Back-gated Field-effect Transistors Research Articles

Janus Electronic Devices with Ultrathin High-κ Gate Dielectric Directly Integrated on 1T'-MoTe2.

Integrating high-quality dielectrics with two-dimensional (2D) transition metal chalcogenides (TMDCs) is crucial for high-performance electronics. However, the lack of dangling bonds on 2D material surfaces complicates direct dielectric deposition. We propose using atomic layer deposition (ALD) to integrate ultrathin high-κ dielectric directly on 1T'-MoTe2 surfaces, facilitating the creation of high-performance back-gated field-effect transistors (FETs). Exploiting 1T'-MoTe2's natural oxidation in ambient conditions, we directly deposit dense and uniform HfO2 dielectric films below 5 nm, achieving an equivalent oxide thickness (EOT) of 0.97 nm. The resulting back-gate transistors, with a monolayer MoSSe on HfO2/1T'-MoTe2, show a current on/off ratio over 105 and operate at low voltages (<1 V), indicating high gating efficiency and a charge carrier mobility of 2.93 cm2V-1s-1. Additionally, we demonstrate a 6 × 5 bottom-gated array of MoSSe transistors using all-1T'-MoTe2 electrodes, achieving an 86.7% sample yield. Our approach also enables the creation of various integrated logic circuits such as inverters, NAND, and NOR gates. This research offers a feasible method for integrating high-κ dielectric films using industrially compatible ALD processes, providing excellent thickness control, uniformity, and scalability for 2D electronic devices.

High-Fidelity Transfer of 2D Semiconductors and Electrodes for van der Waals Devices.

As traditional silicon-based materials almost reach their limits in the post-Moore era, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been regarded as next-generation semiconductors for high-performance electrical and optical devices. Chemical vapor deposition (CVD) is a widely used technique for preparing large-area and high-quality TMDCs. Yet, it suffers from the challenge of transfer due to the strong interaction between 2D materials and substrates. The traditional PMMA-assisted wet etching method tends to induce damage, wrinkles, and inevitable polymer residues. In this work, we propose an etch-free and clean transfer method via a water intercalation strategy for TMDCs, ensuring a high-fidelity, wrinkle-free, and crack-free transfer with negligible residues. Furthermore, metal electrodes can also be transferred via this method and back-gate field-effect transistors (FETs) based on CVD-grown monolayer WSe2 with van der Waals (vdW) metal/semiconductor contacts are fabricated. Compared to the PMMA-assisted transfer method (∼1.2 cm2 V-1 s-1 hole mobility with ∼2 × 106 ON/OFF ratio), our high-fidelity transfer method significantly enhances the electrical performance of WSe2 FET over one order of magnitude, achieving a hole mobility of ∼43 cm2 V-1 s-1 and a high ON/OFF ratio of ∼5 × 107 in air at room temperature.

WS2 Nanotube Transistor for Photodetection and Optoelectronic Memory Applications.

Nanotube and nanowire transistors hold great promises for future electronic and optoelectronic devices owing to their downscaling possibilities. In this work, a single multi-walled tungsten disulfide (WS2) nanotube is utilized as the channel of a back-gated field-effect transistor. The device exhibits a p-type behavior in ambient conditions, with a hole mobility µp ≈ 1.4 cm2V-1s-1 and a subthreshold swing SS ≈ 10 V dec-1. Current-voltage characterization at different temperatures reveals that the device presents two slightly different asymmetric Schottky barriers at drain and source contacts. Self-powered photoconduction driven by the photovoltaic effect is demonstrated, and a photoresponsivity R ≈ 10 mAW-1 at 2V drain bias and room temperature. Moreover, the transistor is tested for data storage applications. A two-state memory is reported, where positive and negative gate pulses drive the switching between two different current states, separated by a window of 130%. Finally, gate and light pulses are combined to demonstrate an optoelectronic memory with four well-separated states. The results herein presented are promising for data storage, Boolean logic, and neural network applications.

