Eilam Yalon

Energy Dissipation in Monolayer MoS2 Electronics

The advancement of nanoscale electronics has been limited by energy dissipation challenges for over a decade. Such limitations could be particularly severe for two-dimensional (2D) semiconductors integrated with flexible substrates or multilayered processors, both being critical thermal bottlenecks. To shed light into fundamental aspects of this problem, here we report the first direct measurement of spatially resolved temperature in functioning 2D monolayer MoS2 transistors. Using Raman thermometry, we simultaneously obtain temperature maps of the device channel and its substrate. This differential measurement reveals the thermal boundary conductance of the MoS2 interface with SiO2 (14 ± 4 MW m-2 K-1) is an order magnitude larger than previously thought, yet near the low end of known solid-solid interfaces. Our study also reveals unexpected insight into nonuniformities of the MoS2 transistors (small bilayer regions) which do not cause significant self-heating, suggesting that such semiconductors are less sensitive to inhomogeneity than expected. These results provide key insights into energy dissipation of 2D semiconductors and pave the way for the future design of energy-efficient 2D electronics.

Towards ultimate scaling limits of phase-change memory

Data storage based on a reversible material phase transition (e.g. amorphous to crystalline) has been studied for nearly five decades. Yet, it was only during the past five years that some phase-change memory technologies (e.g. GeSbTe) have been approaching the physical scaling limits of the smallest possible memory cell. Here we review recent results from our group and others, which have achieved sub-10 nm scale PCM with switching energy approaching single femtojoules per bit. Fundamental limits could be as low as single attojoules per cubic nanometer of the memory material, although approaching such limits in practice appears strongly limited by electrical and thermal parasitics, i.e. contacts and interfaces.

Spatially Resolved Thermometry of Resistive Memory Devices

The operation of resistive and phase-change memory (RRAM and PCM) is controlled by highly localized self-heating effects, yet detailed studies of their temperature are rare due to challenges of nanoscale thermometry. Here we show that the combination of Raman thermometry and scanning thermal microscopy (SThM) can enable such measurements with high spatial resolution. We report temperature-dependent Raman spectra of HfO2, TiO2 and Ge2Sb2Te5 (GST) films, and demonstrate direct measurements of temperature profiles in lateral PCM devices. Our measurements reveal that electrical and thermal interfaces dominate the operation of such devices, uncovering a thermal boundary resistance of 28u2009±u20098 m2K/GW at GST-SiO2 interfaces and an effective thermopower 350u2009±u200950 µV/K at GST-Pt interfaces. We also discuss possible pathways to apply Raman thermometry and SThM techniques to nanoscale and vertical resistive memory devices.

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.

Rapid Flame Synthesis of Atomically Thin MoO3 down to Monolayer Thickness for Effective Hole Doping of WSe2

Two-dimensional (2D) molybdenum trioxide (MoO3) with mono- or few-layer thickness can potentially advance many applications, ranging from optoelectronics, catalysis, sensors, and batteries to electrochromic devices. Such ultrathin MoO3 sheets can also be integrated with other 2D materials (e.g., as dopants) to realize new or improved electronic devices. However, there is lack of a rapid and scalable method to controllably grow mono- or few-layer MoO3. Here, we report the first demonstration of using a rapid (<2 min) flame synthesis method to deposit mono- and few-layer MoO3 sheets (several microns in lateral dimension) on a wide variety of layered materials, including mica, MoS2, graphene, and WSe2, based on van der Waals epitaxy. The flame-grown ultrathin MoO3 sheet functions as an efficient hole doping layer for WSe2, enabling WSe2 to reach the lowest sheet and contact resistance reported to date among all the p-type 2D materials (∼6.5 kΩ/□ and ∼0.8 kΩ·μm, respectively). These results demonstrate that flame synthesis is a rapid and scalable pathway to growing atomically thin 2D metal oxides, opening up new opportunities for advancing 2D electronics.

Effect of oxygen vacancies and strain on the phonon spectrum of HfO2 thin films

The effect of strain and oxygen deficiency on the Raman spectrum of monoclinic HfO2 is investigated theoretically using first-principles calculations. 1% in-plane compressive strain applied to a and c axes is found to blue shift the phonon frequencies, while 1% tensile strain does the opposite. The simulations are compared, and good agreement is found with the experimental results of Raman frequencies greater than 110u2009cm−1 for 50u2009nm HfO2 thin films. Several Raman modes measured below 110u2009cm−1 and previously assigned to HfO2 are found to be rotational modes of gases present in air ambient (nitrogen and oxygen). However, localized vibrational modes introduced by threefold-coordinated oxygen (O3) vacancies are identified at 96.4u2009cm−1 computationally. These results are important for a deeper understanding of vibrational modes in HfO2, which has technological applications in transistors and particularly in resistive random-access memory whose operation relies on oxygen-deficient HfOx.

Temperature Dependent Thermal Boundary Conductance of Monolayer MoS2 by Raman Thermometry

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 MoS2 with AlN and SiO2, 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 MW m-2 K-1 near room temperature, increasing as ∼ T0.65 in the range 300-600 K. The similar TBC of MoS2 with the two substrates indicates that MoS2 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.

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.

Validation and Extension of Local Temperature Evaluation of Conductive Filaments in RRAM Devices

Local temperature plays a key role in resistive switching random access memory devices. We have previously presented a method for measuring the local filament temperature on a nanometric scale using an MIS bipolar transistor structure. Here, a more detailed analysis of the method is presented. A new calibration technique improves the accuracy of the extracted temperature. Alternative device structures allow validation of the method by the extraction of temperature that equals the ambient temperature when no self-heating occurs as well as extension of the obtained temperature range. The accuracy, prospects, and limitations of the method are discussed.

Research Update: Recent progress on 2D materials beyond graphene: From ripples, defects, intercalation, and valley dynamics to straintronics and power dissipation

This review article was based on contents discussed in the 5th annual Graphene and Beyond workshop held at Pennsylvania State University. The workshop was sponsored by Corning, Morgan Advanced Materials, APL Materials, CVD Equipment Corporation, and FEI. The authors thank the following funding agencies for supporting their research: NSF, DOE, DOD, AFOSR, and SRC. The authors also thank NEWLIMITS, a center in nCORE, a Semiconductor Research Corporation (SRC) program sponsored by NIST and thank LEAST, one of six STARnet Centers, a SRC program sponsored by MARCO and DARPA. (NSF; DOE; DOD; AFOSR; SRC; NIST; MARCO; DARPA)