Strain-driven light-polarization switching in deep ultraviolet nitride emitters

Residual strain plays a critical role in determining the crystalline quality of nitride epitaxial layers and in modifying their band structure; this often leads to several interesting physical phenomena. It is found, for example, that compressive strain in Al_xGa_1-xN layers grown on Al_yGa_1-yN (x<y) templates results in an anticrossing of the valence bands at considerably much higher Al composition than expected. This happens even in the presence of large and negative crystal field splitting energy in the Al_xGa_1-xN layers. A judicious magnitude of the compressive strain can support vertical light emission (out of the c-plane) from Al_xGa_1-xN quantum wells up to x 0.80, which is desirable for the development of deep ultraviolet light-emitting diodes designed to operate at or below 250nm with transverse electric polarization characteristics.