Dislocation density control in high-purity germanium crystal growth

abstract High-purity germanium (HP-Ge) crystals were grown in hydrogen atmosphere by Czochralski method. Thecontrol of dislocation density in high-purity germani um crystal growth was studied. It could be shown that bycontrol of the temperature gradient during crystal growth the dislocation density dis tribution in the crystal canbe controlled to a degree which allows for the growth of crystal ful ﬁlling detector requirements. Crystals withdiameters of 3.5 and 9 cm were grown according to the relationship between axial temperature gradient anddislocation density to be able to meet the requirements of detector fabrication by having dislocation density inthe range 2000–7000cm 2 .& 2013 Elsevier B.V. All rights reserved. 1. IntroductionIdentiﬁcation of the general properties of neutrinos and thenature of the mysterious dark matter are currently two of the mostimportant pursuits in fundamental physics. Both neutrinos anddark matter are integral to physics beyond the Standard Model.However, very important properties could not yet be determinedabout the general properties of neutrinos such as their absolutemass, magnetic moment, identiﬁcation as Dirac or Majoranaparticles, or even the number of species. Even less is known aboutthe nature and quantity of dark matter in the universe. Gaining adeeper understanding of both neutrinos and dark matter is a veryimportant research in the 21st century. Germanium-based experi-ments have recently reported a possible dark matter signature(CoGeNT) [1] and have claimed discovery of neutrinoless doublebeta decay [2], making germanium the preferred target for use infuture experiments. Five research groups: Super-CDMS, CRESST,GERDA, MAJORANA, CDEX [3–8], have been designed to detectdark matter and/or neutinoless double-beta decay utilizing large-scale HP-Ge detectors with ultra-low internal radioactive back-grounds. The sensitivity of germanium-based experiments isrelated to the size of the HP-Ge crystals. Large diameter (43 in.)HP-Ge crystals have bigger volume to surface ratio for collectingthe signal in particular from weakly interacting massive particles(WIMPs), a plausible dark matter candidate.HP-Ge crystals grown in a surface environment can be con-taminated by cosmic rays that generate long-lived isotopes inHP-Ge detectors [9,10]. The long-lived isotopes can limit thesensitivity for next generation experiments. To minimize theseunwanted long-lived isotopes, the best solution is to zone-reﬁnegermanium ingots, grow crystals and fabricate detectors in anunderground environment where cosmic rays are largely shieldedby rock overburden. With reduced cosmic ray exposure, the HP-Gedetectors will be preferable for dark matter and neutrinolessdouble-beta decay experiments. The growth of HP-Ge crystals inan underground environment is a critical step for undergroundexperiments utilizing HP-Ge detectors. However, much researchand development must be performed on the surface to facilitatenon-commercial growing of detector grade crystals.Currently, the largest detector-grade HP-Ge crystal weighs4.8 kg with a diameter of 10 cm and a length of 12 cm [11,12].Growing HP-Ge detector grade crystal in this size presents achallenge. In 1970s, HP-Ge crystals with 2 3.5 cm in diameterwere discussed [13,14]. However, from then on, there is less workfocus on the research of crystal growth, especially for large size(46 cm in diameter) crystals. At present, only few companieshave the disclosed technology to grow large HP-Ge crystals.Large Ge crystals are difﬁcult to grow, especially via the radiofrequency (RF) heating method, since the thermal ﬁeld will bequite different from the resistance heating method that is used forelectronic and IR germanium crystals [15]. However, radio fre-quency induction heating provides the cleanest environment forHP-Ge crystal growth. The HP-Ge crystals must be grown in highlypure hydrogen atmosphere [16]. Hydrogen has a large convectiveheat transfer coefﬁcient, large thermal conductivity and lowerviscosity. These attributes make the thermal ﬁeld difﬁcult to becontrolled. Hydrogen is easy to dissolve into germanium melt. Itcan form hydrogen di-vacancies (V