Epitaxial growth and self-organized superlattice structures in AlGaN films grown by plasma assisted molecular beam epitaxy

Aluminum gallium nitride alloys were grown by molecular beam epitaxy and their film composition, structure and microstructure were investigated by Rutherford backscattering spectroscopy, Atomic Force Microscopy, X-ray diffraction, Transmission Electron Microscopy and High Resolution Electron Microscopy. The kinetics of growth was investigated and the result show that at the growth temperature of 750 °C the sticking coefficient of Ga varies monotonically from practically 0 to 1 as the growth varies from group-III to -V rich regime. Correspondingly, the surface morphology changes from atomically smooth in the group-III regime to relatively rough in the nitrogen-rich regime. The X-ray diffraction and TEM studies revealed the existence of three types of spontaneously formed superlattice structures, along the [0001] direction, with periodicities of 2-, 7- and 12-monolayers. While the two-monolayer ordering is preferred under group-V rich conditions of growth, the 7- and 12-monolayer ordering were observed under group-III rich conditions of growth. Off-axis X-ray diffraction shows the absence of in-plane ordering and that periodic stacking faults are not present in the superlattice structures.