Multi-Bit Analog Transmission Enabled by Electrostatically Reconfigurable Ambipolar and Anti-Ambipolar Transport

Various analog applications, such as phase switching, have been demonstrated using either ambipolar or anti-ambipolar transport in two-dimensional materials. However, the availability of only one transport mode severely limits the application scope and range. This work demonstrates electrostatically reconfigurable and tunable ambipolar and anti-ambipolar transport in the same field-effect transistor using a photoactive ambipolar WSe2 channel with gate-controlled channel and Schottky barriers. This enables the realization of in-phase, out-of-phase, and double-frequency sinusoidal output signals under dark and illumination conditions. The output waveforms were used to generate phase-, frequency-, and amplitude-modulated analog schemes for 2- and 3-bit data transmission. Evaluation of all possible schemes for their power consumption, error probability, and implementation complexity highlights the importance of switching between ambipolar and anti-ambipolar modes of transport for best transmission performance. A dual-metal contact transistor with improved linearity for harmonic and excess power suppression demonstrates further performance enhancement. Generic device architecture and operation makes this work adaptable to any ambipolar material amenable to electrostatic control

Atomistic Modeling of van der Waals Heterostructures with Group‑6 and Group‑7 Monolayer Transition Metal Dichalcogenides for Near Infrared/Short-wave Infrared Photodetection

In this work, heterostructures formed with vertical stacking of two-dimensional (2D) layered materials are systematically studied. Considering near infrared (NIR)/short-wave-infrared (SWIR) photodetection, van der Waals (vdW) heterostructures with various possible combinations of group-6 and group-7 monolayer transition metal dichalcogenides (TMDs) are explored. Single-layer distorted 1T ReS2, being a dynamically stable semiconducting material, is adopted as the group-7 constituent. On the other hand, as group-6 constituents, five different semiconducting monolayer TMDs, viz., MoS2, WS2, MoSe2, WSe2, and MoTe2 have been chosen. A rational selection of group-6 TMDs based on intrinsic properties of individual materials as well as their heterointerfaces with single-layer ReS2 is demonstrated here to obtain type-II vdW heterostructures which can ensure efficient generation, separation, and collection of charge carriers resulting in significant improvement in photodetection metrics

Accurate Threshold Voltage Reliability Evaluation of Thin Al₂O₃ Top-Gated Dielectric Black Phosphorous FETs Using Ultrafast Measurement Pulses

Few-layer black phosphorus (BP) has attracted significant interest in recent years due to electrical and photonic properties that are far superior to those of other two-dimensional layered semiconductors. The study of long term electrical stability and reliability of black phosphorus field effect transistors (BP-FETs) with technologically relevant thin, and device-selective, gate dielectrics, stressed under realistic (closer to operation) bias and measured using state-of-the-art ultrafast reliability characterization techniques, is essential for their qualification and use in different applications. In this work, air-stable BP-FETs with a thin top-gated dielectric (15 nm Al2O3, SiO2 equivalent thickness of 5 nm) were fabricated and comprehensively characterized for threshold voltage (Vth) instability under negative gate bias stress at various measurement delays (tm), stress biases (VGSTR), temperatures (T), and stress times (tstr) for the first time. Thin top-gated oxide enables low VGSTR that is closer to the operating condition and ultrafast Vth measurements with low delay (tm = 10 μs, due to high drain current) that ensure minimal recovery. The resultant time kinetics of Vth degradation (ΔVth) shows fast saturation at longer stress times and low-temperature activation energy. Vth instability in these top-gated devices is suggested to be dominated by hole trapping, which is modeled using first-order equations at different VGSTR and T. It is shown that measurements using larger tm show lower degradation magnitude that do not saturate due to recovery artifacts and give inaccurate estimation of hole trap densities. Conventional, thick, and global back-gated oxide BP-FETs were also fabricated and characterized for varying tm (1 ms being the lowest due to a low drain current level for thick oxide), VGSTR, and T to benchmark our top-gated results. Nonsaturating ΔVth in the back-gated devices is shown to result from recovery artifacts due to the large tm (1 ms and greater) values. Finally, using a VGSTR and T-dependent first-order model, we show that the top-gated Al2O3 BP-FETs with scaled gate oxide thickness can match state-of-the-art Si reliability specifications at operating voltage and room/elevated temperature

