氮化鎵部分應力釋放發光二極體之量子效率下降效應之研究

Abstract

近幾年在氮化鎵發光二極體上做表面粗化一直都是一個很熱門的題目。不少人提出各式各樣的表面粗化技術來提高出光強度，包括像多長一層粗化p型氮化鎵、在p型氮化鎵上直接做粗化或者是在透明導電層上粗化，都是能夠提升光萃取效率的粗化技術。不過至今基本上一般的研究都認為表面粗化只是單純提高光萃取效率而已，認為表面粗化與發光二極體的內部量子效率是沒有關係的。此外，目前常用定義發光二極體內部量子效率的方法主要是做低溫光激發螢光(photoluminescence)量測，這是假設在非常低的環境溫度時(接近0K)，進行非輻射復合的載子數量會趨近於零，所以幾乎全部載子都會進行輻射復合，也因此可以定義此時的內部量子效率是100%。我們認為這個量測方法雖然簡單易用，但此定義方法其實低估了很多其他的因素，在探討不同結構的發光二極體內部量子效率時，其實是很難去量測其絕對強度的。在本論文裡我們證明了就算在極低的環境溫度下，除了非輻射復合這個行為之外，仍然會有其他的因素來影響發光二極體的內部量子效率，在氮化鎵發光二極體因應力所造成的量子侷限史塔克效應(Quantum Confined Stark Effect)就是一個會影響內部量子效率的關鍵重要因素。在氮化鎵發光二極體裡，因為氮化鎵和氮化銦鎵的晶格常數不匹配，會導致在兩者之交界面產生一定程度的應力，而應力的產生會使得量子井裡額外產生一個內建電場，這個內建電場就會使得電洞跟電子之間重疊的波函數減少，進而造成內部量子效率的下降，所以我們可以藉由降低應力來提高發光二極體的內部量子效率。 在本論文中，我們成功利用表面粗化技術來讓發光二極體達到部分應力釋放。我們更明確證明表面粗化技術除了提高光萃取效率之外，由於達到部分應力的釋放，我們看到表面粗化結構之氮化鎵發光二極體其內部量子效率確實是比平坦型發光二極體還高。我們更利用拉曼散射量測技術去計算粗化p型氮化鎵之前與之後在氮化鎵/氮化銦鎵層裡應力的改變量並同時觀察表面粗化和平坦型發光二極體在光學特性的改變。最後，我們利用低溫PL量測來證明兩種不同結構發光二極體在量子侷限史塔克效應上的改變。 本論文另有討論發光二極體內應力的多寡與量子效率下降效應的關係。固態照明發展至今遇到了一個很大的問題叫“量子效率下降效應”(efficiency droop)，這個現象主要是描述氮化鎵系列之發光二極體的發光效率會在很低的電流密度就達到最大值，隨著工作電流的繼續提高，發光二極體的發光效率會持續劇烈地下降。量子效率下降效應會造成發光二極體在許多產品應用上的困難，甚至更會成為固態照明進一步在全球照明市場更深入發展的障礙。發光二極體與現在的大眾照明白幟燈泡相比，有著高亮度且更能節省能源的優點。這也代表著如果我們能有效解決量子效率下降效應，其實就是為整個地球減少大量資源和能源上的浪費。目前有很多學者提出許多不同的理論來解釋造成量子效率下降效應發生的原因，但至今仍然沒有非常明確的答案，也因此研究量子效率下降效應是一個毫無疑問非常重要的課題。 在本論文中，我們發現發光二極體裡所含的應力多寡與量子效率下降效應有一定程度上的關係，在實驗結果裡可以看到含較少應力的發光二極體除了其內部量子發光效率會更高外，其量子效率下降效應的嚴重程度也會越少。在本論文的第三章，我們做了發光二極體在不同接面溫度和環境溫度下的實驗，其結果顯示越高的接面溫度會導致越嚴重的量子效率下降效應。

External light extraction has been a very popular topic in the past few years for researchers in the field of GaN based light emitting diodes(LEDs). Various surface roughening techniques have been proposed to enhance extraction efficiency, include the growth of an additional rough p-GaN layer, the texturing on transparent conducting contacts and on p-GaN layer. On the other hand, the root causes of the internal quantum efficiency of an LED have been considered separately from those of the external light extraction. A commonly employed method to find internal quantum efficiency of a LED is by assuming complete frozen of defects states at very low temperature (~0K). The internal quantum efficiency is therefore assumed to be 100% as the injected carriers are all contributed to radiative recombination. Despite the simplicity, the method underestimates other factors for the internal quantum efficiency. Also, as will be explained in this work, it becomes difficult to identify the absolute internal quantum efficiency between devices with various structures. In addition to nonradiative light emission, other factor that affects internal quantum efficiency in a GaN based LED is the strain induced quantum confined stark effect (QCSE). For a GaN based LED epi-structure, lattice mismatch between InGaN and GaN results in strain. It creates the internal electric field that leads to the separation of electrons and holes in the quantum well region and decreases the internal quantum efficiency. In the literature, the influence of QCSE is observed to be suppressed from nanorods by etching through the LED epilayers, or from the LED structure on a pattern sapphire substrate . In this work, partially strain relaxation from the LEDs with surface textured p-GaN layer has been observed. The effect of surface roughening on external light extraction is thus correlated to the improvement of internal quantum efficiency due to relaxed strain. We first performed Raman measurement to study the strain in the InGaN/GaN layers with the top p-GaN layer textured. The optical properties were then explored by comparing the textured p-GaN device with the planar one. And finally, the effect of QCSE on both textured and planar p-GaN devices was analyzed by photoluminescent (PL) measurement at low temperature. A major obstacle for solid-state lighting is so-called 'efficiency droop', the way that the efficiency falls at high drive currents. Its origins were unclear. There are several mechanisms and theories proposed to explain the origin of droop effect, but up to the present, the physical origin of efficiency droop is still under hot debate. In this thesis, we identify the polarization field due to the strain resulted from lattice mismatch have closely relationship with droop effect. On the purpose of verification, we fabricated the surface-textured InGaN/GaN LED in order to achieve partial strain relaxation. In experiment result, the Raman scattering measurement and the blue shift of peak wavelength prove us that the reduction of strain is a key factor of droop effect and show that less strain in MQWs will have better internal quantum efficiency and have better LEDs’ droop performance. It prove that surface roughness not only improve the extraction efficiency but also the increase internal quantum efficiency. In addition, we also make ITO surface-textured LED to verify whether the behavior of droop change or not and the result show that it indeed have the better efficiency droop performance. Furthermore, by junction and ambient temperature experiment, we show that the electrical stress induced junction temperature raise of LED also have close relationship with efficiency droop. Finding the origin of efficiency droop is a very critical and important issue in our society now. Fabricating droop-free LEDs could be the key breakthrough to unlocking the general lighting market and make significant contributions to our human society.