本論文主要利用電子束蒸鍍技術來蒸鍍氧化鋅鋁(Aluminum Zinc Oxide, AZO)之透明導電薄膜，文中首先對於不同摻雜鋁含量之氧化鋅鋁薄膜進行材料之熱穩定性之光電特性分析，接著探討氧化鋅鋁在有無氮化銦鎵(InGaN)接觸層之P型氮化鎵(p-GaN)上之歐姆接觸特性研究，最後將AZO薄膜蒸鍍在有無氮化銦鎵接觸層之氮化鎵藍色發光二極體上，探討其光電特性差異。 本實驗採用電子束蒸鍍(electron-beam evaporation)系統來成長AZO薄膜，首先將不同摻雜鋁元素含量的氧化鋅鋁透明導電膜(ZnO:Al=95%:5%)及(ZnO:Al=90%: 10%)蒸鍍於藍寶石基板(Sapphire)上，在氮氣環境下不同溫度熱處理1分鐘後， 5wt.% AZO與10wt.% AZO薄膜之穿透率，在可見光波段380 nm~700 nm皆可高於90%。但在電阻率方面，在900℃氮氣環境下熱處理1分鐘後，5wt.% AZO之電阻值會從3.6×10- 4 Ω-cm上升到6.85×10-4 Ω-cm，相對的10wt.% AZO薄膜之電阻值會從3.7×10-4 Ω-cm上升到1.12×10-3 Ω-cm ，由實驗結果得知5wt.% AZO薄膜在熱穩定性上有較佳的表現。 將5wt.% AZO當作透明導電薄膜，直接成長於具有氮化銦鎵接觸層之P型氮化鎵上(InGaN/p-GaN)，可得到線性之歐姆接觸特性，並在氮氣環境下不同溫度(500℃~900℃)熱處理1分鐘後，特徵電阻值並沒有明顯改變(~1.35×10-2 Ω-cm2)，這是因為InGaN接觸層在AZO薄膜與p-GaN間形成電流穿隧機制提供較佳的歐姆接觸。將5wt.% AZO薄膜直接成長在P型氮化鎵上，無法直接得到線性之歐姆接觸特性，但在900°C熱處理1分鐘後，可得到線性之歐姆接觸特性，其特徵接觸電阻值(ρC)約為1.37×10-2 Ω-cm2，其原因為熱處理後，其AZO與P型氮化鎵接面處會產生鎵元素空缺，提高P型氮化鎵表面濃度導致空乏區變窄，使電子有較大的穿隧機率，因而降低其接觸電阻，得到線性歐姆接觸特性。 比較AZO成長在具有氮化銦鎵接觸層之氮化鎵藍色發光二極體上(AZO/InGaN/LED)與AZO成長在氮化鎵藍色發光二極體(AZO/LED)之光電特性可發現，在20mA注入下，AZO/InGaN/LED與AZO/LED其順向導通電壓(Vf)分別為3.3V與5.25V，且其光輸出功率( light output power )分別為4.32 mW與4.20 mW，在900°C熱處理1分鐘後，其AZO/InGaN/LED 順向導通電壓和光輸出功率沒有太大的差異，這是因為具有InGaN接觸層之氮化鎵藍色發光二極體與AZO薄膜有良好的歐姆接觸特性和熱穩定性，相對的AZO/LED之順向導通電壓可以改善到3.24 V，但是其光輸出功率卻會下降到3.28 mW，其順向導通電壓改善的原因為經由熱處理後，AZO與P型氮化鎵之特徵接觸電阻值下降所導致，而光輸出功率下降的原因是因為熱處理後，AZO薄膜阻值上升且AZO/LED之接觸電阻值下降造成電流侷限效應(current crowding effect)。 經由本研究可知，AZO薄膜具有高光穿透率、低電阻值和薄膜熱穩定性，應用於發光二極體上，AZO在具有氮化銦鎵接觸層之氮化鎵藍色發光二極體表現出極佳的熱穩定性，相對的在沒有氮化銦鎵接觸層之氮化鎵藍色發光二極體，透過熱處理製程，即可達到良好的歐姆接觸特性，但其光輸出功率卻有下降的問題，我們預期未來可利用電流阻障層使得電流均勻分佈達到改善其光輸出功率下降的問題。In this study, the conductive transparent ZnO:Al (AZO) thin films with 5wt% and 10wt% Al2O3 were deposited at 350 oC by electron-beam evaporator. The optoelectronic characteristics of AZO films were investigated. The resistivity of as-deposited AZO thin films with 5wt% and 10wt% Al2O3 content were about 3.6×10-4 and 3.7×10-4 Ω-cm, respectively. Optical transmittances of both AZO films are more than 90% in visible regions. However, resistivity of AZO thin films with 5wt% and 10wt% Al2O3 content were increased to 6.85×10-4 Ω-cm and 1.12×10-3 Ω-cm after 900 oC thermal annealing, respectively. This indicates that AZO films with 5wt% Al2O3 content shows better thermal stability than AZO films with 10wt% Al2O3 content. AZO films with 5wt% Al2O3 content were deposited on InGaN/p-GaN and p-GaN. Samples were than annealed in the temperature range of 500-900 oC. As-deposited AZO film onto InGaN/p-GaN exhibits an ohmic characteristic. The specific contact resistance is about 1.35×10-2 Ω-cm2. No significant change on specific contact resistance can be observed after 500-900 oC thermal annealing. On the other hand, the as-deposited AZO film onto p-GaN exhibits a non-ohmic characteristic. However, ohmic characteristics can be obtained after 500-900 oC thermal annealing. The specific contact resistance decreased with increasing the annealing temperature. XPS results show that a Ga 2p core-level peak shifted toward the valence-band edge by 0.3 eV, indicating that the surface Fermi level was shifted toward the valence-band edge. This suggests that the surface barrier height for hold injection from AZO to p-GaN can be lowered by thermal annealing, thus resulting in a drastic reduction in specific contact resistance. AZO thin films with 5wt% Al2O3 content were employed as transparent contact layers (TCL) of LED with InGaN contact layer (AZO/InGaN/LED) and LED W/O InGaN contact layer (AZO/LED). The forward voltages (Vf) at 20mA of the InGaN/LED and LED with as-deposited AZO TCL are 3.3V and 5.25V, respectively. After 900 oC thermal annealing in N2 ambient for 1 minute, the forward voltage of the AZO/LEDs decreased to 3.24V while the forward voltage of AZO/InGaN/LED was not affected by thermal annealing. The reduction of forward voltage of AZO/LED is attributed to the fact that the specific contact resistance was decreased with increasing the annealing temperature. In addition, the output power at 20mA of AZO/InGaN/LED decreased only slightly from 4.32mW to 4.2mW after 900 oC annealing. With the same annealing condition, it was found that output power decreased obviously from 4.2mW to 3.3mW for the AZO/LED. It is attributed to the current crowding effect that is more serious with increasing the annealing temperature. This problem is expected to solve by using the current blocking layer under the p-electrode. The material of AZO shows the superior thermal stability and is highly promising that the AZO thin film deposited by electron-beam evaporator could be a transparent electrode application on GaN-based light emitting diode.
