In this work we study the nature of the band gap in GeSn alloys for use in silicon-based lasers. Special attention is paid to Sn-induced band mixing effects. We demonstrate from both experiment and ab-initio theory that the (direct) Γ-character of the GeSn band gap changes continuously with alloy composition and has significant Γ-character even at low (6%) Sn concentrations. The evolution of the Γ-character is due to Sn-induced conduction band mixing effects, in contrast to the sharp indirect-to-direct band gap transition obtained in conventional alloys such as Al1−xGaxAs. Understanding the band mixing effects is critical not only from a fundamental and basic properties viewpoint but also for designing photonic devices with enhanced capabilities utilizing GeSn and related material systems.

Abstract-The efficiency limiting mechanisms in type-I GaInAsSb-based quantum well (QW) lasers, emitting at 2.3 µm, 2.6 µm and 2.9 µm, are investigated. Temperature characterization techniques and measurements under hydrostatic pressure identify an Auger process as the dominant non-radiative recombination mechanism in these devices. The results are supplemented with hydrostatic pressure measurements from three additional type-I GaInAsSb lasers, extending the wavelength range under investigation from 1.85-2.90 μm. Under hydrostatic pressure, contributions from the CHCC and CHSH Auger mechanisms to the threshold current density can be investigated separately. A simple model is used to fit the non-radiative component of the threshold current density, identifying the dominance of the different Auger losses across the wavelength range of operation. The CHCC mechanism is shown to be the dominant non-radiative process at longer wavelengths (> 2 μm). At shorter wavelengths (< 2 μm) the CHSH mechanism begins to dominate the threshold current, as the bandgap approaches resonance with the spin-orbit split-off band.

From a systematic study of the threshold current density as a function of temperature and hydrostatic pressure, in conjunction with theoretical analysis of the gain and threshold carrier density, we have determined the wavelength dependence of the Auger recombination coefficients in InGaAsSb/GaSb quantum well lasers emitting in the 1.7–3.2 µm wavelength range. From hydrostatic pressure measurements, the non-radiative component of threshold currents for individual lasers was determined continuously as a function of wavelength. The results are analysed to determine the Auger coefficients quantitatively. This procedure involves calculating the threshold carrier density based on device properties, optical losses, and estimated Auger contribution to the total threshold current density. We observe a minimum in the Auger rate around 2.1 µm. A strong increase with decreasing mid-infrared wavelength (<2 µm) indicates the prominent role of intervalence Auger transitions to the split-off hole band (CHSH process). Above 2 µm, the increase with wavelength is approximately exponential due to CHCC or CHLH Auger recombination, limiting long wavelength operation. The observed dependence is consistent with that derived by analysing literature values of lasing thresholds for type-I InGaAsSb quantum well diodes. Over the wavelength range considered, the Auger coefficient varies from a minimum of ≲ 1 × 10−16cm 4 s−1 at 2.1 µm to ∼8 × 10−16cm4 s−1 at 3.2 µm.

In this paper, we discuss how the deliberate and controlled introduction of strain can be used to improve the performance of semiconductor lasers. We show how strain-induced modifications of the electronic band structure give rise to significant changes in the valence band of III-V semiconductors which have been used to produce devices with lower threshold currents and higher efficiencies. We furthermore illustrate how the strain limit of semiconductor layers can be overcome by using strain compensation techniques and how this is being widely adopted in lasers based on a number of emerging III-V systems, enhancing device efficiency and output power and extending the wavelength of operation. We show how strained layers are also being used to optimize the performance of mid-infrared lasers through band offset control. Finally, we show how strain may be used to facilitate the production of lasers on silicon through controlling the conduction band valley splitting in group IV semiconductors or through the development of novel direct bandgap III-V systems that may be grown lattice matched to silicon. Such systems are expected to be of significant potential for the future convergence of electronic and photonic devices and highlight the ongoing importance of strain engineering in photonic devices.

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