arXiv subjects

Zhun-Yong Ong

Publications and source records attributed to Zhun-Yong Ong.

At least 37 records · Page 2Linked to original sources

arXiv2015

Realization of Room-Temperature Phonon-limited Carrier Transport in Monolayer MoS2 by Dielectric and Carrier Screening

We show that by combining high-k dielectric substrate and high density of charge carriers, Coulomb impurity can be effectively screened, leading to an unprecedented room-temperature mobility of ~150cm2/Vs in monolayer MoS2. The high sample quality enables us to quantitatively extract the mobility components limited by Coulomb impurities, intrinsic and surface optical phonons, and study their scaling with temperature, carrier density and dielectric constant. The excellent agreement between our theoretical analysis and experimental data demonstrates unambiguously that room-temperature phonon-limited transport is achieved in monolayer MoS2, which is a necessary factor for electronic device applications.

cond-mat.mtrl-sci↗

arXiv2015

High-Performance Monolayer WS2 Field-effect Transistors on High-k Dielectrics

The combination of high-quality Al2O3 dielectric and thiol chemistry passivation can effectively reduce the density of interface traps and Coulomb impurities of WS2, leading to a significant improvement of the mobility and a transition of the charge transport from the insulating to the metallic regime. A record high mobility of 83 cm2/Vs (337 cm2/Vs) is reached at room temperature (low temperature) for monolayer WS2. A theoretical model for electron transport is also developed.

cond-mat.mtrl-sci↗

arXiv2014

Anisotropic charged impurity-limited carrier mobility in monolayer phosphorene

The room temperature carrier mobility in atomically thin 2D materials is usually far below the intrinsic limit imposed by phonon scattering as a result of scattering by remote charged impurities in its environment. We simulate the charged impurity-limited carrier mobility $μ$ in bare and encapsulated monolayer phosphorene. We find a significant temperature dependence in the carrier mobilities ($μ\propto T^{-γ}$) that results from the temperature variability of the charge screening and varies with the crystal orientation. The anisotropy in the effective mass leads to an anisotropic carrier mobility, with the mobility in the armchair direction about one order of magnitude larger than in the zigzag direction. In particular, this mobility anisotropy is enhanced at low temperatures and high carrier densities. Under encapsulation with a high-$κ$ overlayer, the mobility increases by up to an order of magnitude although its temperature dependence and its anisotropy are reduced.

cond-mat.mes-hall↗

arXiv2014

Enhancement and reduction of one-dimensional heat conduction with correlated mass disorder

Short-range order in strongly disordered structures plays an important role in their heat conduction property. Using numerical and analytical methods, we show that short-range spatial correlation (with a correlation length of $Λ_{m}$) in the mass distribution of the one-dimensional (1D) alloy-like random binary lattice leads to a dramatic enhancement of the high-frequency phonon transmittance but also increases the low-frequency phonon opacity. High-frequency semi-extended states are formed while low-frequency modes become more localized. This results in ballistic heat conduction at finite lengths but also paradoxically higher thermal resistance that scale as $\sqrt{Λ_{m}}$ in the $L\rightarrow\infty$ limit. We identify an emergent crossover length ($L_{c}$) below which the onset of thermal transparency appears. The crossover length is linearly dependent on but is two orders of magnitude larger than $Λ_{m}$. Our results suggest that the phonon transmittance spectrum and heat conduction in a disordered 1D lattice can be controlled via statistical clustering of the constituent component atoms into domains. They also imply that the detection of ballistic heat conduction in disordered 1D structures may be a signature of the intrinsic mass correlation at a much smaller length scale.

cond-mat.mes-hall↗

arXiv2014

Strong Thermal Transport Anisotropy and Strain Modulation in Single-Layer Phosphorene

Using first-principles calculations and non-equilibrium Green's function method, we investigate the ballistic thermal transport in single-layer phosphorene. A significant crystallographic orientation dependence of thermal conductance is observed, with room temperature thermal conductance along zigzag direction being 40 percent higher than that along armchair direction. Furthermore, we find that the thermal conductance anisotropy with the orientation can be tuned by applying strain. In particular, the zigzag-oriented thermal conductance is enhanced when a zigzag-oriented strain is applied but decreases when an armchair-oriented strain is applied; whereas the armchair-oriented thermal conductance always decreases when either a zigzag- or an armchair-oriented strain is applied. The present work suggests that the remarkable thermal transport anisotropy and its strain-modulated effect in single-layer phosphorene may be used for thermal management in phosphorene-based electronics and optoelectronic devices.

