Day 3 Session 3.2

Introduction: Two-dimensional (2D) electronics can enable FETs down to a few nanometers. However, these devices require scalable insulators which should form high-quality interfaces with 2D channels and maintain low gate leakage currents for sub-1nm equivalent oxide thickness (EOT). Previously used amorphous oxides result in poor interfaces with 2D materials, while hBN has mediocre dielectric properties (ε < 5, E_g = 6eV) [1]. As a promising alternative, we suggest the use of the crystalline ionic insulator CaF₂ (ε = 8.43, E_g = 12.1eV) which forms van der Waals interfaces with 2D semiconductors [2]. At the moment, CaF₂ can be grown by molecular-beam epitaxy (MBE) down to a few nanometers thickness [3] and appears promising for chemical vapour deposition (CVD) [4] and atomic-layer deposition (ALD) [5]. Here we discuss our recent progress [3, 6, 7] on ultra-thin CaF₂ which presents a universal platform for 2D devices. In particular, we demonstrate nanoscale MoS₂ FETs with L =50-60nm and a record-thin ∼2nm CaF₂ insulator (EOT∼0.9nm) which exhibits near-ideal subthreshold swing (SS).

Growth and structure of CaF₂ films: MBE of CaF₂ on Si(111) has been known for over three decades [8]. However, previously used high-temperature growth (up to 800◦C), which targeted high crystallinity, results in the formation of triangular pinholes [9]. To address this problem, we have succeeded in growing CaF₂ at 250◦C [6] after careful cleaning of Si(111) substrates [10]. This allowed us to obtain pinhole-free homogeneous CaF₂ films with 1−2nm thickness. For instance, the atomic-force microscopy (AFM) image in Fig.1a shows that the surface of CaF₂ is flat with clearly visible atomic steps, and the transmission electron microscopy (TEM) images from Fig.1b,c confirm that the 2nm thick CaF₂ film consists of triple F-Ca-F monolayers (1ML=0.315nm, Fig.1c). Despite low-temperature MBE growth, reflection high energy electron diffraction (RHEED) patterns (Fig.2) contain distinct streaks which indicates high crystallinity of our CaF₂ films grown on atomically clean Si(111).

High-quality CaF₂ for device applications: An important advantage of CaF₂ is its inert (111) surface which forms well-defined interfaces with 2D materials [2] (Fig.3a). This is in contrast to amorphous oxides like HfO₂ which contain dangling bonds (Fig.3b). Also, CaF₂ has a reasonably high permittivity (ε = 8.43), an extremelywide bandgap (E_g = 12.1eV) and forms high band offsets with Si and 2D semiconductors (Fig.3c). Thus, predicted leakage currents through CaF₂ are lower [11] than those through HfO₂, hBN and SiO₂ (Fig.3d), and fit the end-of-roadmap requirement. To understand leakage mechanisms through CaF₂,we measured the I−V curves of Au/CaF₂/Si(111) diodes [3,6,12]. We found that the results can be reproduced with theoretical tunneling models considering thickness fluctuations [3] and transverse momentum conservation [12] (Fig.3e). The latter becomes important because of the (111) orientation and further suppresses tunneling through CaF₂ [12]. Next we analyzed the light emission of hot electrons which are injected through CaF₂ to p-Si under accummulation (VSi >0) and can be involved in recombination (RR), intra-band (IB) and intra-band direct (IB-d) radiative transitions (Fig.4a) [3,13]. We found that activation thresholds measured for different photon energies ħω are close to their estimated values (Fig.4b) and concluded that carrier injection through CaF₂ is elastic, i.e. takes place without energy dissipations. This means that our CaF₂ layers are mostly defect-free and thus perfectly suited for device applications.

MoS₂ FETs with 2nm CaF₂: Fig.5 demonstrates the use of Si(111)/CaF₂ substrates for MoS₂ FETs [7]. Devices have been produced by transferring bilayer CVD-grown MoS₂ films onto CaF₂ and shaping Ti/Au electrodes with ebeam lithography. First we fabricated hundreds of large-area (L =400 - 800nm) MoS₂ FETs with 2nm thick CaF₂ [7] and confirmed the device structure using scanning electron microscopy (SEM, Fig.5a) and TEM (Fig.5b). Then we found that the best devices exhibit SS down to 90mV/dec and record-high on/off current ratios up to 107 for 2nm thick insulators (Fig.5c). Finally, we extended the source Ti/Au area and shaped nanoscale CaF₂/MoS₂ FETs with L =50-60nm(Fig.6). These devices exhibit excellent drain current control by the gate bias VG and near-ideal SS down to 60mV/dec, which slightly degrades at larger V_D where the device reaches the full on state.

Conclusions: We fabricated high-quality crystalline 1−2nm CaF₂ films and successfully used them for MoS₂ FETs with record-thin gate insulators. For the first time we demonstrated MoS₂ FETs with simultaneously sub-1nm EOT insulators and sub-100nm channel length and found that these devices can exhibit near-ideal SS. Our results suggest that Si(111)/CaF₂ substrates provide a universal platform for future 2D nanoelectronics, which has enormous potential due to the possibility of direct growth of 2D semiconductors on CaF₂ [14]

78^th Device Research Conference

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Researchers should cite this work as follows:

• Yury Yuryevich Illarionov, A.G. Banshchikov, Theresia Knobloch, D.K. Polyushkin, S. Wachter, V.V. Fedorov, M. Stöger-Pollach, M.I. Vexler, N.S. Sokolov, T. Grasser (2020), "A3 Crystalline Calcium Fluoride: A Record-Thin Insulator for Nanoscale 2D Electronics," https://nanohub.org/resources/34171.

  BibTex | EndNote

1. 2D materials and devices
2. devices
3. DRC 2020
4. FET
5. insulators
6. MBE growth
7. nanoelectronics