In Situ Electrical Characterization of Phase Transformations in Si during Indentation

Abstract

An in situ electrical characterization technique is used to study details of the deformation behavior of crystalline silicon during nanoindentation. The experimental arrangement involves the measurement of current flow through a reverse-biased Schottky diode and exploits a sharp transition from a Schottky to an Ohmic contact that accompanies the formation of a metallic Si-II phase directly under the indenter. This electrical technique is particularly sensitive to the nature and extent of the local Si-I to Si-II phase transformation and allows such changes to be directly correlated with features in nanoindentation load-unload curves, using both spherical and Berkovich indenters. Interestingly, for spherical indentation, the onset of a transformation to a metallic Si-II phase is observed before the so-called "pop-in" event occurs during loading. Furthermore, after the "pop-in" event, fine structure in the electrical behavior suggests that extrusion of the ductile metallic Si-II phase from under the indenter may occur when the transformed area exceeds that of the indenter contact. Indeed, the in situ electrical measurements have provided considerable insight into the evolution of deformation processes during indentation loading and unloading of Si. During unloading, metallic Si-II transforms to less electrically conducting phases of Si. We suggest that, although Si-III and Si-XII are the preferred low pressure phases during pressure release, as diamond anvil studies show, a-Si is often obtained during fast unloading rates as a result of a high kinetic barrier to nucleation of the crystalline phases. Furthermore, we suggest that the pop-out occurs for slow unloading rates as a result of spontaneous nucleation and growth of the crystalline phases at a critical pressure.