Abstract
The relative importance of atomic defects and electron transfer in explaining conductivity at the crystalline interface has been a topic of debate. Metallic interfaces with similar electronic properties produced by amorphous oxide overlayers on [Y. Chen et al., Nano Lett. 11, 3774 (2011); S. W. Lee et al., Nano Lett. 12, 4775 (2012)] have called in question the original polarization catastrophe model [N. Nakagawa et al., Nature Mater. 5, 204 (2006)]. We resolve the issue by a comprehensive comparison of (100)-oriented substrates with crystalline and amorphous overlayers of of different thicknesses prepared under different oxygen pressures. For both types of overlayers, there is a critical thickness for the appearance of conductivity, but its value is always 4 unit cells (around 1.6 nm) for the oxygen-annealed crystalline case, whereas in the amorphous case, the critical thickness could be varied in the range 0.5 to 6 nm according to the deposition conditions. Subsequent ion milling of the overlayer restores the insulating state for the oxygen-annealed crystalline heterostructures but not for the amorphous ones. Oxygen post-annealing removes the oxygen vacancies, and the interfaces become insulating in the amorphous case. However, the interfaces with a crystalline overlayer remain conducting with reduced carrier density. These results demonstrate that oxygen vacancies are the dominant source of mobile carriers when the overlayer is amorphous, while both oxygen vacancies and polarization catastrophe contribute to the interface conductivity in unannealed crystalline heterostructures, and the polarization catastrophe alone accounts for the conductivity in oxygen-annealed crystalline heterostructures. Furthermore, we find that the crystallinity of the layer is crucial for the polarization catastrophe mechanism in the case of crystalline overlayers.
- Received 29 January 2013
DOI:https://doi.org/10.1103/PhysRevX.3.021010
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Published by the American Physical Society
Popular Summary
Insulating polar oxides, consisting of charged layers [e.g., (100) (LAO) as layers of and ], have generated a great deal of excitement in the last decade. At the interface of the polar LAO with a nonpolar insulating oxide (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.
