Fabrication and Characterization of λ ≈ 14μm GaAs/AlGaAs Quantum Cascade Lasers

Abstract

Quantum cascade lasers (QCLs) are semiconductor lasers that emit light from the mid-infrared (mid-IR) to the terahertz region through intersubband transitions. Mid-IR QCLs with wavelengths ranging from 5 − 12μm are currently commercially available and robust for use. However, for long wavelength mid-IR QCLs between 14− 20μm, there are currently no commercially available solutions, and they require more research and development for room temperature continuous wave operation. This thesis introduces a novel intersubband structure design for a λ ≈ 14μm GaAs/AlGaAs QCL on a GaAs-substrate. Optimizing the fabrication recipe of the λ ≈ 14μm QCLs improved the mesa resistance and adhesion. The best mesa resistance was obtained with Ni(50A)/Ge(334A)/Au(666A) annealed at 400◦C with Ti(350A)/Au(3000A) on top. The specific mesa resistance was measured to be 3 × 10^−3kΩcm^2. The best adhesion was achieved with Ti(350A)/Au(2500A) as the electric contact. Some experimental challenges, such as the thinning of metal on the sidewalls of the ridges due to the uneven deposition of the sputterer and narrow dielectric windows, were encountered. Adjustments have been made in the fabrication process to address these problems. Following a successful ridge waveguide fabrication using the developed recipe, LIV characterization of the 14 μm QCL was performed. Lasing behavior was observed with the ridge laser with a cavity length of 2.25 mm and ridge width of 45 μm. The threshold current density was around 5 kA/cm^2 with a roll-over current and voltage at 9.5 kA/cm^2 and 50V, respectively, at 80K.