Monolayer Gate Research Articles

Self-assembled monolayer gate doping and their detail deep cryogenic characterization of GaN/Si HEMTs

Wide bandgap GaN/AlGaN/GaN/SOI high-electron-mobility transistors (GaN/Si HEMTs) have recently grabbed interest in the semiconductor field as outstanding contenders for microwave and radio-frequency (RF) power amplification systems with their powerful capacities. Though the negative threshold voltage (Vth) property of GaN limits the practical application as an electronic system for static power consumption consideration, the current study demonstrates a novel method that can adjust Vth higher and without damaging the crystal lattice of the GaN channel materials by self-assembly monolayer doping. Also, the fabricated GaN/AlGaN/GaN/SOI HEMT devices exhibit excellent efficiency with enhancement of mobility and subthreshold slope in cryogenic temperatures using Fluorine-contained monolayer doping. This could unlock additional process options for the development of next-generation GaN/AlGaN/GaN/SOI HEMT with high mobility, catering to the advancements in 6G communication and circuits for low-orbit satellites.

Promoting sensitivity and selectivity of SF6 gas sensor based on metal (Rh, Pd, Ag)-doped monolayer GaTe: A DFT study

In this study, a novel gas-sensitive material based on transition metal (Rh, Pd and Ag) doped GaTe monolayer is proposed. The adsorption properties of Rh-GaTe, Pd-GaTe and Ag-GaTe single molecule membranes on four SF6 decomposition gases (H2S, SO2, SOF2, SO2F2) were investigated by using density functional theory (DFT). The adsorption energy, QT, DOS, work function and energy band structure were analyzed. For further analysis, differential charge density, molecular orbital theory and recovery time were also investigated. The results show that Rh-GaTe, Pd-GaTe and Ag-GaTe are promising materials for gas detection.

Efficient spin filtering through Fe4GeTe2-based van der Waals heterostructures.

Utilizing ab initio simulations, we study the spin-dependent electronic transport characteristics within Fe4GeTe2-based van der Waals heterostructures. The electronic density of states for both free-standing and device-configured Fe4GeTe2 (F4GT) confirms its ferromagnetic metallic nature and reveals a weak interface interaction between F4GT and PtTe2 electrodes, enabling efficient spin filtering. The ballistic transport through a double-layer F4GT with a ferromagnetic configuration sandwiched between two PtTe2 electrodes is predicted to exhibit an impressive spin polarization of 97% with spin-up electrons exhibiting higher transmission probability than spin-down electrons. Moreover, we investigate the spin transport properties of Fe4GeTe2/GaTe/Fe4GeTe2 van der Waals heterostructures sandwiched between PtTe2 electrodes to explore their potential as magnetic tunnel junctions in spintronic devices. The inclusion of monolayer GaTe as a 2D semiconducting spacer between F4GT layers results in a tunnel magnetoresistance of 487% at a low bias and decreases with increasing bias voltage. Overall, our findings underscore the potential of F4GT/GaTe/F4GT heterostructures in advancing spintronic devices based on van der Waals materials.

Effect of the Strength of Carbon Nanotube Network on the Efficiency of CNFET Based on Self-Assembled Molecular Monolayer Gate Dielectric

Effect of the Strength of Carbon Nanotube Network on the Efficiency of CNFET Based on Self-Assembled Molecular Monolayer Gate Dielectric

Effect of Organic / Inorganic Gate Materials on the Organic Field-Effect Transistors Performance

The choice of gate dielectric materials is fundamental for organic field effect transistors (OFET), integrated circuits, and several electronic applications. The operation of the OFET depends on two essential parameters: the insulation between the semiconductor layer and the gate electrode and the capacitance of the insulator. In this work, the electrical behavior of a pentacene-based OFET with a top contact / bottom gate was studied. Organic polyvinyl alcohol (PVA) and inorganic hafnium oxide (HfO2) were chosen as gate dielectric materials to lower the operation voltage to achieve the next generation of electronic applications. In this study, the performance of the OFET was studied using monolayer and bilayer gate insulators. To model and analyze a device's electrical properties, MATLAB was used. Two main parameters were studied: switching ratio (Ion/Ioff) and subthreshold swing (SS), as well as the effect of dielectric capacitance on the gate dielectric materials. The PVA/HfO2 bilayer gate dielectric gave the best results in Ion/Ioff ratio, SS and transconductance of 9.05´10-7, -1.52, and -4.99 x10-5A/V respectively, which is because the dielectric capacitance has increased.

