Morphological evolution and strain relaxation of Ge islands grown on chemically oxidized Si(100) by molecular-beam epitaxy

Abstract

We have previously demonstrated that high-quality Ge can be grown on Si by the touchdown process, where chemically oxidized Si is exposed to a Ge molecular beam. The causes of strain relaxation in the Ge epilayer were also proposed and discussed. Herein, we present a detailed analysis on the morphological evolution and strain relaxation of nanoscale Ge islands on SiO2-covered Si in order to identify the mechanisms by which the high-quality epilayer forms. During the touchdown, the Ge seeds are anchored to the underlying Si. This immobility of Ge islands gives rise to a unique bimodal size distribution during coarsening. Three events are observed during coalescence: (1) merging of two small (<10nm) islands largely driven by surface diffusion, (2) merging of a small island and a big island (∼50nm), and (3) merging of two big islands. The coalescence of two small islands is characterized by the formation of twins or stacking faults at the two merging fronts. In contrast, no stacking fault or grain boundary results from the coalescence of large islands, indicating that their coalescence is favorable in achieving high-crystalline quality. For the Ge epilayer, the compressive strain exists mostly within 2nm from the heterojunction, resulting in a fully relaxed Ge epilayer. We attribute the relaxation to three mechanisms: (1) the strain at the junction pad decays below the critical limit within 2nm, a scale exactly comparable with the dimension of individual Ge–Si junction; (2) the remaining SiO2 serves as artificially introduced 60° dislocations; and (3) the intermixing between Ge and Si at the heterojunction reduces the effective lattice mismatch.