Effect of Carrier Localization on Electrical Transport and Noise at Individual Grain Boundaries in Monolayer MoS2

The Electrical Behaviors of Grain Boundaries in Polycrystalline Optoelectronic Materials

Polycrystalline optoelectronic materials are widely used for photoelectric signal conversion and energy harvesting and play an irreplaceable role in the semiconductor field. As an important factor in determining the optoelectronic properties of polycrystalline materials, grain boundaries (GBs) are the focus of research. Particular emphases are placed on the generation and height of GB barriers, how carriers move at GBs, whether GBs act as carrier transport channels or recombination sites, and how to change the device performance by altering the electrical behaviors of GBs. This review introduces the evolution of GB theory and experimental observation history, classifies GB electrical behaviors from the perspective of carrier dynamics, and summarizes carrier transport state under external conditions such as bias and illumination and the related band bending. Then the carrier scattering at GBs and the electrical differences between GBs and twin boundaries are discussed. Last, the review describes how the electrical behaviors of GBs can be influenced and modified by treatments such as passivation or by consciously adjusting the distribution of grain boundary elements. By studying the carrier dynamics and the relevant electrical behaviors of GBs in polycrystalline materials, researchers can develop optoelectronics with higher performance.

Physical vapor deposition-free scalable high-efficiency electrical contacts to MoS2

Fermi-level pinning caused by the kinetic damage during metallization has been recognized as one of the major reasons for the non-ideal behavior of electrical contacts, forbidding reaching the Schottky-Mott limit. In this manuscript, we present a scalable technique wherein Indium, a low-work-function metal, is diffused to contact a few-layered MoS2flake. The technique exploits a smooth outflow of Indium over gold electrodes to make edge contacts to pre-transferred MoS2flakes. We compare the performance of three pairs of contacts made onto the same MoS2flake, the bottom-gold, top-gold, and Indium contacts, and find that the Indium contacts are superior to other contacts. The Indium contacts maintain linearI-Vcharacteristics down to cryogenic temperatures with an extracted Schottky barrier height of ∼2.1 meV. First-principle calculations show the induced in-gap states close to the Fermi level, and the damage-free contact interface could be the reason for the nearly Ohmic behavior of the Indium/MoS2interface.

Benchmarking Noise and Dephasing in Emerging Electrical Materials for Quantum Technologies

As quantum technologies develop, a specific class of electrically conducting materials is rapidly gaining interest because they not only form the core quantum-enabled elements in superconducting qubits, semiconductor nanostructures, or sensing devices, but also the peripheral circuitry. The phase coherence of the electronic wave function in these emerging materials will be crucial when incorporated in the quantum architecture. The loss of phase memory, or dephasing, occurs when a quantum system interacts with the fluctuations in the local electromagnetic environment, which manifests in "noise" in the electrical conductivity. Hence, characterizing these materials and devices therefrom, for quantum applications, requires evaluation of both dephasing and noise, although there are very few materials where these properties are investigated simultaneously. Here, the available data on magnetotransport and low-frequency fluctuations in electrical conductivity are reviewed to benchmark the dephasing and noise. The focus is on new materials that are of direct interest to quantum technologies. The physical processes causing dephasing and noise in these systems are elaborated, the impact of both intrinsic and extrinsic parameters from materials synthesis and devices realization are evaluated, and it is hoped that a clearer pathway to design and characterize both material and devices for quantum applications is thus provided.

Rapid Growth of Monolayer MoSe2 Films for Large-Area Electronics

The large-scale growth of semiconducting thin films on insulating substrates enables batch fabrication of atomically thin electronic and optoelectronic devices and circuits without film transfer. Here an efficient method to achieve rapid growth of large-area monolayer MoSe₂ films based on spin coating of Mo precursor and assisted by NaCl is reported. Uniform monolayer MoSe₂ films up to a few inches in size are obtained within a short growth time of 5 min. The as-grown monolayer MoSe₂ films are of high quality with large grain size (up to 120 μm). Arrays of field-effect transistors are fabricated from the MoSe₂ films through a photolithographic process; the devices exhibit high carrier mobility of ≈27.6 cm² V^-1 s^-1 and on/off ratios of ≈10⁵. The findings provide insight into the batch production of uniform thin transition metal dichalcogenide films and promote their large-scale applications.

