Gavin R. Bell

Surface morphology evolution during the overgrowth of large InAs–GaAs quantum dots

The effects of GaAs overgrowth on the structural properties of large low-growth-rate InAs quantum dots (LGR-QDs) grown on GaAs(001) are examined using in situ scanning tunneling microscopy. Strongly anisotropic surface diffusion produces a characteristic valley-ridge structure above the LGR-QDs and the surface is not planarized even after a cap thickness >400 A. The evolution of surface morphology proceeds very differently to the case of smaller conventional growth rate QDs capped under the same conditions, due to the different initial strain states of the QDs.

Physical Vapor Deposition of Metal Nanoparticles on Chemically Modified Graphene: Observations on Metal–Graphene Interactions

The growth of metallic nanoparticles formed on chemically modified graphene (CMG) by physical vapor deposition is investigated. Fine control over the size (down to ∼1.5 nm for Au) and coverage (up to 5 × 10(4) μm(-2) for Au) of nanoparticles can be achieved. Analysis of the particle size distributions gives evidence for Au nanocluster diffusion at room temperature, while particle size statistics differ clearly between metal deposited on single- and multilayer regions. The morphology of the nanoparticles varies markedly for different metals (Ag, Au, Fe, Pd, Pt, Ti), from a uniform thin film for Ti to a droplet-like growth for Ag. A simple model explains these morphologies, based only on consideration of 1) the different energy barriers to surface diffusion of metal adatoms on graphene, and 2) the ratio of the bulk cohesive energy of the metal to the metal-graphene binding energy. Understanding these interactions is important for controlling nanoparticle and thin-film growth on graphene, and for understanding the resultant charge transfer between metal and graphene.

Weak mismatch epitaxy and structural feedback in graphene growth on copper foil

AbstractGraphene growth by low-pressure chemical vapor deposition on low cost copper foils shows great promise for large scale applications. It is known that the local crystallography of the foil influences the graphene growth rate. Here we find an epitaxial relationship between graphene and copper foil. Interfacial restructuring between graphene and copper drives the formation of (n10) facets on what is otherwise a mostly Cu(100) surface, and the facets in turn influence the graphene orientations from the onset of growth. Angle resolved photoemission shows that the electronic structure of the graphene is decoupled from the copper indicating a weak interaction between them. Despite this, two preferred orientations of graphene are found, ±8° from the Cu[010] direction, creating a non-uniform distribution of graphene grain boundary misorientation angles. Comparison with the model system of graphene growth on single crystal Cu(110) indicates that this orientational alignment is due to mismatch epitaxy. Despite the differences in symmetry the orientation of the graphene is defined by that of the copper. We expect these observations to not only have importance for controlling and understanding the growth process for graphene on copper, but also to have wider implications for the growth of two-dimensional materials on low cost metal substrates.

Cubic MnSb : epitaxial growth of a predicted room temperature half-metal

Epitaxial films including bulklike cubic and wurtzite polymorphs of MnSb have been grown by molecular beam epitaxy on GaAs via careful control of the Sb4/Mn flux ratio. Nonzero-temperature density functional theory was used to predict ab initio the half-metallicity of the cubic polymorph and compare its spin polarization as a function of reduced magnetization with that of the well known half-metal NiMnSb. In both cases, half-metallicity is lost at a threshold magnetization reduction, corresponding to a temperature T* 350 K, making epitaxial cubic MnSb a promising candidate for efficient room temperature spin injection into semiconductors.

Spin transport in germanium at room temperature

Spin-dependent transport is investigated in a Ni/Ge/AlGaAs junction with an electrodeposited Ni contact. Spin-polarised electrons are excited by optical spin orientation and are subsequently used to measure the spin dependent conductance at the Ni/Ge Schottky interface. We successfully demonstrate electron spin transport and electrical extraction from the Ge layer at room temperature.

