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The quasi-ballistic hole transport in germanium and silicon nanowires was theoretically investigated by solving the Boltzmann transport equation taking account of phonon scattering in an atomistic framework. Comparison of quasi-ballistic hole transport capabilities between germanium and silicon nanowires showed that the transmission coefficients for the two materials are similar. Then, the behavior of forward and backward current fluxes was analyzed focusing on the impact of energy relaxation process. The slower energy relaxation of holes in germanium nanowires leads to a longer distance where backscattering enables holes to return to the source. This cancels the benefit of the longer mean free path of holes in germanium nanowires, resulting in similar transmission coefficients in germanium and silicon nanowires. [ABSTRACT FROM AUTHOR]

Lattice parameters of high-dose ion-implanted 4H-SiC were investigated with reciprocal space mapping (RSM). N, P, Al, or (C+Si) ions were implanted into lightly doped epilayers to form a (330-520) nm-deep box profile with concentrations of 1019-1020atoms/cm3. After activation annealing at 1800 °C, RSM measurements were conducted. The RSM images for (0008) reflection revealed that high-dose ion implantation causes c-lattice expansion in implanted layers, irrespective of ion species. In addition, crystallographic tilt was observed after high-dose ion implantation. The tilt direction is the same for all the samples investigated; the c-axis of the implanted layers is inclined toward the ascending direction of the off-cut. The c-lattice mismatch and the tilt angle increased as the implantation dose increases, indicating that the implantation damage is responsible for the lattice parameter change. From these results and transmission electron microscopy observation, the authors conclude that the c-lattice mismatch and the crystallographic tilt are mainly caused by secondary defects formed after the ion-implantation and activation-annealing process. [ABSTRACT FROM AUTHOR]

Premise of research. Plant diversification can be a result of many different factors, such as environment or genome size variation. Further, the presence of phenotypic or genomic variation could highlight the presence of a cryptic lineage. Here, we tested whether phenotypic and genomic differences support the presence of a cryptic lineage in the South African populations of Helichrysum odoratissimum (L.) Sweet. Methodology. We used estimates of genetic structure from microsatellite data and cytogenetic data from 17 sampled populations, paired with morphological measurements from representative populations, to test for the presence of cryptic lineages. Pivotal results. Ploidy level estimates for the populations that span the species' South African distribution indicated that 15 of the 17 populations contain either putative DNA-diploid or DNA-hexaploid individuals and that two populations contain both putative DNA-diploids and DNA-hexaploids. Population genetic analyses of seven microsatellite loci showed reduced admixture (gene flow) between populations of different ploidies, as well as across three geographic regions (Mpumalanga, the Drakensberg, and the Western Cape). We found leaf size differences between plants of different ploidy; plants with larger genome sizes exhibit longer and wider leaves than plants with smaller genome sizes. Conclusions. While the role of polyploidy in global floral diversification is well accepted, the extent to which polyploidy contributes to specific diverse regions, such as South Africa, is less well known. Our results suggest that morphologically variable populations of H. odoratissimum are distributed across distinct geographic regions in South Africa that coincide with different genome sizes. The Knsyna area of the Western Cape harbors a center of genomic diversity, as well as a cryptic hexaploid subspecies, possibly due to the variability in rainfall patterns, as summer rainfall changes over to winter rainfall in this region. This study contributes to the emerging work surrounding the microhabitat differences of ploidies within the context of establishment and subsequent divergence. [ABSTRACT FROM AUTHOR]

The dominance of natural hazards in our environment is very high. Only when natural events occur does one realize their power and consequences. The risk potential of natural hazards and their possible consequences have prompted scientific studies of torrent events and barrier structures. This paper presents a method that allows the assessment of uncertain stress forces and resisting elements. The method is based on a probabilistic approach and includes uncertain parameters regarding acting and resistance in a risk analysis. This way of modelling also enables time changes of the barrier resistance to be taken into account. Basic information on the types of torrential barriers used is given and traditional assessment methods and loads are described. [ABSTRACT FROM AUTHOR]

