Low-temperature Al-induced crystallization of amorphous Ge

This work reports on the low-temperature crystallization of hydrogenated amorphous germanium (a-Ge:H) films induced by aluminum. A series of aluminum-doped a-Ge:H films ([Al∕Ge]∼10−6–10−2 range) were deposited onto crystalline silicon substrates at 220°C by the cosputtering technique under the same nominal conditions, except for the Al∕Ge concentration. Raman scattering and infrared transmission spectroscopy were used for the structural characterization. The analysis of experimental data indicates that as-deposited Al-doped a-Ge:H films having an Al relative concentration between 1 and 2at.% crystallize spontaneously. Aluminum contents below this range induce a partial crystallization of the films, whereas [Al∕Ge]>2at.% does not induce any crystallization. The mechanisms involved in the crystallization of these Al-doped a-Ge:H films were also investigated after thermal annealing treatments up to a temperature of 500°C. Since the films are hydrogenated, the influence of hydrogen in the crystallization process was considered in detail. The ensemble of the data leads us to associate the induced crystallization with the coordination of, and the local order around, aluminum atoms in the a-Ge:H network. A microscopic mechanism behind the low-temperature crystallization is proposed. The present research indicates that both fourfold coordinated aluminum atoms and hydrogen species are fundamental in the crystallization phenomenon: the former acting as crystallization seeds, and the latter determining the dynamics of the process.