STM STUDY OF INITIAL-STAGE OF GE EPITAXY ON SI(001)

A scanning tunneling microscope (STM) was employed to study the superstructures and the formation process of islands at the initial stage of epitaxial growth of Ge layers on the Si(001) surface. Amount of Ge deposition was varied from one half of a monolayer (ML) to 4 ML. At a low Ge coverage the Ge layers were developed at step edges and on terraces forming two-dimensional (2D) islands and exhibited c(4 x 2) and p(2 x 2) structures on the Si substrate at 300-degrees-C. At a higher coverage of about 2 ML, a new superstructure with a periodicity of about 7 times of the unit cell was observed and the new cells were found to be the cores of the 3D islands. The number of islands increased and the islands grew with Ge coverage. Most Ge islands disappeared by annealing at 500-degrees-C and the 2D superstructure appeared again. Formation and growth mechanisms of the 3D Ge islands and the relaxation of the stress due to Si-Ge lattice mismatch will be discussed in analyzing the observed STM images.