Probing Non-Stoichiometry in GaAsBi and GaAsNBi Alloys Using Local-Electrode Atom Probe Tomography

Due to the significant bandgap narrowing induced by incorporation of dilute fractions of N and Bi, dilute nitride-bismide alloys are of interest for optoelectronic devices operating in the near- to mid-infrared range. We recently reported that Bi incorporation into GaAsNBi alloys during molecular-beam epitaxy requires low substrate temperatures which also promote the incorporation of excess As. To minimize excess As incorporation, we determined the substrate temperature needed to eliminate the non-stoichiometric GaAs diffraction peak in x-ray rocking curves (XRC). The resulting [excess As] is below the XRC-detectable limit of ~10¹⁹ cm^-3. To explore excess As incorporation and alloy non-stoichiometry below this limit, we are utilizing local-electrode atom-probe (LEAP) tomography, with LEAP field evaporation conditions optimized for the nearest-stoichiometry of GaAs. Our LEAP data analysis reveals similar V/III ratios for GaAs, GaAsBi, and GaAsNBi, in spite of differences in their effective permittivities that yield variations in field evaporation during LEAP tomography. We discuss the role of arsenic ion species assignments in the LEAP data analysis on apparent stoichiometries reported for a variety of mixed anion and mixed cation III-V semiconductor alloys.