A. A. Baski

The structure of silicon surfaces from (001) to (111)

Abstract We describe the structure of silicon surfaces oriented between (001) and (111) as determined by scanning tunneling microscopy (STM) and first-principles, total-energy calculations. In addition to reviewing and reproducing the structures reported for the few surfaces previously studied, we describe a number of additional surfaces in order to provide a complete overview of the (001)-to-(111) surface morphology. As the sample orientation is titled from (001) to (111) (ϑ=0 to 54.7°), the surface morphology varies as follows: (1) Si(001) to Si(114) = (001)-like surfaces composed of dimers separated by steps (both rebonded and nonrebonded); (2) Si(114) to Si(113) =mesoscale sawtooth facets composed of the stable (114)−2 × 1 and (113)−3 × 2 planes; (3) Si(113) to Si(5 5 12) =mesofacets composed of (113)−3 × 2 and (5 5 12)-like planes; (4) Si(5 5 12) to ∼Si(223) =nanoscale sawtooth facets composed of (5 5 12)-like and unit-cell-wide (111)−7 × 7 planes; and (5) ∼Si(223) to Si(111)=(111)−7 × 7 terraces separated primarily by single- and triple-layer steps. The change in the surface morphology is accompanied by a change in the composition of surface structural units, progressing from (001)-like structures (e.g. dimers, rebonded steps, and tetramers) to (111)-like structures (π-bonded chains, adatoms and dimer-chain walls). The resultant morphology is a delicate balance between the reduction of dangling bond density achieved by the formation of these structural units, and the resulting surface stress associated with their unusual bond angles and bond lengths.

Highly efficient electron field emission from graphene oxide sheets supported by nickel nanotip arrays.

Electron field emission is a quantum tunneling phenomenon whereby electrons are emitted from a solid surface due to a strong electric field. Graphene and its derivatives are expected to be efficient field emitters due to their unique geometry and electrical properties. So far, electron field emission has only been achieved from the edges of graphene and graphene oxide sheets. We have supported graphene oxide sheets on nickel nanotip arrays to produce a high density of sharp protrusions within the sheets and then applied electric fields perpendicular to the sheets. Highly efficient and stable field emission with low turn-on fields was observed for these graphene oxide sheets, because the protrusions appear to locally enhance the electric field and dramatically increase field emission. Our simple and robust approach provides prospects for the development of practical electron sources and advanced devices based on graphene and graphene oxide field emitters.

The Origin of the Superstructure in Bi2Sr2CaCu2O8+δ as Revealed by Scanning Tunneling Microscopy

Real-space images with atomic resolution of the BiO plane of Bi2Sr2CaCu2O8+δ were obtained with a scanning tunneling microscope. Single-crystal samples were cleaved and imaged under ultrahigh vacuum conditions at room temperature. The images clearly show the one-dimensional incommensurate superstructure along the b-axis that is common to this phase. High-resolution images show the position of the Bi atoms, revealing the structural nature of the superlattice. A missing row of Bi atoms occurs either every nine or ten atomic sites in both (110) directions, accounting for the measured incommensurate periodicity of the superstructure. A model is proposed that includes missing rows of atoms, as well as displacements of the atomic positions along both the a- and c-axis directions.

A Stable High-Index Surface of Silicon: Si(5 5 12)

A stable high-index surface of silicon, Si(5 5 12), is described. This surface forms a 2 x 1 reconstruction with one of the largest unit cells ever observed, 7.7 angstroms by 53.5 angstroms. Scanning tunneling microscopy (STM) reveals that the 68 surface atoms per 2 x 1 unit cell are reconstructed only on a local scale. A complete structural model for the surface is proposed, incorporating a variety of features known to exist on other stable silicon surfaces. Simulated STM images based on this model have been computed by first-principles electronic-structure methods and show excellent agreement with experiment.

Epitaxial lateral overgrowth of (112¯2) semipolar GaN on (11¯00)m-plane sapphire by metalorganic chemical vapor deposition

The authors report the growth of semipolar (112¯2) GaN films on nominally on-axis (101¯0) m-plane sapphire substrates using metal organic chemical vapor deposition. High-resolution x-ray diffraction (XRD) results indicate a preferred (112¯2) GaN orientation. Moreover, epitaxial lateral overgrowth (ELO) of GaN was carried out on the (112¯2) oriented GaN templates. When the ELO stripes were aligned along [112¯0]sapphire, the Ga-polar wings were inclined by 32° with respect to the substrate plane with smooth extended nonpolar a-plane GaN surfaces and polar c-plane GaN growth fronts. When compared with the template, the on-axis and off-axis XRD rocking curves indicated significant improvement in the crystalline quality by ELO for this mask orientation (on-axis 1700arcsec for the template, 380arcsec for the ELO sample, when rocked toward the GaN m axis), as verified by transmission electron microscopy (TEM). For growth mask stripes aligned along [0001]sapphire with GaN m-plane as growth fronts, the surface was...

