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Effects of ionizing radiation on the layered semiconductor tungsten diselenide
- 1. Effects of ionizing radiation on the layered semiconductor: Tungsten diselenide (WSe2) Roger C. Walker II March 31st, 2017 M.S. Thesis Defense Committee: Dr. Robinson (adviser), Dr. Mohney, and Dr. Das
- 2. Tungsten diselenide (WSe2) for space electronics 2 Sources: ACS Nano, 2015, 9 (2), pp 2080–2087; Nano Lett., 2016, 16 (3), pp 1896–1902;” NASA's Fermi Sees Gamma Rays from 'Hidden' Solar Flares”, NASA Press Release, Jan 30 2017,; IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 55, NO. 4, AUGUST 2008; Proc. IEEE, 76 (11), 1423–1442; Ultramicroscopy 42-44 (1992) 683-688 Radiation tolerance of WSe2 requires further examination Single-layer WSe2 is a two- dimensional material (2DM) Requirements for space electronics • Minimized weight and volume • High performance / quality • Low power consumption • Radiation tolerance Particle type Energy range (MeV) Photon Up to 4 × 103 Trapped electron 10-3 – 10 Trapped proton 10-3 – 4 × 102 Solar proton Up to 103 Cosmic ray (Protons and ions) Generally 10 – 104 Up to 3 × 1014 Radiation tolerance = resistance to damage from energetic particles Space radiation hazards Prior studies for WSe2 looked at impact of keV ions (e.g. argon) Impact crater Corrugated surface 5 x 1011 ions/cm2 1015 ions/cm2
- 3. Experimental 3 Exposure to X-rays and electrons in UHV Left image sources: ; ACS Nano, 2015, 9 (2), pp 2080–2087; K S Novoselov and A H Castro Neto 2012 Phys. Scr. 2012 014006; right image source: “An Introduction to Surface Analysis by XPS and AES” by J.F. Watts and J. Wolstenholme Growth of nanoscale WSe2 via MOCVD W(CO)6 + DMSe / H2Se + H2 Main characterization technique is X-ray photoelectron spectroscopy (XPS) High energy charged particle exposure Reprinted from: http://mibl.engin.umich.edu/ Mechanical exfoliation from bulk WSe2 crystals
- 4. Stability of WSe2 in the XPS environment 4 MOCVD-grown WSe2 was exposed to 1.486 keV X-rays and low energy electrons for 24 hours Main expected impact: ionization from X-rays (positive charge buildup) Choice of substrate should influence surface charging Material Band gap (eV) GP 0 WSe2 1.2 to 2.2 SiC 3.1 Al2O3 8.5 Ultra-thin MOCVD WSe2 → reduced X-ray absorption Larger band gap → expected greater positive charge buildup
- 5. Stability of WSe2 in the XPS environment 5No dependence on substrate band gap → WSe2 growth is key WSe2 on GP WSe2 on SiC WSe2 on Al2O3 MOCVD-grown WSe2 was exposed to 1.486 keV X-rays and low energy electrons for 24 hours Track ionization via shifts in the binding energy of electrons
- 6. Effect of 2 MeV proton exposure to WSe2 on SiC 6 High energy charged particles have two effects on materials • Ionization (charge buildup due to electron ejection) • Displacement (point defect formation – e.g. vacancies) Ionization is expected in the WSe2 Both are expected in the SiC Ionization in WSe2 above a threshold exposure level WSe2 SiC Vacancies formed deep in SiC induce a color change
- 7. Influence of proton energy on damage to WSe2/SiC 7 Reducing proton energy = two effects expected • More ionization in the WSe2 and SiC • Damaged SiC region moves closer to the surface Lower proton energy ≠ increased shift in binding energy for WSe2 Proton energy and exposure Average energy loss in WSe2 (eV/nm) Average energy loss in SiC (eV/nm) Depth of travel (µm) 40 keV proton, 1016 /cm2 154 184.6 0.258 200 keV proton, 1016 /cm2 145 145.3 1.2 1 MeV proton, 1016 /cm2 79.2 61.6 10.8 2 MeV proton, 1016 /cm2 56 38.9 32 Proton energy loss is set by its initial energy, influences resulting damage Trend in SiC is unclear
