High-mobility perovskite BaSnO3 films are of significant interest as new wide bandgap semiconductors for power electronics, transparent conductors, and as high mobility channels for epitaxial integration with functional perovskites. Despite promising results for single crystals, high-mobility BaSnO3 films have been challenging to grow. Here, we demonstrate a modified oxide molecular beam epitaxy (MBE) approach, which supplies pre-oxidized SnOx. This technique addresses issues in the MBE of ternary stannates related to volatile SnO formation and enables growth of epitaxial, stoichiometric BaSnO3. We demonstrate room temperature electron mobilities of 150 cm2 V−1 s−1 in films grown on PrScO3. The results open up a wide range of opportunities for future electronic devices.

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The magnetotransport properties of epitaxial films of Cd_{3}As_{2}, a paradigm three-dimensional Dirac semimetal, are investigated. We show that an energy gap opens in the bulk electronic states of sufficiently thin films and, at low temperatures, carriers residing in surface states dominate the electrical transport. The carriers in these states are sufficiently mobile to give rise to a quantized Hall effect. The sharp quantization demonstrates surface transport that is virtually free of parasitic bulk conduction and paves the way for novel quantum transport studies in this class of topological materials. Our results also demonstrate that heterostructuring approaches can be used to study and engineer quantum states in topological semimetals.

We demonstrate the formation of semimetal graphite/semiconductor Schottky barriers where the semiconductor is either silicon (Si), gallium arsenide (GaAs) or 4H-silicon carbide (4H-SiC). Near room temperature, the forward-bias diode characteristics are well described by thermionic emission, and the extracted barrier heights, which are confirmed by capacitance voltage measurements, roughly follow the Schottky-Mott relation. Since the outermost layer of the graphite electrode is a single graphene sheet, we expect that graphene/semiconductor barriers will manifest similar behavior.PACS numbers: 81.05. UW, 73.30.+y, Metal-semiconductor contacts are ubiquitous in semiconductor technology not only because they are unavoidable, but also because the associated (Schottky) barriers to electronic transport across the metal-semiconductor interface can be tuned by judicious choice of materials and processing techniques [1]. The most prominent property of a Schottky barrier is its rectifying characteristic; the barrier acts like a diode with large currents flowing for forward bias and significantly smaller currents flowing for reverse bias [2]. If low resistance and "ohmic" (linear) I-V characteristics are desired, then materials and/or processing techniques are chosen to assure that the Schottky barrier height (SBH) φ B is small compared to temperature (i.e., φ B << k B T ). Semimetal rather than metal electrodes can also be used. For example, epitaxial ErAs/InAlGaAs diodes fabricated by molecular beam epitaxy have barrier heights that can be tuned over a wide range by adjusting composition and doping[3].Here we report on the use of highly oriented pyrolytic graphite (HOPG) as the semimetal in semimetal/semiconductor Schottky barriers. We demonstrate rectifying characteristics on three different n-type semiconductors each of which is uniquely suited to specific applications: namely Si, with its robust oxide, to field gated transistors, GaAs, with its direct band gap, to spintronic and optical applications and SiC, with its high thermal conductivity and breakdown strength, to high power/frequency devices. Advantageously the HOPG contact, which can be applied at room temperature, causes minimal disturbance at the semiconductor surface for two reasons: the graphene sheets of the graphite are robustly impervious to diffusion of impurity atoms[4] and the Van der Waals force of attraction is relatively weak. Since φ B is related to an interfacial dipole layer associated with bond polarization[1], we infer that barrier properties are determined primarily by the outermost layer of the HOPG contact, i.e., a single layer graphene (SLG) sheet. Accordingly, our results anticipate similar phenomenology using two-dimensional (2D) graphene rather than three-dimensional (3D) graphite. * Corresponding author: afh@phys.ufl.eduOther examples demonstrating SLG-like properties in graphite include ARPES evidence for the precursor influence of K-point Dirac fermions[5] and a pronounced temperature-dependent upturn in the in-plane resistivity (ρ ab...

