Characteristics of dislocations in ZnO layers grown by plasma-assisted molecular beam epitaxy under different Zn/O flux ratios

We have investigated the characteristic of the dislocations in the ZnO layers grown on c sapphire by the plasma-assisted molecular beam epitaxy under the different Zn/O flux ratios. The ZnO layers were characterized by the transmission electron microscopy (TEM) and the high-resolution x-ray diffraction (HRXRD). The TEM and HRXRD experiments revealed that the major threading dislocations (TDs) in the ZnO layers are the edge dislocations running along the c axis with Burgers vector of 1/3<11–20>. The TD densities are determined to be 6.9×10⁹, 2.8×10⁹, and 2.7×10⁹ cm^-2, for O-rich, stoichiometric, and Zn-rich grown ZnO, respectively. Different from the O-rich grown ZnO where the dislocations run along the c-axis, several dislocations in the stoichiometric and the Zn-rich grown ZnO are inclined to 20°∼30° from the c-axis. By considering the slip system in the wurtzite-structure ZnO, the glide planes of the dislocations are close to (10-10) for the O-rich grown ZnO and close to (10-11) for the stoichiometric and Zn-rich grown ZnO. Furthermore, the thickness of the interface dislocations in t- he O-rich grown ZnO is much thinner than in the stoichiometric and Zn-rich grown ZnO. In addition, the most probable origin of the inclined dislocations including c/a ratios, growth rate, and initial growth stage of high temperature (HT)-ZnO layers are also discussed.