Influence of Oxygen Content and Post-Deposition Annealing on Structural and Sensing Characteristics of $\hbox{Tm}_{2}\hbox{O}_{3}$ Thin Membranes for pH Detection

In this paper, we describe an electrolyte-insulator-semiconductor (EIS) device prepared from Tm
₂
O
₃
sensing membranes deposited on Si (100) substrates through 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 10/20, 15/15, and 20/10; temperatures ranging from 700 to 900°C). The Tm
₂
O
₃
EIS prepared under an Ar/O
₂
flow ratio of 15/15 with subsequent annealing at 800°C exhibited a higher sensitivity (58.02 mV/pH, in the solutions from pH 2 to 12), a smaller hysteresis voltage (2 mV in the pH loop 7 ¿ 4 ¿ 7 ¿ 10 ¿ 7), and a lower drift rate (1.04 mV/h in the pH 7 buffer solution) than those of the other conditions. We attribute this behavior to the optimal oxygen content in this oxide film forming a strong (622) crystallographic orientation of Tm
₂
O
₃
, a thin low-k interfacial layer, and a large surface roughness.