Nucleation control of high crystal quality heteroepitaxial Sc_0.4Al_0.6N grown by molecular beam epitaxy

High ScN fraction ScxAl1-xN has promise in important application areas including wide bandwidth RF resonators and filters, and ferroelectric devices such as non-volatile memory, but demands high crystal quality. In this work, the role of the nucleation layer (NL), ScxAl1-xN growth temperature, and strain management to preserve the wurtzite crystal structure are investigated to maximize both acoustoelectric and ferroelectric material properties for high ScN fraction ScxAl1-xN grown on SiC substrates. A 5 nm AlN nucleation layer reduces the x-ray diffraction 0002 reflection full width at half maximum (FWHM) for a Sc0.32Al0.68N film by almost a factor of 2, and reducing the growth temperature to 430 degrees C enables a Sc0.40Al0.60N film with a FWHM of 4100 arcsec (1.1 degrees) while being only 150 nm thick. Grading the initial ScxAl1-xN layer from x = 0.32 to 0.40 suppresses the formation of rock-salt grain nucleation at the Sc0.40Al0.60N lower interface and reduces the anomalously oriented grain density by an order of magnitude. Increasing the total ScxAl1-xN growth thickness to 500 nm produces an average x = 0.39 ScxAl1-xN layer with a FWHM of 3190 arcsec (0.89 degrees) and an anomalously oriented grain areal fill factor of 1.0%. These methods enable the lowest heteroepitaxial ScxAl1-xN FWHM reported for x similar to 0.4, with layer thicknesses and defect densities appropriate for high frequency (>10 GHz) filter applications.