2D fin field-effect transistors integrated with epitaxial high-k gate oxide.

Precise integration of two-dimensional (2D) semiconductors and high-dielectric-constant (k) gate oxides into three-dimensional (3D) vertical-architecture arrays holds promise for developing ultrascaled transistors1-5, but has proved challenging. Here we report the epitaxial synthesis of vertically aligned arrays of 2D fin-oxide heterostructures, a new class of 3D architecture in which high-mobility 2D semiconductor fin Bi2O2Se and single-crystal high-k gate oxide Bi2SeO5 are epitaxially integrated. These 2D fin-oxide epitaxial heterostructures have atomically flat interfaces and ultrathin fin thickness down to one unit cell (1.2 nm), achieving wafer-scale, site-specific and high-density growth of mono-oriented arrays. The as-fabricated 2D fin field-effect transistors (FinFETs) based on Bi2O2Se/Bi2SeO5 epitaxial heterostructures exhibit high electron mobility (µ) up to 270 cm2 V-1 s-1, ultralow off-state current (IOFF) down to about 1 pA µm-1, high on/off current ratios (ION/IOFF) up to 108 and high on-state current (ION) up to 830 µA µm-1 at 400-nm channel length, which meet the low-power specifications projected by the International Roadmap for Devices and Systems (IRDS)6. The 2D fin-oxide epitaxial heterostructures open up new avenues for the further extension of Moore's law.

Single-crystalline van der Waals layered dielectric with high dielectric constant.

The scaling of silicon-based transistors at sub-ten-nanometre technology nodes faces challenges such as interface imperfection and gate current leakage for an ultrathin silicon channel1,2. For next-generation nanoelectronics, high-mobility two-dimensional (2D) layered semiconductors with an atomic thickness and dangling-bond-free surfaces are expected as channel materials to achieve smaller channel sizes, less interfacial scattering and more efficient gate-field penetration1,2. However, further progress towards 2D electronics is hindered by factors such as the lack of a high dielectric constant (κ) dielectric with an atomically flat and dangling-bond-free surface3,4. Here, we report a facile synthesis of a single-crystalline high-κ (κ of roughly 16.5) van der Waals layered dielectric Bi2SeO5. The centimetre-scale single crystal of Bi2SeO5 can be efficiently exfoliated to an atomically flat nanosheet as large as 250 × 200 µm2 and as thin as monolayer. With these Bi2SeO5 nanosheets as dielectric and encapsulation layers, 2D materials such as Bi2O2Se, MoS2 and graphene show improved electronic performances. For example, in 2D Bi2O2Se, the quantum Hall effect is observed and the carrier mobility reaches 470,000 cm2 V-1 s-1 at 1.8 K. Our finding expands the realm of dielectric and opens up a new possibility for lowering the gate voltage and power consumption in 2D electronics and integrated circuits.

Strain-Free Layered Semiconductors for 2D Transistors with On-State Current Density Exceeding 1.3 mA µm-1.

High-mobility and air-stable two-dimensional (2D) Bi2O2Se semiconductor holds promise as an alternative fast channel material for next-generation transistors. However, one of the key challenges remaining in 2D Bi2O2Se is to prepare high-quality crystals to fabricate the high-performance transistors with a high on-state current density. Here, we present the free-standing growth of strain-free 2D Bi2O2Se crystals. An ultrahigh Hall mobility of 160 000 cm2 V-1 s-1 is measured in strain-free Bi2O2Se crystals at 2 K, which enables the observation of Shubnikov-de Haas quantum oscillations and shows substantially higher (>4 times) mobility over previous in-plane 2D crystals. The fabricated 2D transistors feature an on-off current ratio of ∼106 and a record-high on-state current density of ∼1.33 mA µm-1, which is comparable to that of commercial Si and Ge n-type field-effect transistors (FETs) for similar channel length. Strain-free 2D Bi2O2Se provides a promising material platform for studying novel quantum phenomena and exploration of high-performance low-power electronics.

