探討隨機合金和厚度擾動下垂直傳輸與橫向擴散的綠光多重量子井發光二極體之三維模型

Abstract

在這篇論文中，我們分析綠光發光二極體的合金擾動、量子井厚度擾動和濃度擾動的三維模擬模型。近年來，研究顯示合金擾動因為局部的能態和位能擾動造成的滲透而影響了電性。再者我們發現為了更正確的模擬，考慮量子井的厚度擾動或大尺度的量子井濃度擾動是必須的。在我們比較有或沒有厚度擾動的結構後，我們發現當模型考慮了厚度擾動或濃度擾動是可以讓模擬的啟動電壓變小也更接近實驗結果。在量子井中厚度比較薄的區域，因為極化電場而有較小能障，讓載子可以由較小的能障區域注入，使得啟動電壓變小。然而厚度擾動的缺點是降低了主動區體積的大小，導致在相同電流下局部的電流密度比沒有厚度擾動的還要大。因此較低的內部量子效率和比較強的效率下降，會發生在量子厚度較薄的區域比量子井厚度較厚的區域大很多的時候。 最後我們試著在擴散模型中觀察橫向電流擴散受到位能擾動的影響。我們比較量子井有或沒有隨機合金擾動和量子井厚度擾動之間的差異。我們模擬顯示合金擾動會降低電子和電洞的擴散長度二到三倍。電子會被限制在小於一微米的長度，而電洞會被限制在小於一百奈米的長度。然而當我們進一步的考慮厚度擾動，會發現擴散長度被限制在週期的厚度擾動中。

In this thesis, a full 3D simulation model is applied to analyze the alloy fluctuation, QW thickness or composition fluctuation in the green LEDs. Recently, studies show that alloy fluctuation could influence electric property due to localized state or the percolation due to the fluctuation potential. Furthermore, we found that in order to make the prediction more accurately, the QW thickness fluctuation or large scale QW composition fluctuation will be needed. After we compare the structure with and without thickness fluctuation, we have found that models with (thickness fluctuation/or composition fluctuation) can make the turned-on voltage obtained by the simulation smaller and match well to the experimental result. The thinner of QW provides a smaller barrier induced by polarization field and it allows carriers to be injected at this low barrier side so that the turn-on voltage will be smaller. However, the drawback for thickness fluctuation is the reduction of active volume size so that local carrier density at the same current is much larger than the case without thickness fluctuation. Therefore, a smaller IQE and a stronger droop was observed if the ratio of thin QW to thick QW region was too large. Finally, we tried to under the influence of lateral carrier diffusion to the potential fluctuation through the diffusion model. We compared the differences between the QW with or without random alloy fluctuation, QW with thickness fluctuation. Our studies show that the fluctuation reduce the diffusion length of electron and holes by 2 or 3 times. Electrons are limited to be less than 1um and holes to be less than 100nm. However, if the thickness fluctuation is further considered, the diffusion length was observed to be limited by the periods of thickness fluctuation.