LAPS with nanoscaled and highly polarized HfO₂ by CF₄ plasma for NH₄⁺ detection

In this article, the detection of ammonium (NH₄⁺) ion using nanoscaled 2-nm thick atomic layer deposition (ALD)-hafnium oxide (HfO₂) films with post rapid thermal annealing (RTA) and carbon tetrafluoride (CF₄) plasma treatments based on light-addressable potentiometric sensor (LAPS) was investigated. 2-nm thick ALD-HfO₂ films with post RTA and CF₄ plasma treatment were fabricated as sensitive membranes, respectively. Measured pNH₄ response from 2-nm thick ALD-HfO₂ LAPS was decreased with increasing annealing temperature and was improved under CF₄ plasma treatment. The optimum pNH₄-sensitivity of 37 mV/pNH₄ was achieved with both 900 °C annealing and 5 min CF₄ plasma on ALD-HfO₂ LAPS. When compared to the same structure without plasma treatment, the sensitivity was improved by approximate fourfold. Based on X-ray photoelectron spectroscopy (XPS) analysis, increased pNH₄-sensitivity was attributed to polar dipole (F–O) formation in ALD-HfO₂ thin films due to the incorporation of fluorine by CF₄ plasma treatment. To assess interferences from other ions (H⁺, Na⁺, K⁺, and Ca²⁺), selectivity coefficients obtained by fixed interference method (FIM) measurements were presented.