Influence of leakage current on temperature performance of GaAs/AlGaAs quantum cascade lasers (original) (raw)
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GaAs Quantum Cascade Lasers: Fundamentals and Performance
Les lasers : applications aux technologies de l'information et au traitement des matériaux, 2002
Quantum engineering of the electronic energy levels and tailoring of the wavefunctions in GaAs/Al x Ga 1−x As heterostructures allows to obtain the correct matrix elements and scattering rates which enable laser action between subbands. This article reviews the state-of-the-art of GaAs based quantum cascade lasers. These new light sources operate, with peak power in excess of 1 W at 77 K, in the 8-13 µm wavelength region, greatly extending the wavelength range of GaAs optoelectronic technology. Waveguides are based on an Al-free design with an appropriate doping profile which allows optical confinement, low losses and optimal heat dissipation. Finally, new active region designs aiming to improve the laser temperature dependence are discussed. Recent results on these devices confirm that the ratio between the conduction band discontinuity and the photon energy (∆E c /E laser) is the dominant parameter controlling their thermal characteristic. The maximum operating temperature of these devices is 280 K for lasers with emission wavelength at ∼11 µm.
Self-consistent scattering theory of transport and output characteristics of quantum cascade lasers
Journal of Applied Physics, 2002
Electron transport in GaAs/AlGaAs quantum cascade lasers operating in midinfrared is calculated self-consistently using an intersubband scattering model. Subband populations and carrier transition rates are calculated and all relevant electron-LO phonon and electron-electron scatterings between injector/collector, active region, and continuum resonance levels are included. The calculated carrier lifetimes and subband populations are then used to evaluate scattering current densities, injection efficiencies, and carrier backflow into the active region for a range of operating temperatures. From the calculated modal gain versus total current density dependencies the output characteristics, in particular the gain coefficient and threshold current, are extracted. For the original GaAs/Al 0.33 Ga 0.67 As quantum cascade structure ͓C. Sirtori et al., Appl. Phys. Lett. 73, 3486 ͑1998͔͒ these are found to be gϭ11.3 cm/kA and J th ϭ6Ϯ1 kA/cm 2 ͑at Tϭ77 K͒, and gϭ7.9 cm/kA and J th ϭ10Ϯ1 kA/cm 2 ͑at Tϭ200 K͒, in good agreement with the experiment. Calculations shows that threshold cannot be achieved in this structure at Tϭ300 K, due to the small gain coefficient and the gain saturation effect, also in agreement with experimental findings. The model thus promises to be a powerful tool for the prediction and optimization of new, improved quantum cascade structures.
Influence of doping density on electron dynamics in GaAs∕AlGaAs quantum cascade lasers
Journal of Applied Physics, 2006
A detailed theoretical and experimental study of the influence of injector doping on the output characteristics and electron heating in midinfrared GaAs/ AlGaAs quantum cascade lasers is presented. The employed theoretical model of electron transport was based on a fully nonequilibrium self-consistent Schrödinger-Poisson analysis of the scattering rate and energy balance equations. Three different devices with injector sheet doping densities in the range of ͑4 -6.5͒ ϫ 10 11 cm -2 have been grown and experimentally characterized. Optimized arsenic fluxes were used for the growth, resulting in high-quality layers with smooth surfaces and low defect densities. A quasilinear increase of the threshold current with sheet injector doping has been observed both theoretically and experimentally. The experimental and calculated current-voltage characteristics are in a very good agreement. A decrease of the calculated coupling constant of average electron temperature versus the pumping current with doping level was found.
Self-consistent thermal simulation of GaAs/Al0.45Ga0.55As quantum cascade lasers
Journal of Computational Electronics, 2012
This paper presents a self-consistent thermal model for quantum cascade lasers (QCLs) that takes into account the nonuniform heat generation distribution in the active region as well as the temperature dependences of the heat generation rate and thermal conductivity. The model extracts the heat generation rate from the electron-optical phonon scattering recorded during the ensemble Monte Carlo (EMC) simulation of electron transport in a single QCL stage at different temperatures. The extracted heat generation rate, in conjunction with temperature-dependent thermal conductivities, enables us to solve the nonlinear heat diffusion equation in a self-consistent manner. The model is used to investigate the cross-plane temperature distribution throughout a 9.4 µm infrared GaAs-based QCL. The nonlinear effects stemming from the temperature dependence of thermal conductivity and the heat generation rate are studied. Finally, the accuracy of using the equivalent uniform heat source with the total power obtained from experiments to model the thermal performance of QCLs is evaluated and discussed.
