Dynamics of ALD-grown ZnO thin films with varying thicknesses and substrate temperatures for optoelectronic applications

This work investigates the synergy between the structural properties and the optical and electrical behaviour of ZnO layers synthesized using diethylzinc (DEZ) and ozone precursors, aiming to evaluate their potential for ultraviolet (UV) sensing and photovoltaic applications. Hexagonal wurtzite-phase zinc oxide (ZnO) thin films, belonging to the P63mc space group, were synthesized using the atomic layer deposition (ALD) technique. ZnO films with thicknesses ranging from 56 nm to 530 nm were deposited on Corning 2947 substrate at substrate temperatures of 150 °C, 200 °C, and 250 °C. X-ray diffraction (XRD) analysis showed that the (002) diffraction peak was the most intense, indicating a strong preferential orientation along the c-axis, corresponding to columnar growth perpendicular to the substrate surface. Thin films with a thickness of 530 nm, deposited at a substrate temperature of 150 °C, exhibited unit cell parameters of a = b = 3.238 Å, c = 5.2 Å, with a calculated unit cell volume of 47.21 ų. Their average crystallite size (D) was estimated to be 36 nm. The films exhibit high optical transparency, reaching up to 95%, and possess a wide bandgap ranging from 3.31 eV to 3.44 eV. The absorption coefficient (α) is > 1 × 10⁵ cm⁻¹ in the UV region and ≤ 1 × 10⁴ cm⁻¹ in the visible region. Hall effect measurements confirmed that the ZnO thin films exhibit n-type carrier conductivity. The resistivity (ρ) ranged from (6.4–53.0) × 10^–2 Ω-cm, the carrier concentration varied from (0.32–5.73) × 10¹⁸ cm⁻³, and mobility ranged from 16.8 to 38.5 cm²/Vs, depending on the film thickness and substrate temperature. The Al/n-ZnO/Al metal–semiconductor–metal (MSM) structures exhibit enhanced photodetection performance, with photosensitivity of 234.4%, a specific detectivity of 5.27 × 10¹¹ Jones, and a responsivity of 1.338 A/W. Their measured response and recovery times are 5.27 s and 7.62 s, respectively.