Kinetically controlled dewetting of thin GaAs cap from an ErAs/GaAs nanoparticle composite layer

Abstract

ErAs metal nanoparticles (NPs) embedded in GaAs have multiple applications in plasmonic, terahertz, and tunneling devices. Growing a high quality thin GaAs layer over the ErAs NP layer is vital to these applications. In this work, the authors study the surface stability of a thin GaAs cap (1–5 nm) annealed in a temperature range of 450–620 °C. The thin GaAs cap covered a single layer of ErAs NPs [0.5–1.33 monolayer (ML)] grown using molecular beam epitaxy on GaAs(001) substrates at 450–500 °C. For 1.33 ML ErAs coverage, although a 1 nm GaAs cap exhibited a root-mean-square surface roughness close to 0.3 nm, the authors expected that 1 nm GaAs was not thick enough to overgrow the NPs in a height of 3–4 nm; thus, a large number of pinholes should be left on the surface. By increasing the GaAs cap thickness to 3 nm, the authors were able to achieve atomically smooth surfaces with few remaining pinholes. At a lower coverage of ErAs, 0.5 ML, the authors were able to achieve atomically flat pinhole-free GaAs caps with a thickness of 3–5 nm. However, the key finding from this study is that distinct changes in the surface morphology occurred upon annealing depending on the film thickness and NP density. In the case of 1 nm GaAs caps, clumps were formed when annealed, whereas in the case of GaAs caps of 3–5 nm in thickness, the GaAs film uncharacteristically dewetted at the ErAs NP/GaAs composite interface. Thermodynamically, this dewetting is driven by the high interfacial energy resulting from the difference in the crystal structure between GaAs and ErAs (zinc-blende and rock salt); however, surface mobility plays an important kinetic role in this process. It has been demonstrated that the dewetting can be prevented by combining a higher As overpressure, a low growth/annealing temperature, lower surface coverage of ErAs NP, and thicker GaAs caps.