cond-mat.mes-hall↗

arXiv2014

Ballistic Heat Conduction and Mass Disorder in One Dimension

It is well-known that in the disordered harmonic chain, heat conduction is subballistic and the thermal conductivity ($κ$) scales asymptotically as $\lim_{L\rightarrow\infty}κ\propto L^{0.5}$ where $L$ is the chain length. However, using the nonequilibrium Green's function (NEGF) method and analytical modeling, we show that there exists a critical crossover length scale ($L_{C}$) below which ballistic heat conduction ($κ\propto L$) can coexist with mass disorder. This ballistic-to-subballistic heat conduction crossover is connected to the exponential attenuation of the phonon transmittance function $Ξ$ i.e. $Ξ(ω,L)=\exp[-L/λ(ω)]$, where $λ$ is the frequency-dependent attenuation length. The crossover length can be determined from the minimum attenuation length which depends on the maximum transmitted frequency. We numerically determine the dependence of the transmittance on frequency and mass composition as well as derive a closed form estimate which agrees closely with the numerical results. For the length-dependent thermal conductance, we also derive a closed form expression which agrees closely with numerical results and reproduces the ballistic to subballistic thermal conduction crossover. This allows us to characterize the crossover in terms of changes in the length, mass composition and temperature dependence, and also to determine the conditions under which heat conduction enters the ballistic regime. We describe how the mass composition can be modified to increase ballistic heat conduction

cond-mat.mes-hall↗

arXiv2014

Mobility enhancement and temperature dependence in top-gated single-layer MoS2

The deposition of a high-$κ$ oxide overlayer is known to significantly enhance the room-temperature electron mobility in single-layer MoS$_{2}$ (SLM) but not in single-layer graphene (SLG). We give a quantitative account of how this mobility enhancement is due to the non-degeneracy of the two-dimensional electron gas system in SLM at accessible temperatures. Using our charged impurity scattering model [Ong and Fischetti, Phys. Rev. B 86, 121409 (2012)] and temperature-dependent polarizability, we calculate the charged impurity-limited mobility ($μ_{\textrm{imp}}$) in SLM with and without a high-$κ$ (HfO$_{2}$) top gate oxide at different electron densities and temperatures. We find that the mobility enhancement is larger at low electron densities and high temperatures because of finite-temperature screening, thus explaining the enhancement of the mobility observed at room temperature. $μ_{\textrm{imp}}$ is shown to decrease significantly with increasing temperature, suggesting that the strong temperature dependence of measured mobilities should not be interpreted as being solely due to inelastic scattering with phonons. We also reproduce the recently seen experimental trend in which the temperature scaling exponent ($γ$) of $μ_{\textrm{imp}}\propto T^{-γ}$ is smaller in top-gated SLM than in bare SLM. Finally, we show that a $\sim37$ percent mobility enhancement can be achieved by reducing the HfO$_{2}$ thickness from 20 to 2 nm.

cond-mat.mtrl-sci↗

arXiv2014

Towards Intrinsic Charge Transport in Monolayer Molybdenum Disulfide by Defect and Interface Engineering

Molybdenum disulfide is considered as one of the most promising two-dimensional semiconductors for electronic and optoelectronic device applications. So far, the charge transport in monolayer molybdenum disulfide is dominated by extrinsic factors such as charged impurities, structural defects and traps, leading to much lower mobility than the intrinsic limit. Here, we develop a facile low-temperature thiol chemistry to repair the sulfur vacancies and improve the interface, resulting in significant reduction of the charged impurities and traps. High mobility greater than 80cm2 V-1 s-1 is achieved in backgated monolayer molybdenum disulfide field-effect transistors at room temperature. Furthermore, we develop a theoretical model to quantitatively extract the key microscopic quantities that control the transistor performances, including the density of charged impurities, short-range defects and traps. Our combined experimental and theoretical study provides a clear path towards intrinsic charge transport in two-dimensional dichalcogenides for future high-performance device applications.

