Extraordinary Radiation Hardness of Atomically Thin MoS2 - ACS

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Functional Nanostructured Materials (including low-D carbon)

Extraordinary Radiation Hardness of Atomically Thin MoS2 Andrew Joseph Arnold, Tan Shi, Igor Jovanovic, and Saptarshi Das ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18659 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Materials & Interfaces

Extraordinary Radiation Hardness of Atomically Thin MoS2 Andrew J. Arnold1, Tan Shi2, Igor Jovanovic2, Saptarshi Das3, 4 1Electrical 2Nuclear

Engineering, Pennsylvania State University, University Park, PA 16802

Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109

3Engineering 4Materials

Science and Mechanics, Pennsylvania State University, University Park, PA 16802

Research Institute, Pennsylvania State University, University Park, PA 16802

Abstract: We demonstrate that atomically thin layered two-dimensional (2D) semiconductors are promising candidates for space electronics owing to their inherent and extraordinary resilience to radiation damage from energetic heavy charged particles. In particular, we found that ultrathin MoS2 nanosheets can easily withstand proton and helium irradiation with fluences as high as ~1016 ions/cm2 and ~1015 ions/cm2, respectively, corresponding to hundreds or thousands of years of unshielded exposure to radiation in space. While radiation effects on 2D material-based field effect transistors (FETs) have been reported in the recent past, none of these studies could isolate the impact of irradiation on standalone ultrathin 2D layers. By adopting a unique experimental approach that exploits the van der Waals epitaxy of 2D materials, we were able to differentiate the effects of radiation on the 2D semiconducting channel from that of the underlying dielectric substrate, semiconductor/substrate interface and metal/semiconductor contact interface, revealing the ultimate potential of these 2D materials. Furthermore, we used a statistical approach to evaluate the effect of radiation damage on critical device and material parameters, including threshold voltage, subthreshold slope, and carrier mobility. The statistical approach lends additional credence to the general conclusions drawn from this study, overcoming a common drawback of methods applied in this area of research. Our findings do not only offer exciting prospects for the

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operation of modern electronics in space, but may also benefit electronics applications in highaltitude flights, military aircrafts, satellites, nuclear reactors, particle accelerators, and other highradiation environments. Additionally, they highlight the importance of evaluating the impact of damage to the substrate and surrounding materials on electrical characteristics during future radiation studies of 2D materials.

Keywords: 2D materials, Transition Metal Dichalcogenides (TMDCs), MoS2, Field Effect Transistor, Radiation Hardness, Proton Radiation, Helium Radiation

Introduction The future of state-supported space exploration will involve crewed missions beyond the lowEarth orbit and manned spaceflight mission to the Moon and the Mars. Furthermore, several private sector companies are promoting space tourism, which has the potential to become a large industry within the next 20 years. However, the extreme space environment demands that adequate technologies be developed to support these ambitious goals. A critical component of any space technology is space electronics, which must be resilient to damage and/or malfunction when exposed to high, persistent fluxes of ionizing radiation, such as high-energy electrons, protons, α particles, heavier ions, X rays, or gamma rays. In particular, energetic heavy ions cannot be adequately shielded in space applications, which necessitates the discovery of radiation hard materials. Note that these materials, at the same time, must provide a platform to facilitate the development of high-performance, low-power, and cost-effective electronic devices, circuits and components. Such materials will also be useful for electronic systems used in other high-radiation


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environments, including high-altitude flights, military aircraft, satellites, nuclear reactors, and particle accelerators.

In this context, two-dimensional (2D) layered transition metal dichalcogenides (TMDCs) such as MoS2 exhibit attractive semiconducting properties, including high mobility,1 and bandgap tunability.2 Due to the advances in modeling and simulation,3 material synthesis4 and device fabrication,5 high-performance MoS2 field-effect transistors (FET),6-8 and more complex digital components such as NAND gate and static random access memory9 have been demonstrated based upon TMDCs. As the feature size in state-of-the-art metal oxide semiconductor (CMOS) technology is approaching its fundamental limit due to the increase in the leakage power density,10 layered TMDCs offer the prospect for development of several alternative low-power device concepts based on straintronics,11 valleytronics,12-13 spintronics,14 quantum tunneling,15-16 negative capacitance,17-19 and brain inspired electronics.20-21 At the same time, they facilitate transistor downsizing,22 which is particularly desirable in space applications and in electronics used for highenergy physics, where reductions of weight and power consumption are sought.23-24 However, due to the atomically thin nature of the TMDC, it is not fully understood how radiation would impact the electronic properties of TMDCs.

