Low-Temperature Thermal Conductivity of Fast-Neutron-Irradiated Silicon and Germanium

Abstract

The thermal conductivity K of initially n-type silicon and germanium irradiated at about 30°C at four fast-neutron doses (1.1×1017, 2.5×1017, 1.7×1018, and 3.4×1018 n/cm2) was measured between 5°K and 300°K. The thermal-conductivity behaviors of the two irradiated materials differ by significant features: Pronounced shifts of the maximum of K to higher temperatures are observed in germanium after irradiations, leading to a depression of K which is more pronounced below than above the maximum. No noticeable shift, however, appears in silicon after bombardment, the depressions of K on both sides of the peak remaining comparable. The additive thermal resistivity (K−1−K0−1) at 20°K is found to increase with the integrated flux φ, approximately as 7.9×10−13 φ0.65 cm·deg/W, and 3.8×10−12 φ0.65 cm·deg/W, for irradiated silicon and germanium, respectively. Similar flux dependences were observed by Vook for 2-MeV electron-irradiated silicon and germanium. Using the Callaway model, it is shown that the experimental data for irradiated silicon can be accounted for by considering only an increase in the τ−1=A′ω4 scattering, the factor A′ increasing linearly with the flux. The additional scattering mechanism is most likely to be associated with strain fields arising from bombardment-induced defects instead of an electron-phonon interaction, unless one assumes for the latter a Rayleigh-type scattering law. In agreement with Vook's results on electron-irradiated germanium, the dominant additional scattering in the present irradiated germanium, at least for the lower doses, appears to be due to electron-phonon interaction. On the basis of the Keyes model, the difference between bombardment-induced scatterings in silicon and germanium is attributed to the predominant effects of irradiation in these materials, silicon approaching an intrinsic behavior and germanium becoming highly p type.