Supercycle-Modulated Transparent Conductive Hf-Doped ZnO Thin Films by Thermal ALD for Photovoltaic Applications

This study investigated the effect of film thickness on the physical characteristics of Hf-doped ZnO (HZO) transparent conducting oxide films deposited by using atomic layer deposition (ALD) at a temperature of 250 °C. HZO films of varying thicknesses were deposited using a supercycle approach to analyze their structural, morphological, optical, and electrical characteristics. Grazing incidence X-ray diffraction (GIXRD) analysis revealed the hexagonal wurtzite crystal structure of the HZO films with a predominant orientation along the (100) orientation. Surface morphology and topographical (scanning electron microscope (SEM) and atomic force microscopy (AFM)) studies revealed an increase in grain size and surface roughness with increasing film thickness. The average optical transmittance of the HZO films exceeded 80% in the visible and near-infrared (NIR) regions. The films demonstrated a competitive resistivity of 2.59 mΩ-cm at a relatively lower film thickness of 41 nm with an optical transmittance of ∼90%, and a minimum resistivity of 1.39 mΩ-cm at higher thicknesses (∼83 nm) with a Hf doping concentration of 3.87 at. %. Additionally, Al/HZO/p-Si/Al heterojunctions demonstrated a transition from rectifying to Ohmic behavior with increasing film thickness as a result of changes in the work function of HZO. From the electrical measurements (I–V) of the device with 18 nm-thick HZO layer, the ideality factor (n) and barrier height (φ_b) were calculated to be 8.18 and 0.49 eV, respectively. The results demonstrated that ALD-grown HZO films are high-quality and promising candidates as transparent conducting oxide layers for photovoltaic device applications, even at lower thicknesses.