Consistent evidence for intra- and inter-columnar transport decoupling in sputter-grown ZnSnN₂ thin films through growth-driven microstructural evolution

ZnSnN₂ is an earth-abundant nitride semiconductor of interest for low-cost thin-film growth, yet the origin of transport degradation in sputter-grown films remains poorly understood. Here, we systematically investigate the growth-duration-dependent structural, electrical, optical, and compositional evolution of sputter-grown ZnSnN₂ thin films. By combining Hall measurements with terahertz time-domain spectroscopy, we obtain consistent evidence for length-scale-dependent transport decoupling, in which Hall measurements are sensitive to macroscopic inter-columnar connectivity, whereas the THz response reflects the local intra-columnar electrical response. Although the Hall mobility, conductivity, and scattering time decrease markedly as the growth duration increases from 12 min to 60 min, the THz-derived local response remains nearly unchanged. Correlative microscopy and diffraction analyses reveal that prolonged growth promotes column widening, increasing column inclination, and progressive inter-columnar void formation, which disrupt electrical connectivity across the film. The 60 min sample correspondingly exhibits the lowest refractive index and the highest extinction coefficient, consistent with pronounced structural degradation. Taken together, these observations support a transport-decoupling scenario in which the dominant transport bottleneck in thick sputter-grown ZnSnN₂ films originates primarily from microstructural evolution of the columnar architecture rather than from substantial deterioration within individual columns. This work establishes a consistency-based, length-scale-aware framework for understanding and mitigating transport degradation in columnar semiconductor thin films.