Kirby K. H. Smithe

Intrinsic electrical transport and performance projections of synthetic monolayer MoS2 devices

We demonstrate monolayer (1L) MoS2 grown by chemical vapor deposition (CVD) with transport properties comparable to those of the best exfoliated 1L devices over a wide range of carrier densities (up to ~1013 cm−2) and temperatures (80–500 K). Transfer length measurements decouple the intrinsic material mobility from the contact resistance, at practical carrier densities (>1012 cm−2). We demonstrate the highest current density reported to date (~270 μA μm−1 or 44 MA cm−2) at 300 K for an 80 nm long device from CVD-grown 1L MoS2. Using simulations, we discuss what improvements of 1L MoS2 are still required to meet technology roadmap requirements for low power and high performance applications. Such results are an important step towards large-area electronics based on 1L semiconductors.

Approaching ballistic transport in monolayer MoS 2 transistors with self-aligned 10 nm top gates

We present the first study of 10 nm self-aligned top-gated field-effect transistors (SATFETs) based on monolayer MoS2. Using a novel fabrication process, we achieve record saturation current, IDsat > 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 MoS2 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.

Photoresponse of Natural van der Waals Heterostructures

Van der Waals heterostructures consisting of two-dimensional materials offer a platform to obtain materials by design and are very attractive owing to unique 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, hexagonal boron nitride, and members of the transition metal dichalcogenide family. Here we present our experimental study of the optoelectronic 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) behaves as a 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 photocarrier 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 a fast photoresponse with a rise time as fast as ∼1 ms. Our study provides a fundamental understanding of the optoelectronic behavior in a complex naturally occurring vdWH, and may pave an avenue toward developing nanoscale optoelectronic devices with tailored properties.

Large array fabrication of high performance monolayer MoS2 photodetectors

Scalable fabrication of high quality photodetectors derived from synthetically grown monolayer transition metal dichalcogenides is highly desired and important for wide range of nanophotonics applications. We present here scalable fabrication of monolayer MoS2 photodetectors on sapphire substrates through an efficient process, which includes growing large scale monolayer MoS2 via chemical vapor deposition (CVD), and multi-step optical lithography for device patterning and high quality metal electrodes fabrication. In every measured device, we observed the following universal features: (i) negligible dark current (Idark 6 10fA); (ii) sharp peaks in photocurrent at ∼1.9eV and ∼2.1eV attributable to the optical transitions due to band edge excitons; (iii) a rapid onset of photocurrent above ∼2.5eV peaked at ∼2.9eV due to an excitonic absorption originating from the van Hove singularity of MoS2. We observe low (6 300%) device-to-device variation of photoresponsivity. Furthermore, we observe very fast rise time ∼0.5 ms, which is three orders of magnitude faster than other reported CVD grown 1L-MoS2 based photodetectors. The combination of scalable device fabrication, ultra-high sensitivity and high speed offer a great potential for applications in photonics.Large array fabrication of high quality photodetectors derived from synthetically grown monolayer transition metal dichalcogenides is highly desired and important for a wide range of nanophotonic applications. We present here large array fabrication of monolayer MoS2 photodetectors on sapphire substrates through an efficient process, which includes growing large scale monolayer MoS2 via chemical vapor deposition (CVD) and multi-step optical lithography for device patterning and high quality metal electrode fabrication. In every measured device, we observed the following universal features: (i) negligible dark current ( Idark≤10 fA), (ii) sharp peaks in photocurrent at ∼1.9 eV and ∼2.1 eV attributable to the optical transitions due to band edge excitons, and (iii) a rapid onset of photocurrent above ∼2.5 eV peaked at ∼2.9 eV due to an excitonic absorption originating from the van Hove singularity of MoS2. We observe a low (≤300%) device-to-device variation of photoresponsivity. Furthermore, we observe a ve...

Effective n-type doping of monolayer MoS 2 by AlO x

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<inf>max</inf>/I<inf>min</inf>) of a device. Here, we demonstrate that AlOx can n-dope monolayer (1L) MoS2 while preserving Imax/Imin and S. The AlO<inf>x</inf> 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.

Direct observation of power dissipation in monolayer MoS 2 devices

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.

Studies of two-dimensional h-BN and MoS 2 for potential diffusion barrier application in copper interconnect technology

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.Interconnect technology: atomically thin h-BN and MoS 2 mitigate Cu diffusionAtomically 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.

Atomically thin diffusion barriers for ultra-scaled Cu interconnects implemented by 2D materials

Sub-1 nm Cu difïusion barriers are realized by using transferred CVD-grown hexagonal boron nitride (h-BN) and directly deposited molybdenum disulfide (MoS2), 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.

Electronic, thermal, and unconventional applications of 2D materials

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.

Electrons, phonons, and unconventional applications of 2D materials

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.