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Effects of Ionizing Radiation on the Layered Semiconductor Tungsten

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Effects of Ionizing Radiation on the Layered Semiconductor Tungsten The Pennsylvania State University The Graduate School College of Earth and Mineral Sciences EFFECTS OF IONIZING RADIATION ON THE LAYERED SEMICONDUCTOR TUNGSTEN DISELENIDE A Thesis in Materials Science and Engineering by Roger Craig Walker II © 2017 Roger Craig Walker II Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science August 2017 The thesis of Roger C. Walker II was reviewed and approved* by the following: Joshua A. Robinson Associate Professor of Materials Science and Engineering Thesis Adviser Saptarshi Das Assistant Professor of Engineering Science and Mechanics Suzanne Mohney Professor of Materials Science and Engineering Materials Science and Engineering Program Chair *Signatures are on file in the Graduate School. ii Abstract The miniaturization of electronic devices has been a critical for the successes of space exploration. Scaling down individual transistors allows integrated circuits to be more powerful and flexible, while reducing their weight and volume. This is highly attractive for space electronics given the cost of launching objects into space. Semiconducting two-dimensional materials (2DMs), such as tungsten diselenide (WSe2), are of recent interest for developing ultimately-scaled transistors. These materials have a layered structure with a thickness of individual atoms at the monolayer limit, and are compatible with existing device fabrication processes. However, the space environment imposes an additional constraint on the device performance of radiation hardness. Space is filled with high energy ionizing radiation – i.e. X- rays, gamma rays, protons, electrons, and heavy ions – that can damage and destroy devices. As such, the stability of WSe2 against various forms of ionizing radiation should be critically examined to determine if it possesses sufficient radiation hardness for space applications. In this work, the stability of WSe2 against X-rays, low energy plasma, protons, and heavy metal ions has been studied. WSe2 was prepared using a combination of transfer via mechanical exfoliation and growth via metal-organic chemical vapor deposition (MOCVD). Mechanical exfoliation is used to transfer micrometer-thick flakes onto substrates. To obtain nanometer thick films and individual layers, MOCVD was used. In this process, WSe2 is formed by controllably reacting gas phase pre-cursors of its constituent elements at elevated temperatures. The pre- cursors react on the surface of a substrate (e.g. silicon carbide (SiC)), nucleate as a WSe2 crystal, and grow into individual domains or coalesced films. X-ray exposure was achieved using the soft X-rays generated in an X-ray photoelectron spectroscopy (XPS) system for 12 hours in ultra- high vacuum. Plasma was generated by the ionization of helium and oxygen gas under medium vacuum conditions using an RF generator. Protons and heavy ions were generated by stripping the electrons from gases of hydrogen, iron, and silver, and were accelerated into the WSe2 samples under high vacuum conditions. Total fluences are varied from 1014 particles/cm2 to ~1018 particles/cm2, which covers the typical expected values for space electronics. Modification to the WSe2 induced by ionizing radiation is primarily studied using XPS, a highly surface sensitive characterization technique. It is found that each form of ionizing radiation has a different impact on the WSe2. Soft X- rays induce a charge in the WSe2 that is dependent on the domain size and surface coverage. Isolated domains of MOCVD-grown WSe2 are subject to a small surface potential on the order of 100 meV, which is attributed to interface states formed at the edges of these domains. Fully coalesced films of MOCVD-grown WSe2 were not significantly affected by X-rays, even at a fluence of 1.86 × 1018 photons/cm2. The low energy plasma can be used to convert the top layers of WSe2 into a mix of tungsten oxide (WOx) and selenium oxide. MOCVD-grown WSe2 is found to be more susceptible to oxidation than the exfoliated material, perhaps due to a greater defect density. Surprisingly, the extent of oxidation is not very sensitive to changes in plasma parameters such as pressure or gas flow rate, suggesting that much of the oxidation is due to air exposure after the treatment. Knowledge of the effects of protons and heavy ions with energies in the mega-electron-volt (MeV) range are the most directly related to knowledge of WSe2 stability in the space environment. It was determined that WSe2 flakes exfoliated onto SiC were not affected by 2 MeV proton bombardment up to a dose of 1015 protons/cm2. Once a dose of 1016 /cm2 had been achieved, there was significant charge transfer between the WSe2 and the SiC substrate that iii