Purification of Ge ingots for growing crystals in developing dark matter detectors

High-purity germanium (HPGe) single crystals having the net impurity level of ~ 10⁹ to 10¹⁰ cm^-3 can be used for the fabrication of high-resolution dark matter detectors. We implement a zone refining technique as a primary purification procedure for growing crystals. The raw germanium ingots in our laboratory have four main impurities, such as Aluminum, Boron, Phosphorous, and Gallium, identified through photothermal ionization spectroscopy (PTIS), with a net impurity level of (10¹³-10¹⁴) cm^-3.The process of zone-refining included a two-step strategy, which included initial purification of the raw germanium ingots in a graphite boat, and then further purification of the zone-refined ingots from the first step in a carbon-coated quartz boat. We have optimized the parameter, zone length, zone travel speed and number of passes for impurities ingots having distinguished the segregation coefficient. Using the Van der Pauw Hall method, we were able to measure the electrical properties of zone refined ingots and analyze the distribution of impurities. Results obtained from graphite boat Ingots have the impurity level of (10¹¹-10¹²) cm^-3, and from quartz boat Ingots (10¹⁰-10¹¹) cm^-3. This work is supported by NSF OISE-1743790 and NSF PHY-1902577.