Characterization of the ScAlMgO₄ cleaving layer by X-ray crystal truncation rod scattering

ScAlMgO₄ - easily cleaved in c-plane - forms a natural superlattice structure of a ScO₂ layer and two Al_0.5Mg_0.5O layers stacking along c-axis. ScAlMgO₄ is one of the RAMO₄-type layered multicomponent oxides and a promising lattice-matching substrate material for InGaN and ZnO. Identification of the topmost layer and the surface atomic structure of the cleaved ScAlMgO₄ (0001) are investigated by the X-ray crystal truncation rod scattering method. It is confirmed that ScAlMgO₄ is cleaved between the two Al_0.5Mg_0.5O layers. The two parts separated at this interlayer are inversion symmetric to each other and without surface charge. This prevents parallel-plate-capacitor-like electrostatic force during the cleavage. Two different mechanisms are proposed for the two types of cleavage caused by the impact of a wedge and by the in-plane stress due to an overgrown thick GaN film. It is also revealed that about 10%-20% of the topmost O atoms are desorbed during a surface cleaning at 600 °C in ultra-high vacuum. Surface observations using reflection high-energy electron diffraction are possible only after the high-temperature cleaning because the electrical conduction caused by the oxygen deficiency prevents the charge-up of the insulating sample.