While germanium (Ge) has long been an important player in microelectronics, strain-engineered Ge heterostructures have recently gained increased attention owing to the intriguing possibility of using strain to change the relative positions of the conduction band valleys and, as a result, go from an indirect to a direct gap material. Time-resolved pump-probe spectroscopy is an excellent tool to provide insight into the fundamental interactions and microscopic dynamics of electrons, holes, phonons, and impurities. In this study, using complementary techniques of single-color time-resolved differential reflectivity and two-color time-resolved differential transmission, we gained insights into the effects of interfacial strain on carrier dynamics in several different Ge-based heterostructures. Our structures include high-quality Ge films interfaced with InGaAs and AlAs grown on semi-insulating GaAs as the substrate. In contrast to traditional microwave photoconductivity decay ($\mu -\mathrm{PCD}$) measurements, which typically have a time resolution on the ns scale, we probe samples on a 100 fs scale and can measure decay times approximately three orders of magnitude faster than previous $\mu -\mathrm{PCD}$ measurements. Therefore, our results are sensitive to the effects of both carrier recombination as well as electron intervalley and hole interband scattering.

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• Carrier dynamics
• Electronic structure
• Growth
• Narrow band gap systems
• Semiconductors
• Band structure methods
• Drude model
• Free-electron model
• Photoexcitation
• Strain engineering
• Time-resolved infrared spectroscopy