Although high-energy radiation has shown to be an effective tool to change the morphology25 and optical properties of TMD materials,26-27 here we focused on the study of MoS2 FET electronic properties. Radiation effects on TMDC-based FETs have been investigated in several prior studies. Fox et al.27 and Stanford et al.28 studied helium-ion induced damage to MoS2 and WSe2, respectively, through Raman spectroscopy and electrical measurement. Spatially tunable transport



behavior in monolayer WS2 and WSe2 using focused helium ion beam has also been reported in a recent study.29 Kim et al.30 studied the evolution of I-V characteristics of back gated MoS2 FETs irradiated with 10-MeV protons with a fluence in the range of 1012 -1014 ions/cm2. The degradation of the device characteristics was explained by a combined effect of oxide-charge traps generated in the SiO2 and interface states, which is similar to the total ionizing effect on traditional silicon MOSFETs.31 In the work of Dhakras et al.,32 a reconfigurable WSe2 FET was fabricated with three buried gates to tune the channel polarity in order to suppress the impact of Schottky barriers on the subthreshold characteristics. With proton fluence from 1011 to 1014 ions/cm2, it was found that oxide charges dominated the post-exposure device characteristics, leading to a significant degradation of the subthreshold slope (SS). In the experiments reported by Lu et al.,33 MoS2 FETs were irradiated by 30-keV electrons. A transition from negative voltage shift to positive voltage shift with increasing dose was observed and explained by the dominance of oxide charges at low doses and interface traps at high doses. Besides protons and electrons, total ionizing effects on TMD materials induced by 10-keV X rays and detrimental effects induced by energetic heavy ions have also been reported.34-35 In addition to TMD materials, ionizing radiation effects on FETs based upon 2D heterostructures and other lower dimensional materials, such as MoS2/graphene,36 graphene,37 or carbon nanotubes,38-39 have also been investigated in prior research.

Interestingly, none of the studies reported to date highlight the impact of irradiation on standalone ultrathin 2D layers, and instead they focus on the degradation of underlying oxide substrate and its subsequent impact on the electrical characteristics of the FETs based on the 2D materials. Using traditional approaches such as I-V measurement, it is sometimes challenging to evaluate the radiation hardness of 2D materials, since interface traps and oxide charges can induce opposite


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shift of the I-V curve, leading to a compensating effect. The positive charges created inside the dielectric induce a positive shift of the I-V curve, whereas interface traps from n-type channel material induce a negative shift, because interface traps are usually below the Fermi level and are therefore negatively charged.31 Furthermore, the conclusions can be significantly different if the thickness and/or composition of the dielectric substrates are changed.40 In this work, we, therefore, designed a novel experimental approach that exploits the van der Waals (vdW) epitaxy of the layered TMDCs in order to decouple the effects of irradiation on the 2D semiconducting channel from that on the underlying dielectric substrate, semiconductor/substrate interface, and metal/semiconductor contact interface. We found that ultrathin MoS2 nanosheets can withstand proton and helium fluences as high as ~1016 ions/cm2 and ~1015 ions/cm2, respectively with minimal damage. Furthermore, we adopted a statistical approach to reliably compare the irradiation-induced changes while evaluating the impact of irradiation on critical device and material parameters such as the subthreshold slope, threshold voltage, and electron mobility of MoS2 FETs.

