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E Wistrela, I. Schmied, M. Schneider, M. Gillinger, P.M. Mayrhofer, A. Bittner, U. Schmid:
"Impact of sputter deposition parameters on the microstructural and piezoelectric properties of Cr x Al 1−x N thin films";
Thin Solid Films, 648 (2018), 76 - 82.

In the field of MEMS, aluminum nitride (AlN) in the wurtzite phase is an important functional material due to its
piezoelectricity. To further enhance device performance, transition metal alloying of AlN is proposed to achieve
a higher piezoelectric response. In this work, the effect of moderate chromium (Cr) doping (up to x =
(5.0 ± 0.6) at.%) as well as the influence of changing deposition pressures and gas composition are in-
vestigated, focusing on the microstructure, morphology and piezoelectric response of sputter deposited
Cr x Al 1−x N thin films. X-ray diffraction analyses reveal a wurtzite type structure with highest degree in c-axis
orientation for lowest deposition pressures and Cr concentrations. With increasing deposition pressure, a de-
terioration in crystal quality and the formation of other crystal orientations within the films are observed. The
presence of crystallite orientations away from the 〈002〉 orientation is linked to the presence of randomly
oriented, conical-shaped grains which are embedded in the fine grainy thin film surface topography as indicated
by atomic force microscopy. By using piezo force microscopy, these grains are identified to cause a reduction in
the effective piezoelectric response. Furthermore, we show that combining both the analysis of orientation di-
stribution and surface morphology, is by far more reliable for estimating the piezoelectric response of thin films
than the sole use of one method.

http://dx.doi.org/10.1016/j.tsf.2018.01.003