- 8. Influence of proton energy on damage to WSe2/SiC 8 Reducing proton energy = two effects expected • More ionization in the WSe2 and SiC • Damaged SiC region moves closer to the surface Proton energy and exposure Average energy loss in WSe2 (eV/nm) Average energy loss in SiC (eV/nm) Depth of travel (µm) 40 keV proton, 1016 /cm2 154 184.6 0.258 200 keV proton, 1016 /cm2 145 145.3 1.2 1 MeV proton, 1016 /cm2 79.2 61.6 10.8 2 MeV proton, 1016 /cm2 56 38.9 32 40 keV = drastic reduction in signal from SiC C-C SiC SiC C-OC=O Proton energy loss is set by its initial energy, influences resulting damage
- 9. Proton effects on WSe2/SiC band alignment 9 Alignment type is preserved, offset value is alteredInitial: Type I alignment 6H-SiC 3.1 eV 1.2 eV Exfoliated WSe2 Initial VBO ≈1.2 eV 𝑉𝐵𝑂 = 𝐸𝑆𝑖 2𝑝 𝑊𝑆𝑒2/𝑆𝑖𝐶 − 𝐸 𝑊 4𝑓 𝑊𝑆𝑒2/𝑆𝑖𝐶 + 𝐸 𝑊 4𝑓 𝑊𝑆𝑒2 − 𝐸 𝑉𝐵𝑀 𝑊𝑆𝑒2 − 𝐸𝑆𝑖 2𝑝 6𝐻−𝑆𝑖𝐶 − 𝐸 𝑉𝐵𝑀 6𝐻−𝑆𝑖𝐶 Band alignment can be altered by protons, and measured using XPS data 200 keV → Ionization in SiC 2 MeV → Ionization in WSe2
- 10. Heavy metal ion exposure of WSe2 on SiC 10 Proton energy and exposure dpa in WSe2 40 keV proton, 1016 /cm2 0.003 200 keV proton, 1016 /cm2 0.0005 1 MeV proton, 1016 /cm2 0.00005 2 MeV proton, 1016 /cm2 0.00005 2.5 MeV Fe ion, 1016 /cm2 11.7 5 MeV Fe ion, 1016 /cm2 6.9 4 MeV Ag ion, 1016 /cm2 24.4 More displacement damage expected, characterized by displacements per atom (dpa) Displacement damage = selenium ejection → tungsten oxidation in air WSe2 WOx WSe2 WOx WSe2 WOx WSe2 WSe2 WSe2
- 11. C-C SiC C-O C-C SiC C-O C-C SiC C-O SiC SiC SiC SiOx SiOx SiOx Heavy metal ion exposure of WSe2 on SiC 11 Displacement damage also occurs in the SiC substrate, affects the XPS spectra Formation of silicon oxide Amorphization of SiC
- 12. Heavy metal ion exposure of WSe2 on SiC 12 The surface chemistry may change over time due to the initial destabilization → Re-analyze samples after storage in medium vacuum WSe2 continues to degrade in storage SiC does not appear to degrade further
- 13. Conclusions • Thin films of MOCVD WSe2 are stable against soft X-ray exposure in ultra-high vacuum • Discontinuities and vertical features reduce this stability • Future work would further explore this effect • Exfoliated WSe2 ionizes in response to protons, is destabilized by heavy metal ions • Threshold exposure needed for ionization of WSe2 • Band alignment is modified due to ionization • Ion exposure leads to oxidation when exposed to air • Future work would analyze the differences in radiation tolerance between exfoliated and MOCVD-grown WSe2, and impact on devices 13
- 14. Acknowledgements • This research was funded by the Defense Threat Reduction Agency under grant HDTRA1-14-1-0037 • I would like to thank the following people for their help and support • Staff from MCL, the Nanofab, and MIBL • Collaborators from the University of Michigan: Dr. Igor Jovanovic, Tan Shi • The Robinson group members, esp. Dr. Ganesh Bhimanapati for helping with the XPS study • My friends and family 14
- 15. Thanks for listening! 15 Image source: http://arseniic.deviantart.com/art/How-do-you-organize-a-space-party-289404903
