Algorithm based high composition-controlled growths of GeSn on GaAs (001) via molecular beam epitaxy

The growth of high-composition GeSn films of the future will likely be guided via algorithms. In this study we show how a logarithmic-based algorithm can be used to obtain GeSn compositions up to 16 % on GaAs (001) substrates via molecular beam epitaxy. Within we demonstrate composition targeting and logarithmic gradients to achieve pseudomorph GeSn compositions near 11% before partial relaxation of the structure and a continued gradient to 16 % GeSn. Using algorithmic-based control and calibration, the ability to consistently and easily grow GeSn compositions above 20 % will likely become very possible. In this report, we use X-ray diffraction and atomic force microscopy to analyze and demonstrate some of the possible growths that can be produced with the enclosed algorithm

Bi incorporation and segregation in the MBE-grown GaAs-(Ga,Al)As-Ga(As,Bi) core–shell nanowires

Incorporation of Bi into GaAs-(Ga,Al)As-Ga(As,Bi) core–shell nanowires grown by molecular beam epitaxy is studied with transmission electron microscopy. Nanowires are grown on GaAs(111)B substrates with Au-droplet assisted mode. Bi-doped shells are grown at low temperature (300 °C) with a close to stoichiometric Ga/As flux ratio. At low Bi fluxes, the Ga(As,Bi) shells are smooth, with Bi completely incorporated into the shells. Higher Bi fluxes (Bi/As flux ratio ~ 4%) led to partial segregation of Bi as droplets on the nanowires sidewalls, preferentially located at the nanowire segments with wurtzite structure. We demonstrate that such Bi droplets on the sidewalls act as catalysts for the growth of branches perpendicular to the GaAs trunks. Due to the tunability between zinc-blende and wurtzite polytypes by changing the nanowire growth conditions, this effect enables fabrication of branched nanowire architectures with branches generated from selected (wurtzite) nanowire segments

Метод семантичної верифікації застосувань у технології GPGPU

An application development and verification method for massively parallel systems using NVIDIA GPUs is proposed. The method allows creating models at different levels of abstraction using the apparatus of marked transition systems. The compositions (product) of such systems are transformed into a Petri net, which are then analyzed by appropriate means. The proposed method allows specifying model properties by temporal logic formulas. This allows studying the properties of massively parallel systems which is almost impossible to analyze manually, since the number of execution threads in the latest NVIDIA video adapter architectures (Pascal, Volta, Turing, Ampere) is measured in hundreds of thousands or millions.Предложен метод разработки и верификации приложений для систем с массовым параллелизмом на основе видеоадаптеров от компании NVIDIA, который позволяет создавать абстракции различных уровней с помощью аппарата размеченных транзиционных систем. Композиции таких систем трансформируются в сети Петри, которые далее анализируются соответствующими средствами. Метод позволяет создавать модели на различных уровнях абстракции, а их свойства могут специфицироваться формулами темпоральной логики. Это позволяет исследовать свойства систем с массовым параллелизмом, которые практически невозможно анализировать вручную, так как количество потоков в новейших архитектурах видеоадаптеров (Pascal, Volta, Amper, Тьюринг), выделенных для выполнения кода, измеряется сотнями тысяч или миллионами.Запропоновано метод розроблення та верифікації застосувань для систем з масовим паралелізмом на основі відеоадаптерів від компанії NVIDIA, який дозволяє створювати абстракції різних рівнів за допомогою апарата розмічених транзиційних систем. Композиції таких систем трансформуються в мережі Петрі, які далі аналізуються відповідними засобами. Метод також дає змогу створювати моделі на різних рівнях абстракції, а їх властивості можуть специфікуватися формулами темпоральної логіки. Це дозволяє досліджувати властивості систем з масовим паралелізмом, які майже неможливо аналізувати вручну, оскільки кількість потоків у новітніх архітектурах відеоадаптерів (Pascal, Volta, Amper, Тюрінг), виділених для виконання коду, вимірюється сотнями тисяч або мільйонами

Structural and Magnetoresistive Properties of Nanometric Films Based on Iron and Chromium Oxides on the Si Substrate

Abstract Ultraviolet photons of KrF laser (248 nm) was used for the synthesis of nanometric films based on iron and chromium oxides (Fe2O3 − X (0 ≤ x ≤ 1) and Cr3 − X O3 − Y (0 ≤ x ≤ 2; 0 ≤ y ≤ 2)) with variable thickness, stoichiometry, and electrical properties. Film deposition was carried out on the silicon substrate Si  at the substrate’s temperature T S = 293 K. X-ray diffraction and X-ray reflectometry analysis were used for the obtained structure characterization. Such a combined investigation reveals the composition and texture for samples investigated and provides useful information about layer thickness and roughness. Fe2O3 − X (0 ≤ x ≤ 1) nanometric films demonstrate the negative magnetoresistance in magnetic fields up 7 kOe. At the same time, for hybrid systems of the alternate layers Fe2O3 − X (0 ≤ x ≤ 1)/Cr3 − X O3 − Y (0 ≤ x ≤ 2; 0 ≤ y ≤ 2), the positive magnetoresistance as well as the magnetic hysteresis and magnetoresistivity switching effect in the low magnetic fields were observed

The Role of the built-in electric field in recombination processes of GaN/AlGaN quantum wells: temperature- and pressure-dependent study of polar and non-polar structures

