A reliable and rapid manufacturing process of molybdenum disulfide (MoS2) with atomic‐scale thicknesses remains a fundamental challenge toward its successful incorporation into high‐performance ...nanoelectronics. It is imperative to achieve rapid and scalable production of MoS2 exhibiting high carrier mobility and excellent on/off current ratios simultaneously. Herein, inhibitor‐utilizing atomic layer deposition (iALD) is presented as a novel method to meet these requirements at the wafer scale. The kinetics of the chemisorption of Mo precursors in iALD is governed by the reaction energy and the steric hindrance of inhibitor molecules. By optimizing the inhibition of Mo precursor absorption, the nucleation on the substrate in the initial stage can be spontaneously tailored to produce iALD‐MoS2 thin films with a significantly increased grain size and surface coverage (>620%). Moreover, highly crystalline iALD‐MoS2 thin films, with thicknesses of only a few layers, excellent room temperature mobility (13.9 cm2 V−1 s−1), and on/off ratios (>108), employed as the channel material in field effect transistors on 6″ wafers, are successfully prepared.
High‐mobility MoS2 thin films with inhibitor‐utilizing atomic layer deposition (iALD) are successfully demonstrated on a wafer scale. As a reliable and rapid manufacturing process, MoS2‐iALD, based on the kinetics of Mo precursor adsorption, enables extraordinary grain growth and surface coverage, leading to an excellent room‐temperature mobility (13.9 cm2 V−1 s−1) and on/off ratios (>108), as a channel material in a field‐effect transistor.
2D vertical stacking and lateral stitching growth of monolayer (ML) hexagonal transition‐metal dichalcogenides are reported. The 2D heteroepitaxial manipulation of MoS2 and WS2 MLs is achieved by ...control of the 2D nucleation kinetics during the sequential vapor‐phase growth. It enables the creation of hexagon‐on‐hexagon unit‐cell stacking and hexagon‐by‐hexagon stitching without interlayer rotation misfits.
van der Waals layered materials have large crystal anisotropy and crystallize spontaneously into two-dimensional (2D) morphologies. Two-dimensional materials with hexagonal lattices are emerging 2D ...confined electronic systems at the limit of one or three atom thickness. Often these 2D lattices also form orthorhombic symmetries, but these materials have not been extensively investigated, mainly due to thermodynamic instability during crystal growth. Here, we show controlled polymorphic growth of 2D tin-sulfide crystals of either hexagonal SnS2 or orthorhombic SnS. Addition of H2 during the growth reaction enables selective determination of either n-type SnS2 or p-type SnS 2D crystal of dissimilar energy band gap of 2.77 eV (SnS2) or 1.26 eV (SnS) as a final product. Based on this synthetic 2D polymorphism of p–n crystals, we also demonstrate p–n heterojunctions for rectifiers and photovoltaic cells, and complementary inverters.
Two-dimensional stacks of dissimilar hexagonal monolayers exhibit unusual electronic, photonic and photovoltaic responses that arise from substantial interlayer excitations. Interband excitation ...phenomena in individual hexagonal monolayer occur in states at band edges (valleys) in the hexagonal momentum space; therefore, low-energy interlayer excitation in the hexagonal monolayer stacks can be directed by the two-dimensional rotational degree of each monolayer crystal. However, this rotation-dependent excitation is largely unknown, due to lack in control over the relative monolayer rotations, thereby leading to momentum-mismatched interlayer excitations. Here, we report that light absorption and emission in MoS2/WS2 monolayer stacks can be tunable from indirect- to direct-gap transitions in both spectral and dynamic characteristics, when the constituent monolayer crystals are coherently stacked without in-plane rotation misfit. Our study suggests that the interlayer rotational attributes determine tunable interlayer excitation as a new set of basis for investigating optical phenomena in a two-dimensional hexagonal monolayer system.
In general, in thermoelectric materials the electrical conductivity σ and thermal conductivity κ are related and thus cannot be controlled independently. Previously, to maximize the thermoelectric ...figure of merit in state-of-the-art materials, differences in relative scaling between σ and κ as dimensions are reduced to approach the nanoscale were utilized. Here we present an approach to thermoelectric materials using tin disulfide, SnS2, nanosheets that demonstrated a negative correlation between σ and κ. In other words, as the thickness of SnS2 decreased, σ increased whereas κ decreased. This approach leads to a thermoelectric figure of merit increase to 0.13 at 300 K, a factor ∼1,000 times greater than previously reported bulk single-crystal SnS2. The Seebeck coefficient obtained for our two-dimensional SnS2 nanosheets was 34.7 mV K(-1) for 16-nm-thick samples at 300 K.