Dependence of GaN polarity on the parameters of the buffer layer grown by molecular beam epitaxy

The polarity of GaN films grown using GaN and AlN buffer layers on sapphire substrates by molecular beam epitaxy were investigated by atomic force microscopy, hot wet chemical etching, and reflection high-energy electron diffraction. We found that the GaN films grown on high temperature AlN (>890 °C) and GaN (770–900 °C) buffer layers invariably show Ga and N polarity, respectively. However, the films grown using low temperature (∼500 °C) buffer layers, either GaN or AlN, could have either Ga or N polarity, depending on the growth rate of the buffer layer.

Epitaxial growth of silver on mica as studied by AFM and STM

The epitaxial growth of Ag(111) on mica was studied using the techniques of atomic force microscopy (AFM) and scanning tunneling microscopy (STM). We examined the morphology of these films primarily with AFM as a function of the substrate growth temperature (75 to 350°C) and film thickness (50 to 300 nm) for slow metal deposition rates of 1 to 2 A/s. In the lower temperature regime near or below 75°C, a “rolling hill” morphology is observed for all film thicknesses. In the higher temperature regime above approximately 75°C, a number of different surface morphological phases exist which depend on both the film thickness and growth temperature. As the film thickness increases, the phases develop in the following order: (1) island, (2) channel, (3) hole and (4) network. The rate of progression through these phases with increasing film thickness is slower for higher substrate temperatures. Although a number of these films appear atomically flat in small regions, holes and other surface features often disrupt the surface flatness on a larger scale. We have found, however, that it is possible to grow films which are atomically flat on the micrometer-scale using higher deposition rates ( ∼ 40 A/s) and high substrate temperatures.

Structure of the Sb‐terminated Si(100) surface

The structure of the Sb‐terminated Si(100) surface has been studied by scanning tunneling microscopy. The images show that the surface is terminated in a symmetric Sb dimer structure. The long‐range order of the Sb‐terminated surface is broken up by a high density of antiphase domain boundaries which accounts for the low intensity of the half‐order spots in the 2×1 low‐energy electron diffraction pattern. Images on single‐domain Si(100) substrates demonstrate that the Sb grows as an additional layer of dimers, rather than substituting for the topmost layer of Si dimers.

Surface photovoltage in undoped n-type GaN

Steady-state and transient surface photovoltage (SPV) in undoped GaN is studied in vacuum and air ambient at room temperature and 400 K with a Kelvin probe. The results are explained within a phenomenological model accounting for the accumulation of photogenerated holes at the surface, capture of free electrons from the bulk over the near-surface potential barrier, and emission of electrons from surface states into the bulk. Simple analytical expressions are obtained and compared with experimental results. In particular, the proposed model explains the logarithmic decay of the SPV after stopping illumination. Internal and external mechanisms of the SPV are discussed in detail. It is shown that an internal mechanism dominates at low illumination intensity and/or small photon energies, while external mechanisms such as charging of a surface oxide layer and photoinduced processes play a significant role for above-bandgap illumination with sufficient intensity.

Surface band bending of a-plane GaN studied by scanning Kelvin probe microscopy

We report the value of surface band bending for undoped, a-plane GaN layers grown on r-plane sapphire by metalorganic vapor phase epitaxy. The surface potential was measured directly by ambient scanning Kelvin probe microscopy. The upward surface band bending of GaN films grown in the [112¯0] direction was found to be 1.1±0.1V. Because polarization effects are not present on a-plane GaN, we attribute such band bending to the presence of charged surface states. We have modeled the surface band bending assuming a localized level of surface states in the band gap on the surface. It should be noted that the band bending observed for a-plane layers is comparable to that obtained on polar c-plane layers, and both a-plane and c-plane GaN films with similar surface treatments demonstrate comparable band bending behavior, indicating that charged surface states dominate band banding in both cases.