Nanoporous Ge surface decomposition under ion bombardment: towards settling the dispute about driving forces

First detailed studies of the nanoporous decomposition of Ge under ion irradiation date back more than 30 years. Irradiated Ge alters its (near-)surface morphology into a nanostructure, which remains stable after irradiation even under thermal treatment up to several hundred degrees Celsius. In recent years, this peculiar transformation of Ge has been studied extensively. However, a conclusive assessment of the driving force for the nanoporous Ge decomposition has not been reached yet.

We show that hole patterns and sponge-like layers of irradiated Ge surfaces originate from the same driving force, namely the kinetics of irradiation-induced defects in amorphous Ge layers. Ge hole patterns reported earlier for irradiation with low ion energies around 5 keV were reproduced for low energy Bi+ but also for Ge+ self-irradiation, which proves that the dominating driving force for morphology evolution cannot originate from the implanted impurities. At higher ion energies up to 100 keV the well-known formation of sponge-like Ge surface layers after heavy ion irradiation was found for Bi+ irradiation and Ge+ self-irradiation, too. The transition from smooth surfaces via hole patterns to sponge-like morphologies with increasing ion energy has been studied in detail. A model based on the kinetics of ion beam-induced defects was developed and implemented in 3D kinetic Monte Carlo simulations, which reproduce the transition from hole patterns to sponge-like layers with increasing ion energy. Finally, the proposed defect kinetics driven mechanism is undergird by a systematic positron annihilation spectroscopy investigation.
The authors acknowledge financial funding by the German Research Foundation via the Research Unit 845 “Selforganized nanostructures induced by low-energy ion beam erosion.”