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Semiconductors require stable doping for applications in transistors, optoelectronics, and thermoelectrics. However, this has been challenging for two-dimensional (2D) materials, where existing approaches are either incompatible with conventional semiconductor processing or introduce time-dependent, hysteretic behavior. Here we show that low temperature (< 200$^\circ$ C) sub-stoichiometric AlO$_x$ provides a stable n-doping layer for monolayer MoS$_2$, compatible with circuit integration. This approach achieves carrier densities > 2x10$^{13}$ 1/cm$^2$, sheet resistance as low as ~7 kOhm/sq, and good contact resistance ~480 Ohm.um in transistors from monolayer MoS$_2$ grown by chemical vapor deposition. We also reach record current density of nearly 700 uA/um (>110 MA/cm$^2$) in this three-atom-thick semiconductor while preserving transistor on/off current ratio > $10^6$. The maximum current is ultimately limited by self-heating and could exceed 1 mA/um with better device heat sinking. With their 0.1 nA/um off-current, such doped MoS$_2$ devices approach several low-power transistor metrics required by the international technology roadmap, To appear in ACS Nano (2021)

Layering two-dimensional van der Waals materials provides unprecedented control over atomic placement, which could enable tailoring of vibrational spectra and heat flow at the sub-nanometer scale. Here, using spatially-resolved ultrafast thermoreflectance and spectroscopy, we uncover the design rules governing cross-plane heat transport in superlattices assembled from monolayers of graphene (G) and MoS2 (M). Using a combinatorial experimental approach, we probe nine different stacking sequences: G, GG, MG, GGG, GMG, GGMG, GMGG, GMMG, GMGMG and identify the effects of vibrational mismatch, interlayer adhesion, and junction asymmetry on thermal transport. Pure G sequences display signatures of quasi-ballistic transport, whereas adding even a single M layer strongly disrupts heat conduction. The experimental data are described well by molecular dynamics simulations which include thermal expansion, accounting for the effect of finite temperature on the interlayer spacing. The simulations show that a change of only 1.5% in the layer separation can lead to a nearly 100% increase of the thermal resistance. Using these design rules, we experimentally demonstrate a 5-layer GMGMG superlattice with an ultralow effective cross-plane thermal conductivity comparable to air, paving the way for a new class of thermal metamaterials with extreme properties.

Drift velocity saturation (at some characteristic value, ${v}_{d}^{\mathrm{sat}}$) is a critical process that limits the ultimate current-carrying capacity of semiconductors at high electric fields $(\ensuremath{\sim}{10}^{4}\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{cm})$ . With the recent emergence of two-dimensional (2D) semiconductors, there is a need to understand the manner in which velocity saturation is impacted when materials are thinned to the monolayer scale. Efforts to determine ${v}_{d}^{\mathrm{sat}}$ are typically hampered, however, by self-heating effects that arise from undesirable energy loss from the active 2D layer to the dielectric substrate that supports it. In this work, we explore this problem for an important 2D semiconductor, namely monolayer molybdenum disulfide ($\mathrm{Mo}{\mathrm{S}}_{2}$). By applying a strategy of rapid (nanosecond duration), single-shot, pulsing, we are able to probe the true hot-carrier dynamics in this material, free of the influence of self-heating of its $\mathrm{Si}{\mathrm{O}}_{2}$ substrate. Our approach allows us to realize high current densities $(\ensuremath{\sim}\mathrm{mA}/\ensuremath{\mu}\mathrm{m})$ in the $\mathrm{Mo}{\mathrm{S}}_{2}$ layers, representing a significant enhancement over prior studies. We similarly infer values for the saturated drift velocity $({v}_{d}^{sat\phantom{\rule{4pt}{0ex}}}\ensuremath{\sim}5\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}{\mathrm{s}}^{\ensuremath{-}1})$ that are higher than those reported in earlier works, in which the influence of self-heating (and carrier injection into oxide traps) could not be excluded. In fact, our estimates for ${v}_{d}^{\mathrm{sat}}$ are somewhat close to the ideal velocity expected for normal (parabolic) semiconductors. Since a proper knowledge of this parameter is essential to the design of active electronic and optoelectronic devices, the insight into velocity saturation provided here should provide useful guidance for such efforts.

