Effect of Ta Doping Concentration on the Optical Band Gap of ZnO/Porous Silicon Heterostructures

UV photodetectors require materials with strong absorption in the ultraviolet region and well-defined band-edge transitions. ZnO/porous-silicon (PS) heterostructures combine the light-trapping properties of PS with the UV selectivity of ZnO. This study investigates the effect of the Ta doping ratio on the optical band gap (Eg) of ZnO/PS heterostructures. PS layers were fabricated on p-type Si wafers by electrochemical anodization at current densities of 5 and 10 mA/cm² for durations of 1 and 2 min. Undoped and Ta-doped (1%, 2%, 3%) ZnO thin films were deposited onto the PS substrates via spin coating. Structural, morphological, and elemental properties were characterized by XRD, SEM, and EDS. Optical properties were evaluated by UV-Vis spectroscopy, and Eg values were obtained from Tauc plots. ZnO crystallized in the hexagonal wurtzite structure with preferred (002) orientation in all heterostructures. EDS confirmed successful Ta incorporation into the ZnO matrix. The optical band gap varied in the range 3.24–4.12 eV. The highest Eg value (4.12 eV) was obtained for 3% Ta-doped ZnO deposited on porous silicon (PS) prepared under anodization conditions of 10 mA/cm² for 2 min. The Burstein–Moss band-filling effect is consistent with the observed band-gap shift at higher doping ratios. The band gap of ZnO/PS-based UV-sensitive heterostructures can be tuned not only through the doping ratio but also through the pore morphology of the PS substrate. Ta doping combined with controlled anodization parameters is an effective strategy for band-gap engineering in these systems.