Annealing-induced precipitate formation behavior in MOVPE-grown GaAs1-x Bi x explored by atom probe tomography and HAADF-STEM.

The effects of a 45 min anneal at 800 °C on the physical properties and microstructure of a five-period GaAs1-x Bi x /GaAs1-y Bi y superlattice with y ≠ x were studied using room-temperature photoluminesence spectroscopy, high-resolution x-ray diffraction, high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM), and atom probe tomography (APT). The anneal resulted in a substantial increase of the photoluminesence intensity over that observed in the as-deposited sample, indicating annihilation of non-radiative recombination centers and stability of the superlattice structure during the anneal. However, some precipitation of Bi from the GaAs1-x Bi x also occurred. The characteristics of phase separation that occurred within these precipitates were investigated in detail by APT and HAADF-STEM. They indicate that the precipitation reaction involves formation of embedded nano-scale liquid droplets that can accelerate local Bi dissolution from the GaAs1-x Bi x matrix by moving through it. Preservation of nanometer scale sharp Bi concentration gradients in the growth direction suggested that very little solid state diffusion of Bi occurred during the anneal. The observed gradient in precipitate number density with distance from the sample surface further supports hypotheses of an enabling role of Ga vacancies in the precipitation process.

Droplet-mediated formation of embedded GaAs nanowires in MBE GaAs(1-x)Bi(x) films.

We have examined the morphology and composition of embedded nanowires that can be formed during molecular beam epitaxy of GaAs(1-x)Bi(x) 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 ∼GaAs embedded in the GaAs(1-x)Bi(x) epitaxial layers. These nanowires extend from a surface droplet to the substrate at a shallow angle of inclination (∼4°). They typically are 4 µm 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 ∼GaAs to GaAs(1-x)Bi(x) appears abrupt. The bottom surfaces of the nanowires appear wavy and the composition change appears to be graded over ∼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 GaAs(1-x)Bi(x) film growth.