Study of Nickel-Germanium intercations in lateral diffusion couples and thin films

Abstract

Electronic devices are shrinking to ever smaller dimensions, a trend technically known as downscaling. The main drive for downscaling is to increase device functionalityand computational capability. However, one challenge that arises as a result of downscaling involves the materials that are best suited as electricalcontacts in nano-scale devices; existing materials and technologies give performance problems as device-size reduces below a certain limit. Germanium, with its high charge-carrier mobility and low freeze-out temperature,stands as a promising candidate for future microelectronics, in which case nickel germanides would be used to make electrical contacts.It is therefore important to thoroughly study and understand thephase formation characteristics of nickel-germanidesin order to foresee and avoid problems that may be encountered both during device integration and operation.In this work, the thermally induced formation of nickel germanides has been investigated using both conventional thin films and lateral diffusion couples. In thin films, two of the nine phases predicted in the Ni-Ge phase diagram, viz.Ni5Ge3 and NiGe, were observed. The phases were formed in the sequence: Ni5Ge3, and then NiGe. Although formation of Ni5Ge3 started during deposition, its growth as a result of heat treatment only commenced around 150 °C and went on until232 °C. The growth of NiGewas found to occur in the temperature range of 223 – 254 °C. A period of co-existence of Ni5Ge3 and NiGe in the presence of unreacted Ni was observed between 223 °C and 232 °C. Using Kissinger plots, activation energies of growth for both Ni5Ge3 and NiGe were determined, yielding values 0.83 ± 0.05 eV and 1.33 ± 0.05 eV respectively.In lateral diffusion couples, lateral interactions were observed at temperatures as low as 300 °C. Our results qualitatively indicated that Ge is the dominant diffusing species during Ni-Gelateral interactions. Three phases, viz.NiGe, Ni3Ge2 and Ni5Ge3, were observed.NiGe and Ni5Ge3 were respectively located inside and outside the original island boundary while the Ni3Ge2 region stretched across the boundary. The NiGe region was observed to have resulted from the exposure of the NiGe that had formed during the initial stages of the interactions while the Ni3Ge2 region resulted both from the dissociation of NiGe (at the NiGe/Ni3Ge2interface) and the transformation of Ni5Ge3(at the Ni3Ge2/Ni5Ge3 interface). Phase growthwas found to obeyasquare-root-of-timelaw, characteristic of diffusion-limitedprocesses. The activation energy of the combined lateral growth of Ni3Ge2 and Ni5Ge3 was found to be 0.9  0.1 eV. The average activation energy corresponding to the rate of exposure of NiGe, in competition with its rate of decomposition, was determined to be 1.1  0.1 eV.Apart from contributing to the reservoir of fundamental knowledge on the Ni-Ge system, it is hoped that the output of this research willhelp in improving the manufacturing capability as well as theoperational stability of future microelectronic devices