Electronic structure and optical properties of GaTe/MoTe2 based vdW heterostructure under mechanical strain and external electric fields

In this work, we systematically designed a two-dimensional (2D) heterostructure based on GaTe and MoTe2 monolayers to circumvent the limitations of the constituents and enhance their performance for various optoelectronic and energy applications. All the calculations are performed using Hybrid-Density Functional Theory (DFT) calculations. We observed that the GaTe/MoTe2 heterostructure has an indirect bandgap (Eg) of 1.5 eV with type-I band alignment. Due to the type-I alignment, electrons and holes both reside in the MoTe2 monolayer, which restricts its use in various potential applications. Therefore, the modulation of electronic structure and optical spectra of 2D-GaTe/MoTe2 heterostructure are investigated in detail under external biaxial strain varying from −9% to +9%, and electric field values between ±1V/Å. We observed, a type II band alignment with high optical visible absorption of order 105 cm−1 within εxy > +2% and Eext between +0.4 V/Å to +1 V/Å, and −0.6 V/Å to −1 V/Å. This suggests that the GaTe/MoTe2 heterostructure has excellent potential for various next–generation optoelectronic and energy applications.

First-principles prediction of the missed Pmmn phase for a GaTe monolayer as a new two-dimensional semiconductor

In this study, we predicted a new semiconducting phase for a GaTe monolayer with remarkable stability using first-principles calculations and the RG2 code, which implements the random sampling strategy combined with space group and graph theory. The new phase (Pmmn) possesses a very similar atomic configuration to the ground state C2/m phase. The calculated total energy of the GaTe monolayer in the Pmmn symmetry is just 4 meV/atom higher than that of the experimentally realized C2/m phase, and exfoliation energy of Pmmn GaTe is lower than that of C2/m phase, which indicates a high probability of synthesizing GaTe in this novel Pmmn configuration in the future. The calculated vibrational spectrum and elastic constants also suggest that the Pmmn configuration of the GaTe monolayer is dynamically and mechanically stable. It is a direct band gap semiconductor with a gap of 2.15 eV, which is very similar to that of the C2/m phase (2.08 eV). The simulated mechanical properties show that the GaTe monolayers in both the Pmmn and C2/m symmetries exhibit anisotropy. Our present work provides a new structural candidate for GaTe monolayers synthesized using the bottom-to-top method. Moreover, such a new structure with Pmmn symmetry can also be considered as a fundamental configuration for predicting other III–VI monolayer semiconductors.

Au-PEDOT/rGO nanocomposites functionalized graphene electrochemical transistor for ultra-sensitive detection of acetaminophen in human urine

A novel graphene electrochemical transistor (GECT) sensor based on Au-poly(3,4-ethylenedioxythiophene)/reduced graphene oxide (Au-PEDOT/rGO) nanocomposites functionalized the gate electrode and monolayer graphene as channel was proposed and constructed for the ultra-sensitive detection of acetaminophen (AP). Au-PEDOT/rGO nanocomposites were synthesized by a simple one-pot method to modify the gate electrode of GECT. With the high catalytic activity of Au nanoparticles, the good conductivity and stability of PEDOT, the large specific surface area and abundant adhesion sites of rGO, the sensitivity and stability of the device for AP detection could be effectively improved. The sensing mechanism of the device was that the electrochemical reactions of the AP on the surface of gate electrode causes the effective gate voltage on the GECT to change, thereby adjusting the carrier concentration and current of the graphene channel. Combined with the excellent catalytic properties of Au-PEDOT/rGO nanocomposites and the high carrier mobility of the graphene channel, the resulting device has remarkable sensing performance for AP, with a detection limit as low as 1 nM and a linear range from 1 nM to 8 mM. In addition, the device has good anti-interference ability and accuracy in the detection of AP in urine samples and tablets, which proved that it could be used to determine AP in human non-invasive and pharmaceutical products. The GECT sensor based on Au-PEDOT/rGO provides an efficient, sensitive and cost-effective sensing platform for AP detection, and is expected to realize in vitro diagnosis of diseases.

