Composition modulations in Al_0.4 Ga_0.6 N layers grown by molecular beam epitaxy

Al_0.4 Ga_0.6 N layers grown by molecular beam epitaxy on (0001) AlN/sapphire composite substrates were examined using transmission electron microscopy. The layers show modulated structures consisting of Al-rich and Ga-rich regions. 1:1 atomic ordering on the (0001) planes was not observed. To rationalize the formation of modulations, we invoke the presence of phase separation induced Al-rich and Ga-rich regions at the AlGaN/composite interface, the development of surface undulations due to the presence of two-dimensional stresses between these regions and asymmetry in the wetting characteristics of the Al-rich and Ga-rich regions. Arguments are also developed to explain the absence of 1:1 ordering in these layers.