Controlling the Leakage-Current of Low Temperature Germanium Detectors Using XeF Dry Etching

Ionization detectors, based on the drift and collection of charges in high purity Germanium crystals (HP-Ge), are commonly used for gamma and particle detection. In the field of very low temperature detectors, several major experiments, like EDELWEISS or CDMS, are using such devices combined with a simultaneous measurement of the detector heat pulses, to search for dark matter in the universe. The leakage current between the charge collecting electrodes of the detectors must be kept as low as possible to preserve performance in terms of high resolution and low threshold. We present results concerning the surface treatment of EDELWEISS HP-Ge crystals, aiming at the reduction of their leakage currents at temperatures below 4 K. Among the explored solutions, a post-process dry-etching using XeF (xenon difluoride) gives very promising and reproducible results. The modification of the Ge surface by this treatment and its effect on the low temperature leakage currents are discussed.