The future scaling of semiconductor devices can be continued only by the development of novel nanofabrication techniques and atomically thin transistor channels. Here we demonstrate ultra-scaled MoS2 field-effect transistors (FETs) realized by a shadow evaporation method which does not require nanofabrication. The method enables large-scale fabrication of MoS2 FETs with fully gated ∼10 nm long channels. The realized ultra-scaled MoS2 FETs exhibit very small hysteresis of current–voltage characteristics, high drain currents up to ∼560 A m−1, very good drain current saturation for such ultra-short devices, subthreshold swing of ∼120 mV dec−1, and drain current on/off ratio of ∼106 in air ambient. The fabricated ultra-scaled MoS2 FETs are also used to realize logic gates in n-type depletion-load technology. The inverters exhibit a voltage gain of ∼50 at a power supply voltage of only 1.5 V and are capable of in/out signal matching.

Two-dimensional semiconductors such as monolayer MoS2 are of interest for future applications including flexible electronics and end-of-roadmap technologies. Most research to date has focused on low-field mobility, but the peak current-driving ability of transistors is limited by the high-field saturation drift velocity, vsat. Here, we measure high-field transport as a function of temperature for the first time in high-quality synthetic monolayer MoS2. We find that in typical device geometries (e.g. on SiO2 substrates) self-heating can significantly reduce current drive during high-field operation. However, with measurements at varying ambient temperature (from 100 to 300 K), we extract electron vsat = (3.4 ± 0.4) × 106 cm/s at room temperature in this three-atom-thick semiconductor, which we benchmark against other bulk and layered materials. With these results, we estimate that the saturation current in monolayer MoS2 could exceed 1 mA/μm at room temperature, in digital circuits with near-ideal thermal ...

Understanding growth, grain boundaries (GBs), and defects of emerging two-dimensional (2D) materials is key to enabling their future applications. For quick, nondestructive metrology, many studies rely on confocal Raman spectroscopy, the spatial resolution of which is constrained by the diffraction limit (∼0.5 μm). Here we use tip-enhanced Raman spectroscopy (TERS) for the first time on synthetic MoSe2 monolayers, combining it with other scanning probe microscopy (SPM) techniques, all with sub-20 nm spatial resolution. We uncover strong nanoscale heterogeneities in the Raman spectra of MoSe2 transferred to gold substrates [one near 240 cm–1 (A1′), and others near 287 cm–1 (E′), 340 cm–1, and 995 cm–1], which are not observable with common confocal techniques and appear to imply the presence of nanoscale domains of MoO3. We also observe strong tip-enhanced photoluminescence (TEPL), with a signal nearly an order of magnitude greater than the far-field PL. Combining TERS with other SPM techniques, we find th...

We report considerable improvement in the hysteresis and reliability of MoS2 field-effect transistors (FETs) achieved by chemical vapor deposition (CVD) of single-layer MoS2 and dielectric encapsulation. Our results show that a high-quality 15-nm thick Al2O3 layer allows for an efficient protection of the devices from adsorbent-type trapping sites. Combined use of the CVD-grown MoS2 as a channel and encapsulation simultaneously leads to at least an order of magnitude smaller hysteresis and up to two orders of magnitude lower long-term drifts of the transistor characteristics. Together with high on/off current ratios ( $\sim 10^{\textsf {9}}$ ) achieved in our devices, this presents a considerable advance in the technology of MoS2 FETs. As such, we conclude that both CVD growth of MoS2 and encapsulation present important technological steps toward reaching commercial quality standards of next-generation two-dimensional (2D) material technologies.