Electronic properties of Janus MXY/graphene (M = Mo, W; X ≠ Y = S, Se) van der Waals structures: a first-principles study

Based on the first-principles calculations, we studied the intrinsic dipole moment and electronic properties of Janus MXY (M = Mo, W; X ≠ Y = S, Se) monolayers, bilayers and heterostructures with graphene, and the possibility of MXY encapsulating graphene. The results show that Janus MXY monolayer has an intrinsic dipole moment and a direct band gap. However, for MXY bilayers strong interlayer coupling will cause direct to indirect band gap transition, and the existence of the dipole moment leads to a significantly large interlayer band offset, being the driving force for the formation of interlayer excitons. In MXY/graphene heterostructures, changes in the direction of intrinsic dipole moment will cause a change in Schottky barrier height and even the transition between p- and n-type Schottky contacts. Independent of the interface atomic layer of Janus MXY, on one hand, the Dirac cone still exists in graphene, proving that MXY is an ideal coating material. On the other hand, the type-II band alignment will disappear as the intrinsic dipole moment disappears, confirming that the intrinsic dipole moment plays a vital role in the formation of a large band offset. Our results provide guidance for the study of interlayer excitonic states, the experimental construction of atomically thin p-n junctions and the encapsulation of graphene.

Optimal architecture for ultralow noise graphene transistors at room temperature

The fundamental origin of low-frequency noise in graphene field effect transistors (GFETs) has been widely explored but a generic engineering strategy towards low noise GFETs is lacking. Here, we systematically study and eliminate dominant sources of electrical noise to achieve ultralow noise GFETs. We find that in edge contacted, high-quality hexagonal boron nitride (hBN) encapsulated GFETs, the inclusion of a graphite bottom gate and long (⪆1.2 μm) channel-contact distance significantly reduces noise as compared to global Si/SiO₂ gated devices. From the scaling of the remaining noise with channel area and its temperature dependence, we attribute this to the traps in hBN. To further screen the charge traps in hBN, we place few layers of MoS₂ between graphene and hBN, and demonstrate that the noise is as low as ∼5.2 × 10^-9μm² Hz^-1 (corresponding to minimum Hooge parameter ∼5.2 × 10^-6) in GFETs at room temperature, which is an order of magnitude lower than the earlier reported values.

Enhanced Piezoelectric Effect Derived from Grain Boundary in MoS2 Monolayers

Recent discovery of piezoelectricity that existed in two-dimensional (2D) layered materials represents a key milestone for flexible electronics and miniaturized and wearable devices. However, so far the reported piezoelectricity in these 2D layered materials is too weak to be used for any practical applications. In this work, we discovered that grain boundaries (GBs) in monolayer MoS₂ can significantly enhance its piezoelectric property. The output power of piezoelectric devices made of the butterfly-shaped monolayer MoS₂ was improved about 50% by the GB-induced piezoelectric effect. The enhanced piezoelectricity is attributed to the additional piezoelectric effect induced by the existence of deformable GBs which can promote polarization and generates spontaneous polarization with different piezoelectric coefficients along various directions. We further made a flexible piezoelectric device based on the 2D MoS₂ with the GBs and demonstrated its potential application in self-powered precision sensors for in situ detecting pressure changes in human blood for health monitoring.

Atomically Sharp Dual Grain Boundaries in 2D WS2 Bilayers

It is shown that tilt grain boundaries (GBs) in bilayer 2D crystals of the transition metal dichalcogenide WS₂ can be atomically sharp, where top and bottom layer GBs are located within sub-nanometer distances of each other. This expands the current knowledge of GBs in 2D bilayer crystals, beyond the established large overlapping GB types typically formed in chemical vapor deposition growth, to now include atomically sharp dual bilayer GBs. By using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging, different atomic structures in the dual GBs are distinguished considering bilayers with a 3R (AB stacking)/2H (AA' stacking) interface as well as bilayers with 2H/2H boundaries. An in situ heating holder is used in ADF-STEM and the GBs are stable to at least 800 °C, with negligible thermally induced reconstructions observed. Normal dislocation cores are seen in one WS₂ layer, but the second WS₂ layer has different dislocation structures not seen in freestanding monolayers, which have metal-rich clusters to accommodate the stacking mismatch of the 2H:3R interface. These results reveal the competition between maintaining van der Waals bilayer stacking uniformity and dislocation cores required to stitch tilted bilayer GBs together.