Interfacial n‑Doping Using an Ultrathin TiO₂ Layer for Contact Resistance Reduction in MoS₂

We demonstrate a low and constant effective Schottky barrier height (Φ_B ∼ 40 meV) irrespective of the metal work function by introducing an ultrathin TiO₂ ALD interfacial layer between various metals (Ti, Ni, Au, and Pd) and MoS₂. Transmission line method devices with and without the contact TiO₂ interfacial layer on the same MoS₂ flake demonstrate reduced (24×) contact resistance (<i>R</i>_C) in the presence of TiO₂. The insertion of TiO₂ at the source-drain contact interface results in significant improvement in the on-current and field effect mobility (up to 10×). The reduction in <i>R</i>_C and Φ_B has been explained through interfacial doping of MoS₂ and validated by first-principles calculations, which indicate metallic behavior of the TiO₂-MoS₂ interface. Consistent with DFT results of interfacial doping, X-ray photoelectron spectroscopy (XPS) data also exhibit a 0.5 eV shift toward higher binding energies for Mo 3d and S 2p peaks in the presence of TiO₂, indicating Fermi level movement toward the conduction band (n-type doping). Ultraviolet photoelectron spectroscopy (UPS) further corroborates the interfacial doping model, as MoS₂ flakes capped with ultrathin TiO₂ exhibit a reduction of 0.3 eV in the effective work function. Finally, a systematic comparison of the impact of selective doping with the TiO₂ layer under the source-drain metal relative to that on top of the MoS₂ channel shows a larger benefit for transistor performance from the reduction in source-drain contact resistance

Few-Layer MoS₂ <i>p</i>‑Type Devices Enabled by Selective Doping Using Low Energy Phosphorus Implantation

<i>P</i>-type doping of MoS₂ has proved to be a significant bottleneck in the realization of fundamental devices such as <i>p-n</i> junction diodes and <i>p</i>-type transistors due to its intrinsic <i>n</i>-type behavior. We report a CMOS compatible, controllable and area selective phosphorus plasma immersion ion implantation (PIII) process for <i>p</i>-type doping of MoS₂. Physical characterization using SIMS, AFM, XRD and Raman techniques was used to identify process conditions with reduced lattice defects as well as low surface damage and etching, 4X lower than previous plasma based doping reports for MoS₂. A wide range of nondegenerate to degenerate <i>p</i>-type doping is demonstrated in MoS₂ field effect transistors exhibiting dominant hole transport. Nearly ideal and air stable, lateral homogeneous <i>p-n</i> junction diodes with a gate-tunable rectification ratio as high as 2 × 10⁴ are demonstrated using area selective doping. Comparison of XPS data from unimplanted and implanted MoS₂ layers shows a shift of 0.67 eV toward lower binding energies for Mo and S peaks indicating <i>p</i>-type doping. First-principles calculations using density functional theory techniques confirm <i>p</i>-type doping due to charge transfer originating from substitutional as well as physisorbed phosphorus in top few layers of MoS₂. Pre-existing sulfur vacancies are shown to enhance the doping level significantly

Polarity-Tunable Photocurrent through Band Alignment Engineering in a High-Speed WSe₂/SnSe₂ Diode with Large Negative Responsivity

Excellent light–matter interaction and a wide range of thickness-tunable bandgaps in layered vdW materials coupled by the facile fabrication of heterostructures have enabled several avenues for optoelectronic applications. Realization of high photoresponsivity at fast switching speeds is a critical challenge for 2D optoelectronics to enable high-performance photodetection for optical communication. Moving away from conventional type-II heterostructure pn junctions towards a WSe2/SnSe2 type-III configuration, we leverage the steep change in tunneling current along with a light-induced heterointerface band shift to achieve high negative photoresponsivity, while the fast carrier transport under tunneling results in high speed. In addition, the photocurrent can be controllably switched from positive to negative values, with ∼104× enhancement in responsivity, by engineering the band alignment from type-II to type-III using either the drain or the gate bias. This is further reinforced by electric-field dependent interlayer band structure calculations using density functional theory. The high negative responsivity of 2 × 104 A/W and fast response time of ∼1 μs coupled with a polarity-tunable photocurrent can lead to the development of next-generation multifunctional optoelectronic devices