Experimental Procedure The difference between the state-of-the-art methods and our unique experimental approach for studying the radiation effect on 2D FETs is elucidated in Figure 1. Most of the previous investigations involved irradiation of complete devices, as shown in Figure 1a. In such an experimental setup, the post-radiation device characteristics arise from the combined radiation damage to the 2D material, source/drain contacts, and dominantly the underlying oxide (typically ~300 nm), which is orders of magnitude thicker than the ultrathin 2D semiconducting channel. Conclusions drawn on the basis of such experiments will widely differ for FETs with different



oxide thicknesses, dielectric materials, and contact metals. In order to eliminate as many of these extrinsic effects as possible, we have used three different samples, each subjected to identical irradiation conditions. Figure 1b shows the first sample, where the MoS2 flakes were exfoliated on a SiO2 substrate and both were subsequently irradiated. Figure 1c shows the second sample, where the MoS2 flakes were irradiated and then transferred to an unirradiated SiO2 substrate using a wet transfer process. Finally, Figure 1d shows the third sample, where unirradiated MoS2 flakes are exfoliated on an irradiated SiO2 substrate. In all instances the FETs were fabricated post irradiation by lithographically defining the source/drain contacts. This new approach eliminates the radiation effects due to contacts and at the same time allows identification and differentiation of the radiation impact due to the 2D semiconducting channel and the SiO2 back gate dielectric. Figure 1e shows the wet transfer process, wherein the substrate holding the irradiated MoS2 flakes is coated with PMMA and immersed in a NaOH solution. Capillary forces cause the PMMA to peel off the substrate and float to the surface of the solution. The vdW nature of the 2D layered materials ensures that the MoS2 flakes are contained in the PMMA film owing to the stronger cohesive force than the SiO2 substrate. Next, the PMMA film holding the flakes is rinsed thrice in deionized water baths and transferred onto the unirradiated target substrate. Finally, the PMMA film is dissolved with acetone. More details on the transfer process can be found in the methods section.

Results and Discussion Figures 2a-d, respectively, show the representative transfer characteristics of MoS2 FETs from preradiation control sample (CS) with neither the flakes nor the substrate being irradiated and postradiation samples with both the flake and the substrate irradiated (BI), only the flake irradiated (FI), and only the substrate irradiated (SI) with 390-keV singly-charged He+ ions at a total fluence


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of 1014 ions/cm2. For FET fabrication, the source/drain contacts were defined using electron beam lithography, followed by electron beam evaporation of 40 nm Ni/ 30 nm Au. More details on the device fabrication and electrical measurement procedures can be found in the methods section. We used SiO2 substrates with heavily doped Si as the back gate for all devices and also kept the device channel lengths constant at 500 nm. There is some observable degradation in the SS of the MoS2 FETs corresponding to the BI sample, whereas minimal changes are observed for the MoS2 FETs corresponding to the FI and SI samples. This could indicate some radiation damage involving the flake/substrate interface, which results in an increase in the interface trap states, but the changes are too small to draw any convincing conclusions. Figures 2e-h show the results for He+ ion irradiation at a fluence of 1015 ions/cm2 for the four configurations described above, i.e. CS, BI, FI, and SI. The MoS2 FETs corresponding to the FI and BI samples demonstrate observable changes in their respective transfer characteristics, which is reflected through degradation of the SS, a large positive shift in the threshold voltage (VT), and a significant reduction in the ON-state current. Since both BI and FI samples involve irradiated flakes, the majority of the changes in the device characteristics must originate from damage to the flakes rather than damage to the oxide substrate. The damage to flakes can lead to an increase of interface traps, which would degrade device performance. In addition, the NaOH transfer process was found to have negligible effect on the electrical characteristics. A more detailed discussion can be found in the supplemental information. The MoS2 FETs corresponding to the SI samples show similar characteristics to the CS with a slight increase in the SS indicating minimal damage to the SiO2 substrate. Figures 2i-l show the results for He+ ion irradiation at a fluence of 1016 ions/cm2 for CS, BI, FI, and SI, respectively. Clearly, at this dose, radiation damage to the device is much more severe. The MoS2 FETs corresponding to the BI and FI samples show very low device currents over the entire back



gate voltage (VBG) sweep range. Examination of the raw data shows that this current is due to gate leakage rather than any gating of the MoS2 channel. This indicates that the MoS2 flakes have been damaged to the point where they no longer support any current conduction. The MoS2 FETs corresponding to the SI sample show a high current over the entire VBG range coupled with elevated gate leakage currents. However, the gate leakage current was found to be several orders of magnitude lower than the source to drain current. Since the flakes were unirradiated, the lack of gating implies that either the oxide has been damaged to a degree that VBG can no longer influence the surface potential of the MoS2, or there has been a massive buildup of positive charges in the oxide leading to a large negative VT shift so that the OFF state is shifted far off the negative side of the window and only the nearly-flat far ON state is visible within the sweep range. Furthermore, the increased gate leakage current points towards physical radiation damage to the oxide. Nevertheless, it is apparent that at this dose, both the flake and the oxide substrate have suffered enough damage that makes the MoS2 FETs unusable.

