Designing Multifunctionality via Assembling Dissimilar Materials: Epitaxial AlN/ScN Superlattices

Abstract

First-principles calculations are performed to investigate the effect of epitaxial strain on energetic, structural, electrical, electronic and optical properties of 1 × 1 AlN/ScN superlattices. This system is predicted to adopt four different strain regions exhibiting different properties, including optimization of various physical responses such as piezoelectricity, electro-optic and elasto-optic coefficients, and elasticity. Varying the strain between these four different regions also allows the creation of an electrical polarization in a nominally paraelectric material, as a result of a softening of the lowest optical mode, and even the control of its magnitude up to a giant value. Furthermore, it results in an electronic band gap that can not only change its nature (direct versus indirect), but also cover a wide range of the electromagnetic spectrum from the blue, through the violet and near ultraviolet, to the middle ultraviolet. These findings thus point out the potential of assembling two different materials inside the same heterostructure to design multifunctionality and striking phenomena.

Ultrahigh energy storage density in epitaxial AlN/ScN superlattices

“…Moreover, one can also estimate the intrinsic breakdown field of such systems empirically, which depends on the band gap [47]. Our predicted gap of 3.1 eV after correction (to account for the typical underestimation of LDA) of the 1×1 AlN/ScN superlattice ground state [27] yields an intrinsic breakdown field, E break , of 6.3 MV/cm, which will be the maximum fields considered here. Note that the estimated breakdown field is reasonable since (1) Yazawa et al reported the highest applied electric field (∼6 MV/cm) in epitaxial Al 0.7 Sc 0.3 N film [28]; and (2) Yasuoka et al also reported very large coercive fields of the order of 4-7 MV/cm and maximum applicable electric fields of the order 5-10 MV/cm in Al 1−x Sc x N films [29].…”

“…This would require the nonlinear dielectrics to have large ε r and high E break [11][12][13][14][15][16]. A promising candidate is the III-V semiconductor-based systems made by mixing AlN and ScN to form Al 1−x Sc x N solid solutions or AlN/ScN superlattices, that have been attracting much attention due to their potential for high piezoelectric and electro-optic responses [20][21][22][23][24][25][26][27][28][29]. Note that a recent experiment observed a ferroelectric switching in Al 1−x Sc x N films, with a remnant polarization reaching a very large value -in excess of 1.0 C/m 2 [25] -which contrasts with the case of pure AlN that is of wurtzite-type structure [30,31] and is polar but not ferroelectric.…”

“…Here, first-principles calculations are performed on (001) epitaxial 1×1 AlN/ScN superlattices within the local density approximation (LDA) to the density functional theory (DFT), as implemented in the ABINIT package [32]. The epitaxial strain is defined as η in = (a − a eq )/a eq , where a eq is the inplane lattice constant corresponding to the equilibrium P 6m2 phase of bulk 1×1 AlN/ScN superlattices [27]. A 6 × 6 × 4 k-point mesh and a plane-wave kinetic energy cutoff of 50 Hartree and of 60 Hartree are employed for structural relaxation.…”

“…For the 1×1 AlN/ScN superlattices under different epitaxial strains, previous studies have predicted the existence of different phases [26,27], including a wurtzite-derived structure (polar P 3m1 space group with a polarization along the c-axis) and a hexagonal-derived structure (non-polar P 6m2 space group). They are related to each other by a continuous change in the internal parameter (u) and the axial ratio (c/a) [27]. Figure 1(a) shows the P -E curves for strain values larger than −0.5% (from compressive to tensile), with the hexagonal-derived structure as ground state [see Fig.…”

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

“…[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.…”