Thermoelectric power factors of defective scandium nitride nanostructures from first principles

TL;DR

This work addresses how microscopic defects in ScN influence electronic transport and thermoelectric performance, a major source of experimental variability. It adopts a Landauer-NEGF framework applied to ScN nanowires with defects (N-site vacancies and O substitutions, including stacking faults) and couples first-principles DFT-derived electronic structures to compute the transmission function $\mathcal{T}(E)$, conductance $G$, and Seebeck coefficient $S$ via $zT=\frac{\sigma S^2 T}{\kappa}$ and related moments $L_n$. The study identifies two defect categories—contiguous N vacancies (v+v) and oxygen impurities adjacent to stacking faults (O+sf)—as primary modulators of the Seebeck profile and conductivity, and shows that these defects can partially account for experimental variations across samples by independently affecting $\sigma$ and $S$. The results offer a mechanistic framework for defect engineering in ScN thermoelectrics, linking atomistic defect configurations to measurable transport properties and guiding synthesis toward optimized $zT$ in nanostructured ScN.

Abstract

The thermoelectric properties of scandium nitride are strongly influenced by structural and electronic factors arising from defects and impurities. Nevertheless, the mechanisms by which these microscopic features affect transport are not yet fully understood. Experiments show a large variability in the electronic transport properties, with a strong dependence on the experimental conditions, and attempts to improve thermoelectric efficiency often lead to conflicting effects. In this work, we employ the Landauer approach to analyze the effects of different kinds of structural defects and impurities on electronic transport in scandium nitride. This approach allows us to relate the transport mechanisms to the structural and electronic modifications introduced in the lattice, with atomistic resolution. In light of these new insights, we propose a rationale relating part of the experimental variability to its microscopic origin.

Figures (13)

• Figure 1: The Landauer model adapted to NW systems. The nanowire (NW) is divided into an infinite series of principal layers along its length, one layer acting as the central conductor and the other ones constituting the semi-infinite leads.
• Figure 2: Schematic representations of (a) a stacking fault, (b) a point defect, and (c) a combined configuration of a planar stacking fault and a point defect.
• Figure 3: Electronic structures of pure ScN three-dimensional (3D) and NW systems. Panels a and b: Calculated electronic band structures and densities of states (DOS) of pure ScN in its 3D bulk form, represented along the high-symmetry lines of (panel a) the primitive (p) face-centered cubic system (2 atoms per cell) and (panel b) a conventional (c) simple cubic system (8 atoms per cell). Panels c, d, and e: Calculated electronic band structures represented along the NW direction and DOS of pure ScN NW systems, as described in the text: (c) (1×)1×1+s, (d) (1×)3×3+s, and (e) (1×)5×5+s. To simplify the panels, we use the concise symbols (c) 111, (d) 133, and (e) 155, respectively. The shapes of the Wigner-Seitz cells of the different referred lattices are schematically presented in blue.
• Figure 4: Electronic band structures along the axial direction of defected (1×)5×5+s ScN NW systems. Panel a: with an O atom replacing the central N site (155O). Panel b: with a vacancy replacing the central N site (155v).
• Figure 5: A section of the 155v NW system. In gray: real-space representation at the $\Gamma$-point of the wavefunction associated with the electronic state whose band connects the points ($\Gamma$, $-0.58$) and (X, $0.01$) in the 155v electronic structure (Fig. \ref{['fig:bands_155O_155v']}, panel b). The wavefunction is represented as its contribution to the electronic charge density at an isovalue of 0.003 a.u., using the XCrySDen visualization tool Kokalj1999.