cond-mat.mtrl-sci↗

arXiv2014

Theoretical analysis of high-field transport in graphene on a substrate

We investigate transport in graphene supported on various dielectrics (SiO2, BN, Al2O3, HfO2) through a hydrodynamic model which includes self-heating and thermal coupling to the substrate, scattering with ionized impurities, graphene phonons and dynamically screened interfacial plasmon-phonon (IPP) modes. We uncover that while low-field transport is largely determined by impurity scattering, high-field transport is defined by scattering with dielectric-induced IPP modes, and a smaller contribution of graphene intrinsic phonons. We also find that lattice heating can lead to negative differential drift velocity (with respect to the electric field), which can be controlled by changing the underlying dielectric thermal properties or thickness. Graphene on BN exhibits the largest high-field drift velocity, while graphene on HfO2 has the lowest one due to strong influence of IPP modes.

cond-mat.mes-hall↗

arXiv2013

Ballistic to diffusive crossover of heat flow in graphene ribbons

Heat flow in nanomaterials is an important area of study, with both fundamental and technological implications. However, little is known about heat flow in two-dimensional (2D) devices or interconnects with dimensions comparable to the phonon mean free path (mfp). Here, we find that short, quarter-micron graphene samples reach ~35% of the ballistic heat conductance limit up to room temperature, enabled by the relatively large phonon mfp (~100 nm) in substrate-supported graphene. In contrast, patterning similar samples into nanoribbons (GNRs) leads to a diffusive heat flow regime that is controlled by ribbon width and edge disorder. In the edge-controlled regime, the GNR thermal conductivity scales with width approximately as ~W^{1.8+/-0.3}, being about 100 W/m/K in 65-nm-wide GNRs, at room temperature. Manipulation of device dimensions on the scale of the phonon mfp can be used to achieve full control of their heat-carrying properties, approaching fundamentally limited upper or lower bounds.

cond-mat.mes-hall↗

arXiv2012

Effect of Grain Boundaries on Thermal Transport in Graphene

We investigate the influence of grain boundaries (GBs), line defects (LDs), and chirality on thermal transport in graphene using non-equilibrium Green's functions. At room temperature the ballistic thermal conductance is ~4.2 GW/m^2/K, and single GBs or LDs yield transmission from 50-80% of this value. LDs with carbon atom octagon defects have lower thermal transmission than GBs with pentagon and heptagon defects. We apply our findings to study the thermal conductivity of polycrystalline graphene for practical applications, and find that the type and size of GBs play an important role when grain sizes are smaller than a few hundred nanometers.

cond-mat.mtrl-sci↗

arXiv2012

Charged Impurity Scattering in Graphene Nanostructures

We study charged impurity scattering and static screening in a top-gated substrate-supported graphene nanostructure. Our model describes how boundary conditions can be incorporated into scattering, sheds light on the dielectric response of these nanostructures, provides insights into the effect of the top gate on impurity scattering, and predicts that the carrier mobility in such graphene heterostructures decreases with increasing top dielectric thickness and higher carrier density. An increase of up to almost 60 percent in carrier mobility in ultrathin top-gated graphene is predicted.

cond-mat.mes-hall↗

arXiv2012

Theory of Interfacial Plasmon-Phonon Scattering in Supported Graphene

One of the factors limiting electron mobility in supported graphene is remote phonon scattering. We formulate the theory of the coupling between graphene plasmon and substrate surface polar phonon (SPP) modes, and find that it leads to the formation of interfacial plasmon-phonon (IPP) modes, from which the phenomena of dynamic anti-screening and screening of remote phonons emerge. The remote phonon-limited mobilities for SiO$_{2}$, HfO$_{2}$, h-BN and Al$_{2}$O$_{3}$ substrates are computed using our theory. We find that h-BN yields the highest peak mobility, but in the practically useful high-density range the mobility in HfO$_{2}$-supported graphene is high, despite the fact that HfO$_{2}$ is a high-$κ$ dielectric with low-frequency modes. Our theory predicts that the strong temperature dependence of the total mobility effectively vanishes at very high carrier concentrations. The effects of polycrystallinity on IPP scattering are also discussed.

