Effects of HfO2 encapsulation on electrical performances of few-layered MoS2 transistor with ALD HfO2 as back-gate dielectric

Modifying the Power and Performance of 2-Dimensional MoS2 Field Effect Transistors

Over the past 60 years, the semiconductor industry has been the core driver for the development of information technology, contributing to the birth of integrated circuits, Internet, artificial intelligence, and Internet of Things. Semiconductor technology has been evolving in structure and material with co-optimization of performance-power-area-cost until the state-of-the-art sub-5-nm node. Two-dimensional (2D) semiconductors are recognized by the industry and academia as a hopeful solution to break through the quantum confinement for the future technology nodes. In the recent 10 years, the key issues on 2D semiconductors regarding material, processing, and integration have been overcome in sequence, making 2D semiconductors already on the verge of application. In this paper, the evolution of transistors is reviewed by outlining the potential of 2D semiconductors as a technological option beyond the scaled metal oxide semiconductor field-effect transistors. We mainly focus on the optimization strategies of mobility (μ), equivalent oxide thickness (EOT), and contact resistance (R_C ), which enables high ON current (I_on ) with reduced driving voltage (V_dd ). Finally, we prospect the semiconductor technology roadmap by summarizing the technological development of 2D semiconductors over the past decade.

Atomic Layer Deposition of Metal Oxides and Chalcogenides for High Performance Transistors

Atomic layer deposition (ALD) is a deposition technique well-suited to produce high-quality thin film materials at the nanoscale for applications in transistors. This review comprehensively describes the latest developments in ALD of metal oxides (MOs) and chalcogenides with tunable bandgaps, compositions, and nanostructures for the fabrication of high-performance field-effect transistors. By ALD various n-type and p-type MOs, including binary and multinary semiconductors, can be deposited and applied as channel materials, transparent electrodes, or electrode interlayers for improving charge-transport and switching properties of transistors. On the other hand, MO insulators by ALD are applied as dielectrics or protecting/encapsulating layers for enhancing device performance and stability. Metal chalcogenide semiconductors and their heterostructures made by ALD have shown great promise as novel building blocks to fabricate single channel or heterojunction materials in transistors. By correlating the device performance to the structural and chemical properties of the ALD materials, clear structure-property relations can be proposed, which can help to design better-performing transistors. Finally, a brief concluding remark on these ALD materials and devices is presented, with insights into upcoming opportunities and challenges for future electronics and integrated applications.

Thermal Conductivity of Large-Area Polycrystalline MoSe2 Films Grown by Chemical Vapor Deposition

It is of great importance to understand the thermal properties of MoSe₂ films for electronic and optoelectronic applications. In this work, large-area polycrystalline MoSe₂ films are prepared using a low-cost, controllable, large-scale, and repeatable chemical vapor deposition method, which facilitates direct device fabrication. Raman spectra and X-ray diffraction patterns indicate a hexagonal (2H) crystal structure of the MoSe₂ film. Ellipsometric spectra analysis indicates that the optical band gap of the MoSe₂ film is estimated to be ∼1.23 eV. From the analysis of the temperature-dependent and laser-power-dependent Raman spectra, the thermal conductivity of the suspended MoSe₂ films is found to be ∼28.48 W/(m·K) at room temperature. The results can provide useful guidance for an effective thermal management of large-area polycrystalline MoSe₂-based electronic and optoelectronic devices.

High mobility field-effect transistors based on MoS2crystals grown by the flux method

Two-dimensional (2D) molybdenum disulphide (MoS₂) transition metal dichalcogenides (TMDs) have great potential for use in optical and electronic device applications; however, the performance of MoS₂is limited by its crystal quality, which serves as a measure of the defects and grain boundaries in the grown material. Therefore, the high-quality growth of MoS₂crystals continues to be a critical issue. In this context, we propose the formation of high-quality MoS₂crystals via the flux method. The resulting electrical properties demonstrate the significant impact of crystal morphology on the performance of MoS₂field-effect transistors. MoS₂made with a relatively higher concentration of sulphur (a molar ratio of 2.2) and at a cooling rate of 2.5 °C h^-1yielded good quality and optimally sized crystals. The room-temperature and low-temperature (77 K) electrical transport properties of MoS₂field-effect transistors (FETs) were studied in detail, with and without the use of a hexagonal boron nitride (h-BN) dielectric to address the mobility degradation issue due to scattering at the SiO₂/2D material interface. A maximum field-effect mobility of 113 cm²V^-1s^-1was achieved at 77 K for the MoS₂/h-BN FET following high-quality crystal formation by the flux method. Our results confirm the achievement of large-scale high-quality crystal growth with reduced defect density using the flux method and are key to achieving higher mobility in MoS₂FET devices in parallel with commercially accessible MoS₂crystals.