Abstract: AlN has been grown on 4H-SiC m-plane and a-plane substrates by RF-plasma-assisted molecular-beam epitaxy. Similarities and differences between the two growth orientations are discussed. High-quality AlN can be obtained in both orientations when grown in the metastable 4H-crystal structure. For both epilayer orientations Al-rich conditions were necessary to stabilize the 4H-polytype. 4H-AlN for both growth directions shows very narrow X-ray rocking curves widths less than 100 arcsec. a-plane 4H-AlN exhibited a much smoother morphology than m-plane 4H-AlN. On the other hand, reflection high-energy diffraction intensity oscillations in the initial growth stage were observed only for m-plane AlN. [Copyright &y& Elsevier]

Abstract: ZrB2(0001) crystals grown by the rf-floating zone technique were characterized by X-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and atomic force microscopy. These characteristics were investigated as a function of thermal cleaning temperature up to 1000°C in vacuum for as-received substrates as well as substrates treated ex situ in HF aqueous solution. The HF treatment process removed the ZrO2 native oxide layer present on as-received substrates and resulted in ZrB2(0001) surfaces exhibiting long-range order. Upon annealing the HF-treated surface in high vacuum, two types of reconstructions were observed: an incommensurate reconstruction from 650 to 900°C related to residual H2 gas, and n × n reconstructions at 1000°C, possibly related to oxygen. [Copyright &y& Elsevier]

Genome sizes for 127 Macaronesian endemic angiosperms from 69 genera and 32 families were estimated using propidium iodide flow cytometry. Only about 30-fold variation in 1C-values was found, ranging from 0.32 pg inEchium bonnetiito 9.52 pg inScilla dasyantha. Taxa with very small DNA amounts (1C = 1.4 pg) were the most dominant group (71.7%), whereas the frequency of other categories was much lower (18.9% and 9.4% in taxa with small (1.41-3.50 pg) and intermediate 1C-values (3.51-14.00 pg), respectively). Comparisons of average C- and Cx-values between Macaronesian endemics and non-Macaronesian representatives always revealed significantly smaller amounts in the former group at various taxonomic levels (genus, family, major phylogenetic lineage). Potential relationship between nuclear DNA content and insular burst of speciation is suggested owing to the marked prevalence of very small genomes among angiosperms that underwent rapid adaptive radiation. Merging all the genome size data on Macaronesian angiosperms available shows that this flora represents the best covered plant assemblage from the phytogeographic point of view. [ABSTRACT FROM AUTHOR]

Growth of group-III nitrides was carried out on zirconium diboride (ZrB2) (0 0 0 1) substrates for the first time. ZrB2 is a semi-metal compound and have a hexagonal crystal structure. The a-axis lattice constant of ZrB2 is 3.169 Å, which is almost lattice-matched to GaN and AlN. GaN and AlN were grown on mirror-polished ZrB2 (0 0 0 1) substrate directly by molecular beam epitaxy using elemental Al and Ga, and radio frequency plasma-excited active nitrogen. Epitaxial growth was confirmed by in situ reflection high-energy electron diffraction observation and X-ray diffraction pole-figure measurement. The surface morphology of 1.4 μm-thick GaN layer was rough. On the other hand, that of 0.9 μm-thick AlN layer was specular. [Copyright &y& Elsevier]

The ploidy level of plants of the Vaccinium sect. Oxycoccus (HILL) W.D.J. KOCH sampled in the Czech Republic, Germany, Austria and Poland was determined by chromosome counting and/or by flow cytometry. Forty-five characters were measured and scored in the morphometric analysis. Principal component analysis, cluster analysis, canonical discriminant analysis and classificatory discriminant analysis were used in the statistical analyses. Diploid (2n=24), tetraploid (2n=48) and hexaploid (2n=72) populations were confirmed and a new ploidy level pentaploid hybrid plants (2n=60) was revealed. Results of the multivariate morphometric analysis support the separation of the two native species. Diploid V. microcarpum (TURCZ. exRUPR.) SCHMALH. differs from the polyploids by smaller size of petals, shorter style and stamens (stamens have long filaments and short anthers), glabrous pedicels, mostly solitary flowers, earlier flowering and by occurrence predominantly in Polytrichum strictum tufts. The low taxonomic significance of some features often used in keys for their separation (shape of fruits, insertion of prophylla, pubescence of filaments) was confirmed. V. oxycoccos L. includes three ploidy levels. The hexaploids represent the most abundant ploidy level in the area studied. They show a slightly bigger size of petals, longer bracts, prophylla, style, sepal tips and wider seeds in comparison with the tetraploids. Pentaploid cranberries are hitherto known only from the Czech Republic. They differ particularly in the low proportion of fully-developed tetrads. [ABSTRACT FROM AUTHOR]

