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 (FO) 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.