The effect and nature of the radiation induced oxide-interface traps on the performance of the Yb₂O₃ MOS device

Highlights

• The radiation effect on the structural and electrical properties of Yb₂O₃ MOS device.

• XPS depth profiles of the Yb₂O₃/Si were obtained before and after irradiation.

• Grain sizes of the structures were calculated before and after irradiation.

• The structural changes causing oxide and interface traps were detailed.

• A relationship was established between radiation response and structural changes.

Abstract

Modifications in the crystal properties, chemical compositions and bond contents as a result of irradiation of Yb₂O₃/Si structures annealed at different temperatures in the dose range of 1–50 kGy were investigated. Beside, comprehensive results on the effect of the structural changes on radiation response of the MOS capacitors produced with these structures was presented in this study. 122 nm-thick Yb₂O₃ films were grown on p-type Si by RF magnetron sputtering, and the Yb₂O₃/Si structures were annealed at 200 °C, 400 °C, 600 °C, 800 °C under nitrogen ambient. The radiation exposure disrupts the crystalline properties of the film. Yb 4 d and O 1s spectra were taken from different depths in the Yb₂O₃/Si structures with X-ray photoelectron spectroscopy. Reasons of the right-side shift in the capacitance voltage (C–V) curves of the as-deposited and 200 °C-Yb₂O₃ MOS capacitors with radiation exposure were determined as increasing 2+ oxidation state occurring with the trapping of the electrons in the Yb³⁺ defect centers, decreasing Si–Si bond contents causing the positive charge trapping at Yb₂O₃/Si interface, existence of the hydrogen defect precursors. Events causing the left-side shift of the C–V with radiation exposure may be the silicate layer developing at the interface, increase in the trapping of the positive charges in the Si–Si defect centers, and decreasing Yb–Yb and 2+ contents. The radiation responses of the 400 °C, 600 °C, 800 °C-Yb₂O₃ MOS capacitors could not be measured at the frequencies lower than 2 MHz due to high charge trapping, high binding energies of the Yb–O and Yb³⁺ peaks, increasing 2+ oxidation content. The C–V curve shifted to the left-side at the relatively lower dose of 1 kGy for only device composed of 200 °C-Yb₂O₃ film. The sensitivities of the 200 °C-Yb₂O₃ MOS capacitor were found to be 16.3 mV/Gy for 70 Gy.

Introduction

Nowadays, systems that can measure radiation dose with high sensitivity are needed in many areas from the nuclear power plant to radiotherapy clinics. Electronic/active dosimeters are superior to passive systems because they enable instant monitoring of dose information. RadFETs (Radiation sensing field effect transistor), also known as pMOS dosimeters or p-channel MOSFET, attract attention due to their superior properties such as small size, low power consumption, less sensitive to the temperature with respect to diodes, radiation responses independent of dose rate up to 10⁸ Gy/s (Rosenfeld, 2006; Yilmaz et al., 2017). This study focuses on improving the sensitivity of the RadFETs to low doses.

The gate oxide layer, which is the dose-sensitive part of the sensor, is composed of SiO₂ in the commercial RadFETs due to easy production and excellent interface quality with Si (Tataroğlu and Altındal, 2006). In order to improve the sensitivity to low doses, it is necessary to find an alternative dielectric to SiO₂. In our previous studies conducted for this purpose, the sensitivity of MOS capacitors produced with the rare earth oxides of Er₂O₃, Gd₂O₃, La₂O₃, Yb₂O₃ were found to be higher than the SiO₂ device (Kahraman et al., 2016; Kahraman and Yilmaz, 2017; Kahraman and Yilmaz, 2017, 2018; Yilmaz and Kaya, 2016). However, among these dielectrics, the device with the gate oxide layer of Yb₂O₃ is characterized with similar sensitivity in the low (100 kHz) and high frequency regions (1 MHz), the continuous shift of the C–V curves to the higher negative voltages with increasing dose due to predominantly positive charge traps (which is desirable) and high sensitivity. However, the problem in this device is that the mid-gap voltage shifted to different directions in various frequencies at a dose of only 0.5 Gy due to interface trap charges or the border trap charges located at vicinity of the high-k/Si interface (Kahraman and Yilmaz, 2017). The determination of the problem causing this instability at low doses is extremely important for finding the solution.

Since the temperature in the medium increases with the heat transfer in the result of radiation interaction, it is still a mystery how the structural properties of the Yb₂O₃ dielectric will change and their effect on the radiation response of the device, because the thermal annealing was not applied to the Yb₂O₃ dielectric in our previous study.

The studies on Yb₂O₃ dielectric have examined the high-k/Si interface property with X-ray photoelectron spectroscopy (XPS) after radiation exposure and have not revealed a comprehensive link between the structural analysis and radiation responses up to now. In the literature, the focus of most of the studies with high-k dielectrics is CMOS technology, which is gradually scaled down (Lok et al., 2018; Yue et al., 2009; Zhu et al., 2004). It is also important that the information obtained in this context be transferred to the radiation sensor size.

The aim of this study is to investigate the structural modifications of Yb₂O₃/Si samples annealed at different temperatures and irradiated at different radiation doses and to understand the effect of these changes on the radiation responses of the Yb₂O₃ MOS capacitors. The Yb₂O₃/Si structures annealed at four different temperatures were irradiated at 1 kGy, 25 kGy and 50 kGy. The crystal properties of the samples were investigated with XRD technique. Chemical composition, bond structures, binding energies and oxidation states were evaluated with XPS depth profiles in order to understand the effect of the structural modification of both the oxide layer and interface on the electrical properties. The capacitance-voltage (C–V) curves of the Yb₂O₃ MOS device were measured at the same doses, and the results were evaluated with structural analyses after the variation in the oxide-interface trap charge densities were determined. To our best knowledge, it is the first study in which the effect of radiation on the structural modifications of the Yb₂O₃ dielectric is examined in detail.