Gavin Richard Bell

The University of Warwick · Department of Physics

Oxide-free surfaces of polycrystalline Cu are prepared using acetic acid etching after chemical-mechanical polishing. UV ozone treatment is shown to increase the work function of the cleaned Cu by up to 0.5 eV. There is also a large reduction in quantum efficiency at 265 nm. Cu sheet can be easily masked from ozone exposure by Si or glass, meaning...

The photoemission quantum efficiency of InSb and InP with clean (001) surfaces.

The magneto-electronic properties of ferromagnetic MnSb grown by molecular beam epitaxy can be dominated by the presence of a surface state in the minority spin bandgap when the surface is Sb-terminated. The material resistivity is 120 µΩ.cm at 295 K, and although this is determined by the majority spin population, the anisotropic magnetoresistance...

The construction and calibration of a high vacuum system for thin film growth and in situ quantum efficiency (QE) measurement are described. Surface cleaning by in situ argon ion sputtering and annealing is supported. The QE measurement is based on an external 265 nm LED and in situ positively biased collector grid. The system is applied to two met...

Undoped and In‐doped ZnSb thin films are deposited on rigid glass and flexible polyimide (Kapton) by physical vapor deposition. Detailed structural and chemical characterization is performed along with measurement of electrical and optical properties. These properties are very similar for films on glass and Kapton. Flexible ZnSb films show remarkab...

One route to the improvement of thermoelectric material performance is nanostructuring. Herein, BiSb thin films are grown by molecular‐beam epitaxy on two types of InP(001) substrate: flat epiready wafer, and nano‐patterned by ion bombardment and annealing (IBA). The effects of IBA on InP substrate are presented, and the structural and electrical p...

The performance expected from the next generation of electron accelerators is driving research into photocathode technology as this fundamentally limits the achievable beam quality. The performance characteristics of a photocathode are most notably: normalised emittance, brightness and energy spread. Ultra–thin oxide films on metal substrates have...

Metal photocathodes are widely utilized as electron sources for particle accelerators for their ease of use, high durability, and fast response time. However, the high work function (WF) and low quantum efficiency (QE) typically observed in metals necessitate the use of high power deep UV lasers. Metal oxide ultra-thin films on metals offer a route...

While the phenomenon of metal substrate adatom incorporation into molecular overlayers is generally believed to occur in several systems, the experimental evidence for this relies on the interpretation of scanning tunneling microscopy (STM) images, which can be ambiguous and provides no quantitative structural information. We show that surface X-ra...

Antimony ultra-thin films in tensile strain are grown on InAs(111)B substrates and studied in situ using surface X-ray diffraction. The detailed atomic structures of two highly crystalline Sb(0001) films are derived, with thicknesses of 19 and 4 bilayers. Features considered in structural modelling include interfacial intermixing, surface roughness...

While the phenomenon of metal substrate adatom incorporation into molecular overlayers is generally believed to occur in several systems, the experimental evidence for this relies on the interpretation of scanning tunnelling microscopy (STM) images, which can be ambiguous and provides no quantitative structural information. We show that surface X-...

New alloys from the III-V family are presented. Undoped (0001)-oriented AlGaNAs epitaxial layers with 3%–16% Al and with constant As concentration equals 0.6% were grown by molecular beam epitaxy. Photoelectron spectroscopy, and secondary ion mass spectrometry confirm As incorporation into AlGaN and show that a small amount of As causes valence ban...

Contaminants in water were studied using ultraviolet absorption with light emitting diode and deuterium lamp sources, and a thresholding detector. The absorption spectra of potassium hydrogen pthalate, clothianidin, tryptophan, thiamethoxam, uric acid and metaldehyde were obtained in the range 200–360 nm. Only metaldehyde was not suitable for detec...

Band bending and Fermi level pinning at GaN(0001) surfaces have proved controversial despite their fundamental importance. By combining Kelvin probe, surface photovoltage and X-ray photoemission measurements, we clarify how the Fermi level pinning affects the band bending for n- and p-type GaN(0001). The presence of two different mid-gap pinning le...

Spin-coating of poly(ethylenimine) (PEI) has been used to reduce the work function of GaAs (001), (110), (111)A and (111)B. The magnitude of the reduction immediately after coating varies significantly from 0.51 eV to 0.69 eV and depends on the surface crystal face, on the GaAs bulk doping and on the atomic termination of the GaAs. For all samples,...

