Laser engineering of ITO/ZnO/ITO structures for photodetector applications

ZnO nanomaterials have received much attention due to their suitability for applications such as gas sensors, UV detectors, and solar collectors. However, the functionality of ZnO in optical applications is often limited by its wide bandgap (3.15 eV) which restricts the response to shorter wavelengths. In view of this limitation, there has been much interest in tuning the optical properties of ZnO through defect engineering. In this work, we show that processing ZnO thin films with nanosecond (ns) laser irradiation is a simple and effective way to introduce interband defects lowering the bandgap and increasing the sensitivity of ITO/ZnO/ITO photodetector structures at longer wavelengths. In particular, we show that the concentration of oxygen vacancies in ZnO is proportional to laser fluence below 700 mJ/cm2, but that an increase in laser fluence above this value results in thermal heating that anneals the film and lowers the relative abundance of oxygen vacancies. On the other hand, the surface morphology of ZnO does not change significantly even though the fluence reaches 700 mJ/cm2. To illustrate the way in which laser processing can be utilized to improve the optical properties of ZnO films in photodetector applications, we have fabricated transparent ITO/ZnO/ITO stacked structures and measured their response at various optical wavelengths. We find that processing with ns laser radiation is effective in enhancing the responsivity and detectivity of these devices at blue (460 nm) and UV (390 nm) wavelengths. The response of the photodetector is also increased at green wavelengths (570 nm) and red wavelengths (620 nm) when processed with laser fluences in the 480–600 mJ/cm2 range.