Nanoscale-Barrier Formation Induced by Low-Dose Electron-Beam Exposure in Ultrathin MoS₂ Transistors

Abstract

Utilizing an innovative combination of scanning-probe and spectroscopic techniques, supported by first-principles calculations, we demonstrate how electron-beam exposure of field-effect transistors, implemented from ultrathin molybdenum disulfide (MoS), may cause nanoscale structural modifications that in turn significantly modify the electrical operation of these devices. Quite surprisingly, these modifications are induced by even the relatively low electron doses used in conventional electron-beam lithography, which are found to induce compressive strain in the atomically thin MoS. Likely arising from sulfur-vacancy formation in the exposed regions, the strain gives rise to a local widening of the MoS bandgap, an idea that is supported both by our experiment and by the results of first-principles calculations. A nanoscale potential barrier develops at the boundary between exposed and unexposed regions and may cause extrinsic variations in the resulting electrical characteristics exhibited by the transistor. The widespread use of electron-beam lithography in nanofabrication implies that the presence of such strain must be carefully considered when seeking to harness the potential of atomically thin transistors. At the same time, this work also promises the possibility of exploiting the strain as a means to achieve "bandstructure engineering" in such devices.

Negative Differential Conductance & Hot-Carrier Avalanching in Monolayer WS2 FETs

“…This in turn provides favorable conditions for the observation of negative differential conductance, by suppressing the likelihood of carriers being transferred into the T valleys at low fields 4 In our simulations, we have systematically raised the T valleys and find that negative differential conductance begins when the inter-valley separation (Δ) is as little as 100 meV (i.e. when it is increased by just 25% relative to the unstrained case), corresponding to a strain level of just 1% 31 , 32 This estimate is consistent with recent scanning-microscopy studies of MoS 2 FETs, which have revealed the presence of unintended strain in their channels, introduced during nanofabrication 33 By assuming Δ = 110 meV, in Fig. 3(b) we compute the fraction of the carriers that remain in the K valley as a function of the electric field.…”

Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers

“…MoS 2 layers can be significantly activated, functionalized, and modified. The irradiation-induced defects are beneficial for several applications including: solar cells [19,[194][195][196], batteries [54,197,198], supercapacitors [199], thin film transistors [15,16,80,86,110,138,[200][201][202][203][204][205], sensors [17,55,56,106,[206][207][208][209][210], hydrogen generators [8,47,50,69,134,176,177,183,211,212], and applications in thermoelectrics [213][214][215][216], piezotronics [63], valleytronics [65], and environments [45,49]. MoS 2 2D layers were damaged simultaneously under ...…”

“…To understand the irradiation-induced defects well, the synthesis/preparation details of the as-prepared MoS 2 layers will be described in each section in addition to irradiation. [75] mono-layer electron 60 keV 10 6 -10 9 electron /nm 2 STEM beam 400-700 • C in vacuum induced 2H/1T phase transition [76] ∼10 layer electron 3-15 keV n/a EPMA in vacuum broke the inversion symmetry [77] mono-layer electron 80 keV n/a TEM beam in vacuum removed top and bottom S atoms [78] mono-layer electron 200 keV 3000 electrons/nm 2 /s TEM beam in vacuum created S vacancies, increased electric resistance [79] mono-layer electron 15 keV 280 µC/cm 2 EBL in vacuum produced local strain and changed band structure [80] mono-layer electron 80 keV 40 A/cm 2 TEM beam in vacuum produced holes and Mo 5 S 3 nanoribbons [81] amorphous 5-7 layer electron 1 keV 1-10 min EBI in vacuum crystallized [82] mono-layer U 238 1.14 GeV 4000 ions/cm 2 heavy ion accelerator in vacuum total damaged [83] micron thickness Ar + 500 eV 2.26 × 10 15 ions/cm 2 plasma UHV produced S vacancies [84] bi-layer Ar + 500 eV 10 14 -10 15 ions/cm 2 plasma UHV produced S vacancies and MoS 6 vacancy clusters [84] mono-layer Ar + 500 eV 2.26 × 10 15 ions/cm 2 plasma UHV damaged [84] 200 µm thickness proton 3.5 MeV 5 × 10 18 ions/cm 2 Singletron facility RT preserved lattice structure, produced defects, changed magnetic moments [85] few-layer proton 10 MeV 10 12 -10 14 ions/cm 2 MC-50 cyclotron n/a decreased electrical conductance [86] mono-layer proton 100 keV 10 12 -10 15 particles/cm 2 LEAF n/a created defects [87] bi-layer proton 100 keV 6 × 10 14 particles/cm 2 LEAF n/a created defects [87] bulk He 2+ 1.66 MeV 900 MGy ion accelerator n/a changed Raman scattering slightly [88] nanosheet He 2+ 1.66 MeV 900 MGy ion accelerator n/a invariant [88] few-layer He 2+ 30 keV 10 18 ions/cm 2 FIB beam in vacuum milled or damaged [89] mono-layer He 2+ 3.04 MeV 8 × 10 13 particles/cm 2 PTA n/a produced defects [90] mono-layer He + 30 keV 10 12 -10 16 ions/cm 2 HIM in vacuum produced S vacancies [91] mono-layer He + 30 keV 10 13 -10 17 ions/cm 2 NFM in vacuum ...…”

“…Matsunaga et al [80] irradiated triangular-shaped MoS 2 mono-layers grown on SiO 2 /Si substrates under an electron beam with an acceleration voltage of 15 kV and an areal dose of 280 µC/cm 2 charge. Figure 19a shows an optical image of a mono-layer MoS 2 crystal.…”

How defects influence the photoluminescence of TMDCs

“…According to DFT calculations, the bandgap of a MoS2 monolayer shrinks with tensile strain but broadens with compressive strain (Fig. 6(a)) [88][89][90][91]. Chalcogen vacancies generally lead to compressive strain and therefore locally increase the bandgap [92].…”