Despite much interest in applications of two-dimensional (2D) fabrics such as MoS2, to date most studies have focused on single or few devices. Here we examine the variability of hundreds of transistors from monolayer MoS2 synthesized by chemical vapor deposition. Ultraclean fabrication yields low surface roughness of ∼3 A and surprisingly low variability of key device parameters, considering the atomically thin nature of the material. Threshold voltage variation and very low hysteresis suggest variations in charge density and traps as low as ∼1011 cm−2. Three extraction methods (field-effect, Y-function, and effective mobility) independently reveal mobility from 30 to 45 cm2/V/s (10th to 90th percentile; highest value ∼48 cm2/V/s) across areas >1 cm2. Electrical properties are remarkably immune to the presence of bilayer regions, which cause only small conduction band offsets (∼55 meV) measured by scanning Kelvin probe microscopy, an order of magnitude lower than energy variations in Si films of comparab...

Demonstration of metamaterials with ultrahigh thermal resistance by phonon-level engineering of heterogeneous 2D monolayers., Heterogeneous integration of nanomaterials has enabled advanced electronics and photonics applications. However, similar progress has been challenging for thermal applications, in part due to shorter wavelengths of heat carriers (phonons) compared to electrons and photons. Here, we demonstrate unusually high thermal isolation across ultrathin heterostructures, achieved by layering atomically thin two-dimensional (2D) materials. We realize artificial stacks of monolayer graphene, MoS2, and WSe2 with thermal resistance greater than 100 times thicker SiO2 and effective thermal conductivity lower than air at room temperature. Using Raman thermometry, we simultaneously identify the thermal resistance between any 2D monolayers in the stack. Ultrahigh thermal isolation is achieved through the mismatch in mass density and phonon density of states between the 2D layers. These thermal metamaterials are an example in the emerging field of phononics and could find applications where ultrathin thermal insulation is desired, in thermal energy harvesting, or for routing heat in ultracompact geometries.

The temperature coefficient of resistance (TCR) of thin metal lines is often used for applications in thermometry [1], phase change [2], or even thermal accelerometers [3]. However, the TCR of metals drops sharply in films thinner than ~10 nm due to strong surface scattering, preventing their use as ultra-fast thermal sensors, which require reduced thermal mass. In contrast, here we demonstrate for the first time that atomically-thin two-dimensional (2D) materials maintain large TCRs ( $>0.5\%$ ) even at sub-nm thickness. Fundamentally, these experiments illustrate that transport in high-quality 2D materials remains phonon-limited, unlike ultra-thin metals. From an application standpoint, high TCR in atomically-thin films is expected to enable fast thermal sensors.

Mos2 is a two-dimensional (2D) semi-conductor which is now considered as a channel material in 2D FETs [1]–[3]. Recently we found that single-layer (1L) Mos2 FETs grown by chemical vapour deposition (CVD) and encapsulated with high-quality Al 2 O 3 layer exhibit superior performance and reliability compared to all previously reported 2D FETs [2]. However, while the $I_{\mathrm{on}}/I_{\mathrm{off}}$ ratio $(\sim 10^{9})$ of the devices [2] already fulfills commercial standards, their reliability is still poorer than for Si devices [4]. In order to understand the physical origin of these aspects, here we analyze the impact of annealing at 300°C and Al 2 O 3 deposition on the performance and hysteresis dynamics of 1L Mos2 FETs.

We investigate four monolayer transition metal dichalcogenides (TMDs) — MoS 2 , WS 2 , MoSe 2 , and WSe 2 — transferred to silicon substrates as possible sub-nanometer copper diffusion barriers compatible with back-end-of-line temperatures. Based on top-down scanning electron microscope (SEM) and cross-section transmission electron microscope imaging, we demonstrate that the W-based TMDs act as diffusion barriers up to 360°C, while Mo-based TMDs fail at temperatures as low as 300°C. Analysis by SEM indicates that points of failure occur as pinholes, suggesting that mechanical damage may be the origin of failure. Further analysis by X-ray photoemission spectroscopy on as-grown TMDs reveals all four to be chemically unaffected by copper at temperatures as high as 600°C, indicating that directly-grown TMDs still have potential as sub-nanometer diffusion barriers.

