In-situ TEM Study of Thermal Stabilities of Metastable Silicon Phases

The cubic diamond structure of silicon (Si-I) undergoes a phase transformation to a tetragonal structure (Si-II) at a pressure level of 11.2~12 GPa. This transition is not reversible; a mixture of body-centered cubic Si-III (bc8) and rhombohedral Si-XII (r8) structures or amorphous silicon (a-Si) may form during pressure release [1]. In the last decade, depth-sensing indentation has proven to be a powerful tool for studying phase transformations in silicon under highly deviatoric stresses. However, the transmission electron microscopy (TEM) analysis of various Si structures formed during nanoindentation has been largely inhibited by the difficulties involved in sample preparation. Recently, with the aid of focused ion beam and other advanced TEM sample preparation techniques, a few groups have performed successful TEM studies on residual silicon indentations [2-6]. However, the structures formed under load and the exact transformation mechanisms between different phases are still not fully understood. Furthermore, the thermal stabilities of metastable phases (such as Si-III and Si-XII) formed within nanoindentations have not been reported. In this paper, a plan-view TEM work on silicon nanoidentations is presented. This work is then used to discuss phase transformation mechanisms during unloading and to investigate the stability of metastable silicon phases during heating in TEM.