ANALYSIS OF COMPOSITION FLUCTUATIONS ON AN ATOMIC-SCALE IN AL0.25GA0.75N BY HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY

Composition fluctuations in the Al0.25Ga0.75N layer of an AlGaN/GaN tr ansistor structure grown by plasma induced molecular beam epitaxy on A l2O3(0001) at a growth temperature of 870 degrees C were studied by di gital analysis of lattice images (DALI) of high-resolution transmissio n electron microscopy (HRTEM) cross-section images. DALI exploits the linear dependence of the lattice parameters on the Al content by apply ing Vegard's law. Detecting the distances between intensity maxima pos itions in the micrograph which can be considered as a fingerprint of t he local lattice parameters quantitatively derives composition profile s on an atomic scale. In the HRTEM cross-section image different areas were observed in the Al0.25Ga0.75N layer with either homogeneous or ' 'striped'' contrast, Tn the striped areas the analyses indicate a stro ng periodic decomposition with a period of 1 nm consisting of 1 ML Al0 .8Ga0.2N and about 3 ML Al0.07Ga(0.93)N. The regions with homogeneous contrast do not exhibit significant composition fluctuations. (C) 1998 American Institute of Physics.