Thursday, May 23, 2013

1305.5016 (Z. Q. Liu et al.)

Origin of the two-dimensional electron gas at LaAlO3/SrTiO3 interfaces:
The role of oxygen vacancies and electronic reconstruction

Z. Q. Liu, C. J. Li, W. M. Lü, X. H. Huang, Z. Huang, S. W. Zeng, X. P. Qiu, L. S. Huang, A. Annadi, J. S. Chen, J. M. D. Coey, T. Venkatesan, Ariando
Popular Summary:Polar oxides, consisting of charged layers (e.g., (100) LaAlO3 (LAO) as layers of LaO+1 and AlO2-1) have generated a great deal of excitement in the last decade. At an interface of a polar (LAO) with a non-polar insulator SrTiO3 (STO) a discontinuity in charge polarization is created resulting in a potential that builds up linearly with the number of polar overlayers. To avoid this potential buildup, a charge transfer to the interface occurs and this creates a two dimensional electron gas (2DEG-P) which has captivated the attention of researchers in this field. However, STO can be made conducting if oxygen vacancies are created on its surface (2DEG-V). The relative role of these two mechanisms and their contribution to the 2DEG have been hot topics. In this work we show that with crystalline overlayers (without oxygen post-annealing) both mechanisms contribute to the 2DEG. That is because when STO has an overlayer (either amorphous or crystalline) containing a strong oxygen affinity element such as Al, 2DEG-V can be created through oxygen vacancies arising from the strong chemical affinity of Al to oxygen. However, there is no 2DEG-P in the case of amorphous overlayers. 2DEG-V can be removed by oxygen post-annealing and therefore 2DEG-P alone accounts for the interface conductivity in oxygen-annealed crystalline LAO/STO heterostructures. Futhermore, it was found that high crystallinity of the overlayer is crucial for the polarization catastrophe mechanism (2DEG-P) in the case of crystalline LAO overlayers. We also show that these two types of carriers are different in their behavior: 2DEG-P is degenerate while 2DEG-V is thermally activated. These findings guide us how to create high mobility 2DEG at oxide interfaces with the carriers of choice, which may be important for the future of oxide electronics.
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