Excitation and imaging of GHz surface acoustic waves in Al1−xScxN grown on SiC by molecular-beam epitaxy

Abstract

Scandium incorporation significantly boosts the piezoelectric properties of wurtzite AlN thin films. Synthesis of wurtzite Al1−xScxN thin films on SiC, although underexplored, can lead to benefits in terms of epitaxial quality and performance of radio frequency (RF) devices such as surface-acoustic-wave (SAW) filters. In this work, we grow 200 nm-thick Al1−xScxN thin films (0≤x≤0.18) without buffer layers by plasma-assisted molecular-beam epitaxy (PAMBE) on semi-insulating 6H- and 4H-SiC substrates. The epitaxial thin films exhibit both (0001) surface orientation and in-plane epitaxial alignment across the whole investigated composition range. A full-width at half-maximum of 0.3° is recorded for the Al0.82Sc0.18N film in the 0002 x-ray rocking curve. Consistent with the well-established enhancement of the effective electromechanical coupling coefficient keff2 by ternary alloying, we observe a tenfold improvement of keff2 for SAW devices operating above 6 GHz in the Al0.82Sc0.18N film compared to the AlN binary film grown and processed under consistent conditions. In addition, we visualize the impact of possible film cracking on SAW propagation utilizing atomic-force microscopy mapping with nanometer-scale spatial resolution. Our report demonstrates that PAMBE-grown Al1−xScxN/SiC yields high epitaxial quality and is a promising platform for GHz SAW technology, with potential extensions to other RF devices that require submicrometer thickness.