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.