Yukio H. Ogata

Electrodeposition of Noble Metals into Ordered Macropores in p-Type Silicon

The electrodeposition of noble metals, i.e., platinum, palladium, and gold, into macroporous p-type silicon was examined. For platinum and palladium, the electrodeposition proceeded preferentially from the pore bottom to the opening when sodium chloride was used as a supporting electrolyte. When sodium sulfate was used as a supporting electrolyte, the electrodeposition mainly proceeded at pore openings, leading to plugging. For gold electrodeposition, a condition for achieving pore filling from the bottom was not found in either the NaCI or Na 2 SO 4 solutions. Pore depth was another key factor to achieve continuous filling by electrodeposition. As the depth of pores became deeper, the electrodeposition proceeded preferentially from the bottom. The effect of mass transfer in pores was also investigated by changing the concentration of metal ions and applied potential.

Electrochemical stabilization of porous silicon multilayers for sensing various chemical compounds

Porous silicon rugate filters are fabricated and investigated for their ability to sense chemical species. The durability of the filter is tested by allowing the structure to undergo many cycles of adsorption and desorption of vapor-phase ethanol molecules. The characteristic reflectivity peak of the structure exhibits a relative blueshift of 2.7% after 86 adsorption∕desorption cycles. The observed shift is ascribed to the formation of silicon dioxide, which has a lower refractive index than that of silicon. In order to stabilize the structure against oxidation expected from cycling and environmental exposure, the filter is subjected to electrochemical oxidation in an aqueous sulfuric acid electrolyte. The treatment dramatically improves stability of the sensor; a relative blueshift of <0.4% is observed after 100 adsorption∕desorption cycles for this sensor. The sensitivity of the sensor is also affected by electrochemical oxidation: the response to saturated ethanol in air changes from Δλ=100nm to Δλ=70n...

Electrochemical preparation of a rugate filter in silicon and its deviation from the ideal structure

Porous silicon single layers are formed by anodization of silicon in a hydrofluoric acid solution at different current densities. An accurate estimation of the etch rates and refractive indices is performed. The refractive index-depth profile of a porous silicon rugate filter (PSRF) prepared by sinusoidally modulating the current density during etch is investigated. The produced PSRF is found to have a periodic pseudo-sinusoidal variation of refractive index with depth. The sinusoidal current density waveform that was employed generates a nonideal refractive index-depth profile with an asymmetric period. It is found that this deformed refractive index profile affects the quality of the characteristic resonant peak of the rugate filter. A simple modification of the current density waveform can be implemented, which allows one to achieve a true sinusoidal refractive index-depth profile characteristic of a rugate filter. The modified current density waveform allows the generation of a PSRF with a low index c...