Wafer-Scale Epitaxial Growth of an Atomically Thin Single-Crystal Insulator as a Substrate of Two-Dimensional Material Field-Effect Transistors.

As the electron mobility of two-dimensional (2D) materials is dependent on an insulating substrate, the nonuniform surface charge and morphology of silicon dioxide (SiO2) layers degrade the electron mobility of 2D materials. Here, we demonstrate that an atomically thin single-crystal insulating layer of silicon oxynitride (SiON) can be grown epitaxially on a SiC wafer at a wafer scale and find that the electron mobility of graphene field-effect transistors on the SiON layer is 1.5 times higher than that of graphene field-effect transistors on typical SiO2 films. Microscale and nanoscale void defects caused by heterostructure growth were eliminated for the wafer-scale growth of the single-crystal SiON layer. The single-crystal SiON layer can be grown on a SiC wafer with a single thermal process. This simple fabrication process, compatible with commercial semiconductor fabrication processes, makes the layer an excellent replacement for the SiO2/Si wafer.

New Insights into Mechanism of Surface Reactions of ZnO Nanorods During Electrons Beam Irradiation.

This study provides new insight into mechanisms of ionic reactions on the surface of ZnO nanorod networks, which could result in enhanced performance in optical or molecular sensors. The current- voltage characteristics of ZnO nanorod network devices exhibit typical nonlinear behavior in air, which implies the formation of a Schottky barrier when metals are used as contacts. The conductance of the device increased significantly in vacuum, which can be explained by the desorption of hydroxyl groups at very low pressure. While physisorbed water or oxygen-related ions can detach from the ZnO surface during evacuation, exposure to high energy in the electron beam is believed to detach the chemisorbed anions of O- and O-2 from the surface of ZnO nanorods, which releases more electrons into the channel. The increase in available electrons enhances the conductance of the ZnO nanorods. Slow initialization of the conductance under ambient conditions indicates that the ionic re-adsorption is inactive under these conditions. Thus, the electron irradiation process can be used to reset the surface ionic molecules on metal oxide nano-structures by tuning the surface potential prior to the passivation process.

Evenly transferred single-layered graphene membrane assisted by strong substrate adhesion.

We explored the transfer of a single-layered graphene membrane assisted by substrate adhesion. A relatively larger adhesion force was measured on the SiO2 substrate compared with its van der Waals contribution, which is expected to result from the additional contribution of the chemical bonding force. Density functional theory calculations verified that the strong adhesion force was indeed accompanied by chemical bonding. The transfer of single-layered graphene and subsequent deposition of the dielectric layer were best performed on the SiO2 substrate exhibiting a larger adhesion force. This study suggests the selection and/or modification of the underlying substrate for proper transfer of graphene as well as other 2D materials similar to graphene.

Improved electroluminescence of quantum dot light-emitting diodes enabled by a partial ligand exchange with benzenethiol.

In this study, benzenethiol ligands were applied to the surface of CdSe@ZnS core@shell quantum dots (QDs) and their effect on the performance of quantum dot light-emitting diodes (QD-LEDs) was investigated. Conventional long-chained oleic acid (OA) and trioctylphosphine (TOP) capping ligands were partially replaced by short-chained benzenethiol ligands in order to increase the stability of QDs during purification and also improve the electroluminescence performance of QD-LEDs. The quantum yield of the QD solution was increased from 41% to 84% by the benzenethiol ligand exchange. The mobility of the QD films with benzenethiol ligands approximately doubled to 2.42 × 10(-5) cm(2) V(-1) s(-1) from 1.19 × 10(-5) cm(2) V(-1) s(-1) compared to the device consisting of OA/TOP-capped QDs, and an approximately 1.8-fold improvement was achieved over QD-LEDs fabricated with bezenethiol ligand-exchanged QDs with respect to the maximum luminance and current efficiency. The turn-on voltage decreased by about -0.6 V through shifting the energy level of the QDs with benzenethiol ligands compared to conventional OA and TOP ligands.

Effects of Sn doping on the growth morphology and electrical properties of ZnO nanowires.

This study examines the effects of doping ZnO nanowires (NWs) with Sn on the growth morphology and electrical properties. ZnO NWs with various Sn contents (1-3 at.%) were synthesized using the vapor-liquid-solid method. Scanning electron and transmission electron microscopy analyses showed that all of the Sn-doped NWs grew in a bamboo-like morphology, in which stacking faults enriched with Sn were periodically inserted. We fabricated a hybrid film of InZnO sol-gel and Sn-doped ZnO NW networks to characterize the effects of Sn doping on the electrical properties of the NWs. With increasing doping density, the carrier concentration increases significantly while the mobility decreases greatly. The resistivity remains scattered, which suggests that Sn doping in ZnO is not an effective method for the enhancement of conductivity, since Sn does not readily incorporate into the ZnO structure.

