Electron accumulation and doping in InN and InGaN alloys

InN and group III nitride materials have attracted great interest due to their potential applications for optoelectronic devices, as the range of band gaps cover the ultra-violet to the near infrared. InN and all In-rich InxGa1−xN alloys exhibit a surface electron accumulation layer. This is due to the unusually low conduction band minimum (CBM) at the Brouillon zone centre (Γ-point) with respect to the charge- neutrality level. Electron accumulation has been observed at the surface of almost all n-type and p-type InN, making proof of p-type doping of this material very difficult. Routine characterization of p-type conductivity of Mg-doped samples using single-field Hall effect is prevented by the presence of a surface inversion space-charge layer, and hence the surface electron-rich region dominates the measurements. In this thesis, the results of investigations on non-polar InN surfaces, Mg-doped InN surfaces and a range of InxGa1−xN alloys across the composition entire range are presented. Considerable improvement of the quality of a- and m-plane InN thin films has been achieved using free standing GaN substrates in conjunction with a GaN buffer layer and grown by PAMBE. Using a combination of infrared reflectivity (FTIR), x-ray photoemission spectroscopy (XPS) and electrochemical capacitance voltage (ECV) measurements, the surface space charge properties of these samples have been investigated. The surface Fermi level has been determined to be lower than previously observed on non- cleaved InN samples. Additionally a high carrier concentration has been found on the non-polar InN, close to the interface with the GaN buffer layer, associated with unintentionally incorporated oxygen impurities. The increased concentration of oxygen impurities near the InN/GaN interface, confirmed by secondary ion mass spectrometry (SIMS), is due to the relatively low growth temperature (380 - 450 ◦C) used to produce the non-polar InN films. XPS has been also used in the investigations of Mg-doped InN. A significant lowering of the surface Fermi level has been observed with increasing Mg-doping for the highest Mg concentration (> 1 × 1019 cm−3) indicating a highly desirable reduction in the degree of surface electron accumulation. While for moderate Mg concentrations the surface Fermi level is at the previously determined ‘universal’ value of ~ 1.4 eV above the valence band maximum, for [Mg]=1.2×1020 cm−3, a value of 0.83 eV is found. As a consequence, for [Mg]> 1 × 1019 cm−3 the donor surface state density increases while the surface electron density decreases enormously, resulting in a transition from electron accumulation to almost just hole depletion layer. This reduction of electron accumulation in high Mg-doped InN can be improved by additional surface treatment, therefore results of a series of sulfur treated Mg-doped InN sample are also reported in this thesis. Finally, the electronic properties of InxGa1−xN alloys with a composition range of 0.20 >= x >= 1.00 have been investigated, using XPS and FTIR. The transition from electron accumulation to electron depletion has been observed at a composition of x = 0.20, while for x >= 0.20 an increasing electron accumulation with decreasing Ga fraction has been observed

Slow equivariant lump dynamics on the two sphere

The low-energy, rotationally equivariant dynamics of n CP^1 lumps on S^2 is studied within the approximation of geodesic motion in the moduli space of static solutions. The volume and curvature properties of this moduli space are computed. By lifting the geodesic flow to the completion of an n-fold cover of the moduli space, a good understanding of nearly singular lump dynamics within this approximation is obtained.Comment: 12 pages, 3 figure

Activation of sperm motility in the euryhaline tilapia Sarotherodon melanotheron heudelotii (Dumeril, 1859) acclimatized to fresh, sea and hypersaline waters

The effects of osmolality and ions were examined on motility of sperm from males of Sarotherodon melanotheron heudelotii acclimatized in tanks at salinities set at 0, 35 and 70 g L-1. The range of osmolality that enabled sperm activation, shifted and broadened as the maintenance salinity of broodfish increased. The requirement of extracellular Ca2+ for activation of sperm motility increased when the maintenance salinity of broodfish was higher

Formation of singularities for equivariant 2+1 dimensional wave maps into the two-sphere

