Role of oxygen on structure and piezoelectric properties of Al0.65Sc0.35 N thin films

Al1-xScxN thin films have emerged as a technologically important material for several piezoelectric and ferroelectric device applications ranging from acoustic filters and microelectromechanical systems to memories. Such applications require materials to be grown with high structural quality and optimal properties. However, it is challenging to completely eliminate oxygen contamination, which is persistently present in Al1-xScxN thin films produced by sputtering deposition, particularly with increasing Sc content. In this study, we examine the role of oxygen in the structure and its effect on the piezoelectric and dielectric properties of Al0.65Sc0.35N thin film. Our theoretical analysis and experimental results suggest that different oxygen concentrations can have contrasting effects on the piezoelectric properties of Al0.65Sc0.35N thin films. Oxygen concentration in the range of 1.2 to 4.3 at. % severely degrades the structural coherence of the films and consequently reduces the piezoelectric coefficient from 10.5 to 8.2 pm/V. Electrical characterization and density-functional theory (DFT) calculations indicate the existence of defects of ON type and 3ON-VAl type, depending on oxygen concentration. Our DFT calculations further suggest that at low oxygen concentrations where the substitutional defect of ON is favored, the piezoelectric coefficient can be nearly 1.5 times higher compared to its counterpart without oxygen.