Comparative Analysis of λ≈9µm GaAs/AlGaAs Quantum Cascade Lasers with Different Injector Doping

Abstract

An experimental and theoretical comparative analysis of the output characteristics of λ ≈ 9m GaAs/Al0.45Ga0.55As quantum cascade lasers based on single and double phonon resonance depopulation mechanisms were presented. The layer structures were grown with solid source molecular beam epitaxy and consist of 48 or 36 active stages embedded in a symmetrical plasmon enhanced waveguide. From the wafers, ridge waveguide lasers were fabricated by optical lithography and dry etching. The theoretical model is based on a fully non-equilibrium Schrödinger- Poisson self-consistent analysis of the coupled scattering rate and single-temperature energy balance equations, taking all relevant electron-LO phonon, electron-electron and electron-ionised impurity scattering processes into account. Single phonon resonance devices exhibit clear current saturation, simultaneously with a decrease of the optical power. In the moderate doping regime, a quasi-linear dependence of both the threshold and saturation current densities on injector doping, were measured, in a very good agreement with theoretical predictions. Double phonon resonance lasers exhibit ‘saturation’ mechanism evident from their decrease in optical power, but without pronounced current saturation. Previously reported saturation of the ‘maximal’ current under higher injector doping in single phonon resonance lasers, is also observed in the double phonon resonance structure for injector sheet doping above 8x1011cm-2.

Influence of the active region design on output characteristics of GaAs/AlGaAs quantum cascade lasers in a strong magnetic field

“…One can expect, however, that introducing additional QWs in the injector [21] and the reduction of emission energy would both improve the temperature performance of 2LO GaAs QCLs. For the electric field value of 48 kV cm −1 and in the absence of the magnetic field, the active region energies amount to 6 meV, resulting in a lasing wavelength of λ = 9.36 µm, in good agreement with the measured value of λ = 9.3 µm [14]. If this structure is subjected to a magnetic field, the same oscillatory behaviour of the total electron relaxation rate with B is observed (this time for transitions from the ground state of the topmost subband (state (4, 0)) into the three sets of LLs belonging to lower subbands, as illustrated in figure 3(a)).…”

“…Its active region is designed to have a double-LO phonon depopulation mechanism. However, after the very recent experimental realization [14] this structure appeared to offer similar performance as the TQW structure at low temperatures (J th ∼ 3.7 kA cm −2 for a 48 period structure, comparable to J th ∼ 2.9 kA cm −2 for a 36 period TQW structure, at T = 77 K, with the injector sheet doping of 4×10 11 cm −2 ). At higher temperatures the 2LO structure had a somewhat inferior performance, with the maximum operating temperature of 260 K, lower than the TQW.…”

“…In view of that, the active subbands corresponding to the laser transition are now labelled as n = 4 and n = 3. This so-called 'double-LO' structure, presented in figure 1(b), is predicted to have a decreased lower laser level lifetime and consequently a higher degree of population inversion [8,14].…”

Doping effects on electrical and dynamical characteristics of GaAs / AlGaAs quantum cascade laser

“…The doping level in the active region is an important parameter with particular influence on the dynamical working range of QCLs. Until now, very few experimental investigations have been presented including the influence of the injector doping of InP-based [15] and GaAs-based [16] QCL threshold current.…”