Huasheng Wu

Reduction of threading defects in GaN grown on vicinal SiC(0001) by molecular-beam epitaxy

We observe a significant reduction of threading dislocations in GaN grown on vicinal substrates of SiC(0001). Using scanning tunneling microscopy, we find films grown on vicinal substrates maintain the surface misorientation of the substrate and display terraces with straight edges. On top of the terraces there is no spiral mound, which is the main feature found for films grown on singular substrates. Transmission electron microscopy studies confirm that threading screw dislocations are reduced by two orders of magnitude while edge dislocations are reduced by one order.

Synthesizing tungsten oxide nanowires by a thermal evaporation method

Tungsten oxide W18O49 nanowires with diameters of 10–20nm were synthesized with high yield by thermal evaporation in a tube furnace. By heating tungsten trioxide powder at 900°C in vacuum (5×10−3torr), W18O49 nanowires with diameters of 10–20nm and lengths up to micrometers were produced with high yield on the Au-coated Si substrates located in the low temperature zone (550–600°C) of the furnace. The morphology, composition, and crystal structure of the nanowires were characterized by various methods. The conditions and the mechanism of W18O49 nanowire growth are discussed.

Transition between wurtzite and zinc-blende GaN: An effect of deposition condition of molecular-beam epitaxy

GaN exists in both wurtzite and zinc-blende phases and the growths of the two on its (0001) or (111) surfaces are achieved by choosing proper deposition conditions of molecular-beam epitaxy (MBE). At low substrate temperatures but high gallium fluxes, metastable zinc-blende GaN films are obtained, whereas at high temperatures and/or using high nitrogen fluxes, equilibrium wurtzite phase GaN epilayers resulted. This dependence of crystal structure on substrate temperature and source flux is not affected by deposition rate. Rather, the initial stage nucleation kinetics plays a primary role in determining the crystallographic structures of epitaxial GaN by MBE.

InN island shape and its dependence on growth condition of molecular-beam epitaxy

During molecular-beam epitaxy of InN films on GaN(0001) surface, three-dimensional (3D) islands are observed following an initial wetting layer formation. Depending on deposition condition, the 3D islands take different shapes. Pyramidal islands form when excess nitrogen fluxes are used, whereas pillar-shaped islands are obtained when excess indium fluxes are employed. The pillar-shaped islands are identified to represent the equilibrium shape, whereas the pyramidal ones are limited by kinetics. As the size of islands increases, their aspect ratio shows a decreasing trend, which is attributed to a gradual relaxation of strain in the layer by defects.

Reflection high-energy electron diffraction intensity oscillations during growth of GaN(0001)A by plasma-assisted molecular beam epitaxy

Abstract Reflection high-energy electron diffraction (RHEED) intensity oscillations have been observed during radio-frequency plasma-assisted molecular beam epitaxy of GaN on its (0001) A face. The starting A (Ga) face was prepared by growing a micrometer-thick GaN layer directly on a low-index 6H–SiC(0001) substrate at 650°C. RHEED intensity oscillations are measured with substrate temperatures less than 550°C in both Ga-limited and N-limited growth conditions. In the N-limited condition, an initial transient high-frequency oscillation is observed before it reaches a steady-state frequency. If the Ga flux is subsequently stopped while keeping the N flux unchanged, a few extra oscillations are recorded. Scanning tunneling microscopy images of surfaces quenched during growth show triangular-shaped islands, verifying a two-dimensional growth mode. At substrate temperatures greater than 550°C, neither island nucleation nor intensity oscillation is observed, suggesting a step-flow growth mode.

Dislocation network at InN/GaN interface revealed by scanning tunneling microscopy

For heteroepitaxy of InN on GaN(0001) by molecular-beam epitaxy, the lattice misfit strain is relieved by misfit dislocations (MDs) formed at the interface between InN and GaN. Imaging by scanning tunneling microscopy (STM) of the surfaces of thin InN epifilms reveals line feature parallel to ⟨112¯0⟩. Their contrast becomes less apparent for thicker epifilms. From the interline spacing as well as a comparison with transmission electron microscopy studies, it is suggested that they correspond to the MDs beneath the surface. The STM contrast originates from both the surface distortion caused by the local strain at MDs and the electronic states of the defects.

STRUCTURAL PROPERTIES OF GaN FILMS GROWN ON THE 6H-SiC(0001)

Ab initio total energy calculations are performed to determine the interface structure of GaN films grown on the 6H-SiC(0001) substrate. The results show that the GaN film is of the wurtzite structure and has the Ga-polarity. It is also shown that stacking mismatch boundaries (SMBs) caused by the coalescence of GaN islands grown on stepped terraces of the 6H-SiC(0001) surface may be removed by stacking faults as the film grows. The types of SMBs on a stepped 6H-SiC(0001) surface are discussed.

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A quantitative structural determination of the Ga-polar 1×1 (0001) surface of GaN is performed by quantitative low energy electron diffraction (LEED). The global best-fit structure is obtained by a new frozen LEED approach connected to a simulated annealing algorithm. The global minimization frozen (GMF) LEED search finds that the ordered structure consists of 1 ML of Ga adatoms at atop sites above Ga-terminated bilayers. The Ga adatoms are bonded with a Ga–Ga bond length of 2.51 A. The spacings within surface bilayers show a weak oscillatory trend, with the outmost bilayer thickness expanding to 0.72 A and the next bilayer thickness contracting to 0.64 A, compared to the bulk thickness of 0.65 A. The interlayer spacing between the first and second bilayers is 1.89 A, while the next interlayer spacing is 1.94 A, compared to the bulk value of 1.95 A. These results are compared with data from other theoretical and experimental studies.

SUBSTRATE

A high quantity of tungsten oxide nanosheets were synthesized by oxidizing tungsten plates with potassium hydrate as the catalyst and tungsten plate as the substrate. The structural and geometrical properties were characterized by various techniques. It was found that the crystalline nanosheets have a WO 3 structure with thicknesses of 30–50 nm and widths up to tens of micrometers. There exist two characteristic acute angles of about 37° or 51° on the nanosheet plane. The formation of these angles and the growth mechanism were discussed.

STRUCTURE DETERMINATION OF THE 1 × 1 GaN(0001) SURFACE BY QUANTITATIVE LOW ENERGY ELECTRON DIFFRACTION

We examine the differences between low-energy electron-diffraction patterns (holograms) and optical holograms. We show that electron-diffraction patterns in solids are not analogous to optical holograms because of strong dynamical factors. We also show that low-energy electron holograms can be inverted by a large-wave-number small-angle integral transformation. The grid sizes in wave number and angular spaces used in the transformation are derived.