Comparative In Situ Study of Pressure- and Strain-Induced Phase Transformations in Silicon

We report a comparative in situ study of pressure- and strain-induced phase transformations (PTs) in silicon (Si), the most important electronic material. Synchrotron X-ray diffraction (XRD) studies are conducted on three types of Si particles (micron, 100 nm, and 30 nm), and their equations of state for various Si phases are investigated using a He pressure-transmitting medium [1]. The Si-I phase of 100 nm Si is observed to be less compressible than that of micron and 30 nm Si. A correlation between the direct and inverse Hall-Petch effects of particle size on yield strength and pressure for strain-induced PTs is predicted theoretically and confirmed experimentally for the Si-I→Si-II PT [2]. For 100 nm particles, the strain-induced PT (Si-I→Si-II) initiates at 0.3 GPa under both compression and shear, whereas it begins at 16.2 GPa under hydrostatic conditions. The Si-I→Si-III PT starts at 0.6 GPa but does not occur under hydrostatic pressure. The pressure in the small Si-II and Si-III regions of micron and 100 nm particles is approximately 5–7 GPa higher than in Si-I. The phase fractions and crystallite sizes of various Si phases are reported for the first time [2]. Retaining Si-II and single-phase Si-III at ambient pressure and obtaining a reverse Si-II→Si-I PT demonstrate the potential for manipulating different synthetic paths.