Esaki Diodes in van der Waals Heterojunctions with Broken-Gap Energy Band Alignment

Abstract

Esaki's discovery of NDR in heavily doped semiconducting germanium p-n junctions in 1958 was the first experimental evidence of quantum mechanical tunneling transport of electrons in all-condensedmatter systems 3,4 . This discovery motivated Giaever's tunneling experiments that proved the existence of the superconductive energy gap predicted by the then-newly formulated Bardeen-Cooper Schrieffer (BCS) theory of superconductivity 5 . After these initial breakthroughs, tunneling in various classes of crystalline matter has been observed, and forms the basis for several practical applications. For example,Josephson junctions exploit tunneling in superconductors for exquisitely sensitive magnetic flux detectors in superconducting quantum interference devices (SQUIDs) 6 , and are now being investigated as the building blocks of quantum computers 7 . Electron tunneling forms the basis for low-resistance ohmic contacts to heavily doped semiconductors for energy-efficient transistors, as low-loss cascade elements in multi-junction solar cells, and for coherent emission of long-wavelength photons in quantum-cascade lasers 8,9 . In addition to such practical applications, the extreme sensitivity of tunneling currents to various electronic, vibrational, and photonic excitations of solids makes tunneling spectroscopy one of the most sensitive probes for such phenomena 10 .Recently, interband tunneling in semiconductors has been proposed as the enabler for a new class of semiconductor transistors called tunnel field-effect transistors (TFETs) that promise very low-power operation. The heart of such devices is an Esaki tunnel diode, with preferably a near broken-gap band alignment at the source-channel heterojunction 11,12 . As these heterojunction TFETs are scaled down to the nanometer regime, the increase in bandgap barrier due to quantum confinement may significantly prohibit the desired tunneling currents, because tunneling current decreases exponentially with the barrier height. Layered semiconductors with a sizable bandgap and a wide range of band alignments can potentially avoid such degradation, and have been proposed as ideally suited for such applications 13,14 .This class of devices distinguishes themselves from the graphene-based SymFET by offering a desired low off-current 15,16 . Compared to traditional 3D heterojunctions, such structures are expected to form high-quality heterointerfaces due to the absence of dangling bonds [1][2][3]20 . The weak vdW bonding in principle does not suffer from lattice mismatch requirements and makes strain-free integration possible.Among the previous reports on vdW solids, heterojunctions of type-I (straddling) and type-II (staggered) band alignments have been demonstrated [17][18][19] . The Esaki diode device structure is schematically shown in Fig. 1a. The devices are fabricated using a dry transfer process with flake thicknesses of ~50-100 nm for both BP and SnSe 2 24 . BP is the p-type semiconductor, and SnSe 2 the n-type semiconductor of the vdW Esaki diode. A detailed de...

Booming Development of Group IV–VI Semiconductors: Fresh Blood of 2D Family

“…Therefore, there is still a huge room for further study on the controlled synthesis of high‐quality and large‐scale single crystalline 2D GIVMCs. Moreover, in‐situ growth of vertical ( Figure 11 a) and lateral (Figure 11b) heterostructures are very important for exploring the novel physical properties, considering that most of the reports on 2D GIVMCs is currently based on the transferred heterojunctions,130, 243, 244 which may harm the electronic properties231, 240 due to the dangling bonds and adsorbates at the interface. In addition, graphene is an ideal template to promote the nucleation and growth of other 2DLMCs crystals for producing functional hybrid structures via CVD method (Figure 11c),198, 204, 207 which may result in the modulation of the electric and optical properties coupled with graphene.…”

“…Then they fabricated p‐n heterostructures based on synthesized SnS and SnS 2 as shown in Figure 7 a. Lately, Xing and co‐workers243 for the first time demonstrated a room temperature Esaki tunnel diodes based on exfoliated black phosphorous (BP) and tin diselenide (SnSe 2 ) which possess a broken‐gap energy band offset. The device construction began with the cleavage of BP flakes onto a 285 nm SiO 2 /Si substrate via scotch tape.…”

“…Xing and co‐workers243 for the first time demonstrated a room temperature Esaki tunnel diodes based on exfoliated p‐type black phosphorous (BP) and n‐type tin diselenide (SnSe 2 ) which possess a broken‐gap energy band offset. Javey and co‐workers244 further fabricated 2D–2D tunneling FETs based on exfoliated WSe 2 and SnSe 2 heterostructures employing ZrO 2 as the gate dielectric, allowing the scaling of gate oxide to improve the subthreshold swing of the device (Figure 8h,i).…”

Independent Band Modulation in 2D van der Waals Heterostructures via a Novel Device Architecture

“…However, due to the absence of a bandgap, graphene‐based TFETs are unable to obtain a high on/off ratio of the current. TMD‐based TFET devices have also been experimentally demonstrated, including MoS 2 /WSe 2 ,24, 32, 33 MoS 2 /BP,34, 35 SnSe 2 /BP,36 SnSe 2 /WSe 2 ,37 and ReS 2 /BP 35. In these devices, gate‐tunable tunneling current governed by the band‐to‐band tunneling (BTBT) mechanism can be observed through electrostatic gating of energy‐band alignment and carrier density under specific device architectures.…”

Magnitude of the current in 2D interlayer tunneling devices

“…11,12,13 A number of experimental efforts to produce such devices have been made, although the quality of the results considerably lag those in the G/h-BN/G system. 14,15,16 Nevertheless, it is important to consider the TMD-based devices, since they offer a wider range of operating modes with potential application in electronic circuits and systems. 17 Specifically, the TMD-based interlayer tunneling devices can be utilized for 2D-2D tunneling, just as for G/h-BN/G, and sharp resonant peaks exhibiting NDR are expected (though not yet experimentally observed).…”