Silicon antireflection is realized with vertical-aligned SiNWs by using improved metal-induced etching technique. The spectral responses of the transmission, reflection, and absorption characteristics for the SiNWs of different lengths are investigated. In order to realize short SiNWs to provide sufficiently low reflection, a post chemical etching process is developed to make the nanowires have a larger length fluctuation and/or tapered structure. The use of short SiNWs can allow a faster process time and avoid the sub-bandgap absorption that frequently occurs in long nanowires. Short SiNWs can also provide more compatible material structure and fabrication procedures than long ones can for applying to make optoelectronic devices. Taking the applications to solar cells as examples, the SiNWs fabricated by the proposed technique can provide 92% of solar weighted absorption with about 720 nm long wires because of the resultant effective graded index and enhanced multiple optical scattering from the random SiNW lengths and tapered wires after KOH etching.

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A complete investigation of holographic photonic crystal structures has been conducted. From both theoretical and experimental results, profiles of resultant patterns under different process conditions can be estimated and controlled. The use of antireflection layers is crucial for realizing submicron photonic crystals with good uniformity over a large area. We successfully realize submicron-scale photonic crystal templates on silicon substrates with an aspect ratio of 2.5 and good quality by a laser holography technique. The samples are highly uniform in an area of Ͼ2 ϫ 2 cm 2 and present good reproducibility. A lift-off process is performed to transfer inversed pillar patterns into a chromium hard mask for the following dry etching into silicon substrates. A single-step deep reactive ion etching with controlled mixture of Ar/ SF 6 / C 4 F 8 gases is used to directly transfer pillar patterns into silicon. Transferred patterns with a high aspect ratio and vertical sidewalls ͑no scalloping͒ are demonstrated over a large area.

The use of elliptical photonic crystals (PhCs) simplifies the fabrication procedure and provides a wide fabrication tolerance over circular holes when PhCs are realized with holographic exposure techniques. Thus, it can increase the manufacturing yield and reduce the processing cost. We systematically analyze the photonic bandgap (PBG) maps of hexagonal PhC slabs with elliptical air-holes. Despite the reduction in the PBG for elliptical PhCs, the PBG is large enough for most applications. The variation of the midgap frequency and PBG of elliptical PhCs against fluctuation in the fabrication parameters is calculated. The results verify good tolerance to fabrication error for elliptical PhCs. The elliptical PhCs provide directional PBGs, which can be used to realize new functionalities for PhC-based circuits. We also demonstrate the use of laser holography to form submicron-scale PhCs with elliptical air-holes by using negative photoresist. PhC templates with a lattice constant of 375 nm and a high aspect ratio (>3) are demonstrated. The PhC patterns have good uniformity and high reproducibility over an area of larger than 2 cm × 2 cm.

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