On an SiO2-patterned c-plane sapphire substrate, GaN domains were grown with their polarity controlled in accordance with the pattern. While N-polar GaN was grown on hexagonally arranged circular openings, Ga-polar GaN was laterally overgrown on mask regions due to polarity inversion occurring at the boundary of the circular openings. After etching of N-polar GaN on the circular openings by H3PO4, this template was coated with 40-nm Si by sputtering and was slightly etched by KOH. After slight etching, a thin layer of Si left on the circular openings of sapphire,but not on GaN, was oxidized during thermal annealing and served as a dielectric mask during subsequent regrowth. Thus, the subsequent growth of GaN was made only on the existing Ga-polar GaN domains, not on the circular openings of the sapphire substrate. Transmission electron microscopy analysis revealed no sign of threading dislocations in this film. This approach may help fabricating an unholed and merged GaN film physically attached to but epitaxially separated from the SiO2-patterned sapphire.
Controllable growth and facile transferability of a crystalline film with desired characteristics, acquired by tuning composition and crystallographic orientation, become highly demanded for advanced flexible devices. Here the desired crystallographic orientations and facile transferability of a crystalline film can be achieved by “thru‐hole epitaxy” in a straightforward and undemanding manner with no limitation on the layer number and polarity of a 2D space layer and the surface characteristics. The crystallographic alignment can be established by the connectedness of the grown material to the substrate through a small net cross‐sectional area of thru‐holes, which also allows the straightforward detachment of the grown material. Thru‐hole epitaxy can be adopted for the realization of advanced flexible devices on large scale with desired crystallographic orientation and facile transferability.
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