Vapor–solid–solid growth of Ge nanowires from GeMn solid cluster seeds
We describe the fabrication of Ge nanowires during a single co-deposition step of Ge and Mn at high temperature. In these experimental conditions, a phase separation occurs and two different phases Ge and Ge1 − xMnx are formed with Ge1 − xMnx in the shape of small clusters distributed randomly in the Ge matrix. Because of the high deposition temperature, a new Ge1 − xMnx phase with low eutectic point is stabilized; this phase is different from the one (commonly Ge3Mn5) stabilized at lower temperature. During the growth process at 350 °C, the crystalline clusters remain solid but they are highly mobile and can float at the surface, serving as seeds to direct the growth of crystalline Ge nanowires from the vapor. The sketch steps of NWs formation are first the phase separation with formation of specific Ge1 − xMnx critical nuclei with low eutectic point and second the growth of Ge NWs directed by the Ge1 − xMnx solid cluster seeds. Ge NWs growth is forced along particular crystalline axis by the cluster seeds that lower the interfacial energy Ge/Ge1 − xMnx and the energy formation of the germanium crystal stabilizes the cluster position at the tip of the NWs. The density of NWs can be tuned by varying the nominal Mn concentration since this density is related to the number of clusters with the specific Ge1 − xMnx phase (with low eutectic point). The single step MBE process presented here has the main advantage to fully avoid any incorporation of unintentional impurity into Ge nanowires (apart from Mn atoms) and could be applied to several other systems. This work also provides new insights into the vapor–solid–solid growth mechanisms of Ge NWs.