Copper interconnects in modern integrated circuits require a barrier layer to prevent Cu diffusion into surrounding dielectrics. However, conventional barrier materials like TaN are highly resistive compared to Cu and will occupy a large fraction of the cross-section of ultra-scaled Cu interconnects due to their thickness scaling limits at 2–3 nm, which will significantly increase the Cu line resistance. It is well understood that ultrathin, effective diffusion barriers are required to continue the interconnect scaling. In this study, a new class of two-dimensional (2D) materials, hexagonal boron nitride (h-BN) and molybdenum disulfide (MoS2), is explored as alternative Cu diffusion barriers. Based on time-dependent dielectric breakdown measurements and scanning transmission electron microscopy imaging coupled with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy characterizations, these 2D materials are shown to be promising barrier solutions for Cu interconnect technology. The predicted lifetime of devices with directly deposited 2D barriers can achieve three orders of magnitude improvement compared to control devices without barriers. Atomically thin h-BN and MoS2 may provide a viable alternative to conventional barrier materials in Cu interconnects. A team led by Zhihong Chen at Purdue University utilized two-dimensional crystals to mitigate Cu diffusion into the dielectric, a known cause of chip failure. By means of time-dependent dielectric breakdown measurements to investigate the diffusion barrier properties of atomically thin h-BN and MoS2, they recorded a substantial improvement of the time-to-breakdown, owing to a reliability enhancement of the dielectric underneath Cu under normal operating conditions. A number of structural and electrical characterizations, including scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy confirmed that two-dimensional h-BN and MoS2 films effectively prevent Cu diffusion, highlighting their potential applicability as sub-nanometer barrier for interconnect technology.

We demonstrate the first 1-transistor-1-resistor (1T1R) memory cell using the atomically thin molybdenum disulfide (MoS2) field-effect transistor (FET) and resistive random access memory (RRAM). This 1T1R demonstration realizes a key milestone for tight integration of memory with logic in a monolithic 3D integrated chip. The monolayer MoS2 is grown by chemical vapor deposition (CVD), suitable for wafer-scale fabrication. The MoS 2 FETs have ON-state current of 190 μA/μm at V d = 2.5 V, showing strong driving capability for RRAM. Metal-oxide RRAMs are fabricated at low process temperature, compatible with MoS 2 FET fabrication. 1T1R measurements show higher resistances, and less resistance and voltage variation compared with measurements using only the RRAM. The multiple resistance states obtained for pulsed reset measurements show promise for in-memory computing and neuromorphic computing applications.

The electrical and thermal behavior of nanoscale devices based on two-dimensional (2D) materials is often limited by their contacts and interfaces. Here we report the temperature-dependent thermal boundary conductance (TBC) of monolayer MoS$_2$ with AlN and SiO$_2$, using Raman thermometry with laser-induced heating. The temperature-dependent optical absorption of the 2D material is crucial in such experiments, which we characterize here for the first time above room temperature. We obtain TBC ~ 15 MWm$^-$$^2$K$^-$$^1$ near room temperature, increasing as ~ T$^0$$^.$$^6$$^5$ in the range 300 - 600 K. The similar TBC of MoS$_2$ with the two substrates indicates that MoS$_2$ is the "softer" material with weaker phonon irradiance, and the relatively low TBC signifies that such interfaces present a key bottleneck in energy dissipation from 2D devices. Our approach is needed to correctly perform Raman thermometry of 2D materials, and our findings are key for understanding energy coupling at the nanoscale.

This invited talk will present recent highlights from our research on two-dimensional (2D) materials including graphene, boron nitride (h-BN), and transition metal dichalcogenides (TMDs). The results span from fundamental measurements and simulations, to device- and several unusual system-oriented applications which take advantage of unique 2D material properties. Basic electrical, thermal, and thermoelectric properties of 2D materials will also be discussed.

Doping of two-dimensional (2D) semiconductors often utilizes charge transfer techniques that are not compatible with standard CMOS fabrication and are unstable over time. Sub-stoichiometric oxides have demonstrated stable 2D material doping [1], but often degrade the subthreshold swing (S) and current on/off ratio (I max /I min ) of a device. Here, we demonstrate that AlOx can n-dope monolayer (1L) MoS2 while preserving Imax/Imin and S. The AlO x doping significantly reduces the contact resistance (to 480 Ω·μm) while preserving the mobility (∼34 cm2V−1s−1) and S, ultimately achieving record on-current of 700 μA/μm for a monolayer semiconductor. We also present a model for the effect of interface traps on the transfer characteristics, which explains the experimentally obtained results.

