Among various semiconducting metal oxide materials, ZnO has re-ceived great attention due to its wide band gap (3.37 eV), large exciton bind energy (60 meV), high transparency in the visible wavelength range, excellent luminescent properties, high chemical stability and relatively low price. ZnO has been used in many applications, such as gas sensors [1], photocatalyst [2], solar cell [3], piezoelectric devices [4], photodetector [5–8], and LED [9]. ZnO has a hexagonal wurtzite structure with the main growth direction toward c-axis.