Transition from Schottky to Ohmic contacts in Janus MoSSe/germanene heterostructures

Computational Insights into Schottky Barrier Heights: Graphene and Borophene Interfaces with H- and H́-XSi2N4 (X = Mo, W) Monolayers

The two-dimensional (2D) semiconducting family of XSi2N4 (X = Mo and W), an emergent class of air-stable monolayers, has recently gained attention due to its distinctive structural, mechanical, transport, and optical properties. However, the electrical contact between XSi2N4 and metals remains a mystery. In this study, we inspect the electronic and transport properties, specifically the Schottky barrier height (SBH) and tunneling probability, of XSi2N4-based van der Waals contacts by means of first-principles calculations. Our findings reveal that the electrical contacts of XSi2N4 with metals can serve as the foundation for nanoelectronic devices with ultralow SBHs. We further analyzed the tunneling probability of different metal contacts with XSi2N4. We found that the H-phase XSi2N4/metal contact shows superior tunneling probability compared to that of H́-based metal contacts. Our results suggest that heterostructures at interfaces can potentially enable efficient tunneling barrier modulation in metal contacts, particularly in the case of MoSi2N4/borophene compared to MoSi2N4/graphene and WSi2N4/graphene in transport-efficient electronic devices. Among the studied heterostructures, tunneling efficiency is highest at the H and H́-MoSi2N4/borophene interfaces, with barrier heights of 2.1 and 1.52 eV, respectively, and barrier widths of 1.04 and 0.8 Å. Furthermore, the tunneling probability for these interfaces was identified to be 21.3 and 36.4%, indicating a good efficiency of carrier injection. Thus, our study highlights the potential of MoSi2N4/borophene contact in designing power-efficient Ohmic devices.

One-step sputtering of MoSSe metastable phase as thin film and predicted thermodynamic stability by computational methods

We present the fabrication of a MoS2-xSex thin film from a co-sputtering process using MoS2 and MoSe2 commercial targets with 99.9% purity. The sputtering of the MoS2 and MoSe2 was carried out using a straight and low-cost magnetron radio frequency sputtering recipe to achieve a MoS2-xSex phase with x = 1 and sharp interface formation as confirmed by Raman spectroscopy, time-of-flight secondary ion mass spectroscopy, and cross-sectional scanning electron microscopy. The sulfur and selenium atoms prefer to distribute randomly at the octahedral geometry of molybdenum inside the MoS2-xSex thin film, indicated by a blue shift in the A1g and E1g vibrational modes at 355 cm-1 and 255 cm-1, respectively. This work is complemented by computing the thermodynamic stability of a MoS2-xSex phase whereby density functional theory up to a maximum selenium concentration of 33.33 at.% in both a Janus-like and random distribution. Although the Janus-like and the random structures are in the same metastable state, the Janus-like structure is hindered by an energy barrier below selenium concentrations of 8 at.%. This research highlights the potential of transition metal dichalcogenides in mixed phases and the need for further exploration employing low-energy, large-scale methods to improve the materials' fabrication and target latent applications of such structures.

Transition from Schottky to Ohmic contacts in 2D Ge/GaAs heterostructures with high tunneling probability

The metal-semiconductor (M-S) contact is usually an Ohmic contact or a Schottky contact, which greatly affects the electronic properties of devices, and it remains a huge challenge to realize a low-resistance Ohmic contact in a metal-semiconductor junction (MSJ). Herein, we systematically studied the band structures, electrostatic potential, charge transfer, Schottky barrier height of carriers, effective carrier masses, and tunneling probability of carriers of a germanene (Ge)/GaAs MSJ. The transition from the Schottky to the Ohmic contact can be caused by applying certain biaxial strains or electric fields, which weakens the Fermi level pinning (FLP) effect and reduces contact resistance. Meanwhile, the electron injection efficiency of Ge/(GaAs)As MSJ (PTB > 27%) is far superior to that of other two-dimensional (2D) vdW MSJs. This work indicates that Ge/GaAs heterostructures are the most compatible for applying high-effective 2D electronic nanodevices under controllable conditions.

First-principles study of controllable contact types in Janus MoSH/GaN van der Waals heterostructure

The search for contact materials with low contact resistance and tunable Schottky barrier (SB) height of two-dimensional (2D) materials is important for improving the electronic performance. Inspired by the recently synthesized metallic Janus MoSH, this study employs first-principles calculations to investigate the electronic structure, mechanical properties, and interface characteristics of Janus MoSH/GaN and MoHS/GaN van der Waals (vdW) heterostructures. We find that both heterostructures exhibit isotropic mechanical properties and form p-type Schottky barrier contacts (p-ShC) and the SB height of MoHS/GaN is smaller than that of the MoSH/GaN heterostructure. The variation in SB height and contact type under biaxial strain and electric field is also studied for both vdW heterostructures, respectively. Compared to the MoSH/GaN heterostructure, the MoHS/GaN heterostructure can transition to Ohmic contact (OhC) under biaxial strain and electric field, making the S-face contact of MoSH with GaN a more effective contact approach. These findings could provide a new pathway for the design of controllable Schottky nanodevices and high-performance electronic devices on GaN-based vdW heterostructures.

