Yoshiaki Kanamori

BROADBAND ANTIREFLECTION GRATINGS FABRICATED UPON SILICON SUBSTRATES

We fabricated a two-dimensional subwavelength structured (SWS) surface upon a crystal silicon substrate. The SWS surface was patterned by electron beam lithography and etched by an SF(6) fast atom beam. The SWS grating had a conical profile, the period was 150 nm, and the groove was approximately 350 nm deep. The reflectivity was examined at 2002500-nm wavelengths. At 400 nm the reflectivity decreased to 0.5% from the 54.7% of the silicon substrate. We also used HeNe laser light to examine the reflectivity as a function of the incident angle.

100 nm period silicon antireflection structures fabricated using a porous alumina membrane mask

An ordered anodic porous alumina membrane has been used as a lithographic mask of SF6 fast atom beam etching to generate a 100 nm period antireflection structure on a silicon substrate. The antireflection structure consists of a deep hexagonal grating with 100 nm period and aspect ratio of 12, which is a fine two-dimensional antireflection structure. In the wavelength region from 400 to 800 nm, the reflectivity of the silicon surface decreases from around 40% to less than 1.6%. The measured results are explained well with the theoretical results calculated on the basis of rigorous coupled-wave analysis.

Antireflective subwavelength structures on crystalline Si fabricated using directly formed anodic porous alumina masks

A simple fabrication technique for subwavelength structured (SWS) surfaces by means of anodic porous alumina masks directly formed on Si substrates was proposed and demonstrated. By this technique, SWS surfaces were fabricated on polished single-crystalline Si and chemically etched as-cut multicrystalline Si wafers. Smoothly tapered SWS surfaces with a periodicity of 100nm and a height of 300–400nm were obtained. A low reflectivity below 1% was observed from 300to1000nm for both of the wafers, in agreement with numerical simulation. After thermal annealing at 800°C, the reflectivity of the SWS surface increased to 3%.

Fabrication of Transmission Color Filters Using Silicon Subwavelength Gratings on Quartz Substrates

We investigate theoretically and experimentally transmission color filters using silicon subwavelength gratings on quartz substrates. Each grating area is 120 mum-square, which is suitable pixel size for displays and multichannel detectors. In the fabrication, electron beam lithography and fast atom beam etching are used. The grating periods are 400, 350, and 440 nm for the red, green, and blue filters, respectively. The transmission spectrum obtained from a coupling between an incident light and the submicrometer periodic grating matches with human color perception. The transmittances of 71.1%, 58.1%, and 59.3% are obtained for the red, green, and blue filters, respectively

Enhancement of light trapping in thin-film hydrogenated microcrystalline Si solar cells using back reflectors with self-ordered dimple pattern

In this article, the light-trapping effect of textured back surface reflectors in thin-film Si solar cells is investigated. A unique substrate with a periodic dimple pattern has been developed by utilizing anodic oxidation of Al as a self-ordering process. n-i-p hydrogenated microcrystalline Si (μc-Si:H) cells fabricated on the Al substrate with a period of 0.9 μm show an improved infrared response compared to those fabricated on randomly textured substrates. A high short circuit current density of 24.3 mA/cm2 has been achieved in a 1-μm-thick μc-Si:H cell by adopting the patterned Al substrate.

Etching submicrometer trenches by using the Bosch process and its application to the fabrication of antireflection structures

This paper reports solutions to the issues of profile control, microloading effect and suppression of the sidewall roughness of submicrometer trenches by modifying the regular conditions of the Bosch process that is often employed in the inductively coupled plasma (ICP) deep reactive ion etching (DRIE) system. Additionally, under the modified processing conditions, a high efficient antireflection structure can be fabricated.

A wavelength-selective add-drop switch using silicon microring resonator with a submicron-comb electrostatic actuator

Electrostatic comb-drive micro actuator with submicron comb fingers was connected to silicon microring resonator to consist a wavelength-selective add-drop switch at 1.5 microm wavelength with variable coupling mechanism. A 500 nm wide 260 nm thick 63.4 microm long silicon microring waveguide was suspended in air with low-loss suspension arms. The air gap between the microring and the input/output waveguides was adjusted by the voltage applied to the comb actuator to vary the coupling efficiency. Transmittance from the input port to a drop port was varied 32.9dB by applying the voltages from 0V to 28.2V. At 28.2V, transmittance from the input port to a through port decreased by 7.83 dB from that at 0V, and 55% of the intensity was transmitted to the drop port. The full-width-half-maximum bandwidth of the dropped light was 0.5 nm, corresponding to a Q-value of 3150.

Wide-Angle Antireflection Effect of Subwavelength Structures for Solar Cells

The angle-dependent reflectivity of several surface structures was analyzed and evaluated with the viewpoint of solar cell applications. Numerical analysis showed that a Si subwavelength structure (SWS) maintains a lower reflectivity at large incident angles than conventional light-trapping techniques such as a random pyramid texture, and that it can contribute to increasing the output power of solar cells under oblique irradiation. This wide-angle antireflection effect was demonstrated by fabricating test crystalline Si cells with several surface structures including a SWS and measuring their angle-dependent short-circuit current densities.

Control of guided resonance in a photonic crystal slab using microelectromechanical actuators

A wavelength-selective variable-reflection filter is proposed, in which a guided resonance in a photonic crystal (PC) slab is controlled by microelectronic mechanical actuators. A narrow wavelength response is generated by the guided resonance of PC slab, and the variable optical response at the wavelength is caused by evanescent-wave coupling to a substrate. The coupling is controlled by microelectromechanical actuators. The proposed filter has been fabricated by silicon micromachining and the mechanism has been confirmed.

Tuning of the thermal radiation spectrum in the near-infrared region by metallic surface microstructures

This paper reviews our recent research on thermal radiation from metallic surface microstructures to develop selective radiators for thermophotovoltaic generation. Numerical simulation showed that two different peaks appeared on the emissivity spectra of metallic gratings. One was originated from surface plasmon polaritons, which was dependent on the grating period and angle. The other peak was explained by the microcavity effect which arose from each microcavity on a grating surface. The microcavity effect became dominant with deepening gratings, and showed suitable properties for selective radiators in thermophotovoltaic generation: a high emissivity within the visible and near-infrared regions, and angle independence. We developed two kinds of two-dimensional metallic gratings based on Si and W with the period of 1.0–2.0 µm. The W gratings composed of rectangular microcavities displayed a high emissivity in the near-infrared region as expected from the calculation results. It was confirmed experimentally that the W selective radiators have advantages for high-power and high-efficiency thermophotovoltaic systems.