: Nawrodt, R.; Zimmer, A.; Koettig, T.; Schwarz, C.; Heinert, D.; Hudl, M.; Neubert, R.; Thürk, M.; Nietzsche, S.; Vodel, W.; Seidel, P.; Tünnermann, A.

Abstract
Future gravitational wave detectors will be limited by different kinds of noise. Thermal noise from the coatings and the substrate material will be a serious noise contribution within the detection band of these detectors. Cooling and the use of a high mechanical Q-factor material as a substrate material will reduce the thermal noise contribution from the substrates. Silicon is one of the most interesting materials for a third generation cryogenic detector. Due to the fact that the coefficient of thermal expansion vanishes at 18 and f 25 K the thermoelastic contribution to the thermal noise will disappear. We present a systematic analysis of the mechanical Q-factor at low temperatures between 5 and 300 K on bulk silicon (100) samples which are boron doped. The thickness of the cylindrical samples is varied between 6, 12, 24, and 75 mm with a constant diameter of 3 inches. For the 75 mm substrate a comparison between the (100) and the (111) orientation is presented. In order toobtain the mechanical Q-factor a ring-down measurement is performed. Thus, the substrate is excited to resonant vibrations by means of an electrostatic driving plate and the subsequent ring-down is recorded using a Michelson-like interferometer. The substrate itself is suspended as a pendulum by means of a tungsten wire loop. All measurements are carried out in a special cryostat which provides a temperature stability of better than 0.1 K between 5 and 300 K during the experiment. The influence of the suspension on the measurements is experimentally investigated and discussed. At 5.8 K a highest Q-factor of 4.5 * 108 was achieved for the f 4.9 kHz mode of a silicon (1 00) substrate with a diameter of 3 inches and a thickness of 12 mm.