非極性氮化銦鎵/氮化鎵發光二極體的效率改善和特性研究

Abstract

相較於成長於極性的多重量子井，成長於非極性方向的多重量子井由於可規避掉量子侷限史塔克效應，所以被視為極有潛力開發成為同時具有高量子效率和低波長飄移的光電元件。然而非極性a-面氮化鎵成長於r-面藍寶石基板時，會因為極大的晶格不匹配產生大量的差排密度，這將限制非極性光電元件的發光效率。因此本論文首要提出兩種可行性高的磊晶方法來優化氮化鎵成長並減少材料缺陷，其中之ㄧ為利用氮化銦鎵超晶格結構彎曲向上竄升的差排，讓它們彼此消滅，進而降低差排密度。另外一種方法為在氮化鎵奈米柱使用側向磊晶成長，可以在側向磊晶成長的區域得到高品質的氮化鎵。由穿透式電子顯微鏡的觀察，可以觀察到此兩種方法都可以有效降低在a-面氮化鎵內部的差排密度。隨後，我們個別成長多重量子井於此兩種磊晶方法的基板上，並且以變激發強度變溫度光激發螢光光譜研究其光學特性，包含熱活能計算、內部量子效率估算、載子再結合係數等等。由材料品質分析和光學特性評估，氮化鎵奈米柱側向磊晶成長具有較高的潛力製作非極性發光二極體。從電激發光量測中，和發光二極體直接成長於藍寶石基板相較之下，發光二極體成長於奈米柱側向磊晶基板可以提升發光強度十倍左右。另外我們也嘗試成長藍綠光非極性發光二極體於高品質的m-面氮化鎵基板，並且觀察到其量子效率較不隨電流注入而有所衰退，這和傳統極性發光二極體會產生很高的量子效率衰退明顯不同。由數值模擬分析，我們認為量子效率衰退的主要原因和極性多重量子井本身具有很高的內建電場相關。

Wurtzite (Al,In,Ga)N alloys grown in nonpolar direction are the candidates with the most potential for the fabrication of such devices since high internal quantum efficiency and a small blue-shift of the emission peak will be expected due to elusion of the quantum confined Stark effects (QCSE). However, in general, there is very high dislocation density in a-plane GaN grown on r-plane sapphire because of the large lattice mismatch, which would restrict the performance of nonpolar light-emitting devices. Therefore, in this thesis, we propose two approaches to reduce dislocation density. One is the InGaN/GaN superlattices. The dislocation lines that run parallel to the growth direction are bowed as a result of the coherent strain present in the superlattices. Then, the dislocations move laterally and are consequently eliminated with other dislocations of the opposite Burgers vector. The other is the nanorod epitaxial lateral overgrowth (NRELOG). We can achieve high crystal quality a-plane GaN in the lateral overgrowth region. From the observation of transmission electron microscope (TEM), the dislocation density in a-plane GaN is indeed reduced by these two kinds of epitaxial approaches. Subsequently, we grow the multiple quantum wells (MQWs) on these two kinds of template and then utilize the excitation power and temperature dependent photoluminescence (PL) to investigate their optical properties, including thermal activation energy, internal quantum efficiency and carrier recombination coefficients. Taking into account the material quality and optical characteristics, we conclude that the method of NRELOG has higher potential to fabricate the nonpolar light-emitting diodes (LEDs). From electroluminescence (EL) measurement, the output power of NRELOG LEDs exhibit approximately 10-fold enhancement compared to that of as-grown LEDs. Furthermore, we also try to grow blue-green LED structure on high-quality freestanding m-plane GaN substrate and observe that the efficiency of m-plane LEDs can be retained with current increasing, which is apparently different to conventional c-plane LEDs with high efficiency droop. By numerical simulated analysis, we attribute the origin of efficiency droop to built-in internal electric fields of c-plane polar MQWs.