Atomistic Understanding of the Ferroelectric Properties of a Wurtzite‐Structure (AlN)_n/(ScN)_m Superlattice

Abstract

The wurtzite (WZ) structure AlN is a well-known piezoelectric material with a spontaneous polarization (P s ) as high as %130 μC cm À2 . [1] However, such a high P s cannot be switched reversibly by the electric field due to its presumably higher coercive field (E c ) than its typical breakdown field (E bd ). [2] This problem has severely limited the usefulness of AlN in ferroelectric devices. Such a high E c could be positively related to the high switching barrier (E a ) for the P s switching. Several other oxide materials with the WZ structure had also been examined as potential candidates for ferroelectricity. [3,4] Among them, doped ZnO has shown ferroelectricity, [5][6][7] which suggests that its E c is lower than its E bd . The doping of Sc into AlN was experimentally reported recently, allowing the (Al,Sc)N solid solution (SS) to significantly decrease the E c and making ferroelectric switching feasible. The E c and the remanent polarization (P r ) decreased with the increasing fraction of ScN and ranged from 1.5 to 4.5 MV cm À1 and 85-110 μC cm À2 , respectively. [8] The reason for the ferroelectricity manifestation was presumed to be the reduction of E a through ScN alloying in the theoretical study using a disordered (Al,Sc)N SS. [9] To avoid the configuration-dependent property variation, the AlN/ScN superlattice (SL), the ordered structure, was also recently studied. [10,11] However, both studies focused on the effect of strain in a fixed composition of an (AlN) 1 /(ScN) 1 SL. The problem of the concomitant reduction in the P s (and P r ) by the incorporated Sc in the (Al,Sc)N might be less severe in SL than in SS. Despite the importance of this aspect, it has not been studied yet.Therefore, in this study, we expanded the previous theoretical studies by encompassing various compositions of the (AlN) n / (ScN) m SL and examining the configuration effects by comparing them with those of the (Al,Sc)N SS, based on ab initio calculations. The composition and configuration effects on the ferroelectric behavior were explored by considering the atomic structure, phase stability, and key properties (E a and P s ) required for ferroelectric application at the same time. The calculated E a is a measure of the experimental E c , [12] and the calculated P s corresponds to the experimental P r , considering the almost ideal P-E hysteresis curve observed in (Al,Sc)N. [8,13] Both E a and P s were found to have decreased as the ScN fraction increased in the SL and SS structures. However, the degree of reduction in SL was less significant than in SS, as seen in the analysis of the discrepancies in the atomic structure and energy landscape during the ferroelectric switching. The insights from this study were used to propose a clear strategy for tailoring the ferroelectric properties by controlling the composition and structure. Results and DiscussionThe most stable phases of AlN and ScN are the WZ and rock salt (RS), respectively. Both materials have a layered hexagonal phase (H-phase) as a metastable state. The detail...

From Fully Strained to Relaxed: Epitaxial Ferroelectric Al1‐xScxN for III‐N Technology

“…Besides distorting the lattice, Sc has an additional effect on the switching behavior, e.g., an introduction of local structural instability which facilitates switching. [22] Contrarily, the coercive field dependence on the a-lattice parameter is more congruent for the two series, as depicted in Figure 8c. To conclude, the coercive field is heavily influenced by epitaxial strain.…”

Ferroelectricity and Piezoelectric Response of (Sc,Y)N/(Al,Ga,In)N Monolayer Alternating Stacked Structures by First‐Principles Calculations

“…It means that the realistic ferroelectric switching barriers of w ‐Sc 0.5 In 0.5 N and w ‐Y 0.5 In 0.5 N are larger than the values of Δ E , which is also observed in (AlN) n /(ScN) m superlattice and MgS cell. [ 32,33 ] However, the E eb of w ‐Sc 0.5 In 0.5 N and w ‐Y 0.5 In 0.5 N are still smaller than the conventional perovskite ferroelectric material PbTiO 3 (about 0.2 eV unit cell −1 ). [ 9 ]…”

“…MgS cell. [32,33] However, the E eb of w-Sc 0.5 In 0.5 N and w-Y 0.5 In 0.5 N are still smaller than the conventional perovskite ferroelectric material PbTiO 3 (about 0.2 eV unit cell À1 ). [9] The coercive field E c can be calculated by the following formula.…”

Piezoelectric effect and polarization switching in Al$_{1-x}$Sc$_x$N

“…of allowing lattice relaxation can be obtained by comparing with the results of Ye et al, 24 who employed the solid-state nudge elastic band method (ss-NEB) 25 that allows for cell relaxation. For pure AlN the ss-NEB result for the switching barrier is 0.23 eV/f.u., about half of our value.…”