High-mobility two-dimensional electron gas in γ-Al2O3/SrTiO3 heterostructures

The origin of the two-dimensional electron gas (2DEG) in the interface between gamma-Al2O3 (GAO) and SrTiO3 (STO) (GAO/STO) as well as the reason for the high mobility of the 2DEG is still in debate. In this paper, the electronic structures of [001]-oriented GAO/STO heterostructures with and without oxygen vacancies are investigated by first-principles calculations based on density functional theory. The calculation results show that the necessary condition for the formation of 2DEG is that the GAO/STO heterostructure have an interface composed of Al and TiO2 layers. For the heterostructure without oxygen vacancies on the GAO side, the 2DEG originates from the polar discontinuity near the interface, and there is a critical thickness for the GAO film, below which the 2DEG would not present and the heterostructure exhibits insulator characteristics. For the case that only the GAO film contains oxygen vacancies, the polar discontinuity near the interface disappears, but the 2DEG still exists. In this situation, the critical thickness of the GAO film for 2DEG formation does not exist either. When the GAO film and STO substrate both contain oxygen vacancies, it is found that the 2DEG is retained as long as the oxygen vacancies on the STO side are not very close to the interface. The low-temperature mobilities of the 2DEGs in these GAO/STO heterostructures are considered to be governed by the ionized impurity scattering, and similar to 3 to similar to 11 times as large as that in LaAlO3/SrTiO3 heterojunction. The high mobility of the 2DEG is mainly due to the small electron effective mass in GAO/STO heterostructures.

Automated summary

The electronic structures and formation conditions of 2DEG in GAO/STO heterostructures were investigated by first-principles calculations. It was found that the formation of 2DEG requires an interface composed of Al and TiO2 layers, and there is a critical thickness for the GAO film. The high mobility of 2DEG in these heterostructures is attributed to the small electron effective mass.