Enhanced morphological and thermal stabilities of nickel germanide with an ultrathin tantalum layer studied by ex situ and in situ transmission electron microscopy.

The formation and morphological evolution of germanides formed in a ternary Ni/Ta-interlayer/Ge system were examined by ex situ and in situ annealing experiments. The Ni germanide film formed in the Ni/Ta-interlayer/Ge system maintained continuity up to 550°C, whereas agglomeration of the Ni germanide occurred in the Ni/Ge system without Ta-interlayer. Through microstructural and chemical analysis of the Ni/Ta-interlayer/Ge system during and after in situ annealing in a transmission electron microscope, it was confirmed that the Ta atoms remained uniformly on the top of the newly formed Ni germanide layer during the diffusion reaction. Consequently, the agglomeration of the Ni germanide film was retarded and the thermal stability was improved by the Ta incorporation.

Three-Dimensional Surface Treatment of MoS2 Using BCl3 Plasma-Derived Radicals.

The realization of next-generation gate-all-around field-effect transistors (FETs) using two-dimensional transition metal dichalcogenide (TMDC) semiconductors necessitates the exploration of a three-dimensional (3D) and damage-free surface treatment method to achieve uniform atomic layer-deposition (ALD) of a high-k dielectric film on the inert surface of a TMDC channel. This study developed a BCl3 plasma-derived radical treatment for MoS2 to functionalize MoS2 surfaces for the subsequent ALD of an ultrathin Al2O3 film. Microstructural verification demonstrated a complete coverage of an approximately 2 nm-thick Al2O3 film on a planar MoS2 surface, and the applicability of the technique to 3D structures was confirmed using a suspended MoS2 channel floating from the substrate. Density functional theory calculations supported by optical emission spectroscopy and X-ray photoelectron spectroscopy measurements revealed that BCl radicals, predominantly generated by the BCl3 plasma, adsorbed on MoS2 and facilitated the uniform nucleation of ultrathin ALD-Al2O3 films. Raman and photoluminescence measurements of monolayer MoS2 and electrical measurements of a bottom-gated FET confirmed negligible damage caused by the BCl3 plasma-derived radical treatment. Finally, the successful operation of a top-gated FET with an ultrathin ALD-Al2O3 (∼5 nm) gate dielectric film was demonstrated, indicating the effectiveness of the pretreatment.

Microstructure and dielectric characteristics of high-k tetragonal ZrO2 films with various thicknesses processed by sol-gel method.

High-k ZrO2/Si films were fabricated by a sol-gel method and the effects of the thickness of ZrO2 on the phase formation, interface chemical structure, and dielectric performance were studied. The 0.1 M precursor sol was prepared by using Zr acetylacetonate, coated, dried on Si substrates, and finally annealed at 500 degrees C. The thickness of ZrO2 was varied in the range from 7 to 51 nm by repeating the coating and drying sequences. The deposited ZrO2 was amorphous for the sample with a thickness of -7 nm, but tetragonal (t-) phases appeared as the thickness increased. As the thickness increased, the flat-band voltage and hysteresis width in the capacitance-voltage curves increased. The sol-gel deposited ZrO2 dielectrics showed a high k value (-33) due to the formation of the t-phase, while retaining gate leakage current levels of less than -4.0 x 10(-5) A/cm2 at 1 MV/cm.

Simultaneous plasma and thermal treatments of ITO surfaces for organic solar cells.

Organic solar cells' power conversion efficiency was improved by combined thermal and CF4 plasma surface treatments of indium tin oxide (ITO). This was further enhanced by adding O2 to the CF4 plasma, which increased atomic fluorine concentration. Organic solar cells were produced with layered structures of ITO, spin-cast thin film of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) mixture (100 nm), and Al (100 nm) as top electrode. While CF4 plasma surface treatment alone significantly improved power conversion efficiency (PCE), from 0.34% to 1.99%, further enhancements occurred with the combined thermal treatment, up to 2.24%. 20% O2 addition to the CF4 plasma resulted in greatest improvement (up to 2.34%) due to optimized surface fluorination. Measured work functions of surface treated ITO increased from 4.80 to 5.17 eV with the plasma and thermal treatments. Such treatments reduce the energy barrier for hole injection at the ITO surface by increasing ITO's work function.

Surface modification of carbon post arrays by atomic layer deposition of ZnO film.