Field enhancement induced by surface defects in two-dimensional ReSe2 field emitters.

The field emission properties of rhenium diselenide (ReSe2) nanosheets on Si/SiO2 substrates, obtained through mechanical exfoliation, have been investigated. The n-type conduction was confirmed by using nano-manipulated tungsten probes inside a scanning electrode microscope to directly contact the ReSe2 flake in back-gated field effect transistor configuration, avoiding any lithographic process. By performing a finite element electrostatic simulation of the electric field, it is demonstrated that the use of a tungsten probe as anode, at a controlled distance from the ReSe2 emitter surface, allows the collection of emitted electrons from a reduced area that furtherly decreases by reducing the tip-sample distance, i.e. allowing a local characterization of the field emission properties. Experimentally, it is shown that the turn-on voltage can be linearly reduced by reducing the cathode-anode separation distance. By comparing the measured current-voltage characteristics with the numerical simulations, it is also shown that the effective field enhancement on the emitter surface is larger than expected because of surface defects. Finally, it is confirmed that ReSe2 nanosheets are suitable field emitters with high time stability and low current fluctuations.

C60 Functionalized Single‐Walled Carbon Nanotube‐based Phototransistor for Efficient Detection of Visible Light Under Appropriate Gate Electrostatics

AbstractThe current study concerns highly reliable visible light sensing by C60 fullerene functionalized single‐walled carbon nanotube (SWCNT) based phototransistor. The absorbance of visible light (532 nm) increases significantly due to the formation of an interlink between the SWCNT network and C60 clusters. Photogenerated excess electrons in SWCNT are trapped by the C60 clusters and increase the effective hole concentrations in the SWCNT channel, which eventually improves the photoconductive gain. C60‐SWCNT channel is further integrated into the back gated field effect transistor (FET) structure in which 90 nm SiO2 dielectric thickness is used. The responsivity toward visible light is further enlarged with appropriate negative gate electrostatic. In order to ensure the morphological and structural behavior of C60‐SWCNT, various microscopic and spectroscopic characterizations are performed. The C60‐SWCNT phototransistor exhibits responsivity of 1.219 A W−1 at Vgs = – 21 V. The detectivity and rise/fall time of C60‐SWCNT came around to be 2.34 × 1010 Jones, 86.42 ms/3.35 ms at the same Vgs. The maximum external quantum efficiency (EQE) of the C60‐SWCNT phototransistor is very high, ≈274%. In the overall study, a comprehensive discussion is introduced on the effect of variable gate potential on the performance of the C60‐SWCNT phototransistor.

Batch production of centimeter-scale monolayer MoS2 with low nucleation by face-to-face metal precursor supply

Transition-metal dichalcogenides (TMDs) show great potential for next-generation semiconductor due to its atomic level thickness, tunable band gap and high carrier mobility. However, high-quality monolayer MoS2 (ML-MoS2) films require stringent growth conditions during preparation. Here, we report a face-to-face metal precursor supply approach during chemical vapor deposition (CVD) to batch production of uniform ML-MoS2 films. Mo foils provide a stable concentration of metal precursors leading to uniform film generation with low nucleation. Comprehensive characterization techniques, including spectroscopic, surface topography and atomic scale analysis confirm the centimeter-scale homogeneity and high crystallographic quality of the monolayer films. Back-gate field effect transistors (FET) fabricated by ML-MoS2 exhibit excellent electrical performance. This methodology provides a pathway for the growth of TMDs, which should promote their applications in high-performance electronics or other fields.

