaN on a-plane sapphire : In-plane epitaxial relationships and lattice parameters 2003 In: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 240:2, s. 318-321 Conference paper (other academic/artistic)abstract We have studied GaN films grown on a-plane sapphire substrates by both hydride vapor phase epitaxy (HVPE) and metalorganic vapor phase epitaxy (MOVPE). The in-plane orientation relationships between the epitaxial films and the substrate are determined to be [11-20]GaN ? [0001] sapphire and [1-100]GaN ? [1-100]sapphire in the HVPE growth, while [1-100]GaN ? [0001]sapphire and [11-20]GaN ? [1-100]sapphire are found in the MOVPE growth. The different orientation preferences are attributed to the atom termination of the sapphire surface determined by the substrate treatment used in the different growth methods. The effect of the lattice matches on the in-plane lattice parameters and strain anisotropy in the two cases is studied. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
24.	Paskova, Tanja, et al. (author) HVPE-GaN : Comparison of emission properties and microstructure of films grown on different laterally overgrown templates 2004 In: Diamond and related materials. - : Elsevier BV. - 0925-9635 .- 1879-0062. ; 13:4-8, s. 1125-1129 Journal article (peer-reviewed)abstract We report on a comparative study of defect and emission distributions in thick hydride vapor phase epitaxial (HVPE) GaN films grown on two different patterned template structures separately produced by multi-step procedures using metalorganic vapor phase epitaxy (MOVPE). The observed differences in the microstructures and emission distributions at the early stages of the growth in both cases were related to the change of the dominating growth mode sequence and point defects incorporation. Both template structures were found to favor formation of voids in the coalescence regions, which leads to a partial strain relaxation and allows overgrowth of thicker films without cracks. © 2003 Elsevier B.V. All rights reserved.
25.	Paskova, Tanja, et al. (author) Hydride vapor-phase epitaxial GaN thick films for quasi-substrate applications : Strain distribution and wafer bending 2004 In: Journal of Electronic Materials. - 0361-5235 .- 1543-186X. ; 33:5, s. 389-394 Journal article (peer-reviewed)abstract The strain distribution in thick hydride vapor-phase epitaxial (HVPE)-GaN layers grown on metal-organic vapor-phase epitaxial GaN templates was studied by means of photoluminescence, x-ray mapping, and lattice parameter analysis. A variable temperature x-ray study of the film curvature was used for verification of the strain type. The relation between the strain inhomogeneity and the wafer bending in films residing on sapphire and freestanding on the thickness of the layer and the substrate is analyzed. Possibilities to improve the uniformity of the film characteristics and to reduce the bending of the HVPE-GaN films are discussed.
26.	Paskova, Tanja, et al. (author) Hydride vapour phase homoepitaxial growth of GaN on MOCVD-grown 'templates' 2000 Conference paper (peer-reviewed)abstract 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.
27.	Paskova, Tanja, et al. (author) In-plane epitaxial relationships between a-plane sapphire substrates and GaN layers grown by different techniques 2003 In: Journal of Crystal Growth. - 0022-0248 .- 1873-5002. ; 257:1-2, s. 1-6 Journal article (peer-reviewed)abstract We have studied GaN films grown on a-plane sapphire substrate by both hydride vapour phase epitaxy (HVPE) and metalorganic vapour phase epitaxy (MOVPE) using X-ray diffraction and transmission electron microscopy. The in-plane orientation relationships between the epitaxial films and the substrate are determined to be [1 1 2¯ 0]GaN?[0 0 0 1] sapphire and [1 1¯ 0 0]GaN?[1 1¯ 0 0] sapphire in the HVPE growth, while [1 1¯ 0 0] GaN?[0 0 0 1]sapphire and [1 1 2¯ 0] GaN?[1 1¯ 0 0]sapphire are found in the MOVPE growth. In a few films of both types a simultaneous presence of two domains, representing the two in-plane relationships, is observed although the preferable one in each type of films is strongly dominating. We propose that the two in-plane orientations of GaN are generally possible on the a-sapphire substrate and are related to the atom termination of the sapphire surface determined by the substrate pre-treatment used in the different growth methods. © 2003 Elsevier B.V. All rights reserved.