Smooth CrSb(0001) films have been grown by molecular beam epitaxy on MnSb(0001) – GaAs(111) substrates. CrSb(0001) shows (2 × 2), triple domain (1 × 4) and (√3×√3)R30° reconstructed surfaces as well as a (1 × 1) phase. The dependence of reconstruction on substrate temperature and incident fluxes is very similar to MnSb(0001).

A “hybrid” surface growth mode is studied by in situ scanning tunneling microscopy and ex situ synchrotron X‐ray diffraction. Heteroepitaxial growth of MnSb(0001) on GaAs(111) leads to the direct formation of islands, but the islands themselves grow in a local layer‐by‐layer fashion, having low aspect ratio and flat tops. Strained and relaxed islan...

We show direct evidence for the impact of Heusler/semiconductor interfaces atomic structure on the spin transport signals in semiconductor-based lateral spin-valve (LSV) devices. Based on atomic scale Z-contrast scanning transmission electron microscopy and energy dispersive x-ray spectroscopy we show that atomic order/disorder of Co2FeAl0.5Si0.5 (...

MnSb layers have been grown on InxGa1-xAs(1 1 1) A virtual substrates using molecular beam epitaxy (MBE). The effects of both substrate temperature (Tsub) and Sb/Mn beam flux ratio (JSb/Mn) were investigated. The surface morphology, layer and interface structural quality, and magnetic properties have been studied for a 3 × 3 grid of Tsub and JSb/Mn...

The depth-resolved chemical structure and magnetic moment of Co₂FeAl_0.5Si_0.5 thin films grown on Si(111) have been determined using x-ray and polarized neutron reflectometry. Bulk-like magnetization is retained across the majority of the film, but reduced moments are observed within 45 Å of the surface and in a 25 Å...

• Download high-res image (224KB) • Download full-size image Dr. Gavin Bell is a Reader in the Department of Physics at the University of Warwick, UK. His research interests combine epitaxy and nano-materials growth with surface/interface science. He works on semiconductors and thin-film magnetic/topological materials for spintronics, photocathode...

Ammonia borane (NH3:BH3) is commonly used as a stoichiometric source of nitrogen and boron for the growth of hexagonal boron nitride (h-BN) by chemical vapour deposition (CVD). We use in situ gas analysis by mass spectrometry to investigate the active chemical components that evolve when an ammonia borane source is heated, and study how these compo...

The lateral ordering of arrays of self-assembled InAs-GaAs quantum dots (QDs) has been quantified as a function of growth rate, using the Hopkins-Skellam index (HSI). Coherent QD arrays have a spatial distribution which is neither random nor ordered, but intermediate. The lateral ordering improves as the growth rate is increased and can be explaine...

Atomic resolution scanning transmission electron microscopy reveals the presence of an antiphase boundary in the half-metallic Co2Fe(Al,Si) full Heusler alloy. By employing the density functional theory calculations, we show that this defect leads to reversal of the sign of the spin-polarization in the vicinity of the defect. In addition, we show t...

Atomic resolution scanning transmission electron microscopy reveals the presence of an antiphase boundary in the half-metallic Co2Fe(Al,Si) full Heusler alloy. By employing the density functional theory calculations, we show that this defect leads to reversal of the sign of the spin-polarization in the vicinity of the defect. In addition, we show t...

Halfmetal-semiconductor interfaces are crucial for hybrid spintronic devices. Atomically sharp interfaces with high spin polarisation are required for efficient spin injection. In this work we show that thin film of half-metallic full Heusler alloy Co2FeSi0.5Al0.5 with uniform thickness and B2 ordering can form structurally abrupt interface with Ge...

The ferromagnetic material MnSb can exist in two polymorphs in epitaxial thin‐film form, namely niccolite n‐MnSb and cubic c‐MnSb. We investigate the behavior of these polymorphs using grazing incidence depth‐dependent in‐plane X‐ray diffraction. The in‐plane lattice parameter evolution of a nominal 3000 Å thin film reveals a small near‐surface com...

We have investigated the growth by molecular beam epitaxy (MBE) of GaAs on MnSb. MnSb epilayers on GaAs(111), GaAs(001) and of thickness 50 nm were used as substrates for GaAs films. The MBE growth of GaAs was monitored in situ using synchrotron X‐ray diffraction and reflection high energy electron diffraction. We show data on strain relaxation and...