Synthesis and characterization of 1T′ and 2H phase coexistence 2D Re1-xMoxS2 alloy films and their application for photodetectors

Alloying is considered an efficient strategy to engineer the crystal structure, modulate the energy band structure and charge carrier concentration by adjusting the elemental composition of transition metal dichalcogenides (TMDs). Here, the two-dimensional (2D) Re1-xMoxS2 alloy films with tunable composition and phase structure were prepared on mica substrates by chemical vapor deposition (CVD) method. The back-gated field effect transistors (FETs) based on 2D Re1-xMoxS2 alloy films were prepared. The Mo component x in 2D Re1-xMoxS2 alloy films was varied from 0, 0.2, 0.4, 0.5, 0.6, 0.8–1 by changing mole ratio of ReO3 to MoO3 while the total number of moles remains the same. The Mo composition-dependent morphology phase structure, electrical transport properties and photoelectric properties of Re1-xMoxS2 alloy films were studied in detail. The results indicate that the 1T′ phase Mo-doped ReS2 and 2H phase Re-doped MoS2 coexist, and the ratio of 2H phase Re-doped MoS2 to 1T′ phase Mo-doped ReS2 can be modulated by changing Mo component x in Re1-xMoxS2 alloy films. 1T′ phase Mo-doped ReS2 exhibits the p-type conduction feature while 2H phase Re-doped MoS2 shows the n-type conduction behavior. The Re1–xMoxS2 alloy devices show obvious photoelectric response to 405, 532, 635 and 808 nm lasers irradiation. The Re0.5Mo0.5S2 (x= 0.5) has the maximum responsivity R of 1024.1 mA/W and external quantum efficiency EQE of 239.3% under 532 nm laser irradiation.

N‐Type GaSe Thin Flake for Field Effect Transistor, Photodetector, and Optoelectronic Memory

AbstractThe family of 2D chalcogenide semiconductors has been growing rapidly. Metal monochalcogenides, for instance, can enable new possibilities in functional electronics and optoelectronics. A Gallium Selenide (GaSe) thin flake is used to fabricate a back gated field effect transistor (FET) with n‐type conduction behavior and wide hysteresis at the ambient conditions. The device shows high mobility up to 28 cm2 V−1 s−1 with Ion/Ioff ratio over 103. Under the laser exposure, the device shows a decrease in the threshold voltage and a left‐shift of the transfer characteristic with a slight increase in the current. The transfer characteristic exhibits a hysteretic behavior with hysteresis width linearly dependent on the applied gate voltage. Moreover, the GaSe‐based FET shows a photo response with a photoresponsivity of 475 mAW−1 and detectivity of 4.6 × 1012 Jones. The photocurrent rise and decay times are 0.1 and 1.3 s, respectively. Furthermore, the GaSe FET device can be used as a performant memory device with well separated states and memory window enhanced by the laser exposure, confirming an optoelectronic memory class.

A WOx/MoOx hybrid oxide based SERS FET and investigation on its tunable SERS performance.

Active control of the surface-enhanced Raman scattering (SERS) enhancement shows great potential for realizing smart detection of different molecules. However, conventional methods usually involve time-consuming structural design or a sophisticated fabrication process. Herein, we reported an electrically tunable field effect transistor (FET) comprising a WOx/MoOx hybrid as the SERS active layer. In the experiment, WOx/MoOx hybrids were first prepared by mixing different molar ratios of WOx and MoOx oxides. Then, R6G molecules were used as Raman reporters, showing that the intensity of the SERS signal observed on the most optimal hybrids (molar ratio = 1 : 3) could be increased by two times as high as that observed on a single WOx or MoOx based substrate, which was ascribed to enhanced charge transfer efficiency by the constructed nano-heterojunction between the WOx and MoOx oxides. Thereafter, a back-gate FET was fabricated on a SiO2/Si substrate, and the most optimal WOx/MoOx hybrid was deposited as the gate channel and the SERS active layer. After that, a series of gate biases (from -15 V to 15 V) were implemented to actively tune the SERS performance of the FET. It is evident that the SERS EF can be further tuned from 2.39 × 107 (-15 V) to 6.55 × 107 (+10 V), which is ∼7.4/4.1 times higher than that observed on the pure WOx device (8.81 × 106) or pure MoOx (1.61 × 107) device, respectively. Finally, the mechanism behind the electrical tuning strategy was investigated. It is revealed that a positive voltage would bend the conduction band down, which increased the electron density near the Fermi level. Consequently, it triggered the resonance charge transfer and significantly improved the SERS performance. In contrast, a negative gate voltage attracted the holes to the Fermi level, which deferred the charge transfer process, and caused the reduction of the SERS enhancement.

