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 70 Ge͑99.99%͒, show intrinsic behavior at room temperature with the exception of a p-type sample with ͉N d-N a ͉Х2ϫ10 16 cm Ϫ3. The ''undoped'' samples exhibit a T 3 dependence at low temperatures, basically determined by boundary scattering. The maximum value of ͑which falls in the range between 13 K and 23 K͒ is found to be a monotonically decreasing function of the isotopic mass variance parameter g. The maximum m measured for the most highly enriched 70 Ge͑99.99%͒ sample is 10.5 kW/mK, one order of magnitude higher than for natural germanium. The experimental data have been fitted with the full Callaway theory, modified by treating transverse and longitudinal modes separately, using three free adjustable parameters for each set of modes to represent anharmonic effects plus the calculated contributions from isotopic and boundary scattering. For the isotopically purest 70 Ge͑99.99%͒ sample, dislocation scattering, or a similar mechanism, must be added in order to fit the data. We have also checked the effect of various surface treatments on the thermal conductivity in the low temperature region. The highest values of are found after polish etching with a SYTON suspension. ͓S0163-1829͑97͒00539-0͔

The thermal conductivity κ(T ) of single crystals of silicon with two different isotopic compositions: natural and 99.983% enriched 28 Si, was investigated in the temperature range from 0.5 K to 300 K. The enriched 28 Si sample has very high thermal conductivity maximum of 290 W cm −1 K −1 at Tmax = 26.5 K, about 7.5 times higher relative to the conductivity of nat Si with natural isotope abundance. The isotope effect decreases with temperature increase, being 10 ± 2% at room temperature. The data are discussed briefly within the Ambegaocar's theory of isotope effect.

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The thermal conductivity of isotopically enriched 28 Si (enrichment better than 99.9%) was redetermined independently in three laboratories by high precision experiments on a total of 4 samples of different shape and degree of isotope enrichment in the range from 5 to 300 K with particular emphasis on the range near room temperature. The results obtained in the different laboratories are in good agreement with each other. They indicate that at room temperature the thermal conductivity of isotopically enriched 28 Si exceeds the thermal conductivity of Si with a natural, unmodified isotope mixture by 10±2 %. This finding is in disagreement with an earlier report by Ruf et al. At ∼26 K the thermal conductivity of 28 Si reaches a maximum. The maximum value depends on sample shape and the degree of isotope enrichment and exceeds the thermal conductivity of natural Si by a factor of ∼8 for a 99.982% 28 Si enriched sample. The thermal conductivity of Si with natural isotope composition is consistently found to be ∼3% lower than the values recommended in the literature. PACS: 66.70.+f, 63.20.Mt, 74.25.Fy, 65.40.Ba, 65.90.+i Keywords: silicon, stable isotopes, thermal conductivity INTRODUCTIONPhonon scattering due to the presence of different isotopes in an otherwise pure crystal (no chemical defects or dislocations) has been identified as a mechanism that strongly affects the thermal conductivity κ [1−3]. The availability of larger quantities of highly isotope-enriched materials recently revived the interest in this effect in order to study the mechanisms underlying the thermal conductivity and possibly to improve material properties [4−8].Recently, the observation of a significantly enhanced thermal conductivity of isotopically enriched 28 Si near room temperature by Capinski [9] and Ruf et al. [10] has generated large interest. In these studies, it was found that the thermal conductivity of isotopically enriched 28 Si is enhanced by about 60% over that of silicon with natural isotopic composition. This rather high isotope effect attracted considerable attention concerning both, fundamental physics and applications. For technical applications, a significantly enlarged thermal conductivity at room 2 temperature would be of interest for high performance electronic devices. [11]. From the fundamental physics aspect, the experimental results were unexpected since the observed isotopic effect was significantly larger than the prediction of simple theoretical estimates [10−13] and more advanced model calculations [14]. On the other hand, theoretical papers [15−17] were published with the results supporting the data of refs. [9,10]. A large isotopic effect at room temperature is in principle only possible if normal phonon-phonon scattering processes play an important role in determining the formation of the non-equilibrium distribution function of phonons at these temperatures.The large room temperature isotope effect was questioned following the results of an experimental study by Gusev et al. [18] which indicate that th...

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