Transistors based on exfoliated 2D materials tend to have significant device-to-device variations in subthreshold slope, threshold voltage, and electron mobility due to differences in the flake thickness. This must be taken into account before deriving any meaningful conclusion regarding radiation hardness of the 2D materials. We, therefore, fabricated and measured at least 20 devices for each irradiation configuration discussed above. Figure 3a-d shows the overlaid transfer curves of all these devices for the four irradiation configurations at the 1015 ions/cm2 dose. It is apparent that, while there are variations in SS, VT, and ON-state current between the different devices corresponding to any given configuration, the trends discussed earlier still hold true. The only exception is in the SI sample shown in figure 3d, where approximately half of the MoS2 FETs


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cannot be switched OFF within the VBG range tested, whereas the other half show device characteristics similar to the CS. Figure 3e shows the mean and standard deviation for the SS, VT, and electron mobility (μFE) extracted from the peak transconductance i.e. the slope of the IDS versus VGS characteristics for all four samples. In order to elucidate the physical mechanism of the radiation damage, one can describe the change in VT and the SS as follows:41 Δ𝑉𝑇 = ―

Δ𝑄𝐼𝑇 𝐶𝑂𝑋

(

―

Δ𝑄𝐹

(1)

𝐶𝑂𝑋

𝐶𝐷 𝐶𝐼𝑇 𝑘𝑇 𝑆𝑆 = 2.3 1+ + 𝑞 𝐶𝑂𝑋 𝐶𝑂𝑋

)

(2)

Here, 𝐶𝑂𝑋 is the oxide capacitance, Δ𝑄𝐼𝑇 is the change in interface trap charge, Δ𝑄𝐹 is the change in fixed oxide charge, 𝐶𝐷 is the depletion capacitance which can be neglected for an ultra-thin body fully depleted channel like the MoS2, 𝐶𝐼𝑇 is the interface trap capacitance, 𝑘 is the Boltzmann constant, 𝑇 is the temperature, and 𝑞 is the electronic charge. It is apparent that any induced charges at the interface or in the oxide will directly shift the VT and modify the SS. Furthermore, since VT depends on both the interface and oxide charges while the SS is only influenced by interface charges, it is possible to determine whether the damage is dominated by the oxide or the interface. Following this argument, one can conclude from the severe degradation of the mean SS in the MoS2 FETs from ~1.81 V/decade in CS to ~10.3 V/decade and ~8.9 V/decade in the BI and FI samples, respectively, that significant amount of interface trapped charges were produced. A simple calculation using Equation (2), suggests ~5X increase in 𝐶𝐼𝑇. Moreover, large positive threshold shifts from a mean value of ~-41 V in CS to ~12 V in BI and ~16 V in FI indicate that the trapped charges must be negative following Equation (1) and dominate over the oxide charges since positive oxide charges would lead to negative threshold shifts. In SI samples, we also observed a SS increase from ~1.81 V/decade to ~4.63 V/decade. The increase of the SS is due to



the generation and migration of oxide charges to near-interfacial locations. These near-interfacial oxide charges can form defects known as border traps, which have similar electric behavior as interface traps and therefore lead to the SS increase.42 Based on the magnitude of SS increase in SI, FI, and BI samples, we can see that interface states caused by the irradiation of flake contribute more to the SS increase than the border traps induced by the irradiation of the substrate.