Suppressed Carrier Recombination in Janus MoSSe Bilayer Stacks: A Time-Domain Ab Initio Study

Janus transition metal dichalcogenides (TMDs) have recently emerged as a new class of two-dimensional materials with a vertical dipole moment. Here, using time-domain ab initio simulations, we show that electron-hole recombination can be substantially suppressed via different stacking orientations of bilayer MoSSe. Despite having a larger net dipole moment, a S-Se/S-Se oriented MoSSe bilayer has a shorter carrier lifetime due to strong nonadiabatic coupling and a small band gap. The electron-hole recombination is coupled to the interlayer out-of-plane motion. In contrast, the opposite vertical dipoles weaken interlayer interactions in symmetric oriented MoSSe bilayers. Consequently, initial and final states are localized within different layers, and this significantly suppresses carrier recombination, resulting in an order of magnitude longer excited carrier lifetime in Se-S/S-Se oriented MoSSe bilayers. Our simulations provide theoretical insights into the carrier dynamics and suggest a way to enhance the carrier lifetime in Janus TMDs for efficient energy harvesting.

Phonon-Suppressed Auger Scattering of Charge Carriers in Defective Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides (TMDs) draw strong interest in materials science, with applications in optoelectronics and many other fields. Good performance requires high carrier concentrations and long lifetimes. However, high concentrations accelerate energy exchange between charged particles by Auger-type processes, especially in TMDs where many-body interactions are strong, thus facilitating carrier trapping. We report time-resolved optical pump-THz probe measurements of carrier lifetimes as a function of carrier density. Surprisingly, the lifetime reduction with increased density is very weak. It decreases only by 20% when we increase the pump fluence 100 times. This unexpected feature of the Auger process is rationalized by our time-domain ab initio simulations. The simulations show that phonon-driven trapping competes successfully with the Auger process. On the one hand, trap states are relatively close to band edges, and phonons accommodate efficiently the electronic energy during the trapping. On the other hand, trap states localize around defects, and the overlap of trapped and free carriers is small, decreasing carrier-carrier interactions. At low carrier densities, phonons provide the main charge trapping mechanism, decreasing carrier lifetimes compared to defect-free samples. At high carrier densities, phonons suppress Auger processes and lower the dependence of the trapping rate on carrier density. Our results provide theoretical insights into the diverse roles played by phonons and Auger processes in TMDs and generate guidelines for defect engineering to improve device performance at high carrier densities.

Electronic Doping Controlled Migration of Dislocations in Polycrystalline 2D WS2

Migration of dislocations not only determines the durability of large-scale nanoelectronic and opto-electronic devices based on polycrystalline 2D transition-metal dichalcogenides (TMDCs), but also plays an important role in enhancing the performance of novel memristors. However, a fundamental question of the migration dependence on the electronic effects, which are inevitable in practical field-effect transistors based on 2D TMDCs, and its interplay with different dislocations, remains unexplored. Here, taking WS₂ as an example, first-principle calculations are used to show that the electronic contributions arising from defect states can greatly influence the migration barriers of dislocations. The barrier height can be reduced by as much as 50%, which is mainly attributed to the change in electronic occupation and the band energy of defect levels controlled by electronic chemical potential (Fermi level). The reduced barriers in turn lead to significantly enhanced migration, and thus the plasticity. Since defect levels from dislocations locate deep inside the bandgap, the doping-induced tuning of barrier height can be achieved at relatively low doping concentration through either chemical doping or electrode gating. The effective electromechanical coupling in 2D TMDCs can provide new opportunities in material engineering for various potential applications.

Why Chemical Vapor Deposition Grown MoS2 Samples Outperform Physical Vapor Deposition Samples: Time-Domain ab Initio Analysis

Two-dimensional transition metal dichalcogenides (TMDs) have drawn strong attention due to their unique properties and diverse applications. However, TMD performance depends strongly on material quality and defect morphology. Experiments show that samples grown by chemical vapor deposition (CVD) outperform those obtained by physical vapor deposition (PVD). Experiments also show that CVD samples exhibit vacancy defects, while antisite defects are frequently observed in PVD samples. Our time-domain ab initio study demonstrates that both antisites and vacancies accelerate trapping and nonradiative recombination of charge carriers, but antisites are much more detrimental than vacancies. Antisites create deep traps for both electrons and holes, reducing energy gaps for recombination, while vacancies trap primarily holes. Antisites also perturb band-edge states, creating significant overlap with the trap states. In comparison, vacancy defects overlap much less with the band-edge states. Finally, antisites can create pairs of electron and hole traps close to the Fermi energy, allowing trapping by thermal activation from the ground state and strongly contributing to charge scattering. As a result, antisites accelerate charge recombination by more than a factor of 8, while vacancies enhance the recombination by less than a factor of 2. Our simulations demonstrate a general principle that missing atoms are significantly more benign than misplaced atoms, such as antisites and adatoms. The study rationalizes the existing experimental data, provides theoretical insights into the diverse behavior of different classes of defects, and generates guidelines for defect engineering to achieve high-performance electronic, optoelectronic, and solar-cell devices.

