Faceting changes of InAs self-assembled nano islands induced by strain modulation

Abstract

The self-assembly of pyramidal-faceted InAs quantum dots on strain-engineered GaAs surfaces was systematically studied. The growth was carried out using molecular beam epitaxy (MBE), where the islands self-assemble through strain relaxation in the lattice-mismatched InAs/[GaAs/In_xGa_1-xAs] heterostructure, following a mechanism akin to the Stranski-Krastanov (SK) growth mode. The epilayer/substrate strain f_s was modulated by varying the thickness (Σ) of the spacer GaAs layer. Reflection high-energy electron diffraction (RHEED) patterns taken in real-time revealed that the critical thickness (H_c) decreases with Σ. At the initial stages of growth, chevron-like RHEED spots indicated the formation of spikier islands for small Σ. However, by the end of the self-assembly process, the QDs converged to a similar geometry, becoming independent of f_s. The experimental data were used as input for numerical simulations to map the biaxial strain (ε_xx) distribution throughout the heterostructure. The results demonstrated that f_s primarily affects the initial stages of nucleation. However, upon completion of QD formation a strong dependence on the pyramidal shape and vertex angle emerges, the biaxial strain ε_xx inside the QDs remains unaffected by Σ or externally induced strain.