HfO2-based ferroelectric thin films deposited via atomic layer deposition have been extensively studied as promising candidates for next-generation ferroelectric devices. The conversion of an ...amorphous Hf1-xZrxO2 film to the ferroelectric phase (non-centrosymmetric orthorhombic phase) has been achieved through annealing using a post-thermal process. However, in this study, we present the first report of ferroelectricity of hafnium-zirconium-oxide (HZO) thin films deposited via atomic layer deposition using cyclopentadienyl-based precursors without additional post-thermal processing. By increasing the deposition temperature using a cyclopentadienyl-based cocktail precursor, the conditions of the as-deposited HZO thin film to crystallize well with an orthorhombic phase were secured, and excellent ferroelectric properties with a large remanent polarization (2Pr ∼ 47.6 μC cm-2) were implemented without crystallization annealing. The as-deposited HZO thin film possessed very stable ferroelectric properties without a wake-up effect or significant fatigue up to 106 cycles. Futhermore, we demonstrated the applicability to devices using negative capacitance and non-volatile memory characteristics. This result suggests that a new strategy can be applied to ferroelectric devices where subsequent processing temperature constraints are required, such as back-end-of-line processes and ferroelectric-based flexible device applications.
2D layered materials with sensitive surfaces are promising materials for use in chemical sensing devices, owing to their extremely large surface‐to‐volume ratios. However, most chemical sensors based ...on 2D materials are used in the form of laterally defined active channels, in which the active area is limited to the actual device dimensions. Therefore, a novel approach for fabricating self‐formed active‐channel devices is proposed based on 2D semiconductor materials with very large surface areas, and their potential gas sensing ability is examined. First, the vertical growth phenomenon of SnS2 nanocrystals is investigated with large surface area via metal‐assisted growth using prepatterned metal electrodes, and then self‐formed active‐channel devices are suggested without additional pattering through the selective synthesis of SnS2 nanosheets on prepatterned metal electrodes. The self‐formed active‐channel device exhibits extremely high response values (>2000% at 10 ppm) for NO2 along with excellent NO2 selectivity. Moreover, the NO2 gas response of the gas sensing device with vertically self‐formed SnS2 nanosheets is more than two orders of magnitude higher than that of a similar exfoliated SnS2‐based device. These results indicate that the facile device fabrication method would be applicable to various systems in which surface area plays an important role.
A novel approach for fabricating self‐formed active‐channel devices based on 2D semiconductor materials with very large surface areas is proposed and their potential gas sensing ability is examined. The device exhibits extremely high response values and this novel fabrication method is expected to find use in various applications where surface area plays an important role in function.
Plants must perceive and rapidly respond to changes in ambient temperature for their successful reproduction. Here we demonstrate that Arabidopsis SHORT VEGETATIVE PHASE (SVP) plays an important role ...in the response of plants to ambient temperature changes. The loss of SVP function elicited insensitivity to ambient temperature changes. SVP mediates the temperature-dependent functions of FCA and FVE within the thermosensory pathway. SVP controls flowering time by negatively regulating the expression of a floral integrator, FLOWERING LOCUS T (FT), via direct binding to the CArG motifs in the FT sequence. We propose that this is one of the molecular mechanisms that modulate flowering time under fluctuating temperature conditions.
The dielectric properties of the Si-doped Zr1-xHfxO2 thin films were investigated over a broad compositional range with the goal of improving their properties for use as DRAM capacitor materials. The ...Si-doped Zr1-xHfxO2 thin films were deposited on TiN bottom electrodes by atomic layer deposition using a TEMA-Zr/TEMA-Hf mixture precursor for deposition of Zr1-xHfxO2 film and Tris-EMASiH as a Si precursor. The Si stabilizer increased the tetragonality and the dielectric constant; however, at high fractions of Si, the crystal structure degraded to amorphous and the dielectric constant decreased. Doping with Si exhibited a larger influence on the dielectric constant at higher Hf content. A Si-doped Hf-rich Zr1-xHfxO2 thin film, with tetragonal structure, exhibited a dielectric constant of about 50. This is the highest value among all reported results for Zr and Hf oxide systems, and equivalent oxide thickness (EOT) value of under 0.5 nm could be obtained with a leakage current of under 10(-7) A·cm(-2), which is the lowest EOT value ever reported for a DRAM storage capacitor system without using a noble-metal-based electrode.
ZnO thin films were deposited via atomic layer deposition (ALD) using H2O and H2O2 as oxidants with substrate temperatures from 100°C to 200°C. The ZnO films deposited using H2O2 (H2O2‐ZnO) showed ...lower growth rates than those deposited with H2O (H2O‐ZnO) at these temperature range due to the lower vapor pressure of H2O2, which produces fewer OH− functional groups; the H2O2‐ZnO films exhibited higher electrical resistivities than the H2O‐ZnO films. The selection of H2O2 or H2O as oxidants was revealed to be very important for controlling the electrical properties of ALD‐ZnO thin films, as it affected the film crystallinity and number of defects. Compared to H2O‐ZnO, H2O2‐ZnO exhibited poor crystallinity within a growth temperature range of 100‐200°C, while H2O2‐ZnO showed a strong (002) peak intensity. Photoluminescence showed that H2O2‐ZnO had more interstitial oxygen and fewer oxygen vacancies than H2O‐ZnO. Finally, both kinds of ZnO thin films were prepared as transparent resistive oxide layers for CIGS solar cells and were evaluated.
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