This paper demonstrates the molecular beam epitaxial growth of metallic full-Heusler Ni2TiSn precipitates within semiconducting NiTiSn host matrix, and discusses the structural and chemical stability of this biphasic epitaxial composite and its influence on the electronic properties. Structural properties of the epitaxial films were characterized by in-situ reflection high-energy electron diffraction and ex-situ by X-ray diffraction and transmission electron microscopy. The results indicate the presence of a secondary phase with full Heusler ordering within half-Heusler host matrix. Both the parent half-Heusler and the secondary full-Heusler phases in the epitaxial films are strained from their bulk lattice parameters. Electronic properties show an increase in resistivity at low Ni excess.

B. Shojaei,1 P. J. J. O’Malley,2 J. Shabani,3 P. Roushan,2 B. D. Schultz,4 R. M. Lutchyn,5 C. Nayak,5 J. M. Martinis,2 and C. J. Palmstrom1,3,4,* 1Materials Department, University of California, Santa Barbara, California 93106, USA 2Department of Physics, University of California, Santa Barbara, California 93106, USA 3California NanoSystems Institute, University of California, Santa Barbara, California 93106, USA 4Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106, USA 5Microsoft Research, Station Q, University of California, Santa Barbara, California 93106, USA (Received 2 December 2015; published 1 February 2016)

The discovery of topological insulators, materials with bulk band gaps and protected cross-gap surface states in compounds such as Bi 2 Se 3 , has generated much interest in identifying topological surface states (TSSs) in other classes of materials. In particular, recent theoretical calculations suggest that TSSs may be found in half-Heusler ternary compounds. If experimentally realizable, this would provide a materials platform for entirely new heterostructure spintronic devices that make use of the structurally identical but electronically varied nature of Heusler compounds. Here we show the presence of a TSS in epitaxially grown thin films of the half-Heusler compound PtLuSb. Spin- and angle-resolved photoemission spectroscopy, complemented by theoretical calculations, reveals a surface state with linear dispersion and a helical tangential spin texture consistent with previous predictions. This experimental verification of topological behaviour is a significant step forward in establishing half-Heusler compounds as a viable material system for future spintronic devices.

A study of scattering mechanisms in gate tunable two dimensional electron gases confined to InAs/(Al,Ga)Sb heterostructures with varying interface roughness and dislocation density is presented. By integrating an insulated gate structure the evolution of the low temperature electron mobility and single-particle lifetime was determined for a previously unexplored density regime, 1011–1012 cm−2, in this system. Existing theoretical models were used to analyze the density dependence of the electron mobility and single particle lifetime in InAs quantum wells. Scattering was found to be dominated by charged dislocations and interface roughness. It was demonstrated that the growth of InAs quantum wells on nearly lattice matched GaSb substrate results in fewer dislocations, lower interface roughness, and improved low temperature transport properties compared to growth on lattice mismatched GaAs substrates.

We present a systematic study of the properties of room temperature deposited TiN films by varying the deposition conditions in an ultra-high-vacuum reactive magnetron sputtering chamber. By increasing the deposition pressure from 2 to 9 mTorr while keeping a nearly stoichiometric composition of Ti1−xNx (x = 0.5) without substrate heating, the film resistivity increases, the dominant crystal orientation changes from (100) to (111), grain boundaries become clearer, and the strong compressive in-plane strain changes to weak tensile in-plane strain. The TiN films absorb a high concentration of contaminants including hydrogen, carbon, and oxygen when they are exposed to air after deposition. With the target–substrate distance set to 88 mm the contaminant levels increase from ∼0.1% to ∼10% as the pressure is increased from 2 to 9 mTorr. The contaminant concentrations also correlate with in-plane distance from the center of the substrate and increase by roughly two orders of magnitude as the target–substrate distance is increased from 88 to 266 mm. These contaminants are found to strongly influence the properties of TiN thin films. For instance, the resistivity of stoichiometric films increases by around a factor of 5 as the oxygen content increases from 0.1% to 11%. These results strongly suggest that the energy of the sputtered TiN particles plays a crucial role in determining the TiN film properties, and that it is important to precisely control the energy of these particles to obtain high-quality TiN films. Superconducting coplanar waveguide resonators made from a series of nearly stoichiometric films grown at pressures from 2 to 7 mTorr show a substantial increase in intrinsic quality factor from ∼104 to ∼106 as the magnitude of the compressive strain decreases from nearly 3800 MPa to approximately 150 MPa and the oxygen content increases from 0.1% to 8%. Surprisingly, the films with a higher oxygen content exhibit lower loss, but care must be taken when depositing at room temperature to avoid nonuniform oxygen incorporation, which presents as a radially dependent resistivity and becomes a radially dependent surface inductance in the superconductor.

Epitaxial single crystal nanocomposites comprised of rare-earth arsenide nanoparticles embedded in GaAs (001) layers produce a larger change in lattice parameter than expected from the lattice parameters of relaxed films. Despite similar cubic structures and lattice parameters, elongation of the interfacial bond length between the two materials induces additional strain causing an expansion in the nanocomposite lattice. The interface bond length is material dependent with an average atomic layer spacing at the ErAs:GaAs interface of 1.9 Å while the spacing at the ScAs:GaAs interface is only 1.4 Å. Implications for lattice matching various single crystal epitaxial nanostructures in semiconductors are discussed.