Charge Transfer Properties of Heterostructures Formed by Bi2 O2 Se and Transition Metal Dichalcogenide Monolayers.

Atomically thin bismuth oxyselenide (Bi2 O2 Se) exhibits attractive properties for electronic and optoelectronic applications, such as high charge-carrier mobility and good air stability. Recently, the development of Bi2 O2 Se-based heterostructures have attracted enormous interests with promising prospects for diverse device applications. Although the electrical properties of Bi2 O2 Se-based heterostructures have been widely studied, the interlayer charge transfer in these heterostructures remains elusive, despite its importance in harnessing their emergent functionalities. Here, a comprehensive experimental investigation on the interlayer charge transfer properties of two heterostructures formed by Bi2 O2 Se and representative transition metal dichalcogenides (namely, WS2 /Bi2 O2 Se and MoS2 /Bi2 O2 Se) is reported. Kelvin probe force microscopy is used to measure the work functions of the samples, which are further employed to establish type-II band alignment of both heterostructures. Photoluminescence quenching is observed in each heterostructure, suggesting high charge transfer efficiency. Time-resolved and layer-selective pump-probe measurements further prove the ultrafast interlayer charge transfer processes and formation of long-lived interlayer excitons. These results establish the feasibility of integrating 2D Bi2 O2 Se with other 2D semiconductors to fabricate heterostructures with novel charge transfer properties and provide insight for understanding the performance of optoelectronic devices based on such 2D heterostructures.

Uniform High-k Amorphous Native Oxide Synthesized by Oxygen Plasma for Top-Gated Transistors.

The integration of high-k gate dielectrics with two-dimensional (2D) semiconducting channel materials is essential for high-performance and low-power electronics. However, the conformal deposition of a uniform high-k dielectric with sub-1 nm equivalent oxide thickness (EOT) and high interface quality on high-mobility 2D semiconductors is still challenging. Here, we report a facile approach to synthesize a uniform high-k (εr ∼ 22) amorphous native oxide Bi2SeOx on the high-mobility 2D semiconducting Bi2O2Se using O2 plasma at room temperature. The conformal native oxide can directly serve as gate dielectrics with EOT of ∼0.9 nm, while the original properties of underlying 2D Bi2O2Se is preserved. Furthermore, high-resolution area-selective oxidation of Bi2O2Se is achieved to fabricate discrete electronic components. This facile integration of a high-mobility 2D semiconductor and its high-k native oxide holds high promise for next-generation nanoelectronics.

High-Mobility Flexible Oxyselenide Thin-Film Transistors Prepared by a Solution-Assisted Method.

Two-dimensional (2D) semiconductors hold great promise in flexible electronics because of their intrinsic flexibility and high electrical performance. However, the lack of facile synthetic and subsequent device fabrication approaches of high-mobility 2D semiconducting thin films still hinders their practical applications. Here, we developed a facile, rapid, and scalable solution-assisted method for the synthesis of a high-mobility semiconducting oxyselenide (Bi2O2Se) thin film by the selenization and decomposition of a precursor solution of Bi(NO3)3·5H2O. Simply by changing the rotation speed in spin-coating of the precursor solution, the thicknesses of Bi2O2Se thin films can be precisely controlled down to few atomic layers. The as-synthesized Bi2O2Se thin film exhibited a high Hall mobility of ∼74 cm2 V-1 s-1 at room temperature, which is much superior to other 2D thin-film semiconductors such as transition metal dichalcogenides. Remarkably, flexible top-gated Bi2O2Se transistors showed excellent electrical stability under repeated electrical measurements on flat and bent substrates. Furthermore, Bi2O2Se transistor devices on muscovite substrates can be readily transferred onto flexible polyvinyl chloride (PVC) substrates with the help of thermal release tape. The integration of a high-mobility thin-film semiconductor, excellent stability, and easy transfer onto flexible substrates make Bi2O2Se a competitive candidate for future flexible electronics.

