Y. Ohno

Growth rate of silicon nanowires

We have measured the growth rate of silicon nanowires (SiNWs), which were grown at temperatures between 365 and 495u2009°C via the vapor-liquid-solid (VLS) mechanism. We grew SiNWs using gold as catalysts and monosilane (SiH4) as a vapor phase reactant. Observing SiNWs by means of transmission electron microscopy, we have found that SiNWs with smaller diameters grow slower than those with larger ones, and the critical diameter at which growth stops completely exists. We have estimated the critical diameter of SiNWs to be about 2 nm. We have also measured the temperature dependence of the growth rate of SiNWs and estimated the activation energy of the growth of SiNWs to be 230kJ∕mol.

Field-ion microscopy of GaAs and GaP

Abstract Clean surfaces of GaAs and GaP were studied by field-ion microscope (FIM). Field-ion images with ordered surfaces were first obtained in pure hydrogen, neon-50% hydrogen and pure neon gases at 78 K, by using channeltron electron multiplier arrays (CEMA). The field-ion images of GaAs were quite similar to those of GaP with respect to the surface structure and the image contrast. They showed the anisotropies of the ion emission and the surface structure between the [111] and [ 1 1 1 ] orientations. Ring steps expected from a spherical surface were observed on the (111) and {100} planes, but not on the [ 1 1 1 ] and {110} planes. The regional brightness of the FIM patterns was discussed in terms of the Knor and Muller model and the atomic and electronic structures of the surface. The image field of these crystals was much lower than that of metals usually used in FIM. For example, the image field strength for the hydrogen and GaAs system was about 1.1 V/A. The reduction of the field necessary to image was also discussed in terms of the field penetration effect.

Origin of a pair of stacking faults in pseudomorphic ZnSe epitaxial layers on GaAs

We have revealed the origin of typical extended defects in semiconductor heterostructures with heterovalent interfaces, namely pairs of stacking faults in pseudomorphic ZnSe epitaxial layers grown on a GaAs(001) substrate. We have taken structural images of the defects by means of high-resolution transmission electron microscopy. Analyzing the images combined with first-principles energy calculations, we have clarified the atomistic structure of the extended defects: An intrinsic stacking fault on (111) intersects the other one on (111) on the interface between the epitaxial layer and the substrate, forming an intersecting line along [110]. Around the intersecting line, we have found that Ga, As, and Se atoms form the specific reconstructed structure with relatively low formation energy, which corresponds well to a reconstructed surface structure on GaAs(001) with excess As and additional Se atoms. We have therefore attributed the stability of extended defects to the reconstructed surface structure on the substrate formed before epitaxial growth.

Observation of silicon surface nanoholes by scanning tunneling microscopy

Abstract We have studied electron-irradiation-induced defects created on an electron exit surface of a Si thin film by means of scanning tunneling microscopy (STM). Several electron-irradiated areas with different electron doses are provided for STM observation. Transmission electron microscopy (TEM) observation reveals a number of silicon-surface-nanoholes of 2–3 nm in diameter and about 5 nm apart in an irradiated area whenever it receives the dose larger than 1.5×10 24 e/cm 2 , while no distinctive TEM contrast of defects is observed in an area with lower dose. STM observation has shown that electron-irradiated surfaces are rougher than a nonirradiated surface. Examining the depth distribution of the areas with different doses, we have found that each irradiated surface exhibits two depth levels which are attributed to a rough surface and a bottom of surface nanoholes, respectively. Even in an area with the lowest dose (1.5×10 22 e/cm 2 ) in this experiment we have observed distinctive STM contrasts, the arrangement and sizes of which are similar to those of the well-developed surface nanoholes observable by TEM. This STM observation shows that the arrangement of nanoholes on an electron exit surface is set up at the very early stage, followed by the excavating of nanoholes under prolonged electron irradiation. We suggest that nanoholes exist in the early stage when only a few atomic layers are removed from the initial surface.