Bulk single crystal and polycrystalline samples of NiSb, MnSb, and NiMnSb have been grown and characterized. The lattice parameter of NiMnSb was found to be 5.945 ± 0.001 Å, around 0.25% larger than previous reports. The surface preparation of these materials was investigated using x-ray photoelectron spectroscopy. Wet etching with HCl and argon io...

Multilayer structures comprised of a central layer of the binary transition metal pnictide MnSb and outer layers of standard semiconductors have been grown using molecular beam epitaxy (MBE). The growth of the GaAs overlayers was characterized using a combination of reflection high energy electron diffraction (RHEED) and in situ surface x-ray diffr...

The simultaneous use of symmetric and grazing incidence X-ray diffraction in the characterisation of a pnictide material is demonstrated using MnSb epi-layers grown on compound semiconductor substrates. This combination of diffraction geometries enables a comprehensive determination of the lattice parameters and complex structural behaviour of MnSb...

The deposition of Mn on to different III–V surfaces can generates both self-assembled nanostructures and alloyed surface reconstructions. These are analyzed by electron diffraction, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) for different InSb and GaAs surfaces. The overall behavior can be explained by mass transport and...

We demonstrate a new mechanism in the early stages of sub-monolayer epitaxial island growth, using Monte Carlo simulations motivated by experimental observations on the growth of graphene on copper foil. In our model, the substrate is "dynamically rough", by which we mean (i) the interaction strength between Cu and C varies randomly from site to si...

We show that Co2FeAl0.5Si0.5film deposited on Si(111) has a single crystal structure and twin related epitaxial relationship with the substrate. Sub-nanometer electron energy loss spectroscopy shows that in a narrow interface region there is a mutual inter-diffusion dominated by Si and Co. Atomic resolution aberration-corrected scanning transmissio...

Using density functional theory calculations motivated by aberration-corrected electron microscopy, we show how the atomic structure of a fully epitaxial Co2MnSi/Ag interfaces controls the local spin-polarization. The calculations show clear difference in spin-polarization at Fermi level between the two main types: bulk-like terminated Co/Ag and Mn...

Covalent functionalization of graphene offers opportunities for tailoring its properties and is an unavoidable consequence of some graphene synthesis techniques. However, the changes induced by the functionalization are not well understood. By using atomic sources to control the extent of the oxygen and nitrogen functionalization, we studied the ev...

Summary abstract of recent work on Mn interactions with reconstructed GaAs and InSb surfaces.

The transition between the a(1 × 3) and c(4 × 4) surface reconstructions of InSb(0 0 1) has been carefully monitored by reflection high energy electron diffraction as a function of temperature and Sb2 flux, without incident In flux. Arrhenius-like behaviour is observed across the whole range of Sb2 fluxes and temperatures, allowing accurate interna...

slides from invited talk @ photoemission facility meeting 2009 about surface preparation of MnSb and NiMnSb

extended abstract - SemiconNano2015 presentation

extended abstract SemiconNano2015 invited talk

extended abstract SemiconNano2015 invited talk

extended abstract for SemiconNano2015 invited talk

extended abstract for SemiconNano2015 invited talk

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...

We investigate the origin of the spatial regularity of arrays of InAs quantum dots (QDs) grown on GaAs(001). The Hopkins-Skellam index (HSI) is used with a newly developed calculation algorithm to quantify the spatial regularity both of QDs and of nm-sized surface reconstruction territories (SRTs) present in the In x Ga1− x As wetting layer prior...

The cubic polymorph of the binary transition metal pnictide (TMP) MnSb, c-MnSb, has been predicted to be a robust half-metallic ferromagnetic (HMF) material with minority spin gap ≳1 eV. Here, MnSb epilayers are grown by molecular beam epitaxy (MBE) on GaAs and In0.5Ga0.5As(111) substrates and analyzed using synchrotron radiation X-ray diffraction....

The deposition of Mn on to reconstructed InSb and GaAs surfaces, without coincident As or Sb flux, has been studied by reflection high energy electron diffraction, atomic force microscopy and scanning tunnelling microscopy. On both Ga- and As-terminated GaAs(0 0 1), (2 × n) Mn-induced reconstruction domains arise with n = 2 for the most well ordere...

