MBE growth of GaSb-based alloys for mid-infrared semiconductor diode lasers
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Mid-infrared lasers in the 3-5 µm range are important for wide variety of applications including trace gas sensing, infrared counter measures, free space optical communications, etc. GaSb-based type-I quantum well (QW) diode lasers are an attractive choice due to their relatively simple design and growth tolerances, as compared with quantum cascade lasers and interband cascade lasers. Excellent diode lasers have been demonstrated for wavelengths up to ~3.0 µm, employing GaInAsSb/AlGaAsSb QW active regions. But, device performance tends to degrade at longer wavelengths, due to Auger recombination and decreasing QW valence band offsets. In this work we look into the feasibility of using highly strained GaInAsSb/GaSb QWs as active regions for diode lasers operating at wavelengths beyond 3.0 µm. Heavy strain in the QW can improve valence band offset and also increase the splitting between the heavy and light hole bands which can help minimize Auger recombination. Through optimized molecular beam epitaxy (MBE) growth conditions we were able to incorporate up to 2.45 % compressive strain in these QWs enabling laser operation up to 3.4 µm at room temperature. An alternate path to extend the emission wavelength is to incorporate dilute quantities of nitrogen into the QW. Incorporating dilute quantities of substitutional nitrogen into traditional III-V’s strongly reduces the bandgap of the alloy. The advantage for the case of GaSb based dilute-nitrides is that the bandgap reduction is almost exclusively due to the lowering of the conduction band leaving the valence band offsets unaffected; thus providing a path to mitigating hole leakage while extending the emission wavelength. Although GaSb-based dilute-nitrides are a potentially elegant solution for extending the operating wavelength of GaSb-based type-I QW diode lasers, the luminescence efficiency of this material system has been relatively poor. This is most likely due to the presence of a high concentration of point defects, like nitrogen substitutional clusters. Through careful optimization of MBE growth conditions and post growth annealing, we demonstrate improved luminescence efficiency. With further optimization this material system can potentially extend the emission wavelength of GaSb-based type-I QW diode lasers even further into the mid-infrared spectrum.