Probing the effect of point defects on the leakage blocking capability of Al0.1Ga0.9N/Si structures using a monoenergetic positron beam

Vacancy-type defects in Al0.1Ga0.9N were probed using a monoenergetic positron beam. Al0.1Ga0.9N layers with different carbon doping concentrations ([C] = 5 x 10(17) -8 x 10(19) cm(-3)) were grown on Si substrates by metalorganic vapor phase epitaxy. The major defect species in Al0.1Ga0.9N was determined to be a cation vacancy (or cation vacancies) coupled with nitrogen vacancies and/or with carbon atoms at nitrogen sites (C(N)s). The charge state of the vacancies was positive because of the electron transfer from the defects to C-N-related acceptors. The defect charge state was changed from positive to neutral when the sample was illuminated with photon energy above 1.8 eV, and this energy range agreed with the yellow and blue luminescence. For the sample with high [C], the charge transition of the vacancies under illumination was found to be suppressed, which was attributed to the trapping of emitted electrons by C-N-related acceptors. With increasing [C], the breakdown voltage under the reverse bias condition increased. This was explained by the trapping of the injected electrons by the positively charged vacancies and C-N-related acceptors

The evidence of quasi-free positronium state in GiPS-AMOC spectra of glycerol

We present the results of processing of Age-Momentum Correlation (AMOC) spectra that were measured for glycerol by the Gamma-induced positron spectroscopy (GiPS) facility. Our research has shown that the shape of experimental s(t) curve cannot be explained without introduction of the intermediate state of positronium (Ps), called quasi-free Ps. This state yields the wide Doppler line near zero lifetimes. We discuss the possible properties of this intermediate Ps state from the viewpoint of developed model. The amount of annihilation events produced by quasi-free Ps is estimated to be less than 5% of total annihilations. In the proposed model, quasi-free Ps serves as a precursor for trapped Ps of para- and ortho-states

Quantum Monte Carlo study of a positron in an electron gas

Quantum Monte Carlo calculations of the relaxation energy, pair-correlation function, and annihilating-pair momentum density are presented for a positron immersed in a homogeneous electron gas. We find smaller relaxation energies and contact pair-correlation functions in the important low-density regime than predicted by earlier studies. Our annihilating-pair momentum densities have almost zero weight above the Fermi momentum due to the cancellation of electron-electron and electron-positron correlation effects

Energetics of positron states trapped at vacancies in solids

We report a computational first-principles study of positron trapping at vacancy defects in metals and semiconductors. The main emphasis is on the energetics of the trapping process including the interplay between the positron state and the defect's ionic structure and on the ensuing annihilation characteristics of the trapped state. For vacancies in covalent semiconductors the ion relaxation is a crucial part of the positron trapping process enabling the localization of the positron state. However, positron trapping does not strongly affect the characteristic features of the electronic structure, e.g., the ionization levels change only moderately. Also in the case of metal vacancies the positron-induced ion relaxation has a noticeable effect on the calculated positron lifetime and momentum distribution of annihilating electron-positron pairs.Comment: Submitted to Physical Review B on 17 April 2007. Revised version submitted on 6 July 200

Stable fourfold configurations for small vacancy clusters in silicon from ab initio calculations

Using density-functional-theory calculations, we have identified new stable configurations for tri-, tetra-, and penta-vacancies in silicon. These new configurations consist of combinations of a ring-hexavacancy with three, two, or one interstitial atoms, respectively, such that all atoms remain fourfold. As a result, their formation energies are lower by 0.6, 1.0, and 0.6 eV, respectively, than the ``part of a hexagonal ring'' configurations, believed up to now to be the lowest-energy states

Modeling the momentum distributions of annihilating electron-positron pairs in solids

Measuring the Doppler broadening of the positron annihilation radiation or the angular correlation between the two annihilation gamma quanta reflects the momentum distribution of electrons seen by positrons in the material.Vacancy-type defects in solids localize positrons and the measured spectra are sensitive to the detailed chemical and geometric environments of the defects. However, the measured information is indirect and when using it in defect identification comparisons with theoretically predicted spectra is indispensable. In this article we present a computational scheme for calculating momentum distributions of electron-positron pairs annihilating in solids. Valence electron states and their interaction with ion cores are described using the all-electron projector augmented-wave method, and atomic orbitals are used to describe the core states. We apply our numerical scheme to selected systems and compare three different enhancement (electron-positron correlation) schemes previously used in the calculation of momentum distributions of annihilating electron-positron pairs within the density-functional theory. We show that the use of a state-dependent enhancement scheme leads to better results than a position-dependent enhancement factor in the case of ratios of Doppler spectra between different systems. Further, we demonstrate the applicability of our scheme for studying vacancy-type defects in metals and semiconductors. Especially we study the effect of forces due to a positron localized at a vacancy-type defect on the ionic relaxations.Comment: Submitted to Physical Review B on September 1 2005. Revised manuscript submitted on November 14 200

