N-type MoS2 Research Articles

GeAs as an emerging p-type van der Waals semiconductor and its application in p–n photodiodes

van der Waals (vdW) layered materials have shown great potential for future optoelectronic applications owing to their unique and variable properties. In particular, two-dimensional layered materials enable the creation of various circuital building blocks via vertical stacking, e.g. the vertical p–n junction as a key one. While numerous stable n-type layered materials have been discovered, p-type materials remain relatively scarce. Here, we report on the study of multilayer germanium arsenide (GeAs), another emerging p-type vdW layered material. We first verify the efficient hole transport in a multilayer GeAs field-effect transistor with Pt electrodes, which establish low contact potential barriers. Subsequently, we demonstrate a p–n photodiode featuring a vertical heterojunction of a multilayer GeAs and n-type MoS2 monolayer, exhibiting a photovoltaic response. This study promotes that 2D GeAs is a promising candidate for p-type material in vdW optoelectronic devices.

Ultrasensitive Phototransistor Based on Laser-Induced P-Type Doped WSe2/MoS2 Van der Waals Heterojunction

Out-of-plane p-n heterojunctions based on two-dimensional layered materials (2DLMs) with unusual physical characteristics are attracting extensive research attention for their application as photodetectors. However, the present fabrication method based on 2DLMs produces out-of-plane p-n homojunction devices with low photoresponsivity and detectivity. This work reports an ultrasensitive phototransistor based on a laser-induced p-doped WSe2/MoS2 van der Waals heterojunction. The laser treatment is used for p-doping WSe2 nanoflakes using high work function WOx. Then, an n-type MoS2 nanoflake is transferred onto the resulting p-doped WSe2 nanoflake. The built-in electric field of p-doped WSe2/MoS2 is stronger than that of pristine WSe2/MoS2. The p-n junction between p-doped WSe2 and MoS2 can separate more photogenerated electron–hole pairs and inject more electrons into MoS2 under laser illumination than pristine WSe2/MoS2. Thus, a high photoresponsivity (R) of ~1.28 × 105 A·W−1 and high specific detectivity (D*) of ~7.17 × 1013 Jones are achieved under the illumination of a 633 nm laser, which is approximately two orders higher than the best phototransistor based on a WSe2/MoS2 heterojunction. Our work provides an effective and simple method to enhance photoresponsivity and detectivity in two-dimensional (2D) heterojunction phototransistors, indicating the potential applications in fabricating high-performance photodetectors based on 2DLMs.

Monolithic 3D integration of back-end compatible 2D material FET on Si FinFET

The performance enhancement of integrated circuits relying on dimension scaling (i.e., following Moore’s Law) is more and more challenging owing to the physical limit of Si materials. Monolithic three-dimensional (M3D) integration has been considered as a powerful scheme to further boost up the system performance. Two-dimensional (2D) materials such as MoS2 are potential building blocks for constructing upper-tier transistors owing to their high mobility, atomic thickness, and back-end-of-line (BEOL) compatible processes. The concept to integrate 2D material-based devices with Si field-effect transistor (FET) is technologically important but the compatibility is yet to be experimentally demonstrated. Here, we successfully integrated an n-type monolayer MoS2 FET on a p-type Si fin-shaped FET with 20 nm fin width via an M3D integration technique to form a complementary inverter. The integration was enabled by deliberately adopting industrially matured techniques, such as chemical mechanical planarization and e-beam evaporation, to ensure its compatibility with the existing 3D integrated circuit process and the semiconductor industry in general. The 2D FET is fabricated using low-temperature sequential processes to avoid the degradation of lower-tier Si devices. The MoS2 n-FETs and Si p-FinFETs display symmetrical transfer characteristics and the resulting 3D complementary metal-oxide-semiconductor inverter show a voltage transfer characteristic with a maximum gain of ~38. This work clearly proves the integration compatibility of 2D materials with Si-based devices, encouraging the further development of monolithic 3D integrated circuits.

