Self‑Catalyzed Growth of Vertical GaSb Nanowires on InAs Stems via Metal‑Organic CVD

Background

III–V semiconductor nanowires are increasingly regarded as the cornerstone of next‑generation electronic, optical, and quantum technologies, owing to their distinctive electronic, optical, and geometrical characteristics. Among III‑V compounds, antimonides stand out for their narrow direct bandgap, minimal carrier effective mass, and exceptional carrier mobility, rendering them ideal for mid‑ to long‑wave infrared photodetectors, low‑power high‑speed transistors, and fundamental quantum research. However, the heavy atomic mass, low Sb volatility, and low melting point of III‑antimonides make nanowire growth notoriously challenging.

GaSb nanowires, in particular, have largely relied on Au catalysts, which introduce deep‑level recombination centers and deteriorate electronic performance. Therefore, catalyst‑free growth is highly desirable. Vertical growth is further complicated by nucleation barriers on substrates, typically mitigated by short stems of another material. Recent MBE work demonstrated self‑catalyzed GaSb on GaAs stems, but no catalyst‑free MOCVD growth on InAs stems has been reported. In this study, we present the self‑catalyzed growth of GaSb on InAs stems by MOCVD on Si(111). The low lattice mismatch (0.6 %) and type‑II broken‑gap alignment between InAs and GaSb enable a new axial heterostructure platform for tunneling devices, high‑speed CMOS, electron‑hole hybridization, and exciton‑spin physics.

By carefully controlling precursor fluxes, we achieve vertical GaSb nanowires with smooth sidewalls. A two‑step TMGa/TMSb flow strategy preserves Ga droplets during the initial stage, then boosts axial growth. At 500 °C the GaSb nanowires exhibit larger diameters and lengths than at 520 °C, attributable to radial growth dynamics and Gibbs‑Thomson supersaturation effects. TEM reveals InAs stems with mixed wurtzite/zinc‑blende polytypes, while the GaSb segments are defect‑free zinc‑blende.

Methods

Nanowire Growth

The heterostructure nanowires were synthesized in a close‑coupled shower head MOCVD reactor (AIXTRON Ltd.) at 133 mbar. Trimethylindium (TMIn) and TMGa served as III precursors; arsine (AsH₃) and TMSb as V precursors. Ultra‑high‑purity H₂ was the carrier gas (12 slm). Si(111) wafers were pre‑annealed at 635 °C, then cooled to 400 °C under AsH₃ to generate (111)B‑like surfaces. InAs stems were grown at 545 °C for 45 s (TMIn: 1.0×10⁻⁶ mol/min, AsH₃: 2.0×10⁻⁴ mol/min). The fluxes were switched to TMGa/TMSb, and the substrate was cooled to the desired GaSb growth temperature. Samples were finally cooled to room temperature with TMSb as a protective layer.

Characterization Methods

Scanning electron microscopy (SEM, Nova Nano SEM 650) assessed morphology. TEM (JEM‑2010F, 200 kV) coupled with energy‑dispersive spectroscopy (EDS) probed crystal structure and composition. Raman spectroscopy (Jobin‑Yvon HR Evolution, 532 nm laser, 0.36 mW, ~1 µm spot) examined optical quality.

Results and Discussion

Figure 1 illustrates the axial growth scheme and precursor sequence. Growth proceeds via a self‑catalyzed VLS mechanism, with the catalyst transitioning from In to Ga upon flux change. The larger Ga droplets on InAs stems explain the thicker GaSb segments. To prevent droplet slippage, initial low TMGa/TMSb flows (0.35×10⁻⁶ mol/min and 2.0×10⁻⁶ mol/min, V/III ≈ 5.7) are applied for 15 min. Subsequently, the flows are doubled (0.7×10⁻⁶ mol/min and 4.0×10⁻⁶ mol/min) to enhance axial growth, maintaining the same V/III ratio. This two‑step protocol yields vertical GaSb nanowires on InAs stems.

The axial growth scheme of GaSb on InAs stems, with growth at 520 °C.

SEM images at 480, 500, 520, and 545 °C (Fig. 2a–d) show temperature‑dependent behavior. At 480 °C, GaSb grows radially or planarly; at 500–520 °C axial growth is achieved; at 545 °C the InAs stems decompose, causing nanowires to fall. Optimal growth requires careful temperature control.

