AFM and RHEED study of Ge/Si(001) quantum dot modification by Si capping

Atomic force microscopy (AFM) images of 6 Ge equivalent monolayers (EML) deposited on Si(001) at 500°C by solid source molecular beam epitaxy reveal the formation of ∼2 nm high islands with a height/base dimension ratio close to 1/10. Clear differences are observed, by both reflection high-energy electron diffraction and AFM, according to the Si capping conditions of such hut clusters. At room temperature the impinging Si adatoms homogeneously cover the Ge clusters due to very low Si and Ge surface diffusions nearly preserving the corrugation of uncovered Ge clusters. In contrast, at 500°C and a low Si deposition rate (∼2 EML min−1), a surface smoothing occurs after deposition of a thickness as low as 5 Si EML. A Si adatom diffusion from the apex towards the bottom of the islands concomitant with a Ge lateral segregation lead to a rapid collapse of the buried islands. This scenario is supported by the observation of a (2×9) superstructure, characteristic of the formation of SiGe alloy.