Why Do Thick MOCVD-Grown beta-Ga2O3 Epilayers on (001) Substrates Crack: Crystallographic Origin
Thick, defect free epitaxial layers grown using industry standard techniques are a fundamental requirement for the widespread adoption of fully vertical power devices based on ultra wide bandgap gallium oxide (Ga2O3). However, metal-organic chemical vapour deposition (MOCVD) of such layers on native beta-Ga2O3 substrates with the largest diameter (001) orientation remains relatively unexplored, and the origins of the reported surface roughening and cracking with increasing thickness are not yet fully understood. To address this, we report a systematic study of MOCVD grown beta-Ga2O3 epilayers deposited at growth rates of ~3.5 um/h, with thicknesses from 0.3 to 3.5 um. The epilayers exhibit a relatively smooth but striated surface morphology, with progressively increasing nanometre-scale roughness beyond coalescence and crack formation observed from ~1.8 um thickness. High resolution X ray diffraction reveals that, despite growth on (001) substrates, the epilayers adopt a predominantly (-401)-oriented structure from the earliest stages of growth. Rocking curve analysis further indicates a higher degree of in-plane twist than tilt, both decreasing with increasing epilayer thickness. While the epilayer and substrate are lattice-matched along the [010] in plane direction, the epitaxial alignment in the orthogonal epilayer [104] in plane direction imposes, in theory, a maximum tensile in-plane strain of approximately +4.1% arising from the underlying lattice mismatch, thereby driving crack formation perpendicular to this direction. Our results suggest that this epitaxial relationship is likely associated with faceted reconstruction of the (001) substrate surface during annealing, driven by the minimisation of surface energy under oxygen-rich MOCVD growth conditions.