Enhanced Electromechanical Coupling in Yb-Substituted III–V Nitride Alloys

Abstract

Group-III nitride alloys are currently used in various microwave communication applications because of the giant enhancement in electromechanical coupling after alloying with rocksalt nitrides such as YbN or ScN. Herein, the Yb-substitution-induced enhancement for electromechanical coupling in wurtzite III−V nitrides is studied via theoretical calculations and experiments. The substitution-induced mechanical softening and local strain can enhance electromechanical coupling. The mechanical softening shows less dependence on the parent AlN or GaN, which is considered to be caused by the Yb−Yb pair interaction in the c-axis, and the difference of electromechanical coupling between the GaN-and AlNbased alloys mainly comes from the enhancement effect of Yb substitution for the piezoelectric response. The largest change in the piezoelectric response relative to the parent nitride is observed in the GaNbased alloy, and is mainly attributed to a small piezoelectric constant of the parent GaN, which makes GaN as the parent nitride difficult to improve its electromechanical coupling coefficient compared with AlN-based alloys. Moreover, our calculations reveal that the substitutional element with a closer ionic size to the host cation is easier to substitute into the host nitride and produces a larger internal strain to partly contribute to the enhancement in the piezoelectric response. This can serve as a simple guideline to identify alloying components in a search for a massive increase in electromechanical coupling.

Synthesis and Calculations of Wurtzite Al1–xGdxN Heterostructural Alloys

“…In comparison, other studies have computationally predicted x = 0.25 for Al 1−x Sc x N 10 and x = 0.1−0.15 for Al 1−x Yb x N 69 at T = 4000 K. It is not clear why Yb solubility is significantly lower compared to other group-3 elements. 32,69 From a DFT calculation perspective, Yb is a particularly challenging f-electron element that exhibits mixed valence (Yb 2+ , Yb 3+ ); Yb_3 pseudopotential is not available with VASP distribution, and Yb_2 is a valid choice only when Yb exists predominantly as Yb 2+ , which is not the case in Al 1−x Yb x N. The use of the Yb pseudopotential, where the 4f electrons are treated as valence electrons, requires careful consideration of the magnetic ordering. We can conclude that a large family of M-rich wurtzite Al 1−x M x N heterostructural alloys can be realized at effective T that can be accessed with nonequilibrium growth techniques such as sputtering.…”

Synthesis and Calculations of Wurtzite Al1−xGdxN Heterostructural Alloys

“…In comparison, other studies have computationally predicted x = 0.25 for Al 1−x Sc x N 10 and x = 0.1 -0.15 for Al 1−x Yb x N 61 at T = 4000 K. It is not clear why Yb solubility is significantly lower compared to other group-3 elements. 33,61 From a DFT calculation perspective, Yb is a particularly challenging f -electron element that exhibits mixed valence (Yb 2+ , Yb 3+ ); Yb_3 pseudopotential is not available with VASP distribution, and Yb_2 is a valid choice only when Yb exists predominantly as Yb 2+ , which is not the case in Al 1−x Yb x N. The use of the Yb pseudopotential, where the 4f electrons are treated as valence electrons, requires careful consideration of the magnetic ordering. We can conclude that a large family of M -rich wurtzite Al 1−x M x N heterostructural alloys can be realized at effective T that can be accessed with non-equilibrium growth techniques such as sputtering.…”

Characteristics of c-axis oriented GeAlN films and applications to higher-order mode BAW resonators

“…Metal-doped AlN films have recently attracted attention as new piezoelectric films for BAW resonators because they can lead to improved performance compared with AlN film BAW resonators. Electromechanical coupling factors can be enhanced in BAW resonators based on Sc-doped, [4][5][6][7][8] Yb-doped, 9,10) and MgZr-doped 11,12) AlN films. AlN or metal-doped AlN film BAW resonators with resonance frequencies above 5 GHz are being developed for 5G and beyond 5G communication systems.…”