arose from damage to the SiC. The SiC was observed to turn black due to the large generation of vacancies corresponding to a deep acceptor lying 1.1 eV below the conduction band edge. The damage to SiC and charging in WSe2 are expected to permanently disrupt device operation, despite a lack of physical damage to the WSe2. No physical changes such as oxidation occurred in the WSe2 due to this bombardment, suggesting that it is highly stable against cosmic radiation that mostly consists of protons. As such, the choice of substrate will be critical in tuning the radiation hardness of WSe2-based devices. Additionally, this damage threshold is several orders of magnitude greater than what was observed for damage to MoS2-based devices on SiO2. As such, top-gated 2DM-based devices using semi-insulating SiC as a substrate are suggested for future miniaturized space electronics. 1 MeV protons are found to have a qualitatively similar effect as the 2 MeV protons. By using lower energy protons (i.e. 0.2 MeV and 0.04 MeV), the vacancy-rich region in the SiC is found to move closer to the sample surface, and the charging in the WSe2 is reduced. 16 2 In contrast, exposure to heavy metal ions such as iron and silver at a fluence of 10 ions/cm leads to significant physical damage for both the exfoliated WSe2 and the SiC substrate. The WSe2 is converted into a mixture of Se-poor WSe2 and WOx due to selenium volatilization during the bombardment and oxidation once exposed to air. The material continues to lose selenium and gain oxygen over time even when stored in medium vacuum. The SiC turns black and becomes fully amorphous. Significant concentrations of silicon oxide are generated after air exposure, suggesting that carbon is preferentially sputtered out from the sample due to ion bombardment. However, the SiC is stable under storage in medium vacuum and does not significantly change from its damaged state. The valence band spectra revealed that the heterostructure undergoes a semiconducting to metallic transition due to the extensive damage. As such, a device in the space environment based on the WSe2/SiC heterostructure would be expected to be permanently destroyed after exposure to this fluence of heavy metal ions. Complimentary proton and ion studies using MOCVD-grown material are in the planning stages and highly recommended as future work. This work has elucidated some of the physical responses of WSe2 to ionizing radiation. This material appears to be robust against soft X-rays but is susceptible to low energy plasma, high energy protons, and high energy ions. Further work determining the sensitivity of WSe2-based devices to X-rays, protons and ions is recommended to measure the changes to the electronic properties of this material, as well as critical points of failure in the devices. To accomplish this, the stability of the WSe2/dielectric interface must also be studied, and the compatibility of high-k dielectric materials, such as HfO2, with 2DMs must be determined. iv Table of Contents List of Figures ............................................................................................................................... vii List of Tables .................................................................................................................................. x Acknowledgements ........................................................................................................................ xi Chapter 1 – Introduction and Literature Review ............................................................................ 1 1.1 A Brief Review of Two-Dimensional Materials ................................................................................. 1 1.2 Electronic Properties of Tungsten Diselenide (WSe2) ........................................................................ 6 1.3 The Near-Earth Space Radiation Environment ................................................................................... 8 1.4 Radiation Damage to Semiconducting Materials and Devices ......................................................... 12 1.5 Radiation Damage to WSe2 ............................................................................................................... 15 Chapter 2 – Experimental Procedure ............................................................................................ 18 2.1 Mechanical exfoliation of WSe2 ....................................................................................................... 18 2.2 Metal-organic chemical vapor deposition (MOCVD) of WSe2 ........................................................ 19 2.3 Stability of MOCVD-grown WSe2 in the XPS environment ............................................................ 21 2.4 Oxygen plasma treatment of WSe2 ..................................................................................................
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