Novel Catalytic Etching Process for Ge Nanowires Fabrication

Ge nanowires (NW) due to their enhanced mobility can improve the electrical and optical properties of electronics [1] and photovoltaics [2] devices, while being compatible with CMOS technology. Several techniques are currently used to fabricate Ge NWs, such as molecular beam epitaxy [3] or chemical vapor deposition. However, homogeneous Ge NWs production by these methods, on a large area of the substrate, is still an open issue. For instance, using a catalyst such as gold introduces impurity levels inside the semiconductor band gap, which can alter the electronic transport properties of the wires. Metal assisted chemical etching (MAcE) process is a simple and economically favored method currently used to fabricate Si NWs, offering a large variety of controllability over the Si wires parameters. However, MAcE is not effective for Ge NWs fabrication. This fact can be attributed to several factors such as dangling bonds on the Ge surface; water solubility of the germanium oxide, which ceases the mass transfer process and to the disruption of the self-generated field in the metal catalyst. In the present work, we have succeeded in the fabrication of Ge NWs by developing a novel method we called anodic metal assisted catalytic etching (AMAcE), which is a combination of the anodic catalytic etching, and MAcE process. By using this method, we have achieved Ge wires with diameters ranging from 10 to 300 nm and with a length up to 10 microns. In the AMAcE, the current density in the etching process is controlled by the metal catalyst, as well as by an external bias source. The fabrication rate of Ge wires is fast compared to typical rates reported for the MAcE process of Si. Hence, Ge wires were found detached from the wafer and dispersed on its surface. Several Ge wafers, with different dopant types and resistivity values have been tested with the AMAcE process, which requires several step of preparation of the substrate before to start the fabrication of Ge wires. The morphology of the Ge wires (SEM) revealed a dense distribution of Ge wires, randomly dispersed on the Ge substrate. The structural investigation by high transmission electron microscopy (HRTEM) is still in progress while larger diameter Ge NWs have been selected for fabrication of metal contacts by Pt, in order to investigate their electronic transport properties. Either focused ion beam or electron beam lithography have been used for contacts deposition. The surface structures of these wires will play an important role in the optical and electronic transport properties of the wires such as the case of Si NWs. Hence, further studies on the parameters involved in the process still seems necessary. We believe that this method can open a new horizon in fabrication of economically favored Ge NWs