Van der Waals Coupled Organic Molecules with Monolayer MoS2 for Fast Response Photodetectors with Gate-Tunable Responsivity

As a direct-band-gap transition metal dichalcogenide (TMD), atomic thin MoS₂ has attracted extensive attention in photodetection, whereas the hitherto unsolved persistent photoconductance (PPC) from the ungoverned charge trapping in devices has severely hindered their employment. Herein, we demonstrate the realization of ultrafast photoresponse dynamics in monolayer MoS₂ by exploiting a charge transfer interface based on surface-assembled zinc phthalocyanine (ZnPc) molecules. The formed MoS₂/ZnPc van der Waals interface is found to favorably suppress the PPC phenomenon in MoS₂ by instantly separating photogenerated holes toward the ZnPc molecules, away from the traps in MoS₂ and the dielectric interface. The derived MoS₂ detector then exhibits significantly improved photoresponse speed by more than 3 orders (from over 20 s to less than 8 ms for the decay) and a high responsivity of 430 A/W after Al₂O₃ passivation. It is also demonstrated that the device could be further tailored to be 2-10-fold more sensitive without severely sacrificing the ultrafast response dynamics using gate modulation. The strategy presented here based on surface-assembled organic molecules may thus pave the way for realizing high-performance TMD-based photodetection with ultrafast speed and high sensitivity.

Sulfur Adatom and Vacancy Accelerate Charge Recombination in MoS2 but by Different Mechanisms: Time-Domain Ab Initio Analysis

Two-dimensional transition metal dichalcogenides (TMDs) have appeared on the horizon of materials science and solid-state physics due to their unique properties and diverse applications. TMD performance depends strongly on material quality and defect morphology. Calculations predict that sulfur adatom and vacancy are among the most energetically favorable defects in MoS₂ with vacancies frequently observed during chemical vapor deposition. By performing ab initio quantum dynamics calculations we demonstrate that both adatom and vacancy accelerate nonradiative charge carrier recombination but this happens through different mechanisms. Surprisingly, holes never significantly populate the shallow trap state created by the sulfur adatom because the trap is strongly localized and decoupled from free charges. Charge recombination bypasses the hole trap. Instead, it occurs directly between free electron and hole. The recombination is faster than in pristine MoS₂ because the adatom strongly perturbs the MoS₂ layer, breaks its symmetry, and allows more phonon modes to couple to the electronic subsystem. In contrast, the sulfur vacancy accelerates charge recombination by the traditional mechanism involving charge trapping, followed by recombination. This is because the hole and electron traps created by the vacancy are much less localized than the hole trap created by the adatom. Because the sulfur adatom accelerates charge recombination by a factor of 7.9, compared to 1.7 due to vacancy, sulfur adatoms should be strongly avoided. The generated insights highlight the diverse behavior of different types of defects, reveal unexpected features, and provide the mechanistic understanding of charge dynamics needed for tailoring TMD properties and building high-performance devices.

Analysis of noise generation and electric conduction at grain boundaries in CVD-grown MoS2 field effect transistors

Grain boundaries in a chemical vapour deposition (CVD)-grown monolayer of MoS₂ induce significant effects on the electrical and low frequency noise characteristics of the MoS₂. Here, we investigated the electrical properties and noise characteristics of MoS₂ field effect transistors (FETs) made with CVD-grown monolayer MoS₂. The electrical and noise characteristics of MoS₂ FETs were analysed and compared for the MoS₂ channel layers with and without grain boundaries. The grain boundary in the CVD-grown MoS₂ FETs can be the dominant noise source, and dependence of the extracted Hooge parameters on the gate voltage indicated the domination of the correlated number-mobility fluctuation at the grain boundaries. The percolative noise characteristics of the single grain regions of MoS₂ were concealed by the noise generated at the grain boundary. This study can enhance understanding of the electrical transport hindrance and significant noise generation by trapped charges at grain boundaries of the CVD-grown MoS₂ devices.