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

TL;DR

This work advances high-mobility oxide interfaces by optimizing pulsed laser deposition of γ-Al$_2$O$_3$ on SrTiO$_3$. Through four growth protocols, it shows that very low deposition pressures ($P\approx 10^{-6}$ mbar) paired with high laser fluence ($F>3\mathrm{\,J/cm^2}$) promote surface oxygen-vacancy formation in SrTiO$_3$, yielding high-mobility 2DEGs with crystalline, epitaxial films (e.g., Sample 9 with $\mu^{10K}=1.6\times10^4$ cm$^2$/Vs and $d_{\gamma-Al_2O_3}=8.44$ nm). The authors develop an oxygen-dynamics framework across six growth regimes to explain vacancy creation and quenching, and they propose room-temperature transport indicators (Rs$^{RT}$ and RRR(10 K)) to rapidly screen mobility potential without low-$T$ measurements. While Protocol 1 achieves the highest mobility, subsequent protocols improve reproducibility and automate growth, emphasizing the role of low surface pressure before deposition. Overall, the study provides a practical pathway to high-mobility γ-Al$_2$O$_3$/SrTiO$_3$ interfaces with implications for high-frequency devices, transparent conductors, and spin–orbit logic.

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 $γ$-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 $γ$-Al$_2$O$_3$/SrTiO$_3$ interfaces, and a description of the underlying growth optimisation. Furthermore, indicative features of high-mobility $γ$-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 ($μ^{10K} = 1.6 \times 10^4 \mathrm{cm}^2/\mathrm{Vs}$). 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 $γ$-Al$_2$O$_3$/SrTiO$_3$ interfaces. $γ$-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.

Figures (4)

• Figure 1: Pressure-control for increased reproducibility and crystal structure. a) Schematic of the $\mathrm{\gamma}$-Al2O3/SrTiO3 crystal structure grown with the PLD setup illustrated in b). c-f) Examples of temperature (brown) and pressure (purple) profiles on Protocols 1-4, respectively.
• Figure 2: Indicative values for high mobility prediction. a) Sheet resistance, Rs, as a function of temperature (T) for a high mobility (brown) and a low mobility (purple) $\mathrm{\gamma}$-Al2O3/SrTiO3 sample. The electron mobility at 10 K ($\mathrm{\mu}$10$\,$K) as a function of carrier concentration at 10$\,$K ($n_s^{10 \,K}$) in b), as a function of sheet resistance at room temperature ($R_s^{RT}$) in c), and as a function of residual resistance ratio (RRR(10$\,$K)) in d). Inset in c) displays the $n_s^{10\,K}$-dependency of $R_s^{RT}$ The brown points are data and the purple dashed line a log-log linear fit. Carrier densities and mobilities are extracted through a single electronic band model as displayed in Supplementary Material Fig. S1.
• Figure 3: Structural data of $\mathrm{\gamma}$-Al2O3/SrTiO3 Sample 9. a) AFM after $\mathrm{\gamma}$-Al2O3 deposition on SrTiO3. b) X-ray reflectivity data and fit (solid brown line) yielding a $\mathrm{\gamma}$-Al2O3 thickness of $d_{\gamma\text{-}Al_2O_3} = 8.44 \,\pm\,0.4$ nm. c) X-ray diffraction showing the SrTiO3(002) and $\mathrm{\gamma}$-Al2O3(004) peaks. Sample 9 was grown using protocol 1. The y-axes in b) and c) is logarithmic intensity.
• Figure 4: Oxygen dynamics through the growth protocols. a) Illustration of the full timespan of the growth process separated into six different regimes. b) SrTiO3 surrounded by oxygen molecules, as is the case during initial annealing. c) SrTiO3 in a low-pressure oxygen environment as the pulsed laser deposition chamber. d) same as c) but with an oxygen vacancy in the topmost layer. The arrows in b-d) indicates the direction and magnitude of the oxygen atoms diffusion to restore equilibrium, not drawn to scale. e) Bombardment of SrTiO3 in the initial part of the pulsed laser deposition. Here $\vv{\mathrm{v}}$ is the velocity vector of the plasma species. f) Oxygen scavenging by the Al atoms of $\mathrm{\gamma}$-Al2O3 that have higher oxygen affinity than Ti and Sr in SrTiO3. g) Oxygen recombination from the atmosphere. The arrows in f) and g) do not indicate forces but movements.