Intervalley scattering in GaAs–AlAs quantum cascade lasers

Abstract

We have investigated the importance of intervalley (Γ–Χ) electron transfer between Γ-point quantum well states and X-point barrier states in GaAs-based quantum cascade lasers with indirect band gap AlAs barriers. A series of samples has been studied in which the energy separation between the coupled injector/upper laser levels and the lowest confined X state in the injection barrier is varied. We demonstrate that for lasing to occur, electron injection into the upper laser level must proceed via Γ states confined below the lowest X state in the injection barrier. The limit this places on the minimum operating wavelength (λ≈8 μm) for the present laser design is overcome by utilizing a double injection barrier to achieve lasing at λ=7.2 μm.

The quantum cascade laser: A versatile high-power semiconductor laser for mid-infrared applications

“…Unfortunately, GaAs QCLs suffer from the fact that AlGaAs is an indirectgap semiconductor above about 45% Al composition. Once the minimum of the X-valley in the injection barrier becomes lower than the ⌫-point upper-laser level, the device performance degrades rapidly [42]. This effect limits the operation wavelength of a GaAS QCL to the 8 mm to 20 mm range.…”

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

“…Experimental results for the importance or otherwise of Γ −X scattering seem confusing: Some authors claim that Γ −X scattering can be appreciable, for example inhibiting lasing in AlGaAs QCLs depending on the relative energies of the X and Γ states [27], and strongly influences the I−V characteristics (depending on the barrier thickness) [28,29]. Other authors, however, claim that intervalley scattering is negligible [30] and plays no significant role in the behaviour of QCLs.…”

Inefficiency of intervalley transfer in narrow InGaAs/AlAsSb quantum wells

“…The reason for this are energy levels stemming from indirect valleys of either well or barrier material that are located below the upper lasing state. This has been shown to inhibit sufficient population inversion and lasing in GaAs/AlGaAs QCLs, since electrons are efficiently transferred to the X-states of the AlGaAs [6]. However, very recently there have been several breakthroughs, and quantum cascade lasers with wavelengths around 3.0 µm were demonstrated in three different material systems: InGaAs/InAlAs [7], InGaAs/AlAsSb [8], and InAs/AlSb [9].…”