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GaAsBi has attracted research for near-infrared (NIR) optoelectronics because bismuth incorporation causes a far greater band gap reduction per unit strain than indium incorporation. The bismuth atoms induce the formation of many localised states near the valence band maximum, which can take part in radiative transitions and result in a large broadening of the luminescence spectrum. The large linewidths observed in GaAsBi are typically seen as a disadvantage of the material and researchers have focussed on reducing the density of localised states.
Superluminescent light emitting diodes with peak emission centred around 1050 nm are useful for ophthalmology applications such as OCT since these wavelengths are less strongly absorbed by ocular media. In this case, a large LED spectral linewidth leads to an improved axial resolution in OCT, enabling better imaging and subsequent analysis by doctors. Commercial LED based OCT light sources operating at 1050nm rely on emission from both ground and excited states in InGaAs quantum wells and have a linewidth around 70nm. State-of-the-art OCT light sources based on multiple layers of InAs self-assembled quantum dots have achieved linewidths of 160nm.
Existing unoptimised GaAsBi single quantum well structures grown in our group by molecular beam epitaxy with a peak wavelength of 1050nm have a spectral linewidth of around 67 nm, which nearly matches the commercial LEDs used for OCT. This is despite our devices only containing emission from the ground state in the quantum wells. With careful control of the bismuth content and well thickness in future devices, the linewidth of GaAsBi based devices could match or exceed the state-of-the-art for NIR broadband light sources.
In this work we study the applicability of GaAsBi quantum well LEDs as a competitor to InGaAs quantum well and InAs quantum dot LEDs for broadband NIR light sources. We show simulations of LED structures to find the optimum LED design parameters that will give the broadest linewidth centred on 1050nm while retaining an approximately Gaussian emission shape. The growth challenges associated with growing the structures are also discussed.
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