Several radiation damage mechanisms are possible in 2D materials. First, radiation-induced localized heating and electrostatic charging are temporary effects without any permanent physical damage and should not influence electrical measurements. In terms of the structural change, collisions between ions and MoS2 lattice atoms will atomic displacement, creating vacancies, Frenkel pairs, or more complicated structural disorder. S vacancies are the most probable defects according to simulation.43 Furthermore, ionizing radiation may cause initial bonds to break and form new bonds between the lattice atoms and any contaminants that are present.44 While it is difficult to identify the dominant radiation damage mechanism, the drop of field effect electron mobility (μFE) from a mean value of ~22 cm2/Vs to ~2 cm2/Vs, i.e. by almost an order of magnitude in both BI and FI samples, suggests that one or more of these mechanisms are occurring. The decrease of mobility indicates the formation of additional scattering sites when atomically thin MoS2 flakes are exposed to 390 keV He+ ion irradiation with a total fluence of 1015 ions/cm2. For SI samples, we note that the creation of oxide charges does not lead to a decrease in mobility. Interestingly, in spite of such high radiation dose, the performance of MoS2 FETs is still reasonably good, with ON-state currents of several tens of μA/μm and ON/OFF ratios exceeding 106.


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The radiation effects described above cannot happen in the MoS2 on the SI sample since it is not irradiated. For BI and SI samples, the degradation of the I-V curve is also associated with the radiation effects in SiO2. First, heavy charged particles can create collisional damage to the SiO2 lattice by elastic collisions with substrate nuclei. Next, ionization from heavy charged particles can also lead to the buildup of charges by creating electron-hole pairs. Holes will move toward the interface by hopping through localized charge states and a fraction of the holes is trapped to form oxide-trapped charges.31 In the studies of traditional MOSFETs, it has also been shown that hydrogen ions can be released from SiO2 and act as holes to form interface traps31, which originate from oxygen vacancy at the interface with a week Si-Si bond.45 The response of oxide-trapped charges and interface-trapped charges is challenging to uncover in electrical measurement due to their dependence on bias conditions, time, oxide properties, and measurement frequency (lowfrequency or high frequency measurement).46

In our results, the SI sample shows significantly more device-to-device variation than the other samples, with approximately half of the devices having a large negative VT shift. The MoS2 flakes used in devices on the SI sample were measured with atomic force microscopy (AFM) and were found to have thicknesses in the range of 1-10 nm. No observable dependence of the device characteristics on the flake thickness was found based on the analysis of the data presented here. The BI sample substrate was irradiated under the same conditions, but did not show the variation. The main differences between the two samples are the exposed surface during the ion irradiation and the lack of SiO2 and MoS2 interaction during the irradiation. However, because the beam is raster scanned across the sample, minimal variations should result. Even if variations were introduced in this way, they should not be exclusive to one sample. As a result, no clear conclusion



can be drawn on the cause of this variation at present time. A more detailed discussion of the large variation in the transfer curves and its lack of dependence on the flake thickness, including the statistical analysis of the effects of the thickness on the device characteristics can be found in the supplemental information.

In order to compare the effects of different radiation types, we also exposed MoS2 flakes to 2-MeV proton radiation at a fluence of 1.26 × 1016 ions/cm2 using the same procedure as the previously discussed measurements. The proton fluence was selected to match the same dose level (total energy absorbed per mass) of helium ions at 1015 ions/cm2. Since the ratio of total stopping power between 390-keV He ions and 2-MeV protons is about the same for MoS2, SiO2, and Si, the energy deposited in each layer is approximately the same in the two irradiation conditions. However, the fraction of energy going to nuclear stopping power is lower for protons, and they also have a longer penetration depth. According to SRIM (Stopping and Range of Ions in Matter) calculations, the nuclear stopping power of 390-keV He+ is about 50 times larger than that of 2-MeV protons. The ion properties in the irradiated materials are shown in Table 1. More details can be found in the Methods section. All four sample configurations contained at least 20 devices with 500 nm channel lengths. The overlaid transfer characteristics of the MoS2 FETs corresponding to CS, BI, FI, and SI samples are, respectively, shown in Figure 4a-d. Additionally, the mean values and standard deviations of the SS, VT, and μFE are shown in Figure 4e. The effects are less prominent than those seen in the 1015 He+ ions/cm2 samples. The BI and FI samples show an increase in the mean SS to ~6 V/decade and ~4.7 V/decade, respectively, compared to 1.8 V/decade in CS corresponding to the formation of interface trap charges. The lower increase of SS compared to 1015 He+ ions/cm2 could be due to the lower nuclear stopping power of 2-MeV protons, which leads to less


