Influence of PH3 exposure on silicon substrate morphology in the MOVPE growth of III-V on silicon multijunction solar cells

Dual-junction solar cells formed by a GaAsP or GaInP top cell and a silicon bottom cell seem to be attractive candidates to materialize the long sought-for integration of III-V materials on silicon for photovoltaic applications. One of the first issues to be considered in the development of this structure will be the strategy to create the silicon emitter of the bottom subcell. In this study, we explore the possibility of forming the silicon emitter by phosphorus diffusion (i.e. exposing the wafer to PH3 in a MOVPE reactor) and still obtain good surface morphologies to achieve a successful III-V heteroepitaxy as occurs in conventional III-V on germanium solar cell technology. Consequently, we explore the parameter space (PH3 partial pressure, time and temperature) that is needed to create optimized emitter designs and assess the impact of such treatments on surface morphology using atomic force microscopy. Although a strong degradation of surface morphology caused by prolonged exposure of silicon to PH3 is corroborated, it is also shown that subsequent anneals under H-2 can recover silicon surface morphology and minimize its RMS roughness and the presence of pits and spikes

Advanced transmission electron microscopy investigation of defect formation in movpe-growth of gap on silicon using arsenic initial coverage

Integration of GaP layers on silicon substrates using AsH3 pre-exposure followed by a PH3-based GaP epitaxial growth allows the development of very promising processes for the photovoltaic industry, although many of the growth routines using this approach suffer from reproducibility issues when transferred to a new epitaxial system, leading to poor quality layers. This fact reveals a lack of knowledge on the mechanisms behind the formation of the most common planar defects (stacking faults and microtwins) and their dynamics for GaP/Si Metal Organic Vapor Phase Epitaxy using AsH3 and PH3. Therefore, in this work, a set of GaP/Si samples with a similarly high defect density grown between 700 °C and 725 °C, are analyzed by means of high-resolution scanning transmission electron microscopy and electron energy loss spectroscopy. The results presented show contaminant-free Si surfaces for temperatures above 725 °C, ruling out the hypothesis of contaminant as the origin of these planar defects. Regarding the interface Si/GaP, the GaP growth starts, in all the samples, with Ga Si bonds. Additionally, no traces of As are found, which reinforces the hypothesis of an effectively displacement of As on Si surface by Ga atoms at high temperature. Finally, it is observed complex chemical structures in the origin of the microtwins and the cause of the origin of these defects seems to be a localized gallium depletion at the GaP/Si interface

Effect of strain rate on tensile mechanical properties of high-purity niobium single crystals for SRF applications

An investigation of the mechanical properties of high-purity niobium single crystals is presented. Specimens were cut with different crystallographic orientations from a large grain niobium disk and uniaxial tensile tests were conducted at strain rates between 10-4 and 103 s-1. The logarithmic strain rate sensitivity for crystals oriented close to the center of a tensile axis inverse pole figure (IPF) is ~0.14 for all strain rates. The strain at failure (ranging from 0.4 to 0.9) is very sensitive to crystal orientation and maximal at ~10-2 s-1 for crystals oriented close to the center of an IPF. The high anisotropy observed at quasi-static strain rates decreased with increasing strain rate. The activation of multiple slip systems in the dynamic tests could account for this reduction in anisotropy. A transition from strain hardening to softening in the plastic domain was observed at strain rates greater than approximately 6 × 10-2 s-1 for crystals oriented close to the center of a tensile axis IPF. Shear bands were observed in specimens with orientations having similarly high Schmid factors on both {110} and {112} slip families, and they are correlated with reduced ductility. Crystal rotations at fracture are compared for the different orientations using scanning electron microscopy images and EBSD orientation maps. A rotation toward the terminal stable [101] orientation was measured for the majority of specimens (with tensile axes more than ~17° from the [001] direction) at strain rates between 1.28 × 10-2 and 1000 s-1.The authors would like to acknowledge the work of CERN's Materials, Metrology and Non-Destructive Testing (EN-MME-MM) section for granting access to their equipment for specimen preparation and scanning electron microscope (SEM) analyses. The authors would also like to thank Mr. Larry Vladic of Elite Motion LLC for lending us the high-speed camera during the high strain rate tests performed ASU. This Marie Sklodowska-Curie Action (MSCA) Innovative Training Network (ITN) receives funding from the European Union's H2020 Framework Programme under grant agreement no. 764879. T.R. Bieler, D. Kang, E. Pai Kulyadi, P. Eisenlohr, C. Kale, and K.N. Solanki acknowledge support from DOE/OHEP grant DE-SC0009962

