Analysis of Bi Distribution in Epitaxial GaAsBi by aberration-corrected HAADF-STEM

The Bi content in GaAs/GaAs1 − xBix/GaAs heterostructures grown by molecular beam epitaxy at a substrate temperature close to 340 °C is investigated by aberration-corrected high-angle annular dark-field techniques. The analysis at low magnification of high-angle annular dark-field scanning transmission electron microscopy images, corroborated by EDX analysis, revealed planar defect-free layers and a non-homogeneous Bi distribution at the interfaces and within the GaAsBi layer. At high magnification, the qHAADF analysis confirmed the inhomogeneous distribution and Bi segregation at the GaAsBi/GaAs interface at low Bi flux and distorted dumbbell shape in areas with higher Bi content. At higher Bi flux, the size of the Bi gathering increases leading to roughly equiaxial Bi-rich particles faceted along zinc blende {111} and uniformly dispersed around the matrix and interfaces. FFT analysis checks the coexistence of two phases in some clusters: a rhombohedral pure Bi (rh-Bi) one surrounded by a zinc blende GaAs1 − xBix matrix. Clusters may be affecting to the local lattice relaxation and leading to a partially relaxed GaAsBi/GaAs system, in good agreement with XRD analysis

Molecular Beam Epitaxy Growth and Properties of GaAsBi and AlAsBi

GaAsBi alloys have been extensively studied in recent years, and the highest Bi concentration yet reached has been 22 %. Many photoelectric devices using this material have been produced, such as quantum well lasers, LEDs, solar cells, etc. The Bi incorporated into AlAs is expected to change the bandgap from indirect to direct. There are only a few theoretical reports on AlAsBi, however, experimental research results are seldom reported. In this chapter, we review the molecular beam epitaxy of GaAsBi and analyze the growth mechanism. Besides, we present the synthesis of AlAsBi by molecular beam epitaxy. The growth temperature, As/Ga flux ratio, Bi flux and the growth rate all have great influence on the Bi incorporation. Bismuth atoms play a surfactant role under As-rich conditions and an anti-surfactant role under Ga-rich conditions. Droplets tend to be formed on the surface of GaAsBi alloys due to the atomic size mismatch between Bi atoms and As atoms. The high-angle annular dark-field mode of scanning transmission electron microscopy images confirm Bi atoms cluster exsiting in GaAsBi films. Furthermore, we show the optical properties of GaAsBi and discuss the localized states induced by Bi. The photoluminescence wavelength of GaAsBi redshifts with increasing Bi concentration. The bandgap of GaAsBi is insensitive to temperature, which is important for developing un-cooled lasers. We discuss the influence of Bi incorporation on the electric and transport properties of GaAsBi. The types of dominant point defects induced by Bi incorporation are analyzed. The measurement results of the electron effective mass demonstrate that Bi incorporation not only changes the valence band but also has non-negligible influence on the conduction band in GaAsBi. For AlAsBi, we review the theoretical simulations and present the molecular beam epitaxy growth without substrate rotaion to investigate the influence of\ua0As/Al flux raio and the Bi flux on\ua0Bi incorporation