Development of large size high-purity germanium crystal growth

Growing large diameter (∼15cm) dislocation-free crystals for the GEODM experiment at DUSEL is a challenging task. A crystal of such high purity and large size has never been grown before and the process will require much research and development. In pursuing this goal, we are growing high-purity germanium crystals by the Czochralski method in our laboratory at USD in order to understand the details of the growing process, especially for large diameter crystals. Using a radio frequency (RF) heating method, germanium crystals with two different orientations (along 〈100〉 and 〈111〉) have been grown. The growth characteristics of 〈100〉 crystals have been investigated to understand the impact of the thermal field on the size of the grown crystals. In order to establish the optimal thermal field for growing large high-purity germanium crystals, three different radial thermal fields have been studied. Furthermore, the dislocation density in the grown crystals was measured using an optical microscope and was determined to be about 1×103–4×103/cm2, which falls in the required range for a high-purity germanium detector. X-ray diffraction tests on the surfaces of (100) indicate the grown crystals are of high quality.