Influence of frequency and gamma irradiation on the electrical characteristics of Er₂O₃, Gd₂O₃, Yb₂O₃, and HfO₂ MOS-based devices

The unique physical, chemical, and electronic properties of rare earth oxides have been of immense interest to replace SiO₂ as a dielectric material in metal–oxide–semiconductor (MOS)-based sensors applications to accurately measure the radiation dosage and increase sensor sensitivities in as diverse applications as space radiation, nuclear physics, medical diagnostics, radiation cancer therapy, and personal dosimetry devices. Hence, the electrical characteristics of oxides prior to and after irradiation of MOS-based devices are needed since they are the backbone of the devices such as MOSFETs and ICs. In addition, an understanding of the behaviour of high-k dielectric oxides in an MOS configuration is necessary since the radiation-induced damage occurs in the bulk oxide film and/or near the oxide–semiconductor interface resulting in creation of lattice defects. Hence, MOS structures with the rare earth oxides of Er₂O₃, Gd₂O₃, Yb₂O₃, and a transition metal oxide of HfO₂ were produced by RF magnetron sputtering to determine (a) the structure of the films, (b) dielectric constants, (c) capacitance versus voltage behaviour of Er₂O₃, Gd₂O₃, Yb₂O₃, and HfO₂ prior to and after irradiation of the devices in the dose range of 0–76 Gy. The experimental results were analysed with a theoretical framework on the energy band diagram and the radiation effects on the electrical characteristics of the MOS capacitors. The characteristics of the devices were evaluated by using effective oxide charge density (\( Q_{\text{EFF}} \)), variation in the oxide trapped charge density (\( \Delta N_{\text{ox}} \)), and interface trapped charge density (\( \Delta N_{\text{it}} \)). In addition, barrier height (\( \phi_{\text{b}} \)), image force barrier lowering (\( \Delta \phi_{\text{b}} \)), acceptor concentration (\( N_{\text{a}} \)) were calculated before and after irradiation and examined the nature of interface states. The radiation responses of the Er₂O₃ and HfO₂ MOS capacitors did not show a stable behaviour with an increase in radiation dose due to possible neutral electron trap centres. Contrary to expectations, we infer that more negative charges are trapped in Gd₂O₃-based device than positive charges with an increase in radiation dose. The C–V curves of the Yb₂O₃ MOS capacitor shifted in the same direction at both 100 kHz and 1 MHz, and as expected, positive charge traps in the structure are more efficient than negative charges. The observed sensitivities of Yb₂O₃ MOS capacitors are 4–7 times higher than those of SiO₂, and the sensitivities of the Yb₂O₃ MOS capacitors with a total radiation dose of 70 Gy were found to be around 28.08 mV/Gy at both 100 kHZ and 1 MHz frequencies. The Yb₂O₃ appears to be a promising dielectric candidate for developing a new generation of radiation sensors with an excellent interface quality when compared to rare earth mixed oxides such as silicates, transition metal oxides, and the silicates based on transition metals, Al₂O₃, and BiFeO₃. Our review of the literature suggests that while the radiation damage has been assessed comprehensively based on the C–V characteristics, microstructural characterization of the irradiated films and their interfaces is lacking even though the quality of oxide/Si interface is the most important feature of the devices. The electrical data should be correlated with the inferences from XPS, AFM, TEM, XRD, and other techniques. Further progress requires selection and validation of material properties based on theoretical calculations and predications, utilization of diverse thin film processing and characterization techniques, determining the effect of thickness on the properties of MOS capacitors, a thorough understanding of the interfaces, effect of frequency on the MOS capacitors and the interface characteristics, effect of radiation on the physical, interfacial, and electrical characteristics of MOS capacitors, and preparation and characterization of sensors based on thin films of novel mixed oxides and silicates of different chemistries.