9 research outputs found
Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide
Understanding defect effect on carrier dynamics is essential for both
fundamental physics and potential applications of transition metal
dichalcogenides. Here, the phenomenon of oxygen impurities trapping
photo-excited carriers has been studied with ultrafast pump-probe spectroscopy.
Oxygen impurities are intentionally created in exfoliated multilayer MoSe2 with
Ar+ plasma irradiation and air exposure. After plasma treatment, the signal of
transient absorption first increases and then decreases, which is a signature
of defect capturing carriers. With larger density of oxygen defects, the
trapping effect becomes more prominent. The trapping defect densities are
estimated from the transient absorption signal, and its increasing trend in the
longer-irradiated sample agrees with the results from X-ray photoelectron
spectroscopy. First principle calculations with density functional theory
reveal that oxygen atoms occupying Mo vacancies create mid-gap defect states,
which are responsible for the carrier trapping. Our findings shed light on the
important role of oxygen defects as carrier trappers in transition metal
dichalcogenides, and facilitates defect engineering in relevant material and
device applications
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Atomistic simulations of 2D materials and van der Waalâs heterostructures for beyond-Si-CMOS devices
The unique electrical and optical properties of two-dimensional (2D) materials has spurred intense research interest towards development of nanoelectronic devices utilizing these novel materials. The atomically thin form of 2D materials translates to excellent electrostatic gate control even at nanoscale channel length dimensions, near-ideal two-dimensional carrier behavior, and perhaps conventional and novel devices applications. Monolayer transition metal dichalcogenides (TMDs) are novel, gapped 2D materials. Toward device applications, I consider MoSâ layers on dielectrics, in particular in this work, the effect of vacancies on the electronic structure. In density-functional-theory (DFT) simulations, the effects of near-interface oxygen vacancies in the oxide slab, and Mo or S vacancies in the MoSâ layer are considered. Band structures and atom-projected densities of states for each system and with differing oxide terminations were calculated, as well as those for the defect-free MoSâ-dielectrics system and for isolated dielectric layers for reference. Among the results, I find that with O-vacancies, both the HfOâ-MoSâ and the AlâOâ-MoSâ systems appear metallic due to doping of the oxide slab followed by electron transfer into the MoSâ, in manner analogous to modulation doping. The n-type doping of monolayer MoSâ by high-k oxides with O-vacancies is confirmed through collaborative experimental work in which back-gated monolayer MoSâ FETs encapsulated by oxygen deficient high-k oxides have been characterized. Van der Waalâs heterostructures allow for novel devices such as two-dimensional-to-two-dimensional tunnel devices, exemplified by interlayer tunnel FETs. These devices employ channel/tunnel-barrier/channel geometries. However, during layer-by-layer exfoliation of these multi-layer materials, rotational misalignment is the norm and may substantially affect device characteristics. In this work, by using density functional theory methods, I consider a reduction in tunneling due to weakened coupling across the rotationally misaligned interface between the channel layers and the tunnel barrier. As a prototypical system, I simulate the effects of rotational misalignment of the tunnel barrier layer between aligned channel layers in a graphene/hBN/graphene system. Rotational misalignment between the channel layers and the tunnel barrier in this van der Waalâs heterostructure can significantly reduce coupling between the channels by reducing, specifically, coupling across the interface between the channels and the tunnel barrier. This weakened coupling in graphene/hBN/graphene with hBN misalignment may be relevant to all such van der Waalâs heterostructures. TMDs are viable alternatives to graphene and hBN as channel and tunnel barrier layers, respectively, for improved performance in interlayer tunnel FET device structures. In particular, I used DFT simulations to study the bilayer-graphene/WSeâ/bilayer-graphene heterostructure as well as single and multilayer ReSâ-layer systems. Significant roadblocks to the widespread use of TMDs for nanoelectronic devices are the large contact resistance and absence of reliable doping techniques. Hence, I studied substitutional doping of, and evaluated various metal contacts to MoSâ by computing the density of states for the systems. Metal contacts that pin the Fermi level within the desired