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displacement damage and lower production of interface trapped charges. As with the devices exposed to 1015 He+ ions/cm2, a fraction of the devices on the SI sample show a large negative VT shift which is accompanied by a large SS increase. This increases the average SS, even though many of the devices show similar SS values to the CS. The BI and SI samples also show a slight negative VT shift to ~-51 V and ~-52 V, respectively, from ~-41 V in the CS, which does not appear in the FI sample. This indicates that the positive fixed oxide charges dominate over negative interface traps for BI samples, in accordance with Equation (1). The 2-MeV protons have a higher charge yield (the fraction of electron-hole pairs that escape the initial recombination) than 390keV He+ ions, leading to a greater number of holes generated in the oxide layer.47 Thus, more holes and fewer interface trapped charges (revealed by the less SS increase) were created compared to irradiation with 1015 He ions/cm2, which leads to the negative VT shift observed in the BI and SI samples. Furthermore, there were only minimal reductions to the mean values of μFE to 18.3 cm2/Vs and 19.2 cm2/Vs for the irradiated MoS2 flakes corresponding to the BI and FI samples, respectively, compared to 22 cm2/Vs in the CS. The lower magnitude of the changes compared to the 1015 He+ ions/cm2 samples, despite the higher dose, can be explained by the differences in the ion masses and energies. Protons have lower mass than He+ ions and consequently have a lower nuclear stopping power, which reduces their collision damage and the generation of interface states. We observed again in SI samples that the generation of oxide charges has minimal effect on the degradation of mobility. By comparing the two experiments of the same ionizing dose but different nuclear stopping, we showed that mobility decreases with sputtering yield, which is associated with ion nuclear stopping. Although accurate trap and oxide charge density cannot be determined from our IV measurement, estimation of the interface trap capacitance and radiation



induced total charge follows the expected trend. More calculation details including Monte Carlo simulation results can be found in the supplementary information.

At a He+ fluence of 1015 cm-2 and proton fluence of 1.26 × 1016 cm-2, based on literature, we expect the major defects in MoS2 to be sulfur vacancies and small defect clusters. Defect configurations and compositional changes of MoS2 and other similar TMD materials before and after proton and helium ion irradiation have been investigated in several prior works. Although the proton and He+ energies are different in some studies, their conclusions can still be considered applicable to this work by making comparisons at the same dose. First, according to DFT and molecular dynamics simulations of monolayer MoS2 upon He+ irradiation at various energies, single and double sulfur vacancies are the most probable types of defects, because the displacement threshold of S atom is calculated to be six times smaller than that of Mo atom.43 S adatoms and Frenkel pair can also be created, according to the same study. The MoS2 Raman peak broadening and stoichiometry change measured from EDX as a function of 30-keV He+ fluence was studied by Fox et al.27 If the results are scaled to the same dose for 30-keV and 390-keV He ions, a slight change of the Raman spectra and compositional ratio at a 390-keV He+ fluence of 1015 cm-2 is expected. In our previous investigation of proton irradiation of exfoliated WSe2,48 we observed no appearance of W or Se oxidation peak in XPS at a proton fluence of up to 1017 cm-2 with 2-MeV protons, which is the same proton energy as used in the present study. Only a slight shifting of core-level W and Se peaks was observed via XPS, which indicates charge transfer upon proton irradiation. Due to the similarity between MoS2 and WSe2, similar results are expected here. STEM was used to image WSe2 after 25-keV He+ irradiation with a range of fluences (1014-1017 cm-2) in the work of Stanford et al.29 With increasing fluence, it was shown that the defect morphology changed from isolated,


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predominately Se, vacancies to large coalescent defects. If the fluence is scaled to reflect the same nuclear stopping power, isolated sulfur vacancies can be expected at a He+ fluence of 1015 cm-2 in our experiment.

