Carrier leakage via interface-roughness scattering bridges gap between theoretical and experimental internal efficiencies of quantum cascade lasers

Abstract

When conventionally calculating carrier leakage for state-of-the-art quantum cascade lasers (QCLs), that is, LO-phonon-assisted leakage from the upper laser level via electron thermal excitation to high-energy active-region (AR) states, followed by relaxation to low-energy AR states, ∼18%-wide gaps were recently found between calculated and experimentally measured internal efficiency values. We incorporate elastic scattering [i.e., interface-roughness (IFR) and alloy-disorder scattering] into the carrier-leakage process and consider carrier leakage from key injector states as well. In addition, the expressions for LO-phonon and IFR-triggered carrier-leakage currents take into account the large percentage of thermally excited electrons that return back to initial states via both inelastic and elastic scattering. As a result, we find that the gaps between theoretical and experimental internal efficiency values are essentially bridged. Another finding is that, for the investigated state-of-the-art structures, IFR scattering causes the total carrier leakage to reach values as much as an order of magnitude higher than conventional inelastic scattering-only leakage. The developed formalism opens the way to significantly increase the internal efficiency (i.e., to more than 80%) via IFR-scattering engineering, such that maximum wall-plug efficiencies close to projected fundamental, both-facets values (e.g., 42% at λ = 4.6 μm) can be achieved. By employing this formalism, we reached a 4.6 μm-emitting-QCL preliminary design for suppressing IFR-triggered carrier leakage, which provides an internal efficiency of 86% as well as a projected single-facet wall-plug efficiency value of 36% at a heatsink temperature of 300 K.

High-Power Mid-Infrared (λ∼3-6 μm) Quantum Cascade Lasers

“…Being intersubband-transition devices relying on the quantum-mechanical tunneling of electrons, QCLs are inherently sensitive to interfacial properties such as compositional grading and roughness. The electronic-states lifetimes determine the population inversion [5] and characterize the carrier-leakage mechanisms [6]. The carrier lifetimes themselves are determined by inelastic (i.e., LO-phonon scattering) and elastic scattering (i.e., alloy-disorder (AD) and interfaceroughness [IFR] scattering [5,6]).…”

“…The electronic-states lifetimes determine the population inversion [5] and characterize the carrier-leakage mechanisms [6]. The carrier lifetimes themselves are determined by inelastic (i.e., LO-phonon scattering) and elastic scattering (i.e., alloy-disorder (AD) and interfaceroughness [IFR] scattering [5,6]). While LO-phonon and AD scattering determine the upper-laser (ul) level lifetime, IFR scattering generally determines the lower-laser (ll) level lifetime [6].…”

“…The carrier lifetimes themselves are determined by inelastic (i.e., LO-phonon scattering) and elastic scattering (i.e., alloy-disorder (AD) and interfaceroughness [IFR] scattering [5,6]). While LO-phonon and AD scattering determine the upper-laser (ul) level lifetime, IFR scattering generally determines the lower-laser (ll) level lifetime [6]. In addition, IFR scattering is key in triggering carrier leakage from the ul-level and injector-region states [6].…”

“…While LO-phonon and AD scattering determine the upper-laser (ul) level lifetime, IFR scattering generally determines the lower-laser (ll) level lifetime [6]. In addition, IFR scattering is key in triggering carrier leakage from the ul-level and injector-region states [6]. Finally, interfacial compositional grading leads to deviations of the emission wavelength from design targets based on "square well" approximations.…”

“…Such 4.6-5.0 m-emitting STA QCLs have demonstrated significant advantages in device design offered by the flexibility to control, via metal-organic chemical vapor deposition (MOCVD) crystal growth, the composition (and strain) of each layer within a QCL structure. Such flexibility in crystal growth can be obtained with gas-source molecular beam epitaxy (GS-MBE) and has been used to design linearly-tapered active-region QCLs [2], so called tapered-active (TA) QCLs [7,8] which, however, inherently have lower ΔE values than STA-type device [7]; thus, higher carrier leakage [6].…”