  The direct detection of MeV-scale dark matter (DM) particles hinges on
achieving an exceptionally low energy detection threshold. Germanium (Ge)
detectors, meticulously tailored with precise impurity compositions, hold the
potential to enhance sensitivity to energy levels below the sub-electronvolt
(sub-eV) range. This study explores the behavior of residual impurities
inherent to Ge detectors at helium temperatures, unveiling a captivating
freeze-out phenomenon leading to the formation of excited localized states
known as dipole states. Using compelling evidence from relative capacitance
measurements obtained from two detectors, we elucidate the transition of
impurity atoms from free charge states to these dipole states as the
temperature drops from 11 K to 6.5 K. Our investigation comprehensively covers
the intricate formation of these dipole states in both n-type and p-type
impurities. Furthermore, we shed light on the electric field generated by these
dipole states, revealing their ability to trap charges and facilitate the
creation of cluster dipole states. Confirming findings from previous
measurements, we establish that these excited dipole states exhibit a binding
energy of less than 10 meV, offering an exceptionally low detection threshold
for MeV-scale DM. Building upon this concept, we propose the development of a
1-kg Ge detector with internal charge amplification, an innovative approach
poised to surpass electrical noise and enable the detection of MeV-scale DM
with unprecedented sensitivity.
