Role of two- and three-dimensional surface structures in InAs-GaAs(001) quantum dot nucleation

Rapid-quench scanning tunneling microscopy has been used to monitor the growth by molecular beam epitaxy of InAs quantum dots (QD’s) on GaAs(001) at and near the critical coverage $\left({\theta }_{\mathrm{crit}}\right).\mathrm{}$ Large two-dimensional (2D) islands do not transform into 3D islands at ${\theta }_{\mathrm{crit}}$ but instead coexist with small, irregular 3D islands of height 6–12 Å (2–4 ML) which contain ∼150 atoms or more. These features develop rapidly (within 0.05 ML of ${\theta }_{\mathrm{crit}})$ into regular mature QD’s with an average volume $>1\mathrm{}\times {10}^4$ atoms. The results provide important constraints for existing theories of the $2\overrightarrow{D}3\mathrm{D}$ growth mode transition in this complex heteroepitaxial system.