80°‑tilted SEM images of GaSb nanowires grown at 480, 500, 520, and 545 °C for 20 min. Inset images (b, c) show higher magnification. Red circles (d) mark residual InAs stems.

Statistical analysis (Fig. 3) reveals that larger diameter nanowires grow longer, consistent with lower supersaturation in smaller droplets (Gibbs‑Thomson effect). At 500 °C, average diameter ~206 nm and length ~330 nm; at 520 °C, diameter ~168 nm and length ~275 nm. Lower growth temperature reduces droplet supersaturation, enhancing axial growth.

Diameter and length distributions for GaSb nanowires grown at 500 °C (blue) and 520 °C (red).

The axial growth rate follows v∝(Δμ/k_BT)², where Δμ is the supersaturation determined by Sb concentration in the Ga droplet. Smaller droplets have higher vapor pressure, reducing Δμ and slowing growth. These findings align with prior GaAsP, InAs/InSb, and InGaSb studies.

Comparison with GaSb grown directly on Si versus with InAs stems (Fig. 4) demonstrates the essential role of stems. Direct growth on Si favors planar orientation due to Sb surfactant effects, while InAs stems enable vertical nanowires with smooth sidewalls and uniform diameters. The InAs segment diameter matches the GaSb top, indicating enhanced radial growth around the short stem, likely due to adatom diffusion from the substrate.

80°‑tilted SEM images: (a) GaSb without stems, (b) and (c) with short InAs stems. Inset images (b, c) show higher magnification.

TEM analysis (Fig. 5) confirms defect‑free zinc‑blende GaSb and reveals the InAs stem’s mixed WZ/ZB polytype with numerous planar defects. These defects can limit carrier mobility, underscoring the advantage of Sb incorporation for crystal‑phase engineering.

(a) Low‑magnification TEM of an InAs/GaSb nanowire. (b)–(e) HRTEM images of selected regions. Inset in (b) and (e) show FFT patterns.

EDS line scans (Fig. 6) show the droplet contains predominantly Ga with trace In, confirming the self‑catalyzed mechanism. The GaSb segment exhibits a 1:1 Ga:Sb ratio, while the InAs section displays residual In and As due to incomplete precursor shut‑off.

EDS line scan along the nanowire and compositional maps for Ga, Sb, In, and As. Two points of quantitative analysis are marked.

Raman spectra (Fig. 7) exhibit TO and LO phonon modes at ~225 cm⁻¹ and ~233 cm⁻¹ for the nanowires, slightly downshifted from bulk GaSb (226.5 cm⁻¹ and 235.2 cm⁻¹). The downshift likely arises from surface defects rather than quantum confinement, given the large nanowire diameters. The clear presence of both modes confirms high optical quality.

Raman spectra: bulk GaSb (red) vs. GaSb nanowires (blue). Green lines show multi‑peak Lorentzian fit.

Conclusions

We have demonstrated self‑catalyzed MOCVD growth of vertical GaSb nanowires on InAs stems. The two‑step TMGa/TMSb protocol preserves Ga droplets and then enhances axial growth. At 500 °C the nanowires exhibit larger diameters and lengths than at 520 °C, a trend governed by radial growth kinetics and Gibbs‑Thomson supersaturation. TEM confirms defect‑free zinc‑blende GaSb atop mixed‑phase InAs stems, while Raman indicates excellent optical quality. This scalable method is applicable to other antimonide nanowires and opens pathways for nanowire‑based devices and quantum research.

Abbreviations

CMOS

Complementary metal‑oxide‑semiconductor

EDS

Energy dispersive spectroscopy

FFT

Fast Fourier transform

LO

Longitudinal optical

MBE

Molecular beam epitaxy

MOCVD

Metal‑organic chemical vapor deposition

SEM

Scanning electron microscopy

SF

Stacking fault

TEM

Transmission electron microscopy

TMGa

Trimethylgallium

TMSb

Trimethylantimony

TO

Transversal optical

TP

Twin plane

VLS

Vapor‑liquid‑solid

ZB

Zinc‑blende