Associate Professor, Institute for Materials Research, Tohoku University
Atomic-scale control of oxide heteroepitaxy is of growing importance from the viewpoints of both fundamental physics and device applications. Many intriguing physical phenomena such as high-Tc superconductivity, ferromagnetism, ferroelectricity, and thermoelectricity occur in naturally layered structures of transition metal oxides, giving rise to emergent interests for “epitaxial” design of new compounds upon the atomic-scale layer-by-layer growth of artificial superlattices.
We developed an ultra-high vacuum-pulsed laser deposition system equipped with reflection high-energy electron diffraction. In this system, rapid optimization of the growth temperature can be carried out by using an infrared semiconductor laser heater that makes a large temperature gradient, enabling to realize atomically abrupt heterointerfaces and nearly identical oxygen stoichiometry. Using this technique, we studied magneto transport properties of high-mobility electrons at polar oxide heterointerfaces. In the first case, we created a metallic state in an atomically abrupt heterointerface between two band insulators, SrTiO3 and LaAlO3, in which naturally arising polarity discontinuity introduces high-mobility electrons in SrTiO3. As a result, dramatic magnetoresistance oscillations appeared at low temperatures. Rotation angle dependence of the oscillations in the magnetic field suggested three-dimensionality of the conducting electrons. Second, we fabricated ZnO/MgZnO heterostructures to attain sufficiently high electron mobility to observe the quantum Hall-effect. In this case, two-dimensional electron gas (2DEG) was formed in the heterointerfaces due to strong built-in potential arising from a spontaneous polarization mismatch. The density of 2DEG could be controlled in a wide range by tuning the Mg content in the MgZnO barriers.
These results have implications for all oxide heterointerfaces, including magnetic tunnel junctions, Josephson junctions, oxides on semiconductors, etc. In particular, the high mobilities achieved present the possibility to combine the world of oxides (superconductors, multiferroics, colossal magnetoresistance) with the world of semiconductor heterostructures, including quantum Hall physics. This work is also a demonstration of the power of the field of nanoscience to create new physical phenomena. This and other recently developed techniques to tailor new materials, atom-by-atom, should continue to reveal new effects at an ever-accelerating pace.