H2 O-Etchant-Promoted Synthesis of High-Quality Graphene on Glass and Its Application in See-Through Thermochromic Displays.

Direct growth of graphene on glass can bring an innovative revolution by coupling the complementary properties of traditional glass and modern graphene (such as transparency and conductivity), offering brand new daily-life related applications. However, preparation of high-quality graphene on nonmetallic glass is still challenging. Herein, the direct route of low sheet resistance graphene on glass is reported by using in situ-introduced water as a mild etchant and methane as a carbon precursor via chemical vapor deposition. The derived graphene features with large domain sizes and few amorphous carbon impurities. Intriguingly, the sheet resistance of graphene on glass is dramatically lowered down to ≈1170 Ω sq-1 at the optical transmittance ≈93%, ≈20% of that derived without the water etchant. Based on the highly conductive and optical transparent graphene on glass, a see-through thermochromic display is thus fabricated with transparent graphene glass as a heater. This work can motivate further investigations of the direct synthesis of high-quality graphene on functional glass and its versatile applications in transparent electronic devices or displays.

Wafer-Scale Growth of Single-Crystal 2D Semiconductor on Perovskite Oxides for High-Performance Transistors.

Emerging two-dimensional (2D) semiconducting materials serve as promising alternatives for next-generation digital electronics and optoelectronics. However, large-scale 2D semiconductor films synthesized so far are typically polycrystalline with defective grain boundaries that could degrade their performance. Here, for the first time, wafer-size growth of a single-crystal Bi2O2Se film, which is a novel air-stable 2D semiconductor with high mobility, was achieved on insulating perovskite oxide substrates [SrTiO3, LaAlO3, (La, Sr)(Al, Ta)O3]. The layered Bi2O2Se epilayer exhibits perfect lattice matching and strong interaction with perovskite oxide substrates, which enable unidirectional alignment and seamless mergence of multiple seeds into single-crystal continuous films free of detrimental grain boundaries. The single-crystal Bi2O2Se thin films show excellent spatial homogeneity over the entire wafer and allow for the batch fabrication of high-performance field-effect devices with high mobilities of ∼150 cm2 V-1 s-1 at room temperature, excellent switching behavior with large on/off ratio of >105, and high drive current of ∼45 µA µm-1 at a channel length of ∼5 µm. Our work makes a step toward the practical applications of high-mobility semiconducting 2D layered materials and provides an alternative platform of oxide heterostructure to investigate novel physical phenomena.

Low Residual Carrier Concentration and High Mobility in 2D Semiconducting Bi2O2Se.

The air-stable and high-mobility two-dimensional (2D) Bi2O2Se semiconductor has emerged as a promising alternative that is complementary to graphene, MoS2, and black phosphorus for next-generation digital applications. However, the room-temperature residual charge carrier concentration of 2D Bi2O2Se nanoplates synthesized so far is as high as about 1019-1020 cm-3, which results in a poor electrostatic gate control and unsuitable threshold voltage, detrimental to the fabrication of high-performance low-power devices. Here, we first present a facile approach for synthesizing 2D Bi2O2Se single crystals with ultralow carrier concentration of ∼1016 cm-3 and high Hall mobility up to 410 cm2 V-1 s-1 simultaneously at room temperature. With optimized conditions, these high-mobility and low-carrier-concentration 2D Bi2O2Se nanoplates with domain sizes greater than 250 µm and thicknesses down to 4 layers (∼2.5 nm) were readily grown by using Se and Bi2O3 powders as coevaporation sources in a dual heating zone chemical vapor deposition (CVD) system. High-quality 2D Bi2O2Se crystals were fabricated into high-performance and low-power transistors, showing excellent current modulation of >106, robust current saturation, and low threshold voltage of -0.4 V. All these features suggest 2D Bi2O2Se as an alternative option for high-performance low-power digital applications.

Exploiting Two-Dimensional Bi2 O2 Se for Trace Oxygen Detection.