The applicability of atomic layer deposition (ALD) process to the carbon microelectromechanical system technology was studied for a surface modification method of the carbon post electrodes. A conformal coating of the ALD-ZnO film was successfully demonstrated on the carbon post arrays which were fabricated by the traditional photolithography and subsequent two-step pyrolysis. A significant Zn diffusion into the underlying carbon posts was observed during the ALD process. The addition of a sputter-deposited ZnO interfacial layer efficiently blocked the Zn diffusion without altering the microstructure and surface morphology of the ALD-ZnO film.

Unveiling the Origin of Robust Ferroelectricity in Sub-2 nm Hafnium Zirconium Oxide Films.

HfO2-based ferroelectrics are highly expected to lead the new paradigm of nanoelectronic devices owing to their unexpected ability to enhance ferroelectricity in the ultimate thickness scaling limit (≤2 nm). However, an understanding of its physical origin remains uncertain because its direct microstructural and chemical characterization in such a thickness regime is extremely challenging. Herein, we solve the mystery for the continuous retention of high ferroelectricity in an ultrathin hafnium zirconium oxide (HZO) film (∼2 nm) by unveiling the evolution of microstructures and crystallographic orientations using a combination of state-of-the-art structural analysis techniques beyond analytical limits and theoretical approaches. We demonstrate that the enhancement of ferroelectricity in ultrathin HZO films originates from textured grains with a preferred orientation along an unusual out-of-plane direction of (112). In principle, (112)-oriented grains can exhibit 62% greater net polarization than the randomly oriented grains observed in thicker samples (>4 nm). Our first-principles calculations prove that the hydroxyl adsorption during the deposition process can significantly reduce the surface energy of (112)-oriented films, thereby stabilizing the high-index facet of (112). This work provides new insights into the ultimate scaling of HfO2-based ferroelectrics, which may facilitate the design of future extremely small-scale logic and memory devices.

Effects of Plasma Damage Removal on Direct Contact Resistance and Hot-Electron-Induced Punch Through (HEIP) of PMOSFETs.

In order to reduce contact resistance (Rc) of the source/drain region in nanoscale devices, it is essential to overcome the increasing leakage and hot-electron-induced punch through (HEIP) degradation. In this paper, we propose a simple in situ Si soft treatment technique immediately after direct contact (DC) etching to reduce Rc and minimize HEIP degradation. We found by analysis with a transmission electron microscope, that 10 s of treatment reduced the plasma damaged layer by 19%, which resulted in 10.5% reduction of the P+ contact resistance. For comparison, the P + Rc was reduced by 6.5% when the doping level of the plug implantation was increased by 25%, but the HEIP breakdown voltage (VHEIP) by AC stress was greatly reduced by more than 80 mV, increasing the standby leakage current of DRAM devices. In the case of removing the plasma damage layer, not only did VHIEP not decrease until after 10 s, but also the reduction in Rc was larger than with the plug enhancement. The effect of the plasma damaged layer on HEIP was verified through the plug effect and gate induced drain leakage measurement, based on the distance between the gate and DC for each process. This simple in situ technique not only removed byproducts and the plasma damaged amorphous layer, but it also affected the effective implantation of dopants in subsequent plug processes. It was also cost effective because the process time was short and no extra process steps were added.

Characteristics of an Amorphous Carbon Layer as a Diffusion Barrier for an Advanced Copper Interconnect.

The size of the advanced Cu interconnects has been significantly reduced, reaching the current 7.0 nm node technology and below. With the relentless scaling-down of microelectronic devices, the advanced Cu interconnects thus requires an ultrathin and reliable diffusion barrier layer to prevent Cu diffusion into the surrounding dielectric. In this paper, amorphous carbon (a-C) layers of 0.75-2.5 nm thickness have been studied for use as copper diffusion barriers. The barrier performance and thermal stability of the a-C layers were evaluated by annealing Cu/SiO2/Si metal-oxide-semiconductor (MOS) samples with and without an a-C diffusion barrier at 400 °C for 10 h. Microstructure and elemental analysis performed by transmission electron microscopy (TEM) and secondary ion mass spectroscopy showed that no Cu diffusion into the SiO2 layer occurred in the presence of the a-C barrier layer. However, current density-electric field and capacitance-voltage measurements showed that 0.75 and 2.5 nm thick a-C barriers behave differently because of different microstructures being formed in each thickness after annealing. The presence of the 0.75 nm thick a-C barrier layer considerably improved the reliability of the fabricated MOS samples. In contrast, the reliability of MOS samples with a 2.5 nm thick a-C barrier was degraded by sp2 clustering and microstructural change from amorphous phase to nanocrystalline state during annealing. These results were confirmed by Raman spectroscopy, X-ray photoelectron spectroscopy and TEM analysis. This study provides evidence that an 0.75 nm thick a-C layer is a reliable diffusion barrier.

