All-2D ReS2 transistors with split gates for logic circuitry.

Two-dimensional (2D) semiconductors, such as transition metal dichalcogenides (TMDs) and black phosphorus, are the most promising channel materials for future electronics because of their unique electrical properties. Even though a number of 2D-materials-based logic devices have been demonstrated to date, most of them are a combination of more than two unit devices. If logic devices can be realized in a single channel, it would be advantageous for higher integration and functionality. In this study we report high-performance van der Waals heterostructure (vdW) ReS2 transistors with graphene electrodes on atomically flat hBN, and demonstrate a NAND gate comprising a single ReS2 transistor with split gates. Highly sensitive electrostatic doping of ReS2 enables fabrication of gate-tunable NAND logic gates, which cannot be achieved in bulk semiconductor materials because of the absence of gate tunability. The vdW heterostructure NAND gate comprising a single transistor paves a novel way to realize "all-2D" circuitry for flexible and transparent electronic applications.

Intimate Ohmic contact to two-dimensional WSe2 via thermal alloying.

The most important interface in semiconductor devices is the interface between the semiconductor and the first layer of the metal contact. However, the van der Waals (vdWs) gap in two-dimensional (2D) materials hindered the formation of an intimate contact between the 2D material and the metal electrode, limiting the device performances. We demonstrated a gapless Ohmic contact to 2D WSe2 by forming a Pt-W-Se alloy, which significantly improved the device performances (contact resistance, current on/off ratio, output current density, field-effect mobility, and hysteresis) of the 2D WSe2 field-effect transistor. The contact resistance to 2D WSe2 was reduced by more than seven orders of magnitude after thermal alloying. The disappearance of the vdW gap confirmed by scanning transmission electron microscopy enhanced the hole conduction and quenched the electron conduction. Our strategy of metallurgical alloying is effective to form a low-resistance stable Ohmic contact to WSe2, which paves the way for utilization of the full potential of 2D materials.

High-energy proton irradiation damage on two-dimensional hexagonal boron nitride.

The dielectric layer, which is an essential building block in electronic device circuitry, is subject to intrinsic or induced defects that limit its performance. Nano-layers of hexagonal boron nitride (h-BN) represent a promising dielectric layer in nano-electronics owing to its excellent electronic and thermal properties. In order to further analyze this technology, two-dimensional (2D) h-BN dielectric layers were exposed to high-energy proton irradiation at various proton energies and doses to intentionally introduce defective sites. A pristine h-BN capacitor showed typical degradation stages with a hard breakdown field of 10.3 MV cm-1, while h-BN capacitors irradiated at proton energies of 5 and 10 MeV at a dose of 1 × 1013 cm-2 showed lower hard breakdown fields of 1.6 and 8.3 MV cm-1, respectively. Higher leakage currents were observed under higher proton doses at 5 × 1013 cm-2, resulting in lower breakdown fields. The degradation stages of proton-irradiated h-BN are similar to those of defective silicon dioxide. The degradation of the h-BN dielectric after proton irradiation is attributed to Frenkel defects created by the high-energy protons, as indicated by the molecular dynamics simulation. Understanding the defect-induced degradation mechanism of h-BN nano-layers can improve their reliability, paving the way to the implementation of 2D h-BN in advanced micro- and nano-electronics.

Enhancing ambipolar carrier transport of black phosphorus field-effect transistors with Ni-P alloy contacts.

Reducing power consumption and leakage current in complementary metal-oxide semiconductors (CMOSs) has gained importance for further increasing the transistor density. An effective strategy to achieve this is to use ambipolar carrier transport that can exploit both holes and electrons in a single transistor. We report the enhancement of ambipolar behavior in black phosphorus (BP) field-effect transistors (FET) by forming a low-resistance Ni2P alloy contact via low-vacuum annealing at 250 °C, where the transformation of BP into Ni2P alloy selectively occurred at the source/drain electrodes with the BP channel remaining pristine. The N-channel current on/off ratio and field-effect electron carrier mobility of BP FETs were improved by 98% and 1290%, respectively. Our results suggest that high-performance ambipolar BP FETs with low-resistance ohmic contacts can be achieved via low-temperature vacuum annealing for next-generation CMOS applications.

A facile method for highly uniform GaN-based nanorod light-emitting diodes with InGaN/GaN multi-quantum-wells.

We report on a simple and reproducible method for fabricating InGaN/GaN multi-quantum-well (MQW) nanorod light-emitting diodes (LEDs), prepared by combining a SiO2 nanosphere lithography and dry-etch process. Focused-ion-beam (FIB)-deposited Pt was contacted to both ends of the nanorod LEDs, producing bright electroluminescence from the LEDs under forward bias conditions. The turn-on voltage in these nanorod LEDs was higher (13 V) than in companion thin film devices (3 V) and this can be attributed to the high contact resistance between the FIB-deposited Pt and nanorod LEDs and the damage induced by inductively-coupled plasma and Ga + -ions. Our method to obtain uniform MQW nanorod LEDs shows promise for improving the reproducibility of nano-optoelectronics.

Electroluminescence from InGaN/GaN multi-quantum-wells nanorods light-emitting diodes positioned by non-uniform electric fields.

We report that the nanorod light-emitting diodes (LEDs) with InGaN/GaN multi-quantum-wells (MQWs) emitted bright electroluminescence (EL) after they were positioned and aligned by non-uniform electric fields. Firstly, thin film LED structures with MQWs on sapphire substrate were coated with SiO(2) nanospheres, followed by inductively-coupled plasma etch to create nanorod-shapes with MQWs, which were transferred to the pre-patterned SiO(2)/Si wafer. This method allowed us to obtain nanorod LEDs with uniform length, diameter and qualities. Dielectrophoretic force created by non-uniform electric field was very effective at positioning the processed nanorods on the pre-patterned contacts. After aligned by non-uniform electric field, we observed bright EL from many nanorods, which had both cases (p-GaN/MQWs/n-GaN or n-GaN/MQWs/p-GaN). Therefore, bright ELs at different locations were observed under the various bias conditions.

Nanostructured n-ZnO / thin film p-silicon heterojunction light-emitting diodes.

Electroluminescence (EL) was obtained from a p-Si (100) thin film/nanostructured n-ZnO heterojunction diode fabricated by a simple dielectrophoresis (DEP) method. The Si substrate was pre-patterned with electrodes and an insulating separation layer by a standard photolithographic process. ZnO nanostructures were formed by a simple solution chemistry and subsequently transferred to the pre-patterned substrate. Application of the DEP force at a frequency of 100 kHz and 6 V peak-to-peak voltage allowed precise positioning of the ZnO nanostructures at the edge of the metal electrodes. The physically formed p-Si (100) thin film/nanostructured n-ZnO heterojunction displayed multi-color emission from the ZnO near band edge as well as emission from defective states within the ZnO band gap.