Crack free ZnO thick film on sapphire substrate without GaN template grown by MVPE

A crack-free ZnO film with thickness of ∼65 μm is obtained on sapphire substrate without GaN template by metal vapor phase epitaxy. The residual stress is investigated in terms of thermal and non-thermal stress. Comparing the calculated values of the thermal stress with the experimental data of the non-thermal stress, it is shown that the non-thermal stress is much larger than the thermal stress. Annealing was used to reduce the non-thermal stress. A re-growth process was employed, using annealed ZnO sample as pseudo-substrate. The result indicates that the residual stress in annealed ZnO sample was reduced to half, and the critical thickness is improved to ∼70 μm in the ZnO film grown on annealed ZnO pseudo-substrate, compared with ∼15 μm for ZnO grown directly on sapphire substrate. In addition, crystal quality is greatly improved, determined by double-crystal X-ray diffraction.