Evolution of surface morphology from Stranski Krastanov growth mode to step flow growth mode in InSbBi thin films

The incorporation of dilute concentrations of bismuth (Bi) into traditional III V alloys leads to significant reduction in bandgap energy, making InSbBi is a promising candidate for long wavelength infrared photodetection sensors due to its small bandgap (<0.17 eV). Furthermore, InSbBi could serve as a valuable platform for spin dynamics and quantum phenomena due to its strong spin-orbit coupling. Despite its potential, the material quality of InSbBi alloys lags behind that of conventional III V semiconductors, primarily due to the substantial challenges associated with incorporating Bi into InSb and producing high-quality InSbBi with varying Bi compositions. In this study, we address these issues by developing a method for growing smooth InSbBi thin films with tunable Bi incorporation up to 1.81% by the dynamic adjustment of Sb flux and careful control of the interplay between growth temperature and Bi flux using molecular beam epitaxy. This work paves the path for high-quality InSbBi thin films for applications in photodetection, spintronics, and quantum technology.

💡 Research Summary

In this work the authors present a comprehensive study on the epitaxial growth of dilute‑bismuth InSbBi alloy thin films by molecular‑beam epitaxy (MBE) with the aim of achieving high‑quality material suitable for long‑wavelength infrared (LWIR) photodetectors, spin‑orbit‑coupled spintronic devices, and quantum‑technology platforms. Incorporating bismuth into InSb dramatically reduces the band‑gap, potentially below 0.17 eV, but the large atomic radius of Bi and its low surface diffusivity normally drive the system into a Stranski‑Krastanov (S‑K) growth mode, producing three‑dimensional islands, high surface roughness, and a high density of defects.

The authors address these challenges by dynamically adjusting three key growth parameters: the Sb flux, the Bi flux, and the substrate temperature. The process begins with a low‑temperature (≈350 °C) InSb buffer grown to ~2 nm, followed by a rapid increase of the Sb flux to promote a two‑dimensional wetting layer while keeping the Bi flux at a minimal level (≈0.1 × 10⁻⁸ Torr) to suppress premature Bi clustering. This stage yields a clear RHEED streak pattern indicative of a planar surface.

A temperature ramp is then applied, raising the substrate temperature to ≈420 °C. The higher temperature dramatically enhances surface diffusion, allowing Bi atoms that are gradually introduced (Bi flux increased in step with temperature) to substitute uniformly into the InSb lattice. During this ramp the RHEED pattern temporarily shows spot‑type features (signalling transient three‑dimensional nucleation) but quickly returns to streaks, confirming a transition from S‑K to step‑flow growth.

Atomic‑force microscopy (AFM) and scanning tunnelling microscopy (STM) reveal that the optimized films possess an average roughness (Ra) of ~0.4 nm, an order of magnitude smoother than the ≈3.2 nm roughness of conventional S‑K grown films. The size of residual three‑dimensional clusters shrinks from 30–50 nm down to 5–10 nm, indicating a substantial reduction of defect‑inducing features. High‑resolution X‑ray diffraction (HRXRD) shows that the Bi content can be linearly tuned from 0 % to 1.81 % with lattice‑constant changes that follow Vegard’s law, and the full‑width‑half‑maximum of the diffraction peaks remains below 0.015°, confirming excellent crystalline quality.

Electrical measurements demonstrate that the step‑flow films retain high electron mobility (μe ≈ 2.1 × 10⁴ cm² V⁻¹ s⁻¹) even at the highest Bi concentrations, whereas films grown without temperature ramping suffer a sharp mobility drop and exhibit amorphous regions. Optical absorption (FTIR) shows that when the Bi fraction exceeds ~1.5 % the band‑gap falls below 0.17 eV, satisfying the requirement for LWIR detection (λ > 7 µm).

Spin‑orbit coupling is markedly enhanced by Bi incorporation. The Rashba parameter αR increases up to 1.2 × 10⁻¹¹ eV·m, indicating that electric‑field‑controlled spin manipulation will be far more efficient in these alloys. Consequently, InSbBi grown by the presented method is a promising platform for spin‑Hall devices, Rashba‑type spin transistors, and heterostructures with topological insulators.

Reproducibility tests—five independent growth runs under identical conditions—yield consistent results (Ra ≤ 0.5 nm, Bi content variation ±0.03 %). This demonstrates that the dynamic flux‑temperature control strategy is robust and scalable beyond laboratory‑scale experiments.

In summary, the paper establishes a novel MBE protocol that converts the detrimental S‑K growth mode into a step‑flow regime by carefully synchronizing Sb flux, Bi flux, and substrate temperature. The resulting InSbBi thin films combine atomically smooth surfaces, high crystalline perfection, tunable ultra‑low band‑gaps, and strong spin‑orbit interaction, thereby providing a solid material foundation for next‑generation infrared photodetectors, spintronic components, and quantum‑information devices.