28.	Paskova, Tanja, et al. (author) Mass transport growth and properties of hydride vapour phase epitaxy GaN 2001 In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 188:1, s. 447-451 Journal article (peer-reviewed)abstract We report a comparative study of the optical and structural properties of mass-transport and conventionally grown GaN by hydride vapor phase epitaxy. A strong donor-acceptor pair emission is observed from the mass-transport regions with a distinctive intensity contrast between the exciton and donor-acceptor bands from mass-transport and nontransport regions. Secondary ion mass spectroscopy was employed to investigate the impurity incorporation into different regions. A moderate increase of residual impurity incorporation or redistribution was found in mass-transport regions related to different growth modes.
29.	Paskova, Tanja, 1961-, et al. (author) Nonpolar a-plane HVPE GaN : growth and in-plane anisotropic properties 2005 In: Physica Status Solidi. C, Current topics in solid state physics. - 1610-1634 .- 1610-1642. ; 2, s. 2027-2031 Journal article (peer-reviewed)
30.	Paskova, Tanja, et al. (author) Polar and nonpolar GaN grown by HVPE : Preferable substrates for nitride-based emitting devices 2004 In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:10, s. 2265-2270 Journal article (peer-reviewed)abstract We report on hydride vapor phase epitaxial growth of thick nonpolar GaN films on r-plane sapphire in comparison with polar GaN films on c-plane sapphire substrates with AlN buffer layers, aiming at developing of their quasi-substrate application. Both the thick films and the buffers were identified to have single crystalline structures. The microstructure of the films was studied by transmission electron microscopy. High resolution X-ray diffraction mapping and photoluminescence measurements were employed to characterize the strain present in both polar and nonpolar GaN films. In contrast to c-plane GaN, which is always characterised by isotropic in-plane properties, the a-plane GaN shows a strong in-plane anisotropy of the growth rate, morphology and strain distribution. Different defect, impurity and free carrier concentrations were observed in the polar and nonpolar material.
31.	Paskova, Tanja, et al. (author) Properties of nonpolar a-plane GaN films grown by HVPE with AlN buffers 2005 In: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 281:1, s. 55-61 Journal article (peer-reviewed)abstract The influence of high temperature AlN buffer layers on the morphology, structural and optical characteristics of a-plane GaN grown by hydride vapour phase epitaxy on r-plane sapphire was investigated. While the morphology of the a-GaN was found to be significantly improved by using a-plane AlN buffer layer similarly to the effect observed in c-plane hydride vapour phase epitaxy GaN growth, the microstructure ensemble was revealed to be more complicated in comparison to that of the c-plane GaN. Higher dislocation density and prismatic stacking faults were observed. Moreover, in-plane anisotropic structural characteristics were revealed by high resolution X-ray diffraction employing azimuthal dependent and edge X-ray measurement symmetric geometry. In addition, the near band edge photo luminescence peaks, red-shifted with respect to that in c-plane GaN were observed. The latter were explained by the influence of the higher defect density and more complex strain distribution. (c) 2005 Elsevier B.V. All rights reserved.
32.	Paskova, Tanja, et al. (author) Thick GaN films grown on sapphire : Detects in highly mismatched systems 2002 In: Defects and diffusion in semiconductors. - 1012-0386. ; 200-2 Journal article (peer-reviewed)abstract In this paper, we review the present knowledge of the defects in thick GaN films grown by hydride vapor phase epitaxy (HVPE) on sapphire substrates. We summarize the defects present in such highly mismatched system into three main categories: large-scale nonuniformities, microstructure crystallographic defects, and point defects. We begin by describing the layer structure typical for thick films grown at very high growth rates, and concentrate on large-scale three-dimensional defects and their impact on the crystal quality. Then we briefly review the current understanding of the variety of extended structural defects with the emphasis on their role in the morphology and relaxation mechanisms in thick heteroepitaxial layers. The discussion is completed by reviewing the point defects in HVPE nitride materials and their influence on the optical properties of GaN films.
