臺灣博碩士論文加值系統

光調制反射光譜(Photoreflectance modulation spectroscopy)的調制原理是利用激發光打在樣品表面上，激發價帶的電子躍遷產生電子電洞對，因此改變樣品表面電場，藉由此來達到調制的目的。不同的樣品有不同的能隙值，所以激發光(pump beam)能量必須大於半導體的能隙。由於本實驗樣品中含有高能隙的材料AlGaN[1] ，以往使用的氦鎘雷射光能量值無法激發材料的電子電洞對，因此須選擇短波長的氬離子雷射(λ=300nm)，四倍頻的Nd:YAG雷射(λ=266nm)，或是二極體(diode)雷射(λ=262nm)[2-4] 。本實驗我們採用汞燈(λ=254nm)裝置作為調制電場的激發光源，由於汞燈裝置具有短波長、發光效果穩定、購置成本低廉等優點，但其為一擴充光源，以致無法聚焦到一小點，因此無法作為一般光譜如PL(photoluminescence)的激發光源。本實驗樣品為GaN thin film與AlGaN/GaN 異質結構(heterostructure)。此篇論文將利用汞燈與氦鎘雷射作為激發光源，計算樣品內調制電場(δF)，比較PR光譜圖有何差變。並且利用汞燈裝置作為激發光源，對AlxGa1-xN/GaN異質結構作PR光譜探測，所得到光譜圖當光能量值大於AlGaN能隙(energy gap)時，將出現Franz-Keldysh oscillation，分析Franz-Keldysh 振盪的極值可以求得AlxGa1-xN中x值與內建電場(built-in electric field)值。

Photoreflectance (PR) spectra of a GaN thin film and an AlGaN/GaN heterostructure were measured by using a HeCd laser or a mercury lamp as a pump beam. The wavelengths (λ) of the HeCd laser and the mercury lamp are 325 nm and 253.7 nm, respectively. The energy of the HeCd laser is lower than band-gap energy of AlxGa1-xN (x > 0.2) so that electron-hole pairs cannot be generated in the AlGaN layer. Hence, the PR of the AlGaN was measured by using Argon ion laser (λ=300 nm) or quadrupled Nd:YAG (λ=266 nm) rather than HeCd laser in the previous works. In this work, the mercury lamp (λ=254 nm)was used as the pump beam. The problem with using the mercury lamp as the pump beam is because it is a diffused source so that it cannot be focused to a small spot. Nevertheless, defocused pump and probe beams were used in the PR measurement to improve signal to noise ratio. Hence, the diffused property of the mercury lamp is not a hindrance to the PR measurements.

第一章　導論及相關理論.......................1
1.1 前言......................................1
1.2 能帶......................................2
1.3 鍵結......................................5
第二章　調制光譜.............................7
2.1 調制光譜之簡介...............................7
2.2 調製光譜學的機制.............................9
2.3 電子躍遷理論...............................10
2.4 界電函數與折射率的關係........................13
2.5 低電場調制.................................16
2.6 中電場調制(穿隧效應).........................19
2.7 Franz-Keldysh Oscillation 與asymptotic form.........22
第三章　樣品介紹與特性分析..................28
3.1 實驗樣品介紹........................................28
3.2 應變的產生..........................................29
3.3 空乏電場(Fd).........................................34
第四章　實驗設計與分析......................38
4.1 實驗架構與調制原理..................................38
4.2 PR不同激發光源調制電場強度(δF)分析.................44
第五章　實驗結果與討論......................47
5.1 PR不同激發光源調制電場強度(δF)計算.................47
5.2 實驗圖形分析........................................51
第六章　結論..............................58
Reference..................................59

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