Strained Hexagonal ScN: A Material with Unusual Structural and Optical Properties

Local-density approximation calculations are performed to predict properties of compressively strained hexagonal ScN. This material is found to exhibit a large electromechanical response, a structural phase transition from a nonpolar to a polar structure, and a variation of the band gap in the entire visible light range when continuously changing the compressive strain. Microscopic effects responsible for these anomalies are revealed and discussed. Suggestions on how to practically grow ScN-based materials having such unusual properties are also provided.

Figure 1

Schematic representation of hexagonal ScN for three different strains, corresponding to a compression of the in-plane lattice constant with respect to its equilibrium value. The black spheres denote N atoms, while the Sc atoms are represented by white spheres. The bold lines join a given N atom to its nearest Sc neighbors. (a) shows the equilibrium hexagonal ScN ($\eta =0$), which adopts a nonpolar and nearly fivefold coordinated structure. (b) displays the phase corresponding to a compressive strain of 3.4%, for which a phase transition from a nonpolar to a polar structure is predicted to occur. The arrow indicates that N atoms have all moved down with respect to their internal coordinates associated with (a). (c) represents the phase corresponding to $\eta =6.4\%$ and for which the number of atomic nearest neighbors becomes nearly 4 rather than 5: the bond between N atoms and Sc atoms located in the (0001) plane above them is broken.

Figure 2

Structural properties of hexagonal ScN as a function of the compressive strain $\eta$. (a) and (b) display the axial ratio $c/a$ and the internal parameter $u$, respectively. The arrows indicate the regions of particular interest discussed in the text.

Figure 3

Electromechanical and optical properties of hexagonal ScN as a function of the compressive strain $\eta$. (a) and (b) display the $e_{33}$ piezoelectric coefficient and the lowest-in-energy optical transitions from occupied to unoccupied electronic levels, respectively. These optical transitions are estimated by increasing the LDA predictions by a constant value of 1.3 eV, which is the energetic difference between the experimental [15] and LDA [16] band gaps in rocksalt ScN.