1,721,118 research outputs found
Keep it simple and switch to pure tellurium
No full textResearchers, inspired by neurobiological functions and architecture of the human brain, have been developing neuromorphic computing by using artificial neurons and synapses to perform processing and storage in the same physical place. However, no conceptual technology can become reality without hardware. The class of memory devices known as phase change (PC) memories is expected to enable more efficient learning algorithms in neuromorphic computers, owing to their capability to also work as a processing unit (1). On page 1390 of this issue, Shen et al. (2) use pure elemental tellurium to build an electrical switch with a large drive current and rapid switching speed that can be used to efficiently operate PC cells in the cross-point memory architecture (see the figure). By creating the logic gate out of a single element, the design gets around challenges presented by material stoichiometry and selective elemental migration. The single-element design represents a step toward high-density, fast, and nonvolatile PC memories
Annealing effects on faceting of InAsGaAs (001) quantum dots
No full textThe aspect ratio and faceting evolution of quantum dots grown at 500 degrees C were studied as a function of postgrowth annealing temperature. We show that faceting and aspect ratio strictly depend on growth conditions. The evolution toward {136} and {137} facets is kinetically limited and occurs under different experimental conditions. Furthermore long annealing procedures lead to the occurrence of low aspect ratio domes different from those forming at higher growth temperatures
In-line correlation and ordering of InAs/GaAs multistacked Quantum Dots structures
No full textSeveral multilayer InAs/GaAs(001) samples were grown in a Molecular Beam Epitaxy chamber under critical growth conditions in order to induce the self-assembling of chains of InAs Quantum Dots over mounded GaAs surfaces. As the number of deposited layers was increased, an increasing in-line ordering was observed. Finite Element Method simulations confirmed this trend which is driven mainly by the propagation of the elastic strain field through the layers. On the other hand, the morphological features of the surface contribute to improving the alignment of the InAs Quantum Dots in every chain
Scaling behavior of GaAs and GaMnAs quantum rings grown by droplet epitaxy
No full textThe transition from the liquid phase of Ga droplets to the formation of GaAs and GaMnAs
quantum rings has been studied as a function of temperature. We show that different aggregation
processes involve the GaAs (GaMnAs) island and the droplet formation. Furthermore, the aspect
ratio of the islands exhibits an anomalous scaling law related to a tendency to aggregate in the
vertical direction
Infrared surface absorption in Si(111)2x1 observed with reflectance anisotropy spectroscopy
No full textOptical transitions between surface states associated with dangling bonds in Si(111)2X1 have been measured by means of reflection anisotropy spectroscopy in the near infrared. The results confirm those previously obtained with other optical techniques (namely surface differential reflectivity and photothermal deflection spectroscopies). The method does not require oxidation of the surface and thus opens the way to studying a number of problems in surface physics, including the temperature dependence of surface transitions in Si(111)2x1 and Ge(111)2x1
Selective growth of InAs quantum dots on GaAs driven by as kinetics
No full textIn this paper we examine experimentally and theoretically how the As flux direction and intensity influences the recently observed selective growth of InAs quantum dots on the rippled surface of GaAs, where the dots line up only against well determined slopes of the mounded GaAs surface facing the impinging Arsenic flux. We have previously shown that the observed phenomenon is exclusively related to the As constituent, challenging the widespread belief that As plays only a minor role in the dot formation. The selective growth is obtained by Molecular Beam Epitaxy at high growth temperature and under a high As/In flux ratio. To interpret the experimental results, we use a newly developed kinetic model incorporating new features: (i) anions are explicitly considered; (ii) cations and anions follow a different kinetics; (iii) the dot surface is distinguished by the dot bulk and atoms are transferred back and forth between the two phases depending on the surface interaction with the environment. We find that even a very small difference in the As flux intensity impinging over two substrate regions produces a cation current flow from one region to the other, so that the dots form only on the region exposed to the largest flux
Anisotropy of the GaAs(001)-β2(2×4) surface from high-resolution electron energy loss spectroscopy
No full textHigh-resolution electron energy loss spectroscopy has been applied to investigate the anisotropy of the GaAs(001)-beta2(2x4) reconstruction. Measurements have been performed on high-quality samples grown in situ by molecular beam epitaxy. The loss intensity is different in the directions parallel and perpendicular to dimers, particularly close to the fundamental gap. We discuss our data in comparison to reflectance anisotropy spectra in the attempt of identifying the source of the anisotropy close to the bulk critical points where surface and bulk like transitions take place. We find direct evidence of transitions involving the dimers of the top atomic layer. The set of data taken on As-decapped surfaces indicates that the intensity of the structures is markedly affected by the As cap removal. The exposure to increasing amounts of molecular oxygen modifies the spectral features in a way consistent with their surface origin.High-resolution electron energy loss spectroscopy has been applied to investigate the anisotropy of the GaAs(001)-beta2(2x4) reconstruction. Measurements have been performed on high-quality samples grown in situ by molecular beam epitaxy. The loss intensity is different in the directions parallel and perpendicular to dimers, particularly close to the fundamental gap. We discuss our data in comparison to reflectance anisotropy spectra in the attempt of identifying the source of the anisotropy close to the bulk critical points where surface and bulk like transitions take place. We find direct evidence of transitions involving the dimers of the top atomic layer. The set of data taken on As-decapped surfaces indicates that the intensity of the structures is markedly affected by the As cap removal. The exposure to increasing amounts of molecular oxygen modifies the spectral features in a way consistent with their surface origin
Epitaxial growth of GeTe/Sb2Te3 superlattices
No full textIn the quest for superlattices with engineered interfaces for disruptive applications such as neuromorphic computing, we present a dedicated study on the epitaxial growth of nominal GeTe/Sb2Te3 chalcogenide superlattices (CSL) on the Sb passivated Si(111) − (√3 × √3)R30°− Sb surface. Intermixing at the GeTe and Sb2Te3 interface is assessed by X-Ray diffraction and reflectivity. A new growth procedure with element flux interruptions is proposed to engineer the degree of intermixing and the tailoring of the GeSbTe layers into Sb-rich compositions
Crystallization Study of Ge-Rich (GeTe) m (Sb 2 Te 3 ) n Using Two-Step Annealing Process
No full textA combination of X-ray diffraction and Raman scattering is employed to investigate (GeTe) m (Sb 2 Te 3 ) n alloys crystallized by thermal annealing from the amorphous (a-) phase. The films are deposited by molecular beam epitaxy on Si(111) substrates. First, a series of a-GeSbTe (GST) films of different composition is deposited and studied by Raman spectroscopy to identify the Ge-rich features of the alloys. Second, the crystallization properties of Ge 10 Sb 2 Te 13 are studied upon different annealing conditions. The aim of this work is to develop a procedure to avoid segregation of Ge and GeTe at the GST crystallization onset (T x ). This is here achieved by means of an incubation step at temperature lower than T x . The crystallization onset T x increased to 270 °C, which is about 160 °C higher compared to a reference GeTe sample, while the alloy always crystallizes in the stable Ge 1 Sb 2 Te 4 composition. The increase of T x is observed for all annealing conditions, regardless the amount of crystalline Ge segregated. For the optimized annealing treatment, the presence of Ge nanocrystals along with crystalline GeSbTe is unveiled by Raman measurements, paving the way for the control of the microstructure and electrical/thermal properties of Ge-rich alloys
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