Influence of oxygen content on the structural and pH-sensitive properties of thin Nd₂O₃ electrolyte-insulator-semiconductor

We investigate the structural properties and sensing characteristics of thin Nd₂O₃ sensing membranes deposited on silicon substrates by means of reactive sputtering. X-ray diffraction 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). The thin Nd₂O₃ electrolyte-insulatorsemiconductor devices prepared under a 15/10 flow ratio 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→4→7→ 10→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.