Whether polarization catastrophe or oxygen vacancies is responsible for the remarkable emergence of a two-dimensional electron gas at the interface of the insulating oxides, polar LaAlO3 and nonpolar SrTiO3, has been hotly debated. Using a series of experiments that compare the electrical properties of amorphous and crystalline LaAlO3/SrTiO3 heterostructures, researchers discover that the answer depends on the structure of the LaAlO3 overlayer.
Insulating 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 the interface of the polar LAO with a nonpolar insulating oxide SrTiO3 (STO), a two-dimensional electron gas emerges. Two mechanisms, each supported by experimental observations, have been put forward to explain this phenomenon. One focuses on the discontinuity in charge polarization that the interface creates: Such a discontinuity results in an electric potential that builds up linearly with the number of polar LAO layers. As a response of the composite material to this potential buildup, a charge transfer to the interface occurs and leads to a two-dimensional electron gas (2DEG-P). The other mechanism is based on the fact that STO can be made conducting if charged oxygen vacancies are created on its surface (2DEG-V). The relative role of these two mechanisms and their contributions to the 2DEG has been a topic of hot debate. In this work, based on a series of systematic experiments, we provide a clear resolution to this debate.
We show that the structure of the LAO overlayers is the key to the resolution. While in samples with crystalline LAO overlayers both mechanisms contribute to the 2DEG, the 2DEG-P is absent in samples with amorphous overlayers. The 2DEG-V arises through the strong chemical affinity of Al to oxygen, but it can be eliminated by high-oxygen-pressure annealing. In contrast, the 2DEG-P is very robust against oxygen annealing and can only be removed when the number of the LAO layer is reduced below the critical thickness. A high crystallinity of the overlayer is thus essential in the polarization-based mechanism. We have also gained the understanding that the 2DEG systems generated by these two different mechanisms are fundamentally different: 2DEG-P is degenerate whereas 2DEG-V is thermally activated.
The findings reported here should also guide us in how to create high-mobility 2DEG at oxide interfaces with the carriers of choice—a potential that may be exploited for the future of oxide electronics.
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, and Ariando