Retraction notice to “Two-dimensional GaTe monolayer as a potential gas sensor for SO2 and NO2 with discriminate optical properties” [Superlattices and Microstructures 135C (2019) 106245

Retraction notice to “Two-dimensional GaTe monolayer as a potential gas sensor for SO2 and NO2 with discriminate optical properties” [Superlattices and Microstructures 135C (2019) 106245

First-principle calculation of optical properties of HfTeSe[formula omitted], HfTe[formula omitted]Ge, GeTe, GaTe, and HfTe[formula omitted] monolayers

In current years, researchers consecrated much regard to nonlinear photonic crystals as their employment in photonic bandgap compounds for testing and moving light. The growth of study in nonlinear optics is intimately connected to the fast technological advancements that have happened in associated fields such as optical communication and optical storage devices. In this framework, we investigate the optical properties such as absorption, conductivity, refractive index, and dielectric function of HfTeSe4, HfTe4Ge, GeTe, GaTe, and HfTe5 monolayers, using density functional theory. We found that the imaginary part ɛ2(ω) of the dielectric function leaves at neighboring 20.00 eV, which corresponds to the energy loss function. In this framework, we have obtained these compounds become transparent when the frequency of the incident light is higher than the plasma frequency (20.00 eV). Further, we reach the highest reflectivity for the HfTe5 sheet, which has a value of around 0.5% at the energy of 2.00 eV.

Controlled Growth of Large-Sized and Phase-Selectivity 2D GaTe Crystals.

GaTe has recently attracted significant interest due to its direct bandgap and unique phase structure, which makes it a good candidate for optoelectronics. However, the controllable growth of large-sized monolayer and few-layer GaTe with tunable phase structures remains a great challenge. Here the controlled growth of large-sized GaTe with high quality, chemical uniformity, and good reproducibility is achieved through liquid-metal-assisted chemical vapor deposition method. By using liquid Ga, the rapid growth of 2D GaTe flakes with high phase-selectivity can be obtained due to its reduced reaction temperature. In addition, the method is used to synthesize many Ga-based 2D materials and their alloys, showing good universality. Raman spectra suggest that the as-grown GaTe own a relatively weak van der Waals interaction, where monoclinic GaTe displays highly-anisotropic optical properties. Furthermore, a p-n junction photodetector is fabricated using GaTe as a p-type semiconductor and 2D MoSe2 as a typical n-type semiconductor. The GaTe/MoSe2 heterostructure photodetector exhibits large photoresponsivity of 671.52 A W-1 and high photo-detectivity of 1.48 × 1010 Jones under illumination, owing to the enhanced light absorption and good quality of as-grown GaTe. These results indicate that 2D GaTe is a promising candidate for electronic and photoelectronic devices.