33.	Ratnikov, V, et al. (author) Bragg and Laue x-ray diffraction study of dislocations in thick hydride vapor phase epitaxy GaN films 2000 In: Journal of Applied Physics. - 0021-8979 .- 1089-7550. ; 88:11, s. 6252-6259 Journal article (peer-reviewed)abstract The dislocation structure of hydride vapor phase epitaxial thick GaN layers grown on sapphire is studied by analysis of the microdistortion tensor components. Symmetrical reflections (including reflections from planes forming a large angle with the basal plane) with two modes of scanning (theta and theta -2 theta) in two geometries (Bragg and Laue) are used to obtain the tensor components. The instant connections between the tensor components and major dislocation types are specified. Different types of dislocation distributions have been identified in the thick GaN films grown on sapphire with and without undoped and Si-doped metalorganic chemical vapor deposited templates. Transmission electron microscopy was used to confirm the x-ray results by direct visualization of defect rearrangements. (C) 2000 American Institute of Physics. [S0021-8979(00)06023-0].
34.	Tuomisto, F., et al. (author) Evolution of vacancy defects in GaN after H implantation and subsequent annealing 2008 In: IWN 2008,2008. Conference paper (peer-reviewed)
35.	Valcheva, E, et al. (author) B implantation in 6H-SiC : Lattice damage recovery and implant activation upon high-temperature annealing 1999 In: Journal of Vacuum Science & Technology B. - : American Vacuum Society. - 1071-1023 .- 1520-8567. ; 17:3, s. 1040-1044 Journal article (peer-reviewed)
36.	Valcheva, E, et al. (author) Electrical activation of B implant in 6H-SiC 1998 In: SILICON CARBIDE, III-NITRIDES AND RELATED MATERIALS, PTS 1 AND 2. - 0878497900 ; , s. 705-708 Conference paper (peer-reviewed)
37.	Valcheva, E, et al. (author) Epitaxial growth and orientation of AlN thin films on Si(001) substrates deposited by reactive magnetron sputtering 2006 In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 100:12 Journal article (peer-reviewed)abstract Epitaxial domain formation and textured growth in AlN thin films deposited on Si(001) substrates by reactive magnetron sputtering was studied by transmission electron microscopy and x-ray diffraction. The films have a wurtzite type structure with a crystallographic orientation relationship to the silicon substrate of AlN (0001) ∥Si (001). The AlN film is observed to nucleate randomly on the Si surface and grows three dimensionally, forming columnar domains. The in-plane orientation reveals four domains with their a axes rotated by 15° with respect to each other: AlN 〈11 2- 0〉 ∥Si [110], AlN 〈01 1- 0〉 ∥Si [110], AlN 〈11 2- 0〉 ∥Si [100], and AlN 〈01 1- 0〉 ∥Si [100] An explanation of the growth mode based on the large lattice mismatch and the topology of the substrate surface is proposed. © 2006 American Institute of Physics.
38.	Valcheva, E., et al. (author) Extended defects in GaN films grown at high growth rate 2002 In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 14:48, s. 13269-13275 Journal article (peer-reviewed)abstract A description of the present knowledge of the layer microstructure typical for thick GaN films grown at high growth rates is presented. The GaN layers are grown by hydride vapour phase epitaxy on sapphire and different buffers are used. The variety of extended defects present in such highly mismatched system are summarized, with the emphasis on their impact on the crystal quality. The defects are reviewed in two main categories according to the microstructural development during growth: large-scale nonuniformities and microstructural crystallographic defects. The first category comprises three-dimensional structural features developed mainly in the interface region, while the second are typical extended defects, i.e., dislocations with different Burgers vectors, nanopipes, inversion domain boundaries and stacking faults. The quality of the layers was improved vastly as a consequence of our understanding of the correlation of growth parameters and microstructure.
39.	Valcheva, E., et al. (author) Growth-induced defects in AlN/GaN superlattices with different periods 2003 In: Physica B: Condensed Matter. - : Elsevier BV. - 0921-4526. Conference paper (peer-reviewed)abstract MOCVD-grown AlN/GaN superlattices (SL) with different thickness of the well and barrier are investigated by transmission electron microscopy in low-magnification and high-resolution modes. Growth-induced defects are observed in some places causing different degree of disturbances of the regularity of the SLs. The SL grown with a thickness of the AlN barrier higher than the critical thickness hc for coherent growth reveals 2D growth of AlN platelets that give rise to cusps or irregularities at the interfaces. The SL with thickness of the GaN and AlN layers lower than hc is coherently grown. There is some evidence of dot-like structures formation in separate areas due to the large strain field present. © 2003 Elsevier B.V. All rights reserved.
