三維數值模擬探討發光二極體中考慮奈米微結構的載子傳輸研究

Abstract

根據實驗結果顯示，有許多奈米尺度的結構擾動存在於三元化合 物氮化銦鎵量子井以及氮化鋁鎵電子阻擋層中。而這寫結構擾動的 尺度從個位數奈米尺度(隨機合金分布擾動)、數十奈米尺度(不完整 量子井結構)、或數百奈米尺度(V型缺陷)。而這些奈米尺度的結構 擾動將會顯著地影響載子傳輸以及輻射發光複合。因此，我們有必 要使用一個適當的模型去分析其影響。本篇論文中，我們將使用我 們實驗室開發的三維有限元素帕松與漂移-擴散模型去分析這些主 題。首先，為了瞭解極化位勢壘如何影響氮化鎵元件系統中的載子 傳輸現象，我們將考慮隨機合金分佈擾動去分析n-i-n氮化銦鎵量子 井系統、n-i-n鋁化銦鎵量子位勢壘系統以及使用不同種電子阻擋層 的發光二極體去分析結構中的傳導帶位能分佈、電流-電壓表現以及 內部量子效應。結果顯示考慮隨機合金分佈擾動可以更能擬合實驗 電流-電壓曲線，也顯示在較薄的磊晶層中，隨機合金分佈擾動將會 更顯著地影響載子局域以及傳輸。此外，我們使用二維以及三維的 模型去分析常存於綠光發光二極體中量子井不完整的結構。結果顯 示，考慮了不完整的量子井結構的計算結果更能接近實際元件的表 現。有了較能適當描述綠光發光二極體電性的模型，我們更可進一 步分析綠光發光二極體中其他物理行為與表現。在最後一章，我們將探討考慮V型缺陷在氮化銦鎵藍光發光二極體中的物理特性。我 們將使用三維應力-應變計算程式以及載子傳輸模型去分析結構中的 電流走向，並更進一步討論對量子效率以及電流-電壓的表現。而模 擬結果顯示，在V型缺陷濃度較低的側壁量子井會提供較小的極化 位勢壘以及較淺的位能井。因此V型缺陷可以幫助載子注入，並幫 助載子注入平面的主動區，使得載子可以避免被非輻射複合中心所 影響。而相較於傳統量子井發光二極體結構，V型缺陷能使載子較 能平均的分佈在平面量子井中。此外，我們更進一步地結合隨機合 金分佈擾動模型以及V型缺陷結構，去探討V型缺陷對接通電壓和量 子效率表現的影響。V型缺陷的結構將會使存有V型缺陷的發光二極 體相較於沒有存有V型缺陷的元件，能提供更多載子路徑且幫助載 子的注入，使得含有V型缺陷的結構有著更小的接通電壓以及更高 的內部量子效率。此外，考慮合金分佈不均的發光二極體以及V型 缺陷模型的接通電壓計算結果更能解釋實驗觀察的接通電壓。本章 的最後一部份，我們將探討不同大小的V型缺陷對元件的影響。不 同大小的V型缺陷元件受陷阱所致的非輻射複合中心影響程度不同 以及有著不同大小的量子井發光區域，將會對有著不一樣的內部量 子效率的表現。V型缺陷的特殊結構不只能提供額外的電洞流的路 徑也能防止載子在線差排缺線中非輻射複合。 關鍵字: 藍綠色發光二極體, 合金含量波動, 非完整量子井結構, V型缺陷, 氮化鎵, 氮化銦鎵, 氮化鋁鎵

The experimental results show that there are nano-scale composition fluctuations existing in the ternary alloy of InGaN quantum wells (QWs) and AlGaN electron blocking layer (EBL). The scales of fluctuations are ranging from the units nanometer scale (random alloy fluctuations), tens nanometer scale (imperfect QWs), or hundreds nanometer scale (V-pits). The existence of nano-scale fluctuations will affect the carrier transport and radiative recombination strongly. Therefore, we need to develop a suitable model to analyze these effects. In this thesis, we applied our inhouse 3D FEM Poisson and drift-diffusion solver to analyze these problems. In the beginning, to understand how the piezoelectric barrier influence the carrier injection in GaN device system, we took the n-i-n InGaN system, n-i-n AlGaN quantum barrier (QB) and light emitting diodes (LEDs) with different EBLs to analyze the conduction band potential distribution, I-V performance and internal quantum efficiency (IQE) by considering the random alloy fluctuation. The results show a better fit in I-V curve and reveal that the random alloy fluctuation will affect the carrier confinement and transport significantly, epecially in a thinner epi-layer case. Besides, the imperfect QWs which commonly exist in the green emission LEDs are modeled by our 2D and 3D simulation programs. According to the calculated results, we can more approach the experimental IV performance by considering imperfect QW structures. With properly modeling the electric property, this model could provide a basis for further modeling other physical properties in green LEDs. In the last part, a V-pit embedded inside the blue InGaN LED was studied. A 3D strain-stress sovler and carrier transport model were employed to study the current path, where the quantum efficiency and turn-on voltage will be discussed. Our calculated results show that the shallow sidewall QWs will provide extra hole current flow paths, and make the carrier distribution more uniform along lateral QWs than traditional planar MQWs, which have high piezoelectric barriers make carriers hard to flow through. In addition, the random alloy fluctuation model is applied in the V-pit structure to compare the turn-on voltage and quantum efficiency with planar structure LEDs. The sidewall structure will provide more percolation paths for carriers and improve the carrier injection so that the V-pit LEDs perform smaller turn-on voltage and higher simulated IQE value than planar MQW LEDs. Moreover, the simulated turn-on voltage of the V-pit LED with the random alloy fluctuation model can be pushed earlier to appropriately explain the experimental data. In the last part of this section, the carrier transport by considering the size effect is studied. The variation of the internal quantum efficiency (IQE) for different V-pit sizes is due to the trap-assisted nonradiative recombination and QW areas. The V-pit structure would not only enhance the hole percolation length but act as a potential barrier to prevent carriers from nonradiatively recombining in threading dislocations (TDs). Keywords: blue-green light emitting diode, alloy fluctuation, imperfect quantum well structure, V-shaped pit, GaN, InGaN, AlGaN