Enhancement of Piezoelectric Response in Scandium Aluminum Nitride Alloy Thin Films Prepared by Dual Reactive Cosputtering

Adv. Mater. 2009, 21, 593–596 2009 WILEY-VCH Verlag Gm The industrial demand for higher-temperature piezoelectric sensors is drastically increasing, for the control of automobile, aircraft, and turbine engines and the monitoring of furnace and reactor systems, because environmental problems, such as carbon dioxide (CO2) and nitrogen oxide (NOx) reduction, are becoming more globally serious. The sensors are also desirable for health monitoring coal-fired electric-generation plants and nuclear plants. It is generally known that piezoelectric materials with a higher Curie temperature possess a lower piezoelectric coefficient. Furthermore, the results of a study (Fig. 1) of the relationship between maximum use temperature and piezoelectric coefficient d33 shows that the piezoelectric materials with a higher maximum use temperature possess a lower piezoelectric coefficient d33. [3–9] For example, the Curie temperature and piezoelectric coefficient d33 of lead zirconium titanate (PZT), which is widely used in many electronic devices, are 250 8C and 410 pCN , respectively. The maximum use temperature and d33 of aluminum nitride (AlN), which is a typical hightemperature piezoelectric material, are 1150 8C and 5.5 pCN . It is difficult to achieve a good balance between high maximum use temperature and large piezoelectricity in a material, and no effective piezoelectric materials with these characteristics have yet been found. In this communication, we report a hightemperature piezoelectric material that exhibits a good balance between high maximum use temperature and large piezoelectricity. This was achieved by the combination of the discovery of a phase transition in scandium aluminum nitride (ScxAl1 xN) alloy thin films and the use of dual co-sputtering, which leads to nonequilibrium alloy thin films. Sc0.43Al0. 57N alloys exhibit a large piezoelectric coefficient d33 of 27.6 pCN , which is at least 500% larger than AlN. The large piezoelectric coefficient d33 is the highest piezoelectric response among the tetrahedrally bonded semiconductors, despite the fact that the crystal structure of scandium nitride (ScN) is rock-salt (nonpolar). Moreover, the large piezoelectricity is not changed by annealing at 500 8C for 56 h under vacuum. This work demonstrates the new route to design of this high-temperature piezoelectric material. ScN has a rock-salt structure (nonpolar). However, Takeuchi reported the existence of a (meta)stable wurtzite structure in ScN, and the possible fabrication of Sc-IIIA-N nitrides by firstprinciples calculations. Farrer et al. predicted that the wurtzite structure is unstable in ScN, and that the hexagonal structure is (meta)stable in ScN, unlike the wurtzite structure. The piezoelectric responses of hexagonal ScxGa1 xN and ScxIn1 xN alloys can be enhanced by an isostructural phase transition (from wurtzite to layered hexagonal). However, the piezoelectric responses and Curie temperatures of the nitride alloys have not yet been confirmed by experiments. AlN, GaN, and InN are IIIA nitrides and have a wurtzite structure (polar). In particular, the thermal stability and piezoelectricity of AlN are the highest among the IIIA nitrides. AlN is a piezoelectric material compatible with the Complementary metal–oxide– semiconductor (CMOS) manufacturing process, and is a promising material for integrated sensors/actuators on silicon substrates. Wurtzite and rocksalt structures have rather different lattice forms and unit sizes. The formation of