TiO₂ Inverse Opals Fabricated Using Low‐Temperature Atomic Layer Deposition

Abstract

Following the work of John [1] and Yablonovitch, [2] the study of photonic crystals (PCs) has become an important area of research for applications in optoelectronics and electromagnetics, as well as chemical and biological sensors. Formation of a complete photonic bandgap (PBG) requires a three-dimensional (3D) periodic structure exhibiting high refractiveindex contrast. PCs based on infiltration of self-assembled opals are promising structures, [3±5] and a full PBG at infrared wavelengths has been produced in a silicon PC. [6,7] Also, transparent materials with indexes ranging from~1.4 (SiO 2 ) to 3.8 (Sb 2 S 3 ) have been used to form inverse opals with pseudophotonic bandgaps (PPBGs) and potentially full PBGs in the visible-wavelength range.[8±12] Two-dimensional (2D) and 3DPC structures are being extensively modeled, and these studies show that changes in the structures, such as shifting the distribution of dielectric material, can significantly improve PBG properties. The importance of the precise placement of the dielectric material is demonstrated by the inverted ªshellº structure, where the opal is infiltrated with a conformal shell-like coating, leaving small air pockets in the face-centered cubic (fcc) interstitial sites. For example, in a silicon inverse shell opal, the width of the PBG can be increased from 4.25 % to 8.6 %. [13] Similarly, the PBG width can also be increased to 9.6 % by formation of a non-close-packed structure. [14] Thus, the performance of these structures critically depends on precisely and accurately placed high-dielectric material, and the fabrication of these optimized structures will require a highly controllable infiltration method. Similarly, even for 2D and 3D photonic crystals not based on the opal architecture, highly controllable deposition methods will be imperative for maximizing desired photonic effects in real structures. Atomic layer deposition (ALD) allows formation of low porosity, conformal films with submonolayer control.[15] These features make ALD ideal for infiltration, and we have successfully used the technique to create ZnS/Mn inverse opals. [16,17] A similar layer-by-layer method has also been demonstrated to increase the mechanical stability and oxide-filling fraction of a SiO 2 opal using SiCl 4 and H 2 O precursors. [18] In this paper, we extend the ALD studies to TiO 2 , which has long been a candidate material for use in PCs because its refractive index (n) can exceed 2.8 and 2.65 (k = 500 nm) for the rutile and anatase phases, respectively. [3,19±21] Unfortunately, the growth methods used to date have resulted in infiltration filling fractions of, at most, 50 % of the available pore volume. In addition, these opals were infiltrated either by solution precipitation or by nanoparticle co-sedimentation, neither of which offers much precision in placement of the high-dielectric material. Infiltration by ALD holds promise for attaining inverse shell opals that exhibit filling fractions very close to the optimum 90 % of the pore volume. For this study, ...

Broadband high-efficiency dielectric metasurfaces for the visible spectrum

“…In addition, creating high-aspect ratio nanostructures via dry etching of TiO 2 , similar to other dielectric materials, is difficult and can lead to increased sidewall roughness. This ALD process is different from previous techniques (24,25) where the patterns created are generally fixed by a set template (e.g., anodic alumina or inverse opal). Here we are free to define more complex nanostructures because we use ALD directly on exposed EBR.…”

Luminescent and Tunable 3d Photonic Crystal Structures

“…From our previous work, an opal infiltration growth rate of 0.51 Å/cycle was used to estimate the thicknesses of the infiltrated TiO 2 . 15 The evolution of the reflectivity spectra during formation of the non-close-packed structure is shown in Figures 13 and 14, after 160, 280, 400, and 520 ALD cycles.…”

“…Thus, for the growth of TiO 2 , using a custom-built, flow-style, hot-wall ALD reactor, special protocols were developed to obtain high conformity and optimum crystallinity, and to address the requirements of depositing material within a highly porous structure. 15 The initial studies on the growth of the high index rutile phase of titania on planar substrates showed that even for growth temperatures above 600 o C, it was difficult to reproducibly form the rutile phase and that the resulting polycrystalline films exhibited considerable surface roughness which increased with deposition temperature. 16 However, as shown by Figure 1, by significantly decreasing the growth temperature (~100 ºC) very low RMS roughness (0.2 nm) films were achieved.…”

Impact of Atomic Layer Deposition to Nanophotonic Structures and Devices

“…Temperature variations during ALD-growth have significant influence on the surface roughness e.g., in infiltration of silica opals to fabricate TiO 2 inverse opals with spherical size templates ranging from 200 to 440 nm in diameter show different roughness (King et al, 2005a). Atomic Force Microscopy (AFM) studies shows that TiO 2 films yielded a root-mean-square (RMS) roughness of 0.2 nm at 100°C, 2.1 nm at 300°C, and 9.6 nm at 600°C, which shows that low temperature deposition is of much more significance for even filling and preparing extremely smooth interfaces (King et al, 2005a).…”