cond-mat.mes-hall↗

arXiv2011

Effect of substrate modes on thermal transport in supported graphene

We examine thermal transport in graphene supported on SiO2 using molecular dynamics simulations. Coupling to the substrate reduces the thermal conductivity (TC) of supported graphene by an order of magnitude, due to damping of the flexural acoustic (ZA) phonons. However, increasing the strength of the graphene-substrate interaction enhances the TC of supported graphene, contrary to expectations. The enhancement is due to the coupling of graphene ZA modes to the substrate Rayleigh waves, which linearizes the dispersion and increases the group velocity of the hybridized modes. These findings suggest that the TC of two-dimensional supported graphene is tunable through surface interactions, providing a novel possibility for controlled energy flow in nanomaterials.

cond-mat.mes-hall↗

arXiv2011

Reduction of Phonon Lifetimes and Thermal Conductivity of a Carbon Nanotube on Amorphous Silica

We use molecular dynamics simulations to examine the phonon lifetimes in (10,10) carbon nano-tubes (CNTs), both when isolated and when supported on amorphous SiO2 substrates. We deter-mine the Umklapp, normal, boundary and CNT-substrate phonon scattering rates from the com-puted inverse lifetimes. Suspended CNTs have in-plane optical phonon lifetimes between 0.7-2 ps, consistent with recent experiments, but contact with the substrate leads to a lifetime reduction to the 0.6-1.3 ps range. The thermal conductivity of the supported CNT is also computed to be ~30 percent lower than that of the isolated CNT. The thermal boundary conductance estimated from the CNT-substrate phonon scattering rates is in good agreement with that computed from the Green-Kubo relation and with previous experimental results. The results highlight that solid substrates can strongly affect and could be even used to tune the thermal properties of CNTs.

cond-mat.mes-hall↗

arXiv2010

Frequency and Polarization Dependence of Thermal Coupling between Carbon Nanotubes and SiO2

We study heat dissipation from a (10,10) CNT to a SiO2 substrate using equilibrium and non-equilibrium classical molecular dynamics. The CNT-substrate thermal boundary conductance (TBC) is computed both from the relaxation time of the CNT-substrate temperature difference, and from the time autocorrelation function of the interfacial heat flux at equilibrium (Green-Kubo relation). The power spectrum of interfacial heat flux fluctuation and the time evolution of the internal CNT energy distribution suggest that: 1) thermal coupling is dominated by long wavelength phonons between 0-10 THz, 2) high frequency (40-57 THz) CNT phonon modes are strongly coupled to sub-40 THz CNT phonon modes, and 3) inelastic scattering between the CNT phonons and substrate phonons contributes to interfacial thermal transport. We also find that the low frequency longitudinal acoustic (LA) and twisting acoustic (TA) modes do not transfer energy to the substrate as efficiently as the low frequency transverse optical (TO) mode.

cond-mat.mes-hall↗

arXiv2010

Thermal Dissipation and Variability in Electrical Breakdown of Carbon Nanotube Devices

We study high-field electrical breakdown and heat dissipation from carbon nanotube (CNT) devices on SiO2 substrates. The thermal "footprint" of a CNT caused by van der Waals interactions with the substrate is revealed through molecular dynamics (MD) simulations. Experiments and modeling find the CNT-substrate thermal coupling scales proportionally to CNT diameter and inversely with SiO2 surface roughness (~d/Δ). Comparison of diffuse mismatch modeling (DMM) and data reveals the upper limit of thermal coupling ~0.4 W/K/m per unit length at room temperature, and ~0.7 W/K/m at 600 C for the largest diameter (3-4 nm) CNTs. We also find semiconducting CNTs can break down prematurely, and display more breakdown variability due to dynamic shifts in threshold voltage, which metallic CNTs are immune to; this poses a fundamental challenge for selective electrical breakdowns in CNT electronics.

cond-mat.mes-hall↗

arXiv2010

Imaging, simulation, and electrostatic control of power dissipation in graphene devices

We directly image hot spot formation in functioning mono- and bilayer graphene field effect transistors (GFETs) using infrared thermal microscopy. Correlating with an electrical-thermal transport model provides insight into carrier distributions, fields, and GFET power dissipation. The hot spot corresponds to the location of minimum charge density along the GFET; by changing the applied bias this can be shifted between electrodes or held in the middle of the channel in ambipolar transport. Interestingly, the hot spot shape bears the imprint of the density of states in mono- vs. bilayer graphene. More broadly, we find that thermal imaging combined with self-consistent simulation provides a non-invasive approach for more deeply examining transport and energy dissipation in nanoscale devices.

cond-mat.mes-hall↗