An STM study of Ge heteroepitaxial growth on Si(111)√3 x √3-B surfaces

We studied the initial stage of Ge heteroepitaxial growth on boron doped Si(111)root 3 x root 3-B (Si(111)root 3 x root 3-B) surfaces at 600 degrees C. Scanning tunneling microscope (STM) images revealed that two-bilayer (2BL) height twinned islands nucleated at the coverage below one bilayer (1BL). The growth then proceeded in the step-flow mode. During the two-dimensional (2D) layer growth, the growth front surface maintained the root 3 x root 3 reconstruction. However, the root 3-reconstruction dissolved into 5 x 5 and root 3 x root 3 domains, and three-dimensional (3D) islands started to nucleate at 4BL. These transitions occurred in accordance with a steep decrease in the B concentration [B](surf) at the top surface, which we evaluated quantitatively using the contrast change at B atom sites in STM images. Calculating the surface strain as a function of [B](surf), we attributed the surface structural change at 4BL, to the change of the surface strain from tensile to compressive. The 3D island nucleation was also assumed to occur to release the increased Ge/Si misfit strain energy with the decrease in [B](surf). (c) 2008 Elsevier B.V. All rights reserved.