Kazuya Tsujino

Pore formation in silicon by wet etching using micrometre-sized metal particles as catalysts

Au, Pt, or Ag particles with particle sizes of ca. 1 μm were used as catalysts for boring pores in p-type Si(100) wafers by wet etching in aqueous solutions containing hydrofluoric acid and hydrogen peroxide. Boring speed was fastest when Pt particles were used as the catalyst. However, the sidewalls of the pores and the surface of the wafer were covered with a nanoporous silicon layer of ca. 500 nm in thickness, and the pore showed a tapered structure. When micrometre-sized Ag particles were used, no deep pores were formed because the particles were unstable in the solution. In contrast to Pt and Ag particles, Au particles bored straight pores under some conditions. However, the morphology of pores depended on the shape of the Au particles. Spherical Au particles formed straight pores, whereas non-spherical Au particles formed pores with spiral sidewalls. When Au particles formed aggregates consisting of a small number of particles (<10 particles), crooked pores tended to be formed. In contrast, when the aggregates were composed of a larger number of particles, straight pores were formed and the boring speed was faster than the pores formed with isolated Au particles.

Oxidation of Atomically Flat and Hydrogen-Terminated Si ( 111 ) Surfaces by Hydrogen Peroxide

The electrochemical current-voltage curve of a Si(lll) surface measured in an aqueous solution is very sensitive to the surface condition. Using this property, we analyzed the Si(111) surface after chemical oxidation by H 2 O 2· Thermodynamically, Si-Si back bonds of surface Si-H bonds are oxidized more easily than the Si-H bonds. However, when the surface was atomically fattened and hydrogen-terminated, the oxidation rate of the Si-Si back bonds was lowered and the surface Si-H bonds and Si-Si back bonds were oxidized simultaneously. The lowered reactivity of the Si-Si back bonds was attributed to the hindrance of the approach of H 2 O 2 molecules to the Si-Si back bonds by the Si-H bonds regularly stretching perpendicular to the surface. Fourier transform infrared (FTIR) spectra showed that the Si-Si back bonds on step sites were oxidized very easily, and this was attributed to easy access of H 2 O 2 molecules to the Si-Si bonds on the step sites. These results indicate that the regular structure of the atomically flattened Si(lll) surface is the reason for its good stability against chemicals.

Optical properties and crystallinity of ZnO films for application in super-resolution optical discs

An energy-gap-induced super resolution (EG-SR) in optical discs was obtained using ZnO thin films deposited by sputtering. The properties of EG-SR optical discs were found to depend on the conditions of ZnO deposition. To clarify the reason for this, we prepared ZnO thin films under different sputtering conditions and studied their properties by X-ray diffraction, X-ray photoelectron spectroscopy, and UV–visible-light (vis) spectroscopy. Results indicated that oxygen vacancies in the ZnO thin films affect the readout sensitivity of the EG-SR optical discs.