Optical signatures of single ion tracks in ZnO

The optical properties of single ion tracks have been studied in ZnO implanted with Ge by combining depth-resolved hyperspectral cathodoluminescence (CL) and photoluminescence (PL) spectroscopy techniques. The results indicate that ZnO is susceptible to implantation doses as low as 10⁸ to 10⁹ cm^-2. We demonstrate that the intensity ratio of ionized and neutral donor bound exciton emissions [D⁺X/D⁰X] can be used as a tracer for a local band bending both at the surface as well as in the crystal bulk along the ion tracks. The hyperspectral CL imaging performed at 80 K with 50 nm resolution over the regions with single ion tracks permitted direct assessment of the minority carrier diffusion length. The radii of distortion and space charge surrounding single ion tracks were estimated from the 2D distributions of defect-related green emission (GE) and excitonic D⁺X emission, both normalized with regard to neutral D⁰X emission, i.e., from the [GE/D⁰X] and [D⁺X/D⁰X] ratio maps. Our results indicate that single ion tracks in ZnO can be resolved up to ion doses of the order of 5 × 10⁹ cm^-2, in which defect aggregation along the extended defects obstructs signatures of individual tracks.