Session K37: Devices from 2D Materials I - Electronics

High-performance MoS2 Field Effect Transistors Gated by Polycrystalline Ferroelectric Pb(Zr,Ti)O3

In this work, we demonstrated high-performance, non-volatile MoS₂ field effect transistors (FETs) with polycrystalline ferroelectric PbZr_0.35Ti_0.65O₃ back gate. We mechanically exfoliated few-layer MoS₂ flake and transferred it onto 300 nm polycrystalline PZT films, which possesses high dielectric constant of 650, and then fabricated samples into 2-point devices. We characterized the transfer characteristics of the MoS2 FET at gate voltage below the coercive voltage of PZT. The PZT behaves as a high-k dielectric, which can induce high current ratio of 10⁷ in MoS2 within a gate voltage range of ±0.5 V. The devices also showed ultra-low sub-threshold swing (SS) of about 52 mV/dec at room temperature, transcending the 60 mV theoretical limit for classical FET, which may originate from the negative capacitance effect of the ferroelectric gate.
  

High Current Density and Low Contact Resistance P-type MoTe2 Thin Flake FETs Enabled by Oxygen Plasma Doping

Few-layer molybdenum ditelluride (MoTe₂) with a small band gap (~1.0 eV) is found promising for realizing tunnel field effect transistors (TFETs). However, the existence of a large contact resistance at the interface between metal and 2D MoTe₂ has drastically restrains the current density, which hinders its application to high performance electronics and optoelectronics. In this work, we demonstrated a controllable mild oxygen plasma doping method for achieving ultra-high hole density in thin MoTe₂ FET. By increasing the plasma treatment duration, degenerate p-type MoTe₂ FETs with enhanced hole density were obtained from the pristine n-dominated MoTe₂ FETs, achieving low contact resistance of 0.6 kΩ●µm. In order to preserve good on-off ratio and carrier mobility of the device, we exposed the contact area to oxygen plasma. Non-degenerate p-type MoTe₂ FETs with comparable on-state hole transport to the pristine electron transport were achieved without hole mobility degradation.   

Electrical Transport Properties and Raman studies of few-layered Arsenic Doped Black Phosphorus Field-effect Transistor

Arsenic doped black phosphorus (As-BP) has recently emerged as a new semiconducting layered material with tunable electronic and optical properties. Here we will present detail electrical transport properties of few-layered Arsenic doped black phosphorus (As-BP) field-effect transistor fabricated on Si/SiO₂ substrate using Ti/Au contacts. By modulating the back gate voltage, the few-layered As-BP channel exhibit hole doped characteristic with intrinsic charge carrier mobility at room temperature ~300cm²/Vs and it increases to ~600cm²/Vs at low temperature. Moreover, the transistor ON/OFF current ratio is obtained as large as 10⁴ -10⁶.We will also present the polarization resolved Raman scattering studies reveals the anisotropic nature of As-BP.
  

2D materials for reducing contact resistivity of metal-semiconductor junction

As the channel resistance of transistor shrinks with continued scale down of complementary metal-oxide-semiconductor technology nodes, lowering external parasitic resistance, especially the contact resistance (Rc), becomes a critical area of focus. The current approach to lower contact resistance is to reduce width of Schottky barrier by using heavily doped semiconductors. However, this method is approaching its limitation because of the solid solubility limit of charged dopants in Si, high leakage current, and dopant profile control.
Here, we insert 2D materials between metal and semiconductor, such as graphene and hexagonal-boron nitride (h-BN) and transition metal dichalcogenides to reduce height of Schottky barrier and contact resistance. In this approach, 2D layers improve the metal/semiconductor contact by modifying the work function of contact metal and thus reducing Schottky barrier height. With the insertion of 2D materials, we achieved Schottky barrier height of less than 0.1 eV and contact resistivity of few nano-ohm cm2. This proposed 2D inserted technique can be used as sub nano-ohm cm2 contacts for metal/n-Si and low Rc contact with new semiconductor materials beyond Si CMOS technology.   