Epitaxial, strain-engineered Dirac semimetal heterostructures promise tuning of the unique properties of these materials. In this study, we investigate the growth of thin films of the recently discovered Dirac semimetal Cd3As2 by molecular beam epitaxy. We show that epitaxial Cd3As2 layers can be grown at low temperatures (110 °C–220 °C), in situ, on (111) GaSb buffer layers deposited on (111) GaAs substrates. The orientation relationship is described by (112)Cd3As2 || (111) GaSb and [11¯0]Cd3As2 || [1¯01]GaSb. The films are shown to grow in the low-temperature, vacancy ordered, tetragonal Dirac semimetal phase. They exhibit high room temperature mobilities of up to 19300 cm2/Vs, despite a three-dimensional surface morphology indicative of island growth and the presence of twin variants. The results indicate that epitaxial growth on more closely lattice matched buffer layers, such as InGaSb or InAlSb, which allow for imposing different degrees of epitaxial coherency strains, should be possible.

GaN nanowires (NWs) were grown selectively in holes of a patterned silicon oxide mask, by rf-plasma-assisted molecular beam epitaxy (PAMBE), without any metal catalyst. The oxide was deposited on a thin AlN buffer layer previously grown on a Si(111) substrate. Regular arrays of holes in the oxide layer were obtained using standard e-beam lithography. The selectivity of growth has been studied varying the substrate temperature, gallium beam equivalent pressure and patterning layout. Adjusting the growth parameters, GaN NWs can be selectively grown in the holes of the patterned oxide with complete suppression of the parasitic growth in between the holes. The occupation probability of a hole with a single or multiple NWs depends strongly on its diameter. The selectively grown GaN NWs have one common crystallographic orientation with respect to the Si(111) substrate via the AlN buffer layer, as proven by x-ray diffraction (XRD) measurements. Based on the experimental data, we present a schematic model of the GaN NW formation in which a GaN pedestal is initially grown in the hole.

Hexagonal boron nitride (h-BN) is a layered two-dimensional material with properties that make it promising as a dielectric in various applications. We report the growth of h-BN films on Ni foils from elemental B and N using molecular beam epitaxy. The presence of crystalline h-BN over the entire substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used to examine the morphology and continuity of the synthesized films.A scanning electron microscopy study of films obtained using shorter depositions offers insight into the nucleation and growth behavior of h-BN on the Ni substrate. The morphology of h-BN was found to evolve from dendritic, starshaped islands to larger, smooth triangular ones with increasing growth temperature.The previous decade has seen extensive research efforts focused on the novel properties of two-dimensional materials and their associated potential applications. This surge in interest was instigated by the isolation of monolayer graphene for the first time, 1 and has rapidly spread to other materials. 2 One such material, hexagonal boron nitride (h-BN), has been the subject of particular attention.This intense research interest has been driven by the suitability of h-BN for integration into heterostructures with other two-dimensional materials, such as graphene. 3 The first, and perhaps most intuitive way to integrate h-BN and graphene is in vertically stacked heterostructures. In this configuration, the h-BN acts as an insulating layer between electrically active graphene sheets, as well as offering the capability to tune the properties of the graphene layers through moiré effects and other interlayer interactions. 4 This scheme is enhanced by the atomically smooth surface and homogeneous charge potential offered by h-BN, as illustrated by the order of magnitude higher charge a) Author's current address:

Rectification and thermal stability of diodes formed at graphene/GaN interfaces have been investigated using Raman Spectroscopy and temperature-dependent current-voltage measurements. The Schottky barriers formed between GaN and mechanically transferred graphene display rectification that is preserved up to 550 K with the diodes eventually becoming non-rectifying above 650 K. Upon cooling, the diodes show excellent recovery with improved rectification. We attribute these effects to the thermal stability of graphene, which acts like an impenetrable barrier to the diffusion of contaminants across the interface, and to changes in the interface band alignment associated with thermally induced dedoping of graphene. V