Sacrificial-Layer Atomic Layer Deposition for Fabrication of Non-Close-Packed Inverse-Opal Photonic Crystals

Photonic crystals have been studied extensively for their potential to control and manipulate light, similar to the way electrons are controlled in a semiconductor, opening the door to high-speed, low-power, all-optical devices. A common method for fabricating 3D photonic crystals is the infiltration and subsequent inversion of synthetic opal templates to create inverse opals consisting of air spheres in a face-centered-cubic (fcc) dielectric backbone. For a sufficiently high dielectric/ air refractive-index contrast (n > 2.8), the inverse opal is predicted to possess a complete photonic bandgap (PBG), which has been demonstrated for Si and Sb2S3 inverse opals for which n > 3.3. To improve the quality and control of inverse-opal fabrication, several groups have developed surface-limited growth techniques, such as atomic layer deposition (ALD) and certain chemical vapor deposition (CVD) processes. Recently, several groups have reported strategies to increase the width of the complete PBG in inverse opals by controlling the dielectric infiltration and/or modifying the underlying opal template. For example, leaving small air pockets at the centers of the opal void spaces by controlling the filling fraction, f, of the infiltrated dielectric produces inverse shell opals and was predicted to increase the width of the PBG (Dx/x) relative to a fully infiltrated inverse opal (f = 0.26). In a second method, Doosje et al. predicted that the PBG width can be increased by modifying the inverse opal unit cell such that the radius of the air spheres, R, is decreased relative to the original opal-sphere radius (R < Ropal) to form a non-close-packed (NCP) inverse opal. Based on this NCP geometry, a gap between the eighth and ninth photonic bands of almost 10 % is predicted for an optimized silicon (n = 3.45) NCP inverse opal. Thus, control over the construction and shape of the high-dielectric regions in the fcc lattice can be used to significantly affect the optical properties of a 3D photonic crystal. Experimentally, NCP inverse opals can be fabricated by backfilling or conformally adding additional dielectric material into the cavities of the inverse opal. This reduces the air-sphere radius to less than the original opal-sphere radius (R < Ropal), thereby creating an NCP structure. While the NCP structure is predicted to have an enhanced complete PBG for high-dielectric backbones such as Si, for lower-dielectric backbone materials the Bragg peak and high-order flat bands are likewise strongly affected by the NCP structure. For inverse opals formed from low-index backbone materials, conformal backfilling provides a mechanism to statically tune the optical properties such as the width and position of the Bragg peak. However, for both highand low-index structures, backfilling is severely limited by the small pores in conventional inverse opals, which seal too quickly to create an NCP structure and prevent a large degree of tunability. Recently, Míguez et al. and King et al. reported techniques to partially overcome this limitation by using extended heavy presintering of the opal template to increase the necking (or overlap) between neighboring spheres, thus increasing the inverse-pore diameter and allowing a higher degree of backfilling. Although this technique can lead to enhanced properties, heavy presintering is