| 摘要: | 本論文主要探討利用化學氣相層積法所成長的p-i-n 氮化銦鎵/氮化鎵多層量子井和p-i-n氮化銦鎵/氮化鋁銦鎵多層子井的光學特性。主要的工作分為以下幾部份。 我們利用電場調制穿透光譜研究p-i-n氮化銦鎵/氮化鎵多層量子井的室溫光學特性。從實驗光譜的結果、我們分析出銦含量為23 % 的氮化銦鎵/氮化鎵異質結構，其本質極化電場可高達1.7~1.9 MV/cm，同時其電場方向和内建的p-i-n電場方向反向。同時在實驗光譜我們也發現到量子井的禁止態(e1-hh2)。這些實驗結果結合理論計算可以提供我們有關III-氮族量子井相關的背景資訊，比如像異質結構中導帶價帶的能隙比率，極化電場的大小等等。 對氮化銦鎵/氮化鋁銦鎵多層量子井，因為在結構內含有四元化合物，所以我們首先利用二次離子質量譜來確認試片達到預期的量子井結構。從二次離子質量譜還發現高濃度的矽摻雜並不影響銦和鋁在量子井結構的分佈。我們利用電場調制反射光譜來研究這一系列的三元/四元多層量子井光學特性。豐富的微分光譜特性例如拋物線式的量子井能量態位移、訊號強度變化和180 ° 的訊號相位變化均一一呈現在改變外加直流電壓的調制光譜上。我們以調制光譜的基本原理對這些實驗結果給出一個完整的解釋。同時利用訊號強度變化的極小值來判斷量子井內電場強度為零的條件。最後利用波松方程式的自洽計算法來求得整個p-i-n量子井的能帶結構，從而我們得出相關的極化電場為0.21 MV/cm。同時我們比較In0.06Ga0.94N/GaN 和In0.06Ga0.94N/Al0.1In0.02Ga0.88N 這兩種結構發現Al0.1In0.02Ga0.88N/GaN 的自發極化電場強度為0.73 MV/cm。這個值和理論上不考慮非線性效應的理論值吻合暗示著在鋁含量比較多的氮化鋁銦鎵四元化合物、其bowing 係數很小。 同時，不同濃度矽摻雜對極化電場的影響也被考慮。從電場調制反射光譜結果發現對無摻雜和有摻雜的試片、其訊號強度最小值分別為 -0.1 V 和1.1 V。 從實驗結果我們推測造成這個現像的成因主要是在量子井能隙層的摻雜原子改變了能帶結構、而不是因為電場被摻雜所引致的自由載子屏壁。所以我們認為矽摻雜能夠使得量子井零電場發生在較低的操作電壓上，這一點將對光電元件的輻射性複合有所幫助。最後，我們也進行一系列的量子井井寬變化試片的電場調制反射光譜實驗。這一系列試片的結構和之前的氮化銦鎵/氮化鋁銦鎵多層量子井類似，只是改變量子井的井寬。利用已知的極化電場強度，我們理論估算此一系列量子井的約束態。從實驗結果和理論的一致性我們可以推論出它們的行為傾向傳統的量子井行為。同時利用電壓變化的調制光譜來探討III-氮化合物的極化電場是一個有效且值得信賴的技術。 在完成氮化銦鎵/氮化鋁銦鎵量子井的電場調制反射光譜量測後，我們也對未摻雜和摻雜濃度1x1018 cm-3的兩塊試片進行變溫的電激發瑩光光譜量測。電激瑩光的瑩光能量隨溫度變化也呈現一個”s”型的轉折。利用變溫的實驗結果擬合出未摻雜和能障層摻雜試片的σ值分別為9 和10 meV。同時兩者在溫度變化範圍各別為10 K 到 130 K及50 K到130 K時、其能量位移量也個別為10 和 7 meV。從數據的結果顯示量子井能障層的矽摻雜並沒有明顯提高磊晶的品質。最後綜合電激瑩光光譜和電場調制光譜結果，我們得出矽摻雜降低元件的起動電流是由於量子井零電場的條件在較低電壓出現的結論。所以適當的在量子井能隙層摻雜可以提高元件的表現。我們的實驗結果有助於最佳化紫外光--發光二極體的設計，這一點將有助於制造室溫操作的高亮度紫外—發光二極體。 This dissertation comprises the optical characteristic of p-i-n In0.23Ga0.77N/GaN multiple quantum wells (MQWs) and p-i-n InGaN/AlInGaN MQWs grown by metalorganic chemical vapor deposition method. The main works is divided to the following parts. The optical property of In0.23Ga0.77N/GaN MQWs is investigated by means of electrotransmittance at room temperature. The strength of piezoelectric field is estimated to be 1.7~1.9 MV/cm for the sample In0.23Ga0.77N/GaN, and its direction is oriented against the built in electric field and applied reverse bias. Meanwhile, the e1-hh2 forbidden transition state is also observed in the spectra. Such investigation combined with theoretical calculation can furnish us with valuable information about band offsets, piezoelectric electric field and etc. For In0.06Ga0.94N/Al0.1In0.02Ga0.88N MQWs structure, we identified that the ternary well and quaternary barrier by means of secondary ion mass spectroscopy measurement initially, and found that the high density si doping does not influenced the distribution of aluminum and indium. We investigated the InGaN/AlInGaN MQWs structure by means of electroreflectance. Fruitful results of bias-dependence ER feature such as parabolic-like energy shift, intensity variation and 180 ° are displayed. We give a simple explanation about modulation spectroscopy for these phenomenons of bias-dependence ER spectra. And then the in-well flat band is determined by the