Thermally activated leakage current in high-performance short-wavelength quantum cascade lasers
Journal of Applied Physics, 2013
The threshold condition for a 4-level quantum cascade laser (QCL)-active region is formulated to include thermally activated leakage of charge carriers from active region confined states into states with higher energy. A method is described and demonstrated to extract the associated thermal escape current density from measurements at laser threshold. This current is modeled by including both the temperature dependent subband-distribution of charge carriers and longitudinal optical-phonon probability. The method is used to analyze the thermally activated leakage of charge carriers in two short-wavelength strain-compensated InGaAs/InAlAs QCL-structures. The energies of the higher-lying states extracted from the model are in good agreement with the values calculated numerically within the effective-mass approximation. The estimated scattering time for the thermal activation process agrees with the expected value as well. Our approach offers a straightforward and accurate method to analyze and troubleshoot thermally activated leakage in new QCL-active region designs. V C 2013 American Institute of Physics.
2016
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X -valley leakage in GaAs-based midinfrared quantum cascade lasers: A Monte Carlo study
Journal of Applied Physics, 2007
We present a detailed Monte Carlo simulation of electron transport incorporating both Γ- and X-valley states in GaAs-based quantum cascade lasers (QCLs). Γ states are calculated using the K⋅p method, while X states are obtained within the effective mass framework. All the relevant electron-phonon, electron-electron, and intervalley scattering mechanisms are included. We investigate the X-valley leakage in two equivalent-design GaAs/AlGaAs QCLs with 33% and 45% Al-barrier compositions. We find that the dominant X-valley leakage path in both laser structures is through interstage X→X intervalley scattering, leading to a parallel leakage current JX. The magnitude of JX depends on the temperature and occupation of the X subbands, which are populated primarily by the same-stage scattering from the Γ-continuum (Γc) states. At 77 K, JX is small up to very high fields in both QCLs. However, at room temperature the 33% QCL shows a much higher JX than the 45% QCL even at low fields. The reaso...
X -valley leakage in GaAs∕AlGaAs quantum cascade lasers
Applied Physics Letters, 2006
The authors present a Monte Carlo simulation of GaAs∕Al0.33Ga0.67As and GaAs∕Al0.45Ga0.55As quantum cascade lasers (QCLs) that incorporates both Γ- and X-valley transport. The dominant X-valley leakage path in both lasers is interstage X→X scattering. The leakage current is much higher in the 33%-Al QCL, as strong coupling of its weakly localized Γ-valley states to the next-stage continuum Γ states (Γc), followed by strong same-stage Γc→X scattering, ensures high X-valley population and subsequent high X→X leakage current at 300K, even at low fields. Very good agreement with experiment is obtained at both cryogenic and room temperatures.
Optical Engineering, 2010
The equations for threshold-current density J th and external differential quantum efficiency η d of quantum cascade lasers (QCLs) are modified to include electron leakage and the electron-backfilling term corrected to take into account hot electrons in the injector. We show that by introducing both deep quantum wells and tall barriers in the active regions of 4.8-μm-emitting QCLs, and by tapering the conduction-band edge of both injector and extractor regions, one can significantly reduce electron leakage. The characteristic temperatures for J th and η d , denoted by T 0 and T 1 , respectively, are found to reach values as high as 278 and 285 K over the 20 to 90 • C temperature range, which means that J th and η d display ≈ 2.3 slower variation than conventional 4.5-to 5.0-μm-emitting, high-performance QCLs over the same temperature range. A model for the thermal excitation of hot injected electrons from the upper laser level to the upper active-region energy states, wherefrom some relax to the lower active-region states and some are scattered to the upper miniband, is used to estimate the leakage current. Estimated T 0 values are in good agreement with experiment for both conventional QCLs and deep-well QCLs. The T 1 values are justified by increases in both electron leakage and waveguide loss with temperature.
Mechanisms of temperature performance degradation in terahertz quantum-cascade lasers
Applied Physics Letters, 2003
Electron transport in a terahertz GaAs/AlGaAs quantum cascade laser is calculated using a full self-consistent intersubband scattering model. Subband populations, carrier transition rates and current densities are calculated and all relevant intra-and inter-period electron-electron and electron-LO phonon scattering mechanisms are included. Employing an energy balance equation which includes the influence of both electron-LO phonon and electron-electron scattering, the method also enables evaluation of the average electron temperature of the non-equilibrium carrier distributions in the device. In particular, the influence of the lattice temperature on the degradation of population inversion and device performance is investigated. The threshold currents, electric field-current density characteristics, and temperature dependent performance are in good qualitative and quantitative agreement with measurement in a recent experimental realization [Köhler et al, Nature, 417, 156 (2002)]. Calculations indicate that an important mechanism limiting its operating temperature is the increase of leakage current from the injector to low levels in the active region, and this feature should be improved in future designs.