Optical Phonon Limited High Field Transport in Layered Materials

An optical phonon limited velocity model has been employed to investigate high-field transport in a selection of layered 2D materials for both, low-power logic switches with scaled supply voltages, and high-power, high-frequency transistors. Drain currents, effective electron velocities and intrinsic cut-off frequencies as a function of carrier density have been predicted thus providing a benchmark for the optical phonon limited high-field performance limits of these materials. The optical phonon limited carrier velocities of a selection of transition metal dichalcogenides and black phosphorus are found to be modest as compared to their n-channel silicon counterparts, questioning the utility of these devices in the source-injection dominated regime. h-BN, at the other end of the spectrum, is shown to be a very promising material for high-frequency high-power devices, subject to experimental realization of high carrier densities, primarily due to its large optical phonon energy. Experimentally extracted saturation velocities from few-layer MoS2 devices show reasonable qualitative and quantitative agreement with predicted values. Temperature dependence of measured vsat is discussed and found to fit a velocity saturation model with a single material dependent fit parameter.Comment: 8 pages, 6 figure

Surface States Engineering of Metal/MoS2 Contacts Using Sulfur Treatment for Reduced Contact Resistance and Variability

Variability and lack of control in the nature of contacts between metal/MoS2 interface is a major bottleneck in the realisation of high-performance devices based on layered materials for several applications. In this letter, we report on the reduction in Schottky barrier height at metal/MoS2 interface by engineering the surface states through sulphur treatment. Electrical characteristics for back-gated MoS2 field effect transistor structures were investigated for two high work-function metal contacts Ni and Pd. Contacts on MoS2 treated with sulphur exhibited significant improvements in Ohmic nature with concomitant reduction in variability compared to those on untreated MoS2 films leading to a 2x increase in extracted mobility. X-ray Photoelectron Spectroscopy (XPS) measurements, Raman Spectroscopy and comparison of threshold voltages indicated absence of additional doping or structural changes due to sulphur treatment. The Schottky barrier heights were extracted from temperature-dependent transfer characteristics based on the thermionic current model. A reduction in barrier height of 80 and 135 meV extracted for Ni/MoS2 and Pd/MoS2 contacts respectively is hence attributed to the increase in surface states (or stronger Fermi level pinning) due to sulphur treatment. The corresponding charge neutrality levels at metal/MoS2 interface, were extracted to be 0.16 eV (0.17 eV) below the conduction band before (after) Sulphur treatment. This first report of surface states engineering in MoS2 leading to superior contacts is expected to significantly benefit the entire class of devices based on layered 2D materials.Comment: 13 pages, 5 figure

Two-dimensional materials and their role in emerging electronic and photonic devices

Innovation in the field of semiconductor materials and devices have to a large extent underpinned the dramatic developments which have unfolded in the area of information and communication technologies over the past 50 years. The ability to form logic devices, memory elements, light emitting diodes, and lasers directly into semiconducting materials has had a transformative effect on modern society. Looking beyond the era of conventional scaling, the drive towards the internet of things, will require the integration of sensors and optical devices with conventional logic and memory elements. The aim of this article is to give a brief overview of the large-area growth of some 2D transition metal dichalcogenide layered materials by MBE and CVD methods, followed by examples of how these 2D materials can be employed in electron devices and optoelectronic structures and devices

Large-area growth of MoS2 at temperatures compatible with integrating back-end-of-line functionality

Direct growth of transition metal dichalcogenides over large areas within the back-end-of-line (BEOL) thermal budget limit of silicon integrated circuits is a significant challenge for 3D heterogeneous integration. In this work, we report on the growth of MoS2 films (~1-10 nm) on SiO2, amorphous-Al2O3, c-plane sapphire, and glass substrates achieved at low temperatures (350 C-550 C) by chemical vapor deposition in a manufacturing-compatible 300 mm atomic layer deposition reactor. We investigate the MoS2 films as a potential material solution for BEOL logic, memory and sensing applications. Hall-effect/4-point measurements indicate that the ~10 nm MoS2 films exhibit very low carrier concentrations (1014-1015 cm-3), high resistivity, and Hall mobility values of ~0.5-17 cm2 V-1 s-1, confirmed by transistor and resistor test device results. MoS2 grain boundaries and stoichiometric defects resulting from the low thermal budget growth, while detrimental to lateral transport, can be leveraged for the integration of memory and sensing functions. Vertical transport memristor structures (Au/MoS2/Au) incorporating ~3 nm thick MoS2 films grown at 550 C (~0.75 h) show memristive switching and a stable memory window of 105 with a retention time >104 s, between the high-low resistive states. The switching set and reset voltages in these memristors demonstrate a significant reduction compared to memristors fabricated from pristine, single-crystalline MoS2 at higher temperatures, thereby reducing the energy needed for operation. Furthermore, interdigitated electrode-based gas sensors fabricated on ~5 nm thick 550 C-grown (~1.25 h) MoS2 films show excellent selectivity and sub-ppm sensitivity to NO2 gas, with a notable self-recovery at room temperature. The demonstration of large-area MoS2 direct growth at and below the BEOL thermal budget limit, alongside memristive and gas sensing functionality, advances a key enabling technology objective in emerging materials and devices for 3D heterogeneous integration

Realizing P-FETs and Photodiodes on MoS2 through area-selective p-Doping via Vacancy Engineering.

