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

The paper reports on fundamental properties of the GaN/AlN quantum wells (QWs) with nominal subcritical thicknesses of 0.75-2 monolayers (MLs). They are grown by plasma-activated molecular beam epitaxy, varying either the nominal thickness or the gallium-to-nitrogen flux ratio. In situ monitoring reveals difference in 2D nucleation and step-flow growth modes of the QWs. The emission charactestics of QWs with integer thicknesses of 1 and 2 MLs depend weakly on the growth mechanism. In contrast, the intensity and spectral position luminescence of QWs with fractional-ML thicknesses are determined by the growth mechanism. Using ab initio calculations, a phenomenological model is proposed that describes fractional-ML QWs either as arrays of 2D quantum disks or as arrays of 2D quantum ribbons, in cases where 2D nucleation or step-flow growth mechanisms predominate, respectively. This model is generally consistent with experimental data on photo- and cathodoluminescence of heterostructures with multiple (250) GaN/AlN QWs. These heterostructures, when pumped by electrom beam at an energy 12.5 keV with a maximum pulse current of 2 A, exhibit linear current dependences of optical peak powers up to 1 and 37 W for wavelengths of 228 and 256 nm, respectively, making them promising for use as powerfull ultraviolet-C emitters.

💡 Research Summary

The manuscript presents a comprehensive study of sub‑monolayer (fractional‑ML) GaN/AlN quantum wells (QWs) grown by plasma‑activated molecular beam epitaxy (PA‑MBE) and demonstrates their suitability for high‑power ultraviolet‑C (UVC) emitters. The authors systematically vary the nominal GaN thickness (0.75–2 ML) and the Ga‑to‑N flux ratio (ϕGa/ϕN2* = 0.7–2.1) while monitoring growth in real time with reflection high‑energy electron diffraction (RHEED) and ex‑situ with atomic‑force microscopy (AFM). Two distinct 2‑dimensional growth regimes are identified: (i) a 2D nucleation mode under nitrogen‑rich or near‑stoichiometric conditions, characterized by oscillatory RHEED intensity and the formation of isolated 2‑D quantum disks (Q‑disks); and (ii) a step‑flow mode under gallium‑rich conditions, where RHEED intensity decays linearly without oscillations and Ga adatoms migrate to step edges, producing elongated quantum ribbons (Q‑ribbons).

AFM confirms that the nucleation mode yields a smooth terrace‑like surface with occasional nanoclusters only in strongly Ga‑rich growth, whereas the step‑flow mode leads to well‑defined ribbons whose width (ωQR) scales with the Ga/N flux ratio. The authors fabricate 250‑period multiple‑quantum‑well (MQW) stacks of the form {GaN m/AlN 15 ML} with m ranging from 0.75 to 2 ML. High‑resolution X‑ray diffraction, reflectivity, AFM, and scanning transmission electron microscopy verify precise control of layer thickness (±0.25 ML), terrace width (30–40 nm), and surface roughness (0.3–0.4 nm).

First‑principles (ab‑initio) calculations of the electronic structure for various Q‑disk and Q‑ribbon configurations reveal that Q‑disks confine carriers in all three dimensions, leading to a larger quantum‑confinement energy and emission near 228 nm, while Q‑ribbons provide weaker confinement along the ribbon axis, shifting emission to ≈256 nm. Based on these results, a phenomenological model is proposed that maps the growth mode to a specific nanostructure ensemble (disk array vs ribbon array) and predicts the corresponding optical response.

The practical relevance is demonstrated by electron‑beam pumping of the MQW structures (12.5 keV, pulse current up to 2 A). The devices exhibit linear scaling of peak optical power with beam current, delivering up to 1 W at 228 nm (disk‑dominated) and 37 W at 256 nm (ribbon‑dominated). No saturation or thermal roll‑off is observed within the tested current range, indicating efficient carrier recombination and minimal non‑radiative losses.

Overall, the work establishes that fractional‑ML GaN/AlN QWs can be engineered via precise flux control to select either quantum‑disk or quantum‑ribbon morphologies, each offering distinct wavelength and power characteristics. The demonstrated high‑power, narrow‑band UVC emission positions these structures as promising candidates for next‑generation sterilization, water‑purification, and deep‑UV photolithography sources, potentially surpassing conventional AlGaN‑based LEDs in both efficiency and spectral tunability.