Strong band gap reduction in highly mismatched alloy InAlBiAs grown by molecular beam epitaxy

We studied InAlBiAs thin films of less than 300 nm on the InP substrate with Bi composition up to 3.2%. The samples were grown by molecular beam epitaxy at low growth temperature and were shown to be fully strained to the substrate and have smooth interfaces. A bandgap reduction around 47 meV/%Bi was observed via spectroscopic ellipsometry. The valence band anticrossing model was used to fit the experimental data. The bandgaps of InAlBiAs with more than 3% Bi are much lower than the expected values from the model, which could be the result of alloy disorders.