Raman Spectroscopy of Self‑Catalyzed InP/InAs/InP Nanopillars and Nanocones on InP(111)B: Insights from a Simple Substrate‑Tilting Approach

Background

Semiconducting heterostructure nanowires have attracted significant interest over the past decade [1]. Core‑shell, superlattice, and alloy nanowires spanning a broad material space have been fabricated [2–10]. InP–InAs nanowires, in particular, are promising for light‑emitting diodes [14], single‑photon sources [15], photodetectors [16], and heterojunction transistors [17] owing to their tunable bandgap, high carrier mobility, and large breakdown field [18–19]. Device performance hinges on the optical and electronic properties of the nanoscale constituents, which in turn depend on crystallinity, morphology, and composition [20–21]. Raman spectroscopy, a non‑destructive probe, provides insights into phonon confinement and surface optical modes that arise from these structural features [22–26]. Polarization‑dependent Raman scattering has revealed angular dependencies of active modes in highly anisotropic nanowires such as Si [27], GaAs [28], InAs [29,30], GaP [31,32], ZnO [33], and GaN [34]. Recent advances have pushed Raman sensitivity to the single‑molecule level through near‑field surface resonances on engineered substrates or metal‑decorated nanostructures [35–38]. Yet, Raman characterization of self‑catalyzed, wafer‑scale one‑dimensional heterostructures remains underexplored. Variations in Raman peak positions, widths, and intensities encode information about composition, chemical environment, and crystallinity, offering a powerful, non‑destructive diagnostic for as‑grown samples [40]. In this work, we present Raman studies of self‑catalyzed InP/InAs/InP core‑shell nanopillars and nanocones, highlighting how morphology, crystal structure, and scattering geometry influence the vibrational signatures.

Methods

One‑dimensional nanostructures (nanopillars and nanocones) were fabricated via a self‑catalyzed vapor‑liquid‑solid process on InP(111)B using a Veeco D125 MOVPE reactor with trimethylindium (TMIn), tertiarybutylphosphine (TBP), and tertiarybutylarsine (TBA) precursors [13,23,41]. Nanopillars were grown at ~350 °C, nanocones at ~400 °C. Indium droplets were seeded in situ by delivering 5.06 × 10⁻⁵ mol min⁻¹ TMIn for 12 s. Subsequent InP growth used TMIn and TBP flows of 3.74 × 10⁻⁶ and 3.37 × 10⁻⁴ mol min⁻¹ (V/III = 90) for 540 s. After purging with H₂ (10 s) and TBA (180 s) while ramping to 420 °C, the InAs shell was deposited with TBA (9.82 × 10⁻³ mol min⁻¹) and TMIn (8.18 × 10⁻⁵ mol min⁻¹) (V/III = 120) for 10 s. A final InP cap was grown with TMIn (3.73 × 10⁻⁶ mol min⁻¹) and TBP (3.37 × 10⁻³ mol min⁻¹) (V/III = 90) for 60 s. Samples without the InAs step served as pure InP controls (see Fig. 1a and Additional file 1: Figure S1).

Growth morphology of InP/InAs nanostructures. a Schematic layout of InP/In(As,P) multi core‑shell nanopillar and nanocone. b SEM images of top view (upper row) and 45° tilted view (lower row) of InP nanopillars, InP/InAs/InP nanopillars, and InP/InAs/InP nanocones grown on (111)B oriented InP single crystal wafer.

Post‑growth morphology was examined by FEI NOVA 230 field‑emission SEM at 5 kV. Over 30 nanostructures were measured to obtain average height and base diameter. Raman spectra of ensembles of InP/InAs/InP nanopillars or nanocones were recorded in a backscattering, confocal configuration using a Renishaw InVia spectrometer. To avoid laser‑induced damage, substrate tilting was limited to ≤35°. A 514.5 nm laser, with power adjustable from 5 to 25 mW, was focused to ~1 µm. Spectra were acquired with 0.5 cm⁻¹ resolution, calibrated against the Si substrate peak at 520.1 cm⁻¹, and fitted with symmetric Gaussian‑Lorentzian functions to extract peak parameters.

Results and Discussion

Figure 1 illustrates the typical morphologies of InP nanopillars, InP/InAs/InP nanopillars, and InP/InAs/InP nanocones on InP(111)B. All structures are vertically aligned along the <111>B direction, exhibiting slight tapering. Pillars are ~150 nm wide and ≤250 nm tall, while cones are ~50 nm wide and up to 2 µm tall, reflecting the interplay of vapor‑liquid‑solid and vapor‑phase epitaxy at 400 °C [13,41]. Detailed crystallographic analyses are reported elsewhere [42].

Figure 2 presents Raman spectra from InP and InP/InAs/InP cones and pillars, with the incident beam aligned along the nanostructure axis. Reference spectra of InP(111)B and InAs(111)B substrates are also shown. Bulk InP (zinc‑blende) exhibits a single F₂ mode that splits into TO and LO phonons. Wurtzite crystals allow A₁, E₁, E₂H, and E₂L modes, with A₁ and E₁ splitting into LO and TO components [43–44].