Effects of forming gas annealing on the doping of monolayer MoS2 crystals

Here we report the effects of forming gas (5% H2 in N2) annealing on the doping behavior of mechanically-exfoliated, natural n-type MoS2 monolayers. Although the fabricated MoS2 transistors exhibited an enhanced device performance because of the reduced contact resistance after annealing at 200 °C for 2 h, the corresponding Raman and photoluminescence spectra suggested that the forming gas annealing suppressed the n-type doping behavior of MoS2. X-ray photoelectron spectroscopy and Kelvin probe force microscopy measurements consistently suggested a downward movement of the Fermi level of the MoS2 monolayers after the forming gas annealing. These results demonstrate the importance of optimizing the process conditions during the forming gas annealing for the fabrication of MoS2 transistors and devices based on transition metal dichalcogenides.

Fast, Multi-Bit, and Vis-Infrared Broadband Nonvolatile Optoelectronic Memory with MoS2 /2D-Perovskite Van der Waals Heterojunction.

Nonvolatile optoelectronic memory (NVOM) integrating the functions of optical sensing and long-term memory can efficiently process and store a large amount of visual scene information, which has become the core requirement of multiple intelligence scenarios. However, realizing NVOM with vis-infrared broadband response is still challenging. Herein, the room temperature vis-infrared broadband NVOM based on few-layer MoS2 /2D Ruddlesden-Popper perovskite (2D-RPP) van der Waals heterojunction is realized. It is found that the 2D-RPP converts the initial n-type MoS2 into p-type and facilitates hole transfer between them. Furthermore, the 2D-RPP rich in interband states serves as an effective electron trapping layer as well as broadband photoresponsive layer. As a result, the dielectric-free MoS2 /2D-RPP heterojunction enables the charge to transfer quickly under external field, which enables a large memory window (104V), fast write speed of 20 µs, and optical programmable characteristics from visible light (405nm) to telecommunication wavelengths (i.e., 1550nm) at room temperature. Trapezoidal optical programming can produce up to 100 recognizable states (>6 bits), with operating energy as low as 5.1 pJ per optical program. These results provide a route to realize fast, low power, multi-bit optoelectronic memory from visible to the infrared wavelength.

High-Performance Solution-Processed 2D P-Type WSe2 Transistors and Circuits through Molecular Doping.

Semiconducting ink based on 2D single-crystal flakes with dangling-bond-free surfaces enables the implementation of high-performance devices on form-free substrates by cost-effective and scalable printing processes. However, the lack of solution-processed p-type 2D semiconducting inks with high mobility is an obstacle to the development of complementary integrated circuits. Here, a versatile strategy of doping with Br2 is reported to enhance the hole mobility by orders of magnitude for p-type transistors with 2D layered materials. Br2 -doped WSe2 transistors show a field-effect hole mobility of more than 27cm2 V-1 s-1 , and a high on/off current ratio of ≈107 , and exhibits excellent operational stability during the on-off switching, cycling, and bias stressing testing. Moreover, complementary inverters composed of patterned p-type WSe2 and n-type MoS2 layered films are demonstrated with an ultra-high gain of 1280 under a driving voltage (VDD ) of 7V. This work unveils the high potential of solution-processed 2D semiconductors with low-temperature processability for flexible devices and monolithic circuitry.

Electron-phonon calculations using a Wannier-based supercell approach: Applications to the monolayer MoS[formula omitted] mobility

We present a first-principles method to calculate electron-phonon coupling elements in atomic systems, and showcase its application to the evaluation of the phonon-limited mobility of n-type single-layer MoS2. The method combines a density functional theory (DFT) plane-wave supercell approach with a real-space maximally localized Wannier basis. It enables the calculation of electronic structure, phonon displacements with their corresponding frequencies, and real-space electron-phonon coupling elements on the same footing, without the need for density functional perturbation theory (DFPT) or Wannier interpolation. We report a low-field, intrinsic mobility of 274 cm2/V s at room temperature for MoS2, and highlight its dependence on carrier density and temperature. In addition, we compare our findings to the latest available modeling data and put them in perspective with the experimentally measured values. Based on these observations, the mobilities presented in this work appear to be compatible with experimental results, when taking into account other scattering sources. Hence, the proposed approach provides a reliable framework for mobility calculations that can be extended towards large-scale device simulations.

