THERMAL-CONDUCTIVITY OF GERMANIUM-CRYSTALS WITH DIFFERENT ISOTOPIC COMPOSITIONS

We have measured the thermal conductivity of seven germanium crystals with different isotopic compositions in the temperature range between 2 K and 300 K. These samples, including one made of highly enriched Ge -70(99.99%), show intrinsic behavior at room temperature with the exce ption of a p-type sample with /N-d-N-a/ congruent to 2 x 10(16) cm(-3) . The ''undoped'' samples exhibit a T-3 dependence at low temperatures , basically determined by boundary scattering. The maximum value of ka ppa (which falls in the range between 13 K and 23 K) is found to be a monotonically decreasing function of the isotopic mass variance parame ter g. The maximum kappa(m) measured for the most highly enriched Ge-7 0(99.99%) sample is 10.5 kW/mK, one order of magnitude higher than for natural germanium. The experimental data have been fitted with the fu ll Callaway theory, modified by treating transverse and longitudinal m odes separately, using three free adjustable parameters for each set o f modes to represent anharmonic effects plus the calculated contributi ons from isotopic and boundary scattering. For the isotopically purest Ge-70(99.99%) sample, dislocation scattering, or a similar mechanism, must be added in order to fit the data. We have also checked the effe ct of various surface treatments on the thermal conductivity in the lo w temperature region. The highest values of kappa are found after poli sh etching with a SYTON suspension.