Tailoring Side Facets of Vertical [100] InP Nanowires for Innovative Radial Heterostructures

Abstract

Vertical [100] oriented InP nanowires offer a unique set of side facets and cross‑sectional geometries that are not accessible in conventional [111] growth. By systematically varying growth parameters and applying in‑situ post‑growth annealing, we engineered a spectrum of facet combinations—including square, rectangular, elongated hexagon, elongated octagon, and perfect octagon—while preserving a high vertical yield. Two exemplar radial heterostructures grown on these engineered facets demonstrate the platform’s potential for next‑generation optoelectronic devices.

Introduction

The large surface area of nanowires enhances the influence of their side facets on morphology, structure, electrical, thermal, and optical behavior ¹-⁵. In conventional [111] InP nanowires, uniform {0–11} facets give rise to predictable radial heterostructures ⁶, ⁷, whereas non‑uniform facets enable the fabrication of complex architectures such as nanocavities, quantum wells with novel geometries, and twinned superlattice nanotubes ⁸-¹⁴. Facet‑specific surface recombination velocities and nanoscale roughness further modulate carrier dynamics and phonon transport ³, ¹⁵, ¹⁶. The cross‑sectional shape, dictated by facet combinations, also determines optical confinement in nanowire cavities ¹⁷-¹⁹. Importantly, facet engineering offers a route to grow quantum wires and wells without patterned substrates, simplifying device fabrication.

Most III–V nanowires are grown on (111) substrates, yielding wurtzite or zincblende twins with {1–100}, {11–20}, or {111} facets and hexagonal or truncated triangular cross‑sections ²⁰, ²¹. Altering the growth orientation unlocks new facet families and cross‑sections—such as square and octagonal shapes—that are difficult to achieve on other orientations ²²-²⁴. These less‑explored facets could unlock novel applications across optoelectronics, sensing, and catalysis.

In this study, we systematically engineered the side facets of vertical [100] InP nanowires, realizing a range of cross‑sectional shapes while maintaining high vertical yields. We first investigated how growth temperature, V/III ratio, and trimethylindium (TMIn) flux influence facet evolution. Next, we employed in‑situ annealing to access facet combinations unattainable through growth alone. Finally, we demonstrated two radial heterostructure concepts that exploit the engineered facets for optically active devices.

Methods

Nanowires were synthesized in a horizontal flow metal‑organic vapour phase epitaxy (MOVPE) reactor at 15 slm total flow, using TMIn and PH₃ as precursors. Two pre‑growth protocols, previously shown to yield high vertical [100] yields, were employed ²⁴, ²⁶. Au seed particles were deposited via a poly‑L‑lysine layer: 30 nm particles for pre‑growth condition 1 (substrate annealed at 450 °C under 8.93 × 10⁻⁴ mol min⁻¹ PH₃ for 10 min) and 50 nm particles for pre‑growth condition 2 (TMIn pre‑flowed for 15 s at 450 °C). Growth parameters are detailed in Tables 1 and 2; higher TMIn fluxes required proportionally shorter growth times to keep dimensions comparable.

Morphological characterization employed Zeiss Ultra Plus and FEI Helios 600 NanoLab SEM; TEM analysis used JEOL 2100 at 200 kV with microtome‑cut cross‑sections. Photoluminescence (PL) measurements were performed on single nanowires dispersed on sapphire, excited with a 633 nm HeNe laser (≈1 µm spot, 20 µW power) and detected by a nitrogen‑cooled InGaAs detector.

Results and Discussion

In a face‑centred cubic lattice, low‑index facets parallel to the [100] growth axis are {011} and {001}. Figure 1 illustrates the relative orientations of these facets on [100] and [111] substrates, and Table 3 lists the possible cross‑sectional shapes arising from their combinations. Unlike the {0–11} facets of [111] wires, the {011} facets in [100] growth are partially polar; under group‑V rich conditions, A‑polar facets grow faster than B‑polar facets ²⁹-³¹. This anisotropy leads to the asymmetric geometries (types III‑VII) discussed below.

Growth temperature, V/III ratio, and TMIn flux are the dominant factors shaping facets ³⁵. Figures 2a–c show the systematic evolution of facets as each parameter is varied. Increasing temperature from 420 °C to 450 °C transforms the facet set from four {011} facets to four {001} facets via an intermediate octagonal shape. At 475 °C, some nanowires kink toward <111> during cooling, likely due to In depletion from the Au droplet ²⁶; the remaining vertical segment exhibits a slightly elongated shape, indicating limited radial growth and possible surface decomposition ³², ³³.

V/III ratio variations (200–700) did not significantly alter the dominant {001} facets, likely because the ratio range remains high for MOVPE and the {001} facets overgrow any polarity differences. In contrast, increasing TMIn flux (up to ~20×) systematically shifts the facet preference from {001} to {011}. At the highest flux, the nanowires display irregular micro‑facets, suggesting reduced adatom diffusion lengths that hinder incorporation at low‑energy sites ³⁷, ³⁸.

Pre‑growth condition 2 (TMIn pre‑flow) maintained a vertical yield of 65–80 % while promoting {011} facets even at high TMIn flux, enabling the growth of rectangular cross‑sections with larger {001} facets (Fig 3). Further increasing TMIn flux preserved vertical yield (~72 %) but produced irregular faceting similar to the highest‑flux case in Fig 2c.

Post‑growth annealing at 550 °C for 20 s–10 min under PH₃ overpressure allowed surface atoms to migrate toward lower‑energy configurations. For rectangular wires, 20 s annealing already produced an elongated octagon, reducing the circumference‑to‑area ratio. For irregular faceted wires, 210 s annealing induced a transition to low‑index {001} and {011} facets, with 600 s completing the evolution to a symmetric octagon (Fig 4). This annealing pathway minimizes total surface energy while preserving the nanowire height.

Exploiting the engineered facet geometry, we fabricated two novel radial heterostructures. In Figure 5a, a high‑flow In_0.55Ga_0.45As shell preferentially coats the larger {001} facets, forming discrete platelets that are isolated from each other—an arrangement distinct from the tubular quantum wells common in [111] or WZ nanowires ¹⁰, ⁴². In Figure 5b, the same high‑flow InGaAs layer on an elongated octagonal core yields quantum wires along the four {001} edges. A subsequent moderate‑flow InP barrier encapsulates the structure. The resulting single‑nanowire PL (Fig 5c) shows a bright emission at ~1.31 µm from the quantum wires, with negligible core emission, confirming efficient carrier confinement.

Conclusions

We have demonstrated full control over the side facets of vertical [100] InP nanowires, enabling a suite of cross‑sectional shapes from squares to octagons while maintaining high vertical yields. Slow growth rates favor {001} facets, whereas high TMIn fluxes drive the formation of {011} facets. In‑situ annealing further refines facet combinations, yielding symmetric octagons that minimize surface energy. Leveraging these engineered facets, we fabricated two optically active radial heterostructures—separated InGaAs platelets and quantum wires—that showcase the potential of [100] nanowires for complex device architectures. These findings open new avenues for integrating [100] InP nanowires into optoelectronic, photonic, and sensing technologies.

Availability of Data and Materials

The datasets generated and analyzed in this study are available from the corresponding author upon reasonable request.

Abbreviations

MOVPE

Metal‑organic vapour phase epitaxy

PL

Photoluminescence

QW

Quantum well

QWR

Quantum wire

SEM

Scanning electron microscopy

TEM

Transmission electron microscopy

TMIn

Trimethylindium

WZ

Wurtzite

ZB

Zincblende