Quantification of Bi distribution in MOVPE-grown Ga(AsBi) via HAADF STEM

Abstract

The importance of dilute bismide III/V semiconductors increases and their physical properties open up a wide range for applications. Therefore, high quality layers are required, what is difficult to achieve, as these alloys are highly metastable and phase separation can occur. We use HAADF (high angle annular dark field) imaging in aberration-corrected STEM (scanning transmission electron microscopy) to quantify the Bi distribution in MOVPE (metal organic vapor phase epitaxy) grown material at large length scales as well as down to the nanoscale. This is done for different Bi fractions in the solid, which are achieved by changing the MOVPE growth conditions. The composition of the Ga(AsBi) was determined by comparing frozen lattice annular dark field simulations of different Ga(AsBi) supercells with the experimental HAADF STEM images. The derived compositions are in quantitative agreement with results of HR-XRD (high resolution X-ray diffraction) and SIMS (secondary ion mass spectroscopy) of the same samples. We furthermore show a homogeneous Bi distribution for the investigated samples, which contain up to 5% Bi. By separating the group III intensities from the group V intensities in high resolution HAADF STEM images, we can investigate the group V intensity distribution only. Moreover from the statistical evaluation, we conclude that the Bi is distributed homogeneously across the group V lattice positions, confirming the excellent structural quality of the layers. This result is also important for device applications of dilute bismide alloys, as homogeneous layers are a prerequisite for optimizing optoelectronic applications.