Metastable ScAlN and YAlN Thin Films Grown by Reactive Magnetron Sputter Epitaxy

Abstract

Metastable ScxAl1-xN and YxAl1-xN thin films were deposited in an ultra high vacuum system using reactive magnetron sputter epitaxy from elemental Al, Sc, and Y targets in Ar/N2 gas mixture. Their structural, electrical, optical, mechanical, and piezoelectrical properties were investigated by using the transmission electron microscopy, x-ray diffraction, spectroscopic ellipsometry, I-V and C-V measurements, nanoindentation, and two different techniques for piezoelectric characterization: piezoresponse force microscopy and double beam interferometry. Compared to AlN, improved electromechanical coupling and increase in piezoelectric response was found in ScxAl1-xN/TiN/Al2O3 structures with Sc content up to x=0.2. Decreasing the growth temperature down to 400 °C improved the microstructure and crystalline quality of the material. Microstructure of the films had a stronger influence on piezoelectric properties than the crystalline quality, which affected the leakage currents. When x was increased from x=0 to x=0.3, the hardness and reduced Young’s modulus Er showed a decrease from 17 GPa to 11 GPa, and 265 GPa down to 224 GPa, respectively. In ScxAl1-xN/InyAl1-yN superlattices, ScxAl1-xN layers negative lattice mismatched to In-rich InyAl1-yN were found to be stable at higher Sc concentration (x=0.4) than lattice-matched or positive lattice mismatched layers, confirmed by first principle (ab initio) calculations using density-functional formalism. Al-rich YxAl1-xN thin films were synthesized and reported for the first time. Formation of solid solution was observed up to x=0.22 and an increase in growth temperature up to 900°C improved the crystalline quality of the YxAl1-xN films. The band gap of YxAl1-xN decreased from 6.2 eV for AlN down to 4.5 eV (x=0.22) and was shown to follow Vegard’s rule. Refractive indices and extinction coefficients were also determined. Lattice constants of wurtzite YxAl1-xN measured experimentally are in good agreement with theoretical predictions obtained through ab initio calculations. The mixing enthalpy