Microstructure and dielectric properties of piezoelectric magnetron sputtered w-ScxAl1−xN thin films

Abstract

Piezoelectric wurtzite ScxAl1−xN (x = 0, 0.1, 0.2, 0.3) thin films were epitaxially grown by reactive magnetron co-sputtering from elemental Sc and Al targets. Al2O3(0001) wafers with TiN(111) seed and electrode layers were used as substrates. X-ray diffraction shows that an increase in the Sc content results in the degradation of the crystalline quality. Samples grown at 400 °C possess true dielectric behavior with quite low dielectric losses and the leakage current is negligible. For ScAlN samples grown at 800 °C, the crystal structure is poor and leakage current is high. Transmission electron microscopy with energy dispersive x-ray spectroscopy mapping shows a mass separation into ScN-rich and AlN-rich domains for x ≥ 0.2 when substrate temperature is increased from 400 to 800 °C. The piezoelectric response of epitaxial ScxAl1−xN films measured by piezoresponse force microscopy and double beam interferometry shows up to 180% increase by the addition of Sc up to x = 0.2 independent of substrate temperature, in good agreement with previous theoretical predictions based on density-functional theory.

Stabilization of wurtzite Sc0.4Al0.6N in pseudomorphic epitaxial Sc Al1−N/In Al1−N superlattices

“…In our previous study of ScxAl1-xN thin films, we demonstrated a correlation between columnar microstructure and good crystalline quality [10], suggesting that the columnar microstructure in the present ScxAl1-xN/InyAl1-yN superlattices also indicates an improved crystalline quality. Even for the highest investigated Sc concentration (x=0.4) the layered structure is present and well defined.…”

“…This suggests that further improvement in application-relevant material properties would be possible by increasing the Sc concentration. However, experimental studies show a driving force for promoting phase separation into more stable wurtzite AlN and cubic rock-salt ScN at elevated growth temperatures and Sc concentration x≥0.3 [10]. Band gap energy measurements indicate a structural instability around x=0.2, leading to rock-salt ScN grains within ScxAl1-xN matrix in samples deposited at 850 °C [11].…”

Implications of heterostructural alloying for enhanced piezoelectric performance of (Al,Sc)N

“…Subsequent studies took one of two directions: epitaxial stabilization of the rocksalt structure for x > 0.5 with interest in the electronic structure [4] or effects of alloying on piezoelectric properties of the wurtzite phase for x < 0.5 [5]. After increases in the longitudinal piezoelectric strain coefficient (d 33 ) were reported by Akiyama et al, the majority of the reported efforts focused on the effect of deposition parameters on crystal quality [6][7][8][9][10], residual stress [11,12], dielectric properties [6,7,13], and overall electromechanical response [11,14,15]. Motivated by industrial interest in the improved performance in AlN-based piezoelectric microelectromechanical systems (piezoMEMS) devices, the breadth of this work was focused on compositions with 0 < x < 0.4, where single-phase textured wurtzite materials could be synthesized.…”

YxAl1−xN thin films

“…It is chemically and high-temperature stable, and is known for its low acoustic losses, high Q values as well as the highest piezoelectric constant (e 33 =1.46 C/m 2 ) among group III nitride semiconductors [2]. Recently, studies on alloying AlN with ScN reported a significant increase in the piezoelectric response and electromechanical coupling [3][4][5][6][7][8].…”