Droplet-mediated formation of embedded GaAs nanowires in MBE GaAs1-xBix films

摘要

We have examined the morphology and composition of embedded nanowires that can be formed during molecular beam epitaxy of GaAs1-xBix using high angle annular dark field ('Z-contrast') imaging in an aberration-corrected scanning transmission electron microscope. Samples were grown in Ga-rich growth conditions on a stationary GaAs substrate. Ga-rich droplets are observed on the surface with lateral trails extending from the droplet in the [110] direction. Cross-sectional scanning transmission electron microscopy of the film reveals epitaxial nanowire structures of composition similar to GaAs embedded in the GaAs1-xBix epitaxial layers. These nanowires extend from a surface droplet to the substrate at a shallow angle of inclination (similar to 4 degrees). They typically are 4 mu m long and have a lens-shaped cross section with major and minor axes dimensions of 800 and 120 nm. The top surface of the nanowires exhibits a linear trace in longitudinal cross-section, across which the composition change from similar to GaAs to GaAs1-xBix appears abrupt. The bottom surfaces of the nanowires appear wavy and the composition change appears to be graded over similar to 25 nm. The droplets have phase separated into Ga- and Bi-rich components. A qualitative model is proposed in which Bi is gettered into Ga droplets, leaving Bi depleted nanowires in the wakes of the droplets as they migrate in one direction across the surface during GaAs1-xBix film growth.