Conclusion We have demonstrated that despite the VT shift and SS degradation, back gated MoS2 FETs maintain high ON currents and ON/OFF ratios even when exposed to He+ fluence of 1015 ions/cm2 or proton fluence of 1.26 × 1016 ions/cm2. Although the flux of cosmic rays in space varies by altitude, orbit inclination, solar cycle, etc., these doses are higher than the lifetime doses encountered by most satellites in even the most radiation-prone orbits. With minimal shielding, this fluence level corresponds to hundreds or thousands of years of proton and alpha particle irradiation in space.49 We have demonstrated that radiation damage to the oxide and surrounding materials can influence the electrical characteristics 2D FETs and that these effects must be accounted for when performing radiation studies. However, with our new approach, we successfully decoupled the impact of oxide charges and interface states on subthreshold slope increase, mobility decrease, and threshold shift. We observed that at a He+ fluence of 1015 ions/cm2, negative interface states dominated the post-irradiation electrical characteristics whereas at a proton fluence of 1.26 × 1016 ions/cm2, oxide charges became more important. Different degradation behavior caused by protons and helium ions at the same dose level could be explained by the difference in nuclear stopping power, electronic stopping power, and charge yield of two different ion species. Our results provide useful insights on the generation of defects and interface charges that are intrinsic to TMD materials, especially for space applications where heavy charged particles represent a major challenge for shielding. This could also be potentially useful for future



improvement of device fabrication process for transistor-level radiation hardening. The relatively low damage resulting from these high radiation fluences can be attributed to the atomically thin nature of MoS2. Consequently, MoS2 and 2D materials in general can be considered to be excellent candidates for electronic materials in future space applications and other high-radiation environments. Methods Irradiation Conditions The irradiation experiment was performed at the Michigan Ion Beam Laboratory, University of Michigan. Protons and helium ions were used since they are major compositions of cosmic rays and cannot be effectively shielded in space applications. The 390-keV singly-charged helium ions were produced by a 400-kV ion implanter. The 2-MeV protons were produced by a 3-MV tandem Pelletron accelerator. The selected ion energies are high enough to penetrate through the MoS2 FET structure. Although one specific ion energy was used, the total dose can be determined based on the ion stopping power and fluence, and then scaled to the dose in a more complex radiation environment for the estimation of radiation damage. The samples were uniformly raster-scanned at an angle of 7° from normal incidence to avoid channeling effects and we controlled the current to keep the sample surface temperature to be less than 75 C. Control samples were prepared and fabricated with the same procedure as the irradiated samples and went through the same transport process between the fabrication lab and the irradiation facility.

Device Fabrication The MoS2 was exfoliated using the standard tape method onto the target substrates from a natural bulk crystal purchased from SPI supplies. The samples were spun at 4000 rpm for 45 s with MMA


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EL6 photoresist and baked at 150 ˚C for 90 s. A second coat of PMMA A3 was spun with identical conditions and baked at 180 ˚C for 90 s. They were then patterned using a 100 kV Vistec EBPG 5200 electron beam lithography tool with a dose of 300 μC/cm2. The develop consisted of immersion for 60 s in 1:1 MIBK:IPA solution followed by 45 s in IPA. 40 nm Ni/30 nm Au contacts are deposited using a Kurt J. Lesker Lab-18 electron beam evaporation tool. Liftoff was then performed in room temperature acetone with a rinse in IPA.

Flake Transfer The samples were spun with PMMA A6 at 4000 rpm for 45 s. They were then allowed to dry at room temperature for 24 hours to avoid any film stresses that may result from baking. A razor was used to remove the PMMA around the edge of the sample to allow the solution to approach the edge of the PMMA at the SiO2 surface without any irregularities in the PMMA that may have been present at the edge of the sample after the spinning. The substrate with PMMA was then placed in a beaker of 1 molar NaOH solution and allowed to float at the top. The NaOH removed the PMMA film through capillary action, which was then transferred to a series of three deionized water baths using a glass slide. The film was then lifted onto the target substrate and allowed to dry by heating on a hot plate at 50 ˚C for 10 minutes and then at 70 ˚C for 10 minutes. Finally, the dried PMMA film was removed in a bath of room temperature acetone leaving the flakes behind on the new substrate.

Characterization The samples were placed in a Lakeshore CRX-VF probe station and placed under vacuum. They were allowed to pump overnight to remove any hysteresis effects or threshold voltage shifts caused



by exposure of the MoS2 to air. Electrical measurements were taken using a Keysight B1500A parameter analyzer.


