Growth modes and microstructures of ZnO layers deposited by plasma-assisted molecular-beam epitaxy on (0001) sapphire

Transmission electron microscopy and high resolution x-ray diffraction are used to characterize defects in ZnO layers grown by plasma-assisted molecular-beam epitaxy on (0001) sapphire. Two- and three-dimensional types of growth modes are described and the observed mosaic structure is analyzed in each case. It is found that two-dimensional layers exhibit a roughness as low as 6 nm. Their subdomains have small lateral coherence lengths and a mean in-plane misorientation of ±0.4°, leading to an important dislocation density of $1–4×1010 cm−2.$ On the contrary, it is demonstrated that, through numerous interactions between dislocations, the three-dimensional growth mode leads to a better structural quality with a larger lateral coherence length and a smaller in-plane mosaic spread of ±0.07°. The total dislocation density is consequently reduced by 1 order of magnitude down to $3–5×109 cm−2$ and the radical modification of the structure results in a change of the dislocation distribution. Our results thus demonstrate that two-dimensional growth mode and low full width at half maximum for symmetric x-ray diffraction are a not reliable indicator of a good structural quality.