Facet-Dependent Electronic Properties and Interfacial Point Defect Interactions in WS$_2$/ZnO Heterostructures

Aiming at two-dimensional materials for high-efficiency optoelectronics, WS$2$/ZnO heterostructures are computationally screened for their facet-dependent electronic properties and interfacial defect thermodynamics using first-principles hybrid functional calculations. Interface comparison identifies the non-polar ($10\overline{1}0$) $m$-plane as the optimal substrate facet, maintaining a direct 2.42~eV bandgap and a robust type-I band alignment. Isolated sulfur ($\mathrm{V_S}$) and interfacial oxygen ($\mathrm{V_O}$) vacancies introduce deep non-radiative recombination centers. Conversely, zinc vacancies ($\mathrm{V{Zn}}$) act as shallow acceptors near the valence band edge, contributing to unintentional $p$-type behavior. Analysis of defect pairs reveals that neutral vacancies cluster across the van der Waals gap due to favorable binding energies. Under $n$-type conditions, defects stabilize as charged species. Although inter-layer Coulomb repulsion weakens the binding energy of $(\mathrm{V_S} - \mathrm{V_{Zn}})''''$ pairs, their formation energy drops to 2.61~eV under anion-poor conditions, making the $-4$ cluster the most thermodynamically abundant defect pair at the interface. Furthermore, native $\mathrm{V_{Zn}}$ prevents the Fermi level rise typically induced by interstitial hydrogen ($\mathrm{H_i}$), distributing donated electrons into shallow acceptor states and preserving host band edge rigidity. These findings establish a microscopic framework for substrate selection and defect engineering in 2D/3D hybrid light-emitting diodes.