We introduce studies on the origin of different formation of hexagonal boron nitride (hBN)-graphene heterostructures by Chemical Vapor Deposition (CVD) growth, focusing on our theoretical investigation based on the first-principles calculations. We found that the presence or absence of H termination changed the shape of the edges of hBN islands and graphene islands. In addition, we have found the possibility of in-plane growth and layered growth of the heterostructure.

  Oxide vapor phase epitaxy (OVPE) method is an attractive bulk GaN growth method because solid by-products that disturb continuous crystal growth do not form. However, because Ga₂O is used as Ga source in the OVPE method, a high O impurity concentration is a major problem. To understand how to incorporate O impurities and its influence on the optical properties of GaN, we examined surface structures of GaN under OVPE growth conditions and electronic structures of GaN including O-related point and complex defects using first-principles calculations. Difference in O adsorption depending on surface orientation and an influence of the O-related defects on the electronic structure of GaN were discussed.

  We review our first-principles calculations based on the density-functional theory (DFT) that clarify microscopic mechanisms of GaN MOVPE (MetalOrganic Vapor-Phase Epitaxy) growth. First, reactions of source gases and carrier gases in vapor phases are clarified using the transition-state theory, and the structures of surface terraces and steps are identified by the large-scale static and dynamical DFT calculations. We then unveil reaction pathways and corresponding energy barriers in the elementary processes in the growth phenomena, i.e., the decomposition of NH₃ on the terrace, the diffusion of the resulting NH unit, and the incorporation of Ga and N at the step edges, completing the step-flow growth. The presence of relatively weak Ga-Ga bonds on the growing surface is argued to be essential in the GaN epitaxial growth.

  The growth mode of group-III nitride thin films during epitaxial growth are systematically investigated using the macroscopic theory with the aid of empirical interatomic potential calculations and ab initio calculations. The analysis of free energies demonstrates that two-dimensional growth with misfit dislocations (2D-MD) is favorable for GaN (InN) thin films on c-plane AlN (GaN) surface under Ga-rich (In-rich) condition while three-dimensional growth with truncated hexagonal pyramid islands is favorable under N-rich condition. Furthermore, we reveal that the area of 2D-MD in the growth mode phase diagram as functions of surface energy anisotropy and surface energy on (0001) plane expands with decreasing Al (In) composition for AlGaN (InGaN) thin films. The difference in growth mode is caused by the surface energy anisotropy, which strongly depends on the growth condition. The results suggest the surface energy anisotropy forming is a crucial factor for determining the growth modes of group-III nitrides. Several issues to construct “digital twin” for epitaxial growth on the basis of conventional calculation schemes are also discussed.

  Efficient optimization of epitaxial growth can be achieved by utilizing a digital twin that is an alternative to laboratory experiments. In this review, two approaches are presented to make the elemental models of a digital twin based on quantum theory calculations more realistic using machine learning. One is to improve the surface structure model through Bayesian optimization. The other is to improve the gas-phase reaction model through data assimilation. These improved elemental models are expected to enable a more quantitative analysis of growth driving force and impurity contamination.

  Recently, a novel quaternary alloy, (ZnO)_1-x(InN)_x, (ZION), has been attracting attention for optical device applications. But its electronic properties have not cleared yet. In this study, we theoretically investigate the initial growth mechanism and electronic structures of ZION. We use density functional theory to investigate microscopic chemical properties and interacting quasi-band theory to calculate the electronic structures of the alloy system. It was found that the most favorable first step in the growth is that an In atom is adsorbed on the H3 site of an O-polar ZnO substrate, although the H3 site is not a proper site for epitaxial growth. We also calculated self-surfactant energies that cause the adsorbed In atom to migrate to a top site. It is revealed that the migration will be caused by the concurrent adsorption of O and N atoms on the nearest neighbor sites of the H3 site. This mechanism may keep the epitaxial growth. The electronic structures of bulk-ZION are also revealed. Comparing calculated band structures on several composition ratios, it is revealed that ZION exhibits a direct-band gap across the range of compositions and shows a large band gap-bowing. These results are well agreed with experimental results.