Reconfigurable Oxide Nanoelectronics by Tip-induced Electron Delocalization

Abstract

Reconfigurable oxide nanoelectronics, enabled by conductive atomic force microscope (cAFM) lithography, have established complex oxide interfaces as a promising platform for quantum engineering that harnesses emergent phenomena for advanced functionalities. However, this cAFM nanofabrication process can only occur in the air, with simultaneous device decay described under the "water-cycle" writing mechanism. These restrictions pose ongoing challenges for device optimization in the quantum regime at mK temperatures. Here, we demonstrate a "waterless" cAFM lithography approach that is compatible with vacuum and cryogenic environments. Through oxygen vacancy engineering at the LaAlO$_3$/SrTiO$_3$ interface, we have achieved nonvolatile and reconfigurable cAFM control of nanoscale interfacial polaron-electron liquid transition at mK temperatures with an ultrafine line resolution of 0.85 nm. Supported by first-principles calculations and drift-diffusion modeling, we show that tip-controlled oxygen vacancy electromigration plays a key role. This advancement bridges reconfigurable device fabrication and concurrent characterization in situ at mK temperatures, and establishes a versatile Hubbard toolbox for engineering programmable quantum phases in correlated oxides.

Figures (4)

• Figure 1: cAFM lithography at room temperature. (a) Schematic of sample structure and experiment set-up for "waterless" cAFM lithography. The LAO surface layer hosts a programmable $V_{\text{O}}$ reservoir. A sharp AFM tip with different bias voltages locally controls interfacial metal-insulator transition. (b) $V_{\text{O}}$ formation energy profile across the LAO/STO interface, which decreases with increasing LAO thickness. (c) Nanowire writing tests on Sample A in different ambient conditions at room temperature. The slow-decay nanowire written in vacuum is dominated by $V_{\text{O}}$ migration mechanism. (d) Nanowire writing tests on a conventionally grown control sample at room temperature. Writing is unsuccessful in the water-free environment. The fast-decay nanowire written in the air is dominated by the "water-cycle" mechanism.
• Figure 2: Environmental dependence of nanowire decay at room temperature. Nanowires were written on Sample A with 10 V tip bias in vacuum (10$^{-3}$ mbar) at room temperature, then the conductance was monitored under varying environments. (a) Decay in vacuum, followed by dry nitrogen exposure (from 10$^1$ to 10$^3$ mbar), then return to vacuum. The wire was cut at $t\approx6600$ s with -10 V, which resulted in a sharp conductance drop. (b) Decay in vacuum, followed by dry oxygen (or air in (c)) exposure at 10$^1$ to 10$^3$ mbar, then return to vacuum.
• Figure 3: Reconfigurable nanofabrication and characterization at 100 mK. (a) Nanowire writing tests at 100 mK on Sample B. 2.0 V tip bias yields no trace, 2.5 V fails to form a stable nanowire, and 2.7 V writes an ultra-fine nanowire showing negligible decay. Red dots in the inset show nanowire conductance measured as a function of the tip position, cutting across the nanowire with -1 V. The sharp conductance drop can be fitted to a profile $G(x)$ (black curve), and the differential conductance $-(\text{d}G(x)/\text{d}x)$ (blue curve) shows a full-width at half-maximum $\delta x = 0.85$ nm. (b) Repeated nanowire writing and erasing at a same position confirms reconfigurability at 100 mK. (c) Schematic of SketchSET fabrication at 100 mK. Inset shows atomic terrace scanned by custom-built mK-AFM. (d) Color-coded $\text{d}I/\text{d}V_{\text{4t}}$ map as a function of $V_{\text{4t}}$ and $V_{\text{sg}}$ for an SketchSET written on Sample C with a nanowire quantum dot (QD) length of 100 nm at 100 mK.
• Figure 4: Drift-diffusion model of oxygen vacancy migration. (a-c) $N_{\text{OV}}$, $n_{\text{e}}$, and band alignment before writing. In (c),the green line denotes charge transition energy level of polarons, whereas the red line denotes $V_{\text{O}}$ charge transition energy level. (d-f) $N_{\text{OV}}$, $n_{\text{e}}$, and band alignment at the onset of applying a positive bias (solid curves). (g-i) $N_{\text{OV}}$, $n_{\text{e}}$, and band alignment during the writing process (solid curves). The inset of (g) shows a zoomed-in profile near the surface. (j-l) $N_{\text{OV}}$, $n_{\text{e}}$, and band alignment after the writing process (solid curves). The purple dashed curves in (d, g, j) show $N_{\text{OV}}$ before writing for comparison. (m) Total $N_{\text{OV}}$ in LAO layer (red curve) and STO layer (blue curve) as a function of time. (n) Total $n_{\text{e}}$ in STO layer as a function of time. In (m) and (n), a positive voltage is applied between the first two gray dashed lines, and a negative voltage between the last two gray dashed lines. The labels in (m, n) correspond to the stages of $N_{\text{OV}}$ and $n_{\text{e}}$ profile in (a-l).