Elastic modulus and coefficient of thermal expansion of piezoelectric Al1−xScxN (up to x = 0.41) thin films

Abstract

Aluminum scandium nitride (Al1−xScxN with x = 0–0.41) thin films were deposited by reactive pulsed-DC magnetron sputtering on Si(001) and Al2O3(0001) substrates. X-ray diffraction indicated high degree of c-axis orientation in all the films, and based on pole figure measurements, epitaxial relationship could be defined as [101¯0]AlScN//[112¯0]sapphire and (0001)AlScN//(0001)sapphire in films deposited on Al2O3. Piezoelectric coefficient increased up to d33 = 31.6 pC/N in Al0.59Sc0.41N, which is 550% higher than for AlN. The biaxial elastic modulus and the in-plane coefficient of thermal expansion (CTE) as a function of Sc concentration were determined by thermal cycling method: biaxial elastic modulus decreased from 535 GPa in pure AlN to 269 GPa in Al0.59Sc0.41N and CTE was 4.65 × 10−6 K−1 for AlN and 4.29 × 10−6 K−1 for Al0.59Sc0.41N. Additionally, we observed an increase in CTE from 4.18 × 10−6 K−1 at 65 °C to up to 6.38 × 10−6 K−1 at 375 °C for Al0.68Sc0.32N. The experimentally determined CTE and elastic modulus allow a more precise design of Al1−xScxN-based frequency filters which are used in mobile communications and are important parameters for the prediction of device performance at elevated temperatures.