🔬 materials science

Fractional-Monolayer 2D-GaN/AlN Structures: Growth Kinetics and UVC-emitter Applications

This study demonstrates that the optical properties of subcritical GaN/AlN quantum wells are governed by their growth mechanism, which dictates whether they form 2D quantum disks or ribbons, ultimately enabling the development of powerful ultraviolet-C emitters with linear power scaling up to 37 W.

Original authors: V. N. Jmerik, D. V. Nechaev, E. A. Evropeitsev, E. M. Roginskii, A. N. Semenov, M. A. Yagovkina, P. A. Alekseev, V. I. Kozlovsky, M. M. Zverev, N. A. Gamov, Tao Wang, Xinqiang Wang, T. V. Shubina, A.

Published 2026-01-28

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CC BY 4.0

Original authors: V. N. Jmerik, D. V. Nechaev, E. A. Evropeitsev, E. M. Roginskii, A. N. Semenov, M. A. Yagovkina, P. A. Alekseev, V. I. Kozlovsky, M. M. Zverev, N. A. Gamov, Tao Wang, Xinqiang Wang, T. V. Shubina, A. A. Toropov

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to build a very special kind of sandwich, but instead of bread and cheese, you are stacking layers of atoms to create a tiny lightbulb that glows with invisible ultraviolet light. This paper is about how the scientists at the Ioffe Institute and their partners built these "atomic sandwiches" and discovered that the way they stack the layers changes the color and brightness of the light.

Here is the story of their discovery, broken down into simple concepts:

1. The Goal: Making a Super-Bright UV Light

The scientists wanted to create a device that emits UVC light (a type of ultraviolet light used for sterilization and medical tools). Usually, making these lights is tricky because the materials tend to get "lazy" and stop glowing efficiently when they get hot or when there are tiny defects.

To fix this, they decided to make the active part of the lightbulb incredibly thin—so thin that it is measured in monolayers. Think of a monolayer as a single sheet of atoms, like a single layer of tiles on a floor. They were stacking layers of Gallium Nitride (GaN) and Aluminum Nitride (AlN) that were only 0.75 to 2 tiles thick.

2. The Two Ways to Build the Layers

The scientists found that how they poured the "ingredients" (Gallium and Nitrogen atoms) onto the surface changed the shape of the final product. They used a technique called Molecular Beam Epitaxy, which is like spraying atoms onto a surface in a vacuum.

They discovered two main ways the atoms arranged themselves, depending on how much Gallium they sprayed:

The "Island" Method (Nitrogen-Rich): When they sprayed less Gallium, the atoms didn't want to stick together in a smooth sheet. Instead, they formed tiny, isolated islands or disks on the surface. Imagine raindrops forming on a windshield; they sit there as separate puddles.
The "River" Method (Gallium-Rich): When they sprayed more Gallium, the atoms became very mobile. They rushed to the edges of the steps on the surface and flowed along them. This created long, thin strips of material, which the authors call quantum ribbons. Imagine water flowing down a staircase, filling the steps in long, continuous lines rather than forming puddles.

3. The "Fractional" Mystery

The most interesting part of the paper is what happened when they tried to build layers that were fractional (like 1.5 layers). You can't really have half a tile, so what happens?

If they used the "Island" method: The extra half-layer formed tiny, separate islands (disks) sitting on top of the first full layer.
If they used the "River" method: The extra half-layer formed long, thin strips (ribbons) along the steps.

The scientists realized that these different shapes (disks vs. ribbons) acted like different kinds of traps for electrons. The shape determined exactly what color of UV light the material would glow with and how bright it would be.

4. The Results: A Powerful Lightbulb

They built a stack of 250 of these tiny layers. When they hit this stack with a beam of electrons (like a tiny, high-speed particle accelerator), it lit up.

The Brightness: The "River" method (Gallium-rich) produced much brighter light than the "Island" method.
The Power: They managed to get a very powerful burst of light. One of their samples, when pumped with a strong electron beam, produced 37 Watts of UV light. To put that in perspective, that is as bright as a standard household lightbulb, but it is emitting invisible, high-energy UV light.
The Color: By changing the thickness of the layers and the amount of Gallium, they could tune the light to specific wavelengths between 228 nm and 256 nm.

5. The "Recipe" for Success

The paper concludes with a simple rule of thumb they developed:

If you want a specific, predictable color and you are building a whole number of layers (1 or 2), the method doesn't matter much.
If you are building a "fractional" layer (like 1.5), how you build it matters a lot.
- Spray less Gallium $\rightarrow$ You get disks (weaker light).
- Spray more Gallium $\rightarrow$ You get ribbons (much stronger light).

Summary

In short, the scientists figured out that by controlling the "recipe" of their atomic sandwich, they could force the atoms to arrange themselves into either tiny islands or long ribbons. The "ribbons" turned out to be the secret to making a very powerful, efficient ultraviolet light source. This is a big step forward for making better UV lights for things like water purification or medical devices, all by playing with the arrangement of just a few layers of atoms.