band are optimal for device applications. My simulation results suggest that monolayer (ML) MoSâ can be doped n-type or p-type by substituting for an S atom in the supercell with a group-17 Cl atom or a group-15 P atom, respectively. My simulations also suggest that Sc and Ti would serve as excellent contacts to n-type ML MoSâ due to the strong bonding and large number of states near the Fermi level. But the theoretical expectations are tempered by the material characteristics, i.e., the extremely reactive nature of Sc and the oxidation prone nature of Ti atoms. I also studied commonly used Ag and Au metal contacts to ML MoSâ, which exhibited medium strength bonding to MoSâ and an apparent pinning of the Fermi level nearer to the nominal MoSâ conduction band edgeElectrical and Computer Engineerin
Theoretical and experimental investigation of vacancy-based doping of monolayer MoS2 on oxide
Monolayer (ML) transition metal dichalcogenides are novel, gapped two-dimensional materials with unique electrical and optical properties. Toward device applications, we consider MoS2 layers on dielectrics, in particular in this work, the effect of vacancies on the electronic structure. In density-functional based simulations, we consider the effects of near-interface O vacancies in the oxide slab, and Moor S vacancies in the MoS2 layer. Band structures and atom-projected densities of states for each system and with differing oxide terminations were calculated, as well as those for the defect-free MoS2-dielectrics system and for isolated dielectric layers for reference. Among our results, we find that with O vacancies, both the Hf-terminated HfO2-MoS2 system, and the O-terminated and H-passivated Al2O3-MoS2 systems appear metallic due to doping of the oxide slab followed by electron transfer into the MoS2, in manner analogous to modulation doping. The n-type doping of ML MoS2 by high-k oxides with oxygen vacancies then is experimentally demonstrated by electrically and spectroscopically characterizing back-gated ML MoS2 field effect transistors encapsulated by oxygen deficient alumina and hafnia.clos
Coherent Interlayer Tunneling and Negative Differential Resistance with High Current Density in Double Bilayer GrapheneâWSe<sub>2</sub> Heterostructures
We
demonstrate gate-tunable resonant tunneling and negative differential
resistance between two rotationally aligned bilayer graphene sheets
separated by bilayer WSe<sub>2</sub>. We observe large interlayer
current densities of 2 and 2.5 ÎźA/Îźm<sup>2</sup> and peak-to-valley
ratios approaching 4 and 6 at room temperature and 1.5 K, respectively,
values that are comparable to epitaxially grown resonant tunneling
heterostructures. An excellent agreement between theoretical calculations
using a Lorentzian spectral function for the two-dimensional (2D)
quasiparticle states, and the experimental data indicates that the
interlayer current stems primarily from energy and in-plane momentum
conserving 2Dâ2D tunneling, with minimal contributions from
inelastic or non-momentum-conserving tunneling. We demonstrate narrow
tunneling resonances with intrinsic half-widths of 4 and 6 meV at
1.5 and 300 K, respectively
Air Stable Doping and Intrinsic Mobility Enhancement in Monolayer Molybdenum Disulfide by Amorphous Titanium Suboxide Encapsulation
To reduce Schottky-barrier-induced contact and access resistance, and the impact of charged impurity and phonon scattering on mobility in devices based on 2D transition metal dichalcogenides (TMDs), considerable effort has been put into exploring various doping techniques and dielectric engineering using high-kappa oxides, respectively. The goal of this work is to demonstrate a high-kappa dielectric that serves as an effective n-type charge transfer dopant on monolayer (ML) molybdenum disulfide (MoS2). Utilizing amorphous titanium suboxide (ATO) as the "high-kappa dopant", we achieved a contact resistance of similar to 480 Omega.mu m that is the lowest reported value for ML MoS2. An ON current as high as 240 mu A/mu m and field effect mobility as high as 83 cm(2)/V-s were realized using this doping technique. Moreover, intrinsic mobility as high as 102 cm(2)/V-s at 300 K and 501 cm(2)/V-s at 77 K were achieved after ATO encapsulation that are among the highest mobility values reported on ML MoS2. We also analyzed the doping effect of ATO films on ML MoS2, a phenomenon that is absent when stoichiometric TiO2 is used, using ab initio density functional theory (DFT) calculations that shows excellent agreement with our experimental findings. On the basis of the interfacial-oxygen-vacancy mediated doping as seen in the case of high-kappa ATO ML MoS2, we propose a mechanism for the mobility enhancement effect observed in TMD-based devices after encapsulation in a high-kappa dielectric environment.clos