One of the major roadblocks in developing Ge nanowire transistors is in finding low resistance ohmic contacts. This is due to the fact that surface states between a metal and a Ge nanowire contribute to large Fermi level pinning and hence detrimental large Schottky barriers for the devices. Metal alloys with Ge, i.e. germanides, have been suggested as a possible solution. Nickel germanide has shown promise as the germanide of choice due to its thermal stability and low resistivity [1].

One problem which has been identified with Ni-germanides for bulk systems is the agglomeration of the germanide and hence an increase in resistivity [2]. The finite dimensions of the nanorods should eliminate this due to a limited Ni/Ge interface. In-situ TEM annealing at 500°C of Ni germanides supplied first insights into the kinetics of the nanowire Ni germanide formation suggesting a linear growth rate and a sharp interface with the Ge nanowire [3].

Our studies further expand this knowledge by looking at the formation of germanides restricted to 1-dimensional Ge nanorods in contact to a finite source of Ni i.e. Ni cap. The nanorod diameters were scaled down to 20 nm using electron beam lithography (EBL) and reactive ion etching (RIE) using (100), (110) and (111) Ge substrates.

Hence we establish the effect of the Ni-germanide formation with a decreasing interface area, deterministically controlled crystal orientation and dopant level. Ni-capped Ge nanorods with varying size and crystal orientation have been annealed in-situ in the TEM to observe the Ni-germanide formation. The consumption of the Ni as it migrates into the Ge is observed and corresponding germanide growth rates were determined. Other methods of annealing such as microwave and rapid thermal annealing (RTA) will also be presented in this study as a comparison to the in-situ annealing.

1. Brunco, D. P. et al. Germanium MOSFET devices: Advances in materials understanding, process development, and electrical performance. Journal of the Electrochemical Society 155, H552-H561, doi:10.1149/1.2919115 (2008).

2. Lee, J.-W. et al. Enhanced Morphological and Thermal Stabilities of Nickel Germanide with an Ultrathin Tantalum Layer Studied by Ex Situ and In Situ Transmission Electron Microscopy. Microscopy and Microanalysis 19, 114-118, doi:doi:10.1017/S1431927613012452 (2013).

3. Tang, J. et al. Single-crystalline Ni2Ge/Ge/Ni2Ge nanowire heterostructure transistors. Nanotechnology 21, doi:10.1088/0957-4484/21/50/505704 (2010).

Roisin Kelly

Tyndall National Institute, University College Cork

Appearances