Atomistic insight into the significantly enhanced photovoltaic cells of monolayer GaTe2 via two-dimensional van der Waals heterostructures engineering

Abstract Designing new van der Waals (vdW) heterostructures from various two-dimensional transition metal dichalcogenides (TMDs) materials shows outstanding properties, such as an ultrafast charge transfer process and strong interlayer interactions by combining the advantageous properties of the different TMD materials. In this study, using the density functional theory method, we systemically investigate the optical property, band alignment, electronic structures, interface charge transfer, mechanical properties and stability of MTe2/GaTe2 (M = Mo and W) vdW heterostructures as promising photovoltaic solar cells materials. In this work, gallium telluride and MTe2 were used as acceptors and donors in high-quality photovoltaic cells. The calculated binding energies suggest that they were energetically favourable and relatively easy to fabricate under suitable conditions. Moreover, the heterostructures possess exceptional characteristics of enhanced visible light absorption edge (∼104 cm−1), type-II band alignment and strong charge separation. The suitable band alignment leads to maximum power conversion efficiency (PCE) of 22.43 and 22.91%, respectively, which was quite promising for photovoltaic solar cells. The high PCE could be due to the internal built-in electric field at the MTe2/GaTe2 interface, which induces efficient separation of charge carriers. This work offers theoretical support for the design and prediction of next-generation low-cost, highly efficient and promising materials for solar device applications.

Lattice Thermal Transport in Monolayer Group 13 Monochalcogenides MX (M = Ga, In; X = S, Se, Te): Interplay of Atomic Mass, Harmonicity, and Lone-Pair-Induced Anharmonicity.

We perform a systematic study of the lattice dynamics and the lattice thermal conductivity, κ, of monolayer group 13 monochalcogenides MX (M = Ga, In; X = S, Se, Te) by combining an iterative solution for linearized phonon Boltzmann transport equation and density functional theory. Among the competing factors influencing κ, harmonic parameters along with the atomic masses dominate over anharmonicity. An increase in atomic mass leads to a decrease in phonon frequencies and phonon group velocities and consequently in κ. At T = 300 K, the calculated κ values are 54.9, 48.1, 44.3, 25.0, 22.3, and 17.3 W m-1 K-1 for GaS, InS, GaSe, InSe, GaTe, and InTe monolayers, respectively. Further analysis of anharmonic scattering rates and average scattering matrix elements evidences that the anharmonicity characterized by the third-order IFCs in GaS and InS are the largest among all monolayer group 13 monochalcogenides despite the largest κ values. This is attributed to a strong interaction between nonbonding lone-pair s electrons around the S atom and adjacent bonding electrons. In addition, the κ of these monolayers further reduces to 50% for sample sizes 300-400 nm. Our findings provide fundamental insights into thermal transport in monolayer group 13 monochalcogenides and should stimulate further experimental exploration of thermal transport in these materials for possible theromoelectric and thermal management applications.

Computational prediction of electronic and optical properties of Janus Ga2SeTe monolayer

Janus group III monochalcogenide structures, which are predicted to have many promising applications in optoelectronics and photocatalytic water splitting, have been of particular interest recently. In this study, electronic properties and optical characteristics of the Janus Ga2SeTe monolayer under a biaxial strain εb and electric field E were considered using the density functional theory. Our calculations demonstrated that the Janus Ga2SeTe monolayer is dynamically and thermally stable and exhibits a semiconducting characteristic with a moderate direct band gap at equilibrium. The band gap of Janus Ga2SeTe monolayer at equilibrium, which is calculated by PBE method and corrected by HSE06 hybrid functional, is smaller than that of both GaSe and GaTe monolayers. Mulliken population analysis shows that there has been a redistribution of charge during the formation of the Janus structure, especially there is a large difference in charge between the two Ga layers in Janus Ga2SeTe monolayer. The biaxial strain has greatly altered the electronic structure of the Janus Ga2SeTe monolayer and direct–indirect band gap transitions were found at appropriate strain εb. While the effect of the E on electronic properties and especially optical properties is weak, the optical absorbance of the Janus Ga2SeTe monolayer can be enhanced by strain engineering, up to 14.42 × 104 cm−1 at εb=−7% in the near-ultraviolet region. The optical absorbance of the Janus Ga2SeTe monolayer is activated in the visible light region that is following its calculated band gap value. This work not only systematically presents the electronic and optical properties of the Janus Ga2SeTe monolayer in the presence of strain engineering and electric field but can also motivate experimental studies for applications in nanoelectromechanical and optoelectronic devices.

