In Situ Transmission Electron Microscopy Analysis of Copper–Germanium Nanowire Solid-State Reaction

Abstract

A promising approach of making high quality contacts on semiconductors is a silicidation (for silicon) or germanidation (for germanium) annealing process, where the metal enters the semiconductor and creates a low resistance inter-metallic phase. In a nanowire, this process allows to fabricate axial heterostructures with dimensions depending only on the control and understanding of the thermally induced solid-state reaction. In this work, we present the first observation of both germanium and copper diffusion in opposite directions during the solid-state reaction of Cu contacts on Ge nanowires using in-situ Joule heating in a transmission electron microscope. The insitu observations allow us to follow the reaction in real time with nm spatial resolution. We follow the advancement of the reaction interface over time, which gives precious 2 information on the kinetics of this reaction. We combine the kinetic study with ex-situ characterization using model based energy dispersive X-ray spectroscopy (EDX) indicating that both Ge and Cu diffuse at the surface of the created Cu 3 Ge segment and the reaction rate is limited by Ge surface diffusion at temperatures between 360 and 600 • C. During the reaction, germanide crystals typically protrude from the reacted NW part. Their formation can however be avoided using a shell around the initial Ge NW. H a direct Joule heating experiments show slower reaction speeds indicating that the reaction can be initiated at lower temperatures. Moreover, they allow combining electrical measurements and heating in a single contacting scheme, rendering the Cu-Ge NW system promising for applications where very abrupt contacts and a perfectly controlled size of semiconducting region is required. Clearly in-situ TEM is a powerful technique to better understand the reaction kinetics and mechanism of metal-semiconductor phase formation.

In Situ Transmission Electron Microscopy Analysis of Aluminum–Germanium Nanowire Solid-State Reaction

“…We have adapted this model. 29 In this model, four different regimes can govern the incorporation of metal atoms in a semiconducting NW during a thermal annealing. We have described this model in detail, 29 here we just present briefly the four potential rate limitation steps and how the converted region L varies as a function of time t and R NW radius.…”

“…The fabrication process is described in den Hertog et al 28 On the latter membranes, after dispersion of Ge NWs, two parallel metal lines are defined using electron beam lithography on a single Ge NW. This process is described in El hajraoui et al 29 and the metal was deposited by sputtering followed by electron beam induced metal evaporation.…”

Reversible Al Propagation in SixGe1–x Nanowires: Implications for Electrical Contact Formation

“…Therefore, in a situation where a metal diffuses into a NW, we typically expect to see an interdiffused region. On the other hand, in such experiments where a metal enters a NW (for example, with Ni in the case of Si or Ge NWs, 34 , 35 Cu in Ge NWs, 36 , 37 Pt in Si NWs, 38 or Al in Ge NWs 15 , 39 ) a very abrupt interface is observed between the original NW and the converted region. The likely explanation is that the concentration gradient is not present along the NW axis but is present in a surface diffusion channel.…”

In‐Situ Observation of the Formation of NiSi/Ni2Si Heterojunction in SiGe Nanowire with Al2O3 Diffusion Barrier Layer

“…According to Fick's law, the diffusion behavior can be expressed as Equation S1, Supporting Information. [ 21 ] When the reaction time is extremely short, the diffusion distance has a first‐order linear relationship with time. In addition, it is the reaction‐controlled diffusion mechanism.…”