Enhanced Photoconductivity in Thin‐Film Semiconductors Optically Coupled to Photonic Crystals
Enhanced Photoconductivity in Thin‐Film Semiconductors Optically Coupled to Photonic Crystals
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DOI：
10.1002/adma.200700564
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发表时间：
2007-12
影响因子：
29.4
通讯作者：
Paul G. O’Brien;N. Kherani;S. Zukotynski;G. Ozin;E. Vekris;Nicolas Tetreault;A. Chutinan;S. John;A. Mihi;H. Míguez
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文献类型：
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作者：
Paul G. O’Brien;N. Kherani;S. Zukotynski;G. Ozin;E. Vekris;Nicolas Tetreault;A. Chutinan;S. John;A. Mihi;H. Míguez
光子晶体（PCs）是一类非凡的材料，其中折射率的周期性可以被设计来获得前所未有的光学特性，这在其他材料中没有相似之处。pc固有的一些有趣的光学现象包括二次谐波产生、超棱镜效应、光子带隙（PBGs）和“慢光子”。通过在设计中引入破坏折射率周期性的缺陷，也可以实现PC内部的光定位，从而允许创建具有位于PC PBG范围内频率的局部光子模式。pc中缺陷模式限制光子的能力有望为光学电路提供基础设施。此外，通过耦合量子点或光腔，光沿PBG波导的无损传播可以为量子信息处理提供新的途径。考虑到理想晶体是一个向各个方向无限延伸的周期结构，PC的平面实际上可以看作是一个二维晶体缺陷。因此，值得注意的是，在将缺陷引入PC体的情况下，光也可以定位在PC表面。例如，人们早就知道，光可以沿着周期性分层介质的表面以局部模式传播，这有效地起到了一维PC的作用。最近，Miguez和同事研究了耦合到pc的薄膜中的光子表面共振模式。在这项工作中研究的一种结构是通过在二氧化硅逆胶体晶体上沉积均匀的二氧化硅层来制造的。研究结果表明，在PC的带隙内，用一定频率的正常入射光照射该二氧化硅层时，PC/薄膜界面附近的电磁场相对于周围区域的电磁场被放大。这种现象是由于改性PC表面的局部光定位引起的，其表现为光学掺杂或缺陷。与Miguez及其同事所研究的结构相似的结构已被用于提高由形状为pc的染料敏化二氧化钛电极组成的Gratzel电池的效率。观察到的改进最初归因于通过PC结构传播的慢光子；然而，随后有人指出，这一假设并不能解释所有观察到的结果，改进主要来自于结构表面的光定位。本研究报告的目的有两个方面。第一个目标是证明半导体薄膜的光导电性可以通过光学耦合到PC表面来放大。事实上，在这种结构中，电影本身并不需要周期性地结构化。第二个目标是证明，与PC表面耦合的薄膜中的表面共振模式所能获得的光导增强比在薄膜背面沉积完美镜面（PM）所能获得的增强更大。为了解决第一个目标，我们比较了沉积在玻璃衬底上的两种不同结构的氢化非晶硅（a- si:H）薄膜的测量量子效率（QEs）。一种结构是如图1a所示的薄膜- PC结构，其中蛋白石PC沉积在薄的a- si:H薄膜的背面，而secC O M M U N IC O O N
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