Philip M. Campbell

Controlled Doping of Large‐Area Trilayer MoS2 with Molecular Reductants and Oxidants

Highly uniform large-area MoS2 is chemically doped using molecular reductants and oxidants. Electrical measurements, photoemission, and Raman spectroscopy are used to study the doping effect and to understand the underlying mechanism. Strong work-function changes of up to ±1 eV can be achieved, with contributions from state filling and surface dipoles. This results in high doping densities of up to ca. 8 × 10(12) cm(-2) .

Flexible MoS2 Field-Effect Transistors for Gate-Tunable Piezoresistive Strain Sensors

Atomically thin molybdenum disulfide (MoS2) is a promising two-dimensional semiconductor for high-performance flexible electronics, sensors, transducers, and energy conversion. Here, piezoresistive strain sensing with flexible MoS2 field-effect transistors (FETs) made from highly uniform large-area films is demonstrated. The origin of the piezoresistivity in MoS2 is the strain-induced band gap change, which is confirmed by optical reflection spectroscopy. In addition, the sensitivity to strain can be tuned by more than 1 order of magnitude by adjusting the Fermi level via gate biasing.

Enhanced Resonant Tunneling in Symmetric 2D Semiconductor Vertical Heterostructure Transistors.

Tunneling transistors with negative differential resistance have widespread appeal for both digital and analog electronics. However, most attempts to demonstrate resonant tunneling devices, including graphene-insulator-graphene structures, have resulted in low peak-to-valley ratios, limiting their application. We theoretically demonstrate that vertical heterostructures consisting of two identical monolayer 2D transition-metal dichalcogenide semiconductor electrodes and a hexagonal boron nitride barrier result in a peak-to-valley ratio several orders of magnitude higher than the best that can be achieved using graphene electrodes. The peak-to-valley ratio is large even at coherence lengths on the order of a few nanometers, making these devices appealing for nanoscale electronics.

Band structure effects on resonant tunneling in III-V quantum wells versus two-dimensional vertical heterostructures

Since the invention of the Esaki diode, resonant tunneling devices have been of interest for applications including multi-valued logic and communication systems. These devices are characterized by the presence of negative differential resistance in the current-voltage characteristic, resulting from lateral momentum conservation during the tunneling process. While a large amount of research has focused on III-V material systems, such as the GaAs/AlGaAs system, for resonant tunneling devices, poor device performance and device-to-device variability have limited widespread adoption. Recently, the symmetric field-effect transistor (symFET) was proposed as a resonant tunneling device incorporating symmetric 2-D materials, such as transition metal dichalcogenides (TMDs), separated by an interlayer barrier, such as hexagonal boron-nitride. The achievable peak-to-valley ratio for TMD symFETs has been predicted to be higher than has been observed for III-V resonant tunneling devices. This work examines the effect ...

Graphene-Molybdenum Disulfide-Graphene Tunneling Junctions with Large-Area Synthesized Materials

Tunneling devices based on vertical heterostructures of graphene and other 2D materials can overcome the low on-off ratios typically observed in planar graphene field-effect transistors. This study addresses the impact of processing conditions on two-dimensional materials in a fully integrated heterostructure device fabrication process. In this paper, graphene-molybdenum disulfide-graphene tunneling heterostructures were fabricated using only large-area synthesized materials, unlike previous studies that used small exfoliated flakes. The MoS2 tunneling barrier is either synthesized on a sacrificial substrate and transferred to the bottom-layer graphene or synthesized directly on CVD graphene. The presence of graphene was shown to have no impact on the quality of the grown MoS2. The thickness uniformity of MoS2 grown on graphene and SiO2 was found to be 1.8 ± 0.22 nm. XPS and Raman spectroscopy are used to show how the MoS2 synthesis process introduces defects into the graphene structure by incorporating sulfur into the graphene. The incorporation of sulfur was shown to be greatly reduced in the absence of molybdenum suggesting molybdenum acts as a catalyst for sulfur incorporation. Tunneling simulations based on the Bardeen transfer Hamiltonian were performed and compared to the experimental tunneling results. The simulations show the use of MoS2 as a tunneling barrier suppresses contributions to the tunneling current from the conduction band. This is a result of the observed reduction of electron conduction within the graphene sheets.

Material Constraints and Scaling of 2-D Vertical Heterostructure Interlayer Tunnel Field-Effect Transistors

Aggressive scaling of logic devices is quickly approaching the physical limitations of conventional CMOS devices, resulting in the need for novel device architectures. One proposed device is the 2-D interlayer tunnel field-effect transistor (ITFET), which relies on tunneling within a vertical heterostructure of 2-D materials. Steep-slope operation of the ITFET relies on proper band alignment for tunneling between the conduction band of one 2-D electrode and the valence band of the other 2-D electrode. Because of the step-like nature of the density of states of transition metal dichalcogenides (TMDs), the subthreshold slope is infinite for ideal materials. Previous theoretical predictions suggested the possibility for steep-slope operation in TMD-based ITFETs, but did not consider the complete electronic and physical structure of the TMD electrodes. This paper explores the implications of the physical structure of materials, such as lattice constant, on ITFET performance. Further, several design parameters are explored within the MoS2–WSe2 system to develop general design rules for ITFETs based on 2-D materials. Benchmarking is performed of the MoS2–WSe2 ITFET to suggest the potential for both lower power and higher performance than conventional CMOS devices.