Metal contacts with Moire interfaces on WSe2 for ambipolar applications

The rational design of metal contacts on transition metal dichalcogenides can significantly improve the performance of 2D devices. We have previously shown that a Moire interface between n-type monolayer MoS2 and metal contacts enhances the stability of physisorptive interface sites, thereby enabling weaker Fermi level pinning and allowing easier variation of the Schottky barrier height at these interfaces. We extend these calculations to p-type and ambipolar WSe2 contacts in this work. The analysis shows that the Moire interfaces again have a weaker Fermi level pinning, while most metals have chemisorptive sites with stronger pinning. We find that the most stable site of Pd is a Moire site with an unusually low p-type Schottky barrier height (p-SBH), while Au has a metastable low p-SBH. In and Al retain their low n-type SBHs, which together with Pd enable ambipolar contacts by the choice of contact metals, indicating that WSe2 can be used for high-performance ambipolar devices with the rational design of contact metals.

Synthesis of a Selectively Nb-Doped WS2-MoS2 Lateral Heterostructure for a High-Detectivity PN Photodiode.

In this study, selective Nb doping (P-type) at the WS2 layer in a WS2-MoS2 lateral heterostructure via a chemical vapor deposition (CVD) method using a solution-phase precursor containing W, Mo, and Nb atoms is proposed. The different chemical activity reactivity (MoO3 > WO3 > Nb2O5) enable the separation of the growth temperature of intrinsic MoS2 to 700 °C (first grown inner layer) and Nb-doped WS2 to 800 °C (second grown outer layer). By controlling the Nb/(W+Nb) molar ratio in the solution precursor, the hole carrier density in the p-type WS2 layer is selectively controlled from approximately 1.87 × 107/cm2 at 1.5 at.% Nb to approximately 1.16 × 1013/cm2 at 8.1 at.% Nb, while the electron carrier density in n-type MoS2 shows negligible change with variation of the Nb molar ratio. As a result, the electrical behavior of the WS2-MoS2 heterostructure transforms from the N-N junction (0 at.% Nb) to the P-N junction (4.5 at.% Nb) and the P-N tunnel junction (8.1 at.% Nb). The band-to-band tunneling at the P-N tunnel junction (8.1 at.% Nb) is eliminated by applying negative gate bias, resulting in a maximum rectification ratio (105) and a minimum channel resistance (108 Ω). With this optimized photodiode (8.1 at.% Nb at Vg = -30 V), an Iphoto/Idark ratio of 6000 and a detectivity of 1.1 × 1014 Jones are achieved, which are approximately 20 and 3 times higher, respectively, than the previously reported highest values for CVD-grown transition-metal dichalcogenide P-N junctions.

Heterogeneous Integration of Atomically Thin Semiconductors for Non-von Neumann CMOS.

Atomically thin, 2D, and semiconducting transition metal dichalcogenides (TMDs) are seen as potential candidates for complementary metal oxide semiconductor (CMOS) technology in future nodes. While high-performance field effect transistors (FETs), logic gates, and integrated circuits (ICs) made from n-type TMDs such as MoS2 and WS2 grown at wafer scale have been demonstrated, realizing CMOS electronics necessitates integration of large area p-type semiconductors. Furthermore, the physical separation of memory and logic is a bottleneck of the existing CMOS technology and must be overcome to reduce the energy burden for computation. In this article, the existing limitations are overcome and for the first time, a heterogeneous integration of large area grown n-type MoS2 and p-type vanadium doped WSe2 FETs with non-volatile and analog memory storage capabilities to achieve a non-von Neumann 2D CMOS platform is introduced. This manufacturing process flow allows for precise positioning of n-type and p-type FETs, which is critical for any IC development. Inverters and a simplified 2-input-1-output multiplexers and neuromorphic computing primitives such as Gaussian, sigmoid, and tanh activation functions using this non-von Neumann 2D CMOS platform are also demonstrated. This demonstration shows the feasibility of heterogeneous integration of wafer scale 2D materials.

