氮化銦奈米粒之成長與特性分析

Abstract

本篇論文主要是利用原子力顯微鏡(AFM)及螢光光譜(PL)等實驗技術來研究關於InN奈米粒(nano-dots)在不同覆蓋厚度(coverages)的過程中的長晶動力學，以及對形貌和發光特性的研究。 利用原子力顯微鏡，我們觀察到以MOCVD的方式成長InN奈米粒在650o時會呈現兩種不同形貌的奈米粒，分別為平頂(flat-top)和圓頂(dome-shape)的奈米粒，並且隨著覆蓋的厚度改變時，兩種不同形貌的奈米粒會有所消長。從高寬的統計，對平頂的奈米粒而言，自1到3ML時其直徑由133奈米增加至428奈米，從3到12ML其直徑則由428奈米增加至667奈米；而對圓頂的奈米粒而言，自1到3ML時其直徑由143奈米增加至332奈米，從3到12ML其直徑則由332奈米增加至509奈米。兩種不同形貌的奈米粒在1到3ML之橫向成長速度約為~100nm/ML，至於3到12ML則約為20nm/ML，在1到3ML較傾向於橫向的成長，至於3到12ML則較傾向垂直成長。 為釐清兩種形貌的奈米粒產生消長的原因究竟是競爭的機制(competition)，亦或是演化的機制(evolution)，我們進一步對不同形貌的奈米粒分別作角度和表面積對體積做統計。兩種不同形貌的奈米粒在一開始就會有不同的角度，隨覆蓋厚度增加，平頂(flat-top)的奈米粒的角度由7o變化至11o，而圓頂的奈米粒角度約由21o變化到35o時飽和，在6ML時則有一群奈米粒的角度的分佈出現在~21o，出現此一新的群組的角度會隨著覆蓋率的增加而增加，逐漸併入圓頂奈米粒的角度分佈，由於較高的角度在較大的體積時會具有較高的能量，所以推測這一新的角度分佈應是來自於低角度的平頂的奈米粒。由表面積對體積的圖中，超過3x106nm3後，平頂的奈米粒表面積對體積的斜率會逐漸傾斜向圓頂奈米粒之表面積對體積的斜率靠近。這似乎隱含著隨著體積的增加，平頂的奈米粒會傾向於改變其形貌而成長為圓頂的奈米粒。從上面的結果顯示，若以能量的觀點會產生這樣的演化過程，表示在這樣的長晶條件下，形成圓頂的奈米粒會比形成平頂的奈米粒具有較低的能量。

In this thesis, the growth mechanism of InN nanodots on GaN, grown by metalorganic vapor phase epitaxy, at 650oC with different coverages were investigated comprehensively in terms of surface morphological parameters and optical properties. Experimental results indicate that there exhibits concurrently two types of InN nanodots, namely flat-top and dome-shaped islands on the surface, which tend to vicissitude with coverages. The corresponding diameters of InN islands was found to increase sharply from 133 to 428 nm for flat-top island and 143 to 332 nm for dome-shaped island for 1 to 3 ML coverage. Then, the growth became slower in the range of 3-12 MLs with diameter varies from 428 to 667 nm and 332 to 509 nm for flat-top and dome-shaped islands respectively. These dots appear to grow preferentially in lateral direction at initial stage (1-3 MLs) and favor to vertical direction for coverages ranged between 3 and 12MLs. More interesting results were found in contact angle histogram plot. At the beginning, there exhibits two groups of contact angles peaked at 7o and 21o, corresponding to flat-top and dome-shaped islands, respectively. The mean peak values of dome-shape islands tend to move gradually toward higher values with the increase of coverage and finally stabilize at ~11o and ~35o. It is interesting to note that at coverage of 6 MLs one additional group appears at ~21o which turns to increase with its contact angle with increasing coverage and seems to merge completely into group of dome-shape islands at higher coverages. Since the island having higher contact angle possesses lower formation energy, we believe the additional group comes highly probably from flat-top islands. The plot of surface-to-volume ratio further confirms the argument of shape transition of flat-top to dome-shaped island at higher island volume. The slope of flat-top island is found to decrease gradually with volume and finally approaches to a value very close to that of dome-shaped islands.