Browsing by Author "Johnston, Steve"

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    Interplay between strain and thickness on effective carrier lifetime of buffer mediated epitaxial germanium probed by photoconductance decay technique
    Bhattacharya, Shuvodip; Johnston, Steve; Datta, Suman; Hudait, Mantu K. (American Chemical Society, 2023-05-19)
    We report contactless effective minority carrier lifetime of epitaxially grown unstrained and in-plane <110> biaxially tensile-strained (001) germanium (ϵ-Ge) epilayers measured using microwave-reflectance photoconductance decay measurements. Strained Ge epilayers were grown using InxGa1-xAs linearly graded buffers on (001) GaAs substrates. Using homogeneous excitation of unstrained Ge epilayers, thickness-dependent separation of minority carrier lifetime components under low injection conditions yielded a bulk lifetime of 114 ± 2 ns and low surface recombination velocity of 21.3 ± 0.04 cm/s. More notably, an effective minority carrier lifetime of >100 ns obtained from sub-50 nm 1.6% tensile-strained Ge epilayers showed no degradation relative to the unstrained counterpart. Detailed material characterization using X-ray diffractometry revealed successful strain transfer of 0.61 and 0.89% to the Ge epilayers via InxGa1-xAs metamorphic buffers and confirms pseudomorphic growth. Lattice coherence observed at the ϵ-Ge epilayer and InxGa1-xAs buffer heterointerfaces via transmission electron microscopy substantiates the prime material quality achieved. The relatively high carrier lifetimes achieved are an indicator of excellent material quality and provide a path forward to realize low-threshold Ge laser sources.
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    Monolithically Integrated ε-Ge/InxGa1-xAs Quantum Well Laser Design: Experimental and Theoretical Investigation
    Joshi, Rutwik; Johnston, Steve; Karthikeyan, Sengunthar; Lester, Luke F.; Hudait, Mantu K. (IEEE, 2023-10-10)
    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.