Local chemical origin of ferroelectric behavior in wurtzite nitrides

Abstract

Ferroelectricity enables key modern technologies from non-volatile memory to precision ultrasound. The first known wurtzite ferroelectric Al1-xScxN has recently attracted attention because of its robust ferroelectricity and Si process compatibility,...

Synthesis and Calculations of Wurtzite Al1–xGdxN Heterostructural Alloys

“…We attribute the higher substitution level at which we observe phase-pure wurtzite compared to previous reports to different deposition conditions. We have previously extensively optimized for nonequilibrium growth conditions (i.e., deposition temperature, process pressure, gas ratios) based on our past experience with related Al 1– x M x N alloys. ,− , We expect that additional optimization of growth parameters may be able to further increase the amount of Gd 3+ that can be incorporated into AlN.…”

“…We have previously extensively optimized for nonequilibrium growth conditions (i.e., deposition temperature, process pressure, gas ratios) based on our past experience with related Al 1−x M x N alloys. 10,[19][20][21]24 We expect that additional optimization of growth parameters may be able to further increase the amount of Gd 3+ that can be incorporated into AlN.…”

“…M−N bonds are relatively more ionic, with La−N bonds expected to be the most ionic based on electronegativities. As discussed in the context of Al 1−x Sc x N alloys, 24 the polar-covalent Al−N bonds composed of sp 3 -hybridized orbitals are highly directional. In comparison, more ionic M−N bonds tend to be spherically symmetric and nondirectional, which may allow easier incorporation in the Al−N matrix.…”

“…After significant effort, Sc 3+ structure occurs. 10,17,23−25 Recent work has shown that the solubility of Sc 3+ depends both on ionic size and on ionicity, with Sc−N bonds more ionic than Al−N bonds; 24 however, this ionicity effect is difficult to deconvolute from other effects. Other transition-metal cations such as Cr, Zr, Hf, Y, Ta, and Ni have also been investigated as alloys with AlN, and their properties have been studied.…”

“…Due to their exceptional optoelectronic, charge transport, and electromechanical properties, a large class of polar wurtzite AlN-based alloys is already enabling light-emitting diodes (isostructural Al 1– x Ga x N) , and electromechanical resonators (heterostructural Al 1– x Sc x N). , Scandium-substituted AlN has been the subject of intense interest as a promising new tetrahedral ferroelectric material; ,− however, the much smaller size of Al 3+ (∼0.39 Å) compared to that of Sc 3+ (∼0.75 Å) presents significant issues for achieving and controlling high Sc incorporation. After significant effort, Sc 3+ has been substituted into the wurtzite AlN crystal structure at compositions of up to x ≈ 0.43 (i.e., Al 0.57 Sc 0.43 N) before phase separation to the rocksalt ScN end-member crystal structure occurs. ,,− Recent work has shown that the solubility of Sc 3+ depends both on ionic size and on ionicity, with Sc–N bonds more ionic than Al–N bonds; however, this ionicity effect is difficult to deconvolute from other effects. Other transition-metal cations such as Cr, Zr, Hf, Y, Ta, and Ni have also been investigated as alloys with AlN, and their properties have been studied. − While it seems unlikely from an ionic radius perspective that any rare earth cations would be able to incorporate into wurtzite AlN, recent work has shown that Er 3+ and Yb 3+ , much larger cations than Sc 3+ , were substituted successfully at x = 0.015 and up to x ≈ 0.15, respectively. , In GaN, Yb 3+ can incorporate up to x ≈ 0.3, consistent with the larger radius of Ga 3+ (∼0.47 Å) …”