Tunable plasma wave resonant detection of optical beating in high electron mobility transistor

We report on tunable terahertz resonant detection of two 1.55 µm cw-lasers beating by plasma waves in AlGaAs/InGaAs/InP high-electron-mobility transistor. We show that the fundamental plasma resonant frequency and its odd harmonics can be tuned with the applied gate-voltage in the range 75-490 GHz. The observed frequency dependence on gate-bias is found to be in good agreement with the theoretical plasma waves dispersion law.Comment: Applied Physics Letters to be published (2006) -

Lifetimes of Confined Acoustic Phonons in Ultra-Thin Silicon Membranes

We study the relaxation of coherent acoustic phonon modes with frequencies up to 500 GHz in ultra-thin free-standing silicon membranes. Using an ultrafast pump-probe technique of asynchronous optical sampling, we observe that the decay time of the first-order dilatational mode decreases significantly from \sim 4.7 ns to 5 ps with decreasing membrane thickness from \sim 194 to 8 nm. The experimental results are compared with theories considering both intrinsic phonon-phonon interactions and extrinsic surface roughness scattering including a wavelength-dependent specularity. Our results provide insight to understand some of the limits of nanomechanical resonators and thermal transport in nanostructures

High quality single crystal Ge nano-membranes for opto-electronic integrated circuitry

Under the terms of the Creative Commons Attribution (CC BY) license to their work.-- et al.A thin, flat, and single crystal germanium membrane would be an ideal platform on which to mount sensors or integrate photonic and electronic devices, using standard silicon processing technology. We present a fabrication technique compatible with integrated-circuit wafer scale processing to produce membranes of thickness between 60 nm and 800 nm, with large areas of up to 3.5 mm2. We show how the optical properties change with thickness, including appearance of Fabry-Pérot type interference in thin membranes. The membranes have low Q-factors, which allow the platforms to counteract distortion during agitation and movement. Finally, we report on the physical characteristics showing sub-nm roughness and a homogenous strain profile throughout the freestanding layer, making the single crystal Ge membrane an excellent platform for further epitaxial growth or deposition of materials.This work was carried out under the RCUK Basic Technology Programme supported by research Grant Nos. EP/F040784/1, EP/J001074/1, and EP/L007010/1. It also received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement NANOFUNCTION n°257375 alongside funding from TAPHOR (MAT2012–31392) and FP7 Project MERGING (Grant No. 309150). Vibrational property measurements were funded by the ERC under Grant No. 202735, NonContactUltrasonic.Peer Reviewe

Acoustic phonon propagation in ultra-thin Si membranes under biaxial stress field

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.We report on stress induced changes in the dispersion relations of acoustic phonons propagating in 27 nm thick single crystalline Si membranes. The static tensile stress (up to 0.3 GPa) acting on the Si membranes was achieved using an additional strain compensating silicon nitride frame. Dispersion relations of thermally activated hypersonic phonons were measured by means of Brillouin light scattering spectroscopy. The theory of Lamb wave propagation is developed for anisotropic materials subjected to an external static stress field. The dispersion relations were calculated using the elastic continuum approximation and taking into account the acousto-elastic effect. We find an excellent agreement between the theoretical and the experimental dispersion relations.The authors acknowledge financial support from the FP7 project MERGING (grant no. 309150); the Spanish MICINN projects nanoTHERM (grant no. CSD2010-0044) and TAPHOR (MAT2012-31392). JGB gratefully acknowledges support from the Spanish government through a Juan de la Cierva fellowship. MP and AS acknowledge funding from the Academy of Finland (grant no. 252598).Peer Reviewe

Reconstructing phonon mean free path contributions to thermal conductivity using nanoscale membranes

Knowledge of the mean free path distribution of heat-carrying phonons is key to understanding phonon-mediated thermal transport. We demonstrate that thermal conductivity measurements of thin membranes spanning a wide thickness range can be used to characterize how bulk thermal conductivity is distributed over phonon mean free paths. A non-contact transient thermal grating technique was used to measure the thermal conductivity of suspended Si membranes ranging from 15 to 1500 nm in thickness. A decrease in the thermal conductivity from 74% to 13% of the bulk value is observed over this thickness range, which is attributed to diffuse phonon boundary scattering. Due to the well-defined relation between the membrane thickness and phonon mean free path suppression, combined with the range and accuracy of the measurements, we can reconstruct the bulk thermal conductivity accumulation vs. phonon mean free path, and compare with theoretical models