An electroplating-based plasmonic platform for giant emission enhancement in monolayer semiconductors

Two dimensional semiconductors have attracted considerable attention owing to their exceptional electronic and optical characteristics. However, their practical application has been hindered by the limited light absorption resulting from their atomically thin thickness and low quantum yield. A highly effective approach to manipulate optical properties and address these limitations is integrating subwavelength plasmonic nanostructures with these monolayers. In this study, we employed electron beam lithography and electroplating technique to fabricate a gold nanodisc (AuND) array capable of enhancing the photoluminescence (PL) of monolayer MoS$_2$ giantly. Monolayer MoS$_2$ placed on the top of the AuND array yields up to 150-fold PL enhancement compared to that on a gold film. We explain our experimental findings based on electromagnetic simulations

Near-Direct Bandgap WSe₂/ReS₂ Type-II pn Heterojunction for Enhanced Ultrafast Photodetection and High-Performance Photovoltaics

Pn heterojunctions comprising layered van der Waals (vdW) semiconductors have been used to demonstrate current-rectifiers, photodetectors, and photovoltaic devices. However, a direct or near-direct heterointerface bandgap for enhanced photogeneration in high light-absorbing few-layer vdW materials remains unexplored. In this work, for the first time, density functional theory calculations show that the heterointerface of few-layer group-6 transition metal dichalcogenide (TMD) WSe2 with group-7 ReS2 results in a sizable (0.7 eV) near-direct type-II bandgap. The interlayer IR bandgap is confirmed through IR photodetection, and microphotoluminescence measurements demonstrate type-II alignment. Few-layer flakes exhibit ultrafast response time (5 μs), high responsivity (3 A/W), and large photocurrent-generation and responsivity-enhancement at the hetero-overlap region (10–100×). Large open-circuit voltage of 0.64 V and short-circuit current of 2.6 μA enable high output electrical power. Finally, long-term air-stability and facile single contact metal fabrication process make the multifunctional few-layer WSe2/ReS2 heterostructure diode technologically promising for next-generation optoelectronics

Solution-Processed Poly(3,4-ethylenedioxythiophene) Thin Films as Transparent Conductors: Effect of <i>p</i>‑Toluenesulfonic Acid in Dimethyl Sulfoxide

Conductivity enhancement of thin transparent films based on poly­(3,4-ethylenedioxythiophene)–poly­(styrenesulfonate) (PEDOT–PSS) by a solution-processed route involving mixture of an organic acid and organic solvent is reported. The combined effect of p-toluenesulfonic acid and dimethyl sulfoxide on spin-coated films of PEDOT–PSS on glass substrates, prepared from its commercially available aqueous dispersion, was found to increase the conductivity of the PEDOT–PSS film to ∼3500 S·cm–1 with a high transparency of at least 94%. Apart from conductivity and transparency measurements, the films were characterized by Raman, infrared, and X-ray photoelectron spectroscopy along with atomic force microscopy and secondary ion mass spectrometry. Combined results showed that the conductivity enhancement was due to doping, rearrangement of PEDOT particles owing to phase separation, and removal of PSS matrix throughout the depth of the film. The temperature dependence of the resistance for the treated films was found to be in accordance with one-dimensional variable range hopping, showing that treatment is effective in reducing energy barrier for interchain and interdomain charge hopping. Moreover, the treatment was found to be compatible with flexible poly­(ethylene terephthalate) (PET) substrates as well. Apart from being potential candidates to replace inorganic transparent conducting oxide materials, the films exhibited stand-alone catalytic activity toward I–/I3– redox couple as well and successfully replaced platinum and fluorinated tin oxide as counter electrode in dye-sensitized solar cells