Auger recombination is known to be a significant non-radiative process limiting near- and mid-infrared quantum well lasers. The one-dimensional confinement of quantum wells and small band offsets (relative to the bandgap) permits two fundamentally different categories of Auger mechanisms to operate. These mechanisms may be identified as either activated or thresholdless in nature. In this work, we investigate the nature of the dominant Auger mechanism in mid-infrared emitting quantum wells by characterizing a range of type-I InGaAsSb quantum well lasers operating within the 2 - 3 μm wavelength range. The temperature dependence of both the threshold current density and integrated spontaneous emission reveal that the threshold current is dominated by radiative recombination up to a break-point temperature (occurring below 200 K). Beyond the break point temperature, the exponential dependence of the threshold current increases rapidly. The deterioration in the stability of lasing threshold indicates that a thermally activated Auger process is dominant in all devices and is sensitive to the population of heavy-holes in the quantum wells.

Type-II “W”-lasers have made an important contribution to the development of mid-infrared laser diodes. In this paper, we show that a similar approach can yield high performance lasers in the optical communications wavelength range. (GaIn)As/Ga(AsSb) type-II “W” structures emitting at 1255nm have been realised on a GaAs substrate and exhibit low room temperature threshold current densities of 200-300 A cm-2, pulsed output powers exceeding 1 W for 100μm wide stripes, and a characteristic temperature T0 ≈90K around room temperature. Optical gain studies indicate a high modal gain around 15-23 cm-1 at 200-300 Acm-2 and low optical losses of 8±3 cm. Analysis of the spontaneous emission indicates that at room temperature, up to 24% of the threshold current is due to radiative recombination, with the remaining current due to other thermally activated non-radiative processes. The observed decrease in differential quantum efficiency with increasing temperature suggests that this is primarily due to a carrier leakage process. The impact of these processes is discussed in terms of the potential for further device optimisation. Our results present strong figures of merit for near- infrared type-II laser diodes and indicate significant potential for their applications in optical communications. Published in Journal of Physics D: Applied Physics, June 2021, https://doi.org/10.1088/1361-6463/ac0b72.

Type-II GaInAs/GaAsSb "W" quantum well heterostructures on GaAs show strong potential for temperature-stable data communications lasers. Devices emitting at 1255 nm show promising lasing characteristics including room-temperature threshold current densities, Jth < 300 A/cm^2, pulsed output powers >1 W, and a reduced wavelength temperature dependence of 0.31 nm/C. Temperature- and pressure-dependent characterisation techniques are used to determine the roles of radiative and non-radiative recombination. Analysis of these characteristics suggest a reduced influence of non-radiative recombination on the thermal stability of type-II "W"-lasers compared to type-I devices, as will be discussed along with recommendations for future device development.

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. A strong increase with decreasing mid-infrared wavelength (< 2 µm) indicates the prominent role of intervalence Auger transitions to the split-off hole band. Above 2 µm, the increase with wavelength is approximately exponential due to CHCC or CHLH Auger recombination. 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 1x10‑16cm4s-1 at 2.1µm to ~8x10‑16cm4s-1 at 3.2µm.

Type-II (GaIn)As/Ga(AsSb) "W"-lasers offer the possibility to develop efficient and thermally stable near-infrared lasers. In this work, we investigate the temperature- and injection-dependent properties of "W"-lasers operating between 1200-1260 nm and use this to quantify the influence of radiative and non-radiative recombination on device performance.

Although photovoltaic (PV) devices are rated at standard testing conditions (STCs), these STCs are rarely met, either outdoors, or when PV devices are used for indoor applications. Thus, it is beneficial to fully characterise the linearity of PV devices with respect to irradiance. Moreover, high accuracy linearity measurements are essential for reference cells (RCs), as they ensure the precision of the measured irradiance. This work presents a new technique for linearity measurements of PV devices based on digital light processing (DLP). The proposed system uses a digital micromirror device coupled with projection optics and a high-power LED array. By creating a series of patterns projected on the device under test with a specific number of bright and dark pixels, linearity measurements can be implemented through a spatial dithering process. Since the dithering process is mechanical, it is expected that any spectral variability effects for the different dithering levels or electrical non-linearities of the light source are avoided. The developed system can provide thousands of measurement points on the linearity curve of a device in seconds, which is impossible with any other currently established methods. Measurements of RCs with known linearity curves are acquired and are validated by conventional methods. Results demonstrate that the DLP method provides equal measurement accuracy compared to conventional systems, but at significantly higher resolution (points on the linearity curve) and order of magnitude higher measurement speed.