AlN has been grown on 4H-SiC (1-100) substrates by rf-plasma molecular beam epitaxy. The epilayers assume a metastable 4H structure to match the in-plane stacking arrangement of the substrate. Initial two-dimensional nucleation of 4H-AlN is revealed by reflection high-energy electron diffraction. The epitaxial quality is evidenced by narrow x-ray diffraction ω-scan linewidths less than 70 arcsec for both symmetric and asymmetric reflections. The AlN growth surface exhibits a smooth and anisotropic morphology similar to that of GaN (1-100). Large residual stress is present in the epilayers, consistent with incomplete relaxation of misfit strain during growth. [ABSTRACT FROM AUTHOR]

4H-polytype AlN has been grown on a 4H-SiC substrate with the (1120) face via plasma-assisted molecular-beam epitaxy. The microscopic structure of the AlN/4H-SiC interface was examined using high-resolution transmission electron microscopy, and the polytype replication of the 4H structure from the 4H-SiC(1120) substrate was evidently confirmed. The x-ray rocking curve of (1120) diffraction for the single crystalline 4H-AlN epilayer exhibited a very small linewidth of 90 arc sec, suggesting noticeably small tilting around the [1120] direction. The excellent crystalline quality is probably owing to the polytype matching between the 4H-AlN epilayer and the 4H-SiC substrate, which resulted in remarkable reduction of defects at the interface. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Enhancement and control of carrier lifetimes in p-type 4H-SiC have been investigated. In this study, thermal oxidation and carbon ion implantation methods, both of which are effective for lifetime enhancement in n-type SiC, were attempted on 147-μm thick p-type 4H-SiC epilayers. Effects of surface passivation on carrier lifetimes were also investigated. The carrier lifetimes in p-type SiC could be enhanced from 0.9 μs (as-grown) to 2.6 μs by either thermal oxidation or carbon implantation and subsequent Ar annealing, although the improvement effect for the p-type epilayers was smaller than that for the n-type epilayers. After the lifetime enhancement, electron irradiation was performed to control the carrier lifetime. The distribution of carrier lifetimes in each irradiated region was rather uniform, along with successful lifetime control in the p-type epilayer in the range from 0.1 to 1.6 μs. [ABSTRACT FROM AUTHOR]

Impacts of reduction of deep levels and surface passivation on carrier lifetimes in p-type 4H-SiC epilayers are investigated. The authors reported that the carrier lifetime in n-type epilayers increased by reduction of deep levels through thermal oxidation and thermal annealing. However, the carrier lifetimes in p-type epilayers were not significantly enhanced. In this study, in order to investigate the influence of surface passivation on the carrier lifetimes, the epilayer surface was passivated by different oxidation techniques. While the improvement of the carrier lifetime in n-type epilayers was small, the carrier lifetime in p-type epilayers were remarkably improved by appropriate surface passivation. For instance, the carrier lifetime was improved from 1.4 μs to 2.6 μs by passivation with deposited SiO2 annealed in NO. From these results, it was revealed that surface recombination is a limiting factor of carrier lifetimes in p-type 4H-SiC epilayers. [ABSTRACT FROM AUTHOR]

Dependencies of temperature and injection level on carrier lifetimes in 50 μm thick p-type and n-type 4H-SiC epilayers have been investigated. The carrier lifetimes have been measured by differential microwave photoconductance decay measurements at various injection levels and temperatures. In both p-type and n-type epilayers, the carrier lifetimes gradually increased with increasing the injection level, which were naturally expected from the Shockley-Read-Hall (SRH) model, and after taking a maximum, the lifetimes dropped at the very high-injection level. In contrast, the carrier lifetimes exhibited continuous increase with elevating the temperature for both epilayers. In addition, the impact of thermal oxidation process on the carrier lifetimes has been also investigated. The thermal oxidation process, by which the Z1/2 and EH6/7 centers were remarkably reduced that had been observed in n-type 4H-SiC in our previous work, led to the improvement of the carrier lifetimes especially for n-type epilayers. The carrier lifetime reached 4.1 μs in p-type and 6.1 μs in n-type epilayers at 250 °C with an injection level of 1.8×1016 cm-3 through the thermal oxidation processing. [ABSTRACT FROM AUTHOR]