Vacancy complexes in nonequilibrium germanium-tin semiconductors

Understanding the nature and behavior of vacancy-like defects in epitaxial GeSn metastable alloys is crucial to elucidate the structural and optoelectronic properties of these emerging semiconductors. The formation of vacancies and their complexes is expected to be promoted by the relatively low substrate temperature required for the epitaxial growth of GeSn layers with Sn contents significantly above the equilibrium solubility of 1 at.%. These defects can impact both the microstructure and charge carrier lifetime. Herein, to identify the vacancy-related complexes and probe their evolution as a function of Sn content, depth-profiled pulsed low-energy positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy were combined to investigate GeSn epitaxial layers with Sn content in the 6.5-13.0 at.% range. The samples were grown by chemical vapor deposition method at temperatures between 300 and 330 {\deg}C. Regardless of the Sn content, all GeSn samples showed the same depth-dependent increase in the positron annihilation line broadening parameters, which confirmed the presence of open volume defects. The measured average positron lifetimes were the highest (380-395 ps) in the region near the surface and monotonically decrease across the analyzed thickness, but remain above 350 ps. All GeSn layers exhibit lifetimes that are 85 to 110 ps higher than the Ge reference layers. Surprisingly, these lifetimes were found to decrease as Sn content increases in GeSn layers. These measurements indicate that divacancies are the dominant defect in the as-grown GeSn layers. However, their corresponding lifetime was found to be shorter than in epitaxial Ge thus suggesting that the presence of Sn may alter the structure of divacancies. Additionally, GeSn layers were found to also contain a small fraction of vacancy clusters, which become less important as Sn content increases

Thermal Analysis of EPOS components

We present a simulation study of the thermal behaviour of essential parts of the electron-positron converter of the positron source EPOS at the Research Center Dresden-Rossendorf. The positron moderator foil and the upper tube element of the electrostatic extraction einzellens are directly exposed to the primary electron beam (40 MeV, 40 kW). Thus, it was necessary to prove by sophisticated simulations that the construction can stand the evolving temperatures. It was found that thin moderator foils (< 20...40 µm) will not show a too strong heating. Moreover, the temperature can be varied in a wide range by choosing an appropriate thickness. Thus, the radiation-induced lattice defects can at least partly be annealed during operation. The wall of the extraction lens which is made from a stainless steel tube must be distinctly thinned to avoid damage temperatures. The simulations were performed time dependent. We found that the critical parts reach their final temperature after less than a minute

Microscopic identification of native donor Ga-vacancy complexes in Te-doped GaAs

Native vacancies in Te-doped (5×1016–5×1018cm−3) GaAs were investigated by means of positron lifetime and Doppler-broadening coincidence spectroscopy. The experimental data were related to theoretical calculations of the positron lifetime and the annihilation momentum distribution. Monovacancies were observed in all Te-doped GaAs samples under study. It will be shown that they can directly be identified to be Ga-vacancy–TeAs-donor complexes. These complexes are the dominating type of vacancy defects in the doping range under observation.Peer reviewe

Влияние дефектов и примесных атомов на физико-механические свойства наноструктурных покрытий в области границ их раздела

С помощью уникальных методов: аннигиляции позитронов, микропучка протонов, Резерфордовского обратного рассеяния (RBS) ионов, микроанализа с помощью EDSX совместно с SEM, дифракции рентгеновских лучей (XRD), рентгеновской тензометрии (α-sin²ψ), тестов на нанотвердость, модуль упругости, индекса пластичности и скреч-тестов были исследованы наноструктурные покрытия Ti-Si-N; Ti-Hf-Si-N; (Ti-Hf-Zr-V-Nb)N, выращенные катодным вакуумно-дуговым источником. Были получены профили дефектов по глубине покрытий и изучено влияние термического отжига до 600 °С (30 мин) на изменение дефектной структуры, упруго-направленного состояния, фазового состава и перераспределения примесных атомов в результате термодиффузии и процессов сегрегации по границам и субграницам нанозерен. Обнаружено увеличение твердости после отжига, уменьшение величины упруго-деформируемого состояния, перераспределение примесных атомов и дефектов.За допомогою унікальних методів: анігіляції позитронів, мікропучка протонів, Резерфордівського оберненого розсіювання (RBS) іонів, мікроаналіза за допомогою EDSX разом із SEM, дифракції рентгенівських променів (XRD), рентгенівської тензометрії (α-sin²ψ), тестів на нанотвердість, модуль пружності, індекса пластичності та скретч-тестів були досліджені наноструктурні покриття Ti-Si-N; Ti-Hf-Si-N; (Ti-Hf-Zr-V-Nb)N, вирощені катодним вакуумно-дуговим джерелом. Були отримані профілі дефектів по товщі покриттів, та досліджений вплив термічного відпалювання до 600 °С (30 хв) на зміну дефектної структури, пружньо-фазового стану, фазового складу та перерозподілу домішкових атомів в результаті термодифузії та процесів сегрегації по межам та субмежам нанозерен. Знайдено збільшення твердості після відпалювання, зменшення величини пружньо-деформаційного стану, перерозподіл атомів та дефектів.Using unique methods: positron annihilation, proton microbeam, Rutherford backscattering (RBS) of ions, microprobe using EDSX with SEM, X-ray diffraction (XRD), X-ray strain measurement (α-sin²ψ), tests of nanohardness, elastic modulus, index plasticity and scratch tests were investigated nanostructured coatings Ti-Si-N; Ti-Hf-Si-N; (Ti-Hf-Zr-V-Nb)N, grown by cathodic vacuum arc source. Defects’ profiles were obtained at all depth of coating, and effect of thermal annealing at 600 °C (30 min) on the change of the defect structure, elastic-directed state, the phase composition and the redistribution of impurity atoms as a result of thermal diffusion and segregation processes boundaries and sub-boundaries of nanograins were studied. An increase of hardness after annealing, decreasing of the elastic-strain state, the redistribution of impurity atoms and defects were observed