HfSe2 and ZrSe2: Two-dimensional semiconductors with native high-κ oxides by Michal J. Mleczko, Chaofan Zhang, Hye Ryoung Lee, Hsueh-Hui Kuo, Blanka Magyari-Köpe,

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HfSe2 and ZrSe2: Two-dimensional semiconductors with native high-κ oxides by Michal J. Mleczko, Chaofan Zhang, Hye Ryoung Lee, Hsueh-Hui Kuo, Blanka Magyari-Köpe, Robert G. Moore, Zhi-Xun Shen, Ian R. Fisher, Yoshio Nishi, and Eric Pop Science Volume 3(8):e1700481 August 11, 2017 Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

Fig. 1 Physical and electronic structure. Physical and electronic structure. (A) Crystal structure of HfSe2 and ZrSe2 along with reciprocal unit cell (top) outlining high-symmetry points. In-plane ARPES spectra of in situ vacuum-cleaved, Na doped (B) ZrSe2 and (C) HfSe2 reveal a band structure around the VBM (at Γ point) and lower edges of CBM (at M point), along with reciprocal space distributions of degenerate CBM valleys. Dashed red lines are the theoretical band structure from DFT calculations (HSE06 hybrid functional) in the Γ to M direction, adjusted for a ~0.05-eV (~0.12 eV) underestimate for Γ to M energy gaps of ZrSe2 (HfSe2). Michal J. Mleczko et al. Sci Adv 2017;3:e1700481 Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

Fig. 2 Cross-sectional TEM and EDX mapping of native oxide formation. Cross-sectional TEM and EDX mapping of native oxide formation. (A) Cross-sectional TEM image of a smaller (<50 nm thick, <10 μm in lateral size) ZrSe2 flake following 3 days of ambient exposure, demonstrating both top-down and bottom-up oxidation into amorphous ZrOx, which partially consumes individual layers. Insets show that EDX elemental mapping demonstrates displacement of Se by O in oxidized regions. Scale bars, 20 nm. (B) Similar TEM images of a larger (>100 μm), thicker HfSe2 flake after 7 days of ambient exposure, with greater extent of top-down oxidation into HfOx, despite a pristine bottom interface with the Si substrate. Insets show EDX elemental mapping confirming complete chalcogen depletion, illustrated via overlay of O and Se signals. Scale bars, 50 nm. Michal J. Mleczko et al. Sci Adv 2017;3:e1700481 Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

Fig. 3 HfSe2 transistors. HfSe2 transistors. (A) Schematic of HfSe2 device, back-gated through 90-nm SiO2, and with ALD alumina used as both protective encapsulation and a ~25 Å interlayer in an MIS contact scheme (contact layers not to scale; capping alumina over contact metals not shown). (B) High-resolution TEM image of the channel of an 8L-thick HfSe2 device, with evidence of a partially oxidized top layer integrated into the capping oxide. (C) Room temperature transfer curves of HfSe2 transistors with varying channel thickness, demonstrating an on/off current ratio of ~106 at VDS = 1 V. Inset: Linear ID versus VDS plots for short-channel devices (L ≈ 140 nm) with 3L and 8L channels. All curves are dual sweeps from the origin, demonstrating low hysteresis (see arrows). (D) Temperature dependence of transfer curves for the 8L-thick HfSe2 device, from 80 to 300 K. The threshold voltage shifts higher as the sample is cooled. Inset: Field-effect mobilities of encapsulated 3L to 13L (~1.8 to 8.1 nm thick) HfSe2 devices. Michal J. Mleczko et al. Sci Adv 2017;3:e1700481 Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

Fig. 4 ZrSe2 transistors. ZrSe2 transistors. (A) Low-power Raman spectra of ZrSe2 devices encapsulated by AlOx, from 3L to bulk, with minute shifts of A1g (~193 cm−1) and Eg (~145 cm−1) modes as the thickness is reduced (532 nm laser). arb.u., arbitrary units. (B) Hysteresis-free room temperature ID versus VDS forward and reverse sweeps for a 6L-thick ZrSe2 transistor (L = 320 nm) with a current density up to ~20 μA/μm. (C) Transfer curves of back-gated ZrSe2 transistors of varying channel thickness show lower current density in the thinnest (3L) devices. (D) Temperature-dependent transfer curves of the 6L device between 80 and 300 K show improved on/off current ratios with cooling (approaching 107) and forward shift in threshold voltage at lower temperatures. Insets are false-colored SEM micrographs of 3L and 6L devices, with source (S) and drain (D) contacts as labeled. Michal J. Mleczko et al. Sci Adv 2017;3:e1700481 Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

Fig. 5 Interfacial oxide engineering. Interfacial oxide engineering. (A) Configuration of MIS contacts for HfSe2 transistors with 25 Å interlayers of either AlOx or HfOx ALD oxide (global 200 Å alumina capping is used for both). (B) Mean shift in threshold voltage ΔVT from the 300 K baseline for multiple HfSe2 devices cooled to ~80 K, contrasting the effects of AlOx (blue) and HfOx (orange) interlayers. VT was extracted from the linear intercept at the point of peak transconductance. Dashed lines are guides to the eye. Inset shows transfer characteristics of an 8L-thick HfSe2 device with HfOx interlayer at 80 and 300 K, demonstrating a minimal shift of VT and steeper turn-on behavior at lower temperatures. (C) Temperature evolution of estimated interfacial trap density NIT for multiple HfSe2 devices with AlOx interlayers, as calculated from ΔVT (see text). Dashed lines are fits to Arrhenius activation energies of 26 to 32 meV. Michal J. Mleczko et al. Sci Adv 2017;3:e1700481 Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

Fig. 6 Top-gated HfSe2 transistor with native HfO2 dielectric. Top-gated HfSe2 transistor with native HfO2 dielectric. Measured transfer characteristics of a top-gated 7L-thick HfSe2 transistor with 2.5-nm HfOx MIS contacts and 19.5-nm HfO2 top-gated dielectric (EOT ≈ 4.75 nm). The channel length is L ≈ 1 μm, VDS = 1 V, and T = 295 K, and the Si substrate was grounded. DC measurements (solid black lines) reveal ~106 on/off current ratio, despite noticeable hysteresis and SS of ~200 to 350 mV per decade. Pulsed measurements (dashed red lines, with 125 μs pulse width and 10 μs fall/rise times) reduce the hysteresis and achieve higher current density, although the low-current resolution is limited. Inset shows device schematic with source (S), drain (D), and gate (G) electrodes as labeled. Michal J. Mleczko et al. Sci Adv 2017;3:e1700481 Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).