Enhanced Photoconductivity in Thin‐Film Semiconductors Optically Coupled to Photonic Crystals

Abstract

Photonic crystals (PCs) are a remarkable class of materials wherein the periodicity of the index of refraction can be engineered to obtain unprecedented optical properties that have no parallels in other materials. Some interesting optical phenomena inherent to PCs include second harmonic generation, the superprism effect, photonic bandgaps (PBGs), and “slow photons”. It is also possible to achieve light localization within PCs by introducing defects by design that disrupt the periodicity of the index of refraction, allowing the creation of localized photon modes having frequencies that lie within the PBG of the PC. The ability of defect modes in PCs to confine photons is expected to provide the infrastructure for optical circuits. Moreover, lossless light propagation along PBG waveguides with coupling to quantum dots or optical cavities could provide new avenues for quantum information processing. Considering that an ideal crystal is a periodic structure extending to infinity in all directions, the planar surface of a PC can actually be regarded as a 2D crystal defect. Accordingly, it is interesting to note that as in the case of defects introduced into the bulk of a PC, light can also be localized at the PC surface. For instance, it has been known for quite some time that light can propagate in localized modes along the surface of a periodically layered medium, which effectively acts a 1D PC. Recently, Miguez and co-workers have investigated photon surface resonant modes in thin films coupled to PCs. One structure studied in this work has been fabricated by depositing a homogeneous silica layer on top of a silica inverse colloidal crystal. The results of the study indicate that upon illumination of this layer of silica with normally incident light at certain frequencies within the bandgap of the PC, the electromagnetic field near the PC/film interface is amplified with respect to the electromagnetic fields in the surrounding regions. This phenomenon arises from partial light localization at the modified PC surface, which behaves as an optical dopant or defect. Structures similar to those studied by Miguez and co-workers have been used to enhance the efficiency of Gratzel cells comprising dye-sensitized titania electrodes shaped as PCs. The observed improvements were initially attributed to slow photons propagating through the PC structure; however, it has subsequently been pointed out that this hypothesis does not account for all of the observed results and that the improvements arise primarily from light localization at the surface of the structure. The objective of the research reported herein is two-fold. The first objective is to demonstrate that the photoconductivity of a semiconductor film can be amplified by optically coupling the film to a PC surface. Indeed, in this construct, the film does not itself need to be periodically structured. The second objective is to show that the photoconductivity enhancement attainable from surface resonant modes in thin films coupled to PC surfaces is greater than the enhancement that can be achieved by depositing a perfect mirror (PM) onto the backside of this film. To address the first objective, we have compared the measured quantum efficiencies (QEs) of a thin hydrogenated amorphous silicon (a-Si:H) film deposited on a glass substrate for two different structures. One structure is the film–PC construct shown in Figure 1a, wherein an opaline PC is deposited onto the backside of a thin a-Si:H film, whereas the secC O M M U N IC A IO N