Epitaxial single crystal films of praseodymium nickelate (PrNiO3) have been grown by molecular beam epitaxy on LaAlO3(001) substrates. In-situ electron diffraction and ex-situ X-ray diffraction techniques confirm an epitaxial relationship to the underlying substrate. Crystalline quality depends strongly on the substrate temperature and activated oxygen flux during growth with the highest quality films formed at 600 degrees C under high oxygen plasma fluxes. The metal-insulator transition for PrNiO3 films was suppressed as the crystalline quality of the layers was improved. (C) 2013 Elsevier B.V. All rights reserved.

The dependence of the electronic structure of ErSb nanoparticles embedded in GaSb(001) surfaces on particle size is investigated by in situ scanning tunneling microscopy and spectroscopy. By varying growth conditions, the planar dimensions and surface termination of ErSb nanoparticles can be controlled. As the deposition temperature is raised, ErSb nanoparticles become increasingly elongated along the <-110 > directions due to anisotropic surface diffusion. The local density of states is measured by tunneling point spectroscopy. ErSb nanoparticles were found to be semimetallic with no discernible band gap, despite predictions from finite potential quantum confinement calculations that suggest the smallest particles should become semiconducting. DOI: 10.1103/PhysRevB.87.035419

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Spin injection efficiency is shown to strongly depend on the interfacial structure between Fe contacts and ${\text{Al}}_{\text{x}}{\text{Ga}}_{1\ensuremath{-}\text{x}}\text{As}$ in spin-based light emitting diodes. Both the magnitude and sign of the injected carriers are dependent on the atomic structure of the contacts and can be controlled through changes in temperature both during and following growth. We propose that the observed dependence is due to phase formation resulting from Fe/GaAs interfacial reactions. This proposed mechanism is consistent with electronic structure calculations, which show that thin layers of ${\text{DO}}_{3}$ ${\text{Fe}}_{3}\text{Ga}$ at the Fe/GaAs interface can produce the observed sign reversals in the spin polarization of injected carriers.

Photoemission work by Nakamura et al. [J. Appl. Phys. 101, 043516 (2007)] on the GaAs(111)B(√19×√19)R23° surface shows that the surface region contains three different types of As atoms and two different types of Ga atoms. The outstanding feature of their data is the presence of Ga atoms in the outermost layer of the reconstruction, which they conclude is inconsistent with published models. However, there are two published models, which were not identified in the paper, that contain these top-layer Ga atoms. Additionally, one of the two models also contains three distinct types of As surface atoms and two distinct types of Ga surface atoms as identified experimentally by Nakamura et al. [J. Appl. Phys. 101, 043516 (2007)].

Interfacial reactions between epitaxial films of Fe and GaAs were examined using Rutherford backscattering spectrometry, x-ray diffraction, and transmission electron microscopy as a function of postgrowth annealing conditions. At 450°C, two stable binary phases were observed at the Fe∕GaAs(001) interface with DO3Fe3Ga forming near the Fe interface and Fe2As forming near the GaAs interface. The diffusion rate of Fe in the reacted interface was found to be of similar magnitude to that of Ga and As resulting in a rough and intermixed interfacial region.

The magnetization depth profiles of three FeCo/GaAs samples grown at different temperatures and measured before and after annealing were obtained using polarized neutron reflectometry. Prior to annealing, the sample grown at 95 °C had the thickest magnetically degraded interfacial region between the FeCo film and the GaAs substrate. For the sample grown at −15 °C, the magnetic interface was sharp. For all samples, annealing promoted thicker interfacial regions with suppressed magnetization and distinct boundaries with the adjoining (FeCo or GaAs) material. Thus, the magnetic structure of the FeCo/GaAs interfacial region was very sensitive to the conditions of growth and annealing.

The successful molecular beam epitaxial growth of single crystalline ScxEr1−xSb compounds on InAs(1 0 0) substrates has been demonstrated. Ex situ high-resolution X-ray diffraction and Rutherford backscattering spectrometry with ion channeling studies indicate high crystalline quality. The surface reconstruction was monitored in situ by reflection high-energy electron diffraction (RHEED) during the growth and low-energy electron diffraction post growth. The ScxEr1−xSb(1 0 0) surface exhibited a mixed (1×4)/(4×1) reconstruction. The RHEED patterns and the RHEED intensity oscillations during the growth are consistent with an embedded growth mechanism for the initial ScxEr1−xSb growth on InAs.

In situ x-ray photoelectron spectroscopy and scanning tunneling microscopy were combined to examine the formation of the reacted region at the Mn∕GaAs(001) interface during deposition. Thin films of Mn were grown on GaAs c(4×4) surfaces by molecular beam epitaxy at substrate temperatures of 40, 95, and 250°C. The attenuations of the Ga and As photoemission peak intensities as a function of Mn deposition indicate the formation of a reacted layer at the interface with an average composition of Mn0.6Ga0.2As0.2, and submonolayer coverages of As were found to segregate to the sample surface independent of the growth temperature. The extent of the Mn–Ga–As interfacial reactions that occur during the growth of Mn on GaAs strongly depends upon the growth temperature. At growth temperatures of 95°C and below, the interfacial reacted layer reaches a thickness that is sufficient to limit any additional diffusion of Mn into the GaAs. During growth at 250°C the rate of diffusion is higher and the interfacial reacted layer continues to increase in thickness as the rate of diffusion remains similar to the growth rate for depositions ranging up to hundreds of angstroms.