Two-dimensional GaTe/Bi2Se3 heterostructure: A promising direct Z-scheme water splitting photocatalyst

Among various photocatalytic materials, Z-scheme photocatalysts have drawn tremendous research interest due to high photocatalytic performance in solar water splitting. Here, we perform extensive hybrid density functional theory calculations to explore electronic structures, interfacial charge transfer, electrostatic potential profile, optical absorption properties, and photocatalytic properties of a proposed two-dimensional (2D) small-lattice-mismatched GaTe/Bi2Se3 heterostructure. Theoretical results clearly reveal that the examined het-erostructure with a small direct band gap can effectively harvest the broad spectrum of the incoming sunlight. Due to the relative strong interfacial built-in electric field in the heterostructure and the small band gap between the valence band maximum of GaTe monolayer and the conduction band minimum of Bi2Se3 nanosheet with slight band edge bending, these photogenerated carriers transfer via Z-scheme pathway, which results in the photogenerated electrons and holes effectively separating into the GaTe monolayer and the Bi2Se3 nanosheet for the hydrogen and oxygen evolution reactions, respectively. Our results imply that the artificial 2D GaTe/Bi2Se3 is a promising Z-scheme photocatalyst for overall solar water splitting.

Various polymorphs of group III–VI (GaSe, InSe, GaTe) monolayers with quasi-degenerate energies: facile phase transformations, high-strain plastic deformation, and ferroelastic switching

The rare existence of large plastic deformation in brittle covalent systems with little plastic regime can be attributed to the features of covalent bonds, which are rigid with directionality compared with ductile metallic bonds with delocalized electrons. In this work, we show ab initio calculation evidence that group III–VI monolayers such as InSe and GaTe possess a series of metastable polymorphs with similar energies. The energy differences between the ground state and those polymorphs are almost negligible (<kBT at room temperature) and can even be further reduced to zero on critical alloying, which is hitherto unreported. The ground state will be tunable owing to its sensitivity to strain or charge injection, which may be used as shape memories. Those low-energy phases with similar lattice constants in one direction and gradually varying lattice constants in another direction may enable a large plastic region from 0% to 12.6% via the ‘multi-intermediate strategy,’ also rendering large plastic deformation or large-strain ferroelasticity that most ductile metals cannot sustain before a fracture, wherein they can be a series of intermediate states with similar energies that may make the pathway much smoother.

Modulation of electronic and optical properties of GaTe monolayer by biaxial strain and electric field

In this work, the electronic and optical properties of monolayer GaTe under biaxial strain and external electric field were investigated by density functional theory. Our calculated results indicate that the monolayer GaTe is an indirect-semiconductor with an energy gap of 1.41eV at equilibrium. Electronic properties of monolayer GaTe, especially the energy gap, depend greatly on the biaxial strain and external electric field. While the compressive strain slightly increases the energy gap, the tensile strain reduces quite rapidly the energy gap of the monolayer GaTe. In particular, semiconductor–metal phase transition can be observed when the external electric field was introduced. The GaTe monolayer strongly absorbs light in the ultraviolet region and the biaxial strain greatly changes its optical characteristics, especially the compression strain significantly increasing the absorption coefficient of the monolayer. Our results can provide more useful information for the prospect of application of the GaTe monolayer in next-generation optoelectronic devices.

First-principles study on the anisotropic transport of electrons and phonons in monolayer and bulk GaTe: a comparative study

