Zn dots coherently grown as the seed and buffer layers on Si(111) for ZnO thin film: Mechanism, in situ analysis, and simulation

In conventional ZnO/Si heterostructures, a buffer layer is usually required to compensate the mismatch between the host substrate and the grown thin film. However, poor quality of buffer layers might lead to severe crystalline misorientation and defects. In this work, we demonstrate that collective oxidized Zn dots act as buffer and seed layers for the growth of high surface quality ZnO thin films on Si(111) by rf-sputtering, and we further in situ analyze the structural evolution by reflective second harmonic generation (RSHG). The collective Zn dots grown on Si(111) were oxidized with exposure to ozone gas under proper Ultraviolet-C (UVC) irradiation, and then these ZnO shells formed seed layers to promote the nucleation process for subsequent ZnO thin film growth. Besides, RSHG was performed in situ to observe the net symmetrical dipole contribution at each fabrication steps and analyzed the surface quality of the ZnO thin film. Consistent with the analyses of synchrotron x-ray diffraction and atomic force microscopy, the RSHG results analyzed with simplified bond-hyperpolarizability model fitting revealed that well oxidized Zn dot-embedded ZnO films grown on Si(111) exhibit a 3m-symmetrical surface structure, and that excessive oxidation time led to ZnO2 formation and higher roughness. Our results demonstrate the efficient approach toward high-surface-quality ZnO thin film by rf-sputtering, verifying that the quality of ZnO shell covering Zn dot grown on Si(111) is the focal factor for the sequent ZnO thin film growth.