A 2D material-based floating gate device with linear synaptic weight update

Neuromorphic computing is of great interest among researchers interested in overcoming the von Neumann computing bottleneck. A synaptic device, one of the key components to realize a neuromorphic system, has a weight that indicates the strength of the connection between two neurons, and updating this weight must have linear and symmetric characteristics. Especially, a transistor-type device has a gate terminal, separating the processes of reading and updating the conductivity, used as a synaptic weight to prevent sneak path current issues during synaptic operations. In this study, we fabricate a top-gated flash memory device based on two-dimensional (2D) materials, MoS2 and graphene, as a channel and a floating gate, respectively, and Al2O3 and HfO2 to increase the tunneling efficiency. We demonstrate the linear weight updates and repeatable characteristics of applying negative/positive pulses, and also emulate spike timing-dependent plasticity (STDP), one of the learning rules in a spiking neural network (SNN).

Hafnium (IV) oxide obtained by atomic layer deposition (ALD) technology promotes early osteogenesis via activation of Runx2-OPN-mir21A axis while inhibits osteoclasts activity

BACKGROUND

Due to increasing aging of population prevalence of age-related disorders including osteoporosis is rapidly growing. Due to health and economic impact of the disease, there is an urgent need to develop techniques supporting bone metabolism and bone regeneration after fracture. Due to imbalance between bone forming and bone resorbing cells, the healing process of osteoporotic bone is problematic and prolonged. Thus searching for agents able to restore the homeostasis between these cells is strongly desirable.

RESULTS

In the present study, using ALD technology, we obtained homogeneous, amorphous layer of hafnium (IV) oxide (HfO2). Considering the specific growth rate (1.9Å/cycle) for the selected process at the temperature of 90 °C, we performed the 100 nm deposition process, which was confirmed by measuring film thickness using reflectometry. Then biological properties of the layer were investigated with pre-osteoblast (MC3T3), pre-osteoclasts (4B12) and macrophages (RAW 264.7) using immunofluorescence and RT-qPCR. We have shown, that HfO2 (i) enhance osteogenesis, (ii) reduce osteoclastogenesis (iii) do not elicit immune response and (iv) exert anti-inflammatory effects.

CONCLUSION

HfO2 layer can be applied to cover the surface of metallic biomaterials in order to enhance the healing process of osteoporotic bone fracture.

Optimizing Al-doped ZrO2 as the gate dielectric for MoS2 field-effect transistors

In this work, we investigate the effects on the electrical properties of few-layered MoS₂ field-effect transistors (FETs) following Al incorporation into ZrO₂ as the gate dielectrics of the devices. A large improvement in device performance is achieved with the Al-doped ZrO₂ gate dielectric when Zr:Al = 1:1. The relevant MoS₂ transistor exhibits the best electrical characteristics: high carrier mobility of 40.6 cm² V^-1 s^-1 (41% higher than that of the control sample, and an intrinsic mobility of 68.0 cm² V^-1 s^-1), a small subthreshold swing of 143 mV dec^-1, high on/off current ratio of 6 × 10⁶ and small threshold voltage of 0.71 V. These are attributed to the facts that (i) Al incorporation into ZrO₂ can decrease its oxygen vacancies; densify the dielectric film; and smooth the gate dielectric surface, thus reducing the traps at/near the Zr_0.5Al_0.5O _y /MoS₂ interface and the gate leakage current; (ii) adjusting the dielectric constant of the gate dielectric to an appropriate value, which achieves a reasonable trade-off between the gate screening effect on the Coulomb-impurity scattering and the surface optical phonon scattering. These results demonstrate that optimized Zr_0.5Al_0.5O_y is a potential gate dielectric material for MoS₂ FET applications.

In situ transport measurements reveal source of mobility enhancement of MoS2 and MoTe2 during dielectric deposition

Layered transition metal dichalcogenides (TMDs) and other two-dimensional (2D) materials are promising candidates for enhancing the capabilities of complementary metal-oxide-semiconductor (CMOS) technology. Field-effect transistors (FETs) made with 2D materials often exhibit mobilities below their theoretical limit, and strategies such as encapsulation with dielectrics grown by atomic layer deposition (ALD) have been explored to tune carrier concentration and improve mobility. While molecular adsorbates are known to dope 2D materials and influence charge scattering mechanisms, it is not well understood how ALD reactants affect 2D transistors during growth, motivating in situ or operando studies. Here, we report electrical characterization of MoS₂ and MoTe₂ FETs during ALD of MoO_x. The field effect mobility improves significantly within the first five cycles of ALD growth using Mo(NMe₂)₄ as the metal-organic precursor and H₂O as the oxidant. Analyses of the in situ transconductance at the growth temperature and ex situ variable temperature transconductance measurements indicate that the majority of the mobility enhancement observed at the beginning of dielectric growth is due to screening of charged impurity scattering by the adlayer. Control experiments show that exposure to only H₂O or O₂ induces more modest and reversible electronic changes in MoTe2 FETs, indicating that negligible oxidation of the TMD takes place during the ALD process. Due to the strong influence of the first <2 nm of deposition, when the dielectric adlayer may be discontinuous and still evolving in stoichiometry, this work highlights the need for further assessment of nucleation layers and initial deposition chemistry, which may be more important than the bulk composition of the oxide itself in optimizing performance and reproducibility.