Effects of post-deposition annealing and oxygen content on the structural and pH-sensitive properties of thin Nd²O³ films

This paper describes the structural properties and sensing characteristics of thin Nd²O³ sensing membranes deposited on silicon substrates by means of reactive sputtering. X-ray diffraction, X-ray photoelectron spectroscopy, and atomic-force microscopy were used to study the chemical and morphological features of these films as functions of the growth conditions (argon-to-oxygen flow ratios of 20/5, 15/10 and 10/15; temperatures ranging from 600°C to 800°C). The thin Nd²O³ electrolyte-insulator-semiconductor devices prepared under a 15/10 flow ratio with subsequent annealing at 700°C exhibited a higher sensitivity (56.01 mV/pH, in the solutions from pH 2 to 12), a smaller hysteresis voltage (4.7 mV in the pH loop 7 \to 4 \to 7\to 10 \to 7), and a lower drift rate (0.41 mV/h in the pH 7 buffer solution) than did those prepared at the other conditions. We attribute this behavior to the optimal oxygen content in this oxide film forming a high density of binding sites and a small surface roughness.