高品質的單晶氮化鋁(AlN)是深紫外發光二極體的關鍵材料。本研究利用有機金屬化學氣象沉積法(metal-organic chemical vapor deposition, MOCVD)在藍寶石基板上成長高品質AlN。在MOCVD的磊晶過程中，我們利用“調整五三比”及“氣流中斷”的方式，優化AlN的成長速度、晶格均勻度、及表面平整度。“五三比”為五族元素(氮)對三族元素(鋁)的莫爾流量(單位: µM/min)比值，可利用氮與鋁的前驅物(NH3與trimethylaluminum, TMAl)流量調整；“氣流中斷”則是以脈衝(pulsed flow)的方式，將NH3與TMAl通入MOCVD反應爐，可透過氣流開關的時間(on/off duration)調控。根據掃描式電子顯微鏡(scanning electron microscopy, SEM)、原子力顯微鏡(atomic force microscopy, AFM)及X光繞射(x-ray diffraction, XRD)的量測結果，我們發現: 降低五三比，可提升AlN的成長速度；交錯式地中斷NH3與TMAl氣流，則可提升AlN的表面平整度及晶格均勻度。“氣流中斷法”可避免MOCVD的寄生反應，並增加提高鋁原子在基板表面的遷移距離，因而減少差排(dislocations)缺陷的形成。利用優化過的五三比及氣流中斷參數，可以單一磊晶溫度、無需低溫成核層，就能得到高品質的AlN磊晶層。這種磊晶技術能縮短AlN的成長時間、減少前驅物的消耗量，極具應用價值。

High-quality AlN is the key material for deep ultraviolet (UV) light emitting diodes (LEDs). In this project, high-quality AlN is grown on c-plane sapphire substrates by MOCVD. Growth rate, lattice uniformity and surface flatness were optimized by varied V/III ratios and pulsed-flow conditions. V/III ratio is the molar-flow (in µM/min) ratio of the group V precursor (NH3) to group III (TMAl); the pulsed-flow condition is periodic on/off supply of the precursors (NH3 & TMAl) into the reactor. According to the characterizations of SEM, AFM and XRD, it is found that reducing the V/III ratio leads to increased growth rate, and alternative supply of NH3 and TMAl results in enhanced lattice uniformity and surface flatness. We believe the pulsed flows of NH3 and TMAl can effectively avoid the undesired MOCVD parasitic reactions and increase the lateral atomic migration of Al on the substrate surface, both of which can improve the crystal qualities of AlN. The MOCVD conditions proposed here, involving optimized V/III ratio and pulsed-flow of NH3 and TMAl, can produce high-quality AlN epilayer with single-substrate temperature and reduced precursors, being attractive for practical applications.

參考文獻
[1] Nam, K. B. et al. Unique optical properties of AlGaN alloys and related ultraviolet emitters. Appl. Phys. Lett. 84, 5264 (2004).
[2] Strite, S., Lin, M. E. & Morkoc, H. Progress and prospects for GaN and the III-V nitride semiconductors . Thin Solid Films 231, 197-210 (1993).
[3] Mohammad, S. et al. AlGaN/AlN integrated photonics platform for
the ultraviolet and visible spectral range. Optics Express 24, 22, 25415-25423 (2016)
[4] Vurgaftman, I. et al. Band parameters for III–V compound semiconductors and their alloys. J. Appl. Phys. 89, 5815 (2001).
[5] Shengchang, C. et al. Defect Reduction in AlN Epilayers Grown by MOCVD
via Intermediate-Temperature Interlayers. Electronic Materials 44, 217–221(2015).
[6] Stacia, K. & Steven P. DenBaars. Metalorganic chemical vapor deposition of group III nitrides—a discussion of critical issues. Crystal Growth 248, 479–486 (2003).
[7] Jianchang, Y. et al. AlGaN-based deep-ultraviolet light-emitting diodes grown
on High-quality AlN template using MOVPE. Crystal Growth 414, 254–257 (2015).
[8] Y. A. Xi, et al. Very high quality AlN grown on (0001) sapphire by metal-organic vapor phase epitaxy. Appl. Phys. Lett. 89, 103106 (2006).
[9] Masataka, I. et al. High-temperature metal-organic vapor phase epitaxial growth of AlN on sapphire by multi transition growth mode method varying V/III ratio. Jpn. J. Appl. Phys. 45, 8639–8643 (2006).
[10] Feng, Y. et al. Competitive growth mechanisms of AlN on Si (111) by MOVPE. Sci Rep. 4, 6416 (2014).
[11] Seong, T.-Y., Han, J. , Amano, H. & Morkoҫ, H. (Eds.) III-Nitride Based Light Emitting Diodes and Applications. Springer, Singapore (2017).
[12] Xiaojuan, S. et al. In situ observation of two-step growth of AlN on sapphire using high-temperature metal–organic chemical vapour deposition. CrystEngComm. 15, 6066-6073 (2013).