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We have studied transport properties of Nb/Al/AlOx/Nb tunnel junctions with ultrathin aluminum oxide layers formed by (i) thermal oxidation and (ii) plasma oxidation, before and after rapid thermal post-annealing of the completed structures at temperatures up to 550 • C. Post-annealing at temperatures above 300 • C results in a significant decrease of the tunneling conductance of thermallygrown barriers, while plasma-grown barriers start to change only at annealing temperatures above 450 • C. Fitting the experimental I-V curves of the junctions using the results of the microscopic theory of direct tunneling shows that the annealing of thermally-grown oxides at temperatures above 300 • C results in a substantial increase of their average tunnel barriers height, from ∼1.8 eV to ∼2.45 eV, versus the practically unchanged height of ∼2.0 eV for the plasma-grown layers. This difference, together with high endurance of annealed barriers under electric stress (breakdown field above 10 MV/cm) may enable all-AlOx and SiO2/AlOx layered "crested" barriers for advanced floating-gate memory applications.

Abstract. High-temperature x-ray powder diffraction study by the full pattern Rietveld method of orthorhombic CaGeO3 (Pbnm at ambient condition) perovskite confirms the previously observed phase transition at Tc = 520 K. The measured volumetric thermal expansion coefficients are 3.1 x 10 -s (K -1) below Tc and 3.5x 10 .5 (K -1) above Tc. The space group at T>Tc has been tentatively identified as Cmcm. Such a transition involves the disappearance of one of the two octahedral rotations in the (001) plane, and the doubling of the unit cell volume, with c axis unchanged. Although this transition should be of first order from symmetry considerations, the distortion of the Pbnm phase decreases continuously as the temperate approaches Tc and there is no observable volume discontinuity at Tc. The measured heat capacity places an upper limit on the enthalpy of transition of 50 J/mol, which is quite reasonable in terms of the crystallographic nature of this phase transition.