Polarity Control in Group-III Nitrides beyond Pragmatism

$P\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}y$ $e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g$ for applications in electronics and nonlinear optics requires the reliable, controllable growth of uniformly polar layers of a compound semiconductor on a nonpolar substrate. So far, this has been developed on a purely empirical basis. The authors use transmission electron microscopy and density-functional theory to see how exchange of aluminum, oxygen, and nitrogen within a sapphire substrate mediates the polarity of an epitaxial film of group-III nitride. Their results shed new light on familiar concepts like substrate nitridation and low-temperature buffers, and may further help to understand polarity control in oxides and other materials.