Strain relaxation via formation of cracks in compositionally modulated two-dimensional semiconductor alloys

Composition modulation of two-dimensional transition-metal dichalcogenides (TMDs) has introduced an enticing prospect for the synthesis of Van der Waals alloys and lateral heterostructures with tunable optoelectronic properties. Phenomenologically, the optoelectronic properties of alloys are entangled to a strain that is intrinsic to synthesis processes. Here, we report an unprecedented biaxial strain that stems from the composition modulation of monolayer TMD alloys (e.g., MoS2xSe2(1 - x)) and inflicts fracture on the crystals. We find that the starting crystal (MoSe2) fails to adjust its lattice constant as the atoms of the host crystal (selenium) are replaced by foreign atoms (sulfur) during the alloying process. Thus, the resulting alloy forms a stretched lattice and experiences a large biaxial tensile strain. Our experiments show that the biaxial strain relaxes via formation of cracks in interior crystal domains or through less constraint bounds at the edge of the monolayer alloys. Griffith’s criterion suggests that defects combined with a sulfur-rich environment have the potential to significantly reduce the critical strain at which cracking occurs. Our calculations demonstrate a substantial reduction in fracture-inducing critical strain from 11% (in standard TMD crystals) to a range below 4% in as-synthesized alloys.2D alloys: intrinsic strain in MoS 2x Se 2(1-x) ternary crystalsComposition modulation synthesis of ternary alloys of atomically thin transition metal dichalcogenides gives rise to intrinsic biaxial strain. A team led by Ali Adibi at Georgia Institute of Technology reported the onset of a substantial biaxial strain in monolayer MoS2xSe2(1-x) that is intrinsically linked to the two-step composition modulation synthesis used to grow the ternary alloy. As the S atoms replace the Se atoms of the starting MoSe2 host crystal, the resulting alloy forms a stretched lattice and develops a large biaxial tensile strain. Morphological and spectroscopic characterisations suggest that such strain results in the onset of fracture in the crystal, and further relaxes via formation of cracks within the crystal domains. Theoretical modelling indicates that pre-existing cracks give a substantial contribution in weakening the strength of the synthesized van der Waals alloy.

Alloying-induced biaxial strain in ternary alloys of transition-metal dichalcogenides (TMDs) (Conference Presentation)

Alloying has served as a powerful means for tuning the non-vanishing optical bandgap of two-dimensional (2D) transition-metal dichalcogenides (TMDs), a family of 2D materials with optoelectronic properties covering a wide spectral window ranging from visible to near-infrared. In addition to the bandgap engineering, ‘spatial’ modulation of the composition ratio (i.e., x) in a ternary TMD alloy (e.g., MX2xX2(1-x)’; M: transition metal, X, X’: chalcogens) enables formation of lateral heterostructures with complex functionalities within the plane of 2D materials, a new asset that expands the realm of applications in which 2D materials can be incorporated. Despite several demonstrations of alloying in 2D TMDs, the phenomenologically important issue of strain development and its effect on the optical and structural properties of 2D TMD alloys is still missing. Here, we show that alloying processes induce a biaxial tensile strain that acts on the lattice of 2D TMD alloys and affect their optical properties. In addition, we show that such strain inflicts sever fracture of the alloys via formation of sub-micron-sized cracks. Our experimental characterization combined with detailed theoretical modeling suggest the important role of the Van der Waals interaction between the 2D material and the substrate in formation of the alloying-induced strain. Furthermore, we demonstrate the critical role of crystal defects in cracking of the TMD alloys, which further emphasizes the importance of high quality synthesis of 2D TMD crystals for practical applications.

Piezoresistive strain sensing with flexible MoS 2 field-effect transistors

We demonstrate piezoresistive strain sensors based on flexible MoS2 field-effect transistors made from a highly uniform large-area trilayer film. The origin of the piezoresistive effect in MoS2 is explained to be a strain-induced band gap change, as confirmed by optical spectroscopy. The results are in good agreement with recently reported simulations and spectroscopic studies on strained exfoliated MoS2. In addition, the strain sensitivity can be tuned by over one order of magnitude via modulating the MoS2 Fermi level with an applied gate voltage. The gate-tunable gauge factors can be as high as -40, comparable to polycrystalline silicon, but for a much thinner active layer (~2 nm). For practical sensing applications, the gate-tunable piezoresistivity is a useful property of transistor-based devices, because the relative sensitivity to strain can be adjusted by changing the gate voltage.

Dual-gated field-effect transistors made from wafer-scale synthetic few-layer molybdenum disulfide

Molybdenum disulfide (MoS2) has recently received significant attention because of its interesting thickness-dependent properties and its potential as a semiconducting substitute to graphene [1,2]. Most of the studies so far have focused on small (<; 100 microns) exfoliated MoS2 flakes [1-3]. For manufacturable electronics, it is essential to have large-area material that is compatible with standard fabrication processes for high yield and reproducibility. Though significant progress has been achieved using chemical vapor deposition (CVD) [4,5], the formation of high-quality wafer-scale MoS2 of controlled thickness is still a challenge.