Suppressing Non-Radiative Relaxation through Single-Atom Metal Modification for Enhanced Fluorescence Efficiency in Molybdenum Disulfide Quantum Dots.

To enhance the fluorescence efficiency of semiconductor nanocrystal quantum dots (QDs), strategies via enhancing photo-absorption and eliminating non-radiative relaxation have been proposed. In this study, we demonstrate that fluorescence efficiency of molybdenum disulfide quantum dots (MoS2 QDs) can be enhanced by single-atom metal (Au, Ag, Pt, Cu) modification. Four-fold enhancement of the fluorescence emission of MoS2 QDs is observed with single-atom Au modification. The underlying mechanism is ascribed to the passivation of non-radiative surface states owing to the new defect energy level of Au in the forbidden band that can trap excess electrons in n-type MoS2 , increasing the recombination probability of conduction band electrons with valence band holes of MoS2 . Our results open an avenue for enhancing the fluorescence efficiency of QDs via the modification of atomically dispersed metals, and extend their scopes and potentials in a fundamental way for economic efficiency and stability of single-atom metals.

Controllable carrier concentration of two-dimensional TMDs by forming transition-metal suboxide layer for photoelectric devices

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have the potential to drive future innovation in the electronics industry. However, the controllability of charge concentration remains challenging due to the atomically thin channel, which perturbs the charge transport characteristics of nanodevices. Here, we demonstrate a strategy that uses the transition metal suboxide layer to modulate the photoelectrical characteristics of TMD channels. The carrier concentration in an n-type MoS2 channel is reduced from 2.05 × 1012 to 6.15 × 1010 cm−3, while it increases from 1.71 × 1010 to 2.76 × 1012 cm−3 for p-type WSe2 channels. Remarkably, the channel mobility remains unchanged or even slightly improves when the carrier concentration is appropriately tuned. Also, the homogenous channel is modulated into a photovoltaic homojunction with a tenfold enhancement of photoelectrical detectivity and response speed. The controllable strategy provides a simple design principle to realize high-performance 2D semiconductor-based optoelectronic and logic devices.

(Invited) Controlling and Using Optoelectronic Properties of MoS2 and WS2 Monolayers

Transition metal dichalcogenides (TMDCs) are an important class of materials because they can be reduced to 2D, where their quantum confined properties are easily manipulated for various applications. MoS2 is one of the most heavily studied TMDCs and is thermodynamically stable in the semiconducting 2H state. However, these monolayers are more catalytically active and have greater electron conductivity in the unstable, metallic (1T/1T’) state. Recently, we have demonstrated how to control the optoelectronic properties of MoS2 monolayers using n-butyl lithium immersion treatments and functional groups, where shorter treatment times heavily n-type dope MoS2 and longer treatment times fully convert the MoS2 from the semiconducting to metallic (1T/1T’) phase. By adding functional groups, the doped/converted MoS2 is stable for up to two weeks. In addition to MoS2, we have also explored how monolayers of semiconducting WS2 can be used as a contactless humidity sensor via photoluminescence. In this presentation, we will discuss the ways in which MoS2 and WS2 can be tuned for various optoelectronic properties.

