Abstract
We present detailed investigations of the structural, elastic, dielectric, and piezoelectric properties of scandium aluminum nitride (ScxAl1−xN) with the wurtzite crystal structure by means of first-principles calculations based on density functional theory in order to enable a detailed comparison to the corresponding physical properties of GaAlN and InAlN. The goal of our approach is to use atomistic simulations to extract the novel solid state characteristics of ScxAl1−xN crystals by the determination of complete sets of coefficients for the elastic, compliance, and piezoelectric tensor and to confirm the theoretical predictions by experimental measurements of selected tensor coefficients. The calculation of the tensor components is accompanied by a detailed analysis of the crystal structures, e.g., average bond length, bond angles, lattice parameters, and mass density in dependence on alloy composition of ScxAl1−xN. If an increasing number of Al atoms of up to x = 0.5 are replaced by Sc atoms, we observe a nonlinear change of the ratio of lattice parameter c(x)a(x) and average bond angles of about 10% and 5%, respectively, which give an indication of an increasing deviation of the crystal structure of ScxAl1−xN from an ideal hexagonal lattice. As a consequence of the deformed crystal structure and the iconicity of the Sc–N bond, we predict a change in value of the elastic coefficient C33ScAlN(x), the piezoelectric coefficient e33ScAlN(x), and the value of spontaneous polarization PSPScAlN(x) of up to 65%, 150%, and 230%, respectively. Based, on these simulation results, physical features of practical use are derived, like the average bulk, shear, and the Young modulus as well as the reciprocal Young's modulus and Poisson ratio. Furthermore, the spontaneous polarization of ScxAl1−xN is approximated, taking nonlinear effects into account as well as the piezoelectric polarization caused by uniaxial, biaxial, and hydrostatic stresses in dependency on alloy composition and strain. A detailed comparison of the structural and polarization related properties of GaAlN and InAlN allows us to point out the peculiarity of wurtzite ScAlN crystals within the group III-nitrides.
Highlights
Microelectronic energy harvester and acoustic wave devices operate on the basis of the piezoelectric effect, in which mechanical energy is transferred to electrical energy or vice versa.[1,2] The performance of a piezoelectric microelectromechanical system or a piezo-acoustic wave device is determined by the properties of the active layer, such as the piezoelectric coefficients, electromechanical coupling, and mechanical quality factor.[3]
Yanagatani and Suzuki[9] have obtained a significant increase of the coupling coefficient from kt2 = 6.4%–14% by enhancement of Sc concentration up to x = 0.38, proving the potential of thin ScAlN films for high frequency acoustic wave devices. These outstanding results motivate this article in which we present elastic, dielectric, and piezoelectric properties of ScAlN with the wurtzite crystal structure using first-principles calculations based on density functional theory (DFT) added by a detailed experimental analysis of structural, mechanical, and polarization related effects
Because biaxial strain in epitaxial layers of group-III nitride heterostructures grown along the [0001]-axis caused by mismatch of the lattice parameter a and/ or a mismatch of the thermal expansion coefficients of the layer and the substrate directed along the basal plane is of most practical relevance, we focus our further efforts on the simulation of the piezoelectric polarization for this case
Summary
Microelectronic energy harvester and acoustic wave devices operate on the basis of the piezoelectric effect, in which mechanical energy is transferred to electrical energy or vice versa.[1,2] The performance of a piezoelectric microelectromechanical system or a piezo-acoustic wave device is determined by the properties of the active layer, such as the piezoelectric coefficients, electromechanical coupling, and mechanical quality factor.[3]. Yanagatani and Suzuki[9] have obtained a significant increase of the coupling coefficient from kt2 = 6.4%–14% by enhancement of Sc concentration up to x = 0.38, proving the potential of thin ScAlN films for high frequency acoustic wave devices. These outstanding results motivate this article in which we present elastic, dielectric, and piezoelectric properties of ScAlN with the wurtzite crystal structure using first-principles calculations based on density functional theory (DFT) added by a detailed experimental analysis of structural, mechanical, and polarization related effects. We point out novel properties of ScAlN in comparison to the related ternary nitrides InAlN and GaAlN in order to stimulate new designs of application related micromechanical, piezo-acoustic, as well as electronic devices
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