Auger recombination is known to be a significant non-radiative channel in near- and mid-infrared quantum well emitters [1] . As a result, the threshold current density of semiconductor lasers increases substantially with increasing wavelength and temperature, impacting the overall efficiency of a laser-based optoelectronic system. In an Auger process the energy released from an electron-hole recombination is transferred to a third carrier. The one-dimensional confinement of quantum wells and small band offsets (relative to the bandgap) in infrared type-I quantum well geometries permit two fundamentally different categories of Auger mechanisms to operate. We refer to these as activated or thresholdless . In a thresholdless process the absence of an activation energy means that the Auger coefficient of an Auger process varies only weakly with temperature. This contrasts with an activated process where the kinematic threshold causes the Auger coefficient to increase approximately exponentially with temperature. There is no clear consensus in the literature on the nature of Auger recombination in type-I quantum wells, and both have variously been invoked to explain the temperature and wavelength dependence of near- and mid-infrared quantum well lasers [2] , [3] .

Epitaxial growth of III-V lasers on CMOS-compatible Si(001) is a key component in the realisation of photonic integrated circuits. In this work we investigate the mechanisms underpinning the performance of first generation GaSb-based devices grown epitaxially on industry-standard Si.

This chapter reviews the fundamental physics and associated limitations of semiconductor lasers operating across the mid-infrared (MIR) range of 2–20 μm. Using a combination of temperature and hydrostatic pressure dependence techniques, we have shown how short-wavelength 1.9–3.7-μm type I quantum well interband devices are dominated by Auger recombination, the nature and type of which depends upon the wavelength. In the intermediate wavelength range (3–7 μm), interband type II "W" lasers offer significant performance improvements despite the reduced wavefunction overlap. Quantum cascade lasers dominate at the longest MIR wavelengths with performance limited by intervalley scattering and leakage processes.

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 Al 1−x Ga x As. 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.

Type-I quantum well (QW) lasers based on the GaSb material system show attractive characteristics in the mid-infrared [1]. However, as the wavelength (λ) increases in the range of 2–4 μm their performance begins to deteriorate due to increasing Auger recombination [2]. In the Auger process, the energy released from an electron-hole recombination is transferred to a third carrier. In order to develop strategies to suppress Auger recombination, it is crucial to understand the magnitude and nature of the dominant Auger recombination pathway, and their dependencies on the operating λ and temperature (T).

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.

We characterize the performance of 1.2 and 1.3 μm `W' lasers using temperature and high-pressure. Both devices exhibit a high characteristic temperature. Pressure data indicates a reduced influence of Auger recombination compared to conventional type-I devices operating in the same wavelength range.

To harness the advanced fabrication capabilities and high yields of the electronics industry for photonics, monolithic growth and CMOS compatibility are required. One promising candidate which fulfils these conditions is GeSn. Introducing Sn lowers the energy of the direct Γ valley relative to the indirect L valley. The movement of the conduction band valleys with Sn concentration is critical for the design of efficient devices; however, a large discrepancy exists in the literature for the Sn concentration at which GeSn becomes a direct band gap. We investigate the bandgap character of GeSn using hydrostatic pressure which reversibility modifies the bandstructure. In this work we determine the movement of the band-edge under pressure using photocurrent measurements. For a pure Ge sample, the movement of the band-edge is dominated by the indirect L valley with a measured pressure coefficient of 4.26±0.05 meV/kbar. With increasing Sn concentration there is evidence of band mixing effects, with values of 9.4±0.3 meV/kbar and 11.1±0.2 meV/kbar measured for 6% and 8% Sn samples. For a 10% Sn sample the pressure coefficient of 13±0.5 meV/kbar is close to the movement of the direct bandgap of Ge, indicating predominately direct Γ-like character for this GeSn alloy. This further suggests a gradual transition from indirect to direct like behaviour in the alloy as also evidenced from theoretical calculations. The implications of this in terms of optimising device performance will be discussed in further detail at the conference.

PURPOSE:To demonstrate a method of simulating mammography images of the CDMAM phantom and to investigate the coefficient of variation (CoV) in the threshold gold thickness (tT) measurements associated with use of the phantom.METHODS:The noise and sharpness of Hologic Dimensions and GE Essential mammography systems were characterized to provide data for the simulation. The simulation method was validated by comparing the tT results of real and simulated images of the CDMAM phantom for three different doses and the two systems. The detection matrices produced from each of 64 images using CDCOM software were randomly resampled to create 512 sets of 8, 16 and 32 images to estimate the CoV of tT. Sets of simulated images for a range of doses were used to estimate the CoVs for a range of diameters and threshold thicknesses.RESULTS:No significant differences were found for tT or the CoV between real and simulated CDMAM images. It was shown that resampling from 256 images was required for estimating the CoV. The CoV was around 4% using 16 images for most of the phantom but is over double that for details near the edge of the phantom.CONCLUSIONS:We have demonstrated a method to simulate images of the CDMAM phantom for different systems at a range of doses. We provide data for calculating uncertainties in tT. Any future review of the European guidelines should take into consideration the calculated uncertainties for the 0.1mm detail.

Type-I GaInAsSb lasers, emitting between 2–3 µm are investigated using temperature and high pressure characterization techniques. A model of the Auger processes is used to fit the non-radiative component of the threshold current at room temperature, identifying the dominance of different Auger losses across the wavelength range of operation.