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.
Strong evidence for the existence of two valence bands in Pb0.7Sn0.3Te and Pbo.sSno.sTe is inferred from the observation of a kink in the variation of the Hall ratio R~O O~K I R~~O O K , as a function of hole concentration. This existence is also supported by the fact that the variation of the Seebeck coefficient as a function of hole concentration exhibits a minimum. Moreover, non parabolic bands are expected from our Hall mobility data, which indicatep-1 instead of p~p-113 for a parabolic band. According to the existence of two valence bands, the thermal energy gap behavior in Pb-rich alloys (x < 0.3) is discussed. From electrical results in the intrinsic regions, the thermal energy gaps for different compositions ranging from PbTe to x = 0.3 were found to be of the same order. Taking into account the interpretation for PbTe, and in correlation with the observed behavior of the energy gap E,, the energy separation E, between the two valence bands Vl and VZ is expected to increase with increasing Sn content and to decrease with increasing temperature. Data on Sn-rich alloys would lead to a better appreciation of a possible shift of Vz relatively to VI and to the conduction band with increasing Sn content. Rbsumb.-I1 existe de fortes presomptions pour l'existence de deux bandes de valence dans Pb0,7Sno,sTe et Pbo,sSno,sTe d'aprks l'observation d'un maximum dans la variation du rapport de Hall R300 o K / R I o o~~ en fonction de la concentration de trous. Cette existence est confirmee par le fait que la variation du coefficient de Seebeck en fonction de la concentration de trous possede un minimum. De plus, des bandes non paraboliques sont presumks d'aprks nos mesures de mobilite de Hall qui indiquent : p~p-1 au lieu de p~p-113 pour une bande parabolique. Nous discutons le comportement de la bande interdite thermique dans des alliages riches en plomb (x < 0,3) dans l'hypothkse de deux bandes de valence. D'apres les resultats des mesures
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