Effects of nano-void density, size, and spatial population on thermal conductivity: a case study of GaN crystal

The thermal conductivity of a crystal is sensitive to the presence of surfaces and nanoscale defects. While this opens tremendous opportunities to tailor thermal conductivity, a true "phonon engineering" of nanocrystals for a specific electronic or thermoelectric application can only be achieved when the dependence of thermal conductivity on the defect density, size, and spatial population is understood and quantified. Unfortunately, experimental studies of effects of nanoscale defects are quite challenging. While molecular dynamics simulations are effective in calculating thermal conductivity, the defect density range that can be explored with feasible computing resources is unrealistically high. As a result, previous work has not generated a fully detailed understanding of the dependence of thermal conductivity on nanoscale defects. Using GaN as an example, we have combined physically-motivated analytical model and highly-converged large scale molecular dynamics simulations to study effects of defects on thermal conductivity. An analytical expression for thermal conductivity as a function of void density, size, and population has been derived and corroborated with the model, simulations, and experiments

Wavelet-Based Confirmatory Factor Analysis: Monitoring of Damage Accumulation Factors

Abstract Under consideration is further development of the wavelet-based confirmatory factor analysis intended for monitoring of factors responsible for evolution of technical and other systems. According to the proposed approach, the samples of coefficients resulted from discrete wavelet transform of initial parameter time series under study and responsible for different observation periods are considered as values of observed variables in the subsequent confirmatory factor analysis to reveal time history of factor influences and estimate factor interaction. Identification of free factor model parameters is carried out by a novel direct (noniterative) procedure, which is an alternative to traditional local iterative optimization procedures based on the maximum likelihood criteria. A technique for estimating significance of factor model components is discussed. Application of the approach to the analysis of aircraft damage accumulation is given. Analysis of the variance components factor model representing influence of maneuvering load factors occurrences and climatic conditions of basing on aircraft damage accumulation rate revealed differences between national flying training schools to yield statistically significant effect on the process of aircraft damage accumulation under repeated loads

Surface acoustic wave velocity and elastic constants of cubic GaN

We present high-resolution surface Brillouin scattering measurements on cubic GaN layers grown on GaAs substrate. By using a suitable scattering geometry, scattering by surface acoustic waves is recorded for different azimuthal angles, and the surface acoustic wave velocities are determined. A comparison of experimental results with numerical simulations of the azimuthal dependence of the surface wave velocity shows good agreement and allows a consistent set of elastic constants for c-GaN to be determined

Combination of osteogenesis imperfecta and type 1 diabetes mellitus

Osteogenesis imperfecta (OI) is a hereditary connective tissue disorder accompanied by increased bone fragility. Five types of OI are distinguished on the basis of phenotypic manifestations. OI type 1 is characterized by a reduced amount of normal type 1 collagen and is the mildest form. In addition to the fractures, course of disease can be accompanied by short stature, skeletal deformity and joint hypermobility. Although fracture risk decreases with age, such patients needs regular follow-up with an assessment of bone mineral density (BMD) and, if necessary, correction of therapy to improve the quality of life. Type 1 diabetes mellitus (T1DM) is associated with a decreased BMD, which is mostly attributed to insulin deficiency and hyperglycemia, which also increase the risk of fractures. Achieving and stable maintenance of glycemic targets is often challenging, but it is necessary to exclude hyperglycemia as a factor that further worsens the quality of bone. This paper describes a clinical case of an extremely rare combination of type 1 OI and T1DM, two diseases with a pronounced negative effect on bone tissue. The combination of these pathologies requires special management tactics for such patients to reduce the risk of developing new fractures

Алгоритмические программы подбора оптимальных условий роста при выращивании эпитаксиальных слоев на основе GaAs методом МОС-гидридной эпитаксии

The algorithm was worked out, witch allows to define obtained GaAs-based layers characteristics and the most significant epitaxy process technological parameters on the base of half-empiric dependences, if MOCVD process carriage conditions are known.Разработан алгоритм, позволяющий при известных условиях проведения процесса МОГФЭ (металлоорганическая газофазная эпитаксия) определить характеристики получаемых слоев на основе GaAs и важнейшие технологические параметры процесса эпитаксии с использованием полуэмпирических зависимостей

Расчёт устойчивости многослойных квантоворазмерных гетероструктур InGaAs/GaAs к вероятности образования дислокаций несоответствия

The stable, metastable and unstable regions of InGaAs/GaAs heterostructures to the formation of misfit dislocations were determined. The character of the strain distribution in the investigating samples has been estimated and the excess stress change in the quantum wells and barrier layers has been designedОпределены области стабильности, метастабильности и нестабиль- ности многослойных квантово- размерных гетероструктур (ГС) InGaAs/GaAs к образованию дислокаций несоответствия (ДН). Установлен характер распределения напряжений в исследуемых образцах. Построены графики, позволяющие наглядно проследить изменение избыточных напряжений в квантовых ямах и барьерах

