Monolithically Integrated ε-Ge/In_xGa_1-xAs Quantum Well Laser Design: Experimental and Theoretical Investigation

Here, we have analyzed the electrical and optical phenomenon occurring in a ϵ-Ge/InxGa1-xAs quantum well (QW) laser through self-consistent physical solvers calibrated using in-house experimental results. A separate confinement heterostructure QW design is proposed to enable lasing from tensile strained germanium (ϵ-Ge) in the range of 1.55 μm to 4 μm wavelength as a function of QW thickness and indium (In) composition. Different recombination mechanisms were analyzed as a function of tensile strain in ϵ-Ge QW. Minority carrier lifetime and band alignment are key attributes of a QW laser, which were measured using microwave photoconductive decay and X-ray photoelectron spectroscopy (as a function of In composition), respectively. The transition point of Ge to a direct bandgap material is re-affirmed to be at ϵ = 1.6% (In ∼24%) and the transition from type I to type II for ϵ-Ge/InxGa1-xAs QW is found to be at In ∼55%. Also, the transition to a TM mode dominant laser is identified at In ∼15%. Using a tunable waveguide design to optimize confinement as a function of In composition, strain, wavelength, QW thickness, refractive index, and geometry, the ϵ-Ge QW laser design provided a net material gain of ∼2000 cm-1 and a threshold current density of ∼5 kA/cm2, which is an improvement over existing Ge based lasers. The impact of In composition and QW thickness on the band structure, polarized gain spectra, and various lasing metrics were analyzed to show ϵ-Ge/InGaAs QW lasers as promising for integrated photonics.