Biaxial texture development in aluminum nitride layers during off-axis sputter deposition

Polycrystalline aluminum nitride (AlN) layers were deposited by pulsed-dc reactive magnetron sputtering from a variable deposition angle alpha = 0 degrees-84 degrees in 5 mTorr pure N-2 at room temperature. X-ray diffraction pole figure analyses show that layers deposited from a normal angle (alpha=0 degrees) exhibit fiber texture, with a random in-plane grain orientation and the c-axis tilted by 42 degrees +/- 2 degrees off the substrate normal, yielding wurtzite AlN grains with the {10 (1) over bar2} plane approximately parallel (+/-2 degrees) to the substrate surface. However, as alpha is increased to 45 degrees, two preferred in-plane grain orientations emerge, with populations I and II having the c-axis tilted toward and away from the deposition flux, by 53 degrees +/- 2 degrees and 47 degrees +/- 1 degrees off the substrate normal, respectively. Increasing alpha further to 65 degrees and 84 degrees, results in the development of a single population II with a 43 degrees +/- 1 degrees tilt. This developing biaxial texture is attributed to a competitive growth mode under conditions where the adatom mobility is sufficient to cause intergrain mass transport, but insufficient for the thermodynamically favored low energy {0001} planes to align parallel to the layer surface. Consequently, AlN nuclei are initially randomly oriented and form a kinetically determined crystal habit exposing {0001} and {11 (2) over bar0} facets. The expected direction of its highest growth rate is 49 degrees +/- 5 degrees tilted relative to the c-axis, in good agreement with the 42 degrees-53 degrees measured tilt. The in-plane preferred orientation for alpha > 0 degrees is well explained by the orientation dependence in the cross section of the asymmetric pyramidal nuclei to capture directional deposition flux. The observed tilt is ideal for shear mode electromechanical coupling, which is maximized at 48 degrees. (C) 2012 American Vacuum Society. [http://dx.doi.org/10.1116/1.4732129]