Customized Si nanostructures by metal assisted chemical etching

Sandu, Georgiana [UCL] Melinte, Sorin [UCL] Vlad, Alexandru [UCL]

Nanostructuration unfolds exciting vistas for the well-established Si industry. The exceptional properties of Si nanostructures can harness high-performance devices in the fields of electronics, photonics, energy conversion and storage as well as biotechnology. However, the synergies between the nanoscale designer properties and the target applications require precise tuning of the nanostructures such as morphology, size or crystallographic orientation. Metal assisted chemical etching is preferred for the large-scale, dimension-controlled fabrication of vertically aligned Si nanowires (SiNWs) with the crystallographic orientation inherited from the initial Si wafer. Here, we detail on the versatility of metal assisted chemical etching to fabricate morphology and orientation-customized SiNWs. The synthesis method uses p-type (100) Si with Au catalyst layers patterned by colloidal lithography. The etching proceeds in a mixture of HF and H2O2. By altering the etchant component concentrations, we address three different etching regimes of Si: porous <100>, solid <100> and solid <110>. Based on these regimes, several advanced etching schemes are experimented. For instance, alternating between porous and solid regimes offers porosity-modulated SiNWs. In addition, interrupting the etching in the solid <110> regime results in the formation of kinks along the major axis of the nanowires. The location of the kinks can be precisely controlled, while the kink’s angle is found to be dependent of the orientation of the previously etched segment. We further investigate the effect of the geometrical parameters of the patterned holey Au catalyst mask by evaluating three different Au thickness (10, 15 and 30 nm) and three different hole diameters (100, 120 and 150 nm). The observed Si nanostructures are discussed based on a simple mathematical model. This study emphasizes the importance of the metal assisted chemical etching for the synthesis of advanced Si nanostructures.