Stability of Piezoelectric Al_1-xSc_xN Thin Films

Since the discovery of the "anomalous" piezoelectric effect in Sc-doped AlN by Akiyama et al. [1] in 2009, there has been significant interest in Al_1−xSc_xN thin films. The increase of the piezoelectric coefficients has been confirmed to be an intrinsic alloying effect and due to the softening of the lattice [2–4]. AlScN films have been studied e.g. for energy harvesting [3,5], PMUTs [6], RF filters [7], and as tunable layers in optoelectronics [8]. Efforts have been made in volume production of AlScN films [9]. The focus of the research has been on optimizing the piezoelectric properties [1,5,8,10–12]. The reported optimal fraction of Sc is 27%–43%. However, the latest published research has focused on Sc-fractions of less than 30%. The possible phases of the AlScN system are known as wurtzite (w) and rock-salt (c). However, at which Sc-concentration the phase change begins from piezoelectric w-AlScN to non-piezoelectric c-AlScN, is somewhat unclear, as shown in Fig. 1. Moreover, it is unclear how wide the two-phase mixture range is. Studies show the transition occurring at Sc-fraction of 22% [13] to 41% [1]. Moreover, the crystal quality is degraded with increasing Sc content [12,14-16].
In addition, mass separation by spinodal decomposition has been observed experimentally and theoretically [8,13,16]. This can lead to Al and Sc rich areas and to the formation of c-AlScN in films with even lower Sc-concentrations. The onset of spinodal decomposition at 1 100 K is at ca. 6% Sc-fraction according to thermodynamic simulations. However, epitaxial strain increases the allowed amount of Sc. When w-AlScN is strained on AlN, w-AlScN is stable up to 40%. Decomposition has not been observed in all experimental studies probably due to the nature of sputter deposition. Low growth temperatures limit kinetically the diffusion driven decomposition. However, studies have not evaluated the stability of w-AlScN. Almost all studies have focused on as-deposited sputtered films. As sputtering can result in nonequilibrium films, it is possible that the microstructure of w-AlScN changes to a more stable one due to high temperatures in processing or during use. The possible issue is loss of texture or formation of new phases with no or reduced piezoelectricity.
In this study 1 μm thick AlScN samples with Sc-fraction of 30% are sputtered at 450 °C directly on (100) Si. The samples are annealed for 5 h at 400, 600, 850 and 1000 °C in order to induce and determine the temperature threshold for possible changes. Afterwards the microstructure of the samples is characterized with XRD and possible decomposition products are detected with RGA. SEM and EDX is used to study the morphology and composition of the films before and after annealing. The results show that AlScN thin films are stable in annealing. The XRD results (Fig. 2) confirm that the samples are c-axis oriented w-AlScN and show no changes after annealing. The RGA test showed no significant decomposition. The EDX results did not show any mass separation. However, the SEM micrographs (Fig. 3) show changes in the topography of the film after annealing.