在這篇論文裡，為了改善氮化鎵(GaN)發光二極體(light-emitting diodes, LEDs)結構的電流散佈(current-spreading)特性與發光強度，我們分別提出了新穎的元件製程與結構磊晶過程。此外，針對成長在不同偏角度藍寶石基板的氮化鎵發光二極體元件，我們分別對其光電特性及結構磊晶品質做一詳細探討。
為了改善氮化鎵發光二極體結構的電流散佈效率，我們提出了一Mg-doped GaN/ intrinsic InGaN 短週期超晶格(superlattice, SL)結構。此超晶格結構係安插在p型氮化鎵與多重量子井(multiple-quantum well, MQW)間，以分散多重量子井前的電流注入密度，提升電流散佈面積。相較於傳統LED元件(簡稱元件A)，此研發元件(簡稱元件B)在20 mA注入電流下，其輸出光功率(light output power)與外部量子效率(external quantum efficiency, EQE)各增加了25.4及5 %。此外，超晶格結構之應力(可能是磊晶層與基板晶格不匹配所導致)調變特性使元件B之p-GaN表層變得較為平整，金-半接面之接觸特徵阻抗降低，因此導通電壓獲得改善。相較於元件A，元件B之漏電流亦較小,這可能是因為超晶格結構提升了p-GaN的磊晶品質。
另一種可行的方式是在MQW與p-GaN層間加入一薄的高摻雜n-GaN層，在此區域所形成的p-GaN/n-GaN內建能障(energy barrier)會使LED元件具有更好的電流橫向散佈能力。此外，電流阻塞(current crowding)效應的減少抑制了LED元件寄生電阻效應。因此，在20 mA的操作電流之下，此種新穎結構能有效降低導通電壓、串聯電阻、以及提升靜電防護能力。在60 mA操作電流的情況下持續1173小時，本研究之LED元件只有約2±0.4 %的光強度衰退現象。這項結果証實了此新穎結構確實能有效降低因電流阻塞產生熱所帶來的熱破壞。
我們亦希望藉由改變LED元件製程以達到提升電流散佈能力之目的。吾人製作並探討一富含鎵空缺之p-GaN接觸層應用在提升氮化鎵系LED之光電特性。利用熱蒸鍍法覆蓋一層厚度約為1 nm的金在p-GaN接觸層之上，並加以高溫退火(氮氣氛圍下，400 °C，30 min.)且蝕刻，使得p-GaN表面鎵空缺增加，進而改善透明導電膜ITO(indium-tin oxide)和p-GaN的歐姆接觸性質。這是因為金與p-GaN接面的高溫處理過程會使金原子向p-GaN內層擴散。p-GaN層的鎵易與金形成金-鎵固融體，在接下來蝕刻金的過程中此固融體會被一併帶離p-GaN表面，而在p-GaN表面留下高密度的鎵空缺。在20 mA的操作電流之下，此LED結構能有效降低元件導通電壓及動態電阻，而其均勻的電流散佈面積亦將使元件輸出光功率提升約18 %。
此外,當氮化鎵LED成長在偏角度藍寶石基板(c(0001)-軸向m(1-100)-軸偏0.2-1.0 °)時，各結構磊晶成長模式會改變。成長在0.2 °基板的氮化鎵磊晶層其成長以島狀模式(island mode)為主。隨著角度增大，磊晶層的成長模式逐漸轉為微(micro step)至巨型(macro step)階梯形狀。我們發現，不同的成長模式會影響磊晶時MQW中銦(In)成份的不均勻度,即銦-鎵相分離(phase separation)程度。基板偏的角度越大至1 °，相分離愈不明顯，但是元件的光輸出功率降低。這是因為適度的相分離，會形成局部侷限態(localized state)幫助侷限載子，提升元件之內部量子效率。此外，我們發現自然成長(naturally grown)在p-GaN層上的金字塔狀粗糙結構之密度與大小亦會受到偏角度程度的影響，進而影響到元件的抗靜電能力(electro-static discharge tolerance)。

In this dissertation, for the purpose of enhancing current spreading characteristics as well as light emission intensity of GaN-based light-emitting diodes (LEDs), novel device fabrication processes and structure epitaxial techniques are proposed, respectively. Moreover, the optical and electrical properties and crystalline qualities of LED structures which are grown on vicinal sapphire substrates with different tilt angles are discussed in detail.