The lithium stellarator experiment. TJ-II as a benchmark

In the last years, lithium wall conditioning has been carried out in several fusion devices (Tokamaks, Compact Toruses) by different techniques, providing in many instances record values of plasma parameters and enhanced plasma reproducibility. Behind these findings are the low recycling, low contamination and non-coronal enhanced radiative power dissipation properties of Li. Stellarators are best suited for reactor concepts as far as they basically free from extreme thermal load events. However, the potential problem of impurity accumulation must be taken into account. Therefore, the use of low Z elements as PFC would be highly desirable. In the present work, the plasma performance of the TJ-II Heliac under Li-coated wall conditions with lithium is described. Compared to previous coatings, lithium has produced the best plasma performance to date, leading to the achievement of record values in plasma density and energy confinement. In the present work, the impact of the Li coating operation on reactor relevant issues, such as particle exhaust and retention, high-density operation, confinement characteristics and plasma wall interaction issues is stressed.На протязі останніх років обробку стінок літієм було проведено в кількох термоядерних установках (токамаки, компактні тори) з використанням різних методів, що забезпечило у багатьох випадках одержання плазми з рекордними параметрами та покращеним відтворенням результатів. Ці результати пов’язані з низьким рециклінгом, що властивий літію, слабким забрудненням плазми і підвищеним не корональним поглинанням випромінювання. Стеларатори краще за все підходять для концепції реактора, бо в них у принципі не може бути підвищених теплових навантажень. Але треба враховувати можливість накопичення домішок. Тому конче бажано в поверхнях, які повернені до плазми, використовувати елементи з низьким Z. В даній роботі описано експерименти по одержанню плазми в геліаку TJ-II в умовах покриття стінок літієм. Порівняно з покриттями, що використовувалися раніше, літій забезпечив одержання плазми з найкращими на даний час параметрами, дозволивши досягти рекордних значень гущини плазми та утримання енергії. Підкреслена важливість літієвого покриття для таких аспектів реактора, як втрати і затримування частинок, робота при високій гущині, характеристики утримання та взаємодія плазма-стінка.В течение последних лет обработка стенок литием была проведена в нескольких термоядерных установках (токамаки, компактные торы) с использованием различных методов, что обеспечило во многих случаях получение плазмы с рекордными параметрами и улучшенной воспроизводимостью результатов. Эти результаты связаны со свойственными литию низким рециклингом, слабым загрязнением плазмы и повышенным не корональным поглощением излучения. Стеллараторы лучше всего подходят для концепции реактора, ибо у них в принципе не может быть повышенных тепловых нагрузок. Однако, необходимо учитывать возможность накопления примесей. Поэтому весьма желательно в поверхностях, обращённых к плазме, использовать элементы с низким Z. В настоящей работе описаны эксперименты по получению плазмы в гелиаке TJ-II в условиях покрытия стенок литием. По сравнению с покрытиями, которые использовались раньше, литий обеспечил получение плазмы с наилучшими в настоящее время параметрами, позволив достичь рекордных значений плотности плазмы и удержания энергии. Подчёркивается важность литиевого покрытия для таких аспектов реактора, как расход и удерживание частиц, работа при большой плотности, характеристики удержания и взаимодействие плазма-стенка

