Formation mechanisms of single-crystalline InN quantum dots fabricated via droplet epitaxy

This work presents an experimental and theoretical insight into formation mechanisms of single crystalline wurtzite InN quantum dots (QDs) fabricated via metal droplet epitaxy (DE) by employing plasma assisted molecular beam epitaxy. The applied procedure consists of two fabrication stages. During the first stage, the cold substrate (T ≈ 15 °C) is exposed to an impinging In flux, resulting in formation of metallic In droplets on the substrate surface, and then to an impinging active nitrogen flux, resulting in In conversion into polycrystalline InN islands. During the second stage, the substrate, which is still kept exposed to active nitrogen, is heated up to T ≈ 300 °C, to allow for the reorganization of extended polycrystalline InN islands into groups of independent single-crystalline wurtzite InN QDs. This work provides a detailed experimental insight into both fabrication stages and their qualitative explanations within the scopes of adatom surface kinetics (stage I) and total energy per unit crystal volume minimization (stage II). Finally, the formation mechanisms of InN QDs on the three different substrates (Si(1 1 1), Si(0 0 1) and In0.3Ga0.7N/Si(1 1 1)) are compared, and also linked to the formation mechanisms of other more studied nanostructures, such as self-assembled GaN/AlN QDs and self-assembled and selective-area-grown GaN nanowires.