AlGaN/GaN异质结的C-V特性研究
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摘要
近年来,作为第三代半导体的代表,氮化镓(GaN)因广阔的应用前景而备受关注。商业化GaN基蓝光发光二极管(LED)和激光器(LD)的出现,吸引了更多科研机构和企业转向GaN基半导体器件的研发和生产上。其中,AlGaN/GaN异质结基HEMT是新型的微波功率器件,与传统的微波功率器件相比,更适应于高温环境,更能满足高频、高电压和大功率的需要,将是下一代无线通信系统功率放大器的核心元件。
而高性能的器件是以高质量的材料为基础的。虽然在蓝宝石和碳化硅衬底上成功生长出了高质量的GaN基外延层,并且实现了LED等光电子器件的商业化,但是在选取合适的衬底材料上仍然存在着诸多困难。比如,在Si衬底上直接生长GaN基外延层会出现裂纹;很难生长厚度超过1.5?m GaN基外延层;生长AlGaN/GaN HEMT必须要求GaN基外延层厚度在2?m-2.5?m之间,但是在硅衬底上很难生长这么厚GaN基外延层,而且AlGaN/GaN的二维电子气与垒层厚度及Al组份的关系不是很清楚,本论文针对这些问题做了相关研究,主要研究如下:
1、为了解决裂纹问题,在Si(111)衬底上插入AlN缓冲层,探讨了AlN缓冲层生长的温度和厚度对GaN外延层的影响。1)研究了低温(800?C) AlN缓冲层生长的厚度对外延层的影响。通过对样品进行HR-XRD、AFM等表征发现,LT-AlN缓冲层的厚度为30nm时,对应晶体质量最高的GaN外延层样品。2)研究了高温(~1060?C)AlN缓冲层的厚度对GaN外延层的影响,结果表明最好的GaN外延层样品对应HT-AlN缓冲层的厚度为300nm。
2、为实现在2英寸的Si(111)衬底上外延生长出厚度超过1.5?m无裂纹的高质量GaN外延层,除了生长AlN缓冲层外,引入三层LT-AlN中间内插层来消除应力,减少位错。
3、为了研究异质界面的2DEG与AlGaN垒层的厚度和Al组分等参数的关系,在蓝宝石上生长AlGaN/GaN异质结,为提高2DEG的面密度,在异质结界面处引入很薄的一层AlN阻挡层,以更好地局域化异质界面处的二维电子气(2DEG),减小散射几率。表征发现当AlGaN垒层厚度超过20nm以上,2DEG对Al组分效应更敏感。通过适当调整Al组分和AlGaN垒层厚度,可以得到符合要求的AlGaN/GaN基HEMT夹断电压和阈值电压。实验得到的值与理论模拟相一致的结果,为优化AlGaN/GaN基HEMT的性能参数提供了一个有效的方法。
Galliμm nitride (GaN),the present of the third generation of semiconductor materials, has a broad application prospect in the recent years. As the emergence of the commercialized GaN LED and LD, more and more research insititutes and companies were attracted and turned to research and product the GaN semiconductor devices. The AlGaN/GaN HEMT is one of the new type micro wave power devices. In contrast with the traditional micro wave power devices, the AlGaN/GaN HEMT is more adaptable to the high temperatures, and better satisfies the requirements of large voltages and large power, so it will be the core element of the power amplify of the next generation wireless communication system.
The good performances depend on the high qualities of the materials. Although the high quality epitaxy GaN layer can be grown on the sapphire and SiC substrate, and the optoelectronic devices such as LED et.al. have been commercialized. But many difficulties still exist in choosing suitable substrate materials, for instance cracks may form in the GaN expitaxial layer deposited on the Si substrate, it is hard to grow a GaN expitaxial layer with thickness rather than 1.5?m, growing AlGaN/GaN HEMT must require the thickness of the GaN expitaxial layer between 2?m to 2.5?m, but it’s hard to grow such thick GaN expitaxial layer, and the relationship among the two dimensional electronic gas (2DEG), the thickness of barrier layer and Al component is unclear. In this thesis, we have done some works relative to the mentioned above.
1. In order to solve the problem of cracks, AIN buffer layers was inserted on the Si(111) substrate, and the effects of the growing temperatures and the thickness of AIN buffer layers to the GaN epitaxial layer were discussed. 1) We investigated the effects of thickness of AIN buffer layers prepared at the low temperature nearly 800?C on the epitaxial layer. By comparing with the results of HR-XRD、AFM et.al, it was found that the high quality crystal of the GaN epitaxial layer would be obtained with thickness around 30nm. 2) We also studied the similar case with only different temperature at high rang closed to 1060?C, and the results demonstrated that the best epitaxial GaN sample were prepared when the thickness of the HT-AlN buffer was 300nm.
