In this paper we describe an experimental set-up for simultaneous measurements of thermopower and electrical resistivity at temperatures from 100 K to 1300 K. Optimal configuration of electrodes and original mechanical contacts of thermocouples, current leads and potential probes with the sample make it possible to measure a large variety of materials and result in greater flexibility with respect to the sample form and dimensions. Both bulk and thin film samples with resistances in the range from 0 up to 200 k (1 G in case of the resistivity measurement) can be investigated. Precision and high reliability of the system have been proven during more than three years of use. The resistivity and thermopower of pure Pb, Cu and Ni, and a Cr-Si thin film composite are presented as test materials to demonstrate the possibilities and accuracy of this experimental set-up.

The electrical conductivity and the Hall effect are investigated in the temperature region fiom 77 to loo0 K on undoped ZnO single crystals grown by vapour phase transport. The growth conditions were systematically altered to get crystals with definite non-stoichiometry. The electrical properties are related to these growth conditions. The analysis of the Hall data yields the phase boundary ZnO/Zn and a formation enthalpy of 1.5eV for the dominant native donor. This donor is suggested to be the oxygen vacancy. The Hall mobility is analysed with regard of new aspects giving a smaller anisotropy than in earlier analyses. An undotierten durch Gasphasentransport geziichteten ZnO-Einkristallen werden elektrische Leitfiihigkeit und Halleffekt im Bereich von 77 bis 1000 K untersucht. Durch Anderung der Ziichtungsbedingungen konnen definierte Grade der Nichtstoichiometrie erzeugt werden, was in den elektrischen Eigenschaften deutlich zum Ausdruck kommt. Die Analyse der Hallmessungen ergab die Phasengrenze ZnO/Zn sowie eine Bildungsenthalpie von 1,5 eV fur den dominierenden Eigendonator, der als Sauerstoffvakanz interpretiert wird. Die Analyse der Hallbeweglichkeit ergibt eine geringere Anisotropie als in friiheren Arbeiten.

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We have presented systematic cross-plane thermal conductivity (λ) data for the undoped strain-symmetrized Si/Ge superlattices grown on Si(111) with superlattice (SL) period thickness varying from 3.6 to 16 nm. In thin SL period (L⩽7 nm) samples, the data have shown considerable reductions of λ, by more than 50% and 30% compared to the SiGe alloy and to the earlier reported values in (100)-oriented Si/Ge superlattice structures (SLS), respectively. For the thick SL period samples (L>10 nm), λ has shown a tendency to saturate at the SiGe alloy value. This is understood as, with increasing L, the SLS breaks and the SiGe alloying starts to grow. This structural behavior is clearly observed in the cross-plane transmission electron microscope images as well. In addition to these, for the thin SL period (L⩽7 nm) samples, the data have shown a shallow minimum which is attributed to the competing behavior of the wave nature and the classical particle nature of the localized phonons. Nevertheless, the present study of thermal conductivity on undoped strain-symmetrized Si/Ge SLs in (111) orientation suggests that an enhancement of thermoelectric figure-of-merit Z is possible.