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.
Epitaxial thin films combining InAs quantum dots (QDs) with ErAs metallic nanoparticles (MNPs) are emerging device materials with the potential for wide ranging optoelectronic applications, which are based on coupling the plasmonic properties of MNPs with the absorption/emission characteristics of QDs. Several critical parameters influence the formation of these hybrid structures, including the MNP-QD size/shape, their separation, placement, and relative orientation, all of which critically depend on the strain field due to the QDs. We have recently developed a growth procedure aimed at optimizing these parameters, wherein the hybrid structures are grown epitaxial on (001) GaAs substrates by molecular beam epitaxy. Specifically, following the growth of InAs QD and GaAs spacer layer, the ErAs MNP is achieved in two steps: (1) co-deposition of InAs and ErAs to trigger preferential NP nucleation over underlying QDs due to associated strain field and (2) subsequent evaporation of In to form ErAs MNPs.In this submission we present transmission electron microscope (TEM) studies aimed at obtaining quantitative information on many of the parameters mentioned above. In particular, we have applied recent advancements in TEM based techniques for defect characterization, and the examination of composition and strain distribution in the GaAs spacer layer surrounding the MNP-QD region. These studies were performed on MNP/QD hybrid layers in which the Er deposition rate was systematically changed by adjusting the cell temperature from 1140C to 1185C. The GaAs spacer layer thickness between the NP/QD was maintained at nominal value of 15 nm and NP/QD layer was separated by a GaAs layer of nominal thickness of 150 nm. The samples were examined using aberration corrected high resolution TEM and Z-contrast imaging techniques. In addition compositional analysis of the constituent layers was performed by X-ray energy dispersion spectroscopy (XEDS). Figure 1 is a bright-field image of the overall structure showing the layers grown at different Er effusion cell temperatures. The image on the right is a magnification of the region indicated by the green arrow showing good alignment between the NP and the underlying QD. Several regions in the image were further analysed to examine the spatial correlation between NPs and the QDs. However Fig.1 also shows the presence of stacking faults in NPs formed by the co-deposition, which is not typically observed in NPs grown by conventional methods (Fig. 2). Figure 3 shows XEDS maps obtained from regions surrounding the NP/QD layers which indicates the presence of a thin In layer beneath the NP, thereby indicating that the temperature for evaporation was not high enough to completely get rid of deposited In. A comparison of the HAADF image and Er map also shows the locations from where the stacking faults nucleate. More detailed high-resolution TEM studies showing the strain field around these defects and the spatial correlation between the QDs and MNPs will be presented [1].
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