TEM investigation of contact loading induced phase transformation in silicon

Uniaxial compression in a diamond anvil cell (DAC) and indentation tests were used to achieve high contact pressures on silicon samples. Transmission electron microscopy (TEM) investigations, supplemented by depth-sensing indentation, Raman microspectroscopy and X-ray icrodiffraction (Micro-XRD), were carried out to study pressure induced phase transformations in silicon. It has been believed that a phase transformation from a cubic diamond structure (Si-I) to a β-tin structure (Si-II) takes place during nanoindentation. It was discovered that with slow unloading, the β-tin structure could be transformed into two metastable crystalline phases, Si-III and Si-XII. These two structurally similar phases were distinguished by using selected area diffraction and high resolution imaging, and their spatial distributions within indentation were determined in dark field TEM micrographs. The thermal stabilities of metastable silicon phases produced by nanoindentation were thoroughly studied by annealing experiments. Inparticular, we suggested the formation of a new silicon phase, referred SI-XIII, during heating at elevated temperatures. In situ heatingexperiments showed that different transformation paths exist for the metastable silicon phases present in the indentations. In looselyconstrained areas, Si-III and Si-XII often directly collapse into an amorphous structure, whereas in relatively thicker areas of ion-milled samples, the new Si-XIII structure and Si-IV may form. The phase transformation sequence during heating of indented samples was established as Si-XII → Si-III → Si-XIII → Si-IV/a-Si → Si-I. In addition, DAC experiments provide a means to monitor the pressure induced phase transformation in silicon, with the aid of Raman spectroscopy. Consistent with nanoindentation work, DAC experiments showed similar phase transformation paths during a compression-decompression cycle and upon subsequent heating. In particular, formation of Si-II was confirmed in DAC tests. However, the formation of Si-XIII in DAC samples is much lessprobable than in nanoindentation. Finally, a novel technique for investigation of indented materials was developed through the successfulapplication of TEM observations in silicon, enabling direct microscopic study of structural changes under contact loading in a wider range of materials. It was also shown that characterization methods such as TEM,micro-XRD, Raman spectroscopy, nanoindentation, and DAC are complementary in the comprehensive analysis of materials deformation and structural changes induced by contact loading.Ph.D., Materials Science and Engineering -- Drexel University, 200