2. Not only the A1N buffer layer should grow on the 2-inch Si(111) substrate , but also three LT-A1N intermediate inserted layers should be introduced to eliminate the stress and reduce the numbers of dislocations, for growing the high quality epitaxial GaN layer which is thicker than 1.5?m and nearly crack-free on the 2-inch Si(111) substrate.
3. So as to study the relationships among 2DEG, the thickness of AlGaN barrier layers, Al component in AlGaN barrier and other parameters, the AlGaN/GaN heterojunction was grown on sapphire; for the sake of increasing the surface density of 2DEG, a very thin AIN barrier layer was introduced on the heterogeneous interface, so the 2DEG can be best constrained on the surface and the scatter ratio was reduced. After comparing with the results, we found that 2DEG seem to be more sensitive to Al-content when the barrier thickness exceeds 20 nm. By adjusting the barrier thickness and Al content, suitable pinch-off voltage and threshold voltage can be obtained for the operation of AlGaN/GaN HEMTs. The expeirmental are consistent with theoretical simulations, and can provide a reliable way in performance parameters optimization for AlGaN/GaN based HEMT.
而高性能的器件是以高质量的材料为基础的。虽然在蓝宝石和碳化硅衬底上成功生长出了高质量的GaN基外延层,并且实现了LED等光电子器件的商业化,但是在选取合适的衬底材料上仍然存在着诸多困难。比如,在Si衬底上直接生长GaN基外延层会出现裂纹;很难生长厚度超过1.5?m GaN基外延层;生长AlGaN/GaN HEMT必须要求GaN基外延层厚度在2?m-2.5?m之间,但是在硅衬底上很难生长这么厚GaN基外延层,而且AlGaN/GaN的二维电子气与垒层厚度及Al组份的关系不是很清楚,本论文针对这些问题做了相关研究,主要研究如下:
1、为了解决裂纹问题,在Si(111)衬底上插入AlN缓冲层,探讨了AlN缓冲层生长的温度和厚度对GaN外延层的影响。1)研究了低温(800?C) AlN缓冲层生长的厚度对外延层的影响。通过对样品进行HR-XRD、AFM等表征发现,LT-AlN缓冲层的厚度为30nm时,对应晶体质量最高的GaN外延层样品。2)研究了高温(~1060?C)AlN缓冲层的厚度对GaN外延层的影响,结果表明最好的GaN外延层样品对应HT-AlN缓冲层的厚度为300nm。
2、为实现在2英寸的Si(111)衬底上外延生长出厚度超过1.5?m无裂纹的高质量GaN外延层,除了生长AlN缓冲层外,引入三层LT-AlN中间内插层来消除应力,减少位错。
3、为了研究异质界面的2DEG与AlGaN垒层的厚度和Al组分等参数的关系,在蓝宝石上生长AlGaN/GaN异质结,为提高2DEG的面密度,在异质结界面处引入很薄的一层AlN阻挡层,以更好地局域化异质界面处的二维电子气(2DEG),减小散射几率。表征发现当AlGaN垒层厚度超过20nm以上,2DEG对Al组分效应更敏感。通过适当调整Al组分和AlGaN垒层厚度,可以得到符合要求的AlGaN/GaN基HEMT夹断电压和阈值电压。实验得到的值与理论模拟相一致的结果,为优化AlGaN/GaN基HEMT的性能参数提供了一个有效的方法。
Galliμm nitride (GaN),the present of the third generation of semiconductor materials, has a broad application prospect in the recent years. As the emergence of the commercialized GaN LED and LD, more and more research insititutes and companies were attracted and turned to research and product the GaN semiconductor devices. The AlGaN/GaN HEMT is one of the new type micro wave power devices. In contrast with the traditional micro wave power devices, the AlGaN/GaN HEMT is more adaptable to the high temperatures, and better satisfies the requirements of large voltages and large power, so it will be the core element of the power amplify of the next generation wireless communication system.
The good performances depend on the high qualities of the materials. Although the high quality epitaxy GaN layer can be grown on the sapphire and SiC substrate, and the optoelectronic devices such as LED et.al. have been commercialized. But many difficulties still exist in choosing suitable substrate materials, for instance cracks may form in the GaN expitaxial layer deposited on the Si substrate, it is hard to grow a GaN expitaxial layer with thickness rather than 1.5?m, growing AlGaN/GaN HEMT must require the thickness of the GaN expitaxial layer between 2?m to 2.5?m, but it’s hard to grow such thick GaN expitaxial layer, and the relationship among the two dimensional electronic gas (2DEG), the thickness of barrier layer and Al component is unclear. In this thesis, we have done some works relative to the mentioned above.
