The performance of electronic and optoelectronic devices based on two-dimensional layered crystals, including graphene, semiconductors of the transition metal dichalcogenide family such as molybdenum disulphide (MoS2) and tungsten diselenide (WSe2), as well as other emerging two-dimensional semiconductors such as atomically thin black phosphorus, is significantly affected by the electrical contacts that connect these materials with external circuitry. Here, we present a comprehensive treatment of the physics of such interfaces at the contact region and discuss recent progress towards realizing optimal contacts for two-dimensional materials. We also discuss the requirements that must be fulfilled to realize efficient spin injection in transition metal dichalcogenides.

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Electrical contacts to two-dimensional semiconductors

Electrical metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are found to be the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (Rc). Until now, Fermi level pinning of monolayer TMDCs has been reported only theoretically, although that of bulk TMDCs has been reported experimentally. Here, we report the experimental study on Fermi level pinning of monolayer MoS2 and MoTe2 by interpreting the thermionic emission results. We also quantitatively compared our results with the theoretical simulation results of the monolayer structure as well as the experimental results of the bulk structure. We measured the pinning factor S to be 0.11 and −0.07 for monolayer MoS2 and MoTe2, respectively, suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theoretical prediction, and interestingly similar pinning energy levels between monolayer and bulk Mo...

Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides.

MoS2 and related metal dichalcogenides (MoSe2, WS2, WSe2) are layered two-dimensional materials that are promising for nanoelectronics and spintronics. For instance, large spin–orbit coupling and spin splitting in the valence band of single layer (SL) MoS2 could lead to enhanced spin lifetimes and large spin Hall angles. Understanding the nature of the contacts is a critical first step for realizing spin injection and spin transport in MoS2. Here, we have investigated Co contacts to SL MoS2 and find that the Schottky barrier height can be significantly decreased with the addition of a thin oxide barrier (MgO). Further, we show that the barrier height can be reduced to zero by tuning the carrier density with back gate. Therefore, the MgO could simultaneously provide a tunnel barrier to alleviate conductance mismatch while minimizing carrier depletion near the contacts. Such control over the barrier height should allow for careful engineering of the contacts to realize spin injection in these materials.

Control of Schottky Barriers in Single Layer MoS2 Transistors with Ferromagnetic Contacts

We have performed a detailed study of the intrinsic electrical conduction process in individual monolayers of chemically reduced graphene oxide down to a temperature of 2 K. The observed conductance can be consistently interpreted in the framework of two-dimensional variable-range hopping in parallel with electric-field-driven tunneling. The latter mechanism is found to dominate the electrical transport at very low temperatures and high electric fields. Our results are consistent with a model of highly conducting graphene regions interspersed with disordered regions, across which charge carrier hopping and tunneling are promoted by strong local electric fields.

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Electrical Conduction Mechanism in Chemically Derived Graphene Monolayers

We report a change in the semimetallic nature of single-layer graphene after exposure to oxygen plasma. The resulting transition from semimetallic to semiconducting behavior appears to depend on the duration of the exposure to the plasma treatment. The observation is confirmed by electrical, photoluminescence and Raman spectroscopy measurements. We explain the opening of a bandgap in graphene in terms of functionalization of its pristine lattice with oxygen atoms. Ab initio calculations show more details about the interaction between carbon and oxygen atoms and the consequences on the optoelectronic properties, that is, on the extent of the bandgap opening upon increased functionalisation density.

https://iopscience.iop.org/article/10.1088/0957-4484/21/43/435203/pdf

Bandgap opening in oxygen plasma-treated graphene.

We discuss mechanisms responsible for the reduction of electron thermionic emission current from a Schottky contact to a modulation doped semiconductor compared to a bulk semiconductor. The effects discussed include metal to semiconductor barrier height enhancement due to proposed electron–electron cloud interaction, confined potential of the reduced dimensional systems, and the reduced dimensional nature of the density of states in the semiconductor. These effects describe the observed lowering of the dark current, and hence noise, of a modulation doped heterojunction based photodetector compared to a conventional bulk device.

Effects of electron confinement on thermionic emission current in a modulation doped heterostructure

It has been shown that, in a three-dimensional (3D) to two-dimensional (2D) contact system, the quantized nature of the energy of the 2D system imposes important changes on thermionic emission of carriers from a 3D metal to two-dimensional electron gas (2DEG). Interestingly, in actual devices, barrier heights higher than what is theorized based on the first confined state are measured. In this paper, we introduce an additional mechanism that explains barrier height enhancement in 3D-2D contacts, which is due to the repulsive Coulombic force that is exerted by the 2DEG on the thermionically emitted electrons. An analytical derivation of the barrier height due to this effect is given and total thermionic emission current is derived. These results are particularly important for design and understanding of device behavior for low-noise photodetectors in front end optical receivers. The electron cloud model presented for the reservoir of mobile charges that are free to move in response to charged particle or electromagnetic waves implies that any means of interaction that disturbs the equilibrium of the electron cloud have strong signature at the contact, as well as the temporal response of the induced disturbance. This can be effective for low-power-detection applications.

Barrier enhancement mechanisms in heterodimensional contacts and their effect on current transport

The electron cloud that is formed in the narrow gap material in a modulation-doped heterostructure affects the Schottky contact made to the wide gap material. It also influences absorption and collection of the optically generated carriers. Photocurrent spectra, current–voltage, and current–temperature measurements show that the increase in electron cloud density decreases dark current flow while increasing photoresponsivity. We propose that the Coulombic interaction between the confined electron cloud and the emitted electrons from metal to the wide gap material increases the barrier height. The electric field in the direction of growth due to modulation doping accounts for the increase in photoelectron collection efficiency. Implementation of this effect increases efficiency of photodetectors while, simultaneously, reducing the noise due to dark current.

Electron cloud effect on current injection across a Schottky contact

A photodetector in which Schottky metal laterally contacts the molecular beam epitaxy grown heterointerface of intermediate-temperature GaAs and Al0.24Ga0.76As is reported. The device processed on 400 °C shows very low dark current, less than 25 fA/μm2 (0.5 pA/μm), with a high dc responsivity of about 10 A/W at low optical power levels. The device is process compatible with high electron mobility transistor technology.

Intermediate temperature grown GaAs/AlGaAs photodetector with low dark current and high sensitivity

The structures of systems of different dimensions and their interface can be called heterodimensional devices. In this paper we discuss a family of photodetector devices that are based on embedding three dimensional (3D) to two dimensional (2D) contacts by employing modulation doping of a layered heterostructure and contacting the resultant two dimensional electron gas by Schottky metal. The process of current transport between 3D and 2D is analyzed showing barrier enhancement mechanisms due to electron confinement. Optical spectral behavior is also discussed showing the effect of quantized energy levels as well as the electric field present in these multilayer structures.

A. Anwar

Papers

Effects of electron confinement on thermionic emission current in a modulation doped heterostructure

Barrier enhancement mechanisms in heterodimensional contacts and their effect on current transport

Electron cloud effect on current injection across a Schottky contact

Intermediate temperature grown GaAs/AlGaAs photodetector with low dark current and high sensitivity

Heterodimensional contacts and optical detectors