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[7p-PB6-4] Theory of the oxygen and aluminum vacancies in amorphous Al2O3 gate insulators
Keywords:semiconductor, insulator
There has been a growing interest in gallium nitride (GaN) as next-generation power conversion devices because of their superior material properties such as larger bandgap, higher saturation velocity, and higher breakdown voltage than silicon [M. Kodama, et al., Appl. Phys. Express 1, 021104 (2008)]. In order to realize GaN-MOS devices, it is important to determine the most suitable gate oxide. Various materials for gate oxide have been reported in experiments, for example SiO2, Al2O3 , and HfO2. SiO2 and Al2O3 show larger conduction-band offset and valence-band offset for GaN. On the other hand, Al2O3 and HfO2 possess higher dielectric constant. Especially, Al2O3 is highly promising candidate for gate oxide insulator of GaN-MOS devices, because it displays both properties. However, the leakage current of GaN device with Al2O3 gate insulator is higher than that with SiO2 gate insulator. It has been reported that there are some vacancies of oxygen and aluminum in Al2O3 deposited on GaN. The oxygen and aluminum vacancies are thought to be responsible for the leakage currents.
In this study, we investigated the oxygen and aluminum vacancies in amorphous Al2O3 (a-Al2O3) by the first-principles calculations. The calculations were performed by using the VASP (Vienna ab initio simulation package) code [G. Kresse and D. Joubert, Phys. Rev. B 59, 1758 (1999)], which is based on the density-functional theory with the Perdew-Burke-Ernzerhof-type generalized gradient approximation [J. P. Perdew, et al., Phys. Rev. Lett. 77, 3865 (1996)]. a-Al2O3 was prepared by the simulated annealing. The coordination number of the O in a-Al2O3 is 2,3,4. On the other hand, the coordination number of the Al in a-Al2O3 is 4,5,6. Therefore, we made the vacancy by removing O and Al atoms with each coordination number and checked whether the defect states were formed in the band gap. For O vacancy, we found that the attractive potential was formed around the Al atoms near the O vacancy due to the effects of Al positive ions. As a result, vacancy defect level appeared in a band gap. As for Al vacancy, we found that anti-bonding states of O-O bonds appeared in the band gap. In this presentation, we also discuss charged state dependence and impurity effects of defects in a-Al2O3.
In this study, we investigated the oxygen and aluminum vacancies in amorphous Al2O3 (a-Al2O3) by the first-principles calculations. The calculations were performed by using the VASP (Vienna ab initio simulation package) code [G. Kresse and D. Joubert, Phys. Rev. B 59, 1758 (1999)], which is based on the density-functional theory with the Perdew-Burke-Ernzerhof-type generalized gradient approximation [J. P. Perdew, et al., Phys. Rev. Lett. 77, 3865 (1996)]. a-Al2O3 was prepared by the simulated annealing. The coordination number of the O in a-Al2O3 is 2,3,4. On the other hand, the coordination number of the Al in a-Al2O3 is 4,5,6. Therefore, we made the vacancy by removing O and Al atoms with each coordination number and checked whether the defect states were formed in the band gap. For O vacancy, we found that the attractive potential was formed around the Al atoms near the O vacancy due to the effects of Al positive ions. As a result, vacancy defect level appeared in a band gap. As for Al vacancy, we found that anti-bonding states of O-O bonds appeared in the band gap. In this presentation, we also discuss charged state dependence and impurity effects of defects in a-Al2O3.