In this paper we report on numerical studies of the Cauchy problem for equivariant wave maps from 2+1 dimensional Minkowski spacetime into the two-sphere. Our results provide strong evidence for the conjecture that large energy initial data develop singularities in finite time and that singularity formation has the universal form of adiabatic shrinking of the degree-one harmonic map from $\mathbb{R}^2$ into $S^2$.Comment: 14 pages, 5 figures, final version to be published in Nonlinearit

Timing performance of a double layer diamond detector

In order to improve the time precision of detectors based on diamonds sensors we have built a detector with two scCVD layers connected in parallel to the same amplifier. This work describes the design and the first measurements of such a prototype performed on a particle beam at CERN. With this different configuration we have obtained an improvement larger than a factor of 1.6-1.7 for the timing precision of the measurement when compared to a one layer scCVD diamond detector.Peer reviewe

Relationship between spermatozoa motility parameters, sperm/egg ratio, and fertilization and hatching rates in pufferfish (Takifugu niphobles)

[EN] The use of high quality gametes from both males and females during in vitro fertilization (IVF) trials is an essential step in order to achieve high fertilization and hatching rates. Although aquaculture hatcheries have focused more on egg rather than spermatozoa quality, some studies have demonstrated that sperm quantity and quality have a great influence both on fertilization/hatching success and the subsequent development of the embryo and larvae. In this study we have demonstrated that sperm/egg ratio and sperm quality are factors strongly related to each other in the pufferfish (Takifugu niphobles). Our results suggest that both factors should be taken into account as unique interrelated elements, making possible to obtain high fertilization rates using a successful combination of small amount of high quality sperm or high amount of low quality sperm. In addition, coefficients of correlation and determination among all the sperm motion parameters provided by a CASA system and fertilization/hatching rates were estimated for the first time in a marine species. Positive significant correlations were found in some parameters such as total and progressive motility (0.68 and 0.7 respectively). However, curvilinear velocity (VCL), straight line velocity (VSL) and average velocity (VAP) showed the highest coefficients of correlation (0.82, 0.8, and 0.81, respectively). In this respect, spermatozoa velocity appears to be a key factor in the fertilization process, especially when the number of spermatozoa per egg is limited in the aqueous environment. (C) 2013 Elsevier B.V. All rights reserved.Funded by the Spanish Ministry of Economy and Competitiveness (MINECO; AGL2010-16009). Victor Gallego has a predoctoral grant (MINECO; BES-2009-020310) and has been granted a fellowship (EEBB-I-12-05858) of the MINECO's Spanish Personnel Research Training Programme to carry out this research in the Misaki Marine Biological Station (Miura, Japan). We would like to thank to Dr. Kurokawa for the help and knowledge supplied during this study.Gallego Albiach, V.; Pérez Igualada, LM.; Asturiano Nemesio, JF.; Yoshida, M. (2013). Relationship between spermatozoa motility parameters, sperm/egg ratio, and fertilization and hatching rates in pufferfish (Takifugu niphobles). Aquaculture. 416:238-243. https://doi.org/10.1016/j.aquaculture.2013.08.035S23824341

The geodesic approximation for lump dynamics and coercivity of the Hessian for harmonic maps

The most fruitful approach to studying low energy soliton dynamics in field theories of Bogomol'nyi type is the geodesic approximation of Manton. In the case of vortices and monopoles, Stuart has obtained rigorous estimates of the errors in this approximation, and hence proved that it is valid in the low speed regime. His method employs energy estimates which rely on a key coercivity property of the Hessian of the energy functional of the theory under consideration. In this paper we prove an analogous coercivity property for the Hessian of the energy functional of a general sigma model with compact K\"ahler domain and target. We go on to prove a continuity property for our result, and show that, for the CP^1 model on S^2, the Hessian fails to be globally coercive in the degree 1 sector. We present numerical evidence which suggests that the Hessian is globally coercive in a certain equivariance class of the degree n sector for n>1. We also prove that, within the geodesic approximation, a single CP^1 lump moving on S^2 does not generically travel on a great circle.Comment: 29 pages, 1 figure; typos corrected, references added, expanded discussion of the main function spac