Hysteresis in the transfer characteristics of MoS2 field effect transistors: gas, temperature and photo-irradiation effect.

We report the characteristic behaviors of the hysteresis observed in the transfer characteristics of back-gated field-effect transistors with an exfoliated MoS2 channel under various conditions. We find that the hysteresis is strongly enhanced by temperature, environmental gas, or light irradiation. Our measurements reveal the characteristic hysteresis behaviors in a 1 atm oxygen environment, which we explain as an oxygen molecule facilitated charge acceptor on the MoS2 surface. The decrease in the current value in the ON state of the device may indicate that oxygen molecules are more effective charge acceptors than nitrogen molecules. We conclude that intrinsic defects in MoS2, such as S vacancies, which result in effective adsorbate trapping, play an important role in the hysteresis behavior, in addition to oxygen and nitrogen adsorbates on the passivated device surface. The availability of thermally or photo-generated minority carriers (holes) in MoS2 is increased by both light and temperature. This leads to subsequent processes of positive charge trapping, which intensify the hysteresis.

Optimized electrical properties of p-type field-effect transistors based on WSe2 grown at moderate temperatures

Two-dimensional transition-metal dichalcogenides (TMDCs) have been pursued for high-performance logic electronic devices, and compatibility with silicon complementary metal-oxide-semiconductor (CMOS) technology is essential. Thus, high-quality material synthesis at reduced temperature is a key challenge for TMDC integration with the back-end-of-line silicon CMOS. In this work, TMDCs have been synthesized at temperatures down to 450 °C on SiO2/Si substrates via chemical vapor deposition. This work highlights the necessity of improving metal precursor mass flux during the low-temperature synthesis of TMDC films. Improved electrical characteristics of the back-gated p-type field-effect transistors based on monolayer WSe2 exhibit an on-current of 14 μA/μm and subthreshold swing of 200 mV/dec.

Ultrahigh responsivity of non-van der Waals Bi2O2Se photodetector

Bismuth oxyselenide has recently gained tremendous attention as a promising 2D material for next-generation electronic and optoelectronic devices due to its ultrahigh mobility, moderate bandgap, exceptional environmental stability, and presence of high-dielectric constant native oxide. In this study, we have synthesized single-crystalline nanosheets of Bismuth oxyselenide with thicknesses measuring below ten nanometers on Fluorophlogopite mica using an atmospheric pressure chemical vapor deposition system. We transferred as-grown samples to different substrates using a non-corrosive nail polish-assisted dry transfer method. Back-gated Bi2O2Se field effect transistors showed decent field effect mobility of 100 cm2 V−1s−1. The optoelectronic property study revealed an ultrahigh responsivity of 1.16 × 106 A W−1 and a specific detectivity of 2.55 × 1013 Jones. The samples also exhibited broadband photoresponse and gate-tunable photoresponse time. These results suggest that Bi2O2Se is an excellent candidate for future high-performance optoelectronic device applications.

The Effect of a Vacuum Environment on the Electrical Properties of a MoS2 Back-Gate Field Effect Transistor

Adsorption of gas molecules on the surface of two-dimensional (2D) molybdenum disulfide (MoS2) can significantly affect its carrier transport properties. In this letter, we investigated the effect of a vacuum environment on the electrical properties of a back-gate MoS2 FET. Benefiting from the reduced scattering centers caused by the adsorbed oxygen and water molecules in a vacuum, the current Ion/Ioff ratio of back-gate MoS2 field effect transistor increased from 1.4 × 106 to 1.8 × 107. In addition, the values of field effect carrier mobility were increased by more than four times, from 1 cm2/Vs to 4.2 cm2/Vs. Furthermore, the values of subthreshold swing could be decreased by 30% compared with the sample in ambient air. We demonstrate that the vacuum process can effectively remove absorbates and improve device performances.

