Two- and three-Dimensional Refractive Index Lattices Formed by Laterally Patterned Porous Silicon Layers
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ABSTRACT Porous silicon was used to fabricate refractive index lattices. Patterned n-doping and/ or substrate etching were used to introduce lateral periodicity. By anodizing p-type substrate with an n-doped area, we realized large refractive index contrast two-dimensional lattices with underlying cladding layer. The anodization process showed an effect specific to the small dimensional patterning and this effect has significant influence on the formed refractive index structure. INTRODUCTION Photonic crystals are attracting wide attention because of their novel physics and potential applications [1],[2]. They consist of regular two- or three-dimensional lattices of high and low refractive index with a large refractive index modulation. A number of studies have been made of the fabrication of these lattices [3]-[5]. For the realization of practical photonic waveguides and devices it is still necessary to study fabrication techniques. In this paper we use porous silicon for the fabrication of refractive index lattices. As is well known, the refractive index of porous silicon can be varied over a wide range by changing the anodization current and, as the anodization proceeds downward without affecting the pre-anodized region, it is quite easy to form index graded structures along the depth. Many studies has been made on porous silicon superlattices or one-dimensional lattices [6]-[8]. By combining the porous silicon with lateral patterning we can extend the technique to refractive index lattices with higher dimensions. ANODIZATION PROCESS We used (100) oriented p-type silicon with 5-10 Q "cm resistivity as the substrate. The unpolished back side was Al deposited and sintered to form an ohmic contact. The anodization was carried out in the dark in HF diluted by the same amount of ethanol. The maximum anodization current density was limited to 25mA/cm 2, because at higher current density the anodized layer became fragile. By changing the current density in the 525mA/cm2 range, we obtained refractive indices in the 1.4-1.8 range [9]. This index range is not sufficient for photonic crystal fabrication. But we can get higher refractive index value if we anodize at lower current density than 5mA/cm 2 and, as will be stated later, we can use crystalline silicon which has refractive index over 3 as the high index medium. The porous layer thickness is controlled by the anodization time. Thus a wide range of refractive index and layer thickness is realized by proper choice of the current and anodization time.
69 Mat. Res. Soc. Symp. Proc. Vol. 597 © 2000 Materials Research Society
REFRACTIVE INDEX LATTICE FABRICATION Two-dimensional Lattices It is preferable to use crystalline silicon as the high index medium, because photonic crystals require a large refractive index contrast. We can form small crystalline region by utilizing n-doping (in p-type substrate) and use that as the high index region. In the process of silicon anpdization, holes are needed for oxidation, therefore n-regions remain unaffected during anodization. T
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