This paper presents a Resonance Raman spectroscopy study of ~1 nm diameter HgTe nanowires formed inside single walled carbon nanotubes by melt infiltration. Raman spectra have been measured for ensembles of bundled filled tubes, produced using tubes from two separate sources, for excitation photon energies in the ranges 3.39 to 2.61eV and 1.82 to 1...

In common with rocksalt-type alkali halide phases and also semiconductors such as GeTe and SnTe, SnSe forms all-surface two atom-thick low dimensional crystals when encapsulated within single walled nanotubes (SWNTs) with diameters below ∼1.4 nm. Whereas previous density functional theory (DFT) studies indicate that optimised low-dimensional trigon...

Gold nano-clusters were grown on chemically modified graphene by direct sputter deposition. Transmission electron microscopy of the nano-clusters on these electron-transparent substrates reveals an unusual bimodal island size distribution (ISD). A kinetic Monte Carlo model of growth incorporating a size-dependent cluster mobility rule uniquely repr...

Molecular beam epitaxial growth of ferromagnetic MnSb(0001) has been achieved on high quality, fully relaxed Ge(111)/Si(111) virtual substrates grown by reduced pressure chemical vapor deposition. The epilayers were characterized using reflection high energy electron diffraction, synchrotron hard X-ray diffraction, X-ray photoemission spectroscopy,...

A good agreement was obtained in the evolution of island densities between Scanning Tunneling Microscope observations and Monte Carlo simulations in GaAs(001)-(2x4) homoepitaxy.

Angle‐resolved photoemission spectroscopy (ARPES) and X‐ray photoemission spectroscopy have been used to characterise epitaxially ordered graphene grown on copper foil by low‐pressure chemical vapour deposition. A short vacuum anneal to 200 °C allows observation of ordered low energy electron diffraction patterns. High quality Dirac cones are measu...

Graphene 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 coppe...

Thin films of NiSb(0001) have been grown using molecular beam epitaxy on GaAs(111)B substrates and characterized with a variety of structural and surface-specific techniques supported by density functional theory calculations. Several differences were observed between NiSb and the more widely studied MnSb. A new (4×4) surface reconstruction was see...

Depth-dependent magnetism in MnSb(0001) epitaxial films has been studied by combining experimental methods with different surface specificities: polarized neutron reflectivity, x-ray magnetic circular dichroism (XMCD), x-ray resonant magnetic scattering and spin-polarized low energy electron microscopy (SPLEEM). A native oxide ∼4.5 nm thick covers...

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...

Epitaxial films including bulk-like 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 spin polarization as a function of reduced magnetization for the half-metals NiMnSb and cubic MnSb....

The authors have investigated the initial growth of MnAs layers by step-by-step epitaxy on GaAs(110) and GaAs(001). On both surfaces, MnAs nanocrystals developed as the initial stage of MnAs layer formation. Surprisingly, an ultrahigh density (∼1×1012 cm-2) of the nanocrystals with a height of ∼5 nm and a size of ∼20 nm appeared on GaAs(110). On di...

Organopalladium species ({Pd}) immobilized on an Sterminated GaAs substrate (S/GaAs) effectively catalyzes C-C bond formation in the Mizoroki-Heck reaction with cycle durability. However, the immobilizing mechanism of {Pd} is unknown. In this study, we deposited Pd(OCOCH3)2 on S/GaAs in two different methods, namely dry-physical vapor-deposition an...

The experimental aspects of rapid-quench scanning tunneling microscopy are discussed. In particular, the effects of sample quenching are investigated in atomic-scale studies of the molecular beam epitaxial growth of GaAs. Implications for the study of the heteroepitaxial system InAs-GaAs are discussed.

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 f...

Organopalladium molecules, such as Pd ( C H ₃ C O O )₂ ({Pd}), immobilized on the S-terminated GaAs(001), termed GaAs–S–{Pd} have high catalytic activity and cycle durability in the Mizoroki–Heck reaction. It is thought that the presence of Ga–S bonds in the single atomic layer S-termination is essential for these catalytic pr...

The bulk and surface electronic structure of In2O3 has proved controversial, prompting the current combined experimental and theoretical investigation. The band gap of single-crystalline In2O3 is determined as 2.93±0.15 and 3.02±0.15eV for the cubic bixbyite and rhombohedral polymorphs, respectively. The valence-band density of states is investigat...

