Electron Mobility in γ−Al2O3/SrTiO3

One of the key issues in engineering oxide interfaces for electronic devices is achieving high electron mobility. SrTiO3-based interfaces with high electron mobility have attracted a lot of interest due to the possibility of combining quantum phenomena with the many functionalities exhibited by SrTiO3. To date, the highest electron mobility (140,000 cm/Vs at 2 K) is obtained by interfacing perovskite SrTiO3 with spinel γ-Al2O3. The origin of the high mobility, however, remains poorly understood. Here, we investigate the scattering mechanisms limiting the mobility in γ-Al2O3/SrTiO3 at temperatures between 2 and 300 K and over a wide range of sheet carrier densities. For T > 150 K, we find that the mobility is limited by longitudinal optical phonon scattering. For large sheet carrier densities (> 8 ⋅ 10 cm) the screened electron-phonon coupling leads to room temperature mobilities up to μ~12 cm/Vs. For 5 K < T < 150 K, the mobility scales as ~T consistent with electron-electron scattering limiting the electron mobility. For T < 5 K and at an optimal sheet carrier density of ~4 ⋅ 10 cm, the electron mobility is found to exceed 100,000 cm/Vs. At sheet carrier densities less than the optimum, the electron mobility decreases rapidly and the current flow becomes highly influenced by domain walls and defects in the near-interface region of SrTiO3. At carrier densities higher than the optimum, the SrTiO3 heterostructure gradually becomes bulk conducting and the electron mobility decreases to ~20,000 cm/Vs. We argue that the high electron mobility observed arises from a spatial separation of donors and electrons with oxygen vacancy donors preferentially forming at the interface whereas the itinerant electrons extend deeper into SrTiO3. Understanding the scattering mechanism in γ-Al2O3/SrTiO3 paves the way for creation of high-mobility nanoscale electronic devices.