Silicon plays a prominent role in the production of electronic and semiconductor devices but its phase transformations associated with the production often change the mechanical properties of the material. Nano-deformation of silicon under indentation has therefore been studied extensively and many different phases have been identified on loading and unloading. On the other hand the phase transformations can also be induced by pressure and this has been studied experimentally up to 250 GPa over the last two decades. So far twelve different structural phases, which are either stable or metastable at different pressure ranges, have been reported.
This paper investigates the phase transformations of silicon that occur under triaxial compressive and tensile stresses during loading and unloading using molecular dynamics analysis. The results show that under both triaxial tension and compression the stress varies non-linearly with strain. Under triaxial tension diamond cubic silicon is fully elastic though a portion of the material transforms into a trigonal planar phase at a higher stress whereas under compression it is elastic only at low stress. At a higher stress, for example at 25.6 GPa silicon deforms plastically. It was found that the 6-coordinated atoms that corresponds to β-tin structure begin to form at about 15 GPa and above 29.0 GPa the 6-coordinated arrangement corresponds to a new phase having hexagonal prism structure. This new phase is a portion of the hexagonal closest packing arrangement in which six more atoms in the planar hexagonal array are missing.
However, even at the maximum stress, the total number of transformed atoms is only 11% of the atoms in the simulation model. Out of this 11%, only 3.6% of the atoms are six coordinated and they are mainly found near the sides and the corners of the sample. This may indicate that the hydrostatic stress component alone may not be sufficient to bring about complete phase transformation.