We exploit a high-performing resistive-type trace oxygen sensor based on 2D high-mobility semiconducting Bi2 O2 Se nanoplates. Scanning tunneling microscopy combined with first-principle calculations confirms an amorphous Se atomic layer formed on the surface of 2D Bi2 O2 Se exposed to oxygen, which contributes to larger specific surface area and abundant active adsorption sites. Such 2D Bi2 O2 Se oxygen sensors have remarkable oxygen-adsorption induced variations of carrier density/mobility, and exhibit an ultrahigh sensitivity featuring minimum detection limit of 0.25 ppm, long-term stability, high durativity, and wide-range response to concentration up to 400 ppm at room temperature. 2D Bi2 O2 Se arrayed sensors integrated in parallel form are found to possess an oxygen detection minimum of sub-0.25 ppm ascribed to an enhanced signal-to-noise ratio. These advanced sensor characteristics involving ease integration show 2D Bi2 O2 Se is an ideal candidate for trace oxygen detection.

Bolometric Effect in Bi2 O2 Se Photodetectors.

Bi2 O2 Se is emerging as a photosensitive functional material for optoelectronics, and its photodetection mechanism is mostly considered to be a photoconductive regime in previous reports. Here, the bolometric effect is discovered in Bi2 O2 Se photodetectors. The coexistence of photoconductive effect and bolometric effect is generally observed in multiwavelength photoresponse measurements and then confirmed with microscale local heating experiments. The unique photoresponse of Bi2 O2 Se photodetectors may arise from a change of hot electrons during temperature rises instead of photoexcited holes and electrons. Direct proof of the bolometric effect is achieved by real-time temperature tracking of Bi2 O2 Se photodetectors under time evolution after light excitation. Moreover, the Bi2 O2 Se bolometer shows a high temperature coefficient of resistance (-1.6% K-1 ), high bolometric coefficient (-31 nA K-1 ), and high bolometric responsivity (>320 A W-1 ). These findings offer a new approach to develop bolometric photodetectors based on Bi2 O2 Se layered materials.

Low-Temperature and Rapid Growth of Large Single-Crystalline Graphene with Ethane.

Future applications of graphene rely highly on the production of large-area high-quality graphene, especially large single-crystalline graphene, due to the reduction of defects caused by grain boundaries. However, current large single-crystalline graphene growing methodologies are suffering from low growth rate and as a result, industrial graphene production is always confronted by high energy consumption, which is primarily caused by high growth temperature and long growth time. Herein, a new growth condition achieved via ethane being the carbon feedstock to achieve low-temperature yet rapid growth of large single-crystalline graphene is reported. Ethane condition gives a growth rate about four times faster than methane, achieving about 420 µm min-1 for the growth of sub-centimeter graphene single crystals at temperature about 1000 °C. In addition, the temperature threshold to obtain graphene using ethane can be reduced to 750 °C, lower than the general growth temperature threshold (about 1000 °C) with methane on copper foil. Meanwhile ethane always keeps higher graphene growth rate than methane under the same growth temperature. This study demonstrates that ethane is indeed a potential carbon source for efficient growth of large single-crystalline graphene, thus paves the way for graphene in high-end electronical and optoelectronical applications.

Controlled Synthesis of High-Mobility Atomically Thin Bismuth Oxyselenide Crystals.

Non-neutral layered crystals, another group of two-dimensional (2D) materials that lack a well-defined van der Waals (vdWs) gap, are those that form strong chemical bonds in-plane but display weak out-of-plane electrostatic interactions, exhibiting intriguing properties for the bulk counterpart. However, investigation of the properties of their atomically thin counterpart are very rare presumably due to the absence of efficient ways to achieve large-area high-quality 2D crystals. Here, high-mobility atomically thin Bi2O2Se, a typical non-neutral layered crystal without a standard vdWs gap, was synthesized via a facial chemical vapor deposition (CVD) method, showing excellent controllability for thickness, domain size, nucleation site, and crystal-phase evolution. Atomically thin, large single crystals of Bi2O2Se with lateral size up to ∼200 µm and thickness down to a bilayer were obtained. Moreover, optical and electrical properties of the CVD-grown 2D Bi2O2Se crystals were investigated, displaying a size-tunable band gap upon thinning and an ultrahigh Hall mobility of >20000 cm2 V-1 s-1 at 2 K. Our results on the high-mobility 2D Bi2O2Se semiconductor may activate the synthesis and related fundamental research of other non-neutral 2D materials.

