Extreme low-temperature molecular beam epitaxy of ZnO-based quantum structures Appl. Phys. Lett. 98, 031907 (2011); 10.1063/1.3544575Investigation of the crystallinity of N and Te codoped Zn-polar ZnO films grown by plasma-assisted molecularbeam epitaxy J. Appl. Phys. 108, 093518 (2010); 10.1063/1.3498800High mobility in ZnO thin films deposited on perovskite substrates with a low temperature nucleation layerThe authors report on the growth mechanism of ZnO homoepitaxy at the low-temperature range of 500 C, which is unavailable to obtain high-quality ZnO films in heteroepitaxy. One typical set of ZnO films were grown on (0001) ZnO substrates by molecular-beam epitaxy: a standard structure without buffer and two buffered structures with high-temperature (HT) homobuffer and low-temperature (LT) homobuffer. As a result, the LT homobuffered structure had the outstanding material properties: the surface roughness is 0.9 nm, the full width at half maximum of x-ray rocking curve is 13 arcsec, and the emission linewidth of donor-bound excitons is 2.4 meV. In terms of the theoretical interpretation of the experimentally obtained electron mobilities, it was found that the LT homobuffered structure suffers less from the dislocation scattering and the ionized-impurity scattering compared to the HT homobuffered structure. It is proposed that, in the ZnO low-temperature homoepitaxy, the LT homobuffer plays a key role in inducing the complete termination of dislocations in the homointerface and suppressing the outdiffusion of contaminants and point defects on the ZnO surface, which results in the formation of smooth wetting layer on the homointerface.

Mg-doped p-type GaN epilayers grown by metalorganic chemical vapor deposition exhibit two different types of photoluminescence (PL) characteristics depending on Mg doping rate; the existence of critical Mg concentration is found where both electrical and optical characteristics show an abrupt change in their behavior. In samples with relatively low Mg concentration, a band edge emission, a peak associated with shallow donor-acceptor pair recombination, and a 2.8 eV blue band emission appear in the PL spectrum. Intensities of all three PL peaks decrease with increasing annealing temperature; the blue band quenches most rapidly. In contrast, in samples grown with high ratio of Mg/Ga flow rate, only a strong blue emission band is observed and the intensity of the blue band increases with the annealing temperature suggesting a different origin from the blue band of lightly doped samples. From the annealing temperature dependence of the blue band intensity, we speculate that the emission mechanism of the blue band in relatively lightly Mg-doped GaN epilayers involves Mg-related deep level complexes such as substitutional/interstitial Mg-related complexes. On the other hand, a model involving a formation of pyramidal defects and the presence of Mg-rich inclusions which modifies the dominant optical transition can be a possible explanation for the origin of the blue band in heavily doped ones.