High-density arrays of quantum dots (QDs) can easily be grown by self-assembly.[1–3] Such QDs are strong candidates for advanced semiconductor quantum devices.[4–6] However, the precise mechanism of self-assembly is not well understood, which hampers control over QD size, density, and distribution for particular applications. This prototypical selfassembly system also presents very general challenges for growth modeling over scales from atomic dimensions to hundreds of nanometers. The scanning tunneling microscope (STM) is a powerful tool to investigate growth at the atomic scale and the growth of InAs and GaAs by molecular beam epitaxy (MBE) has been studied invacuo using STM for more than ten years, greatly improving our understanding of fundamental MBE processes.[7–12] In a typical STM–MBE experiment, the vacuum chambers housing the MBE system and STM are separate, and the sample must be cooled from high temperature in the MBE system to room temperature for imaging. Suitable quenching protocols may freeze a growing surface, allowing atomically resolved snapshots of dynamic processes such as island growth [13, 14] or surface alloying [15] to be obtained.