Carrier Anisotropy and Impurity Scattering in Ge at mK Temperatures: Modeling and Comparison to Experiment

Abstract

Improving upon the present background rejection capabilities of the cryogenic germanium detectors for direct dark matter search involves an in-depth comprehension of the charge collection process in these devices. Experimental data point to the combined effects of lattice and impurity scattering on the anisotropy of electron transport in Ge at mK temperatures. A Monte Carlo simulation code has been implemented to incorporate these features in a consistent model for charge collection. In a novel approach to carrier scattering by charged impurities in Ge at cryogenic temperatures, the scattering field of the impurities is treated statistically as a random contribution to the collection field, described by the Holtsmark distribution function with a single adjustable parameter, the mean density of the charged centers. Simulation of charge collection along these lines in devices different by their impurity content shows excellent agreement to experiment. Especially noteworthy is the fact that the strength of impurity scattering is reversed from the known concentration of dopant impurities in the crystals, as the crystal with the higher dopant concentration shows lower scattering at low field than the one with the lower concentration. This raises as an issue for further improvement of these devices, the question of the nature of the scattering centers in high-purity Ge crystals at cryogenic temperatures, associated presumably with deep level impurities or crystal defects. Fig. 1 (Color online) (a) Simulated carrier trajectories for a 60 keV energy deposit from a collimated 241 Am γ source facing the bottom surface of the detector crystal. See Ref. [1] for a full description of the device and of the geometry of the charge collection field especially. Detector bias V p = 2 V; T = 20 mK. Holes are in red and electrons in blue, black, green and magenta depending on their valley of belonging. (b) Same as (a), with the polarity of the electrode biases reversed so that holes instead of electrons are drifted across the full thickness of the detector crystal

Invited Review Article: Physics and Monte Carlo techniques as relevant to cryogenic, phonon, and ionization readout of Cryogenic Dark Matter Search radiation detectors

“…Detailed Monte Carlo studies in CDMS [36,37,56,57,[64][65][66] and EDELWEISS [67,68] detectors can be found in the literature. These studies include discussion of quantities which are relevant in the tuning and their effects on phonon pulse shape, charge transport, TES phase-separation and TES noise.…”

Effects of a Weak Magnetic Field on Carrier Transport in a Cryogenic Germanium Detector for Dark Matter Search

“…There results a complex, bias dependent pattern of charge collection between the different measurement channels. 2,3 Application of a magnetic field leads to further changes in the collection patterns, as figure 3 (a) shows for the field in the direction normal to the detector axis. On the other hand, minute effects only are observed in the c and d channels when the field is turned parallel to the detector axis ( fig.…”

“…Experiments reported elsewhere in these proceedings demonstrate large differences between electron and hole collection in Ge detectors at mK temperatures, with regard to carrier straggling especially. [2][3][4][5] These differences are also reflected in the magnetic field effects ( figs. 2 and 3).…”

“…e-mail: alexandre.broniatowski@csnsm.in2p3.fr which shows that hole transport should be affected appreciably. We investigate these effects on a dark matter detector at 20 mK, using as a source for the magnetic field a permanent (FeNdB) magnet located outside the body of the 3 He/ 4 He dilution refrigerator. Section 2 presents the setup for these experiments.…”

Transport Anisotropy and Impurity Scattering in Ge at Millikelvin Temperatures: Experimental Study

“…A detailed analysis of the charge collection patterns in these devices is presented in a companion paper in these proceedings, 10 based on Monte Carlo simulations of carrier transport taking into account the combined effects on electron motion of intervalley and impurity scattering processes.…”