报告题目: |
Oxidation of nitride surfaces and its effect to device performance |
报告人: |
Maosheng Miao |
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Materials Department, UC Santa Barbara
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报告时间: |
2010-07-12 15:00 |
报告地点: |
物理系三楼报告厅 |
主办单位: |
物理系 |
简介: |
摘要：
Semiconductor surfaces are prone to be oxidized while exposed in air, and there is no exception to nitride-based devices such as LEDs and high electron mobility transistors (HEMTs). Because the oxidation can greatly alter the electronic structure of the surface, it may strongly affect the performance of the devices. In close collaboration with experimental groups in UCSB, we recently investigated the oxidation of group III nitride surfaces using both the state-of-art first principles methods and Schrödinger-Poisson simulations. In order to accurately capture the surface state levels in the gap, we applied a new scheme of hybrid functional method that has showed the capability of yielding accurate band gaps for various semiconductors. Numerous structures with different oxide coverage and different stoichiometry are examined, and their stability is interpreted in terms of driving mechanisms such as the electron counting (EC) rule and oxide-stoichiometry (OS) matching. We demonstrated that the EC and OS are not compatible for polar surfaces and therefore reconstructions that violates the electron counting may form under certain oxidation condition. As a result, the electronic structures of nitride polar surfaces are largely altered by the oxidation that may create high-energy surface donor states. These states fit well to the surface donor region of two-dimension electron gas (2DEG) at AlGaN/GaN heterostructure as observed by experiments. In contrast to polar surface, the EC and OS are compatible for non-polar surfaces and the effect of oxidation to electronic structure and therefore to the device performance is moderate. At last, I will show that the observed surface accumulation layer for InN non-polar surfaces that pins the Fermi level above the conduction band minimum can also be explained by oxidation of the surface.
References:
[1] “Reconstructions and origin of surface states on AlN polar and nonpolar surfaces”, M. S. Miao, A. Janotti and C. G. Van de Walle, Physical review B 80, 155319 (2009).
[2] “Oxidation and the origin of two-dimensional electron gas in AlGaN/GaN heterostructures”, M. S. Miao, J. R. Weber and C. G. Van de Walle, Journal of Applied Physics, 107, 123713 (2010).
[3] “Distribution of surface donor states on etched AlGaN in AlGaN/GaN heterostructures” M. Higashiwaki, S. Chowdhury, B. L. Swenson, U. K. Mishra M. S. Miao, C. G. Van de Walle, submitted to J. Appl. Phys.
[4] “Distributed surface donor states and the two-dimensional electron gas at AlGaN/GaN heterojunction”, L. Gordon, M. S. Miao, S. Chowdhury, M. Higashiwaki, U. K. Mishra, and C. G. Van de Walle, prepared. |
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