本論文主要針對氧化鋅鎵(Ga doped Zinc Oxide；GZO)透明導電薄
膜，應用於藍色發光二極體上之光電特性研究，並與不同的透明導電薄膜(鎳/金；Ni/Au、氧化銦錫；ITO)作比較。先經由探討透明導電薄膜對於P 型氮化鎵之歐姆接觸特性，並將最佳化之結果應用於藍色發光二極體之製程上。由於氧化鋅鎵薄膜直接成長在P 型氮化鎵上，無法得到良好的歐姆接觸。但藉由在氧化鋅鎵薄膜與P 型氮化鎵間成長一層氧化銦錫(ITO)薄膜，可有效改善其接觸特性。在經過最佳之熱處理製程後，Ni/Au、ITO 和ITO/GZO 薄膜得到的特徵接觸電阻值分別為3.05×10-2Ω-cm2、7.8×10-2 Ω-cm2 及1.33×10-1 Ω-cm2。
在發光二極體元件方面，以Ni/Au、ITO 和ITO/GZO 作為透明導電薄膜的各元件，在注入電流為20mA 時，其順向導通電壓(光輸出功率)分別為3.28V(2.1mW)、3.43V(3.4mW)和3.65V(4.5mW)。雖然ITO/GZOLED 在20mA 電流注入下的順向導通電壓比Ni/Au LED 要來的高，但光輸出功率遠高於Ni/Au LED，且功率轉換效率也比Ni/Au LED 高出一倍。這最主要是由於ITO/GZO 薄膜的穿透率較Ni/Au 要來的高，且薄膜厚度較厚的ITO/GZO 薄膜具有較低的侧向電阻有利於電流傳導，有效增加外部量子效率。
此外，為了要更近一歩提升外部量子效率，具有高折射率的GZO 薄膜可藉由稀釋的鹽酸蝕刻GZO 表面形成網狀粗化結構。表面粗化的ITO/GZO LED，在注入電流為20mA 時，可得到順向導通電壓與光輸出功率分別為3.75V 及6.7mW，且功率轉換效率也高達9%。推測造成LED整體光特性提升的原因有兩項：GZO 表面經過鹽酸蝕刻後，方塊凹槽部分的邊緣增加了光取出面積；且鹽酸蝕刻過後的GZO 表面呈現非常粗糙不平整的結構，造成LED 整體外部量子效率的增加。雖然蝕刻過後較薄的GZO 薄膜增加了薄膜側向電阻，造成LED 順向偏壓些微的上升，但由實驗結果可知網狀結構的ITO/GZO 薄膜可以有效的改善元件外部量子效率，進而有效提升元件光輸出功率。

In this study, gallium-doped zinc oxide(GZO)thin film contact to GaN-based light emitting diode(LED) was investigated and compared with Ni/Au and ITO thin film. We demonstrated a series of properties of different transparent contact layers(TCL) onto p-GaN and applied the optimization to the chip process of LEDs. In order to improve the electrical properties of GZO contact layer to p-GaN, ITO thin film was inserted between GZO and p-GaN. In this study, Ni/Au and ITO TCL was also prepared for comparison.The specific contact resistances of Ni/Au, ITO, and ITO/GZO to p-GaN were 3.05×10-2 Ω-cm2, 7.8×10-2 Ω-cm2and, 1.33×10-1 Ω-cm2 , respectively, when they were annealed at temperature of 550°C,600°C,and 600°C,respectively.
Based on the above TCL conditions, InGaN blue LEDs featured with Ni/Au, ITO, and ITO/GZO TCL were labeled as LED-I, LED-II and LED-III,respectively. With an injection current of 20 mA, these LEDs exhibited forward voltages (output powers) of 3.28V(2.1mW), 3.43V(3.4mW) and,3.65V(4.5mW) for LED-I, LED-II and LED-IIIrespectively. Although the 20mA forward voltage of LEDs with planar ITO/GZO composite TCL(LED-III) is slightly higher than LEDs with Ni/Au TCL (LED-I), the output power of the LED-III is far higher than LED-I. Therefore, the wall-plug efficiency was 2 times higher than that of LED-I. This improvement could be attributed to the fact that the ITO/GZO composite TCL has high transparency and a larger thickness as compared to thin Ni/Au TCL. The thick ITO/GZO TCL with low lateral resistance would also act as the current-spreading layer leading to an enhancement of light extraction. In addition, the thick ITO/GZO TCL with high refractive index could be partially etched away using an HCl solution to form a periodic texture on the GZO layer and thereby result in a further enhancement of light extraction efficiency.
In order to further increase the light output power, periodic texture with mesh structure was performed on ITO/GZO TCL, and these LEDs were labeled as LED-IV. The light output power of LED- IV is even higher that of LED-III, exhibiting an output power of around 6.7 mW corresponding to a WPE of around 9 % at an injection current of 20 mA. Comparing LED-III and LED-IV, this marked enhancement of light output or WPE in the LED-IV is due to the texture added to the GZO layer. Two reasons account for the increased extraction of photons. First, in the LED-IV, photon extraction can occur at the etched GZO side wall, whereas LED-III has no such side wall area due to the planar surface. Second, the etched GZO surface is rougher than that of the as-deposited GZO surface. The escape probability of photons generated in the active layer of the LED can be enhanced by increasing the angular randomization of photons at the roughened surface. Considering the electrical property of LED-III and LED-IV, although the surface texture performed on the GZO layer should cause a reduction of lateral cross-section area and hence an increase of lateral resistance, only a slight increase in Vf can be obtained indicating that the texture process would not significantly result in a degradation of the electrical property compared to those of GaN-based LEDs with planar GZO TCL.

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