Ultra-thin free-standing single crystalline silicon membranes with strain control

We report on fabrication and characterization of ultra-thin suspended single crystalline flat silicon membranes with thickness down to 6 nm. We have developed a method to control the strain in the membranes by adding a strain compensating frame on the silicon membrane perimeter to avoid buckling after the release. We show that by changing the properties of the frame the strain of the membrane can be tuned in controlled manner. Consequently, both the mechanical properties and the band structure can be engineered, and the resulting membranes provide a unique laboratory to study low-dimensional electronic, photonic, and phononic phenomena.Peer reviewe

Heat protective coatings on niobium alloys

Розглянуто різноманітні склади і технології отримання теплозахисних покриттів. Аналіз виявив, що при плазмо-дифузійному нанесенні на поверхні ніобієвого сплаву формувалось багатошарове покриття. Високопористий плазмо-напилений шар силіцидумолібдену має значний розкид по товщині (h=100...350 мкм, H20=6880 МПа). При дослідженні мікроструктури зразків з плазмо-дифузійним покриттям після випробувань виявлено, що тріщини в покритті зароджуються в процесі повзучості здебільшого на границі розділу плазмового і дифузійного шарів покриття. Осередком їх зародження є окремі несуцільності в дифузійному шарі в початковому стані. Поширення тріщин відбувається як в плазмовий, так і в дифузний шари покриття. Гальмування зростання тріщин в плазмовому шарі відбувається за рахунок округлого характеру пор і підвищеної пластичності цього шару. Зростання тріщин в глиб зразка, як правило, гальмується боридним підшаром. Перевага плазмо-дифузійної технології забезпечила підвищену пластичність покриття, наявність тонких бар'єрних підшарів, не суцільну структуру покриття, наявність легкоплавких з'єднань, що сприяють заліковуванню дефектів в покритті, підвищенню його корозійної стійкості і опору термовтомлювальному руйнуванню. Поєднання цих властивостей дозволило забезпечити підвищення довговічності в порівнянні з силіцидними і боросиліцидними покриттями в умовах ізотермічної повзучості на повітрі (1400оС, 50МПа) 1,9...3,7 рази і в умовах термоциклічної повзучості (1400 - 250оС, 50 МПа) в 6,8...8,5 раз. Визначено, що застосування дискретної структури дозволить збільшити товщину шару покриттів та забеспечити підвищення їх робочих властивостей.The article shows that during plasma-diffusion deposition, a multilayer coating was formed on the surface of the niobium alloy. A highly porous plasma-sprayed layer of molybdenum silicide has a significant spread in thickness (h=100...350 m, H20=6880 MPa). When studying the microstructure of samples with a plasma-diffusion coating after testing, it was found that cracks in the coating originate in the process of creep, mostly at the interface between the plasma and diffusion layers of the coating. The source of their origin is individual discontinuities in the diffusion layer as delivered. Crack propagation occurs both into the plasma and diffusion layers of the coating. Crack growth in the plasma layer is inhibited due to the rounded nature of the pores and the increased plasticity of this layer. The growth of cracks deep into the sample is, as a rule, inhibited by a boride sublayer. The advantage of plasma-diffusion technology provided an increased plasticity of the coating, the presence of thin barrier sublayers, a discontinuous coating structure, the presence of low-melting compounds that contribute to the healing of defects in the coating, an increase in its corrosion resistance and resistance to thermal fatigue destruction. The combination of these properties made it possible to provide an increase in durability compared to silicide and borosilicide coatings under conditions of isothermal creep in air (1400 °C, 50 MPa) 1.9...3.7 times and under conditions of thermal cyclic creep (1400-250 °C, 50 MPa) in 6.8...8.5 times. It has been determined that the use of a discrete structure will increase the thickness of the coating layer and ensure an increase in their working properties.Рассмотрены различные составы и технологии получения теплозащитных покрытий. Анализ выявил, что при плазменно-диффузионной нанесении на поверхности ниобиевых сплава формировалось многослойное покрытие. Высокопористый плазменно-напыленный слой силицидамолибдена имеет значительный разброс по толщине (h=100...350 мкм, H20=6880 МПа). При исследовании микроструктуры образцов с плазменно-диффузионным покрытием после испытаний обнаружено, что трещины в покрытии зарождаются в процессе ползучести большей частью на границе раздела плазменного и диффузионного слоев покрытия. Очагом их зарождения являются отдельные несплошности в диффузионном слое в состоянии поставки. Распространение трещин происходит как в плазменный, так и в диффузионный слои покрытия. Торможение роста трещин в плазменном слое происходит за счет округлого характера пор и повышенной пластичности этого слоя. Рост трещин вглубь образца, как правило, тормозится боридным подслоем. Преимущество плазменно-диффузионной технологии обеспечило повышенную пластичность покрытия, наличие тонких барьерных подслоев, не сплошную структуру покрытия, наличие легкоплавких соединений, способствующих залечиванию дефектов в покрытии, повышению его коррозион-ной стойкости и сопротивлению термоусталостному разрушению. Сочетание этих свойств позволило обеспечить повышение долговечности по сравнению с силицидными и боросилицидными покрытиями в условиях изотермической ползучести на воздухе (1400 оС, 50 МПа) 1,9...3,7 раза и в условиях термоциклической ползучести (1400 - 250 оС, 50 МПа) в 6,8...8,5 раз. Определено, что применение дискретной структуры позволит увеличить толщину слоя покрытий и обеспечить повышение их рабочих свойств