Accumulation layer profiles at InAs polar surfaces

High resolution electron energy loss spectroscopy, dielectric theory simulations, and charge profile calculations have been used to study the accumulation layer and surface plasmon excitations at the In-terminated (001)-(4×1) and (111)A-(2×2) surfaces of InAs. For the (001) surface, the surface state density is 4.0±2.0×1011 cm−2, while for the (111)A surface it is 7.5±2.0×1011 cm−2, these values being independent of the surface preparation procedure, bulk doping level, and substrate temperature. Changes of the bulk Fermi level with temperature and bulk doping level do, however, alter the position of the surface Fermi level. Ion bombardment and annealing of the surface affect the accumulation layer only through changes in the effective bulk doping level and the bulk momentum scattering rate, with no discernible changes in the surface charge density.

van der Waals epitaxy of monolayer hexagonal boron nitride on copper foil: growth, crystallography and electronic band structure

We investigate the growth of hexagonal boron nitride (h-BN) on copper foil by low pressure chemical vapour deposition (LP-CVD). At low pressure, h-BN growth proceeds through the nucleation and growth of triangular islands. Comparison between the orientation of the islands and the local crystallographic orientation of the polycrystalline copper foil reveals an epitaxial relation between the copper and h-BN, even on Cu(100) and Cu(110) regions whose symmetry is not matched to the h-BN. However, the growth rate is faster and the islands more uniformly oriented on Cu(111) grains. Angle resolved photoemission spectroscopy measurements reveal a well-defined band structure for the h-BN, consistent with a band gap of 6 eV, that is decoupled from the copper surface beneath. These results indicate that, despite a weak interaction between h-BN and copper, van der Waals epitaxy defines the long range ordering of h-BN even on polycrystalline copper foils and suggest that large area, single crystal, monolayer h-BN could be readily and cheaply produced.

Passivation and reconstruction-dependent electron accumulation at sulphur treated InAs(0 0 1) surfaces

The effects of in situ sulphur passivation on the electronic properties of n-type InAs(0 0 1) have been studied using Xray photoemission spectroscopy and high resolution electron energy loss spectroscopy coupled with space-charge layer calculations. Surfaces passivated by sulphur dosing followed by arsenic capping were annealed in vacuum to progressively remove the protective layers. For disordered surfaces with a sulphur coverage of almost 2 monolayers (ML), complex surface plasmon modes were observed due to strong electron accumulation at the surface, with downward band bending around 600 meV. For (2 1) reconstructed surfaces (sulphur coverage <1 ML), the band bending dropped to 325 meV. A 375 C anneal was sufficient to remove all sulphur and regain a clean (4 1) indium-terminated surface with 200 meV downward band bending. We discuss the reconstruction-dependent surface accumulation and some aspects of electrical passivation of surfaces. 2002 Elsevier Science B.V. All rights reserved.

Heteroepitaxial growth of InAs on GaAs(001) by in situ STM located inside MBE growth chamber

The growth of InAs on GaAs(001) is of great interest primarily due to the self-assembly of arrays of quantum dots (QDs) with excellent opto-electronic properties. However, a basic understanding of their spontaneous formation is lacking. Advanced experimental methods are required to probe these nanostructures dynamically in order to elucidate their growth mechanism. Scanning tunneling microscopy (STM) has been successfully applied to many GaAs-based materials grown by molecular beam epitaxy (MBE). Typical STM-MBE experiments involve quenching the sample and transferring it to a remote STM chamber under arsenic-free ultra-high vacuum. In the case of GaAs-based materials grown at substrate temperatures of 400-600^oC, operating the STM at room temperature ensures that the surface is essentially static on the time scale of STM imaging. To attempt dynamic experiments requires a system in which STM and MBE are incorporated into one unit in order to scan in situ during growth. Here, we discuss in situ STM results from just such a system, covering both QDs and the dynamics of the wetting layer.

Composition profiles of InAs–GaAs quantum dots determined by medium-energy ion scattering

The composition profile along the [001] growth direction of low-growth-rate InAs–GaAs quantum dots (QDs) has been determined using medium-energy ion scattering (MEIS). A linear profile of In concentration from 100% In at the top of the QDs to 20% at their base provides the best fit to MEIS energy spectra.