Jun-ichi Niitsuma

Nanoprocessing of diamond using a variable pressure scanning electron microscope

We demonstrated diamond processing using a variable pressure scanning electron microscope. Dents were formed in diamond by electron beam irradiation in air, nitrogen gas, oxygen gas, and water vapor. The processing in oxygen atmosphere showed the highest efficiency. The processing mechanism can be explained in terms of gas ion sputtering and chemical reaction. We used this method to remove residue on a diamond device, which showed that it is practical for fabricating diamond-based nano-devices.

Hydrogen-vacancy related defect in chemical vapor deposition homoepitaxial diamond films studied by electron paramagnetic resonance and cathodoluminescence

Hydrogen-vacancy related defect (H1′) in chemical vapor deposition homoepitaxial diamond films has been investigated by electron paramagnetic resonance and cathodoluminescence. It is found that the concentration of H1′ significantly decreases as the dilution (CH4∕H2) ratio decreases. It is also confirmed that the intensity of free-exciton emission (Iex) increases as the CH4∕H2 ratio decreases. The complementary relationship between Iex and H1′ can be explained by considering that H1′ acts as a nonradiative recombination center which reduces the lifetime of free exciton and Iex. The suppression mechanism of H1′ is discussed by considering the balance between the growth rate and the annihilation rate of H1′ in the subsurface region.

Inhomogeneous distribution of dislocations in a SiGe graded layer and its influence on surface morphology and misfit dislocations at the interface of strained Si∕Si0.8Ge0.2

To improve the quality of a strained Si layer on a SiGe virtual substrate, the distribution of dislocations in a graded SiGe layer is characterized using electron beam induced current sEBICd. A crosshatch pattern of dark and bright bands running along the two k110l directions is observed in an EBIC image taken with a 25-keV-electron beam at 80 K. These dark and bright EBIC bands are attributed, respectively, to high- and low-density dislocation regions in the graded SiGe layer, as is confirmed by transmission electron microscopy. The effects of such an inhomogeneous dislocation distribution on the surface morphology and the generation of misfit dislocations sMDsd at the interface of strained Si/SiGe are investigated. Comparison between the EBIC image and an atomic force microscope image shows that the high-density dislocation regions are correlated with ridges on the surface topography. A chemical etching image shows that most of the MDs lie along the edges of surface ridges. Possible mechanisms of MD generation at the interface of the strained Si/SiGe are proposed.

Improved Micro-Photopatterning on Azo Film for a Practical Nematic Liquid Crystal Bistability

A surface bistable liquid crystal cell and its photolithographic fabrication process were improved toward practical applications. The exposure time for preparing the aligning pattern was reduced from 35 min in our previous study to 400 s by employing a low-molecular-weight azo compound with a higher photosensitivity (<1 J/cm2). The contrast ratio was enhanced from 6:1 in the previous study to 27:1 by reducing the domain size from 2×2 to 1×1 µm2. In view of the conspicuous deviation of the resultant micropattern from the ideal checker form, we discuss the geometric conditions for surface bistability in two-dimensional periodic patterns.

SCANNING NEAR-FIELD OPTICAL MICROSPECTROSCOPY OF SINGLE PERYLENE MICROCRYSTALS

Perylene microcrystals are known to show characteristic change in the exciton fluorescence spectra depending on the crystal size less than about 200nm. However, the origin of the size-dependence is not yet clear. In this work, we have studied on individual microcrystals by a scanning near-field optical microscope (SNOM). The samples were prepared by a modified technique based on reprecipitation method. As a result, we could successfully measure the topographic and fluorescence images of perylene microcrystals with the size of ~200nm by SNOM at room temperature. In the fluorescence spectra of the single microcrystals, free and self-trapped exciton bands were successfully observed but they did not show any critical difference from that of the bulk crystals. Some possibility of environmental difference is discussed.

Isotope effects between hydrogen and deuterium microwave plasmas on chemical vapor deposition homoepitaxial diamond growth

This article shows that replacing hydrogen with deuterium improves the quality of microwave plasma-assisted chemical vapor deposition homoepitaxial diamond. Suppression of point defects in the bulk and of nonepitaxial crystallites and increasing of free-exciton emission intensity were revealed by electron paramagnetic resonance, optical microscopy, and cathodoluminescence, respectively. The isotope effects on the etching rate of diamond by deuterium are also revealed. The isotope effects are discussed from the viewpoint of etching effects.

Luminescent Characteristics of Undoped and Er-Doped ZnO Thin Films

In order to clarify the fundamental luminescent mechanism of undoped and Er-doped ZnO thin films synthesized by sputtering method, cathodoluminescence (CL) from the samples formed on several kinds of substrate were measured. There was no explicit peak identified with luminescence from ZnO crystal defects in undoped sample, on the contrary, three sharp luminescent peaks were observed in the case of Er-doped ZnO film due to the internal transition of the additive Er ions in the CL spectrum. The mechanism was investigated in comparison with photoluminescence (PL).

Near-Infrared Light Emission from of Er-Doped ZnO Thin Film in Micropits Processed on Si Substrate

We have obtained Er-doped ZnO thin film in a micropattern of reverse trapezoids processed on Si substrate by sputtering and ultrafine polishing techniques. Near-infrared light emission was detected successfully from the thin film filling a single micropit with 10 μm square. Transmission electron microscopy (TEM) observation showed epitaxial growth of ZnO crystals along the curvature of the micropit.