In this work, we investigate the epitaxial growth of InAs quantum dots (QDs) on Ge substrates. By varying the growth parameters of growth temperature, deposition thickness and the growth rate of InAs, high density (1.2 x 10(11) cm(-2)) self-assembled InAs QDs were successfully epitaxially grown on Ge substrates by solid-source molecular beam epitaxy and capped by Ge layers. Pyramid- and polyhedral-shaped InAs QDs embedded in Ge matrices were revealed, which are distinct from the lens- or truncated pyramid-shaped dots in InAs/GaAs or InAs/Si systems. Moreover, with a 200 nm Ge capping layer, one-third of the embedded QDs are found with elliptical and hexagonal nanovoids with sizes of 7-9 nm, which, to the best of our knowledge, is observed for the first time for InAs QDs embedded in a Ge matrix. These results provide a new possibility of integrating InAs QD devices on group-IV platforms for Si photonics.

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