Artificial Synapse Based on 1D/2D Hybrid Heterostructure

Molecular functional devices have been investigated through the study of molecular structure and its corresponding discrete orbital states since it determines the transport behaviors. [1,2] In contrast to the conventional strategy, we have demonstrated a novel strategy and design rule for realizing molecular-scale functions based on the energy band engineering in molecular heterojunction with two-dimensional (2D) semiconductors [3,4]. Recently, we have designed molecular heterojunction selector and the nonlinearity design rule with the role of the molecular dipole moment orientation which controls the band bending of the 2D semiconductor [4]. Likewise, various role of the molecular self-assembled monolayer (SAM) in hybrid heterostructure device with 2D semiconductor can give a novel and diverse functionality to the device that it can be a powerful platform for the next-generation functional devices. We implemented ferrocene substituted alkanethiol SAMs, which is a redox active molecule [5], in 2D MoS2 field-effect transistor (FET) structure. The redox state (oxidation or reduction) of the SAM is controlled through the gate bias and the oxidized molecular states (Fc -> Fc+) can act as a localized gate at the interface between molecule/2D semiconductor (MoS2) which the conductance of the n-type MoS2 channel can be modulated. This MoS2 FET with non-volatile multistate MoS2 channel can be suggested as electrochemically programmed molecular heterojunction artificial synapse.[1] L. A. Bumm, J. J. Arnold, M. T. Cygan, T. D. Dunbar, T. P. Burgin, L. Jones, D. L. Allara, J. M. Tour, P. S. Weiss, Science 271, 1705 ( 1996),.[2] H. Song, Y. Kim, Y. H. Jang, H. Jeong, M. A. Reed, T. Lee, Nature 462, 1039 ( 2009).[3] J. Shin, S. Yang, Y. Jang, J. S. Eo, T.-W. Kim, T. Lee, C.-H. Lee, G. Wang, Nat. Commun. 11, 1 (2020).[4] J. S. Eo, J. Shin, S. Yang, T. Jeon, J. Lee, S. Choi, C.-H. Lee, G. Wang, Adv. Sci. 2101390 (2021).[5] Y. Zhao, S. Bertolazzi, M. S. Maglione, C. Rovira, M. Mas-Torrent, P. Samori, Adv. Mater. 32, 2000740 (2020).

Shift of switching threshold in low-dimensional semiconductor-based complementary inverters via inkjet printing

MoS2 crystals grown by chemical vapor deposition are suited for realization of practical 2D semiconductor-based electronics. In order to construct complementary circuits with n-type MoS2, another p-type semiconductor, whose performance can be adjusted corresponding to that of MoS2 in the limited chip area, has to be sought. Herein, we present a method for tuning switching threshold voltages of complementary inverters simply via inkjet printing without changing their channel dimensions. Random networks of inkjet printed single-walled carbon nanotubes are formed as p-channels beside MoS2, and their density and thickness are controlled by varying the number of printed layers. As a result, p-type transistor characteristics as well as inverter characteristics are facilely tuned only by varying the number of printed layers.

Optical and Material Characteristics of MoS2/Cu2O Sensor for Detection of Lung Cancer Cell Types in Hydroplegia

In this study, n-type MoS2 monolayer flakes are grown through chemical vapor deposition (CVD), and a p-type Cu2O thin film is grown via electrochemical deposition. The crystal structure of the grown MoS2 flakes is analyzed through transmission electron microscopy. The monolayer structure of the MoS2 flakes is verified with Raman spectroscopy, multiphoton excitation microscopy, atomic force microscopy, and photoluminescence (PL) measurements. After the preliminary processing of the grown MoS2 flakes, the sample is then transferred onto a Cu2O thin film to complete a p-n heterogeneous structure. Data are confirmed via scanning electron microscopy, SHG, and Raman mapping measurements. The luminous energy gap between the two materials is examined through PL measurements. Results reveal that the thickness of the single-layer MoS2 film is 0.7 nm. PL mapping shows a micro signal generated at the 627 nm wavelength, which belongs to the B2 excitons of MoS2 and tends to increase gradually when it approaches 670 nm. Finally, the biosensor is used to detect lung cancer cell types in hydroplegia significantly reducing the current busy procedures and longer waiting time for detection. The results suggest that the fabricated sensor is highly sensitive to the change in the photocurrent with the number of each cell, the linear regression of the three cell types is as high as 99%. By measuring the slope of the photocurrent, we can identify the type of cells and the number of cells.

Wafer-Scale Demonstration of MBC-FET and C-FET Arrays Based on Two-Dimensional Semiconductors.