Air-stable and area-selective doping strategies have eluded 2D materials and thus been a major bottleneck in realizing the plethora of semiconductor devices which require an built in electric field accessible from a p/n junction. Here, we demonstrate the possibility of p-doping through Vacancy Engineering, which unlike previous reports of molecular/substitutional doping is both area/dopant controllable and air-stable. Through Ar+ ions of appropriate energy and fluence bombarded on exfoliated MoS2, we demonstrate creation of sulfur vacancies that vary the S:Mo stoichiometry from 1.94 to 0.97 and hence controllably introduce p-type doping as verified using in-situ XPS and ex-situ Raman/PL measurements. FETs fabricated on Ar+ bombarded flakes show complete flip in polarity of carrier type from n-type to p-type when compared to Reference samples with the same metal contacts. Furthermore, selective Ar+ Bombardment only on contacts region shows effective hole injection with I-on/I-off>10(3). Finally p/n junctions with Ar+ bombardment performed on one half of the flake demonstrate high rectification ratio (>10(4)), forward currents (similar to 0.6 mA/cm(2)) and excellent photoresponse (I-light/I-dark similar to 10(3)) and responsivity (100-400 mu A/W)

A sub-thermionic MoS2 FET with tunable transport

The inability to scale supply voltage and hence reduce power consumption remains a serious challenge in modern nanotransistors. This arises primarily because the Sub-threshold Swing (SS) of the thermionic MOSFET, a measure of its switching efficiency, is restricted by the Boltzmann limit (k(B)T/q = 60 mV/dec at 300 K). Tunneling FETs, the most promising candidates to circumvent this limit, employ band-to-band tunneling, yielding very low OFF currents and steep SS but at the expense of severely degraded ON currents. In a completely different approach, by introducing concurrent tuning of thermionic and tunneling components through metal/semiconductor Schottky junctions, we achieve an amalgamation of steep SS and high ON currents in the same device. We demonstrate sub-thermionic transport sustained up to 4 decades with SSmin similar to 8.3 mV/dec and SSavg similar to 37.5(25) mV/dec for 4(3) dec in few layer MoS2 dual gated FETs (planar and CMOS compatible) using tunnel injected Schottky contacts for a highly scaled drain voltage of 10 mV, the lowest for any sub-thermionic devices. Furthermore, the same devices can be tuned to operate in the thermionic regime with a field effect mobility of similar to 84.3 cm(2) V-1 s(-1). A detailed mechanism involving the independent control of the Schottky barrier height and width through efficient device architecture and material processing elucidates the functioning of these devices. The Gate Tunable Thermionic Tunnel FET can function at a supply voltage of as low as 0.5 V, reducing power consumption dramatically. Published by AIP Publishing

High-Performance HfO2 Back Gated Multilayer MoS2 Transistors

A new substrate (similar to 30-nm HfO2/Si) is developed for high-performance back-gated molybdenum disulfide (MoS2) transistors. Record drain current I-ds similar to 180 mu A/mu m and transconductance value g(m) similar to 75 mu S/mu m at V-ds = 1 V have been achieved for 1-mu m channel length multilayer MoS2 transistors on HfO2/Si substrate. The transistors on HfO2 substrate show >2.5x enhancement in field effect mobility (mu(FE) similar to 65 cm(2)/V . s) compared with the transistors on SiO2 (mu(FE) similar to 25 cm(2)/V . s) substrate. The intrinsic mobility extracted from Y function technique (mu(FE) similar to 154 cm(2)/V . s) is 3x more than SiO2 substrate. The drastic improvement in transistor performance is attributed to a combination of three factors: 1) efficient gate coupling with an EOT of 6.2 nm; 2) charge impurity screening due to high-k dielectric; and 3) very low contact resistance through sulfur treatment

Butterfly of Assam University Campus in Silchar: Can Academic Institutions Contribute to Conservation of Species Diversity in Northeastern Region of India

Northeast India is amongst most bio-diverse ecological communities although recent developmental activities marred the environment to a great extent. Assam University campus in Silchar is situated in Barak valley of Assam, boasting a variety of habitats supporting invertebrate diversity. Heavy rainfall during monsoon increases vegetation and in turn larval food plants and overall butterfly density. Total 38 butterfly species were identified belonging to 30 genera under 5 families: Nymphalidae having the maximum species richness (58%), followed by Hesperiidae (13%), Lycaenidae (13%), Pieridae (11%) and Papilionidae (5%). This paper focuses on the problems and possible solutions towards butterfly conservation and highlights the role of academic institutions in conserving biodiversity by acting as green spaces for reducing effects of climate change, carbon sequestration and lowering of energy consumption among other benefits

Intrinsic Limit for Contact Resistance in Exfoliated Multilayered MoS2 FET

A new method for the separation of contact resistance (R-contact) into Schottky barrier resistance (R-SB) and interlayer resistance (R-IL) is proposed for multilayered MoS2 FETs. While R-SB varies exponentially with Schottky barrier height (Phi(bn)), R-IL essentially remains unchanged. An empirical model utilizing this dependence of R-contact versus Phi(bn) is proposed and fits to the experimental data. The results, on comparison with the existing reports of lowest R-contact, suggest that the extracted R-IL (1.53 k Omega.mu m) for an unaltered channel would determine the lower limit of intrinsic R-contact even for barrierless contacts for multilayered exfoliated MoS2 FETs