The search for electrolyte materials with high oxygen conductivities is a key step toward reducing the operation temperature of fuel cells, which is currently above 700°C. We report a high lateral ionic conductivity, showing up to eight orders of magnitude enhancement near room temperature, in yttria-stabilized zirconia (YSZ)/strontium titanate epitaxial heterostructures. The enhancement of the conductivity is observed, along with a YSZ layer thickness–independent conductance, showing that it is an interface process. We propose that the atomic reconstruction at the interface between highly dissimilar structures (such as fluorite and perovskite) provides both a large number of carriers and a high-mobility plane, yielding colossal values of the ionic conductivity.

This article presents a study of the influence of the sputtering parameters on the preferred orientation of polycrystalline aluminum nitride thin films. Aluminum nitride films were grown by rf reactive sputtering of an aluminum target in an ${\mathrm{N}}_2/\mathrm{Ar}$ gas mixture for different values of the deposition parameters: total pressure, nitrogen content in the discharge gas, and substrate bias voltage. The preferred orientation of the films was analyzed by x-ray diffraction. Films with different preferred orientations were obtained, ranging from c-axis oriented films to films with the c axis tilted by up to 61.6° from the substrate normal. The different mechanisms influencing the preferred orientation of the films have been considered, especially the transfer of energy to the adatoms on the substrate by particle bombardment. An analysis of the relation between the deposition parameters and the crystal orientation has allowed us to determine the relative importance of the different particles in the supply of energy to the substrate. We have found that Ar ion bombardment of the film during growth is the most influential mechanism on the preferred orientation of the films. As bombardment becomes more energetic, microcrystals in the film tend to grow with the c axis along the surface normal. The energy of Ar bombardment can be best controlled through the substrate bias voltage, a characteristic that we have employed to obtain AlN films exhibiting pure (00.2) preferred orientation and rocking curves with a full width at half maximum as low as 4.2°.

We have observed the apparently anomalous behavior of sputtered AlN thin films that exhibit poor piezoelectric response in spite of having (0002) preferred orientation and narrow rocking curve with full width at half maximum below 2°; however, other AlN films with weak 0002 x-ray diffraction peak and rocking curve as wide as 8° provide a good value of the piezoelectric coefficient $d_{31}$ $(2.1\mathrm{pm}∕\mathrm{V})$ . The critical difference between both types of films is the presence of traces of non-(0002) reflections in the x-ray diffraction patterns of the former. Non-(0002) reflections may be related to defects (probably, inversion domains) that significantly reduce the net piezoelectric field.