Top-gate engineering of field-effect transistors based on single layers of MoS2 and graphene

Field-effect transistors (FETs) were fabricated with use of vertical heterostructures of graphene and MoS2 monolayers with top-gate and back-gate configurations. Nanodiamonds/poly(vinylidene difluoride) (1:1) and SiO2 were used as the top-gate and back-gate dielectric, respectively. The novel top-gate electrode and gate dielectric assembly consisting of polyaniline and nanodiamonds/poly(vinylidene difluoride) were synthesized by a chemical oxidative polymerization approach. Our MoS2 FETs showed a high linear mobility of approximately 212 cm2/V s and a high on/off current ratio of up to 107 at 10 V for top-gated FETs, which is much greater than for conventional back-gated FETs. The devices were stable under elevated temperatures up to 100 °C, with a decrease in the on/off current ratio. The devices showed good stability without any additional encapsulation after an elapsed time of 10 days, as the results on the first and tenth days were identical. This work will pave the way for the design of high-performance FETs for applications in electronics and optoelectronics.

Improving the optoelectronic properties of monolayer MoS2 field effect transistor through dielectric engineering

The reduced dielectric screening in atomically thin two-dimensional materials makes them very sensitive to the surrounding environment, which can be modulated to tune their optoelectronic properties. In this study, we significantly improved the optoelectronic properties of monolayer MoS2 by varying the surrounding environment using different liquid dielectrics, each with a specific dielectric constant ranging from 1.89 to 18. Liquid mediums offer the possibility of environment tunability on the same device. For a back-gated field effect transistor, the field effect mobility exhibited more than two-order enhancement when exposed to a high dielectric constant medium. Further investigation into the effect of the dielectric environment on the optoelectronic properties demonstrated a variation in photoresponse relaxation time with the dielectric medium. The rise and decay times were observed to increase and decrease, respectively, with an increase in the dielectric constant of the medium. These results can be attributed to the dielectric screening provided by the surrounding medium, which strongly modifies the charged impurity scattering, the band gap, and defect levels of monolayer MoS2. These findings have important implications for the design of biological and chemical sensors, particularly those operating in a liquid environment. By leveraging the tunability of the dielectric medium, we can optimize the performance of such sensors and enhance their detection capabilities.

Gate-tunable hysteresis response of field effect transistor based on sulfurized Mo

Hysteresis effects and their tuning with electric fields and light were studied in thin film molybdenum disulfide transistors fabricated from sulfurized molybdenum films. The influence of the back-gate voltage bias, voltage sweep range, illumination, and AlOx encapsulation on the hysteresis effect of the back-gated field effect transistors was studied and quantified. This study revealed the distinctive contribution of MoS2 surface, MoS2/SiO2 interface defects and their associated traps as primary sources of of hysteresis.

Role of density gradients in the growth dynamics of 2-dimensional MoS2 using liquid phase molybdenum precursor in chemical vapor deposition

The key objective of this work was the comparison of MoS2 2-dimensional monolayer flakes grown by Chemical Vapour Deposition with different density gradients in the liquid precursors solution. Structures grown using solutions with iodixanol or glycerol were characterized by Optical Microscopy, Atomic Force Microscopy, Raman spectroscopy and Photoluminescence. Furthermore, electrical properties of structures were studied by characterizing backgated Field-Effect Transistors fabricated on individual flakes.The fundamental outcome of this study is that there are relevant differences between the structures grown with the two different liquid solutions: MoS2 structures grown with glycerol present a large variability in the lateral size of the sharp-corner triangular structure with a good crystalline quality while the employment of iodixanol (the most used substance) causes a poorer crystalline quality, demonstrated by the presence of nanometric pores, with a more homogeneous sizing of the triangular structures.The poor crystalline quality of the iodixanol samples results in reduced light emission efficiency and lower mobility.

