Strain, morphological, and growth-mode changes in AlGaN single layers at high AlN mole fraction

Abstract We report on morphological and residual-strain characteristics of high-AlN-mole-fraction N-polar Al x Ga 1− x N epilayers on sapphire. Nominally relaxed, thick single-alloy layers in the compositional range 0.4 x x ≅0.65), a maximum in growth rate and minimum in surface roughness are found whereas at a somewhat greater flux value (corresponding to x =0.86) a minimum in stress (lateral and vertical) is obtained. The observed growth-mode phenomenology suggests an approach for improving PA-MBE growth of high-AlN-mole-fraction layers of certain AlGaN/GaN structures such as the distributed Bragg reflector. Finally, optical transmission experiments lead to a bandgap bowing parameter in the large- x region of Al x Ga 1− x N of b ≅0.75.