Epitaxial Growth of Ternary Topological Insulator Bi2 Te2 Se 2D Crystals on Mica.

Nanostructures of ternary topological insulator (TI) Bi2 Te2 Se are, in principle, advantageous to the manifestation of topologically nontrivial surface states, due to significantly enhanced surface-to-volume ratio compared with its bulk crystals counterparts. Herein, the synthesis of 2D Bi2 Te2 Se crystals on mica via the van der Waals epitaxy method is explored and systematically the growth behaviors during the synthesis process are investigated. Accordingly, 2D Bi2 Te2 Se crystals with domain size up to 50 µm large and thickness down to 2 nm are obtained. A pronounced weak antilocalization effect is clearly observed in the 2D Bi2 Te2 Se crystals at 2 K. The method for epitaxial growth of 2D ternary Bi2 Te2 Se crystals may inspire materials engineering toward enhanced manifestation of the subtle surface states of TIs and thereby facilitate their potential applications in next-generation spintronics.

Integrated 2D multi-fin field-effect transistors.

Vertical semiconducting fins integrated with high-κ oxide dielectrics have been at the centre of the key device architecture that has promoted advanced transistor scaling during the last decades. Single-fin channels based on two-dimensional (2D) semiconductors are expected to offer unique advantages in achieving sub-1 nm fin-width and atomically flat interfaces, resulting in superior performance and potentially high-density integration. However, multi-fin structures integrated with high-κ dielectrics are commonly required to achieve higher electrical performance and integration density. Here we report a ledge-guided epitaxy strategy for growing high-density, mono-oriented 2D Bi2O2Se fin arrays that can be used to fabricate integrated 2D multi-fin field-effect transistors. Aligned substrate steps enabled precise control of both nucleation sites and orientation of 2D fin arrays. Multi-channel 2D fin field-effect transistors based on epitaxially integrated 2D Bi2O2Se/Bi2SeO5 fin-oxide heterostructures were fabricated, exhibiting an on/off current ratio greater than 106, high on-state current, low off-state current, and high durability. 2D multi-fin channel arrays integrated with high-κ oxide dielectrics offer a strategy to improve the device performance and integration density in ultrascaled 2D electronics.

Surface-bulk coupling in a Bi2Te3 nanoplate grown by van der Waals epitaxy.

We report an experimental study of the effect of coherent surface-bulk electron scattering on quantum transport in a three-dimensional topological insulator Bi2Te3 nanoplate. The nanoplate is grown via van der Waals epitaxy on a mica substrate and a top-gated Hall-bar device is fabricated from the nanoplate directly on the growth substrate. Top-gate voltage dependent measurements of the sheet resistance of the device reveal that the transport carriers in the nanoplate are of n-type and that, with decreasing top gate voltage, the carrier density in the nanoplate is decreased. However, the mobility is increased with decreasing top-gate voltage. This mobility increase with decreasing carrier density in the nanoplate is demonstrated to arise from a decrease in bulk-to-surface electron scattering rate. Low-field magnetotransport measurements are performed at low temperatures. The measured magnetoconductivity of the nanoplate shows typical weak anti-localization (WAL) characteristics. We analyze the measurements by taking surface-bulk inter-channel electron scattering into account and extract dephasing times τφ, diffusion coefficients D of electrons at the top surface and in the bulk, and the surface-bulk scattering times τSB as a function of top-gate voltage and temperature. It is found that the dephasing in the nanoplate arises dominantly from electron-electron scattering with small energy transfers. It is also found that the ratio of τφ/τSB (a measure of the surface-bulk electron coherent coupling) is decreased with decreasing gate voltage or increasing temperature. We demonstrate that taking the surface-bulk coherent electron scattering in our Bi2Te3 nanoplate into account is essential to understand quantum transport measurements at low temperatures.