Direct Observation of 2D Material Interfacial States within Devices Using STEM-EELS

The 2D materials field continues to expand as novel layered materials are synthesized and embedded in various devices. However, the structure of the interface between an embedded 2D material and the rest of the device, which is critical to the device’s performance, remains poorly understood. Few experimental studies have focused on these interfaces likely because the device-embedded 2D material is difficult to access at the atomic scale. Here, we present an experimental study of the bonding interactions at the interface between MoS₂ and Ti contacts within a FET using STEM-EELS. Atomic-resolution STEM images of the Ti-MoS₂ interface show distinct differences in the crystal structure between interacting and non-interacting areas within the FET. Furthermore, EELS, measured layer-by-layer, shows systematic changes in the local density of electronic states at each layer of MoS₂ with distance away from the Ti contact. These results show how contacts affect the innate structure of 2D materials and how thick a 2D material must be to preserve its expected properties, both of which have significant implications for device performance.   

Room-temperature ferroelectricity and switchable diode effect in α-In2Se3 thin layers

Room-temperature stable ferroelectricity is the key for establishing non-volatile high-density memories. However, reaching the thin film limit is a long-term challenge in conventional ferroelectrics due to the critical size effect. With stable layered structure and weak interlayer couplings, van der Waals material is one of the promising candidates for developing ultra-thin 2D ferroelectrics. Here, we report room-temperature intrinsic ferroelectricity in layered α-In₂Se₃ with film thickness as thin as 5 nm. The out-of-plane ferroelectricity is evidenced by the observation of reversible spontaneous electric polarization. The observed domain size is around tens μm², indicating enough robustness for device potential. The electric hysteresis loop measurement suggests a relative low coercive electric field at ~2 X 10⁵ V/cm. We further demonstrate a practical ferroelectric diode in the form of graphene/α-In₂Se₃ heterojunction. The Schottky barrier in the device can be controlled by switching the electric polarization, exhibiting a tunable diode effect. Our results offer a new way for developing novel electronic devices based on 2D ferroelectrics.   

Electronics in Flatland

2D materials such as graphene, transition metal dichalcogenides and topological insulators have opened up avenues in beyond-CMOS device concepts. We will discuss our work involving single or many-particle 2D-2D tunneling, leading to transistors with negative differential resistance. We also explore spintronics in these systems for novel logic and memory devices. We will also discuss the use of these materials in less esoteric, but more practical high frequency, mechanically flexible FETs for IoT applications.
  

Controllable Oxidation for Making Ambipolar or P-type MoOx/MoS2 Transistors

Recently, semiconducting layered materials attract much attention because of prominent potential for making ultrathin body field-effect transistors (FETs). Here, we show controllable oxidation to convert MoS₂ flakes to either ambipolar or p-type FETs. The MoS₂ flakes were heated under ozone exposure for several hours. The surface of oxidized MoS₂ was examined by scanning tunneling microscope. Further, we made FET devices on MoS₂ flakes with thickness less than 30 nm. Before ozone oxidation, the FET devices presented n-type features that are consistent with previous reports. After oxidation, the devices showed Schottky contact properties that were carefully inspected at different bias and gating voltages. The MoS₂ FETs after minor oxidation showed an ambipolar feature and electron transport revealed Mott's 2D variable range hopping. On the other hand, after strong oxidation, the devices presented p-type FET behaviors. The electron transport and transfer characteristics of the p-type FETs were explored as well. From experimental data, we propose a band diagram to explain the oxidation, the formation of MoO_x/MoS₂ heterostructure, and the electron transport behaviors.   

Reconfigurable Diodes Based on Vertical WSe2 Transistors with van der Waals Bonded Contacts

New device concepts can increase the functionality of scaled electronic devices, with reconfigurable diodes allowing the design of more compact logic gates being one of the examples. In recent years, there has been significant interest in creating reconfigurable diodes based on ultra-thin transition metal dichalcogenide crystals. Thanks to their large surface areas, these devices were constructed under planar geometry and device characteristics are controlled by electrostatic gating through rather complex two independent local gates or ionic-liquid doping process. In this talk, we demonstrate similar reconfigurable diode actions in a WSe₂ transistor by only utilizing van der Waals bonded graphene and Co/ h-BN contacts. Towards this, we first characterize the charge injection efficiencies into WSe₂ by graphene and Co/h-BN contacts. While Co/h-BN contact results in nearly Schottky barrier-free charge injection, graphene/WSe₂ interface has an average barrier height of ~ 80 meV. By taking the advantage of the electrostatic transparency of graphene and the different work-function values of graphene and Co/h-BN, we construct vertical devices where we demonstrate different gate-tunable diode actions. Our device architecture reveals the opportunities for exploring new device concepts.   

