Search

Progress in computing architectures is approaching a paradigm shift: traditional computing based on digital complementary metal-oxide semiconductor technology is nearing physical limits in terms of miniaturization, speed, and, especially, power consumption. Consequently, alternative approaches are under investigation. One of the most promising is based on a "brain-like" or neuromorphic computation scheme. Another approach is quantum computing using photons. Both of these approaches can be realized using silicon photonics, and at the heart of both technologies is an efficient, ultra-low power broad band optical modulator. As silicon modulators suffer from relatively high power consumption, materials other than silicon itself have to be considered for the modulator. In this Perspective, we present our view on such materials. We focus on oxides showing a strong linear electro-optic effect that can also be integrated with Si, thus capitalizing on new materials to enable the devices and circuit architectures that exploit shifting computational machine learning paradigms, while leveraging current manufacturing infrastructure. This is expected to result in a new generation of computers that consume less power and possess a larger bandwidth.

Template-assisted selective area growth techniques have gained popularity for their ability to grow epitaxial materials in prefabricated dielectric templates. Confined epitaxial lateral overgrowth (CELO) is one such technique that uses dielectric templates to define the geometry of the grown nanostructures. Two terminal low-temperature magneto-transport measurements were used to determine electronic properties. For doped In0.53Ga0.47As CELO nanostructures, we observe Shubnikov–De Hass oscillations in the longitudinal magnetoresistance and utilize these to estimate effective mass, carrier density, and mobilities. This analysis both reveals the presence of defects in these nanostructures and material variabilities between growth runs. Electron beam lithography and contact deposition for transport measurements were enabled by parasitic growth removal. In the future, this approach can enable other material systems to be explored for confined lateral epitaxy, improve material quality, and investigate a variety of quantum transport phenomenon in such nanoscale devices. [ABSTRACT FROM AUTHOR]

Selectively growing epitaxial material in confined dielectric structures has been explored recently as a pathway to integrate highly mismatched materials on silicon substrates. This approach involves the fabrication of a channel-like structure of dielectric material that from the growth atmosphere reaches down to a small exposed area of the substrate where subsequent growth via metal organic chemical vapor deposition (MOCVD) initiates. The technique, referred to as template assisted selective epitaxy, can also enable the development of novel nanoscale photonic and electronic device structures because of its ability to allow epitaxy to progress in a direction, final size, and aspect ratio defined by the dielectric template, and allows integration of horizontal heterojunction inside the channel. To date, most confined epitaxy work has been detailed on silicon. Due to the reduced chemical and thermal stability of InP compared to Si, additional steps for surface preparation are required. In this work, two different fabrication routes are described on InP substrates: one involving amorphous silicon as a sacrificial layer and deposited SiO2 as top oxide, while the other involves spin coated photoresist and hydrogen-silsesquioxane sourced SiOx. Both routes, leading to similar template structures, are demonstrated and discussed. Homoepitaxy of InP in both types of templates and the integration of an InAs horizontal heterojunction are demonstrated via MOCVD. An increase in growth rate with decreasing template length, increasing template width, and decreasing pattern density is observed. [ABSTRACT FROM AUTHOR]

The correlative use of X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and atom probe tomography (APT) allows the structure of substitutionally alloyed half-Heusler compound CoTi1-xFexSb to be characterized at the nanometer scale. For x = 0.2, 0.3, and 0.5, XRD patterns indicate an epitaxial cube-on-cube growth with no detectable secondary phases and are suggestive of high crystalline quality for all three of the films. This is supported using atomic resolution STEM images, which revealed the absence of secondary phases or polycrystalline regions, thus confirming the half-Heusler structure of the CoTi1-xFexSb films. For all three samples, the APT reconstructions were optimized to resolve the atomic planes in the [001] growth direction allowing the exploitation of reliable datasets. The presence of Fe-rich phases in the samples with Fe atoms sitting on Ti crystallographic sites is revealed. A strong phase separation is observed for x = 0.2 and x = 0.3 and is almost no longer observed for x = 0.5 in good agreement with the magnetic properties of the films. [ABSTRACT FROM AUTHOR]

