InGaAs/AlAsSb Mid-infrared Quantum Cascade Laser

Quantum cascade lasers (QCLs) have proven their capability and versatility with the realization of high performanceQCLs operating in cw with output powers 100≥mW at room temperature for mid-infrared wavelengths. As QCL designs approach short wavelengths, such as 3.3μm, several problems such as electron confinement, intervalley leakage, surface plasmon loss, etc. limit the device efficiency and make room-temperature cw operation difficult to achieve. Although QCLs with wavelengths as short as 3.4–3.8μm have been demonstrated through the use of strain-balanced heterostructures, only pulsed operation at room temperature was realized. Recently, with an improved material design and growth quality, QCLs emitting light atλ~3.5μm showed a cw operation above room temperature. For 3<λ<4μm wavelengths, however, the QCLs are still competing with Sb-based material devices.The In0.53Ga0.47As/Al0.56As0.44Sb material system appears to be the prime candidate for realizing highperformance quantum-cascade lasers (QCLs) with short emission wavelengths.The wavelength limit of a QCL is ultimately set by the conduction-band offset of the material system, and in In0.53Ga0.47As/Al0.56As0.44Sb heterostructures has a value of ~1.6 eV. This value of the band offset should provide enough quantum confinement to achieve laser emission in the 3–5μm atmospheric transmission window, which is of interest due to the potential applications in freespace optical communication links. In0.53Ga0.47As/Al0.56As0.44Sb is lattice matched to InP which is a favorable property since optical confinement can be provided using well-understood InP-based waveguides. Revin et al. have reported intersubband electroluminescence in the rangeλ~3.1–5.3μm from InGaAs/AlAsSb quantumcascade(QC) structures and have observed pulsed mode laser emission atλ~4.3μm up to a maximum temperature of 240 K. Room-temperature electroluminescence atλ~4.5μm from InGaAs/AlAsSb QC structures has recently been reported by Yang et al. who have also demonstrated InGaAs/AlAsSb QCLs emitting at the same wavelength with maximum operating temperatures of up to 400 K in pulsed mode. Thorough investigation of the carrier dynamics in Sb-based QC structures, as well as further improvements in the QCL design, are required to approach the goal of achieving above-room-temperature nearinfrared (λ~3μm) laser emission.We presented a new design of QC structure there,see Fig.4-5 was designed with the aim of improving electron transport through the device and creating sufficient population inversion to achieve room-temperature lasing. In order to facilitate the efficient extraction of electrons from the active region and avoid a bottleneck effect, the ground state was engineered to be part of the injector region miniband and, hence, utilize fast miniband transport for depopulation. Efficient injection into the upper laser level from the injector was ensured by engineering the upper laser level wave function to penetrate into the injector region, effectively becoming the lowest state in the injector miniband. The design slightly differs from the 3QW vertical-transition scheme before and becomes more of a"miniband-to-bound"design. The laser transition takes place between the lowest state in the miniband and a bound state approximately one LO-phonon energy above the next miniband, ensuring that electrons are efficiently injected into the upper laser level and rapidly extracted from the lower laser level into the next period. The lower laser level is confined entirely in the right and central 3QW active region wells This serves to increase the radiative transition element z32 (which is proportional to the local gain g) to 0.9 nm even though the upper laser level in the structure penetrates further into the injector region miniband. At a temperature of 77 K with a field of 130 kV/cm, the structure has a population inversion of 20% compared to the ~2.6% inversion of the structure before at the design field of 128 kV/cm. At 300 K, the population inversion decreases to 8.4% at 130 kV/cm. The maximum total current density increases from 2.2 kA/cm2 to 5.6 kA/cm2 without significantly changing the applied bias or period length and keeping the same sheet doping density. This is due to the lack of an electron bottleneck in the design and improved injection into the upper laser level. The sharp increase in current density at 120 kV/cm is due to the strong coupling between the bottom two miniband states at this value of field.