亞波長透鏡陣列技術於光電半導體元件之應用

The Applications of Subwavelength Semiconductor Lens-array for Optoelectronic Devices

巫漢敏(Han-Min Wu)

指導教授 ： 彭隆瀚

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2009年)

https://doi.org/10.6342/NTU.2009.01087

摘要

本論文發展自組小球顯影技術，即利用小球的自我排列特性，形成單層、雙層緊密排列的小球結構，達到顯影之目的。我們也使用繞射光學量測，得到高階的繞射模態光點與彩虹炫光，驗證我們已有鋪設大面積緊密排列的單層小球技術。

本文也完成氮化鎵三角柱共振腔之製作與光學特性量測。透過自組小球顯影、光致化學蝕刻與氫氧化鉀晶面削蝕製程的結合，製作非極性晶面之氮化鎵次微米三角柱，具有高品質平滑的晶面側壁。在266 nm雷射入射下，得到半高寬0.4 nm的單模共振放光，其Q值達1000以上，Gth約為 2 MW/cm2。搭配二維有限時域差分法的模擬，得到三角柱尺寸與模態波長關係圖，確認氮化鎵三角柱共振腔具備單一模態受激發光之特性，且三角柱晶面側壁的反射率R達到98%以上。

本研究在矽基板與氮化鎵基板上，結合自組小球技術與反應式離子蝕刻技術，製備具拋物面體形貌的週期0.35 um的亞波長透鏡陣列。在正面反射率之量測，450 nm至700 nm的可見光波段中，透鏡陣列的反射率皆小於3%。斜向入射量測中，於可見光波段400~800 nm間，在TE模式反射率維持在10%以下的訴求下，可容許的入射角達到45度；而在TM模式中反射率維持在2.5%以下的訴求下，可容許的入射角更廣達60度。
我們亦完成搭載透鏡陣列之矽基板光偵測元件的製作。在相同製程與照光強度下，光電元件搭載亞波長透鏡陣列後，可得到大角度寬頻的抗反射能力，元件效能得到約40%的提升。

而為了進一步利用折射係數匹配的特性，達成大角度寬頻譜範圍的抗反射表面結構，我們嘗試將具拋物面體形貌的矽基板0.35 um週期透鏡陣列作為濺鍍之基板，沉積不同厚度的氧化鋅薄膜，尋求折射係數的最佳匹配條件，以增強光電元件之效率。由實驗中得知，50 nm厚度的氧化鋅薄膜是目前最佳化的條件，在400~750 nm波段，得到小於1%的正面反射率表現；而在400~800 nm波段，斜向入射之量測結果顯示，TE模式之反射率在30度的斜向入射角之內，可將反射率維持在2.5%以下，TM模式之反射率在45度的斜向入射角之內，可將反射率維持在1%以下，充分展現氧化鋅/透鏡陣列表面結構的效能改善，得到更大角度之寬頻抗反射特性。

關鍵字

亞波長 ； 透鏡陣列 ； 抗反射表面結構

並列摘要

In this dissertation, the colloidal lithography for casting close-packed distribution of self-assembled polystyrene spheres is demonstrated. High-order diffraction pattern and rainbow lines were observed due to close-packed spheres, showing the ordered-structure characteristics.

Single mode stimulated emission from optical pumping of prismlike gallium nitride (GaN) with a side length of 0.75 um is also reported. Using the self-assembled spheres as the mask, we applied the photo-enhanced etching and wet chemical etching to form prism-like GaN submicron cavities with nonpolar facet sidewalls. They were characterized by an average quality factor of above 1000 and an equivalent facet reflectivity of exceeding 98%, which allowed field amplification by repeated internal reflections in the transverse plane and field polarization along the c axis. These properties allow the observation of single mode stimulated emission by optical pumping the GaN prism at threshold intensity of 2 MW/cm2. A slight spectral blueshift of 0.35 nm and narrowing in linewidth of 0.4 nm were observed with the increase of pump intensity. These observations manifested resonant coupling of the band edge emission to a single mode of the prismlike GaN cavity.

Moreover, it is reported the use of recessive size reduction in self-assembled sphere mask with anisotropic etching to form lenslike nipple arrays onto the surface of silicon (Si) and GaN. These devices are shown to exhibit a filling factor near to an ideal close-packed condition and paraboloidlike etch profile with a slope increased proportionally to the device aspect ratio. Specular reflectivity of less than 3% was observed over the visible spectral range for the 0.35-um-period nipple lens arrays. The Si photodetector fabricated with nipple lens arrays exhibited a 40% enhancement in the photo-current measurement compared to the device without the nipple array.

In order to obtain a surface structure with fine-tuned refractive index matching effect and antireflection for wide incident angles over full visible spectrum, we further combined the nipple lens array structure with thin film coating of zinc oxide (ZnO) to seek the optimised surface structure showing the best antireflection effect. It was found that Si nipple array coated with 50-nm-thick ZnO could exhibit specular reflectivity less than 1% over the visible spectral range at vertical incident angle. In the oblique incidence measurements, the ZnO/lens array structure showed suppressed reflection below 2.5% for TE mode at incident angle of 30° and reflection below 1% for TM mode at incident angle of 45°, respectively, further confirming the improvement of antireflection effect on ZnO/lens array structure.

並列關鍵字

Subwavelength ； Lens array ； Antireflection

參考文獻

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