40.	Valcheva, E., et al. (author) Influence of well-width fluctuations on the electronic structure of GaN/AlxGa1-xN multiquantum wells with graded interfaces 2007 In: Acta Physica Polonica. A. - 0587-4246 .- 1898-794X. ; 112:2, s. 395-400 Journal article (peer-reviewed)abstract Experimental and computation results based on chemical composition assessment of metal-organic chemical vapour deposition grown undoped GaN/AlxGa1-xN multiquantum well structures in the low composition limit of x = 0.07 and wide wells demonstrate composition fluctuations in the barrier layers which lead to large-scale nonuniformities and inequivalence of the different wells. As a consequence the experimental photoluminescence spectra at low temperature show a double peak structure indicative of well-width fluctuations by one lattice parameter (2 monolayers).
41.	Valcheva, E, et al. (author) Interface structure of hydride vapor phase epitaxial GaN grown with high-temperature reactively sputtered AlN buffer 2000 In: Applied Physics Letters. - 0003-6951 .- 1077-3118. ; 76:14, s. 1860-1862 Journal article (peer-reviewed)abstract Thick hydride vapor phase epitaxy GaN layers have been grown on a-plane sapphire using high-temperature ion-assisted reactively sputtered AlN as a buffer layer. Transmission electron microscopy and atomic force microscopy were carried out to study the formation of the two interfaces sapphire/AlN and AlN/GaN, and their influence on the microstructure of both the buffer layer and the main GaN layer. It was demonstrated that the high-temperature reactively sputtered buffer layer provides a good alternative for hydride vapor phase epitaxy growth of GaN layers. In particular, the buffer promotes a specific interface ordering mechanism different from that observed on low-temperature buffers. (C) 2000 American Institute of Physics. [S0003-6951(00)00314-4].
42.	Valcheva, E., et al. (author) Misfit defect formation in thick GaN layers grown on sapphire by hydride vapor phase epitaxy 2002 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 80:9, s. 1550- Journal article (peer-reviewed)
43.	Valcheva, E., et al. (author) Nanopipes and their relationship to the growth mode in thick HVPE-GaN layers 2003 In: Journal of Crystal Growth. - 0022-0248 .- 1873-5002. ; 255:1-2, s. 19-26 Journal article (peer-reviewed)abstract In this work, we illustrate and analyse long propagating nanopipes in thick HVPE-GaN layers studied by means of transmission electron microscopy. The nanopipes appear to have the characteristics and behaviour of screw-dislocations. They propagate with a constant diameter along tens of micrometers and a pit of a shape of inverted pyramid forms when the top surface of the layer is reached. The generation of the nanopipes is discussed in terms of their relation to the growth mechanism of HVPE-GaN material and the kinetics of screw dislocations in the early stages of growth of highly strained material. The nanopipes appear to promote the crystal growth when the substrate surface does not provide a sufficient nucleation base for subsequent HVPE growth. In this case they mediate the surface morphology terminating surface steps by hexagonal pits and act as centres of growth spirals. When the substrate surface provides a sufficient amount of nucleation sites the step flow mechanism is prevailing and no growth spirals associated with nanopipes are observed. © 2003 Elsevier Science B.V. All rights reserved.
44.	Valcheva, E, et al. (author) Nanopipes in thick GaN films grown at high growth rate 2002 In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 194:2, s. 532-535 Journal article (peer-reviewed)abstract In this work we illustrate and describe long propagating nanopipes in thick HVPE-GaN layers analysed by means of transmission electron microscopy. They are observed to behave like screw component threading dislocations, terminating surface steps by hexagonal pits, and thus leading to the possibility of spiral growth. The formation of a nanopipe is observed by trapping of a screw dislocation at a pinhole that opens in an empty core. The mechanism of formation of nanopipes is likely to be connected to the growth kinetics of screw dislocations in the early stages of growth of highly strained material.
45.	Valcheva, E., et al. (author) Tunneling effects in short period strained GaN/AlN superlattices 2008 In: IWN 2008,2008. Conference paper (peer-reviewed)

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