Two-dimentional semiconductors have shown potential applications in multi-bridge channel field-effect transistors (MBC-FETs) and complementary field-effect transistors (C-FETs) due to their atomic thickness, stackability, and excellent electrical properties. However, the exploration of MBC-FET and C-FET based on large-scale 2D semiconductors is still lacking. Here, based on a reliable vertical stacking of wafer-scale 2D semiconductors, large-scale MBC-FETs and C-FETs using n-type MoS2 and p-type MoTe2 are successfully fabricated. The drive current of an MBC-FET with two layers of MoS2 channel (20µm/10µm) is up to 60µA under 1V bias. Compared with the single-gate MoS2 FET, the carrier mobility of MBC-FET is 2.3 times higher and the sub-threshold swing is 70% smaller. Furthermore, NAND and NOR logic circuits are also constructed based on two vertically stacked MoS2 channels. Then, C-FET arrays are fabricated by 3D integrating n-type MoS2 FET and p-type MoTe2 FET, which exhibit a voltage gain of 7V/V when VDD = 4V. In addition, this C-FET device can directly convert light signals to an electrical digital signal within a single device. The demonstration of MBC-FET and C-FET based on large-scale 2D semiconductors will promote the application of 2D semiconductors in next-generation circuits.

Polyoxometalate derived p-n heterojunction for optimized reaction interface and improved HER

MoS2 is a typical electrocatalyst for hydrogen evolution reaction (HER), but the HER activity is spoilt by intensive adsorption towards H*, which requires further improvement. For n-type MoS2, the construction of p-n heterojunction with p-type MoO3 can reverse this situation, because inner electronic field in p-n heterojunction can facilitate H* desorption. Based on this hypothesis, p-n heterojunction is built between MoS2 and MoO3 with polyoxometalate compound as precursor. The obtained MoO3/MoS2 exhibits excellent HER activity, which only requires 68 mV to obtain 10 mA/cm2. With MoO3/MoS2 as cathode material and Zn slice as anode, Zn-H+ battery is assembled. Its open circuit voltage achieves 1.11 V with short circuit current 151.4 mA/cm2. The peak power density of this Zn-H+ battery reaches 47.6 mW/cm2. When discharge at 10 mA/cm2, the specific capacity and energy density reach 728 mAh/g and 759 Wh/kg. In this process, H2 production rate of Zn-H+ battery achieves 364 μmol/h with Faradic efficiency 97.8%. It realizes H2 production and electricity generation simultaneously.

An Annealing Accelerator for Ising Spin Systems Based on In-Memory Complementary 2D FETs.

Metaheuristic algorithms such as simulated annealing (SA) are often implemented for optimization in combinatorial problems, especially for discreet problems. SA employs a stochastic search, where high-energy transitions ("hill-climbing") are allowed with a temperature-dependent probability to escape local optima. Ising spin glass systems have properties such as spin disorder and "frustration" and provide a discreet combinatorial problem with a high number of metastable states and ground-state degeneracy. In this work, subthreshold Boltzmann transport is exploited in complementary 2D field-effect transistors (p-type WSe2 and n-type MoS2 ) integrated with an analog, nonvolatile, and programmable floating-gate memory stack to develop in-memory computing primitives necessary for energy- and area-efficient hardware acceleration of SA for Ising spin systems. Search acceleration of >800× is demonstrated for 4 × 4 ferromagnetic, antiferromagnetic, and spin glass systems using SA compared to an exhaustive search using a brute force trial at miniscule total energy expenditure of ≈120 nJ. The hardware-realistic numerical simulations further highlight the astounding benefits of SA in accelerating the search for larger spin lattices.

Hole doping effect of MoS2 via electron capture of He+ ion irradiation

Beyond the general purpose of noble gas ion sputtering, which is to achieve functional defect engineering of two-dimensional (2D) materials, we herein report another positive effect of low-energy (100 eV) He+ ion irradiation: converting n-type MoS2 to p-type by electron capture through the migration of the topmost S atoms. The electron capture ability via He+ ion irradiation is valid for supported bilayer MoS2; however, it is limited at supported monolayer MoS2 because the charges on the underlying substrates transfer into the monolayer under the current condition for He+ ion irradiation. Our technique provides a stable and universal method for converting n-type 2D transition metal dichalcogenides (TMDs) into p-type semiconductors in a controlled fashion using low-energy He+ ion irradiation.