Department of Applied Physics, Kyung Hee University , Gyeonggi-do 17104, Republic of Korea.
Department of Materials Science and Engineering, Seoul National University , Seoul 08826, Republic of Korea.
Nano Lett. 2016 May 11;16(5):3301-8. doi: 10.1021/acs.nanolett.6b00892. Epub 2016 Apr 7.
Two-dimensional high-index-contrast dielectric gratings exhibit unconventional transmission and reflection due to their morphologies. For light-emitting devices, these characteristics help guided modes defeat total internal reflections, thereby enhancing the outcoupling efficiency into an ambient medium. However, the outcoupling ability is typically impeded by the limited index contrast given by pattern media. Here, we report strong-diffraction, high-index-contrast cavity engineered substrates (CESs) in which hexagonally arranged hemispherical air cavities are covered with a 80 nm thick crystallized alumina shell. Wavelength-resolved diffraction measurements and Fourier analysis on GaN-grown CESs reveal that the high-index-contrast air/alumina core/shell patterns lead to dramatic excitation of the low-order diffraction modes. Large-area (1075 × 750 μm(2)) blue-emitting InGaN/GaN light-emitting diodes (LEDs) fabricated on a 3 μm pitch CES exhibit ∼39% enhancement in the optical power compared to state-of-the-art, patterned-sapphire-substrate LEDs, while preserving all of the electrical metrics that are relevant to LED devices. Full-vectorial simulations quantitatively demonstrate the enhanced optical power of CES LEDs and show a progressive increase in the extraction efficiency as the air cavity volume is expanded. This trend in light extraction is observed for both lateral- and flip-chip-geometry LEDs. Measurements of far-field profiles indicate a substantial beaming effect for CES LEDs, despite their few-micron-pitch pattern. Near-to-far-field transformation simulations and polarization analysis demonstrate that the improved extraction efficiency of CES LEDs is ascribed to the increase in emissions via the top escape route and to the extraction of transverse-magnetic polarized light.
二维高折射率对比介质光栅因其形态而表现出非常规的透射和反射特性。对于发光器件，这些特性有助于导模克服全内反射，从而提高向周围介质的外耦合效率。然而，外耦合能力通常受到图案介质提供的有限折射率对比度的限制。在这里，我们报告了强衍射、高折射率对比的空腔工程衬底（CESs），其中六边形排列的半球形空气腔被 80nm 厚的结晶氧化铝壳覆盖。在 GaN 生长的 CES 上进行的波长分辨衍射测量和傅里叶分析表明，高折射率对比的空气/氧化铝核/壳图案导致低阶衍射模式的强烈激发。在 3μm 节距的 CES 上制造的大面积（1075×750μm2）蓝色发射 InGaN/GaN 发光二极管（LED）与最先进的图案化蓝宝石衬底 LED 相比，光功率提高了约 39%，同时保持了与 LED 器件相关的所有电度量。全矢量模拟定量证明了 CES LED 的光功率增强，并显示出随着空气腔体积的扩展，提取效率逐渐增加。这种光提取趋势适用于横向和倒装芯片几何形状的 LED。远场轮廓的测量表明，CES LED 具有显著的光束效果，尽管其图案节距只有几微米。近场到远场变换模拟和偏振分析表明，CES LED 提取效率的提高归因于通过顶部逃逸路径增加的发射以及横向磁极化光的提取。
