Pore growth direction in anodic oxidation: insights from Ga anodization

Porous anodic oxides have long been studied and used as platforms for nanostructure fabrication. In many representative material systems, particularly porous Al₂O₃ and TiO₂, the growth direction of pores or nanotubes is generally understood to follow the macroscopic surface normal, that is, the direction of the applied electric field. At present, pore growth direction has often been treated as a given geometrical feature rather than as an independent subject of investigation. In this review, the pore growth direction in anodically formed oxides is re-examined, focusing on Ga₂O₃ with Al₂O₃ and TiO₂ used as reference systems in which pores align along the surface normal under typical anodizing conditions. In contrast, recent studies using single-crystal Ga have reported that pore growth can deviate from the surface normal and correlate with the crystallographic orientation of the substrate. In this review, pore growth direction refers to the direction in which pores propagate during anodization and is considered separately from the direction of overall oxide thickening. The discussion is placed within the established framework of field-driven ionic migration, involving oxide formation at the metal/oxide interface and field-assisted dissolution at the oxide/electrolyte interface. By comparing the understanding gained for Al₂O₃ and TiO₂ with experimental findings for Ga₂O₃, the present state of knowledge and the limitations regarding crystallographic orientation dependence are discussed. The applicability of the conventional view that pores grow along the electric field direction is considered, and a material-dependent perspective on pore growth direction is outlined.