International audienceIn this paper, we present a comparative analysis of the optical properties of non-polar and polar GaN/AlGaN multi-quantum well (MQW) structures by time-resolved photoluminescence (TRPL) and pressure-dependent studies. The lack of internal electric fields across the non-polar structures results in an improved electron and hole wavefunction overlap with respect to the polar structures. Therefore, the radiative recombination presents shorter decay times, independent of the well width. On the contrary, the presence of electric fields in the polar structures reduces the emission energy and the wavefunction overlap, which leads to a strong decrease in the recombination rate when increasing the well width. Taking into account the different energy dependences of radiative recombination in non-polar and polar structures of the same geometry, and assuming that non-radiative processes are energy independent, we attempted to explain the ‘S-shape’ behavior of the PL energy observed in polar GaN/AlGaN QWs, and its absence in non-polar structures. This approach has been applied previously to InGaN/GaN structures, showing that the interplay of radiative and non-radiative recombination processes can justify the ‘S-shape’ in polar InGaN/GaN MQWs. Our results show that the differences in the energy dependences of radiative and non-radiative recombination processes cannot explain the ‘S-shape’ behavior by itself, and localization effects due to the QW width fluctuation are also important. Additionally, the influence of the electric field on the pressure behavior of the investigated structures was studied, revealing different pressure dependences of the PL energy in non-polar and polar MQWs. Non-polar MQWs generally follow the pressure dependence of the GaN bandgap. In contrast, the pressure coefficients of the PL energy in polar QWs are highly reduced with respect to those of the bulk GaN, which is due to the hydrostatic-pressure-induced increase in the piezoelectric field in quantum structures and the nonlinear behavior of the piezoelectric constant

Gold Nanoparticle Self-Aggregation on Surface with 1,6-Hexanedithiol Functionalization

Here we study the morphology and the optical properties of assemblies made of small (17 nm) gold nanoparticles (AuNPs) directly on silicon wafers coated with (3-aminopropyl)trimethoxysilane (APTES). We employed aliphatic 1,6-hexanedithiol (HDT) molecules to cross-link AuNPs during a two-stage precipitation procedure. The first immersion of the wafer in AuNP colloidal solution led mainly to the attachment of single particles with few inclusions of dimers and small aggregates. After the functionalization of precipitated NPs with HDT and after the second immersion in the colloidal solution of AuNP, we detected a sharp rise in the number of aggregates compared to single AuNPs and their dimers. The lateral size of the aggregates was about 100 nm, while some of them were larger than 1μm. We propose that the uncompensated dipole moment of the small aggregates appeared after the first precipitation and acts further as the driving force accelerating their further growth on the surface during the second precipitation. By having such inhomogeneous surface coating, the X-ray reciprocal space maps and modulation polarimetry showed well-distinguished signals from the single AuNPs and their dimers. From these observations, we concluded that the contribution from aggregated AuNPs does not hamper the detection and investigation of plasmonic effects for AuNP dimers. Meantime, using unpolarized and polarized light spectroscopy, the difference in the optical signals between the dimers, being formed because of self-aggregation and the one being cross-linked by means of HDT, was not detected.publishe

Critical Evaluation of Various Spontaneous Polarization Models and Induced Electric Fields in III-Nitride Multi-Quantum Wells

International audienceIn this paper, ab initio calculations are used to determine polarization difference in zinc blende (ZB), hexagonal (H) and wurtzite (WZ) AlN-GaN and GaN-InN superlattices. It is shown that a polarization difference exists between WZ nitride compounds, while for H and ZB lattices the results are consistent with zero polarization difference. It is therefore proven that the difference in Berry phase spontaneous polarization for bulk nitrides (AlN, GaN and InN) obtained by Bernardini et al. and Dreyer et al. was not caused by the different reference phase. These models provided absolute values of the polarization that differed by more than one order of magnitude for the same material, but they provided similar polarization differences between binary compounds, which agree also with our ab initio calculations. In multi-quantum wells (MQWs), the electric fields are generated by the well-barrier polarization difference; hence, the calculated electric fields are similar for the three models, both for GaN/AlN and InN/GaN structures. Including piezoelectric effect, which can account for 50% of the total polarization difference, these theoretical data are in satisfactory agreement with photoluminescence measurements in GaN/AlN MQWs. Therefore, the three models considered above are equivalent in the treatment of III-nitride MQWs and can be equally used for the description of the electric properties of active layers in nitride-based optoelectronic devices

Polar and Non-Polar Zn_1−xMg_xO:Sb Grown by MBE

The article presents a systematic study of Sb-doped Zn1−xMgxO layers, with various concentrations of Mg, that were successfully grown by plasma-assisted MBE on polar a- and c-oriented and non-polar r-oriented sapphire substrates. X-ray diffraction confirmed the polar c-orientation of alloys grown on c-and a-oriented sapphire and non-polar structures grown on r-oriented substrates. A uniform depth distribution of the Sb dopant at level of 2 × 1020 cm−3 was determined by SIMS measurements. Raman spectroscopy revealed the presence of Sb-related modes in all samples. It also showed that Mg alloying reduces the compressive strain associated with Sb doping in ZnO. XPS analysis indicates that the chemical state of Sb atoms in ZnMgO is 3+, suggesting a substitutional position of SbZn, probably associated with two VZn vacancies. Luminescence and transmission spectra were measured to determine the band gaps of the Zn1−xMgxO layers. The band gap energies extracted from the transmittance measurements differ slightly for the a, c, and r substrate orientations, and the differences increase with increasing Mg content, despite identical growth conditions. The differences between the energy gaps, determined from transmission and PL peaks, are closely correlated with the Stokes shift and increase with the Mg content in the analyzed series of ZnMgO layers