Optically excited artificial synapse based on α-In2Se3 FETs on Ta2O5

Materials and devices for artificial synapses are being increasingly investigated owing to their promise for brain-inspired computing. Here, we demonstrate an optoelectronic synapse with a light-modulated memory capability in back-gated ferroelectric channel field-effect transistors made of multi-layered 2D α-In2Se3 on Ta2O5. The optical tunability is achieved by exploiting the frequency of the optical signal in vertically stacked layers of In2Se3, which generates a unique persistent photoresponse due to trapping at the In2Se3/Ta2O5 interface. For the 527 nm source wavelengths at intensities of 15 mW cm−2 the In2Se3-FET exhibits a high photoresponsivity at 850 AW−1. These devices can replicate synaptic functions such as photo-induced short-term memory, long-term memory and paired-pulse facilitation—all via optical modulation. We also demonstrate common memory effects that occur in the brain, such as memory loss and memory transitions that depend upon the stimulation rate (i.e., the interval between stimulation pulses). These demonstrations provide a simple and effective strategy for fabricating light-stimulated synaptic transistors with memory and learning abilities which are attractive for building vision-inspired neuromorphic systems.

Potassium-doped nano graphene as an intermediate layer for graphene electronics

To suppress the intrinsic carrier density and increase the carrier mobility in graphene on a silicon dioxide (SiO2) substrate, potassium (K)-doped nano graphene was introduced as an intermediate layer between the graphene layer and SiO2 substrate. Back-gate type graphene field effect transistors with four terminal structures were fabricated, and their electrical properties were measured under vacuum. The results showed that the Dirac point shifted from +9.0 to −0.2 V after inserting the K-doped nano graphene. The results suggested that inserting the intermediate layer compensated for the intrinsic holes and achieved an electron doping of 2 × 1012 cm−2. The field-effect mobilities of electrons and holes also increased because the ionized K-atoms in the intermediate layer shielded the electric force from the negatively charged impurities in SiO2. The K density was estimated using x-ray photoelectron spectroscopy to be 1.49 × 1013 cm−2, and the C1s peak shifted by 0.2 eV, which confirms the upward modulation of the graphene Fermi level by the K-doped nano graphene intermediate layer. These results demonstrated the advantages of the intermediate layer on the carrier density and mobility in graphene.

Optimization of Subthreshold Swing and Hysteresis in Hf0.5Zr0.5O2-Based MoS2 Negative Capacitance Field-Effect Transistors by Modulating Capacitance Matching.

Negative capacitance field effect transistors made of Hf0.5Zr0.5O2 (HZO) are one of the most promising candidates for low-power-density devices because of the extremely steep subthreshold swing and high open-state currents resulting from the addition of ferroelectric materials in the gate dielectric layer. In this paper, HZO thin films were prepared by magnetron sputtering combined with rapid thermal annealing. Their ferroelectric properties were adjusted by changing the annealing temperature and the thickness of HZO. Two-dimensional MoS2 back-gate negative capacitance field-effect transistors (NCFETs) based on HZO were prepared as well. Different annealing temperatures, thicknesses of HZO thin films, and Al2O3 thicknesses were studied to achieve optimal capacitance matching, aiming to reduce both the subthreshold swing of the transistor and the hysteresis of the NCFET. The NCFET exhibits a minimum subthreshold swing as low as 27.9 mV/decade, negligible hysteresis (∼20 mV), and the ION/IOFF of up to 1.58 × 107. Moreover, a negative drain-induced barrier lowering effect and a negative differential resistance effect have been observed. This steep-slope transistor is compatible with standard CMOS manufacturing processes and attractive for 2D logic and sensor applications as well as future energy-efficient nanoelectronic devices with scaled power supplies.