1. In order to solve the problem of cracks, AIN buffer layers was inserted on the Si(111) substrate, and the effects of the growing temperatures and the thickness of AIN buffer layers to the GaN epitaxial layer were discussed. 1) We investigated the effects of thickness of AIN buffer layers prepared at the low temperature nearly 800?C on the epitaxial layer. By comparing with the results of HR-XRD、AFM et.al, it was found that the high quality crystal of the GaN epitaxial layer would be obtained with thickness around 30nm. 2) We also studied the similar case with only different temperature at high rang closed to 1060?C, and the results demonstrated that the best epitaxial GaN sample were prepared when the thickness of the HT-AlN buffer was 300nm.
2. Not only the A1N buffer layer should grow on the 2-inch Si(111) substrate , but also three LT-A1N intermediate inserted layers should be introduced to eliminate the stress and reduce the numbers of dislocations, for growing the high quality epitaxial GaN layer which is thicker than 1.5?m and nearly crack-free on the 2-inch Si(111) substrate.
3. So as to study the relationships among 2DEG, the thickness of AlGaN barrier layers, Al component in AlGaN barrier and other parameters, the AlGaN/GaN heterojunction was grown on sapphire; for the sake of increasing the surface density of 2DEG, a very thin AIN barrier layer was introduced on the heterogeneous interface, so the 2DEG can be best constrained on the surface and the scatter ratio was reduced. After comparing with the results, we found that 2DEG seem to be more sensitive to Al-content when the barrier thickness exceeds 20 nm. By adjusting the barrier thickness and Al content, suitable pinch-off voltage and threshold voltage can be obtained for the operation of AlGaN/GaN HEMTs. The expeirmental are consistent with theoretical simulations, and can provide a reliable way in performance parameters optimization for AlGaN/GaN based HEMT.
引文
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[2]刘刚,何笑明.微电子器件与IC设计[M].北京:科学出版社, 2005:18-56.
[3] Chen W, Zhou C, Chen K J, et al. High-current-density high-voltage normally-off AlGaN/GaN hybrid-gate HEMT with low on-resistance [J]. Electron. Lett., 2010, 46(24):1626-1627.
[4]伍国珏(美).半导体器件完全指南[M].北京:科学出版社, 2009:48-87.
[5]刘芳. GaN基器件肖特基接触的新结构和新材料的研究[D].天津大学博士论文, 2009.
[6] Kouvetakis J, An Y J, Dcosta V R, et al. Synthesis of (Hf, Zr) B-2-based heterostructures hybrid substrate systems for low temperature Al-Ga-N integration with Si [J]. J. Mater. Chem., 2008, 18(40):4775-4782.
[7] Wang X L, Chen T S, Xiao H L, et al. High-performance 2 mm gate width GaN HEMTs on 6H-SiC with output power of 22.4 W and 8 GHz. Solid-state Electron., 2008, 52(6):926-929.
[8] Feltin E, Beaumont B, Laügt M, et al. Stress control in GaN grown on silicon (111) by metalorganic vapor phase epitaxy [J]. Appl. Phys. Lett., 2001, 79(20):323-327.
[9] Stringfellow G B. Organometallic vapor phase epitaxy: Second Edition: theory and practice [M]. Academic Press, 1999.
[10] Manasevit H M. Single-crystal GaAs on insulating substrates [J]. Appl. Phys. Lett., 1999, 12:126-159.
[11] Strite S, Morkoc H. GaN, AlN and InN a review [J]. J Vac Sci Technol., B. 1992, 10(4):1237-1266.
[12]陆大成,段树坤.金属有机化合物气相外延基础及应用[M].北京:科学出版社, 2008:1-57.
[13] Wang X, Hu G. et al. Characteristics of high Al content AlxGa1-xN grown by metal organic chemical vapor deposition [J]. Microelectron. J., 2007, 38(8/9):838-841.
[14] Metzger T, Hopler R, Born E. Defect structure of epitaxial GaN films determined by transmission electron microscopy and triple-axis X-ray Diffractometry [J]. Philos. Maga. A., 1998, 77(4):1013-1025.
[15] Dadgar A, Hμms C, Diez A, et al. Growth of blue GaN LED structures on 150-mm Si (111) [J]. J. Cryst. Growth, 2006, 287(2):279-282.
[16]张冠英,梅俊平,解新建,等. MOCVD外延生长GaN材料的技术进展,趋势与展望[J].半导体技术. 2010, 35(3):368-372.
[17] Kuchibhatla S, Rodak L E, Korakakis D, et al. Fourier transform infrared spectroscopy characterization of AlN thin films grown on sacrificial silicon oxide layers via metal organic vapor phase epitaxy [J]. Thin Solid Films, 2010, 519(1):117-121.
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