Step-Climbing Epitaxy of Layered Materials with Giant Out-of-Plane Lattice Mismatch.

Heteroepitaxy with large lattice mismatch remains a great challenge for high-quality epifilm growth. Although great efforts have been devoted to epifilm growth with an in-plane lattice mismatch, the epitaxy of 2D layered crystals on stepped substrates with a giant out-of-plane lattice mismatch is seldom reported. Here, taking the molecular-beam epitaxy of 2D semiconducting Bi2 O2 Se on 3D SrTiO3 substrates as an example, a step-climbing epitaxy growth strategy is proposed, in which the n-th (n = 1, 2, 3…) epilayer climbs the step with height difference from out-of-plane lattice mismatch and continues to grow the n+1-th epilayer. Step-climbing epitaxy can spontaneously relax and release the strain from the out-of-plane lattice mismatch, which ensures the high quality of large-area epitaxial films. Wafer-scale uniform 2D Bi2 O2 Se single-crystal films with controllable thickness can be obtained via step-climbing epitaxy. Most notably, one-unit-cell Bi2 O2 Se films (1.2 nm thick) exhibit a high Hall mobility of 180 cm2 V-1 s-1 at room temperature, which exceeds that of silicon and other 2D semiconductors with comparable thickness. As an out-of-plane lattice mismatch is generally present in the epitaxy of layered materials, the step-climbing epitaxy strategy expands the existing epitaxial growth theory and provides guidance toward the high-quality synthesis of layered materials.

Momentum-matching and band-alignment van der Waals heterostructures for high-efficiency infrared photodetection.

Two-dimensional (2D) infrared photodetectors always suffer from low quantum efficiency (QE) because of the limited atomically thin absorption. Here, we reported 2D black phosphorus (BP)/Bi2O2Se van der Waals (vdW) photodetectors with momentum-matching and band-alignment heterostructures to achieve high QE. The QE was largely improved by optimizing the generation, suppressing the recombination, and improving the collection of photocarriers. Note that momentum-matching BP/Bi2O2Se heterostructures in k-space lead to the highly efficient generation and transition of photocarriers. The recombination process can be largely suppressed by lattice mismatching-immune vdW interfaces. Furthermore, type II BP/Bi2O2Se vdW heterostructures could also assist fast transport and collection of photocarriers. By constructing momentum-matching and band-alignment heterostructures, a record-high QE of 84% at 1.3 micrometers and 76.5% at 2 micrometers have been achieved in BP/Bi2O2Se vdW photodetectors.

Integrated wafer-scale ultra-flat graphene by gradient surface energy modulation.

The integration of large-scale two-dimensional (2D) materials onto semiconductor wafers is highly desirable for advanced electronic devices, but challenges such as transfer-related crack, contamination, wrinkle and doping remain. Here, we developed a generic method by gradient surface energy modulation, leading to a reliable adhesion and release of graphene onto target wafers. The as-obtained wafer-scale graphene exhibited a damage-free, clean, and ultra-flat surface with negligible doping, resulting in uniform sheet resistance with only ~6% deviation. The as-transferred graphene on SiO2/Si exhibited high carrier mobility reaching up ~10,000 cm2 V-1 s-1, with quantum Hall effect (QHE) observed at room temperature. Fractional quantum Hall effect (FQHE) appeared at 1.7 K after encapsulation by h-BN, yielding ultra-high mobility of ~280,000 cm2 V-1 s-1. Integrated wafer-scale graphene thermal emitters exhibited significant broadband emission in near-infrared (NIR) spectrum. Overall, the proposed methodology is promising for future integration of wafer-scale 2D materials in advanced electronics and optoelectronics.