Mid-infrared type-I diode laser design using molecular beam epitaxy

The mid-infrared region of the electromagnetic spectrum, particularly in the wavelength range between 3 and 5 µm, is important for a number of applications in spectroscopy, gas sensing, infrared countermeasures, and communications. Despite these motivations, mid-infrared laser development has lagged behind that of visible and near-infrared technology. This is in part because semiconductor laser sources, while they exist across the mid-infrared, suffer from one or several drawbacks such as high power consumption, high threshold currents, low characteristic temperatures, limited wallplug efficiency, parasitic non-radiative recombination processes, or reduced carrier confinement. The latter impediment, specifically reduced carrier confinement of holes, is endemic to the active regions of GaSb-based type-I quantum-well diode lasers as the optical emission wavelength is extended past 3 µm.

In this work, we present our efforts toward enhancing mid-infrared active regions to extend the emission wavelength of type-I emitters. Through the use of highly-strained, high indium-content quantum wells we demonstrate type-I diode laser operation from aluminum-free active regions up to 3.62 µm, and photoluminescence emission from type-I quantum wells out past 4 µm. Additional studies focused on the effect of using bismuth during the growth of these materials. While increased compressive strain in the quantum well alloy enables greater hole confinement at longer emission wavelengths, it also leads to material roughening and defect formation that restrict the number of and thickness of strained regions that can be grown before material quality irreparably degrades. We observed that by using bismuth as a surfactant during the growth of highly-strained GaIn(As)Sb alloys, material degradation was suppressed as these materials were grown well beyond classical critical thickness limits. We were also able to leverage the epitaxial growth conditions used for highly-strained, high indium-content quantum wells to incorporate dilute amounts of bismuth, up to 3%, into the quantum well materials. The addition of bismuth to the quantum well alloys modifies the valence band to provide additional hole confinement, leading to brighter emitters with up to 34% higher peak intensity. It also resulted in overall lower materials strain without reducing the emission wavelength or performance. This opens a promising approach to overcome strain-related limitations to laser performance and emission wavelength, allowing for device designs with increased numbers of quantum wells and potentially reducing the effects of gain saturation.

An additional path toward improved mid-infrared devices is to switch the quantum well barrier material from GaSb to a lattice-matched AlGaAsSb alloy. This is the same strategy employed for many other mid-infrared type-I diode lasers, albeit for emission wavelengths less than 3.1 µm. By changing the barrier alloy, the quantum well valence band offset is increased, providing stronger hole confinement. Coupling these barriers with the highly-strained, high indium-content quantum wells results in a 3× improvement in peak photoluminescence and a >30% reduction in emission linewidth for quantum wells operating up to 4.2 µm. Using this coupled approach, we propose a laser diode device designed to operate at 4.1 µm.