Van der Waals (vdW) heterostructures consisting of two dimensional materials offer a platform to obtain material by design and are very attractive owing to novel electronic states. Research on 2D van der Waals heterostructures (vdWH) has so far been focused on fabricating individually stacked atomically thin unary or binary crystals. Such systems include graphene (Gr), hexagonal boron nitride (h-BN) and member of the transition metal dichalcogenides family. Here we present our experimental study of the opto-electronic properties of a naturally occurring vdWH, known as Franckeite, which is a complex layered crystal composed of lead, tin, antimony, iron and sulfur. We present here that thin film franckeite (60 nm < d < 100 nm) behave as narrow band gap semiconductor demonstrating a wide band photoresponse. We have observed the band-edge transition at ~ 1500 nm (~830 meV) and high external quantum efficiency (EQE~3%) at room temperature. Laser power resolved and temperature resolved photocurrent measurements reveal that the photo-carrier generation and recombination are dominated by continuously distributed trap states within the band gap. To understand wavelength resolved photocurrent, we also calculated the optical absorption properties via density functional theory. Finally, we have shown that the device has fast photoresponse with rise time as fast as ~ 1 ms. Our study provides a fundamental understanding of the optoelectronic behavior in a complex naturally occurring vdWH and can open up the possibilities of producing new type of nanoscale optoelectronic devices with tailored properties., 10 pages, 5 figures (to be appeared in ACS NANO)

This invited talk will present recent highlights from our research on two-dimensional (2D) materials including graphene, boron nitride (h-BN), and transition metal dichalcogenides (TMDs). The results span from fundamental measurements and simulations, to device- and several unusual system-oriented applications which take advantage of unique 2D material properties. Basic electrical, thermal, and thermoelectric properties of 2D materials will also be discussed.

Sub-1 nm Cu difiusion barriers are realized by using transferred CVD-grown hexagonal boron nitride (h-BN) and directly deposited molybdenum disulfide (MoS 2 ), for the first time. Based on time-dependent dielectric breakdown measurements, the diffusion barrier properties of these 2D materials are explored to address the barrier/liner scaling challenge for the ultra-scaled interconnect technology. The predicted lifetime of devices with directly deposited 2D barriers can achieve 3 orders of magnitude improvement compared to control devices.

We present the first study of 10 nm self-aligned top-gated field-effect transistors (SATFETs) based on monolayer MoS 2 . Using a novel fabrication process, we achieve record saturation current, I Dsat > 400 μA/μm, sub-threshold slope down to 80 mV/dec and equivalent oxide thickness (EOT) ≈ 2.5 nm. Combining transistor modeling with careful gate capacitance and contact resistance measurements, we provide the first analysis of diffusive vs. ballistic transport in monolayer MoS 2 FETs. Results indicate the onset of ballistic transport with transmission up to 0.25 at low temperature. We also suggest a feasible route to advance MoS2 transistors further to the ballistic limit.

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attractive materials for next generation nanoscale optoelectronic applications. Understanding nanoscale optical behavior of the edges and grain boundaries of synthetically grown TMDCs is vital for optimizing their optoelectronic properties. Elucidating the nanoscale optical properties of 2D materials through far-field optical microscopy requires a diffraction-limited optical beam diameter sub-micron in size. Here we present our experimental work on spatial photoluminescence (PL) scanning of large size ( $\geq 50$ microns) monolayer MoS$_2$ grown by chemical vapor deposition (CVD) using a diffraction limited blue laser beam spot (wavelength 405 nm) with a beam diameter as small as 200 nm allowing us to probe nanoscale excitonic phenomena which was not observed before. We have found several important features: (i) there exists a sub-micron width strip ($\sim 500$ nm) along the edges that fluoresces $\sim 1000 \%$ brighter than the region far inside; (ii) there is another brighter wide region consisting of parallel fluorescing lines ending at the corners of the zig-zag peripheral edges; (iii) there is a giant blue shifted A-excitonic peak, as large as $\sim 120$ meV, in the PL spectra from the edges. Using density functional theory calculations, we attribute this giant blue shift to the adsorption of oxygen dimers at the edges, which reduces the excitonic binding energy. Our results not only shed light on defect-induced excitonic properties, but also offer an attractive route to tailor optical properties at the TMDC edges through defect engineering., 10 pages, 4 figures in Journal of Physical Chemistry C, 2016