SiN passivation on AlGaN/GaN heterostructures was carried out using catalytic or plasma-enhanced chemical vapor deposition (Cat-CVD or PECVD), which has been found to increase two-dimensional electron gas (2DEG) density. The 2DEG density can be closely related to AlGaN surface properties via polarization effects. AlGaN potential barrier heights of AlGaN/GaN heterostructures with and without SiN passivation were systematically investigated using x-ray photoelectron spectroscopy (XPS) and capacitance-voltage (C-V) measurements. The results for the XPS and C-V measurements were consistent and demonstrated that a reduction in the AlGaN potential barrier height was actually induced by SiN passivation. Furthermore, Cat-CVD SiN passivation lowered the AlGaN potential barrier height more significantly than PECVD SiN passivation did, suggesting that the passivation method can influence the AlGaN potential barrier height. [ABSTRACT FROM AUTHOR]

A series of m-plane GaN layers with the Ga beam-equivalent pressure (BEP) as the only varied parameter was grown by rf-plasma assisted molecular beam epitaxy on m-plane 4H-SiC substrates using AlN buffer layers. The smoothest growth surfaces and most complete film coalescence were found for the highest Ga BEP corresponding to the Ga droplet accumulation regime. However, better structural quality as assessed by x-ray rocking curves was observed for growth at a lower Ga BEP value below the droplet limit. The variation of rocking curve widths for planes inclined with respect to the epilayer c axis followed a different trend with Ga BEP than those of reflections parallel to the c axis. The GaN layers were found to exhibit a large residual compressive strain along the a axis. [ABSTRACT FROM AUTHOR]

Phonon frequencies of a high-quality AlN layer coherently grown on a 6H-SiC (0001) substrate are investigated by Raman scattering. Owing to the largest strain in our coherent AlN layer among heteroepitaxially grown AlN layers ever reported, phonon frequencies of the E2 (low), E2 (high), and A1 (LO) modes are considerably shifted to 244.5 (−3.3, compared with bulk AlN), 672.1 (+16.3), and 899 (+11)cm-1, respectively. Full widths at half maximum of the phonon modes in the coherent AlN are almost equal to those of high-quality bulk AlN, clearly indicating its high crystalline quality and uniform strain. We discuss accuracy of phonon deformation potentials reported by several other groups thorough comparing our experimental results. [ABSTRACT FROM AUTHOR]

The temperature dependencies of hole density and hole mobility of p-type 4H-SiC obtained by Hall effect measurement were theoretically analyzed taking account of its anisotropic valence band structure. The experimental Hall factor, which was derived from the ratio of theoretical hole density to experimental Hall hole density, was reproduced by theoretical Hall factor computed using the valence band structure and relaxation times of scattering mechanisms. The product of the theoretical Hall factor and drift mobility computed by the same transport model agreed well with the experimental Hall mobility. Based on analyses of the results, it was revealed that the temperature dependence of Hall factor can be explained by considering the anisotropic valence band structure along with consideration of anisotropic relaxation times. The contribution of each scattering mechanism was also discussed, and empirical formulas for Hall and drift mobilities are presented. [ABSTRACT FROM AUTHOR]

Background: The amount of DNA comprising the genome of an organism (its genome size) varies a remarkable 40 000-fold across eukaryotes, yet most groups are characterized by much narrower ranges (e.g. 14-fold in gymnosperms, 3- to 4-fold in mammals). Angiosperms stand out as one of the most variable groups with genome sizes varying nearly 2000-fold. Nevertheless within angiosperms the majority of families are characterized by genomes which are small and vary little. Species with large genomes are mostly restricted to a few monocots families including Orchidaceae. [ABSTRACT FROM PUBLISHER]

