GaN/AlN superlattices consisting of few‐monolayer GaN wells have attracted considerable attention for use in deep‐ultraviolet (DUV) light‐emitting devices. To avoid the formation of droplets and AlGaN interface layers, precise growth control is essential for fabricating superlattices with flat and abrupt interfaces. Herein, GaN/AlN superlattice structures are grown on face‐to‐face‐annealed sputter‐deposited AlN (FFA Sp‐AlN) template substrates using radio‐frequency plasma‐excited molecular beam epitaxy (RF‐MBE) utilizing in situ reflection high‐energy electron diffraction (RHEED) monitoring. Both AlN and GaN are grown under metal‐rich conditions, and subsequently, the droplets are eliminated by droplet elimination by radical beam irradiation (DERI) method for AlN and by growth interruption for GaN. Furthermore, the dependence of AlN thickness on the properties of superlattices is investigated. The AlN thickness changes linearly with the supply time of the Al metal; thus, the AlN thickness is easily controllable. A total of 20‐period GaN/AlN superlattices with flat and abrupt interfaces is fabricated, as confirmed using atomic force microscopy and X‐ray diffraction. Cathodoluminescence with a peak wavelength of 230–260 nm at room temperature is obtained from the fabricated superlattices. Moreover, the emission wavelength shifts with an increase in AlN thickness.

ScAlMgO4 (SAM) has attracted attention as a substrate for nitride semiconductor crystal growth owing to its small a-axis lattice mismatch with GaN and InGaN. In this study, we investigated GaN growth on an SAM substrate via radio-frequency plasma-excited molecular beam epitaxy. By optimizing the growth conditions, GaN with the following epitaxial orientation relations (0001)GaN//(0001)SAM and [11-20]GaN//[11-20]SAM was successfully grown directly on the SAM substrate. The atomically flat and abrupt interface of GaN directly grown on the SAM substrate was observed via high resolution transmission electron microscopy, and uniform GaN growth on a two-inch SAM substrate was also demonstrated.

ScAlMgO4 (SAM) substrates have attracted considerable attention as platforms for GaN growth in recent years because GaN can be grown directly on SAM without any buffer layer. Herein, the effect of the terrace width of SAM substrates on direct GaN growth is investigated using radio‐frequency molecular beam epitaxy (RF‐MBE). A flat and single wurtzite (WZ)‐phase GaN film is grown on a SAM substrate with a large terrace width. In contrast, a rough GaN film with a mixture of WZ and zinc‐blende (ZB) phases is obtained on SAM with a small terrace width. The SAM step height of 0.8 nm corresponds to three GaN molecular layers. Therefore, ZB stacking faults may be generated during the coalescence of islands located on adjacent terraces, although only the WZ‐GaN islands grow in the initial growth stage. This indicates that SAM substrates with a larger terrace width, that is, a smaller off‐cut angle, are preferable for obtaining a flat WZ‐GaN film.