Improved On-Off Ratio in Black Phosphorus Field-Effect Transistors with True Ohmic Contacts

Two-dimensional (2D) materials have emerged in recent years as a candidate for use in electronic and optoelectronic applications due to their unique properties. One such layered 2D material that has attracted much interest is black phosphorus (BP). BP possesses a thickness-dependent band gap of 0.3 - 2.0 eV, making it a suitable material for field effect transistor (FET) applications. Recent studies of BP FETs have shown ambipolar transport characteristics with very high anisotropic hole mobility. However, several issues still remain. Rapid oxidation in ambient conditions and a relatively low ON/OFF ratio of few-layer BP FETs makes them unsuitable for use in low-power digital electronics. In this work, we present a strategy to fabricate BP FETs that simultaneously overcomes these limitations. The channel is fully passivated by hexagonal boron-nitride layers, protecting it from degradation. We also demonstrate that the relatively low ON/OFF ratio of few-layer BP FETs with Schottky contacts is largely caused by the OFF-state leak-current injected from the drain side, which can be suppressed by the achievement of true ohmic contacts while maintaining the large ON-state current. Consequently, we have realized a high ON/OFF ratio exceeding 10⁶ in p-type few-layer BP FETs.   

Pseudospintronics with Phosphorene Nanoribbons

Near-equilibrium electron transport in metallic phosphorene nanoribbons takes place in the states whose wavefunctions are located near the edges of the ribbon. Here, we show that electrical manipulation of these edge states provides a platform for the implementation of two different schemes of pseudospin electronics, a form of electronics based upon manipulation of tunable equivalents of the spin-one-half degree of freedom, i.e., the pseudospin. In each pseudospin electronics scheme, two different devices have been proposed: a pseudospin field-effect transistor (FET) for the purpose of generating pseudospin-polarized current and a pseudospin valve that operates similar to conventional spin valves. In each pseudospin scheme, we investigate the performance of the pseudospin FET and the pseudospin valve. The results presented here provide new avenues for realization of pseudospin electronics.   

High Mobility Palladium Diselenide (PdSe2) Field Effect Transistors Using heavily n-doped Graphene Contacts

Two dimensional materials comprised of puckered pentagonal layers could enable a variety of optoelectronic, piezoelectronic, spintronic, and valleytronic applications due to the low symmetry of the lattice structure. PdSe₂ is one such material of particular interest because of its high electron mobility and excellent chemical stability. However, in spite of its relatively small bandgap, the performance of few-layer PdSe₂ field-effect transistors (FETs) has been largely limited by the presence of a substantial Schottky barrier, which is likely due to Fermi-leveling pinning. In this work, we report the fabrication of high mobility n-type PdSe₂ FETs with hexagonal boron nitride passivated channels and graphene contacts. We achieve a typical room temperature field-effect mobility of ~ 140 cm² V^-1 s^-1 in few-layer PdSe₂ FETs, which increases to ~500 cm² V^-1 s^-1 at 77 K. The low-temperature mobility is further improved by heavily n-doping the graphene contacts.   

Persistent and Reversible Electrostatic Control of Doping in Graphene/BN Heterostructures

Heterostructures of graphene and hexagonal boron nitride (BN) are highly tunable platforms that enable the study of novel physical phenomena and technologically promising nanoelectronic devices. Recent opto-electronic and scanning tunneling spectroscopy studies on graphene/BN heterostructures have shown that optical and local electric field excitation can be used to modify the electronic properties of these heterostructures. These previous techniques rely on an exposed or optically accessible graphene surface and thus are incompatible with standard top gating techniques. To address this issue, we have developed a new technique that uses substrate backgating to induce electric fields strong enough to ionize defects in the supporting bulk BN. We have optimized the effectiveness of our technique by studying its response to the applied electric field duration, BN thickness, and temperature over multiple devices. We will discuss the latest experimental progress on the development of our persistent and reversible electrostatic doping technique, and demonstrate its flexibility when used in conjunction with other doping methods.