Raman spectra of (a) InP(111)B crystal, (b) InAs(111)B crystal, (c) InP nanopillar, (d) InP/InAs/InP nanopillar, and (e) InP/InAs/InP nanocones. Green dots mark the positions of InAs A₁(TO), E₁(TO), A₁(LO), E₁(LO); InP E₁(TO), A₁(LO), E₁(LO); and second‑order modes E₁(2TO), E₁(TO + LO), E₁(2LO).

All spectra from the InP(111)B substrate and InP/InAs/InP nanocones show two peaks at 303.7 cm⁻¹ (TO) and 344.5 cm⁻¹ (LO). InP nanopillars display TO and LO peaks at 303.8 cm⁻¹ and 343.0 cm⁻¹, consistent with wurtzite E₁ TO and E₁ LO. InP/InAs/InP nanopillars exhibit a pronounced LO band broadening, absent in bulk InP. The peaks at 303.8 cm⁻¹ and 341.7 cm⁻¹ correspond to InP E₁ TO and A₁ LO, respectively. LO modes are particularly sensitive to Fröhlich interactions [45]. The 218 cm⁻¹ and 241 cm⁻¹ peaks represent InAs E₁ TO and E₁ LO, respectively. Lower intensities in the nanostructures relative to InAs(111)B suggest core‑shell or alloy formation [13,42]. Red‑shifts and broadenings of InAs E₁ LO and A₁ LO in nanopillars reflect phonon confinement in sub‑nanometer features [48–50]. Second‑order modes (2TO, TO + LO, 2LO) appear between 600–700 cm⁻¹, arising from critical‑point singularities in the two‑phonon density of states [51]. Peaks at 616 and 649 cm⁻¹ match 2TO(Γ) and TO(Γ)+LO(Γ); the 2LO peak is slightly blueshifted, consistent with contributions from the L point [52]. In nanocones, 649 and 684 cm⁻¹ match TO(Γ)+LO(Γ) and 2LO(Γ), while the 619 cm⁻¹ 2TO(Γ) peak is slightly displaced, likely due to the high aspect ratio [53]. All detected peaks are summarized in Table 1.

Effect of substrate tilting on Raman active modes in InP/InAs/InP nanocones.

Figure 4 shows how Raman intensities of InP TO and LO peaks vary with excitation power for different substrate tilts, and the corresponding I(LO)/I(TO) ratios. For nanopillars, a 2–3 cm⁻¹ red‑shift and broadening of InAs E₁ TO and A₁ LO occur when the power is increased from 5 to 25 mW, whereas nanocones exhibit negligible shift (Additional file 1: Figures S2c‑d). Laser heating is therefore minimal. Nanopillars, with larger effective scattering cross‑section, display stronger Raman resonance than nanocones. The integrated intensities increase linearly with power, confirming the absence of heating effects. Substrate tilt enhances TO reflection relative to LO for both morphologies, as seen in Figure 3 and Additional file 1: Figure S2. At 0° tilt, the InP E₁ TO/E₁ LO ratio is similar for nanopillars and nanocones; at 30°, the ratio rises to ~2.3 for nanocones versus ~1.3 for pillars, reflecting changes in photon–lattice cross‑section and surface electric field [49,56]. These results illustrate that combining Raman spectroscopy with simple substrate tilting provides a powerful, non‑destructive tool for identifying growth morphology, crystal structure, and composition in III–V heterostructures with nanometer‑scale resolution.

An excitation power dependence on Raman spectra of InP 1TO and InP 1LO peaks for different substrate tilts. a InP/InAs/InP nanopillars. b InP/InAs/InP nanocones. c Integrated intensity ratio of InP 1TO over InP 1LO excitations.

Conclusion

We have demonstrated Raman spectroscopy of self‑catalyzed InP/InAs/InP core‑shell nanopillars and nanocones grown on InP(111)B. By varying laser power and substrate tilt in a backscattering geometry, we identified InAs E₁ TO, A₁ TO, E₁ LO, and InP E₁ TO, A₁ LO, E₁ LO modes. In contrast to bulk references, the nanostructure ensembles exhibit second‑harmonic Raman features (E₁ 2TO, E₁ LO + TO, E₁ 2LO). Red‑shifts and broadenings of LO modes in nanopillars and pronounced E₁ TO/A₁ LO splitting in nanocones were observed. Raman intensities scale linearly with excitation power, and the I(TO)/I(LO) ratio remains stable except when the substrate is tilted; at 30° the ratio increases to ~2.3 for nanocones. These findings establish a straightforward, non‑destructive Raman protocol—augmented by substrate tilting—to probe shape, crystal structure, and composition in wafer‑scale III–V heterostructures, facilitating their integration into next‑generation nanoelectronics and photonics.

Availability of data and materials

All data generated or analyzed during this study are included in this published article and its supplementary information files.

Abbreviations

As:

Arsine

EDS:

Energy‑dispersive spectroscopy

In:

Indium

LO:

Longitudinal optical phonon

MOCVD:

Metal‑organic chemical vapor deposition

P:

Phosphine

SEM:

Scanning electron microscope

TBA:

Tertiarybutylarsine

TBP:

Tertiarybutylphosphine

TMIn:

Trimethylindium

TO:

Transverse optical phonon

WZ:

Wurtzite crystal structure

ZB:

Zinc blende crystal structure