Effects of sputtering atmosphere on the properties of c-plane ScAlN thin films prepared on sapphire substrate

In this work, scandium aluminum nitride alloy (ScAlN) thin films were prepared on c-sapphire substrates by DC reactive magnetron sputtering with a scandium aluminum alloy (Sc_0.06Al_0.94) target. The crystal orientation and surface morphology were detected by XRD and AFM, respectively. The electrical properties were analyzed by a standard ferroelectric test system and piezoelectric response force microscopy. The results show that the sputtering atmosphere, including N₂/Ar flow ratio and sputtering pressure, appears to be important to influence the crystal quality and electrical properties of ScAlN films. With the N₂/Ar flow ratio increasing from 3.1:7 to 3.6:7, the crystal orientation and surface morphology of ScAlN films firstly improves and then gets worse. Meanwhile, the electrical qualities of the films performs a similar variation. When the sputtering pressure increases from 0.3 to 0.7 Pa, the properties of the films change obviously too, and the best sputtering pressure is determined as 0.5 Pa. Finally, highly c-axis ScAlN films can be obtained with a N₂/Ar flow ratio of 3.4:7 and a sputtering pressure of 0.5 Pa, and the FWHM value of the rocking curve, the RMS roughness, the resistivity, the dielectric constant ɛ_r and piezoelectric constant d₃₃ are 2.6°, 2.650 nm, 2.9 × 10¹² Ω/cm, 12.2 and 8.1 pC/N, respectively.