Abstract: Stimulated by the “anomalous mode” observed in the time-of-flight measurement of phonon signals [J.K. Wigmore, A.G. Kozorezov, H. bin Rani, M. Giltrow, H. Kraus, B.M. Taele, Physica B 316–317 (2002) 589], we have theoretically studied the group velocities and the phonon images in CaWO4. Comparing the phonon caustics of transverse modes in the low-frequency nondispersive and slightly dispersive 1 THz regions, we propose that the anomalous phonon mode is originated from the phonons near the cuspidal edge of the group velocity curve of the fast transverse branch. Also the low-frequency phonon images (two-dimensional maps of phonon group velocities) calculated from the two sets of published elastic-constant data of CaWO4 [J.M. Farley, G.A. Saunders, J. Phys. C: Solid State Phys. 5 (1972) 3021] and [M. Gluyas, F.D. Hughes, B.M. James, J. Phys. D: Appl. Phys. 6 (1973) 2025] are compared. A significant difference is found in the images in the plane owing to the variance of the reported values of of about 40%. [Copyright &y& Elsevier]

Abstract: Thin (∼25nm) zincblende GaN (111) epilayers have been grown on hexagonal ZrB2 (0001) substrates by plasma-assisted molecular-beam epitaxy at temperatures near 600°C. Layers grown in near-stoichiometric conditions exhibit high phase purity; no wurtzite inclusions are detected by high-resolution X-ray diffraction or photoluminescence. For growth in the presence of excess Ga the layers are predominantly zincblende phase, but contain some wurtzite inclusions. Reducing the growth temperature to 400°C results in a much-increased fraction of wurtzite inclusions. The growth of zincblende GaN with high phase purity on hexagonal ZrB2 implies the existence of an underlying physical mechanism rather than simply a result of “growth mistakes.” A possible surface energy-related mechanism by which zincblende GaN (111) can be preferentially nucleated on hexagonal substrates is discussed. [Copyright &y& Elsevier]

Lamium subg. Galeobdolon was investigated in Central Europe to evaluate karyological, morphological, and isozyme variations. Flow-cytometric analyses revealed three ploidy levels: diploid (corresponding to L. flavidum and L. galeobdolon), tetraploid (corresponding to L. argentatum and L. montanum), and triploid (very rare cytotype most probably of hybrid origin). Mixed samples comprising two or more cytotypes / taxa were repeatedly detected. An existence of hybrid individuals between both tetraploid taxa was also supported. Almost no variation at isozyme level was observed in L. argentatum originating from three European countries thus favouring a hypothesis of a monotopic origin. On the contrary, the pilot study detected high isozyme variability in the other taxa. Morphometric analyses confirmed clear separation between L. galeobdolon and L. montanum; bract characters and maximum number of flowers were selected as quantitative features with the highest discriminant power. Serious discrepancies between the original description of L. endtmannii and both karyological and morphological performance of the plants from localities attributed to this species were found. These results allow us to conclude that any taxonomic status of L. endtmannii is not justified. [ABSTRACT FROM AUTHOR]

We calculated the phonon-limited hole mobility in rectangular cross-sectional [001], [110], [111], and [112]-oriented germanium nanowires, and the hole transport characteristics were investigated. A tight-binding approximation was used for holes, and phonons were described by a valence force field model. Then, scattering probability of holes by phonons was calculated taking account of hole-phonon interaction atomistically, and the linearized Boltzmann's transport equation was solved to calculate the hole mobility at low longitudinal field. The dependence of the hole mobility on nanowire geometry was analyzed in terms of the valence band structure of germanium nanowires, and it was found that the dependence was qualitatively reproduced by considering an average effective mass and the density of states of holes. The calculation revealed that [110] germanium nanowires with large height along the [001] direction show high hole mobility. Germanium nanowires with this geometry are also expected to exhibit high electron mobility in our previous work, and thus they are promising for complementary metal-oxide-semiconductor (CMOS) applications. [ABSTRACT FROM AUTHOR]

