James J. Mudd

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.

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.

Effect of oxygen and nitrogen functionalization on the physical and electronic structure of graphene

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 evolution in the structure and properties at the atomic scale. Atomic oxygen reversibly introduces epoxide groups whilst, under similar conditions, atomic nitrogen irreversibly creates diverse functionalities including substitutional, pyridinic, and pyrrolic nitrogen. Atomic oxygen leaves the Fermi energy at the Dirac point (i.e., undoped), whilst atomic nitrogen results in a net n-doping; however, the experimental results are consistent with the dominant electronic effect for both being a transition from delocalized to localized states, and hence the loss of the signature electronic structure of graphene.

Optical absorption by dilute GaNSb alloys: Influence of N pair states

The optical properties of GaNSb alloys with N contents of up to 2.5% have been investigated at room temperature using infrared absorption spectroscopy. The evolution of the absorption onsets with N content has been described using a three level band anticrossing model of the N localized states interactions with the GaSb conduction band. This approach includes the effect of N pair states, which is critical to reproduce the observed optical properties. This confirms theoretical predictions that N pair states have a more pronounced effect on the band dispersion in GaNSb than in GaNAs.

Heteroepitaxial Growth of Ferromagnetic MnSb(0001) Films on Ge/ Si(111) Virtual Substrates

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, and magnetometry. The surface reconstructions, magnetic properties, crystalline quality, and strain relaxation behavior of the MnSb films are similar to those of MnSb grown on GaAs(111). In contrast to GaAs substrates, segregation of substrate atoms through the MnSb film does not occur, and alternative polymorphs of MnSb are absent.

Pinning effect on the band gap modulation of crystalline BexZn1−xO alloy films grown on Al2O3(0001)

We have investigated the influence of Be concentration on the microstructure of BexZn1−xO ternary films (from x = 0 to 0.77), grown on Al2O3(0001) substrates using radio-frequency co-sputtering. With increasing Be concentration, the (0002) X-ray diffraction peak shows a systematic shift from 33.86° to 39.39°, and optical spectroscopy shows a blue-shift of the band gap from 3.24 to beyond 4.62 eV towards the deep UV regime, indicating that Be atoms are incorporated into the host ZnO lattice. During the band-gap modulation, structural fluctuations (e.g. phase separation and compositional fluctuation of Be) in the ternary films were observed along with a significant change in the mean grain size. X-ray photoelectron spectroscopy indicates higher concentrations of metallic Be states found in the film with the smaller grain size. Correlation between these two observations indicates that Be segregates to near grain boundaries. A model structure is proposed through simulation, where an increase in grain growth driving force dominates over the Be particle pinning effect. This leads to further coalescence of grains, reactivation of grain growth, and the uniform distribution of Be composition in the BexZn1−xO alloy films.

Recrystallization of highly-mismatched BexZn1-xO alloys:formation of a degenerate interface

We investigate the effect of thermally induced phase transformations on a metastable oxide alloy film, a multiphase Be(x)Zn(1-x)O (BZO), grown on Al2O3(0001) substrate for annealing temperatures in the range of 600-950 °C. A pronounced structural transition is shown together with strain relaxation and atomic redistribution in the annealed films. Increasing annealing temperature initiates out-diffusion and segregation of Be and subsequent nucleation of nanoparticles at the surface, corresponding to a monotonic decrease in the lattice phonon energies and band gap energy of the films. Infrared reflectance simulations identify a highly conductive ZnO interface layer (thicknesses in the range of ≈ 10-29 nm for annealing temperatures ≥ 800 °C). The highly degenerate interface layers with temperature-independent carrier concentration and mobility significantly influence the electronic and optical properties of the BZO films. A parallel conduction model is employed to determine the carrier concentration and conductivity of the bulk and interface regions. The density-of-states-averaged effective mass of the conduction electrons for the interfaces is calculated to be in the range of 0.31 m0 and 0.67 m0. A conductivity as high as 1.4 × 10(3) S · cm(-1) is attained, corresponding to the carrier concentration n(Int) = 2.16 × 10(20) cm(-3) at the interface layers, and comparable to the highest conductivities achieved in highly doped ZnO. The origin of such a nanoscale degenerate interface layer is attributed to the counter-diffusion of Be and Zn, rendering a high accumulation of Zn interstitials and a giant reduction of charge-compensating defects. These observations provide a broad understanding of the thermodynamics and phase transformations in Be(x)Zn(1-x)O alloys for the application of highly conductive and transparent oxide-based devices and fabrication of their alloy nanostructures.