In this Letter we report on the exploration of axial metal/semiconductor (Al/Ge) nanowire heterostructures with abrupt interfaces. The formation process is enabled by a thermal induced exchange reaction between the vapor–liquid–solid grown Ge nanowire and Al contact pads due to the substantially different diffusion behavior of Ge in Al and vice versa. Temperature-dependent I–V measurements revealed the metallic properties of the crystalline Al nanowire segments with a maximum current carrying capacity of about 0.8 MA/cm2. Transmission electron microscopy (TEM) characterization has confirmed both the composition and crystalline nature of the pure Al nanowire segments. A very sharp interface between the ⟨111⟩ oriented Ge nanowire and the reacted Al part was observed with a Schottky barrier height of 361 meV. To demonstrate the potential of this approach, a monolithic Al/Ge/Al heterostructure was used to fabricate a novel impact ionization device.

Semiconductor nanowires (NWs) are promising candidates for many device applications ranging from electronics and optoelectronics to energy conversion and spintronics. However, typical NW devices are fabricated using electron beam lithography and therefore source, drain and channel length still depend on the spatial resolution of the lithography. In this work we show fabrication of NW devices in a transmission electron microscope (TEM) where we can obtain atomic resolution on the channel length using in‐situ propagation of a metallic phase in the semiconducting NW. The corresponding channel length is independent on the lithography resolution. We show results on semiconducting NW devices fabricated on two different electron transparent Si 3 N 4 membranes: a calibrated heater chip from DENs solution [1] and homemade membranes where the NW‐metal contact is locally heated by Joule heating [2]. We demonstrate a real‐time observation of the metal diffusion in the semiconducting NW. First we present results on in‐situ propagation of aluminum metal in Ge NWs while monitoring the system temperature [3] and by Joule heating while measuring the current through the device. We study the kinetics and rate limiting step by monitoring the position of the reaction front as a function of time. Second we will show characterization of the formed phase at atomic length scales with different (S)TEM techniques (energy dispersive X‐ray spectroscopy, HR(S)TEM) to understand how the metal atoms diffuse and incorporate into the Ge NW at the reaction front and how these parameters relate to the electrical properties of the same interface. Using EDX analysis and comparing with 3D NW model calculations we show that the reacted NW part is pure Al, with a shell of Al 2 O 3 with a low Ge content on both sides of the Al 2 O 3 shell see Fig.2. EDX analysis show that both Al and Ge are diffusing in opposite directions.