P-N Junction Diode Using Plasma Boron-Doped Black Phosphorus for High-Performance Photovoltaic Devices.

This study used a spatially controlled boron-doping technique that enables a p-n junction diode to be realized within a single 2D black phosphorus (BP) nanosheet for high-performance photovoltaic application. The reliability of the BP surface and state-of-the-art 2D p-n heterostructure's gated junctions was obtained using the controllable pulsed-plasma process technique. Chemical and structural analyses of the boron-doped BP were performed using X-ray photoelectron spectroscopy, transmission electron microscopy, and first-principles density functional theory (DFT) calculations, and the electrical characteristics of a field-effect transistor based on the p-n heterostructure were determined. The incorporated boron generated high electron density at the BP surface. The electron mobility of BP was significantly enhanced to ∼265 cm2/V·s for the top gating mode, indicating greatly improved electron transport behavior. Ultraviolet photoelectron spectroscopy and DFT characterizations revealed the occurrence of significant surface charge transfer in the BP. Moreover, the pulsed-plasma boron-doped BP p-n junction devices exhibited high-efficiency photodetection behavior (rise time: 1.2 ms and responsivity: 11.3 mA/W at Vg = 0 V). This study's findings on the tunable nature of the surface-transfer doping scheme reveal that BP is a promising candidate for optoelectronic devices and advanced complementary logic electronics.

Risk factor analysis of postoperative kyphotic change in subaxial cervical alignment after upper cervical fixation.

OBJECTIVELittle is known about the risk factors for postoperative subaxial cervical kyphosis following craniovertebral junction (CVJ) fixation. The object of this study was to evaluate postoperative changes in cervical alignment and to identify the risk factors for postoperative kyphotic change in the subaxial cervical spine after CVJ fixation.METHODSOne hundred fifteen patients were retrospectively analyzed for postoperative subaxial kyphosis after CVJ fixation. Relations between subaxial kyphosis and radiological risk factors, including segmental angles and ranges of motion (ROMs) at C0-1, C1-2, and C2-7, and clinical factors, such as age, sex, etiology, occipital fixation, extensor muscle resection at C2, additional C1-2 posterior wiring, and subaxial laminoplasty, were investigated. Univariate and multivariate logistic regression analyses were conducted to identify the risk factors for postoperative kyphotic changes in the subaxial cervical spine.RESULTSThe C2-7 angle change was more than -10° in 30 (26.1%) of the 115 patients. Risk factor analysis showed CVJ fixation combined with subaxial laminoplasty (OR 9.336, 95% CI 1.484-58.734, p = 0.017) and a small ROM at the C0-1 segment (OR 0.836, 95% CI 0.757-0.923, p < 0.01) were related to postoperative subaxial kyphotic change. On the other hand, age, sex, resection of the C2 extensor muscle, rheumatoid arthritis, additional C1-2 posterior wiring, and postoperative segmental angles were not risk factors for postoperative subaxial kyphosisCONCLUSIONSSubaxial alignment change is not uncommon after CVJ fixation. Muscle detachment at the C2 spinous process was not a risk factor of kyphotic change. The study findings suggest that a small ROM at the C0-1 segment with or without occipital fixation and combined subaxial laminoplasty are risk factors for subaxial kyphotic change.

Efficiency enhancement of polymer solar cells by patterning nanoscale indium tin oxide layer.

The efficiency of polymer solar cells was improved by patterning indium tin oxide (ITO) electrode layer in this work. Light absorbance was enhanced with ITO layer patterning resulting in the improvement of power conversion efficiency of polymer solar cells. The line-and-space grooved patterns of polystyrene layer are formed on the top of 100 nm thick indium tin oxide layer by capillary force lithography process. The surface patterning of the ITO layer were completed with O2 and Ar plasma etching with various step heights of 22 nm to 64 nm. The active layer was fabricated with one-to-one ratio of P3HT (poly-3-hexylthiophene) and PCBM ([6,6]-phenyl C61-butyric acid methyl ester) conjugated polymers on the top of the patterned ITO layer. Efficiency of the polymer solar cell was improved from 0.96% to 1.35% with this approach. We attribute the efficiency improvement to periodic grooved patterns of electrode. The periodic grooved patterns are believed to enhance light trapping resulting in the increase of diffraction and also to increase contact area of the electron-collecting electrode leading to increase of short circuit current.

Novel Conductive Filament Metal-Interlayer-Semiconductor Contact Structure for Ultralow Contact Resistance Achievement.

