Anomalous elongation of c-axis of GaN on Al2O3 grown by MBE using NH3-cluster ions

GaN thin films were grown on Al2O3 (0001) by MBE using NH3-clusters either ionized with the energy of 4-7 eV/molecule (ionized Cluster Beam, i-CB) or un-ionized with the energy of about 0.1 eV/molecule (neutral Cluster Beam, n-CB) at growth temperatures ranging from 390 to 960 °C. The c-axis is extremely elongated but the a-axis is shrunken at the initial growth stage (up to the film thickness of about 10 nm) in GaN grown by the mixture of n- and i-CB under N-rich condition. The films thicker than 30 nm have the relaxed a- and c-axis lengths close to the unstrained values and obey the Poisson relation. GaN grown by i-CB under Ga-rich condition have the relaxed lattice constants obeying the Poisson relation for the film as thin as 6 nm. In GaN grown by the cluster beam (CB) which is not ionized intentionally, both a- and c-axis lengths are almost independent of the film thickness, having nearly the same values as those of the unstrained samples. These characteristics can be ascribed to the nature of interface between the nitrided Al2O3 substrate and epilayer. It is concluded that the films grown by i-CB bond firmly to underlay AlN than the films by n-CB and CB.