QWs signal intensity minimum. Using self-consistent calculation of Poission equation, we studied the quantum well field strength in p-i-n active region. And the polarization-field about 0.21 MV/cm is acquired by the condition of in-well flat band. We also acquired the spontaneous polarization field of Al0.1In0.02Ga0.88N/GaN structure by compared the difference of In0.06Ga0.94N/GaN and In0.06Ga0.94N / Al0.1In0.02Ga0.88N structure. The 0.73 MV/cm spontaneous polarization field is nearly consistent with the Vegard’s law prediction. This results implied that the weak bowing effect on AlGaN-like quaternary alloys. Meanwhile, the si doping effect on polarization field is also taking into account. The ER signal intensity minimums (turning voltage) of un-doped and doped samples are -0.1 V and 1.1 V, respectively. We supposed that the dopant ion in barrier layer is the major factor of influenced the p-i-n built-in electric field but not the free carrier induced by the dopant atoms. Therefore, the doping may cause the flat-band condition happened to lower applied bias, this would benefit to the radiation recombination of the opto-devices. Then that point should be considered on the designing of opto-devices. Finally, electroreflectance measurements for well-width dependence In0.06GaN/Al0.1In0.02Ga0.88N MQWs at room temperature have performed. The indium and aluminum content in well and barrier are the same as the previous samples. The quantum wells ground state is observed at zero volts. Used the known polarization field of this QWs, we obtained the theoretical confined state by an analytic form. The consistency between the theoretical curves and the experimental results presents that the In0.06Ga0.94N/Al0.1In0.02Ga0.88N MQWs structure behaves like that from a conventional QW. We also can claim that the method of acquirement the polarization field strength from the ER signal intensity minimum is trustworthy. After the electroreflectance measurement, temperature dependence electroluminescence for un-doped and 1x1018 cm-3 barrier doped samples is performed. The EL emission energy varying with the temperature exhibited the anomalous “s-shape” shift. In our measurement, the σ value obtained by the fitting are 9 and 10 me for un-doped and barrier-doped sample, respectively. Meanwhile, the blue-shift in the un-doped LEDs between 10 K and 130 K is as small as ~10 meV, which is reasonable consistent with that of the barrier-doped LED (~7 meV). So, the crystal quality of these two LEDs is nearly equal. Comparing the temperature dependence EL results and the ER results, we found that the threshold current is reduced by doping effect due to the flat-band condition is being enhanced. And this is the major factor for the improvement of the diode performance. This analysis makes it possible to optimize the UV-light emitting diodes (LEDs) structures, which is benefit for fabricating high-brightness nitrides-based UV-LEDs operating at room temperatures. |
