2, Air Force Research Laboratory, Albuquerque, New Mexico, United States
Efficient high-performance infrared detection and emission is desired for numerous applications, including missile guidance, gas detection, thermal imaging, and infrared spectroscopy. The incorporation of bismuth in InAs alloys results in a larger bandgap reduction per unit strain than antimony  and provides an efficient means of tuning mid-IR (3 – 5 µm) to long-IR (8 – 12 µm) wavelengths without introducing high levels of strain that can introduce point defects and reduce optical quality. The growth of InAsSbBi and GaAsSbBi quaternary alloys on GaSb substrates permits the designer to independently adjust bandgap and strain by varying the antimony and bismuth mole fractions. GaAsSbBi exhibits a type-II band lineup with InAsSbBi, and heterostructures composed of these bismide quaternary alloys open up the possibility of highly tunable multiple quantum well/superlattice active layers with a type-II band alignment.
The molecular beam epitaxy growth of InAsSbBi and GaAsSbBi on GaSb substrates is investigated, and optimal growth temperatures and group-V fluxes are identified for droplet-free, high-quality crystalline material. A key challenge for the bismide quaternary material systems is identifying growth conditions that yield appreciable bismuth incorporation, defect-free growth, and high optical quality. Since the large Bi atoms tend to surface segregate and not evaporate, they accumulate on the surface and form surface droplets. Therefore bismuth alloys are grown at near-stoichiometric V/III flux ratios of ~ 1.01 and reduced growth temperatures to facilitate incorporation. Both low temperature (280 to 340 °C) and high temperature (430 to 420 °C) growth regimes are explored. The material grown at low temperature exhibits high structural quality and unity bismuth incorporation at 280 °C, but exhibits poor optical quality. On the other hand, the materials grown at high temperature yield greatly improved optical quality and quantum efficiency, but with reduced structural quality due to the formation of Bi droplets. These and other challenges will be presented.
 P. T. Webster, A. J. Shalindar, S. T. Schaefer, and S. R. Johnson, Appl. Phys. Lett. 111, 082104 (2017).