Conserved Mutations in the Pneumococcal Bacteriocin Transporter Gene, blpA, Result in a Complex Population Consisting of Producers and Cheaters

All fully sequenced strains of Streptococcus pneumoniae possess a version of the blp locus, which is responsible for bacteriocin production and immunity. Activation of the blp locus is stimulated by accumulation of the peptide pheromone, BlpC, following its secretion by the ABC transporter, BlpA. The blp locus is characterized by significant diversity in blpC type and in the region of the locus containing putative bacteriocin and immunity genes. In addition, the blpA gene can represent a single large open reading frame or be divided into several smaller fragments due to the presence of frameshift mutations. In this study, we use a collection of strains with blp-dependent inhibition and immunity to define the genetic changes that bring about phenotypic differences in bacteriocin production or immunity. We demonstrate that alterations in blpA, blpC, and bacteriocin/immunity content likely play an important role in competitive interactions between pneumococcal strains. Importantly, strains with a highly conserved frameshift mutation in blpA are unable to secrete bacteriocins or BlpC, but retain the ability to respond to exogenous peptide pheromone produced by cocolonizing strains, stimulating blp-mediated immunity. These “cheater” strains can only coexist with bacteriocin-producing strains that secrete their cognate BlpC and share the same immunity proteins. The variable outcome of these interactions helps to explain the heterogeneity of the blp pheromone, bacteriocin, and immunity protein content

Dispersive readout of reconfigurable ambipolar quantum dots in a silicon-on-insulator nanowire

We report on ambipolar gate-defined quantum dots in silicon on insulator (SOI) nanowires fabricated using a customised complementary metal-oxide-semiconductor (CMOS) process. The ambipolarity was achieved by extending a gate over an intrinsic silicon channel to both highly doped n-type and p-type terminals. We utilise the ability to supply ambipolar carrier reservoirs to the silicon channel to demonstrate an ability to reconfigurably define, with the same electrodes, double quantum dots with either holes or electrons. We use gate-based reflectometry to sense the inter-dot charge transition(IDT) of both electron and hole double quantum dots, achieving a minimum integration time of 160(100) $\mu$s for electrons (holes). Our results present the opportunity to combine, in a single device, the long coherence times of electron spins with the electrically controllable holes spins in silicon.Comment: 5 pages, 4 figure

Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications

Resonant frequencies of the two-dimensional plasma in FETs increase with the reduction of the channel dimensions and can reach the THz range for sub-micron gate lengths. Nonlinear properties of the electron plasma in the transistor channel can be used for the detection and mixing of THz frequencies. At cryogenic temperatures resonant and gate voltage tunable detection related to plasma waves resonances, is observed. At room temperature, when plasma oscillations are overdamped, the FET can operate as an efficient broadband THz detector. We present the main theoretical and experimental results on THz detection by FETs in the context of their possible application for THz imaging.Comment: 22 pages, 12 figures, review pape