Leonardus B. Bayu Aji, Department of Electronic Materials Engineering, Research School of Physics and Engineering, Australian National University, Canberra, 0200, Australia.
Studies of (pure) ion-implanted amorphous silicon (a-Si) show that, during thermal annealing, the Si-Si network undergoes extensive restructuring toward a state of lowest free energy, a process called structural relaxation. However, despite this being the focus of much research, the mechanism(s) that govern the kinetics of the structural relaxation still remain unclear. The work presented here addresses this issue.
Annealing of a-Si leads to a change in its mechanical, vibrational, and electrical properties, whereby these properties are sensitive to the degree of relaxation. The effect of annealing temperatures up to 550ºC on the mechanical behavior of a-Si was studied using nanoindentation by measuring the probability of a pressure-induced phase transition. The changes in the covalent Si-Si network were further tracked using Raman microspectroscopy and electrical measurements.
The mechanical behavior of a-Si is very sensitive to the ‘state’ (relaxed or unrelaxed) of the amorphous network, whereby unrelaxed (as-implanted) a-Si deforms plastically via flow and relaxed (thermally annealed) a-Si via a pressure-induced phase transformation. The probability of phase transformations is found to change significantly in a very narrow temperature range from 300ºC to 350ºC during thermal annealing. In contrast, the reduction in the average tetrahedral bond-angle distortion (Δθb) from 10.8º to 9.4º as observed by Raman microspectroscopy occurs at slightly higher temperatures, namely over a temperature range centered at 370ºC.
Previous experiments have shown that the electrical conduction in amorphous semiconductors occurs via variable-range hopping of the charge carriers in localized states, e.g. dangling bonds, near the Fermi level. This is characterized by Mott's variable-range hopping relation. Thus, conductivity measurements in a-Si as a function of annealing temperature provide additional information on the evolution of the density of the dangling bonds and the role of defect annihilation during structural relaxation. The conductivity, and hence the density of dangling bonds, is found to progressively decrease from 1021 eV-1 cm-3 to 1019 eV-1 cm-3 with annealing temperature. A major decrease in the density of the dangling bonds was observed around an annealing temperature of 250ºC, significantly lower than the temperatures where a reduction in Δθb or the probability for phase transformations occurs.
This strongly suggests that structural relaxation is at least a two-step process involving first a reduction in defects and dangling bond density followed by a reduction in the average tetrahedral bond-angle distortion as second step.
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Cette conférence est présentée par le RQMP Versant Nord du Département de physique de l'Université de Montréal et le Département de génie physique de Polytechnique Montréal.
