Spectroscopy of GaAs∕AlGaAs quantum-cascade lasers using hydrostatic pressure

Abstract

The authors have measured the output spectrum and the threshold current in 9.2 m wavelength GaAs/ Al 0.45 Ga 0.55 As quantum-cascade lasers at 115 K as a function of hydrostatic pressure up to 7.3 kbars. By extrapolation back to ambient pressure, thermally activated escape of electrons from the upper lasing state up to delocalized states of the ⌫ valley is shown to be an important contribution to the threshold current. On the other hand leakage into the X valley, although it has a very high density of states and is nearly degenerate with the ⌫ band edge in the barrier, is insignificant at ambient pressure. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2364159͔Since the realization of the GaAs-based quantumcascade laser ͑QCL͒, 1 an impressive extension of the attainable infrared frequency range has been achieved. It can operate at wavelengths as long as 160 m, 2 or in dual frequency regime up to 215 m. 3 The design of GaAs/ AlGaAs QCLs can be made very flexible by varying the Al content due to naturally occurring near lattice matched material system across the full range of Al contents. Hence, following the terahertz emitting QCL, 4 several laser designs based on 15% Al content in the barriers were presented, approaching high temperature pulsed operation 5 or close to liquid nitrogen temperature cw operation. 6 GaAs-based QCLs emitting in the midinfrared ͑MIR͒ spectral region have so far used Al contents of 33%, 1 45%, 7-9 and 100%, 10-12 respectively, and have been subjected to a high external magnetic field. 13 Pulsed room temperature operation has been reported only for designs with 45% Al content 7-9 and for a hybrid GaAs/ InAs/ AlAs design. 11 Achieving cw operation in MIR GaAs-based QCLs is a very challenging task due to the relatively high threshold current densities ͑I th ͒. Influence of the injector doping, 14 the lattice temperature, 15 and carrier escape via weakly localized ⌫ states 16 were attributed as major limiting factors, for the high temperature operation and attainable gain, that determine the increase of I th and dynamic working range of ϳ9 m GaAs-based QCLs. Nevertheless, cw operation has been reported 10,17 with operating temperatures up to 150 K. However, the output characteristics are still inferior compared to InP MIR QCL. 18 Apart from ⌫-band related scattering, particularly in AlAs material, the importance of ⌫-X transport processes was examined in detail. 19,20 Recently, in the Sb-based QCLs the ⌫-X intervalley scattering is attributed as a possible limiting factor for the emission shorter than ϳ4 m. 21 The temperature sensitivity of the GaAs/ Al x Ga 1−x As QCL's I th decreases as the Al content increases from 33% to 45% in the barrier region. This is attributed to the increase in the activation energy for thermal escape 12 of electrons from the quantum well into the upper ⌫ miniband of the injector, which reduces its magnitude. However, at high Al concentrations the participation of lateral X and L valleys is believed to be an important factor deteriorating the device perfor...

Investigations of carrier scattering into L‐valley in λ = 3.5 µm InGaAs/AlAs(Sb) quantum cascade lasers using high hydrostatic pressure

“…2, T 0 is measured to be $264 K for T < 220 K. However, it drops to $151 K for T > 220 K. The measured total threshold current of the lasers is the sum of all the recombination current contributions, in this case constituting the phonon and leakage currents (the spontaneous emission current is negligible as it is a much slower process) [8,10]. The different recombination processes have different temperature dependences, hence the observed decrease in T 0 indicates the activation of a loss process at higher temperature related to carrier leakage.…”

“…Hydrostatic pressure shifts the G-, L-, and X-bands at different rates, which effectively changes the separation between the physica different minima [9]. This enables a better understanding of indirect inter-valley scattering and potentially allows for design optimization for improved device performance [8,10]. For the InAs/AlSb QCLs studied in Ref.…”

“…The threshold current of laser devices in the absence of direct heterojunction carrier leakage is determined by either radiative or non-radiative recombination processes [7,8]. From the temperature dependence of the threshold current we can obtain some indication of the presence and significance of recombination losses in the devices.…”

The effect of hydrostatic pressure on the operation of quantum cascade lasers

“…Compared to phonon-scattering, the radiative lifetime is relatively long, meaning that the current component related to radiative transitions is very small in QCLs at threshold and hence is neglected in this analysis. The dashed lines (I ph ) in Figure 2 describe the pressure dependence of inter-subband carrier relaxation via longitudinal polar optical phonon scattering calculated using the Frölich Hamiltonian approximation [7]. The theoretical pressure variation of the leakage current is also plotted in the figure for different temperatures and was derived using the temperature dependence of the leakage current at threshold which may be calculated from Eq.…”

Intervalley Scattering and the Role of Indirect Band Gap AlAs Barriers: Application to GaAs/AlGaAs Quantum Cascade Lasers

“…Other authors, however, claim that intervalley scattering is negligible [30] and plays no significant role in the behaviour of QCLs.…”