Direct Growth of III‑Nitride Nanowire LEDs on Amorphous Quartz Using a TiN/Ti Interlayer

Background

Light‑emitting diodes have become the workhorse of display technology, outpacing cold‑cathode fluorescent lamps in energy efficiency and portability. Conventional GaN‑based blue LEDs are typically grown on sapphire, and the drive toward larger‑diameter sapphire boules has introduced manufacturing challenges such as maintaining precise c‑plane orientation and surface flatness [1, 2]. Moreover, planar GaN LEDs suffer from the green‑gap, a decline in quantum efficiency for wavelengths beyond 520 nm.

Attempts to grow III‑nitride materials directly on glass have included gas‑source molecular beam epitaxy (MBE) and sputtering, yielding polycrystalline films that degrade device performance [3, 4, 5]. Samsung’s micromasking and selective MOCVD growth produced near‑single‑crystalline GaN pyramids on glass, yet indium incorporation remained limited due to evaporation during MOCVD [6, 7]. Graphene pre‑buffer layers improved the quality of sputtered InGaN on amorphous glass but added process complexity [8]. These approaches highlight the need for a simpler, scalable method to integrate nitride LEDs with transparent substrates.

III‑nitride nanowires, grown catalyst‑free under nitrogen‑rich conditions, can nucleate spontaneously without a global epitaxial relationship, enabling growth on diverse substrates such as silicon, metal, and silica [9–25]. Their high surface‑to‑volume ratio suppresses threading dislocations and reduces strain in the active region, making them suitable for emissions within the green‑gap [10–16]. The insulating nature of glass, however, complicates the creation of an electrically injected device that remains transparent and conductive.

Here we present the first direct growth of InGaN/GaN nanowires on amorphous quartz using a TiN/Ti interlayer that provides both transparency and conductivity. The resulting nanowire LED emits yellow light (~590 nm) and demonstrates the feasibility of integrating nitride optoelectronics with scalable glass substrates.

Methods

Material Growth

The samples were grown in a Veeco GEN 930 PA‑MBE system under nitrogen‑rich conditions. Commercial double‑polished quartz (≈ 500 µm thick) was cleaned with acetone and isopropanol, then dried with nitrogen. A 20 nm Ti layer was electron‑beam evaporated to serve as a translucent conductive interlayer. Following another solvent clean and two outgassing steps, the substrate entered the growth chamber where a nitrogen plasma (1 sccm, 350 W RF) partially nitridated the Ti into TiN before opening the Ga shutter. N‑type GaN:Si nanowire bases were grown with a Ga BEP of 6.5 × 10⁻⁸ Torr and Si cell temperature of 1165 °C. A two‑step approach—620 °C nucleation for 10 min followed by 770 °C growth—controlled nanowire density. Five InGaN quantum disks (5 × 10⁻⁸ Torr Ga BEP, 5 × 10⁻⁸ Torr In BEP) and GaN barriers were deposited, and a p‑type GaN:Mg layer followed at 310 °C Mg cell temperature.

Optical and Structural Characterization

Photoluminescence (PL) was measured with a 325‑nm HeCd laser (3.74 mW) and ×15 UV objective, achieving an excitation density of ≈ 310 kW cm⁻². Samples were cooled from 77 K to 300 K in a Linkam THMS 6000 cryostat. Transmittance spectra were recorded with a Shimadzu UV‑3600 spectrophotometer using air as reference. SEM images were taken on an FEI Quanta 600, while HRTEM and HAADF‑STEM used a Titan 80‑300 at 300 kV. Elemental maps were obtained via EDS (EDAX) and EELS.

Device Fabrication and Characterization

After cleaning, a 2 µm parylene C layer was deposited by thermal evaporation. Oxygen‑plasma RIE etched the parylene to expose p‑type tips. Ni (5 nm) and ITO (230 nm) were deposited by e‑beam evaporation and RF sputtering, respectively, forming the transparent current‑spreading layer; annealing at 500 °C in Ar improved conductivity. Cl/Ar ICP‑RIE defined mesas, and Ni/Au contacts were added via e‑beam evaporation and lift‑off. L‑I‑V measurements used a Keithley 2400. Infrared thermography with OptoTherm employed a custom emissivity map generated by heating the device to 60 °C; subsequent current‑dependent imaging measured local temperature rise.

