In-situ observation of ion-induced nanoscale patterning on a crystalline Ge(001) surface

Ion-induced surface patterning has turned out to be a highly versatile technique for many applications where large areas of nanostructured surfaces or thin films are required. Both fundamental and applied research may benefit from in-situ studies revealing the kinetics of the patterning process, yielding further insight into the dominant mechanisms and thus enabling to gain precise process control. The surface-sensitive X-ray scattering technique of Grazing Incidence Small Angle X-Ray Scattering (GISAXS) is a well-suited method for such in-situ investigations, allowing for contact-less examination under various external conditions.

Here, we present a real-time in-situ GISAXS investigation of reverse epitaxy patterning in crystalline Ge(001). From the X-ray scattering pattern we deduce the significant morphological parameters of the surface, thus tracking the development of the surface morphology with time during ion irradiation.

These findings are compared with results from simulations based on a continuum equation of the local surface height. Good agreement of the simulation with both experiment and theory was only achieved when including in the continuum equation an additional term for regulating the pattern anisotropy. We then find that a continuum equation considering only diffusive effects reproduces the experimentally observed surface patterning kinetics well.

Observing the kinetics of pattern formation in the non-linear regime, we find that the temporal evolutions of characteristic length and roughness conform to power laws, their exponents agreeing with scaling laws for conserved continuum equations with four-fold symmetry. Moreover, we find that the facet angle kinetics can be described by the Austin-Rickett equation for diffusion-controlled transformation processes, corroborating our assumption of a predominantly diffusive mechanism of pattern formation.