Abstract
We report density-dependent effective hole mass measurements in undoped germanium quantum wells. We are able to span a large range of densities (2.0-11×10¹¹ cm⁻²) in top-gated field effect transistors by positioning the strained buried Ge channel at different depths of 12 and 44 nm from the surface. From the thermal damping of the amplitude of Shubnikov-de Haas oscillations, we measure a light mass of 0.061m_e at a density of 2.2×10¹¹ cm⁻². We confirm the theoretically predicted dependence of increasing mass with density and by extrapolation we find an effective mass of ∼0.05m_e at zero density, the lightest effective mass for a planar platform that demonstrated spin qubits in quantum dots.