Transistors based on two-dimensional (2D) materials often exhibit hysteresis in their electrical measurements, i.e. a dependence of measured current on voltage sweep direction due to charge trapping. Here we demonstrate a simple pulsed measurement technique which reduces this hysteretic behavior, enabling more accurate characterization of 2D transistors. We compare hysteresis and charge trapping in four types of devices fabricated from both exfoliated and synthetic MoS2, with SiO2 and HfO2 insulators, using DC and pulsed voltage measurements at different temperatures. Applying modest voltage pulses (~1 ms) on the gate significantly reduces charge trapping and results in the elimination of over 80% of hysteresis for all devices. At shorter pulse widths (~1 µs), up to 99% of hysteresis is reduced for some devices. Our measurements enable the extraction of a unique value of field-effect mobility, regardless of voltage sweep direction, unlike measurements that rely on forward or backward DC measurements. This simple and reproducible technique is useful for studying the intrinsic properties of 2D transistors, and can be similarly applied to other nanoscale and emerging devices where charge trapping is of concern.

In summary, we demonstrated WSe2 FETs contacted with thin metallic WTe2, at channel lengths down to 90 nm and current saturation up to 60 μA/μm. A temperature-dependent study suggests the presence of an intrinsic vdWg at the 2D-2D contact. Thus, ultra-thin WTe2 could be preferred to graphene as a contact depinning layer. This work was supported in part by the AFOSR, NSF-EFRI, and Stanford SystemX.

We studied power dissipation in 1L MoS2 devices using Raman thermometry for the first time. We uncovered non-uniformities of power dissipation and the important role of the MoS2-substrate interface thermal resistance. These results provide critical insights for thermal design of devices based on 2D materials. This work was supported by the AFOSR, NSF EFRI 2-DARE, and Stanford SystemX.

We demonstrate monolayer MoS2 grown by chemical vapor deposition (CVD) with transport properties comparable to those of the best exfoliated devices over a wide range of carrier densities (up to ~10^13 1/cm^2) and temperatures (80-500 K). Transfer length measurements reveal monolayer mobility of ~20 cm2/V/s on SiO2 substrates at 300 K and practical carrier densities (>2x10^12 1/cm^2). We also demonstrate the highest current density reported to date (~270 uA/um or 44 MA/cm^2) at 300 K for an 80 nm device from CVD-grown monolayer MoS2. Using simulations, we discuss what improvements of monolayer MoS2 are still required to meet technology roadmap requirements for low power (LP) and high performance (HP) applications. Such results are an important step towards large-area electronics based on monolayer semiconductors., to be published in 2D Materials

This talk will give an overview of our recent work on two-dimensional (2D) materials and devices. Particular focus will be placed on high-field transport, device self-heating, and fundamental aspects of thermal (phonon) transport in 2D materials including graphene and MoS2.

Monolayer (1L) two-dimensional (2D) semiconductors such as MoS 2 have garnered attention for highly scaled optoelectronics [1,2], due to their sub-nm thickness and direct band gap. For practical applications, such films must be grown on large areas with good electrical properties. However, the highest reported mobility values of 1L MoS 2 grown by chemical vapor deposition (CVD) to date have been below 20 cm2/V/s, and only on isolated single crystals [3,4]. Here we demonstrate CVD-grown continuous 1L MoS 2 with mobilities comparable to those of exfoliated samples over a range of temperatures (T = 80–500 K) and carrier densities (up to n ≈ 5×1012 cm−2). We also demonstrate the highest current density reported to date (∼275 µA/µm) at 300 K for an 80 nm channel length FET built from CVD-grown 1L MoS 2 .