Recently, monoclinic-phase GaTe has attracted much attention due to its potential applications in nanoelectronics. Despite the experimental research, theoretical studies on the thermal and transport properties, which are necessary to provide information for future applications, are still absent. We have systematically investigated the electronic, phonon and electron transporting, and thermoelectric properties of monolayer and bulk GaTe using first-principles calculations plus the Boltzmann transport equation. At the valence band maximum and conduction band minimum, the effective mass shows large anisotropy as the band dispersions are along different k-paths. The group velocity of acoustic modes also shows large anisotropy owing to the in-plane low-symmetry. Our calculations reveal that the in-plane thermal conductivities, κa and κb, take 3.5 and 8.9 W m-1 K-1, respectively, for the bulk at 300 K, compared to κa = 5.5 and κb = 10.4 W m-1 K-1 of the monolayer. Due to the van der Waals interactions between interlayers, the out-of-plane thermal conductivity is very small, κc = 1.8 W m-1 K-1. The difference between the in-plane thermal conductivities of the bulk and the monolayer can be attributed to the strengthened Umklapp scattering, which is caused by the stiffening of the lowest-frequency optical mode in the bulk. The hole mobilities of the bulk is found to be about 12-35 cm2 V-1 s-1 at 300 K, in good agreement with the experimental results. The monolayer is found to have smaller mobility but larger anisotropy than those of the bulk. Interestingly, the out-of-plane conductivity is anomalously larger than the in-plane one for the bulk, which is attributed to the orbital overlaps between the interlayer Te atoms. Moreover, n-type GaTe is found to have much larger mobility and anisotropy than the p-type one, which is useful for future applications. Compared with the case of monolayer GaTe, thermoelectric performance can be enhanced by one order of magnitude for the bulk GaTe by exploiting the out-of-plane thermal and electrical conductivities.

RETRACTED: Two-dimensional GaTe monolayer as a potential gas sensor for SO2 and NO2 with discriminate optical properties

Abstract First-principles calculations have been executed to explore the electronic and optical properties of adsorbed NO, NO2, CO, CO2, SO2, NH3 and H2S molecules on gallium telluride (GaTe) monolayer. Our outcomes disclose that the energy gap of GaTe monolayer (1.44 eV) changes appreciably ranging between 0.177 and 1.464 eV with the adsorption. Also, it is perceived the adsorption of NO and H2S molecules is accomplished by physisorption. Accordingly, GaTe monolayer is not suitable as a sensor for these molecules. Besides, the adsorption of CO, CO2, and NH3 molecules on GaTe monolayer with lower adsorption energy makes it less practicable as a gas sensor. Furthermore, the GaTe monolayer is more feasible for SO2 sensor with strong binding between SO2 and monolayer. Simultaneously, our conjecture that GaTe monolayer is adequate to sensitively monitor NO2 sensors with moderate adsorption energy. Interestingly, the findings exhibit that compared to other molecules, the loss function and reflectivity of adsorbed CO2 and NO2 on GaTe monolayer are better in the ultraviolet region. Importantly, the light absorption ability of gas adsorbed on GaTe monolayer has been earnestly improved. Hence, the present work proposes that the electronic and optical properties of GaTe monolayer can be stirringly modulated by molecules adsorption.

Performance of Monolayer Blue Phosphorene Double-Gate MOSFETs from the First Principles.

We systematically study the device characteristics of the monolayer (ML) blue phosphorene metal-oxide semiconductor field-effect transistors (MOSFETs) by using ab initio quantum-transport simulations. The ML blue phosphorene MOSFETs show superior performances with ultrashort-channel length. We first predict the ultrascaled ML blue phosphorene MOSFETs with proper doping concentration and underlap structures are compliant with the high-performance (HP) and low-power (LP) requirements of the International Technology Roadmap for Semiconductors in the next decade in the aspects of the on-state current, delay time, and power dissipation. Encouragingly, the performances of the ML blue phosphorene MOSFETs are superior to that of the MOSFETs based on arsenene, antimonene, InSe, etc. in terms of the on-state current at similar device size. We also consider the electron-phonon scattering in 10.2 nm gate ML blue phosphorene MOSFET. It is found that the on-state current with the scattering of the blue phosphorene device is degraded by 25.4 and 23.6% for HP and LP applications, which can also fulfill the HP and LP application target. Therefore, we can deduce that ML blue phosphorene is an alternative channel material to silicon for ultrascaled FETs if the large-scale and high-quality blue phosphorene can be achieved.