Frequency dependent gamma-ray irradiation response of Sm2O3 MOS capacitors

https://doi.org/10.1016/j.nimb.2015.06.037Get rights and content

Abstract

The frequency dependent irradiation influences on Sm2O3 MOS capacitors have been investigated and possible use of Sm2O3 in MOS-based radiation sensor was discussed in this study. To examine their gamma irradiation response over a range of doses, the fabricated MOS capacitors were irradiated up to 30 grays. Capacitance–Voltage (CV) measurements were recorded for various doses and the influences of irradiation were determined from the mid-gap and flat-band voltage shifts. In addition, the degradations of irradiation have been studied by impedance based leakage current–voltage (JV) characteristics. The results demonstrate that JV characteristics have not been significantly change by irradiation and implying that the excited traps have a minor effect on current for given dose ranges. However, the frequency of applied voltage during the CV measurements affects the irradiation response of devices, significantly. The variations on the electrical characteristics may be attributed to the different time dependency of acceptor and donor-like interface states. In spite of the variations on the device characteristics, low frequency measurements indicate that Sm2O3 is a potential candidate to be used as a dielectric layer in MOS based irradiation sensors.

Introduction

The bare metal–oxide–semiconductor (MOS) capacitors are the base of the MOS-based device, e.g., MOSFETs and ICs. Therefore, understanding of the device characteristics of MOS capacitors is important for development of novel MOSFET technology [1]. The studies related to design and characterization of the MOS devices have been performed to its better performance for various filed since the last few decades [2], [3], [4], [5].

MOS devices used for irradiation measurements have attracted special attention recently and have been widely used in most dosimeters for monitoring radiation doses in various environments such as space, nuclear industry, and medical applications. This is mainly due to their linear performance over the intended energy range and superior sensitivity. The existence of an oxide (dielectric) layer in MOS structures makes MOS-based devices more sensitive to irradiation environments. Majority of the radiation-induced damages are located at oxide bulk and/or near the oxide–semiconductor interface. Additionally, the exposure of these structures to high-level particles results in a considerable amount of lattice defects. The generated defects that act as recombination centers or minority/majority carrier trapping centers cause degradation of these device performance and their applications. Therefore, influences of radiation on the electrical characteristics of MOS devices are complicated in nature [5]. Various dielectric materials such as Al2O3 [6], HfO2 [7], [8], TiO2 [9], [10], La2O3 [11] and ZrO2 [12] have been studied for MOS-based technology to substitute conventional SiO2 layer. However, there is not enough knowledge in the literature about their radiation responses. The investigations of finding new alternative dielectric materials are crucial for development of MOS-based devices to be used in irradiation environment. Thus, samarium oxide (Sm2O3) could be one of the suitable dielectrics compared to other dielectric materials due to the fact that it maintains many attractive features in itself alone such as high dielectric constant up to 30 [13], [14] depending on film quality, thermodynamically stable on the underlying Si surface up to 1000 °C preventing the formation of silicide layers and rough surfaces [15], wide band gap [16], and large conduction band offsets [17].

In order to investigate influences of irradiation on the electrical characteristics of Sm2O3 MOS capacitors, the samples were irradiated by using the Co-60 gamma ray source from 0.6 to 30 grays. The electrical characteristics of the device were investigated from capacitance–voltage (CV) and impedance based leakage current density–voltage (JV) measurements at high (1 MHz) and low (100 kHz) frequencies. It is known that frequency dependent charges have crucial effects on the electrical characteristics of MOS capacitors [18], [19], [20] and very few studies exist in the literature for irradiation dispersion on various frequencies. The forgoing two different frequencies have been selected to study influence of frequency dependent interface state to radiation response of device characteristics. The influences of irradiation were determined from the mid-gap and flat-band voltage shifts of capacitance curves and radiation induced impedance based leakage current characteristics.

Section snippets

Experimental details

The Sm2O3 thin films were deposited on n-type (1 0 0) Si substrate with a resistivity of 1–4 ohm cm by reactive RF sputtering from a 4-inch Samarium target with purity of 99.99%. Before the deposition of Sm2O3 layers, the wafers were cleaned with standard RCA cleaning process. Prior to commencing sputtering, the base pressure of the chamber was below 4.0 × 10−4 Pa and sputtering gas pressure was at 1.0 Pa. The pre-sputtering was done for 10 min at 200 W to remove any impurities present on the target

Results and discussions

The capacitance characteristics of fabricated MOS devices provide a very delicate tool to study irradiation effects on the dielectric layer and their interfaces between dielectric and substrate [23]. However, before the study of irradiation response of devices, frequency dispersion of non-irradiated capacitances curves had been investigated. The obtained capacitance curves for low and high frequencies are illustrated in Fig. 1. The measured capacitance increase and flat-band voltage shifts

Conclusion

The frequency dependent irradiation responses of Sm2O3 MOS capacitors have been investigated. The obtained capacitance curves at high frequency exhibits evident irradiation dispersions in accumulation and depletion edge, while the curves measured capacitances at low frequency are almost the same. The variations on the measured capacitance at high frequency may be related to time dependency on generated interface states by irradiation. At lower frequency, almost interface states, being composed

Acknowledgments

The authors would like to thank Middle East Technical University GUNAM and METU Chemistry Department for providing experimental facilities and their generous support.

This work is supported by Abant Izzet Baysal University under Contract Number: AIBU, BAP. 2014.03.02.706 and the Ministry of Development of Turkey under Contract Number: 2012K120360.

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