Leveraging high fluence and low pressure for pulsed laser deposition of high-mobility $\gamma$-Al${}_2$O${}_3$/SrTiO${}_3$ heterostructure growth

Abstract

High-mobility oxide heterostructures could be applied for high-frequency devices, transparent conductors, and spin-orbit logic devices. SrTiO${}_3$ is one of the most studied oxide substrate materials for heterostructures. To date, the highest SrTiO3-based charge carrier mobility at 2 K was measured in the interfacial 2-dimensional electron gas (2DEG) of $\gamma$-Al${}_2$O${}_3$/SrTiO${}_3$. The formation mechanism and origin of the high electron mobility are not yet fully understood. This investigation presents a successful growth protocol to synthesise high mobility $\gamma$-Al${}_2$O${}_3$/SrTiO${}_3$ interfaces, and a description of the underlying growth optimisation. Furthermore, indicative features of high-mobility $\gamma$-Al${}_2$O${}_3$/SrTiO${}_3$, including the room-temperature sheet resistance, are presented. Signs of epitaxial and crystalline growth are found in a high-mobility sample (${\mu }^{10K}=1.6\times 10^4{\mathrm{cm}}^2/\mathrm{Vs}$). Outlining the growth mechanisms and comparing 40 samples, indicates that high-fluence ($F>3\mathrm{J}/{\mathrm{cm}}^2$) and low pressure ($P\approx 1\times 10^{-6}\mathrm{mbar}$) are essential growth parameters for high-mobility $\gamma$-Al${}_2$O${}_3$/SrTiO${}_3$ interfaces. $\gamma$-Al${}_2$O${}_3$ having single-element cations allows higher laser fluences during growth, compared to thin films with multi-element cations such as LaAlO${}_3$, without causing stoichiometric imbalances.