Lithium Diffusion in Germanium: Revisiting the Coefficient in High Purity Crystals for Rare-Event Detectors

Lithium-diffused n+ contacts remain the workhorse for high-purity germanium (HPGe) detectors, yet quantitative control of lithium transport and the stability of Li-diffused layers at room temperature still limit uniformity in complex geometries. Although the Li diffusion coefficient in Ge was investigated more than fifty years ago, those classic measurements were performed on non-HP material whose impurity content and compensation levels differ substantially from today's detector-grade Ge. Motivated by this gap, we revisit the diffusion coefficient using USD grown HPGe test coupons to establish an accurate D(T) for modern detector processing relevant to rare-event physics. Using controlled lithium deposition and thermal schedules on well-characterized p-type crystals, we perform depth-resolved Hall measurements to extract carrier density, mobility, and resistivity profiles and to determine the lithium diffusion coefficient as a function of process conditions. The resulting diffusion lengths and coefficients define practical windows for deposition thickness, anneal time, and etch-back margins tailored to HPGe. In parallel, we have initiated a long-term study of the room-temperature evolution of Li-diffused layers, monitored at multi-month intervals, to assess contact stability during handling and assembly. Together these results provide materials-level guidance for reproducible, low-leakage lithium contacts in both planar test vehicles and upcoming large-mass detector concepts—including ring-contact designs—thereby supporting the next generation of rare-event experiments.