Influence of Cr-doping on microstructure and piezoelectric response of AlN films

The influence of Cr-doping on the microstructure and properties of aluminum nitride (AlN) films has been investigated by means of x-ray diffraction, transmission electron microscopy, x-ray absorption near edge structure, piezoelectric force microscopy and x-ray photoelectron spectroscopy. The results indicate that Cr3+ steadily substitutes for Al3+ in AlN wurtzite lattice without any precipitated secondary phase formation. Moderate Cr-doping has a profound influence on the enhancement of (0 0 2) texture and crystallinity of Cr : AlN films. The residual stress for the Cr : AlN films changes from tension to compressive stress with increasing Cr concentration. The Al0.937Cr0.063N film is almost stress-free and has a piezoelectric constant of -7.1 pm V-1 which is 73% higher than that of the undoped AlN film (-4.1 pm V-1). Besides the enhancement of (0 0 2) texture and crystallinity of the Cr : AlN films, the decrease in Al-polarity regions and the resultant increase in the net polarity orientation by Cr-doping also contribute to the increase in AlN piezoelectric response.