Defect-induced nonpolar-to-polar transition at the surface of chalcopyrite semiconductors

In zinc-blende semiconductors, the nonpolar (110) surface is more stable than all polar surfaces because the formation of the latter requires the creation of charge-neutralizing but energetically costly surface reconstruction. Our first-principles calculations on CuInSe₂ reveal this in the double-zinc-blende (chalcopyrite) structure, the defect-induced reconstructions make the (112)-cation plus (1¯1¯2¯)-anion polar facets lower in energy than the nonpolar (110) plane, despite the resulting increased surface area. We show that this spontaneous facetting results from the remarkable stability of surface defects (Cu vacancy, Cu-on-In antisite) in chalcopyrites, and explains the hitherto puzzling formation of polar microfacets when one attempts to grow epitaxialloy a nonpolar chalcopyrite surface.