Structural Analysis of Mo Thin Films on Sapphire Substrates for Epitaxial Growth of AlN

Aluminum nitride (AlN) thin film/molybdenum (Mo) electrode structures are typically required in microelectromechanical system applications. However, the growth of highly crystalline and c-axis-oriented AlN thin films on Mo electrodes remains challenging. In this study, we demonstrate the epitaxial growth of AlN thin films on Mo electrode/sapphire (0001) substrates and examine the structural characteristics of Mo thin films to determine the reason contributing to the epitaxial growth of AlN thin films on Mo thin films formed on sapphire. Two differently oriented crystals are obtained from Mo thin films grown on sapphire substrates: (110)- and (111)-oriented crystals. The dominant (111)-oriented crystals are single-domain, and the recessive (110)-oriented crystals comprise three in-plane domains rotated by 120° with respect to each other. The highly ordered Mo thin films formed on sapphire substrates serve as templates for the epitaxial growth by transferring the crystallographic information of the sapphire substrates to the AlN thin films. Consequently, the out-of-plane and in-plane orientation relationships among the AlN thin films, Mo thin films, and sapphire substrates are successfully defined.