In the post-Moore era, it is well-known that contact resistance has been a critical issue in determining the performance of complementary metal-oxide-semiconductor (CMOS) reaching physical limits. Conventional Ohmic contact techniques, however, have hindered rather than helped the development of CMOS technology reaching its limits of scaling. Here, a novel conductive filament metal-interlayer-semiconductor (CF-MIS) contact-which achieves ultralow contact resistance by generating CFs and lowering Schottky barrier height (SBH)-is investigated for potential applications in various nanodevices in lieu of conventional Ohmic contacts. This universal and innovative technique, CF-MIS contact, forming the CFs to provide a quantity of electron paths as well as tuning SBH of semiconductor is first introduced. The proposed CF-MIS contact achieves ultralow specific contact resistivity, exhibiting up to ∼×700 000 reduction compared to that of the conventional metal-semiconductor contact. This study proves the viability of CF-MIS contacts for future Ohmic contact schemes and that they can easily be extended to mainstream electronic nanodevices that suffer from significant contact resistance problems.

Carbon nanotube patterning with capillary micromolding of catalyst.

Patterning of multi-walled carbon nanotube (MWNT) in a plasma enhanced chemical vapor deposition (PECVD) chamber has been achieved by catalyst patterning using capillary micromolding process. Iron acetate catalyst nanoparticles were dissolved in ethanol and mold was fabricated with polydimethylsiloxane (PDMS). The ethanol solution containing catalyst nanoparticles was filled into the microchannel formed between PDMS mold and Si-wafer by capillary force. The capillary action of different solvents was simulated by commercial CFD-ACE+ simulation code to determine optimal solvents. Simulated result shows that the choice of solvent was critical in this capillary filling process. After the catalyst patterning, MWNT was grown at 700 approximately 800 degrees C by PECVD process using CH4 and Ar gas in a scale of approximately 10 micro-meters in a tubular inductively coupled plasma reactor. Grown CNTs were analyzed by FE-SEM and Raman Spectroscopy.

Highly Flexible and Transparent Ag Nanowire Electrode Encapsulated with Ultra-Thin Al2O3: Thermal, Ambient, and Mechanical Stabilities.

There is an increasing demand in the flexible electronics industry for highly robust flexible/transparent conductors that can withstand high temperatures and corrosive environments. In this work, outstanding thermal and ambient stability is demonstrated for a highly transparent Ag nanowire electrode with a low electrical resistivity, by encapsulating it with an ultra-thin Al2O3 film (around 5.3 nm) via low-temperature (100 °C) atomic layer deposition. The Al2O3-encapsulated Ag nanowire (Al2O3/Ag) electrodes are stable even after annealing at 380 °C for 100 min and maintain their electrical and optical properties. The Al2O3 encapsulation layer also effectively blocks the permeation of H2O molecules and thereby enhances the ambient stability to greater than 1,080 h in an atmosphere with a relative humidity of 85% at 85 °C. Results from the cyclic bending test of up to 500,000 cycles (under an effective strain of 2.5%) confirm that the Al2O3/Ag nanowire electrode has a superior mechanical reliability to that of the conventional indium tin oxide film electrode. Moreover, the Al2O3 encapsulation significantly improves the mechanical durability of the Ag nanowire electrode, as confirmed by performing wiping tests using isopropyl alcohol.

Electrical properties and thermal stability in stack structure of HfO2/Al2O3/InSb by atomic layer deposition.

Changes in the electrical properties and thermal stability of HfO2 grown on Al2O3-passivated InSb by atomic layer deposition (ALD) were investigated. The deposited HfO2 on InSb at a temperature of 200 °C was in an amorphous phase with low interfacial defect states. During post-deposition annealing (PDA) at 400 °C, In-Sb bonding was dissociated and diffusion through HfO2 occurred. The diffusion of indium atoms from the InSb substrate into the HfO2 increased during PDA at 400 °C. Most of the diffused atoms reacted with oxygen in the overall HfO2 layer, which degraded the capacitance equivalent thickness (CET). However, since a 1-nm-thick Al2O3 passivation layer on the InSb substrate effectively reduced the diffusion of indium atoms, we could significantly improve the thermal stability of the capacitor. In addition, we could dramatically reduce the gate leakage current by the Al2O3 passivation layer. Even if the border traps measured by C-V data were slightly larger than those of the as-grown sample without the passivation layer, the interface trap density was reduced by the Al2O3 passivation layer. As a result, the passivation layer effectively improved the thermal stability of the capacitor and reduced the interface trap density, compared with the sample without the passivation layer.