Multicolor Light Generation in UV‑GaN Photonic Quasicrystal Nanopyramids with Semipolar InGaN/GaN Quantum Wells

Abstract

We report large‑area, high‑quality multicolor emission from a 12‑fold symmetric GaN photonic quasicrystal nanorod array fabricated via nanoimprint lithography and a regrowth process. Optical pumping of In_xGa_{1‑x}N/GaN multiple quantum wells (MQWs) produces efficient blue (460 nm) and green (520 nm) light. Finite‑element simulations confirm strong coupling between the MQW emission and the photonic crystal band‑edge resonances.

Background

Wide‑band‑gap GaN materials underpin a wide spectrum of optoelectronic devices—from LEDs and laser diodes to biosensors and optogenetic tools. A key challenge remains the realization of phosphor‑free white LEDs, which can be achieved through multichip, monolithic, or color‑conversion architectures. GaN nanorod LEDs, owing to their low defect density, reduced internal electric fields, and superior light‑extraction efficiencies, present a compelling pathway toward this goal.

Numerous strategies have been explored to boost extraction efficiency in III‑nitride LEDs, including surface texturing, sapphire microlenses, oblique mesa designs, nanopyramids, graded‑index layers, self‑assembled lithography, colloidal microlens arrays, and photonic crystals. Photonic crystals, especially quasicrystal or defective two‑dimensional configurations, have demonstrated notable improvements in light extraction by opening photonic band gaps that redirect guided modes into radiative ones. Recent studies have also leveraged band‑edge effects in photonic‑crystal lasers to achieve high‑power, single‑mode, coherent emission.

While electron‑beam and laser‑interference lithography can produce intricate photonic‑crystal patterns, nanoimprint lithography (NIL) offers comparable resolution with lower cost and higher throughput, making it attractive for large‑area fabrication. In this work, we demonstrate multicolor emission from a GaN‑based two‑dimensional photonic quasicrystal (PQC) structure fabricated by NIL. The 4 cm × 4 cm PQC pattern, featuring 12‑fold symmetry, has a lattice constant of ~750 nm, pillar diameter of 300 nm, and a depth of ~1 µm. Regrowth of 430‑nm‑tall GaN pyramids and ten‑pair semipolar {10‑11} In_xGa_{1‑x}N/GaN MQWs (3 nm/12 nm) on these pillars completes the device architecture.

Under room‑temperature optical pumping, the device exhibits laser action with a low threshold while simultaneously emitting multiple colors. Prior work from our group reported single‑color lasing from GaN PQC structures; here we extend that platform to multicolor operation, highlighting the cost‑effectiveness and integrability of the NIL‑based fabrication approach.

Methods

Design and Fabrication of the Sample

Figure 2 outlines the fabrication flow: (1) epitaxial growth of a GaN wafer on a C‑plane sapphire substrate using low‑pressure metal‑organic chemical vapor deposition; (2) NIL of the PQC pattern onto a SiO₂/PMMA bilayer mask; (3) transfer of the pattern into the SiO₂ layer by reactive‑ion etching (CHF₃/O₂); and (4) inductively coupled plasma etching (Cl₂/Ar) of the GaN to form the nanorod array. Following pattern transfer, the sidewalls of the nanopillars are passivated with porous SiO₂. Subsequent regrowth at 730 °C creates 430‑nm‑tall GaN pyramids topped with ten‑pair In_xGa_{1‑x}N/GaN MQWs (3 nm/12 nm). Two compositions are employed: In₀.₁Ga₀.₉N/GaN for blue (460 nm) and In₀.₃Ga₀.₇N/GaN for green (520 nm). The etch depth of the nanorods is ~1 µm.

SEM imaging confirms the PQC geometry and the semipolar MQW facets, as shown in Figures 3a–3d. The {10‑11} semipolar planes mitigate the quantum‑confined Stark effect, enhancing radiative efficiency.

Results and Discussion

Samples A (In₀.₁Ga₀.₉N) and B (In₀.₃Ga₀.₇N) were optically pumped with a 325 nm CW He‑Cd laser (~50 mW). Photoluminescence (PL) spectra (Figure 4a) show distinct peaks at 460 nm and 520 nm, with linewidths of 40 nm and 60 nm, respectively. The CIE coordinates (0.19, 0.38) for the blue sample and (0.15, 0.07) for the green sample confirm high‑color‑purity emission. The broader green spectrum arises from defects introduced at higher indium content.

Finite‑element simulations (Figure 5b) reveal transmission spectra for incident angles 0–25°, repeating every 30° due to the 12‑fold symmetry. High‑transmission regions correspond to band‑edge resonances (M₁, M₂, M₃) that align with the MQW emission wavelengths (a/λ ≈ 0.88, 1.0, 1.25). Coupling between these resonances and the MQW emission enhances extraction efficiency and lowers lasing thresholds. The nanorod length (~1 µm) exceeds four times the effective wavelength, ensuring strong mode confinement.

These findings demonstrate that a single PQC device can simultaneously support blue and green lasing, paving the way for integrated multicolor light sources.

Conclusions

We have fabricated a 12‑fold symmetric GaN PQC nanorod array by NIL and achieved efficient blue (460 nm) and green (520 nm) emission from semipolar In_xGa_{1‑x}N/GaN MQWs grown on 430‑nm GaN pyramids. The emission peaks coincide with the PQC band‑edge resonances predicted by FEM, confirming strong light–matter coupling. The low‑cost, scalable NIL process and the inherent advantages of semipolar MQWs make this platform promising for phosphor‑free, multicolor LEDs and laser diodes.

Availability of data and materials

All data supporting the conclusions of this article are included within the manuscript.