We report on the growth of III-V compound semiconductor nanowires by selective-area MOVPE. Position-controlled growth of GaAs nanowire, and InP/InAs/InP core-multi shell nanowires are reviewed. The nanowires are oriented to <111>B or <111>A directions and they have hexagonal cross-section surrounded with {1-10} vertical side facets and (111)B or A top surface. Also, we can control the growth direction to axial or radial by changing the growth parameters. InP/InAs/InP core-multi shell nanowires whose well thickness is of several monolayers are fabricated by using this method. We also review on the growth of vertical GaAs nanowires on Si substrate and fabrication of InGaAs nanowire-FETs.

View full abstract

ZnO has hexagonal wurtzite structure that is characterized by a mixture of ionic and covalent bondings between Zn and O atoms. A Zn atom is surrounded by four O atoms, or vice versa, which are located at the edges of a tetrahedron. Such coordination gives rise to polar symmetry along the c-axis of the hexagonal structure. The polarity affects electrical, optical, mechanical, and chemical properties of ZnO. In 2000, we have reported the successful selective growth of polarity-controlled ZnO films by molecular-beam epitaxy (MBE) using either GaN templates or MgO buffer. We confirmed different growth behaviors on the surfaces of ZnO layers with different polarities. Recently, we have explored the effect of crystal polarity on the formation of ZnO nanostructures. The periodically polarity-inverted (PPI) ZnO templates with submicron line-pitch over a large area were fabricated using MBE by employing MgO buffer layers and lithography technique. We grew ZnO nanorods using chemical transport and condensation method. The ZnO nanorods grew only on Zn-polar regions with the same periodicity as that of PPI template, which eventually produces well-aligned ZnO nanorod arrays.

View full abstract

Growth characteristics of GaAs and InAs nanowhiskers formed by using the vapor-liquid-solid growth method during metal organic vapor-phase epitaxy are reviewed. The nanowhiskers grown along the <111>B crystallographic orientation were as thin as 10-500 nanometers and up to 5 micrometers long. The width of grown nanowhiskers was dependent on the growth temperature and the thickness of Au deposit used as a growth catalyst. The minimum width of the nanowhiskers was estimated to be about several nanometers, which is a growth limit predicted by the Gibbs-Thomson effect. Current-voltage characteristics of selectively grown Si-doped GaAs nanowhiskers were measured. A step-like current change as large as 0.5 microamperes was reproducibly observed on the background current-voltage curve of between 0 and 1.0 microamperes, suggesting that carrier trap levels on the whisker surface might be involved in the change.

View full abstract

A novel nanostructure of semiconductor quantum-well (QW) system is reported. The structure is a fine hollow tube with a micron-order diameter fabricated by rolling several tens of nanometers thick QW layer. For example, although the wall thickness is only 40 nm, the system retains the quantum properties of a QW, and the optical emission from the QW subbands can be clearly observed. In addition, even though the tube-wall thickness is much smaller than the light wavelength, high quality factor optical resonance based on a kind of the whispering gallery modes is observed, which will enable the system to be a new type of low-threshold semiconductor laser.

View full abstract

Electronic properties, related to the correlation between quantum states and Fermi level (E_F) at the semiconductor boundaries, are discussed as one of the essential issues for understanding nanometer-scale physics and its technological application. To understand the mechanism of surface E_F stabilization, the origin of two-dimensional electrons in the native electron accumulation layer was explored using a low-temperature scanning tunneling microscope in ultra-high vacuum at the clean (111)A surface of the undoped InAs thin film grown by molecular beam epitaxy. According to direct comparison of densities between surface defects and accumulated electrons, it was found that the electron accumulation layer was introduced by the two-dimensionally distributed native donor point defects composed of In adatoms and In antisites at the density of 10¹² cm⁻². The mechanism of the surface E_F pinning was explained using a model based on the shallow defect band formation due to highly formed surface donors. It was shown that this surface band was composed of the hydrogenic donor bound states and delocalized due to many-body effects induced by the Mott transition in correlated donor-electron system at the clean surface, and was not composed of the atomic orbitals of point defects or the dangling bonds of surface atoms.

View full abstract

High density arrays of quantum dots (QDs) can easily be grown by ‘self-assembled’ methods. However, the precise mechanism of ‘self-assembled’ is not well understood, which hampers the control over QD size, density and distribution for particular applications. Therefore, in-situ evaluation technique for observing the growth process is necessary and indispensable. STM is a good technique to observe the surface in atomic level but it prevents vibrations and material depositions. So, usually its observation is made after transporting the sample from MBE growth chamber to the STM through a gate valve, resulting that the temperature of the sample is returned to room temperature. Since the real in-situ observation cannot be done with this ordinary method, we develop “STMBE” system in which the STM is placed completely inside MBE growth chamber, and with this system, the surface structure is analyzed centering on the in-situ STM observation of the InAs QD self-assemble process on GaAs(001).

View full abstract

Determination of mechanism of anisotropic growth, dot formation, crystal orientation determination and polarity determination for epitaxial growth are introduced as examples of application of the density functional calculation and the kinetic Monte Carlo simulation. The anisotropic atomic migration for non-polar surface of GaN and ZnO forms stripe-structure during epitaxial growth. The adsorption site and adsorption energy of adatom determine polarity and orientation of compound semiconductor in epitaxial growth.

View full abstract

Adsorption-desorption behavior of Ga on GaAs surfaces such as (001) and (111)B is systematically investigated using our ab initio-based approach, which incorporates chemical potentials in the gas phase of Ga atom or As molecules as a function of temperature and their beam equivalent pressure. Surface phase diagram calculations of the (001)-c(4×4) reveal that the c(4×4)α consisting of Ga-As dimers independently appears on As₂ and As₄ in the gas phase, whereas the c(4×4)β with As dimers is destabilized under As₄. On the basis of this finding, surface phase transition between c(4×4) and (2×4)β2 is discussed in terms of 0.5 monolayer of Ga predeposition. Furthermore, surface phase diagram calculations of the (111)B-(2×2) imply that Ga adsorption induces As trimer desorption on the (2×2) surface. This suggests that Ga adatom acts as a self-surfactant atom to maintain layer-by-layer growth of GaAs without anti-site defect formation.

View full abstract

In dendrochronology, the felling date that affects the construction date of architectures can be estimated by using a statistical average number of sapwood rings only in woods having a recognizable sapwood/heartwood boundary. However, it could not be estimated in discoloured woods previously because of the indistinguishable sapwood. In this paper, we report a method that can discriminate the indistinguishable sapwood from heartwood in a discoloured ancient wood of Hinoki cypress by the direct mapping of molecular ions of specific substances using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The results allowed the determination of the felling date of the discoloured wood. TOF-SIMS has shown to be useful for investigating the distribution of minute amounts of chemical components in woods.

View full abstract