Piezoelectric wurtzite ScxAl1−xN (x = 0, 0.1, 0.2, 0.3) thin films were epitaxially grown by reactive magnetron co-sputtering from elemental Sc and Al targets. Al2O3(0001) wafers with TiN(111) seed and electrode layers were used as substrates. X-ray diffraction shows that an increase in the Sc content results in the degradation of the crystalline quality. Samples grown at 400 °C possess true dielectric behavior with quite low dielectric losses and the leakage current is negligible. For ScAlN samples grown at 800 °C, the crystal structure is poor and leakage current is high. Transmission electron microscopy with energy dispersive x-ray spectroscopy mapping shows a mass separation into ScN-rich and AlN-rich domains for x ≥ 0.2 when substrate temperature is increased from 400 to 800 °C. The piezoelectric response of epitaxial ScxAl1−xN films measured by piezoresponse force microscopy and double beam interferometry shows up to 180% increase by the addition of Sc up to x = 0.2 independent of substrate temperature, in good agreement with previous theoretical predictions based on density-functional theory.

The transverse piezoelectric coefficient e31,f of Al1-xScxN thin films was investigated as a function of composition. It increased nearly 50% from x = 0 to x = 0.17. As the increase of the dielectric constant was only moderate, these films are very suitable for energy harvesting, giving a 60% higher transformation yield (x = 0.17) as compared to pure AlN. A higher doping might even lead to a 100% augmentation. The thickness strain response (d33,f) was found to increase proportionally to the ionic part of the dielectric constant. The e-type coefficients (stress response), however, did not augment so much as the structure becomes softer. As a result, the transverse voltage/strain response (h31,f-coefficient) was raised only slightly with Sc doping. The low dielectric loss obtained at all compositions suggests also the use of Al1−xScxN thin films in sensors.

AlN films are employed in RF filters for wireless communication. We report on enhanced coupling factors kt2 obtained by partial substitution of Al by Sc. Al0.88Sc0.12N films were deposited by reactive magnetron sputtering from an Al0.9Sc0.1 alloy target. They grew in the piezoelectric wurtzite phase with a similar microstructure as pure AlN films. The clamped d33,f increased considerably from 5.1 to 7.8 pm/V. The admittance measured at thin film bulk acoustic wave resonators was fitted to an equivalent circuit model and to 2-dimensional finite element simulation, yielding a kt2 of 7.3% and a quality factor of 650. The material softens considerably.

Sputter deposited Al (1-x) Sc x N thin films with a Sc content from x ¼ 0 to 43 at% are investigated by electron microscopy in order to study and explain the formation and growth of abnormally oriented grains (AOG). It is found that the latter did not nucleate at the interface with the substrate, but at high energy grain boundaries, at which systematically higher Sc concentrations are detected. The AOGs are thus formed during the growth of c-textured grains. They grow faster than those, and finally protrude from the c-textured film surface, having at their end a pyramidal shape with three facets of a hexagonal wurtzite crystal: one (0001) and two (11 20) facets. Process conditions favoring less compact grain boundaries, and lower surface diffusion across grain boundaries are thought to promote nucleation of AOGs. Finally, a 4-step growth mechanism explaining the nucleation from a Sc-rich complexion and proliferation of AOGs with increasing film thickness is proposed.

A measurement setup for the detailed study of the transverse piezoelectric coefficient e 31,f in the converse (actuator) mode was developed. It allows the assessment of the piezoelectric stress in thin films on silicon cantilevers and provides for a correlation of this stress with large and small signal responses to ferroelectric polarization and dielectric response, both as a function of slowly sweeping electric field. This test is important for the understanding of piezoelectric thin films in microelectromechanical systems. The method is illustrated at hand of sol-gel lead-zirconate-titanate (PZT) thin films, and verified also with AlN and AlN-ScN alloy thin films. A 1-µm thick, sol-gel derived PZT(53/47) gradient-free sample showed a response of −18.3 C/m 2 at 100-kV/cm electric field. Reliability tests of PZT thin films were carried out with the same setup in an accelerated manner. The piezoelectric activity did not degrade significantly up to 10 9 unipolar pulses at 100 kHz with an amplitude of −150 kV/cm. The increase in leakage toward the end of the cycles was explained by a thermal runaway effect.[2014-0140]

The use of interdigitated electrodes (IDEs) in conjunction with ferroelectric thin films shows many attractive features for piezoelectric MEMS applications. In this work, growth of {1 0 0}-textured lead zirconate titanate (PZT) thin films was achieved on insulating MgO buffered, oxidized silicon substrates. IDEs were fabricated by lift-off techniques and cantilevers were formed by dicing. The deflection upon application of a sweeping voltage was measured as large signal response in parallel to the ferroelectric polarization (PV loop). Likewise, the small signal piezoelectric response was measured in parallel to the capacitance-voltage (CV) measurement. In this way, a complete picture of the ferroelectric–piezoelectric element was obtained. From the deflection, the in-plane piezoelectric stress in the PZT thin film was derived and, from this, the effective piezoelectric coefficients. For the latter, two types were defined: an engineering type corresponding to the average value along the IDE, which can directly be compared to coefficient of a parallel plate electrode (PPE) capacitor and a second one that approximately yields the idealized coefficient governing between the electrode fingers. The IDE structures were experimentally compared with PPE structures of identical film thickness. The resulting coefficients were of opposite sign, as expected. In spite of a much better polarization loop, the IDE device showed a lower average piezoelectric stress. The estimated peak value between the fingers was about the same as in the PPE device, corresponding to about 20 C m−2. Nevertheless, the result is very promising for cases where compressive piezoelectric stresses are required and for preventing cracking due to large piezoelectric tensile stresses in PPE systems.

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