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Electrochemically

Prepared Pore Arrays for PhotonicCrystal Applications R.B. Wehrspohn and J. Schilling

Introduction In the last few years, photonic crystals have gained considerable interest due to their ability to “mold the flow of light.” 1 Photonic crystals are physically based on Bragg reflections of electromagnetic waves. In simple terms, a one-dimensional (1D) photonic crystal is a periodic stack of thin dielectric films with two different refractive indices, n1 and n2. The two important geometrical parameters determining the wavelength of the photonic bandgap are the lattice constant, a  d1(n1)  d2(n2), and the ratio of d1 to a (where d1 is the thickness of the layer with refractive index n1, and d2 is the thickness of layer n2). For a simple quarter-wavelength stack, the center wavelength  of the 1D photonic crystal would be simply   2n1d1  2n2 d2. In the case of 2D photonic crystals, the concept is extended to either airholes in a dielectric medium or dielectric rods in air. Therefore, ordered porous dielectric materials like porous silicon or porous alumina are intrinsically 2D photonic crystals. Electrochemically grown pores in metals and semiconductors2,3 have been studied for about 50 years. However, only in the last 10 years have intense research efforts enabled the preparation of ordered arrays of pores with pore diameters in the range of a few nanometers to some tens of micrometers. The most studied materials are porous alumina and macroporous silicon. Porous alumina has been known for more than a century, but only in 1995 was it first observed that ordered arrays of porous alumina could be achieved.4 This ordering was initially by self-organization, and the ordered domains were in the micrometer range. However, electron-beam lithography5 and a related new technique, nanoindentation,6 allowed the preparation of

MRS BULLETIN/AUGUST 2001

monodomain porous alumina structures with sizes in the micrometer range. Macroporous silicon was pioneered in the early 1990s by Lehmann and Föll.7 Very regular pore arrays in the micrometer range have been obtained by photolithographic prepatterning. These pores were called macropores (in contrast to microporous silicon, which is a spongelike nanostructured material with photoluminescent properties that was also intensively studied in the early 1990s).8 Moreover, recently a few other semiconductors like InP, GaAs, and GaP have been shown to exhibit micrometer-sized pores.9,10 Whereas standard nanostructuring techniques are limited to small pore aspect ratios (h/d  40), and resolutions are limited by lithographic tools, electrochemically prepared pores exhibit high aspect ratios of 100–10,000 and inherent short-range order. In the following, two materials will be discussed in detail: macroporous silicon and porous alumina. Due to the regular pore arrangements, these materials are extremely well suited as photonic crystals.

Macroporous Silicon Porous silicon formed by the anodization of p-type silicon in hydrofluoric acid has been studied by numerous group