Investigation of the kinetics of crystallisation of Al/a-Ge bilayer by electrical conductivity measurement

The kinetics of aluminium-induced crystallisation of thin amorphous germanium layers have been studied by electrical conductivity measurements. Al/a-Ge bilayers (5/50 nm) were prepared by thermal evaporation onto NaCl substrate at constant temperature (220°C) in a high vacuum (10 −6 Torr), between two electrical contacts, previously prepared on the substrate. The changes in the electrical conductivity due to deposition and phase transformation of amorphous germanium to polycrystalline germanium were recorded as a function of time. Crystallisation starts from aluminium/amorphous germanium interface forming a polycrystalline germanium layer. Using a model of conductors connected in parallel for the system formed by amorphous germanium, polycrystalline germanium and aluminium layers, the time dependence of the crystallisation front motion was calculated. The experiment resulted in a parabolic dependence of the thickness of polycrystalline germanium layer on the annealing time, suggesting that crystallisation of amorphous germanium induced by aluminium is diffusion-controlled. Aluminium concentration profile in depth as measured by AES depth profiling supports further this conclusion.