For improving current-spreading performance of GaN-based LEDs, an Mg-doped GaN/ intrinsic InGaN short-period superlattice (SL) structure is applied. This structure is grown between p-GaN and multiple-quantum well (MQW) layers by means of spreading the current injection density prior to MQW layer, hence a more uniform current spreading becomes possible. As compared with conventional LED devices (denoted as device A), at 20 mA operation current, the studied ones (denoted as device B) exhibit improved light output power and external quantum efficiency (EQE) about 25.4 and 5 %, respectively. In addition, a smoother p-GaN surface is obtained resulted from strain modulation induced by the SL. The strain may mainly come from lattice mismatch between epitaxial layers and the sapphire substrate. An improved Ohmic contact property at metal-semiconductor interface and hence reduced turn-on voltage are found. The device B shows lower forward-/reversed- biased leakage currents than those of the device A. This may due to the better epitaxial quality of p-GaN layer with help of the SL structure.
An another feasible way to improve current-spreading efficiency of LEDs is inserting a thin and highly-doped n-GaN between MQW and p-GaN layers. The naturally-formed p-GaN/n-GaN built-in energy barrier in this region brings about a better current spreading capability. Furthermore, reduction of the current-crowding effect reliefs parasitic resistances of LEDs. At 20 mA operation current, this novel device could effectively reduce operation voltage, parasitic series resistance, and electrostatic discharge tolerance of LEDs. The studied LED device exhibits only 2 ± 0.4 % degradation in power during 1173 hrs aging time at 60 mA operation current. This result could be attributed to the relieved thermal damage induced by current crowding.
We also bring about an enhanced device process to improve current spreading of LEDs. One could fabricate and study a gallium-vacancy-rich p-GaN contact layer applied for GaN-based LEDs. An 1 nm Au thin film was deposited on the p-GaN layer by thermal evaporation followed by thermal-alloyed (400C for 30 min. in nitrogen ambience) and removed processes. This results in increased density of gallium vacancies on the p-GaN layer, and a better Ohmic contact property at p-GaN/ITO interface is achieved. This is because that Au atoms easily diffuse into the p-GaN layer during thermal annealing. Au-Ga solid-phase solution is formed at p-GaN/ITO interface. The followed metal removed process helps taking the solution away, and many gallium vacancies remain on the p-GaN layer. At 20 Ma operation current, reduced turn-on voltage and dynamic resistance are obtained, and improved 18 % light output power due to more uniform current spreading is achieved.
Moreover, when GaN-based LEDs are grown on sapphire substrates with 0.2-1.0 ° tilt angles from c (0001)- toward m (1-100) planes, modes of growth of GaN epitaxial layers will be changed. Epitaxial layers gown on a 0.2 °-tilt substrate exhibit island-dominated growth mode. With increasing tilt angle, growth types are gradually changed from micro- to macro- step modes. It is found that the indium (In) composition and uniformity over whole MQW layers will be influenced by variant growth modes. In composition is more uniform while light output power of LEDs becomes degraded until the tilt angle is increased to 1 °. Generally, power is decreased when In composition becomes relatively uniform. Local regions with non-uniform In composition could be regarded as localized states to help confining free carriers, thus improved internal quantum efficiency (IQE) of LEDs is found. In addition, it is observed that the density and size of naturally-textured pyramidal-shaped structures on p-GaN layer will be influenced by tilt angle of sapphires, and the corresponding electro-static discharge (ESD) endurance of LEDs will also be altered.

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