Microstructural characterization of nonpolar ZnO films grown on sapphire substrates by plasma-assisted molecular beam epitaxy

Abstract

Since ZnO has a direct wide band gap energy of 3.37 eV at room temperature and a high exciton binding energy of 60 meV, it has received considerable attentions for the possible applications to light emitting diodes (LEDs) and laser diodes (LDs) with wavelengths ranging from blue to ultraviolet. For the realization of this possibility, most of ZnO films have been grown on (0001) c-plane sapphire substrates, where the grown ZnO films have a growth direction of <0001>. However, in these <0001> grown ZnO films, a strong built-in electrostatic field appears as a result of spontaneous and piezoelectric polarizations caused by the noncentrosymmetric nature of wurtzite crystal structure along this direction. This polarizations induced electric field results in the quantum confined Stark effects (QCSEs), which have detrimental effects on the performance of the optoelectronic devices. One of the direct ways to eliminate the effects of polarization fields on devices is growing the films with nonpolar directions, e.g. [$10\bar{1}0$] or [$11\bar{2}0$] directions. Although pure ($11\bar{2}0$ a-plane ZnO films have been easily grown on (1\bar{1}02$) r-plane sapphire substrates, pure ($10\bar{1}0$) m-plane ZnO films are difficult to be obtained and other domains such as ($10\bar{1}3$), (0002), and ($11\bar{2}2$) are frequently observed in the m-plane ZnO films grown on ($10\bar{1}0$) m-plane sapphire substrates. In addition to the difficulty in the growth of nonpolar ZnO films, heteroepitaxially grown nonpolar ZnO films usually have high density of structural defects such as threading dislocations, stacking faults, and misfit dislocations. However, comprehensive and systematic microstructural investigations on the nonpolar ZnO films have not been performed sufficiently. Therefore, microstructural characterization of nonpolar ZnO films grown by plasma-assisted molecular beam epitaxy (PAMBE) was performed using transmission electron microscopy (TEM) in this study. Microstr...