The martensitic transformation in silicon—I experimental observations

Abstract

Ribbons of hexagonal material are produced when silicon is indented in the temperature range 400–650°C. This series of three papers, based on two previous preliminary reports [P. Pirouz, R. Chaim and J. Samuels, Proc. 5th Int. Congr. Structure and Properties of Dislocations in Semiconductors, Moscow, U.S.S.R., 1986. Izvestiya Nauk Akademiya S.S.S.R. 51, 753 (1987); P. Pirouz, R. Chaim and U. Dahmen, Mater. Res. Soc. Symp. Proc. 104, 133 (1988)], is a detailed account of an investigation of this phase. In the first paper, the details of the experimental investigation are described. Vickers microindentations have been made in float-zone silicon in the temperature range of 400–500°C. The ribbons of hexagonal silicon are then studied by different techniques of transmission electron microscopy (TEM). Conventional microscopy using strain contrast, selected area diffraction technique, and high resolution electron microscopy (HREM) have been employed to investigate the structure of the hexagonal phase. Using the latter technique, the ribbons are investigated to within a resolution of 0.18 nm and HREM images are compared with simulated ones. HREM is specially useful for studying the faulting within the hexagonal phase and the cubic/hexagonal interface. Analytical microscopy using Electron Energy Loss Spectroscopy (EELS) has been used to show that no oxygen is present in the ribbons within the resolution of this technique. Throughout the paper, it has been attempted to show that the present hexagonal Si is not a thermodynamically stable high pressure phase, but is related to stress relief when twins interact in silicon. In Part II of this series [U. Dahmen, K. H. Westmacott, P. Pirouz and R. Chaim, Acta metall. mater. 38 323 (1990)], a mechanism for the formation of hexagonal silicon by double twinning is presented through a crystallographic analysis. Finally, in Part III [P. Pirouz, U. Dahmen, K. H. Westmacott and R. Chaim, Acta metall. mater. 38, 329 (1990)], a discussion of the results in the context of martensitic transformations and dislocation mechanisms is given and other recent reports of this phase in silicon which has undergone different treatments are discussed.