The conduction band structure and electron effective mass of GaAs nanowires with various cross-sectional shapes and orientations were calculated by two methods, a tight-binding method and an effective mass equation taking the bulk full-band structure into account. The effective mass of nanowires increases as the cross-sectional size decreases, and this increase in effective mass depends on the orientations and substrate faces of nanowires. Among [001], [110], and [111]-oriented rectangular cross-sectional GaAs nanowires, [110]-oriented nanowires with wider width along the [001] direction showed the lightest effective mass. This dependence originates from the anisotropy of the C valley of bulk GaAs. The relationship between effective mass and bulk band structure is discussed. [ABSTRACT FROM AUTHOR]

High-efficiency visible light emission in N-and-B-doped 6H-SiC epilayers was observed in photoluminescence measurements at room temperature. The orange-yellow light emission due to the recombination of donor-acceptor pairs (DAPs) has a broad spectrum with a peak wavelength of 576 nm and a full width at half maximum of 110 nm at 250 K. The high B concentration of more than 1018 cm-3 improves the emission efficiency of the DAP recombination at a high temperature. Compared with the photoluminescence spectrum of GaN at 10 K, a high quantum efficiency of 95% was estimated for the highly B-doped sample. From time-resolved photoluminescence measurements, a DAP recombination time of 5.0 ms was obtained, which is in good agreement with the calculated value by the rate equation with the assumption of a 95% internal quantum efficiency. This is quite promising as a light-emitting medium by optical pumping, as well as monolithic light sources combined with nitride-based light-emitting diodes grown on the DA-doped SiC epilayer. [ABSTRACT FROM AUTHOR]

Abstract Ancient hybridization leading to plastid capture is a likely cause of cytonuclear discordance, although discordance may also be attributed to other mechanisms. Cytonuclear discordance is ubiquitous in a clade of southern African Oxalis known as the Hirta clade, despite a notable lack of documented field hybrids for this genus. The aims of this study were to determine whether hybridization is possible between different Hirta clade species, what the fitness effects on seed set of possible hybridizations are, and to determine the mode of inheritance of plastid DNA in Oxalis. We tested these aims using manual interspecific and conspecific crosses, as well as self-pollination control crosses of species within this clade, conducted in a research collection context. Roughly 10% of interspecific crosses produced seed, most of which were confirmed as unambiguous hybrid plants using DNA sequencing of the Internal Transcribed Spacer (ITS). Low levels of self-compatibility were recorded in several species, while one accession showing very high levels of self-compatibility (accession MO1218). Interspecific crosses produced significantly less seed per capsule compared to conspecific crosses with respective parents, suggesting decreased fitness in the establishment of the F1 hybrids. Selfing seed set was significantly lower than interspecific and conspecific seed set upon removal of MO1218. Using plastid fragment length polymorphisms, Oxalis was found to have predominantly or exclusively maternally inherited plastid DNA. This study establishes the viability of the earliest stages of plastid capture in Oxalis , and provides context relevant to the lack of documented field hybrids in the southern African members of this genus. Future studies should focus on F1 survival and fertility and the maternal fitness effects of subsequent backcrosses to the maternal parent, as well as on hybridization rates under field conditions, to assess whether hybridization occurs at rates sufficient to affect Oxalis evolution in southern Africa. Highlights • Produced the first, unequivocal confirmed Oxalis F1 hybrids • Confirm some of the hybrids to be heteroploid • First unequivocal evidence of self-compatibility in SA Oxalis • Show that plastid DNA is maternally inherited in Oxalis [ABSTRACT FROM AUTHOR]

AlN/GaN short-period superlattices (SPSLs) were grown on 4H-SiC(11̅20) substrates with a 4H-AlN template layer. The thickness of the GaN layers was controlled to be less than 1 nm to suppress the generation of extended defects. The stacking fault and threading dislocation density were found to be 3 x 105 cm-1 and 6 x 107 cm-2, respectively, which were as low as those for a 4H-AlN single layer. The superlattice replicated the polytype of the underlying 4H-AlN layer (4H-SiC substrate), meaning that a 4H-AlN/4H-GaN SPSL was grown. Room-temperature cathodoluminescence measurements revealed that the 4H-AlN/4H-GaN SPSLs exhibited intense luminescence in the deep ultraviolet region of 4.4-5.0 eV, depending on the GaN layer thickness. The emission wavelengths of the SPSLs fairly agreed with the estimation based on the Kronig-Penney model. [ABSTRACT FROM AUTHOR]

