氮化鋁鎵薄膜表面氮化鎵圓盤層之微拉曼及微螢光研究

Abstract

本論文主要利用微拉曼光譜（μ-Raman）、微螢光光譜（μ-PL）及原子力顯微鏡（AFM）等實驗技術去研究有關AlGaN六角丘狀（Hexagonal Hillock）上成長氮化鎵之光學特性（GaN disk）。這些Hillock為成長氮化鋁鎵薄膜時，自然於薄膜表面形成之缺陷，再利用原子層磊晶（ALE growth）的方式，在氮化鋁鎵上成長出氮化鎵層結構。 室溫下不同類型六角丘狀（Hillock）的微螢光光譜，所得氮化鋁鎵譜峰位置有所不同，說明氮化鎵形成在不同鋁組成環境下；而由微拉曼光譜對應來看，氮化鎵層在相對平坦處之完整Hillock處其所承受的壓應力不一致。 針對Hillock內外不同位置作直線掃描，從微拉曼光譜得到一類似W的走勢；而由微螢光光譜的氮化鎵譜峰來看，也是類似W之趨勢。兩光譜顯示Hillock內外的氮化鎵承受不同應力的變化。此外μ-¬Raman及μ-PL兩光譜皆顯示越靠近Hillock正中心存在最大的氮化鎵強度。 對不同尺寸Hillock中心處的氮化鎵層作研究，從微拉曼光譜，我們發現就Mesa-like Hillock及Tent-like Hillock兩類型六角丘狀而言，會隨著Hillock尺寸增加兩類型有著相反的趨勢。就Tent-like Hillock而言，發現在此類型的拉曼譜峰會隨Hillock尺寸增加會向低頻位移；另外微螢光光譜的 也是同樣的趨勢。在此推論是由於氮化鎵隨著Hillock尺寸增加而會受到較小程度的壓縮應力(Compressive strain)所造成的。究其原因是氮化鎵底下之氮化鋁鎵的鋁組成隨Hillock尺寸增加有越小的趨勢。此處由μ-¬ Raman及μ-PL之光譜分析，明顯可看出相互吻合的結果。 上述的研究皆利用定位的技術讓μ-¬Raman及μ-PL的實驗針對同一處作量測。由本論文實驗的結果，可以發現最上層的氮化鎵結構的確和底下氮化鋁鎵在訊號上有所關聯。而從微拉曼光譜了解氮化鎵所承受應力的變化；另一方面也從微螢光光譜氮化鎵譜峰得到互相吻合的驗證。

In this thesis, we studied GaN on different types of Hillocks by μ-Raman, μ-PL and AFM. Hillocks formed on the AlGaN film and the GaN disks were grown on the AlGaN film by ALE method. From room temperature μ-PL spectra, the peak position is different for different types of hillock. This means GaN disks were grown on the regions with different Al content so that GaN disk on the hillocks and on the plane region are under different compressive strain. By line-scanning a cross the hillocks, we observed that the relationship between Raman peak ( or PL peak ) and positions is W-like shape. Both Raman and PL spectra show that the strain inside and outside the hillock are different. In addition, the GaN signals show maximum on the center of the hillock. By comparing μ-Raman spectra of GaN on Mesa-like and Tent-like hillocks, they showed the opposite trend with increasing hillock size. For Tent-like Hillocks, the Raman and peaks of GaN shift to lower frequency with the increasing size. From μ-PL spectra, peak also shifts to lower energy. We suggest that GaN bears the less compressive strain with the increasing hillock size as due to Al composition decrease of AlGaN. It is obvious that μ-Raman spectra are consistent withμ-PL spectra on the same spot. We suggest that GaN disk-like structures are related to AlGaN films. From μ-Raman spectra, the GaN disks are under different strain.