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SUPPORTING INFORMATION Discussion on radiation induced oxide and interface charge; Effect of the NaOH wet transfer process on electrical characteristics; Discussion on large variation in the device characteristics of SI sample irradiated by 1015 He+ ions/cm2 AUTHOR INFORMATION Corresponding Author [email protected], [email protected] Acknowledgement The work of A.J.A. and S.D. was supported by Grant Number FA9550-17-1-0018 from Air Force Office of Scientific Research (AFOSR) through the Young Investigator Program. The authors would also like to acknowledge Fu Zhang for helping with the flake transfer process. The authors would like to thank the staff of the Michigan Ion Beam Laboratory for helping carry out the ion beam experiments.

Author Contributions S.D. and I.J. conceived the idea, designed the experiments and supervised the research. A.J.A. performed device fabrication and electrical measurements and T.S. performed radiation exposure experiments. All the authors analyzed the data, discussed the results, agreed on their implications, and contributed to the preparation of the manuscript. Additional Information: The authors declare no competing financial interest.



Figure Captions

Figure 1: Schematic of Novel Experimental Method. a) Standard approaches use irradiated full transistors making it difficult to isolate radiation effect on 2D materials from that on the oxide substrate and the metal contacts. The new method uses three samples consisting of b) irradiated substrate and flakes, c) irradiated flakes transferred to an unirradiated substrate, and d) exfoliated unirradiated flakes on an irradiated substrate. Furthermore, in the new method, contacts are fabricated after the irradiation to eliminate radiation effects on the contacts from observed electrical characteristics. e) Method for wet transfer of flakes by coating irradiated flakes with PMMA, removing the PMMA/flakes assembly from the substrate in a NaOH bath, rinsing the PMMA in DI water, picking up the floating PMMA with a new substrate, and removing the PMMA with acetone. This transfer process is unique to 2D materials owing to their van der Waals (vdW) epitaxy.

Figure 2: Radiation Damage Due to He+ Exposure. Representative transfer characteristics of MoS2 FETs from an unirradiated control sample (CS) with neither the flakes nor the substrate being irradiated, and post radiation samples with both the flake and the substrate irradiated (BI), only the flake irradiated (FI), and only the substrate irradiated (SI). 390 keV He+ ions were used at a total fluence of a-d) 1014 He ions/cm2, e-h) 1015 He ions/cm2, and i-l) 1016 He ions/cm2.

Figure 3: Statistical Analysis of He+ irradiation at a fluence of 1015 ions/cm2. Overlaid transfer characteristics of all MoS2 FETs corresponding to a) control sample (CS), b) both flakes and substrate irradiated (BI), c) flakes irradiated (FI), and d) substrate irradiated (SI) with 1015 He+


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ions/cm2. e) Mean and standard deviation of subthreshold slope, threshold voltage, and field effect electron mobility for all four sample configurations.

Figure 4: Statistical Analysis of proton irradiation. Overlaid transfer characteristics of all MoS2 FETs corresponding to a) control sample (CS), b) both flakes and substrate irradiated (BI), c) flakes irradiated (FI), and d) substrate irradiated (SI) with 1.26 × 1016 protons/cm2. e) Mean and standard deviation of subthreshold slope, threshold voltage, and field effect electron mobility for all four sample configurations.

Table 1: The stopping power and ranges of protons and helium ions in the irradiated materials estimated from SRIM/TRIM.50 Displacement threshold energy of 31.7 eV for Mo and 5.0 eV for S were used based on the results of MD simulation.43



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Figure 1


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Figure 2



Figure 3


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Figure 4



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Table 1 (dE/dx)elec. in MoS2 (eV/Å)

(dE/dx)nuclear in MoS2 (eV/Å)

(dE/dx)elec. in SiO2 (eV/Å)

(dE/dx)nuclear in SiO2 (eV/Å)

Range in Si (μm)

390 keV helium ion

53.9

3.37 × 10-2

38.7

0.0209

1.66

2 MeV proton

4.30

5.45 × 10-4

3.29

3.86 × 10-4

47.7


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TOC Graphic


Extraordinary Radiation Hardness of Atomically Thin MoS2 - ACS

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