Transparent conductive oxides (TCOs) are critical components in optoelectronic and energy conversion technologies, including silicon heterojunction (SHJ) photovoltaics and photoelectrochemical devices. However, industrial reliance on indium tin oxide (ITO) faces limitations related to indium scarcity and plasma-induced damage during sputtering deposition. To provide a sustainable, low-damage alternative, vanadium-doped zinc oxide (VZO) thin films were synthesized by atomic layer deposition (ALD) using earth-abundant materials. Specifically, vanadium is roughly three orders of magnitude more abundant in the Earth’s crust than indium (1.2×10² vs 2.5×10^-1 ppm), offering a scalable and cost-effective doping strategy [1]. Following the optimization of the ALD sub-cycle ratio, that is, adjusting the DEZ:VTIP ratios ranging from 4:1 to 57:1, Hall effect analysis revealed n-type conductivity with resistivities below 3 mΩ·cm, and a visible transmittance exceeding 80%. In addition, this work systematically analyzes the influence of film thickness (25 to 400 nm) on the structural, electrical, and optical properties of the VZO layers.

Microstructural investigations reveal that film growth follows a competitive mechanism consistent with the Van der Drift model. Near the substrate interface, the films exhibit a preferentially oriented nanocrystalline structure (002 reflections). As thickness increases, the microstructure evolves toward a mixed polycrystalline texture, dominated by [100], [002], and [101] crystalline orientations, with noticeably coarser domains. Cross-sectional analysis confirms the formation of characteristic V-shaped columnar grains that widen continuously from the nucleation region up to 400 nm.

This microstructural development directly correlates with enhanced carrier transport. The electrical resistivity decreases monotonically with thickness, reaching a minimum of 0.31 mΩ·cm for the 400 nm thick film. Extended Drude modeling reveals that carrier transport in thinner layers is restricted by grain-boundary scattering, reflected in a substantial discrepancy between Hall and optical transport parameters. In contrast, in thicker films, electrical and optical resistivities converge, confirming that lateral grain enlargement suppresses extrinsic boundary barriers and shifts transport toward an intrinsic, intra-grain scattering regime [2].

The obtained resistivity of 0.31 mΩ·cm compares favorably with metal-doped ZnO TCO films, deposited either by physical methods (e.g., sputtering) or chemical routes, while benefiting from the low deposition temperature and soft processing inherent to ALD. These results demonstrate that engineering the ALD-grown VZO effectively mitigates extrinsic transport barriers and reduces resistivity without worsening optical transparency, validating VZO as a viable indium-free TCO for damage-sensitive device architectures.

References

[1] W. M. Haynes, D. R. Lide, T. J. Bruno, CRC Handbook of Chemistry and Physics, CRC Press, 2016.

[2] H. C. M. Knoops, B. W. H. van de Loo, S. Smit, M. V. Ponomarev, J.-W. Weber, K. Sharma, W. M. M. Kessels, M. Creatore, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 2015, 33, DOI 10.1116/1.4905086.