Results and Discussion

Structural and Optical Characterization of Nanowires Grown on Quartz

The nanowires comprise ≈ 90 nm n‑GaN, five 7‑nm InGaN disks, 14‑nm barriers, and ≈ 60 nm p‑GaN. Plan‑view SEM (Fig. 1a) shows high‑density, ≈ 100 nm wide, ≈ 250 nm long wires with a density of 9 × 10⁹ cm⁻² and a 78 % fill factor. A two‑step growth optimized nucleation while minimizing coalescence that would introduce non‑radiative centers.

a Plan‑view SEM of as‑grown InGaN/GaN nanowires on quartz. b Bright‑field TEM of the p‑GaN region, with inset diffraction pattern. c HAADF image of a single nanowire. d–f Elemental maps for Ga, Ti, and composite. g–h Interface mapping and EDX/EELS across Ti/TiN and quartz. Scale bar = 25 nm.

Bright‑field TEM (Fig. 1b) confirms crystalline GaN growth on the lattice‑mismatched substrate, while HAADF imaging (Fig. 1c) reveals the five InGaN disks as bright spots. Elemental mapping shows the nanowires nucleate on the TiN layer rather than directly on quartz. TiN, formed during nitridation, is ≈ 10 nm thick and preserves both transparency and conductivity, also acting as a reflector for longer wavelengths [31].

µ‑PL spectra (Fig. 2a) display a broad peak that red‑shifts and broadens from 77 K to 300 K, consistent with Varshni band‑gap shrinkage and exciton‑phonon coupling. Power‑dependent µ‑PL (Fig. 2c) shows negligible blue‑shift, indicating reduced piezoelectric fields and quantum‑confined Stark effect due to radial strain relief in the nanowire geometry.

a Temperature‑dependent PL. b Peak wavelength and FWHM versus temperature. c Power‑dependent PL at 77 K.

Transmittance measurements (Fig. 3) compare bare quartz (≈ 93 % visible), quartz with 20 nm Ti (≈ 22 %), TiN/Ti interlayer (≈ 43 %), and as‑grown nanowires (≈ 40 %). The TiN layer restores transparency, while the active region modestly reduces transmission due to absorption by InGaN disks. Optical photographs (Fig. 3b‑e) illustrate the visual impact of each layer.

a Transmittance curves. b–e Optical images of each sample.

Device Characterization

The LED fabrication flow is summarized in Fig. 4. The final device stack (Fig. 5a) consists of Ni/Au contacts, Ni/ITO current spreaders, the nanowire ensemble with embedded quantum disks, and a TiN/Ti back contact.

Fabrication steps for the nanowire‑on‑quartz LED.

a Device schematic. b Optical micrograph under forward bias. c L‑I‑V curve. d EL spectra versus current. e FWHM and peak shift with bias. f Relative EQE showing droop above 20 mA.

The 500 µm × 500 µm device turns on at ≈ 2.6 V (linear extrapolation of the V‑I curve) with a resistance of ≈ 300 Ω, higher than silicon‑based nanowire LEDs due to the limited conductivity of the thin TiN/Ti layer and inadvertent TiO₂ formation [36]. Light extraction is modest because emission couples into the quartz and is partly back‑scattered. EL measurements reveal a broad 120 nm linewidth; the peak shifts from 650 nm at turn‑on to 590 nm at higher current, a blue‑shift caused by band‑filling while thermal effects counterbalance at high injection. The EQE peaks near 20 mA and then declines, attributed to current crowding and poor heat dissipation in quartz.

Infrared thermography (Fig. 6) shows a device temperature exceeding 60 °C at 35 mA, with heat concentrating around the active region. Efficient phonon transport layers are needed to mitigate thermal roll‑over in future designs.

a Device temperature versus current. b–e Infrared images at 5, 10, 20, and 30 mA.

Color Mixing Experiment

High‑quality white light is critical for displays and lighting. By combining the tunable yellow nanowire LED with RGB laser diodes (LDs) in a transmission configuration, we produced white light with CCT ranging from 2800 K to > 7000 K while maintaining CRI above 55. The yellow LED’s broad spectrum enhances CRI; with only RGB LDs, CRI reached 55.4, but adding the yellow component raised it to 85.1 at 7300 K (Fig. 7e). The setup (Fig. 7a) uses a Thorlabs three‑channel combiner, a 45° mirror, and a GL Spectis 5.0 spectrometer to extract CIE 1931 coordinates.

a Color‑mixing setup. b CCT and CRI versus LED bias. c CCT and CRI versus LD bias. d Spectrum with blue LD + yellow LED. e Spectrum with RGB LDs + yellow LED.

Conclusions

We have demonstrated the growth of InGaN/GaN nanowires directly on amorphous quartz via a TiN/Ti interlayer and fabricated LEDs based on this platform. The nanowire architecture yields highly crystalline nitride material on a glass substrate, enabling yellow‑amber‑red emission (590–650 nm) with a 120 nm FWHM. Leveraging the broad, tunable spectrum, we achieved a wide‑range white‑light source (3000–>7000 K) in a transmission configuration, highlighting the potential of nanowire‑on‑quartz LEDs for scalable, transparent optoelectronic applications.