| 摘要: | 為了生長出用在UV-LED的高品質AlGaN薄膜，在磊後續的高溫層前，必須擁有一個高品質的AlN緩衝層。因此，高溫熱退火是提高其晶體品質的常用方法。然而在退火過程中，AlN的表面降解引起的熱分解是需要處理的關鍵問題。所以本論文研究了空間控制熱退火對於AlN緩衝層表面形態、表面元素組成和晶體品質的影響，並且修改善表面惡化問題。空間控制的手段是通過在熱退火前將兩個面對面的AlN/sapphire晶圓放在石墨坩堝中進行退火。通過計算飽和蒸汽壓以及過飽和度，加上熱力學和平衡常數的推導，闡釋了這些結果的機制。以AFM、EDS、XRD對熱處理後的AlN表面形態、元素組成、晶體品質進行了分析。結果表明，這種方法可以減少表面分解的程度以及減少由氧化鋁管的蒸汽造成的表面氧化。
雖然解決了熱分解的問題，但表面仍有一定的氧含量無法被消除。因此，我們提出了一種利用N2-NH3混合氣體對AlN表面氧化層進行氮化處理的方法。NH3氣體可以作為反應性氣體，將氮原子替換掉表面的氧原子後形成AlN。除了NH3氣體的作用外，N2氣體在氮化過程中也發揮了重要作用，因為當熱退火溫度達到700℃時，N2氣體可以抑制NH3氣體的分解。用Kröger-Vink記數法提出了AlN表面氧化物氮化過程的機制，其可歸因於NH3的表面化學吸附、N3−和O2−向內和向外的交互擴散以及H2、H2O和O2的脫附。在本論文中以密度泛函理論(DFT)和動態蒙地卡羅法(kMC)的數學模型，實現了一步步的氮化過程，也實現了氮化表面的結構組態和其表面能的變化。模擬的結果表明，氧分子的向外擴散是氮化過程的速率決定步驟，引發了後續的氮向內擴散和氧脫附等步驟。;To growing the high quality AlGaN films for the UV-LEDs, it is necessary to possess a quality AlN buffer layer before the epitaxy of the subsequent high temperature layers. Therefore, high temperature annealing is a common way to improve its crystal quality. During the annealing, however, surface degradation of the AlN caused by thermal decomposition is a critical issue need to be dealt with. So, in this dissertation, the effects of space control annealing on the surface morphology, surface elemental composition, and crystal quality of the annealed AlN buffer layers were studied to mend the surface degeneration problem. The means for space control is by using two face to face wafers put in a graphite crucible before the annealing. The mechanism of these results was clarified by the calculation of saturated vapor pressure as well as the supersaturation, and the derivation of thermodynamics and equilibrium constant. The surface morphology, elemental composition, and crystal quality of the annealed AlN were analyzed by AFM, EDS, and XRD. The results showed that this approach could reduce the extent of surface decomposition as well as the surface oxidation arisen from the vapor of the alumina tube.
Although the problem of decomposition was solved, there was still a certain of oxygen content on the surface that was unable to be banished. Hence, we supposed a method that is nitridation of the AlN surface oxide layer by using N2-NH3 gas mixture. This NH3 gas can be served as a reactive gas which will substitute the surface oxygen atoms with nitrogen atoms. In addition to the work of NH3 gas, the N2 gas also played an important role during the nitridation because when the annealing temperature went up to 700℃, it can suppress the decomposition of NH3 gas. The mechanism of the nitridation process of the AlN surface oxide was proposed as follows with Kröger–Vink notation, which could attribute to the surface chemisorption of NH3, interdiffusion of N3− and O2− inward and outward the surface, and the desorption of H2, H2O, and O2. To figure out which step is the RDS, the XPS data fitting was employed to obtain the effective diffusion coefficient, which shows that "D" ̃_"O" is lower than "D" ̃_"N" for every experimental setup. The DFT and kMC method were also performed to realize the step by step process of nitridation and also the surface energy change of each configuration state of nitrided surface. The results of simulation showed that the out-diffusion of oxygen molecules is the RDS of nitridation process that triggered the following steps such as nitrogen diffusion and oxygen desorption. |
