Mechanical properties of ion-implanted amorphous silicon

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We used nanoindentation coupled with finite element modeling to determine the mechanical properties of amorphous Si layers formed by self-ion implantation of crystalline Si at approximately 100 K. When the effects of the harder substrate on the response of the layers to indentation were accounted for, the amorphous phase was found to have a Young’s modulus of 136 ± 9 GPa and a hardness of 10.9 ± 0.9 GPa, which were 19% and 10% lower than the corresponding values for crystalline Si. The hardness agrees well with the pressure known to induce a phase transition in amorphous Si to the denser ␤–Sn-type structure of Si. This transition controls the yielding of amorphous Si under compressive stress during indentation, just as it does in crystalline Si. After annealing 1 h at 500 °C to relax the amorphous structure, the corresponding values increase slightly to 146 ± 9 GPa and 11.6 ± 1.0 GPa. Because hardness and elastic modulus are only moderately reduced with respect to crystalline Si, amorphous Si may be a useful alternative material for components in Si-based microelectromechanical systems if other improved properties are needed, such as increased fracture toughness.

I. INTRODUCTION

Ion implantation can be used to produce special microstructures that can give new insights into the mechanical properties of solids. Implantation can alter the mechanical properties of the target material by producing atomic displacements and by introducing new species, such as by the formation of precipitates1 or transformation to an amorphous state. Amorphous phases have been produced in semiconductors,2,3 ceramic materials,4 and in metals.5 Certain characteristics of implantation, such as direct control over damage levels,6 athermal introduction of essentially any solute to high concentrations, and control of the modified layer thickness and depth, can allow controlled investigations of implanted materials. Amorphous silicon (a-Si) is a prototypical amorphous structure that can offer insight into the mechanical properties of disordered materials. It is readily formed by ion implantation7,8 as well as deposition methods,9 and much is known about its thermodynamic properties.10 However, a-Si formed by implantation appears more uniform than that formed by deposition,9 and any voids in this material are believed to be