Nanosecond laser switching of surface wettability and epitaxial integration of c-axis ZnO thin films with Si(111) substrates

We have achieved integration of polar ZnO[0001] epitaxial thin films with Si(111) substrates where cubic yttria-stabilized zirconia (c-YSZ) was used as a template on a Si(111) substrate. Using XRD (θ-2θ and scans) and HRTEM techniques, the epitaxial relationship between the ZnO and the c-YSZ layers was shown to be [0001]ZnO [111]YSZ and , where the [ ] direction lies in the (0001) plane, and the [ ] direction lies in the (111) plane. Similar studies on the c-YSZ/Si interface revealed epitaxy as 〈111〉YSZ 〈111〉Si and in-plane 〈110〉YSZ 〈110〉Si. HRTEM micrographs revealed atomically sharp and crystallographically continuous interfaces. The ZnO epilayers were subsequently laser annealed by a single pulse of a nanosecond excimer KrF laser. It was shown that the hydrophobic behavior of the pristine sample became hydrophilic after laser treatment. XPS was employed to study the effect of laser treatment on surface stoichiometry of the ZnO epilayers. The results revealed the formation of oxygen vacancies, which are envisaged to control the observed hydrophilic behavior. Our AFM studies showed surface smoothing due to the coupling of the high energy laser beam with the surface. The importance of integration of c-axis ZnO with Si(111) substrates is emphasized using the paradigm of domain matching epitaxy on the c-YSZ[111] buffer platform along with their out-of-plane orientation, which leads to improvement of the performance of the solid-state devices. The observed ultrafast response and switching in photochemical characteristics provide new opportunities for application of ZnO in smart catalysts, sensors, membranes, DNA self-assembly and multifunctional devices.