Direct, Catalyst-Free Growth of High-Quality GaN Nanowires on Indium Tin Oxide–Coated Fused Silica by Plasma‑Assisted MBE

Introduction

Commercial III‑nitride devices predominantly rely on sapphire substrates, which impose limits on scalability and cost. Silica offers a cheaper, industry‑wide alternative, yet its inherent insulating nature demands a transparent, conductive overlayer. Indium tin oxide (ITO) fulfills both criteria, but conventional high‑temperature deposition techniques risk degrading the ITO film. Prior low‑temperature approaches, such as sputtering or plasma‑enhanced chemical vapor deposition (PECVD), often yield polycrystalline GaN with abundant defects. We address this gap by employing PA‑MBE, which supplies reactive nitrogen via RF‑discharged N₂, enabling growth at temperatures that spare the ITO. The high surface‑to‑volume ratio of nanowires mitigates lattice mismatch strain, leading to single‑crystalline, defect‑free structures even on polycrystalline ITO.

Methods

ITO Thin Film Deposition

The ITO film (≈100 nm) was deposited at room temperature using RF magnetron sputtering: argon plasma at 60 W, 2.5 mTorr, and 25 sccm gas flow. Prior to deposition, silica wafers were cleaned in acetone and isopropyl alcohol. The resulting ITO layer exhibited sheet resistance below 10 Ω/□ after annealing.

III‑Nitride Nanowire Growth

Growth was carried out in a Veeco Gen 930 PA‑MBE system. ITO‑coated silica was first annealed in a rapid thermal annealer at 650 °C under Ar for 5 min to improve crystallinity. Subsequent pre‑cleaning steps involved 200 °C and 650 °C bake‑outs in the load lock. Growth parameters: Ga BEP = 1×10⁻⁷ Torr, seed layer at 500 °C, followed by 700 °C nanowire growth. The low substrate temperature prevents ITO degradation while promoting vertical nanowire alignment.

Structural, Optical, and Electrical Characterization

Surface topography and ITO sheet resistance were measured with an Agilent 5500 SPM. For C‑AFM, a 5/5 nm Ni/Au contact was evaporated onto the nanowires, then annealed at 600 °C. A Pt/Ir AFM tip was used to probe current flow under a bias applied to the ITO. Transmission electron microscopy (TEM) and high‑angle annular dark‑field STEM (HAADF‑STEM) were performed on cross‑sectional samples prepared by an FEI Helios Nanolab FIB. Elemental maps were acquired by electron energy loss spectroscopy (EELS). Polarity assessment employed a 40 % KOH etch at room temperature for 1 h. Temperature‑dependent PL was recorded from 10 K to 290 K using a 266 nm laser in a helium‑cooled cryostat. UV‑Vis‑NIR transmittance was measured with a Shimadzu UV‑3600, and X‑ray diffraction (XRD) was conducted on a Bruker D2 Phaser.

Results and Discussion

Thermal stability of ITO was confirmed: annealing up to 700 °C produced sheet resistance <10 Ω/□ but introduced surface roughness (Fig. 1a–d). Nanowires nucleated directly on the rough ITO, maintaining vertical orientation (0001) with a density of 9.3×10⁹ cm⁻² and a fill factor of 73 % (Fig. 2b). KOH etching revealed preferential removal of the N‑polar face, confirming N‑polarity (Fig. 2d). HAADF‑STEM and EELS line scans (Fig. 3a–b) show a sharp interface, with a ~4 nm transition layer likely representing a thin, mixed‑phase GaN interlayer (Fig. 3d). High‑resolution TEM confirms single crystallinity along the c‑axis (Fig. 3c).

PL measurements (Fig. 4a) show a dominant band‑edge emission with negligible yellow luminescence, indicating low defect density. Temperature‑dependent PL (Fig. 4b) follows Varshni behavior; Arrhenius analysis yields an activation energy of 195 meV, and the calculated internal quantum efficiency (IQE) at 10 K is ~67 % (Fig. 4c). Transmittance spectra (Fig. 4d) demonstrate that the GaN nanowires scatter visible light, reducing overall transmittance compared to bare ITO, yet the substrate remains highly transparent. XRD patterns (Fig. 4e) reveal peaks from crystalline ITO and a GaN(0002) reflection, confirming vertical growth on polycrystalline ITO.

C‑AFM of n‑doped nanowires (Fig. 5a–c) shows initial high turn‑on voltage due to a Schottky barrier at the GaN/ITO interface. A subsequent sweep reduces this barrier, yielding measurable current flow across the nanowires (Fig. 5d). This behavior, observed in multiple nanowires, suggests the possibility of electrically robust devices after an initial biasing step.

Conclusions

We have successfully grown single‑crystalline GaN nanowires on ITO‑coated fused silica using PA‑MBE at low temperature. The nanowires exhibit vertical alignment, high crystal quality, N‑polarity, and strong band‑edge PL without yellow luminescence. Conductive n‑doped nanowires demonstrate viable electrical performance, positioning this platform as a promising candidate for transparent, high‑performance optoelectronic devices.

Abbreviations

AFM:

Atomic force microscopy

BEP:

Beam equivalent pressure

C-AFM:

Conductive atomic force microscopy

EELS:

Electron energy loss spectroscopy

FIB:

Focused ion beam

IQE:

Internal quantum efficiency

HAADF:

High‑angle annular dark field

HRTEM:

High‑resolution transmission electron microscopy

ITO:

Indium tin oxide

MOCVD:

Metal organic chemical vapor deposition

PA-MBE:

Plasma‑assisted molecular beam epitaxy

PECVD:

Plasma‑enhanced chemical vapor deposition

PL:

Photoluminescence

RF:

Radio frequency

RTA:

Rapid thermal annealing

sccm:

Standard cubic centimeter per minute

SEM:

Scanning electron microscopy

STEM:

Scanning transmission electron microscopy

TEM:

Transmission electron microscopy

XRD:

X‑ray diffraction