Determining bismuth content in GaAsBi alloys by energy-dispersive X-ray spectroscopy: a case study with multiple sets of k*-factors for analytical transmission electron microscopy

Measuring the bismuth (Bi) content of ternary gallium arsenide bismuthide (GaAsBi) alloys is important because it sensitively influences their bandgap, and Bi is known to segregate vertically to the surface and sometimes also laterally during growth, so elemental distribution maps need to be quantified. A suitable method is mapping of characteristic X-rays based on energy-dispersive X-ray spectroscopy (EDXS) in a scanning transmission electron microscope (STEM). One of the key problems in this alloy system that there are several overlaps of characteristic X-ray lines from the corresponding elements, namely of As Kα with Bi Lα and of a sum peak of Ga Lα and As Lα with Bi Mα, which no standard solid-state detector could distinguish. Routine quantification procedures thus often fail, exhibiting unacceptably large scatter. Here, an iterative procedure using k*-factors is outlined, leading to improved quantification using sets of different X-ray line pairs to be consistent within better than 1% bismuth coverage of the group V sub-lattice, for a range up to 14%.