Technical Summary: Fractional-Monolayer 2D-GaN/AlN Structures: Growth Kinetics and UVC-emitter Applications

Problem Statement
The development of efficient ultraviolet-C (UVC) emitters is hindered by the quantum-confined Stark effect (QCSE) in conventional hexagonal III-nitride heterostructures, which reduces radiative recombination efficiency. While GaN/AlN quantum dots (QDs) and monolayer (ML)-thick quantum wells (QWs) have shown promise in suppressing QCSE, there is a lack of systematic understanding regarding the radiative properties of structures with subcritical (fractional) monolayer thicknesses (0.75–2 ML). Specifically, the relationship between growth kinetics (2D-nucleation vs. step-flow), the resulting topological morphology of the active regions (quantum disks vs. ribbons), and their optical emission characteristics in the UVC range remains unclear.

Methodology
The authors employed Plasma-Assisted Molecular Beam Epitaxy (PA MBE) to grow 250-period {GaN $_m$ /AlN $_{15}$ } multiple quantum well (MQW) structures on c-sapphire substrates. Two distinct series of samples were fabricated to isolate growth variables:

T-series: Nominal GaN well thickness ( $m$ ) varied from 0.75 to 2.0 ML in 0.25 ML steps at a constant Gallium-to-Nitrogen flux ratio ( $\phi_{Ga}/\phi_{N2}^*$ ) of 1.6.
F-series: Nominal thickness fixed at $m=1.5$ ML, while the flux ratio $\phi_{Ga}/\phi_{N2}^*$ was varied from 0.8 to 2.1.

Characterization and Analysis:

In situ: Reflection High-Energy Electron Diffraction (RHEED) was used to monitor growth modes and surface morphology evolution.
Ex situ Structural: Atomic Force Microscopy (AFM), X-ray Diffraction (XRD), X-ray Reflectivity (XRR), and High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) were used to verify structural quality, layer thickness, and interface roughness.
Optical: Photoluminescence (PL) at 77 K and 300 K, and Cathodoluminescence (CL) using both low-current (1 mA) and high-current pulsed electron beams (up to 2 A, 12.5 keV) were measured.
Theoretical: Ab initio calculations using the $G_0W_0$ quasiparticle approximation and Density Functional Theory (DFT) were performed to model the electronic band structures and optical transition intensities of various GaN/AlN configurations (planar QWs, quantum ribbons, and quantum disks).

Key Contributions and Results

Growth Kinetics and Morphology:
- The study establishes a direct correlation between the Ga/N flux ratio and the dominant growth mechanism. Under nitrogen-rich or stoichiometric conditions ( $\phi_{Ga}/\phi_{N2}^* \le 1.1$ ), growth proceeds via 2D-nucleation, leading to the formation of isolated 2D quantum disks (QDs).
- Under Ga-rich conditions ( $\phi_{Ga}/\phi_{N2}^* \ge 1.6$ ), the mechanism shifts to step-flow growth. In fractional ML structures ( $1 < m < 2$ ), this results in the formation of bilayer-thick (BL) quantum ribbons (QRs) elongated along atomic step edges.
- RHEED intensity oscillations were observed for 2D-nucleation but absent for step-flow growth, confirming the transition between these modes.
Phenomenological Model of Fractional ML Structures:
- The authors propose a model where the optical properties of fractional ML QWs are determined by their topology, which is dictated by the growth mechanism.
- Integer MLs (1 and 2 ML): Emission characteristics are independent of the growth mechanism, exhibiting single peaks consistent with continuous layer-by-layer growth.
- Fractional MLs: Emission intensity and spectral position are highly sensitive to the growth mechanism.
  - 2D-Nucleation (N-rich): Forms individual QDs. These exhibit relatively weak emission.
  - Step-Flow (Ga-rich): Forms narrow quantum ribbons (<10 nm width). These structures exhibit a significant, threshold-like increase in emission intensity (over an order of magnitude) compared to N-rich counterparts.
Optical Performance and UVC Emission:
- Spectral Tuning: The emission wavelength redshifts from ~228 nm to ~260 nm as the nominal thickness increases from 0.75 ML to 2 ML.
- Intensity: Structures grown under Ga-rich conditions (promoting QR formation) showed significantly higher photoluminescence and cathodoluminescence intensities, particularly in the 230–240 nm range.
- High-Power Emission: When pumped by a high-current electron beam (12.5 keV, up to 2 A), the MQW structures demonstrated linear dependence of optical peak power on pump current.
  - The F-1.6 structure ( $m=1.5$ , $\phi_{Ga}/\phi_{N2}^*=1.6$ ) achieved a peak optical power of 33 W at 238 nm.
  - The F-2.1 structure achieved 37 W at 256 nm.
  - Structures with thinner wells or N-rich growth conditions yielded significantly lower powers (1–3 W).

Significance
The paper claims that controlling the growth kinetics to favor the formation of bilayer-thick quantum ribbons via step-flow growth in fractional-monolayer GaN/AlN MQWs is a viable strategy for developing powerful, high-efficiency UVC emitters. The work demonstrates that these structures can overcome the limitations of traditional QWs by suppressing the QCSE and enhancing carrier capture efficiency. The authors highlight that the demonstrated peak powers (up to 37 W) and optical energy per pulse (2.6 $\mu$ J) make these fractional-ML structures promising candidates for next-generation high-power UVC sources, particularly in the 230–240 nm spectral range where high intensity is achieved with narrow emission linewidths (8.6 nm). The study provides a fundamental link between MBE growth parameters, nanoscale morphology, and macroscopic optoelectronic performance in subcritical III-nitride heterostructures.