A carbon vacancy defect is one of the most abundant point defects in SiC (as-grown, irradiated, annealed) and of technological importance because the acceptor-like level of a carbon monovacancy (Z 1/2 center: E C – 0.63 eV) works as the primary carrier-lifetime killer in 4H–SiC. The carbon vacancy defects can be preferentially generated by either low-energy electron irradiation or high-temperature treatment in an inert gas ambient. On the other hand, the carbon vacancy defects can be almost eliminated by either a carbon-ion implantation process or thermal oxidation. By combination of these techniques, the density of carbon vacancy defects can be controlled in the wide range from 10 11 cm −3 to 10 15 cm −3 or even higher. [ABSTRACT FROM AUTHOR]

Morphological characters, AFLP markers and flow cytometry were used to investigate the morphological and genetic variability and differentiation of Viola reichenbachiana and V. riviniana in non-metallicolous ( NM) and metallicolous (M) populations. The aims were to clarify the taxonomic status of plants occurring in ore-bearing areas, to determine any relationship in V. reichenbachiana and V. riviniana from sites not polluted with heavy metals, and to examine the genetic variability and differentiation of M and NM populations of both species. Multivariate analyses based on morphological characters showed significant differences between V. reichenbachiana and V. riviniana from non-polluted sites, high levels of intra- and inter-population variability, and the occurrence of inter-specific hybrids. Plants from M populations showed hybrid characters but also fell within the range of V. riviniana or V. reichenbachiana. There were no significant differences in relative genome size between plants from polluted areas and V. riviniana from NM populations. Bayesian analysis of population genetic structure based on AFLP markers distinguished two main groups: V. reichenbachiana and V. riviniana together with the M populations. That analysis also revealed the occurrence of populations of inter-specific hybrids from non-polluted areas. Further Bayesian analysis of V. riviniana including NM and M populations separated all the studied M populations from NM populations. We conclude that plants forming the M populations are well adapted to a metal-polluted environment, and could be considered as stabilised introgressive forms resulting from unidirectional (asymmetric) introgression toward V. riviniana. [ABSTRACT FROM AUTHOR]

The carbon vacancy (Vc) has been suggested by different studies to be involved in the Z1/Z2 defect-a carrier lifetime killer in SiC. However, the correlation between the Z1/Z2 deep level with Vc is not possible since only the negative carbon vacancy (V-c) at the hexagonal site, V-c(h), with unclear negative-U behaviors was identified by electron paramagnetic resonance (EPR). Using freestanding n-type 4H-SiC epilayers irradiated with low energy (250 keV) electrons at room temperature to introduce mainly Vc and defects in the C sublattice, we observed the strong EPR signals of V-c(h) and another S = 1/2 center. Electron paramagnetic resonance experiments show a negative-U behavior of the two centers and their similar symmetry lowering from C3v to C1h at low temperatures. Comparing the 29Si and 13C ligand hyperfine constants observed by EPR and first principles calculations, the new center is identified as V-c(k). The negative-U behavior is further confirmed by large scale density functional theory supercell calculations using different charge correction schemes. The results support the identification of the lifetime limiting Z1/Z2 defect to be related to acceptor states of the carbon vacancy. [ABSTRACT FROM AUTHOR]

Abstract: Contact zones of different cytotypes provide an opportunity to address evolutionary mechanisms underlying the origin, establishment and maintenance of karyological diversity at intraspecific level. We explored the fine-scale distribution of ploidy levels of Oxalis obtusa in seven mixed-ploidy sites in the Western and Northern Cape Provinces, South Africa, and searched for potential isolating barriers promoting assortative mating. Five different ploidy levels (2x, 3x, 4x, 6x and 8x) were detected among 336 samples collected at 112 microsites. The studied sites were inhabited by two (2 sites), three (4 sites) or even all five (1 site) different cytotypes. Despite their sympatric growth, different ploidies show some spatio-ecological segregation. The greatest differences among microsites hosting different cytotypes were found in precipitation parameters. There is a clear altitudinal gradient in ploidy composition in the most ploidy-variable site, Pakhuis Pass. Our results show that a combination of niche partitioning and clumping of same-ploidy individuals due to vegetative reproduction seems to be efficient reproductive barriers, which limit inter-ploidy gene flow in the zones of ploidy contact and contribute to the long-term maintenance of cytotype mixtures in O. obtusa. [Copyright &y& Elsevier]