Overview of recent TJ-II stellarator results

The main results obtained in the TJ-II stellarator in the last two years are reported. The most important topics investigated have been modelling and validation of impurity transport, validation of gyrokinetic simulations, turbulence characterisation, effect of magnetic configuration on transport, fuelling with pellet injection, fast particles and liquid metal plasma facing components. As regards impurity transport research, a number of working lines exploring several recently discovered effects have been developed: the effect of tangential drifts on stellarator neoclassical transport, the impurity flux driven by electric fields tangent to magnetic surfaces and attempts of experimental validation with Doppler reflectometry of the variation of the radial electric field on the flux surface. Concerning gyrokinetic simulations, two validation activities have been performed, the comparison with measurements of zonal flow relaxation in pellet-induced fast transients and the comparison with experimental poloidal variation of fluctuations amplitude. The impact of radial electric fields on turbulence spreading in the edge and scrape-off layer has been also experimentally characterized using a 2D Langmuir probe array. Another remarkable piece of work has been the investigation of the radial propagation of small temperature perturbations using transfer entropy. Research on the physics and modelling of plasma core fuelling with pellet and tracer-encapsulated solid-pellet injection has produced also relevant results. Neutral beam injection driven Alfvénic activity and its possible control by electron cyclotron current drive has been examined as well in TJ-II. Finally, recent results on alternative plasma facing components based on liquid metals are also presented. ISSN:0029-5515 ISSN:1741-432

European Strategy for Particle Physics -- Accelerator R&D Roadmap

The 2020 update of the European Strategy for Particle Physics emphasised the importance of an intensified and well-coordinated programme of accelerator R&D, supporting the design and delivery of future particle accelerators in a timely, affordable and sustainable way. This report sets out a roadmap for European accelerator R&D for the next five to ten years, covering five topical areas identified in the Strategy update. The R&D objectives include: improvement of the performance and cost-performance of magnet and radio-frequency acceleration systems; investigations of the potential of laser / plasma acceleration and energy-recovery linac techniques; and development of new concepts for muon beams and muon colliders. The goal of the roadmap is to document the collective view of the field on the next steps for the R&D programme, and to provide the evidence base to support subsequent decisions on prioritisation, resourcing and implementation.Comment: 270 pages, 58 figures. Editor: N. Mounet. LDG chair: D. Newbold. Panel chairs: P. V\'edrine (HFM), S. Bousson (RF), R. Assmann (plasma), D. Schulte (muon), M. Klein (ERL). Panel editors: B. Baudouy (HFM), L. Bottura (HFM), S. Bousson (RF), G. Burt (RF), R. Assmann (plasma), E. Gschwendtner (plasma), R. Ischebeck (plasma), C. Rogers (muon), D. Schulte (muon), M. Klein (ERL

Latest physics results of TJ-II flexible heliac

This paper is devoted to the presentation of the most relevant recent Physics results obtained in the TJ-II flexible heliac. Firstly ECRH modulation and plasma breakdown studies are summarised; then the particle control techniques used to obtain reproducible discharges with density under control are presented. Transport studies show internal heat transport barriers that reduce heat conductivity to neoclassical values, and ELM-like transport events, similar to those observed in tokamaks and in other stellarators before and during H mode transition. Evidence of ExB sheared has been observed both in the proximity of rational surfaces. Finally, a high resolution Thomsom Scattering system has shown Te and ne profile structures

HE-LHC: The High-Energy Large Hadron Collider – Future Circular Collider Conceptual Design Report Volume 4

In response to the 2013 Update of the European Strategy for Particle Physics (EPPSU), the Future Circular Collider (FCC) study was launched as a world-wide international collaboration hosted by CERN. The FCC study covered an energy-frontier hadron collider (FCC-hh), a highest-luminosity high-energy lepton collider (FCC-ee), the corresponding 100 km tunnel infrastructure, as well as the physics opportunities of these two colliders, and a high-energy LHC, based on FCC-hh technology. This document constitutes the third volume of the FCC Conceptual Design Report, devoted to the hadron collider FCC-hh. It summarizes the FCC-hh physics discovery opportunities, presents the FCC-hh accelerator design, performance reach, and staged operation plan, discusses the underlying technologies, the civil engineering and technical infrastructure, and also sketches a possible implementation. Combining ingredients from the Large Hadron Collider (LHC), the high-luminosity LHC upgrade and adding novel technologies and approaches, the FCC-hh design aims at significantly extending the energy frontier to 100 TeV. Its unprecedented centre-of-mass collision energy will make the FCC-hh a unique instrument to explore physics beyond the Standard Model, offering great direct sensitivity to new physics and discoveries