Mechanism of vertical correlation in Ge/Si(001) islands multilayer structures by chemical vapor deposition

Abstract

The aim of this paper is to study the mechanism of vertical correlation of Ge/Si(001) islands in multilayered structures grown by ultrahigh vacuum chemical vapor deposition. We used in situ reflexion high energy electron diffraction, ex situ atomic force microscopy, photoluminescence spectroscopy, and high resolution and analytical field emission transmission electron microscopy in order to determine the morphological properties of the layered structures at each step of their formation. For Si spacer thickness below 90 nm, a roughness is observed above the top of each Ge island. The local composition of the multilayers measured by x-ray nanoanalysis shows an inhomogeneous Ge distribution in the wetting layers and in the islands. The roughness can then be explained by the existence of a difference of Si growth rate over Ge islands and Ge wetting layers. This roughness on the Si cap surface layer leads, on the one hand, to a preferential nucleation of Ge above each buried island, and consequently to the vertical correlation in multilayers, and, on the other hand, to an increase of island density on upper layers.