Transition from Schottky to ohmic contacts in the C31 and MoS2 van der Waals heterostructure

The utilization of conventional metal contacts has restricted the industrial implementation of two-dimensional channel materials. To address this issue, we conducted first-principles calculations to investigate the interface properties of C₃₁ and MoS₂ contacts. An ohmic contact and a low van der Waals barrier were found in the C₃₁/MoS₂ heterostructure. Our findings provide a promising new contact metal material for two-dimensional nanodevices based on MoS₂.

Ohmic contacts of the two-dimensional Ca2N/MoS2 donor-acceptor heterostructure

In the current stage, conventional silicon-based devices are suffering from the scaling limits and the Fermi level pinning effect. Therefore, looking for low-resistance metal contacts for semiconductors has become one of the most important topics, and two-dimensional (2D) metal/semiconductor contacts turn out to be highly interesting. Alternatively, the Schottky barrier and the tunneling barrier impede their practical applications. In this work, we propose a new strategy for reducing the contact potential barrier by constructing a donor-acceptor heterostructure, that is, Ca₂N/MoS₂ with Ca₂N being a 2D electrene material with a significantly small work function and a rather high carrier concentration. The quasi-bond interaction of the heterostructure avoids the formation of a Fermi level pinning effect and gives rise to high tunneling probability. An excellent n-type Ohmic contact form between Ca₂N and MoS₂ monolayers, with a 100% tunneling probability and a perfect linear I-V curve, and large lateral band bending also demonstrates the good performance of the contact. We verify a fascinating phenomenon that Ca₂N can trigger the phase transition of MoS₂ from 2H to 1T'. In addition, we also identify that Ohmic contacts can be formed between Ca₂N and other 2D transition metal dichalcogenides (TMDCs), including WS₂, MoSe₂, WSe₂, and MoTe₂.

The Role of Permanent and Induced Electrostatic Dipole Moments for Schottky Barriers in Janus MXY/Graphene Heterostructures: a First Principles Study

The Schottky barrier height (ESBH) is a crucial factor in determining the transport properties of semiconductor materials and it directly regulates the carrier mobility in opto-electronics devices. In principle, van...

Two-Dimensional Sb/InS van der Waals Heterostructure for Electronic and Optical Related Applications

Stable configurations with excellent optical adsorption are crucial for the photovoltaics or photocatalysis. Two-dimensional materials with intrinsic electric-field have been proposed suitable for electric and optical device. Here, we have...

Status and prospects of Ohmic contacts on two-dimensional semiconductors

In recent years, two-dimensional materials have received more and more attention in the development of semiconductor devices, and their practical applications in optoelectronic devices have also developed rapidly. However, there are still some factors that limit the performance of two-dimensional semiconductor material devices, and one of the most important is Ohmic contact. Here, we elaborate on a variety of approaches to achieve Ohmic contacts on two-dimensional materials and reveal their physical mechanisms. For the work function mismatch problem, we summarize the comparison of barrier heights between different metals and 2D semiconductors. We also examine different methods to solve the problem of Fermi level pinning. For the novel 2D metal-semiconductor contact methods, we analyse their effects on reducing contact resistance from two different perspectives: homojunction and heterojunction. Finally, the challenges of 2D semiconductors in achieving Ohmic contacts are outlined.

First-principles study on the electronic structures and contact properties of graphene/XC (X = P, As, Sb, and Bi) van der Waals heterostructures

The electrical contacts at the van der Waals (vdW) interface between two-dimensional (2D) semiconductors and metal electrodes could dramatically affect the device performance. Herein, we construct a series of graphene (Gr)/XC (X = P, As, Sb, and Bi) vdW heterostructures, in which XC monolayers have aroused considerable attention recently as an emerging class of 2D semiconductors. The electronic structures and contact properties of Gr/XC vdW heterostructures are investigated systematically using first-principles calculations. The band structures indicate that both Gr/PC and Gr/AsC heterostructures form n-type Schottky contacts with Schottky barrier heights (SBHs) of 0.01 eV and 0.43 eV, respectively, while both Gr/SbC and Gr/BiC heterostructures preferably form Ohmic contacts. The different X atoms result in different work functions, electron flows, charge distributions and orientations of the dipole moment in Gr/XC heterostructures. Moreover, the tunneling probabilities increase with the increasing atom radius of X from P to Bi, indicating the most improved current and smaller contact resistance at the interfaces of Gr/BiC compared to Gr/PC, Gr/AsC and Gr/SbC heterostructures. Our work could provide meaningful information for designing high-performance nanoelectronic devices based on Gr/XC heterostructures.

Modulation of the electronic band structure of silicene by polar two-dimensional substrates

Using the density functional theory (DFT) calculations, we find that  Janus group-III chalcogenide monolayers can serve as a suitable substrate for silicene, and the Dirac electron band properties of silicene are also fully preserved. The maximum opened band gap can reach 179 meV at the Dirac point due to the interaction of silicene and the polar two-dimensional (2D) substrate. In addition, the electronic band structure of the heterostructure can be modulated by applying an electric field where its predicted band gap increases or decreases according to the direction of the applied external electric field. Furthermore, an insight into the electron structures can be understood by analyzing the electron energy-loss (EEL) spectra. From these results, we also predict that heterostructures with polar 2D substrates have broad application prospects in multi-functional devices. Besides, Janus group-III chalcogenide monolayers can be used as good substrates for growing silicene and the modulation of the electronic structure can also be applied to nanodevices and optoelectronic devices.