Self-formed compositional superlattices triggered by cation orderings in m-plane Al_1−xIn _xN on GaN

Immiscible semiconductors are of premier importance since the source of lighting has been replaced by white light-emitting-diodes (LEDs) composed of thermodynamically immiscible In_xGa_1−xN blue LEDs and yellow phosphors. For realizing versatile deep-ultraviolet to near-infrared light-emitters, Al_1−xIn_xN alloys are one of the desirable candidates. Here we exemplify the appearance and self-formation sequence of compositional superlattices in compressively strained m-plane Al_1−xIn_xN films. On each terrace of atomically-flat m-plane GaN, In- and Al-species diffuse toward a monolayer (ML) step edge, and the first and second uppermost < 1 - 1 - 20 > cation-rows are preferentially occupied by Al and In atoms, respectively, because the configuration of one In-N and two Al-N bonds is more stable than that of one Al-N and two In-N bonds. Subsequent coverage by next < 1 - 1 - 20 > Al-row buries the < 1 - 1 - 20 > In-row, producing nearly Al_0.5In_0.5N cation-stripe ordering along [0001]-axis on GaN. At the second Al_0.72In_0.28N layer, this ordinality suddenly lessens but In-rich and In-poor < 1 - 1 - 20 >-rows are alternately formed, which grow into respective {0001}-planes. Simultaneously, approximately 5-nm-period Al_0.70In_0.30N/Al_0.74In_0.26N ordering is formed to mitigate the lattice mismatch along [0001], which grow into approximately 5-nm-period Al_0.70In_0.30N/Al_0.74In_0.26N {10 1 - 2 } superlattices as step-flow growth progresses. Spatially resolved cathodoluminescence spectra identify the emissions from particular structures.