NTL amplification in a SuperCDMS-style detector fabricated from a University of South Dakota-grown germanium crystal
Neganov-Trofimov-Luke (NTL) amplification enables eV-sub-keV detection thresholds for low-mass dark-matter and coherent elastic neutrino-nucleus scattering experiments. We evaluated two University of South Dakota (USD)-grown germanium crystals fabricated as SuperCDMS-style four-channel athermal-phonon detectors and operated near 10 mK. The crystals had electrically active impurity concentrations near $10^10~\mathrm^-3$ and etch-pit densities of $300$-$7000~\mathrm^-2$. Contact-free biasing produced no resolvable amplification. However, after depositing a 600 nm aluminum bias contact, Detector~2 exhibited clear voltage-dependent NTL gain. The reconstructed channel-B energy increased from approximately 3.9 keV at 0 V to 34.9 keV at 40 V, corresponding to an 8.9-fold enhancement. Statistical centroid uncertainties were below 0.3%, while low-bias fits yielded effective prompt- and NTL-phonon response coefficients of $(6.77\pm0.82)%$ and $(4.77\pm0.20)%$, respectively, excluding systematic uncertainties. These results demonstrate the feasibility of USD-grown germanium for SuperCDMS-style high-voltage phonon detectors and highlight the importance of both crystal quality and contact fabrication in achieving NTL amplification.