The bulk and surface electronic structure of In 2 O 3 has proved controversial, prompting the current combined experimental and theoretical investigation. The band gap of single-crystalline In 2 O 3 is determined as 2.93 0.15 and 3.02 0.15 eV for the cubic bixbyite and rhombohedral polymorphs, respectively. The valence-band density of states is inv...

The stoichiometry, microstructure and surface composition of MnSb have been investigated using X-ray photoelectron spectroscopy, electron diffraction and microscopy. Epitaxially grown samples were exposed to ambient air for several weeks and methods for preparing clean, stoichiometric and smooth surfaces were investigated. Air-stored sample surface...

Organopalladium molecules immobilized on the S-terminated GaAs(001), termed GaAs-S-{Pd}, have high catalytic activity and stability in the Heck reaction. It is thought that the presence of Ga-S bonds in the single atomic layer S-termination is essential for these catalytic properties despite the much higher thickness (~100 nm) of the organopalladiu...

The surface reconstruction periodicities of epitaxially grown MnSb (0 0 0 1) and (11¯01) have been determined under a range of beam fluxes and substrate temperatures using reflection high energy electron diffraction. A number of previously unreported surface reconstructions were observed. On MnSb(0 0 0 1), reconstructions ordered from Mn rich to Sb...

Dynamic images of InAs quantum dots (QDs) formation are obtained using a unique scanning tunneling microscope (STM) placed within the growth chamber. These images are interpreted with the aid of kinetic Monte Carlo (kMC) simulations of the QD nucleation process. Alloy fluctuations in the InGaAs wetting layer prior to QD formation assist in the...

The growth of InAs on GaAs(0 0 1) 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 thei...

Growth by molecular beam epitaxy of MnSb on InP(0 0 1) has been studied over a range of substrate temperatures (250 425C) and Sb:Mn flux ratios (2:1 to 8:1). Two growth phases were observed, their predominance mainly dependent on substrate temperature. These have been identified as MnSb (found preferentially at low temperatures) and InSb (found pre...

The properties of InAs quantum dot formation on GaAs(001) using STM within a MBE growth chamber were described. InAs deposited onto an As-terminated GaAs(001) initially forms a two-dimensional (2D) wetting layer (WL). The overall morphology revealed by STMBE is very similar to the results of quenched mode STM. It was shown that it is possible to es...

The plasmon excitations of the degenerate free-electron gas in the surface space-charge region of InSb have been studied using high-resolution electron-energy-loss spectroscopy. Semiclassical dielectric theory simulations of the energy-loss spectra have been performed using the hydrodynamic and Boltzmann-Mermin models. While both analyses include t...

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.

X-ray photoemission spectroscopy has been used to study Ga0.933Mn0.067As surface cleaning using atomic hydrogen and ex situ HCl etching. Both cleaning techniques are found to reduce the Ga and As surface oxides. Quantitative compositional analysis indicates that a Mn rich surface contamination layer of average thickness 3 4 Å remains in both cases....

Reflection high-energy electron diffraction and scanning tunnelling microscopy have been used to study the cleaning of the native oxide from an epi-ready GaAs(0 0 1) substrate using thermally cracked atomic hydrogen (AH). Cleaning at 400 °C results in the formation of a (2 × 4) reconstructed As-terminated surface. A short anneal (5 min) under an As...

The surface reconstructions of InxGa1−xAs alloys grown by molecular beam epitaxy on the (0 0 1) surfaces of GaAs and InAs have been studied by reflection high-energy electron diffraction and scanning tunnelling microscopy. A surface phase diagram is presented for the nominally strain-free alloy as a function of substrate temperature and alloy compo...

Scanning tunneling microscopy (STM) has proved to be an invaluable tool for probing epitaxial growth phenomena in general, and 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...

Arrays of InAs quantum dots (QDs) have been studied using in situ scanning tunneling microscopy (STM) during their growth by molecular beam epitaxy on GaAs(0 0 1). At a substrate temperature of 400 °C under As4 flux, both the QDs and the underlying step-terrace structure of the wetting layer (WL) are found to be static, with neither step-flow nor Q...

Scanning tunneling microscopy has been used to monitor the growth by molecular beam epitaxy of InAs quantum dots (QDs) on GaAs(0 0 1) at and near the critical coverage (θcrit). Direct evidence is obtained for the existence of small irregular 3D islands of height 6–12 Å (2–4 ML) which contain ∼150 atoms or more. These features develop rapidly (withi...