Editor's Notes
- W-Se bond length is ~2.5 Å; P63/mmc symmetry, a = 3.3 Å, c = 13 Å; as-measured layer thickness is ~6.5 Å Argon ions have 5 keV energy here Best device characteristics as monolayer: mobility up to 250, ON/OFF ratio up to 10^9, SS around 300 mV/dec, can go as low as 60 Other studies: selenium is preferentially ejected by helium ions with 25 keV; 60 keV electron beam causes WSe2 -> WSe Space radiation flux goes up to 10^10 for trapped particles and solar protons (depending on energy), cosmic ray flux is ~4
- MIBL image from http://mibl.engin.umich.edu/ X-ray exposure = done in UHV, electrons are generated by a flood gun and magnetically confined, X-ray flux is ~2 * 10^11 photons/mm^2/s Proton and ion exposure = done in HV, beam current is 300 – 500 nA, exposed area is ~ 6 mm x 6 mm, beam is angled at 7 degrees off normal -> flux is ~10^12
- Data in plot calculated via GEANT4 by Tan Shi (U. Michigan) – Geant4 is a simulation toolkit developed by CERN for use in accurately modeling radiation damage from particles with energy of 250 eV up to the PeV range WSe2 band gap decreases with increasing thickness, but saturates after ~10 layers or so
- Total dose is ~10^18 photons/mm^2 = ~10^20 photons/cm^2 Growth conditions: GP = 800 C, 30 minutes, DMSe (4.51 sccm), 0.003 sccm WCO6, 420 sccm H2 SiC = 600 C, 20 minutes, H2Se (4 sccm), 0.0006 sccm WCO6, 225 sccm H2 Sapp = 800 C, 20 minutes, H2Se (4 sccm), 0.0006 sccm WCO6, 196 sccm H2
- Lower energy radiation such as soft X-rays, low energy electrons, and low energy ions do exist in space! However, higher energy particles are more directly significant for space electronics. 6H-SiC crystal structure is P63mc, a = 3.1 Å, c = 15 Å; Si-C bond length is ~ 1.9 Å; note that one of the data points at 10^16 for SiC is of bare SiC Energy transfer is mostly ionization until it loses sufficient energy to interact with nuclei – then displacement may occur
- Data in table calculated via SRIM by Tan Shi (U. Michigan) For compounds, SRIM uses a “core and bond” approach – linearly combine the elements, then add a correction for the chemical bonds Stopping power and proton energy are inversely proportional due to its dependence on particle velocity Stopping power is the sum of ionization and displacement
- Data in table calculated via SRIM by Tan Shi (U. Michigan) For compounds, SRIM uses a “core and bond” approach – linearly combine the elements, then add a correction for the chemical bonds Stopping power and proton energy are inversely proportional due to its dependence on particle velocity Stopping power is the sum of ionization and displacement
- XPS is a common tool for measuring band alignment between semiconductors – for examples between WSe2 and MoS2, HfO2 and SiC, Si and Ge, etc. Band alignment change is due to differential ionization (doping changes in one material and not the other) – interface states affect both materials and cancel out VBO requires bulk samples as well -> only did a control, 200 keV, and 2 MeV
- Data in table calculated via SRIM by Tan Shi (U. Michigan); depth of travel is 1.26 um for 2.5 MeV Fe, 2 um for 5 MeV Fe, and 1.25 um for 4 MeV Fe Heavy ions transfer so much more energy (an order of magnitude more than the 40 keV protons) that they can cause significant damage even at the surface
- The dpa of SiC is about 1/3rd that of WSe2