Intrinsic Correlation between Hardness and Elasticity in Polycrystalline Materials and Bulk Metallic Glasses

Though extensively studied, hardness, defined as the resistance of a material to deformation, still remains a challenging issue for a formal theoretical description due to its inherent mechanical complexity. The widely applied Teter's empirical correlation between hardness and shear modulus has been considered to be not always valid for a large variety of materials. Here, inspired by the classical work on Pugh's modulus ratio, we develop a theoretical model which establishes a robust correlation between hardness and elasticity for a wide class of materials, including bulk metallic glasses, with results in very good agreement with experiment. The simplified form of our model also provides an unambiguous theoretical evidence for Teter's empirical correlation.Comment: 10 pages, 4 figures and 3 table

МЕХАНИЗМ ТРАВЛЕНИЯ GаAs В ПРОЦЕССЕ ЛЕГИРОВАНИЯ ТЕТРАХЛОРИДОМ УГЛЕРОДА В УСЛОВИЯХ МОС-ГИДРИДНОЙ ЭПИТАКСИИ

Recently carbon-doped epitaxial layers become more attractive in most practical applications. In this paper we have considered the mechanisms of etching of CCl<sub>4</sub>-doped GaAs grown by MOCVD in the temperature range of 600–800°C. It has been shown that, as temperature increases, the etching of solid GaAs with the formation of volatile GaCl3 becomes the leading contribution to the growth rate reduction.И зучен процесс травления эпитаксиальных слоев GaAs в процессе легирования тетрахлоридом углерода в технологически привлекательном диапазоне режима роста (t = 600-800°С) в условиях МОС-гидридной эпитаксии. Показано, что с повышением температуры ведущим механизмом становится травление GaAs с образованием летучего GaCl3

ВЛИЯНИЕ УПРУГИХ НАПРЯЖЕНИЙ НА ПРОЦЕСС ПОЛУЧЕНИЯ КВАНТОВЫХ ЯМ InGaAs/AlGaAs МЕТОДОМ МОС-ГИДРИДНОЙ ЭПИТАКСИИ

Strained InGaAs/AlGaAs quantum wells based on GaAs substrates are successfully used in wide optoelectronic applications. In this paper the advantages of these strained wells and their manufacturing difficulties in MOCVD have been described. In order to estimate the quality of InGaAs/AlGaAs quantum wells with and without GaAsP compensation layer the photoluminescence method at room temperature was used. It is shown that optimal thickness and content of InGaAs layers lies in the small metastable region, which was accurately calculated. The effective critical stress, which leads to misfit dislocations formation, was found and equals to 0.014mGaAs (where mGaAs is rigidity modulus in GaAs). The GaAsP compensation layers close to highly strained quantum wells for better reproducibility of the growth method were proposed. Finally, the best position of the GaAsP layer for higher photoluminescence signal has been chosen and discussed.П редставлены результаты по определению границы устойчивости гетероструктур с кван-товыми ямами InGaAs/AlGaAs в процессе МОС-гидридной эпитаксии. Экспериментально доказано наличие критического значения эффективного напряжения, после превышения которого начинается активное дефектообразование. Установлено, что компенсация упругих напряжений в сильнонапряженных квантовых ямах InGaAs приводит к увеличению интенсивности их фото-люминесценции. Показано, что исключение влияния обменного взаимодействия As/P на гете-рограницах квантовых ям позволяет существенно поднять интенсивность излучени

ИССЛЕДОВАНИЕ ПРОЦЕССА ТЕРМИЧЕСКОЙ АКТИВАЦИИ АКЦЕПТОРНОЙ ПРИМЕСИ В ЭПИТАКСИАЛЬНЫХ СЛОЯХ GaN : Mg

The effect of thermal annealing of GaN:Mg layers on acceptor impurity activation has been investigated. Hole concentration increased and mobility decreased with an increase in thermal annealing temperature. The sample annealed at 1000 oC demonstrated the lowest value of resistivity. Rapid thermal annealing (annealing with high heating speed) considerably improved the efficiency of Mg activation in the GaN layers. The optimum time of annealing at 1000 oC has been determined. The hole concentration increased by up to 4 times compared to specimens after conventional annealing.Исследовано влияние условий термического отжига эпитаксиальных слоев GaN : Mg на активацию атомов акцепторной примеси. По результатам измерений методом Холла установлено, что с увеличением температуры отжига концентрация дырок возрастает, а подвижность уменьшается. Наименьшее значение удельного электрического сопротивления получено на образцах, отожженных при температуре 1000 oC. Быстрый термический отжиг (отжиг с высокой скоростью нагрева) позволяет существенно повысить эффективность процесса активации атомов магния в эпитаксиальных слоях GaN. Оптимальное время указанного процесса при температуре 1000 oC составило 1 мин. Показано, что концентрация дырок возросла в 4 раза по сравнению с образцами, подвергнутыми стандартному термическому отжигу