We report the results of experimental and theoretical studies of phonon modes in GaN/AlN superlattices (SLs) with a period of several atomic layers, grown by submonolayer digital plasma-assisted molecular-beam epitaxy, which have a great potential for use in quantum and stress engineering. Using detailed group-theoretical analysis, the genesis of the SL vibrational modes from the modes of bulk AlN and GaN crystals is established. Ab initio calculations in the framework of the density functional theory, aimed at studying the phonon states, are performed for SLs with both equal and unequal layer thicknesses. The frequencies of the vibrational modes are calculated, and atomic displacement patterns are obtained. Raman spectra are calculated and compared with the experimental ones. The results of the ab initio calculations are in good agreement with the experimental Raman spectra and the results of the group-theoretical analysis. As a result of comprehensive studies, the correlations between the parameters of acoustic and optical phonons and the structure of SLs are obtained. This opens up new possibilities for the analysis of the structural characteristics of short-period GaN/AlN SLs using Raman spectroscopy. The results obtained can be used to optimize the growth technologies aimed to form structurally perfect short-period GaN/AlN SLs.

Monolayer (ML)-scale GaN/AlN multiple quantum well (MQW) structures for electron-beam-pumped ultraviolet (UV) emitters are grown on c-sapphire substrates by using plasma-assisted molecular beam epitaxy under controllable metal-rich conditions, which provides the spiral growth of densely packed atomically smooth hillocks without metal droplets. These structures have ML-stepped terrace-like surface topology in the entire QW thickness range from 0.75–7 ML and absence of stress at the well thickness below 2 ML. Satisfactory quantum confinement and mitigating the quantum-confined Stark effect in the stress-free MQW structures enable one to achieve the relatively bright UV cathodoluminescence with a narrow-line (~15 nm) in the sub-250-nm spectral range. The structures with many QWs (up to 400) exhibit the output optical power of ~1 W at 240 nm, when pumped by a standard thermionic-cathode (LaB₆) electron gun at an electron energy of 20 keV and a current of 65 mA. This power is increased up to 11.8 W at an average excitation energy of 5 µJ per pulse, generated by the electron gun with a ferroelectric plasma cathode at an electron-beam energy of 12.5 keV and a current of 450 mA.

Stress evolution was studied in up to 3-µm-thick AlN templates, comprising ~65-nm-thick AlN nucleation (NLs) and thick buffer (BL) layers, grown using different growth modes and conditions by plasma-assisted molecular beam epitaxy (PA MBE) on c-Al₂O₃. Growth of the both NL and BL in a standard PA MBE mode at N-rich conditions (at flux ratio Al/N₂*~0.5) led to instant generation of a relatively high tensile stress (~1.5GPa) which is maintained throughout the entire growth. On the contrary, NLs, grown using a migration-enhanced epitaxy (MEE), demonstrated a transition from the initial compressive stress to the stress-free growth, which is usually observed in the Volmer-Weber films. Further growth of thick AlN BLs on the MEE-NLs at various Al/N₂* ratios revealed a wide variety of stress evolution processes. The BL growth by using metal-modulated epitaxy (MME) at Me-rich conditions with Al/N₂*~1.33 led to a gradual decrease in the initial compressive stress in the 2D AlN layers, whereas standard PA MBE growth of 3D BL at N-rich conditions (Al/N₂*~0.92) exhibited the fast transition from the initial compressive to tensile stress. Moreover, we succeeded in achieving the quasi-stress-free growth of a 3.1-µm-thick AlN BL using the MME growth mode at the optimum flux ratio Al/N₂*=1.05. These results were compared with the results of other authors and explained using a kinetic approach to description of stress evolution during PA MBE of AlN/c-Al₂O₃ templates, taking into account several simultaneously acting competitive mechanisms of continuous generation of tensile and compressive stresses.

We present an extensive theoretical and experimental study to identify the effect on the Raman spectrum due to interface interdiffusion between GaN and AlN layers in short-period GaN/AlN superlattices (SLs). The Raman spectra for SLs with sharp interfaces and with different degree of interface diffusion are simulated by ab initio calculations and within the framework of the random-element isodisplacement model. The comparison of the results of theoretical calculations and experimental data obtained on PA MBE and MOVPE grown SLs, showed that the bands related to A₁(LO) confined phonons are very sensitive to the degree of interface diffusion. As a result, a correlation between the Raman spectra in the range of A₁(LO) confined phonons and the interface quality in SLs is obtained. This opens up new possibilities for the analysis of the structural characteristics of short-period GaN/AlN SLs using Raman spectroscopy.

This article describes GaN/AlN heterostructures for ultraviolet-C (UVC) emitters with multiple (up to 400 periods) two-dimensional (2D)-quantum disk/quantum well structures with the same GaN nominal thicknesses of 1.5 and 16 ML-thick AlN barrier layers, which were grown by plasma-assisted molecular-beam epitaxy in a wide range of gallium and activated nitrogen flux ratios (Ga/N₂*) on c-sapphire substrates. An increase in the Ga/N₂* ratio from 1.1 to 2.2 made it possible to change the 2D-topography of the structures due to a transition from the mixed spiral and 2D-nucleation growth to a purely spiral growth. As a result, the emission energy (wavelength) could be varied from 5.21 eV (238 nm) to 4.68 eV (265 nm) owing to the correspondingly increased carrier localization energy. Using electron-beam pumping with a maximum pulse current of 2 A at an electron energy of 12.5 keV, a maximum output optical power of 50 W was achieved for the 265 nm structure, while the structure emitting at 238 nm demonstrated a power of 10 W.

Results of statistical analysis of the patterns of reflection high-energy electron diffraction (RHEED) measured during molecular beam epitaxy (MBE) of heterostructures of wide-bandgap (Al, Ga, In)N compounds are presented, which can be used for determining the lateral lattice constant (a) of the epitaxial layers. It is established that the error of determination can vary from a minimum of 0.17% up to several percent, depending on the contrast of RHEED patterns, which is determined by the stoichiometry of MBE growth. It is demonstrated that the RHEED data can reveal relaxation of elastic stresses during the growth of short-period {GaN(4 nm)/AlN(6nm)}₃₀/AlN superlattices.