2.1 Gallium antimonide (GaSb) laser diodes

The III-V compound semiconductor material system (AlGaIn)(AsSb) constitutes an alternative basis for the realisation of diode lasers in the mid-IR. GaInAsSb is either latticed matched to GaSb substrates or strained and is used as the active layer with a direct bandgap transition covering the λ 1.8–3.4 μm wavelength region. For the barrier and cladding layers, AlGaAsSb is well suited because of its larger bandgap energy and lower refractive index compared to GaInAsSb. Molecular-beam epitaxy (MBE) is the method of choice and most widely used in the wafer growth. The laser structures are grown on (100)-orientated n-doped GaSb substrates but due to the lower demand for GaSb substrates compared to InP the growth cost is higher and substrate quality lower. Conventional fabrication of distributed-feedback (DFB) lasers incorporating buried gratings for longitudinal mode selection is challenging in the GaSb material system due to the difficulty of epitaxial regrowth with the high Al concentrations in the cladding layers. Separately, processing options are quite limited for GaSb due to the cladding material being hygroscopic.