Over the past few decades, significant progress has been made to manipulate thermal transport in solids. Most of the effort has focused on reducing the phonon mean free path through boundary scattering. Herein, we demonstrate that the phonon confinement effect can also be used as a tool for managing thermal transport in solids. We measured the thermal conductivities of 10–70-nm-thick In0.53Ga0.47As nanofilms and found that the thermal conductivities decrease as the film thickness decreases. However, the reasons for this reduction differ for films with different thicknesses. The thermal conductivity of the 30- and 70-nm-thick In0.53Ga0.47As nanofilms decreases because of severe phonon boundary scattering. Our analysis indicates that phonon confinement occurs in the 10- and 20-nm-thick In0.53Ga0.47As nanofilms, which modifies phonon dispersion leading to changes in the phonon group velocity and the Debye temperature. These experimental and theoretical results could help to elucidate the phonon confinement effect in nanomaterials as well as establish a platform for understanding nanoscale thermal physics. [ABSTRACT FROM AUTHOR]

The effects of post-growth annealing in ultrahigh vacuum (UHV) on the temperature-dependent transport properties of single-crystal, full-Heusler CoFe/MgO/Co2MnSi magnetic tunnel junctions (MTJs) grown by molecular beam epitaxy have been correlated with in-situ X-ray photoelectron spectroscopy (XPS) studies of the MgO/Co2MnSi interface. CoFe and MgO layers were grown on single-crystal Co2MnSi at room temperature and annealed post growth. The structures were found to be epitaxial and single-crystal before and after annealing as assessed by in-situ reflection high-energy electron diffraction (RHEED). While annealing has little effect on RHEED patterns, post-growth annealing at temperatures as low as 200°C has a dramatic effect on tunnel magnetoresistance and transport properties. XPS measurements conducted on MgO/Co2MnSi structures reveal the presence of interfacial Mn and Si oxides which form as a result of the e-beam deposition process used for MgO. Mn oxides are observed to be reduced upon UHV annealing with a corresponding migration of oxygen from the MgO/Co2MnSi interface into the MgO. In contrast to the case of Mn oxides, Si oxides were not significantly reduced following annealing at 300°C. Transport measurements on fabricated MTJs show an increase in the tunneling magnetoresistance ratio and a significant alteration in the interfacial electronic structure with increasing annealing temperature. The changes observed in transport are interpreted to result from a reduction in interfacial oxides and a corresponding reduction in oxygen vacancy defect density in the MgO, consistent with XPS results. [ABSTRACT FROM AUTHOR]

An understanding of the growth of (110) quantum wells (QWs) is of great importance to spin systems due to the observed long spin relaxation times. In this article, we report on the metamorphic growth and characterization of high mobility undoped InAs (110) QWs on GaAs (110) substrates. A low-temperature nucleation layer reduces dislocation density, results in tilting of the subsequent buffer layer and increases the electron mobility of the QW structure. The mobility varies widely and systematically (4000-16 000 cm2/Vs at room temperature) with deposition temperature and layer thicknesses. Low-temperature transport measurements exhibit Shubnikov de-Haas oscillations and quantized plateaus in the quantum Hall regime. [ABSTRACT FROM AUTHOR]

Heusler alloy films Co2MnGa grown on ScErAs/GaAs(001) substrates form stripe domains in the remnant state due to its large strain-induced out-of-plane anisotropy. The dynamics of the stripe domains has been investigated using a microstrip ferromagnetic resonance device in a broad frequency range (∼3–25 GHz). In both in-plane and out-of-plane measurement geometries, three modes were observed in fields less than the saturation field. The resulting frequency/field dispersion curve reveals that one mode has characteristics of an acoustic mode, the other of an optic mode, and with the third being nearly field independent. The acoustic mode continues into the saturated state, while the optic mode does not appear above saturation. The experimental data are analyzed using Wigen’s stripe domain theory. [ABSTRACT FROM AUTHOR]