Work

Dopant size governs valence-band-edge splitting and hierarchical phonon scattering in Cu- and Ag-doped p -type ZnTe

Year: 2026

Type: article

Abstract: Zinc telluride (ZnTe) is a promising wide-bandgap thermoelectric host, but its low intrinsic hole concentration and stiff, highly conductive lattice have historically restricted its figure of merit ( zT ). Here, we report a systematic comparative study of Cu and Ag doping in bulk p -type Zn 1− x M x Te 1+ x (M=Cu, Ag; x = 0, 0.01, 0.05, 0.10) synthesized via melt-quenching and spark plasma sintering, demonstrating that a single design variable—dopant ionic radius—governs both electronic and thermal transport channels. Both elements substitute as monovalent acceptors on Zn 2+ sites, raising the Hall carrier concentration to 4.75 × 10 19 cm −3 at 300 K and ∼3.6 × 10 20 cm −3 at 773 K. First-principles calculations reveal that substitution lowers local tetrahedral site symmetry and splits the threefold-degenerate Te-5 p valence-band edge by 60.5 meV (Cu) and 73.7 meV (Ag)—scaling with ionic radius rather than d -band position, establishing a steric rather than p – d orbital origin. Because both splittings are comparable to k B T ≈58 meV at 673 K, all three valence sub-bands remain thermally co-populated. The measured Seebeck coefficient corroborates this band convergence, yielding a density-of-states effective mass of m DOS * ≈1.3–1.4 m e more than double the single-band value. Concurrently, ionic size mismatch generates 0.45% microstrain, localized nanoscale strain fields, and coherent Ag 2 Te precipitates. This three-tiered phonon scattering hierarchy depresses the lattice thermal conductivity to 0.36 W m −1 K −1 at 873 K—touching the theoretical Cahill minimum limit—while coherent phase boundaries preserve hole mobility. Although Cu doping achieves a marginally higher electronic power factor, Ag doping prevails decisively in thermal transport suppression, yielding a peak figure of merit zT ≈0.75 at 673 K compared to 0.64 at 773 K for Cu. Dopant ionic size thus emerges as a vital design parameter for defect engineering in lead-free wide-bandgap chalcogenides.

Source: Journal of Physics D Applied Physics

DOI: 10.1088/1361-6463/ae9ed5