Novel amorphization process in silicon induced by electron irradiation

We recently found that amorphization is induced in Si by MeV electron irradiation. In order to account for the steady-state diagram under electron irradiation, we propose a phenomenological theory which takes into account the two competing mechanisms, namely amorphization by higher energy recoils and recrystallization by lower energy recoils as well as thermal process. We apply the new amorphization method to fabricating artificial arrangements of the columns of a-Si in a c-Si film, and discuss its use for photonic crystal based on photonic band calculation.

Fabrication of periodic nanohole multilayer structure on silicon surface toward photonic crystal

Abstract We have studied a porous structure on silicon surfaces that is introduced by electron irradiation. The structure consists of nanometer-sized holes arranged on silicon surfaces. Investigating the size and the distribution of surface nanoholes in a temperature range from about 4xa0K to about 600xa0K, we have estimated the porosity on a surface with nanoholes. We have fabricated a periodic dielectric multilayer structure on a silicon surface by introducing nanoholes periodically in one direction: alternating layers with different dielectric constants (the dielectric contrast of about 1.08), spaced by a distance of 100xa0nm. We can form periodic dielectric layers at arbitrary locations on surfaces by scanning an electron beam, changing the periodicity and the dielectric constant by varying irradiation condition.

VLS Growth of Si nanowhiskers on a H-terminated Si{111} surface

We have grown Si nanowhiskers on a Si{1111} surface via the vapor-liquid-solid (VLS) mechanism. The minimum diameter of the crystalline is 3nm and is close to the critical value for the effect of quantum confinement. We have found that many whiskers grow epitaxially or non-epitaxially on the substrate along the 〈112〉 direction as well as the 〈111〉 direction. In our growth procedure, we first deposited gold on a H-terminated Si{111} surface and prepared the molten catalysts of Au and Si at 500°C. Under the flow of high pressure silane gas, we have succeeded in producing the nanowhiskers without any extended defects. We present the details of the growth condition and discuss the growth mechanism of the nanowhiskers extending along the 〈112〉 direction.

Atomistic structure of ZnSe nanowires on ZnSe(001) grown catalytically at low temperatures

Atomistic structure of ZnSe needle‐like nanowires on a ZnSe/GaAs(001) epitaxial layer, grown catalytically by means of molecular beam epitaxy operated at low temperatures (527 or 573 K), was studied. A nanowire was the zinc blende structure. The diameter at the top was in the range from 8 to 20 nm, and the length was about 200 nm. It was implied that the optical property differs from that of the bulk crystals, presumably due to the wire confinement effect.

Mesoscopic characterization of the optical property of antiphase boundaries in CuPt-ordered GaInP 2

We have developed an apparatus for polarized cathodoluminescence (CL) spectroscopy combined with transmission electron microscopy (TEM), that enables us to obtain simultaneously structural data in higher spatial resolution by TEM and polarized luminescence spectra by CL of the same microscopic area. The polarized-CL/TEM method is very useful to study the optical properties of low-dimensional microstructures in semiconducting materials. We have applied the method to examine the optical property of antiphase boundaries in CuPt-ordered GaInP 2 and found, for the first time, the polarized light emission from the APBs whose habit planes are parallel to the (T11) and (1T0) atomic planes.

Field emission from p-type GaAs and GaP crystals

Abstract In order to define field electron emission from p-type GaAs and GaP crystals as the first stage of the applications of a field emission microscope to the surfaces, the emission current-voltage characteristics for the clean surfaces have been investigated as well as the effects of temperature, illumination, anneal, and gold and oxygen adsorption on them. The emission from the clean surfaces has been classified into three groups by the emission c-v characteristics; the linear, the characteristic constructed of two or three straight line segments, and the non linear. It was quite sensitive to temperature and light. The threshold for the light sensitivity was located at the little longer wavelength than that expected from the band gap energy. The emission with nonlinear emission c-v characteristics exhibited a hysteresis, occurring a rapid current rise. High temperature anneal at 1080 K caused the precipitates of dissociated Ga and or inner impurity Au onto a tip surface, which made the emission c-v characteristic shift to a lower voltage side in addition to the decrease of its slope. The same behavior was also obtained by Au adsorption, while oxygen adsorption made the emission c-v characteristic shift to a higher voltage side.