Using electron paramagnetic resonance (EPR), energy levels of the carbon vacancy ( Vc) in 4H-SiC and its negative- U properties have been determined. Combining EPR and deep-level transient spectroscopy we show that the two most common defects in as-grown 4H-SiC--the Z1/2 lifetime-limiting defect and the EH7 deep defect--are related to the double acceptor (2-- |0) and single donor (0|+) levels of Vc, respectively. [ABSTRACT FROM AUTHOR]

To fully understand the contemporary genetic structure of plants, both nuclear and plastid markers are needed. Three chloroplast DNA (cpDNA) lineages, which probably diverged before the major Pleistocene glaciations, have been identified in the circumpolar/circumboreal Vaccinium uliginosum. Here we investigate its nuclear DNA variation using nuclear ribosomal internal transcribed spacer (ITS) sequences, DNA ploidy level measurements and amplified fragment length polymorphisms (AFLPs). We also extend the cpDNA dataset. Two ITS lineages, corresponding to diploids and tetraploids, respectively, were identified. However, both main sequence types apparently occurred in most individual plants but showed ploidy-biased homogenization and possibly reflect paralogy predating the origin of V. uliginosum. The ploidy levels were largely consistent with the cpDNA lineages, suggesting that the initial cpDNA divergence followed early polyploidizations. Five main AFLP groups were identified, consistent with recent glacial refugia in Beringia, western Siberia, the southern European mountains and areas south/east of the Scandinavian and Laurentide ice sheets. Except from the southern European mountains, there has been extensive expansion from all refugia, resulting in several contact zones. Surprisingly, the presumably older ploidy and cpDNA patterns were partly inconsistent with the main AFLP groups and more consistent with AFLP subgroups. A likely major driver causing the inconsistencies is recent nuclear gene flow via unreduced pollen from diploids to tetraploids. This may prevent cytoplasmic introgression and result in overlayed patterns formed by processes dominating at different time scales. The data also suggest more recent polyploidizations, as well as several chloroplast capture events, further complicating this scenario. This study highlights the importance of combining different marker systems to unravel intraspecific histories. [ABSTRACT FROM AUTHOR]

Preferential growth of high-quality cubic GaN on sapphire (0001) substrates was realized at 800 °C under a Ga-rich condition by metal organic molecular beam epitaxy. Hexagonal GaN was grown under a N-rich condition. On the contrary, under the Ga-rich condition, the growing layer changed from a hexagonal phase to a cubic phase as the growth proceeds, which was verified by in situ reflection high-energy electron diffraction. The low-temperature photoluminescence (PL) of this layer was dominated by a sharp and intense excitonic emission of cubic GaN, indicating high crystal quality. The results of PL and x-ray diffraction indicate that the polytype of GaN can be intentionally controlled by changing V/III ratios without serious degradation of crystal quality. In addition, enhancement of cubic phase growth by using a low-temperature grown buffer layer is also discussed. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Impact of step height of silicon carbide (SiC) substrates on heteroepitaxial growth of aluminum nitride (AlN) was investigated. Step-and-terrace structures with various step heights, 6 monolayer (ML), 3ML and 1ML, were formed on 6H–SiC (0001) vicinal substrates by high-temperature gas etching. 2H–AlN layers were grown on the substrate by plasma-assisted molecular-beam epitaxy (MBE) and then these layers were characterized by atomic-force microscopy (AFM) and X-ray diffraction (XRD). High-quality AlN can be grown on SiC substrates with 6ML- and 3ML-height step, while AlN grown on SiC substrates with 1ML-height step exhibited inferior crystalline quality. A model for high-quality AlN growth on SiC substrates with 3ML-height step is proposed. [Copyright &y& Elsevier]