摘要:隨著顯示科技和照明需求的發展,白光發光二極體由於體積小的優點加上理論上具有高效率的優勢,現已逐漸成為應用於液晶螢幕的背光模組以及一些固態照明領域的主流科技。然而,使用三五族氮化銦鎵為基材所研發的發光二極體在相關領域的應用上,在發光效率尤其是綠光和紅光的表現方面仍有著有待突破的瓶頸。而探究這些現象的根本,其實都與發光二極體元件內部的張力和張力所造成的極化場息息相關。因此,在研發設計應用於RGB三原色或非使用磷光粉的白光發光二極體時,二極體元件內部由於晶格不匹配所形成的張力是無可避免要面對的首要問題。而氮化銦鎵量子點和量子線在內部張力的表現明顯較傳統的量子井元件要來的減少許多,應用於白光發光二極體的研發上具有相當的潛力。在本計畫中,主要的目的在建立適當的數值分析模型來分析氮化物之量子線和量子點發光二極體的表現行為,計畫考慮的變因包括載子的流動分析、電子和電洞的發光、激子的產生、晶格內部應力之影響、極化效應和能帶結構之修正等。
本研究計畫分三年執行,第一年的研究方向為延續之前國科會計畫的研究主軸,針對量子點的問題作更深入一步的研究,我們計畫針對量子點內部載子注入的機制,量子點之間的穿遂效應,和各層之間wetting layer與量子點耦合的效應來做進一步分析,期望藉由各相關變因的系統性分析結果找出增進其內部量子效率和發光效率的方法;同時對於量子線的問題,也將同步開發相關模型(如k$\cdot$p model)以計算其能帶結構和分析極化效應所可能造成的影響。第二年的研究重心則是針對量子線或奈米線張力變化的問題發展計算張力的模型,藉以分析氮化銦鎵二極體元件張力之變化,並且以k.p方法和蒙地卡羅方法來計算能帶並分析載子在量子線內部的流動,以及輻射結合和非輻射通道的結合,進一步分析並預測其發光譜線。同時我們會發展解三維的Poisson 方程式和 drift-diffusion方程式的軟體。第三年的研究目標則是完成完整的數值分析軟體的開發,並針對整體元件從電極載子載入、載子在元件內部流動到結合發光的整個過程來作分析和整體優化的設計。期望經由以上之整體規劃,能夠建立完整分析低維度元件的數值模型,同時希望其分析結果能為氮化物發光元件的研發工作提供有價值的參考參數與預測數值,而有助於相關科技研發與應用的進展,並且能更進一步應用於其他元件材料的分析而能有更廣泛的實質貢獻。
Abstract: White light LEDs have become very important technologies for many applications such as lighting and LCD back light modules. However, LED-based lighting systems using III-nitrides suffer from two key shortcomings related to electrical and light efficiency values. These are poor efficiency for green light emission and efficiencies that rapidly degrade at high current injection. All these issues are closely related to the native strain and polarization between InGaN alloy and its substrate. Therefore, in order to make the RGB or white light LEDs without phosphor, it is necessary to overcome the strain and polarization effects. Quantum dot and quantum wire LEDs have less strain issues and have very high potential to emit whole spectrum of visible light. In this proposal, we will build up numerical models for analyzing and designing the nitride-based quantum dot and wired LEDs. The carrier transport, recombination, strain, and polarization effects of InGaN quantum dot/wire will be fully considered in our proposal. In this three-year proposal, we will try to build a sufficient numerical model in analyzing the low dimensional system. For the first year, we will continue our previous NSC proposal on the study of quantum dot devices. We will further study the carrier injection mechanism, the tunneling effect among dots, and the coupling effect between wetting layers and dots. We will try to find the optimum design in fabricating the quantum dot devices to control the light emitting spectrum, and enhance lighting efficiency. Meanwhile, we will start to develop numerical models such as 2D $k\cdot p$ method for quantum wire structures. At the second year, we will start to work on the calculation of the quantum wire structures. We will apply 2D $k\cdot p$ method and modified valence force field method to calculate the band structure of quantum wires, study the effect of strain, and the strain relaxation mechanism inside quantum wires. We will also apply the Monte Carlo technique on studying the carrier transport, radiative and non-radiative recombination mechanisms inside the quantum wire system. At the same time, we will start to develop 3D self-consistent Poisson and drift-diffusion solver which can handle the calculation of most complicated device structures. For the third year, we will finish all the planed numerical models. With all these models, we can study the whole life cycle of carriers from injecting at contact to recombine inside active layers so that we can apply it on optimizing the device performance. In this proposal, we will try to build the capability of analyzing the low dimensional systems. Besides, our models can also be widely applied to study other material systems such as ZnO nano wire, etc..