氮化鋁鎵薄膜表面六角丘狀結構之螢光光譜和拉曼光譜之研究

Abstract

本論文主要針對金屬有機化學氣相磊晶系統(MOCVD system)所成長的氮化鋁鎵薄膜(AlxGa1-xN)做一系列的微螢光光譜(□-PL)和微拉曼光譜(□-Raman)之研究。在顯微鏡下，我們可以在樣品表面觀察到幾種不同形狀的六角丘狀結構(Hillock)，它們的大小分佈是約2~16 □m。 藉由顯微鏡載台的刻度，可以標定Hillock的位置，由此方式我們可以針對同一顆Hillock進行□-PL和□-Raman的量測。因為應力對螢光光譜的影響很小，而且Hillock內外的鋁組成濃度，從EDX (Energy Dispersion X-ray Spectrometer) 所量測出來的結果與微螢光光譜所推算出來的一致，所以將使用微螢光光譜去決定Hillock的鋁組成濃度。 從微螢光光譜的量測中，發現Hillock結構內部會出現額外的發光譜峰(能量位置約∼3.51eV)，有別於該結構外部近帶躍遷的譜峰位置(能量位置約∼3.62eV)。根據微螢光光譜可以推導出Hillock內外的鋁組成濃度分別是約4%和約11%，進而再去計算出此濃度下所對應之沒有受到應力影響的E2模態位置，Hillock內部約在568.5cm-1而其外部約在570.2cm-1。接著使用微拉曼光譜去針對Hillock結構進行實際量測，實驗結果顯示，Hillock結構內部E2模態位置約在570 cm-1，其外部位置約在573 cm-1。比較微螢光光譜和微拉曼光譜之計算和量測結果，發現E2模態位置在Hillock結構內外分別有約1.5 cm-1 和約3 cm-1 的偏差，而此實驗結果顯示，Hillock結構受到了壓縮的應力影響。 我們亦使用微拉曼光譜去分析Hillock結構之形成機制。其結果顯示，Hillock結構內部之E2模態位置約在570 cm-1 而不會隨著聚焦深度變深而改變；聚焦深度越深，隨之出現的是sapphire Eg位於577 cm-1 的訊號。由微拉曼光譜的縱深分析結果，其證明了Hillock結構形成是從AlN緩衝層開始長成。

In this article, we analyzed AlxGa1-xN epilayer which was grown by MOCVD system, with the aid of the micro-photoluminescence (□-PL) and micro-Raman (□-Raman) systems. Under the microscope, we observed several types of hexagonal hillocks on the epilayer with sizes from 2 to 16 □m. By using the microscope to demarcate the hillock position, we can combine the □-PL□and□□-Raman system to study the same hillock on this sample. Because the PL results are relatively insensitive to the strain and the Al fraction from Energy Dispersion X-ray Spectrometer ( EDX ) measurements agrees with that deduced from □-PL whether inside or outside the hillock, so we used □-PL spectra to determine the Al fraction of hillock. From the □-PL spectra, we found that an additional emission peak at (~3.51eV) inside the hillock structure, that differs from the near-band-edge emission (~3.62eV) outside the hillock. According to the PL peak position, the calculated Al fraction is about 4% and 11% inside and outside the hillock. From the Al fraction, we also obtained the strain free E2 mode frequency to be 568.5cm-1 and 570.2 cm-1 inside and outside the hillock, respectively. However, the experimental results of □-Raman spectra show that E2 mode frequency is ~570 cm-1 and ~573 cm-1 inside and outside the hillock, respectively. These are blue shifted by ~1.5 cm-1 and ~3 cm-1 so that hillocks bear compressive stress. We also used □-Raman scattering to investigate how deep hillocks are formed. The results showed that the E2 mode frequency remains at ~570 cm-1 inside hillock, it dose not shift while the focus depth increases. However, the sapphire Eg mode frequency at 577 cm-1 grows obviously with the increasing focus depth. According to the depth analysis, it is evident that the formation of hillock is from the AlN buffer layer.