Substrate Terrace Width Dependence of Direct Growth of GaN on ScAlMgO4 by Radio-Frequency Molecular Beam Epitaxy

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.