Investigation of Optimum Deposition Conditions of Radio Frequency Reactive Magnetron Sputtering of Al_0.7Sc_0.3N Film with Thickness down to 20 nm

Abstract

Ferroelectric aluminum scandium nitride (Al0.7Sc0.3N) has attracted increasing interest due to its high remanent polarization (Pr, >100 µC cm−2) and coercive field (Ec, >5 MV cm−1). The four radio frequency reactive magnetron sputtering conditions (sputtering power, N2 flow ratio, pressure, and temperature) influence the ferroelectric and material properties of 45 nm‐thick Al0.7Sc0.3N deposited on the TiN/SiO2/Si substrate. Crystallinity is enhanced under the deposition conditions with higher adatom energy but deteriorates when the growth condition increases over the optimum. The well‐crystallized films have (002)‐preferred orientation with the in‐plane compressive stress imposed by the peening effect and thermal stress. The imposed compressive stress increases the c0/a0 value, where c0 and a0 mean the c‐ and a–axis lattice parameters, which eventually increases the Ec of the film. Pr increases with the c0/a0 value, but other factors also influence the change. The films with high oxygen concentration show the wake‐up properties due to the large percentage of domain walls and their depinning. Finally, ferroelectricity is confirmed with films down to a thickness of 20 nm. However, the thinnest film shows a higher Ec and lower Pr. These findings imply the presence of non‐ferroelectric interfacial layers, which induce the depolarization effect.

Thermal Stability of the Ferroelectric Properties in 100 nm-Thick Al0.72Sc0.28N

“…Also, it is expected that wake-up behavior could be observed if the Al 0.72 Sc 0.28 N-based capacitor incorporates a significant amount of oxygen during the electrical measurements since they are not performed under vacuum conditions. In contrast to what was observed by Ryoo et al, 44 the capacitors do not return to the same (higher) magnitude of polarization after wake-up above 673 K. No wake-up behavior is observed when the capacitors are measured at 673 K or below, independently from the cooling or heating step. These observations suggest that above 673 K, Al 0.72 Sc 0.28 N undergoes a structural rearrangement, and the wake-up phenomena are possibly related to defects generated by the Sc migration to the grain boundaries or strong oxygen incorporation.…”

“…The reversible change in the intercept value with temperature suggests that there is an activation of defects above 573 K. These defects can enhance the nucleation of new domains and reduce the energy barrier between thermodynamically equivalent states. Possible defect candidates in Al 1– x Sc x N are point defects, such as nitrogen or cation vacancies; extrinsic defects, for instance, due to oxygen incorporation; and crystallographic defects, such as stacking faults, dislocations, or phase and grain boundaries. ,− The decrease in the linear slope when increasing the temperature above 573 K could be associated with a reduction of ν 0 , which depends on the frequency of the optical phonons in the crystal . The reduction of the optical phonon frequency with temperature has been extensively reported in the literature. − The temperature-dependent shift of the optical phonon frequencies depends on anharmonicity in the vibrational potential, thermal expansion, lattice and thermal mismatches, and strain arising from point defects .…”

“…46,66 The hypothesis of a relationship between the increase in lattice constant along the c axis and the Sc segregation to the grain boundaries was already reported in the TEM-EDS analysis performed on Al 0.7 Sc 0.3 N films. 44 Satoh et al 46 observed that the c lattice constant increases when Al 1−x Sc x N consists of a two-phase mixture from wurtzite-type and cubic-type regions as a consequence of easing the c-axis distortion due to the cubic-type structure appearance. However, no evidence of any cubic-type structure was found in the in situ temperature-dependent XRD measurement, thus suggesting an initial Sc migration to the grain boundaries before any possible additional phase appears if Sc segregation is responsible for the lattice expansion along the c axis.…”

“…Grain boundaries can offer more space and coordination for Sc incorporation with respect to the wurtzite-type structure, thus supporting the hypothesis of Sc out-diffusion from the grains. 51 TEM-EDS confirmed the possibility of Sc segregation to the grain boundaries in Al 0.7 Sc 0.3 N. 44,51 A reduction of the Sc-content in the grains would imply an increase in the energy barrier between the polarization states and an increase in the optical phonon frequency due to the lower atomic mass of Al with respect to Sc, thus contributing to the increase in both linear fit parameters. 16,52,53 In agreement with the observation of an irreversible transformation experienced by the ferroelectric capacitors with heating, wake-up behavior appears in those measured at 773 and 873 K. An example of this result is reported in Figure S7 of the Supporting Information.…”

“…In AlN, the polarization orientation can be changed depending on the oxygen incorporation in the layer. 57,58 Ryoo et al 44 reported that in Al 0.7 Sc 0.3 N, wake-up behavior could result from oxygen incorporated during the deposition of the ferroelectric layer. Also, it is expected that wake-up behavior could be observed if the Al 0.72 Sc 0.28 N-based capacitor incorporates a significant amount of oxygen during the electrical measurements since they are not performed under vacuum conditions.…”