Hybrid Epitaxial Al/InGaAs system: Solid-state dewetting and Al facet formation

Abstract

Hybrid superconductor--semiconductor platforms can host subgap electronic excitations such as Andreev bound states (ABSs); in topological regimes, a special zero-energy class, Majorana bound states (MBSs), can emerge. Here we report the growth of epitaxial Al films by molecular-beam epitaxy on $\mathrm{In_{0.75}Ga_{0.25}As}$ under near-room-temperature substrate conditions. Using a combination of AFM/SEM, cross-sectional TEM, and \emph{in situ} RHEED, we map how substrate temperature and Al deposition rate govern film morphology, continuity, and interface quality. We identify a growth window that yields continuous, superconducting Al films with an abrupt $\mathrm{Al}/\mathrm{In_{0.75}Ga_{0.25}As}$ interface and no detectable indium interdiffusion. We further investigate the thermal stability of these films under \emph{in situ} post-growth heating and \emph{ex situ} annealing following surface oxidation. For unoxidized Al, rapid surface diffusion triggers solid-state dewetting at approximately $165\,^\circ\mathrm{C}$, resulting in the formation of $\{111\}$-faceted Al islands. In contrast, the presence of a native oxide largely suppresses dewetting, with failure occurring only locally at surface defects. Annealing above the indium melting point ($156.6\,^\circ\mathrm{C}$) induces significant In surface migration in both cases, leading either to localized interfacial In inclusions beneath Al agglomerates or to uniform surface contamination at sites of localized layer breakdown. Together, these results define growth and annealing conditions for thermally robust epitaxial Al on III--V semiconductors and provide practical guidance for fabricating high-quality superconductor--semiconductor hybrid platforms for quantum devices.

Figures (9)

• Figure 1: RHEED patterns recorded along the $[0\bar{1}\bar{1}]$ and $[0\bar{1}1]$ azimuths: (a) substrate surface after deposition of 6 nm In$_{0.75}$Ga$_{0.25}$As; (b) after $\sim$5 monolayers of Al, showing the evolving surface morphology; and (c) upon completion of $\sim$75 monolayers of Al.
• Figure 2: Substrate temperature profile for sample G1100, detailing the Al growth phase and the subsequent high-temperature dewetting ramp.
• Figure 3: RHEED evolution for Al on In$_{0.75}$Ga$_{0.25}$As during post-deposition heating after $\sim$10 nm of Al. Patterns are shown along $[0\bar{1}\bar{1}]$ (left) and $[0\bar{1}1]$ (right). (a) Streaky In$_{0.75}$Ga$_{0.25}$As surface after the overnight cooldown. (b) Immediately after Al deposition at $\sim 40\,^{\circ}\mathrm{C}$: streaky 2D Al with additional streaks from coexisting variants (red arrows). (c) At $\sim 100\,^{\circ}\mathrm{C}$: sharpened, purely streaky 2D pattern as a single Al orientation dominates. (d) At $\sim 165\,^{\circ}\mathrm{C}$: transition to spotty/3D features with chevrons, consistent with faceting during thermal roughening. (e) Chevrons angle.
• Figure 4: Critical magnetic field measurements for G1001, and G0972 samples of Al.
• Figure 5: (a) Plan-view SEM micrograph of dewetted Al islands. (b) 3D AFM height map (5 $\mu$m $\times$ 5 $\mu$m) of the same surface. The inset indicates the in-plane [011] and [01$\bar{1}$] directions of the (100)-oriented InP substrate; white lines mark the locations of the line-scans. (c) Representative line profiles (black outline) with linear fits to the sidewalls (red/blue), indicating a consistent slope of $m \approx 0.72$, which corresponds to an inclination angle $\theta \approx 35.8^{\circ}$. The schematic overlay illustrates the proposed [110] island normal and the bounding $\{111\}$ facet planes, which are analyzed in the text.