Chang Wan Lee

Layer-Controlled, Wafer-Scale, and Conformal Synthesis of Tungsten Disulfide Nanosheets Using Atomic Layer Deposition

The synthesis of atomically thin transition-metal disulfides (MS2) with layer controllability and large-area uniformity is an essential requirement for their application in electronic and optical devices. In this work, we describe a process for the synthesis of WS2 nanosheets through the sulfurization of an atomic layer deposition (ALD) WO3 film with systematic layer controllability and wafer-level uniformity. The X-ray photoemission spectroscopy, Raman, and photoluminescence measurements exhibit that the ALD-based WS2 nanosheets have good stoichiometry, clear Raman shift, and bandgap dependence as a function of the number of layers. The electron mobility of the monolayer WS2 measured using a field-effect transistor (FET) with a high-k dielectric gate insulator is shown to be better than that of CVD-grown WS2, and the subthreshold swing is comparable to that of an exfoliated MoS2 FET device. Moreover, by utilizing the high conformality of the ALD process, we have developed a process for the fabrication of WS2 nanotubes.

Synthesis of carbon nanotube–nickel nanocomposites using atomic layer deposition for high-performance non-enzymatic glucose sensing

A useful strategy has been developed to fabricate carbon-nanotube-nickel (CNT-Ni) nanocomposites through atomic layer deposition (ALD) of Ni and chemical vapor deposition (CVD) of functionalized CNTs. Various techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), were used to characterize the morphology and the structure of as-prepared samples. It was confirmed that the products possess uniform Ni nanoparticles that are constructed by finely controlled deposition of Ni onto oxygen or bromine functionalized CNT surface. Electrochemical studies indicate that the CNT-Ni nanocomposites exhibit high electrocatalytic activity for glucose oxidation in alkaline solutions, which enables the products to be used in enzyme-free electrochemical sensors for glucose determination. It was demonstrated that the CNT-Ni nanocomposite-based glucose biosensor offers a variety of merits, such as a wide linear response window for glucose concentrations of 5 μM-2 mM, short response time (3 s), a low detection limit (2 μM), high sensitivity (1384.1 μA mM(-1) cm(-2)), and good selectivity and repeatability.

Controllable synthesis of molybdenum tungsten disulfide alloy for vertically composition-controlled multilayer

The effective synthesis of two-dimensional transition metal dichalcogenides alloy is essential for successful application in electronic and optical devices based on a tunable band gap. Here we show a synthesis process for Mo1−xWxS2 alloy using sulfurization of super-cycle atomic layer deposition Mo1−xWxOy. Various spectroscopic and microscopic results indicate that the synthesized Mo1−xWxS2 alloys have complete mixing of Mo and W atoms and tunable band gap by systematically controlled composition and layer number. Based on this, we synthesize a vertically composition-controlled (VCC) Mo1−xWxS2 multilayer using five continuous super-cycles with different cycle ratios for each super-cycle. Angle-resolved X-ray photoemission spectroscopy, Raman and ultraviolet–visible spectrophotometer results reveal that a VCC Mo1−xWxS2 multilayer has different vertical composition and broadband light absorption with strong interlayer coupling within a VCC Mo1−xWxS2 multilayer. Further, we demonstrate that a VCC Mo1−xWxS2 multilayer photodetector generates three to four times greater photocurrent than MoS2- and WS2-based devices, owing to the broadband light absorption.

Improvement of Gas-Sensing Performance of Large-Area Tungsten Disulfide Nanosheets by Surface Functionalization

Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDCs) are promising gas-sensing materials due to their large surface-to-volume ratio. However, their poor gas-sensing performance resulting from the low response, incomplete recovery, and insufficient selectivity hinders the realization of high-performance 2D TMDC gas sensors. Here, we demonstrate the improvement of gas-sensing performance of large-area tungsten disulfide (WS2) nanosheets through surface functionalization using Ag nanowires (NWs). Large-area WS2 nanosheets were synthesized through atomic layer deposition of WO3 followed by sulfurization. The pristine WS2 gas sensors exhibited a significant response to acetone and NO2 but an incomplete recovery in the case of NO2 sensing. After AgNW functionalization, the WS2 gas sensor showed dramatically improved response (667%) and recovery upon NO2 exposure. Our results establish that the proposed method is a promising strategy to improve 2D TMDC gas sensors.

Self-Limiting Layer Synthesis of Transition Metal Dichalcogenides.

This work reports the self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides (2D TMDCs) in the form of MoS2. The layer controllability and large area uniformity essential for electronic and optical device applications is achieved through atomic layer deposition in what is named self-limiting layer synthesis (SLS); a process in which the number of layers is determined by temperature rather than process cycles due to the chemically inactive nature of 2D MoS2. Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale (~10 cm) layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 108. This process is also shown to be applicable to WSe2, with a PN diode fabricated from a MoS2/WSe2 heterostructure exhibiting gate-tunable rectifying characteristics.

Uniform, large-area self-limiting layer synthesis of tungsten diselenide

A process for the self-limited layer synthesis (SLS) of WSe2 on SiO2 substrates has been developed that provides systematic layer number controllability with micrometer-scale (>90%) and wafer-scale (~8 cm) uniformity suitable electronic and optoelectronic device applications. This was confirmed by the fabrication and testing of a WSe2 back-gated field effect transistor (FET) using Pd (30 nm) as the contact metal, which exhibited p-type behavior with an on/off ratio of ~106 and a field-effect hole mobility of 2.2 cm2 V−1 s−1 value, which was higher than has been reported for WSe2-based FETs produced by conventional chemical vapor deposition. On the basis of these results, it is proposed that the SLS method is universally applicable to a range of device applications.

In situ surface cleaning on a Ge substrate using TMA and MgCp2 for HfO2-based gate oxides

Comparative studies of the in situ surface cleaning effect on Ge substrates using trimethyl aluminum (TMA) and dicyclopentadienyl magnesium (MgCp2) were performed. The surface cleaning process is the direct exposure of either a TMA or MgCp2 precursor on a Ge surface prior to the deposition of a HfO2 gate dielectric. Also, we studied a HfO2/Al2O3 and MgO bilayer on uncleaned Ge using the same precursors for comparison with surface treatment. From the correlation of chemical composition, line profile, atomic scale imaging and electrical evaluation, MgCp2 was the most effective method for reducing Ge diffusion into the HfO2 dielectric layer via the efficient surface cleaning process. MgCp2 cleaning produces thermally-stable Ge oxides while TMA cleaning reduces all types of Ge sub-oxides. As a result, the process can form a thermally-stable interface layer primarily composed of Ge3+, leading to better electrical properties than TMA.

A study on the influence of local doping in atomic layer deposited Al:ZnO thin film transistors

Local doping of Al:ZnO into a ZnO matrix was performed vertically at various positions in a thin film using atomic layer deposition, and its influence was investigated by analyzing thin film transistor (TFT) characteristics. The position specific dopant distribution in the films was confirmed by high resolution transmission electron microscopy. It was found that doping specific locations in the active channel layer of a TFT had a different impact on its electrical characteristics. When near the semiconductor/gate dielectric interface, doping had a significant impact on the mobility of the TFT devices, which showed a gradual recovery as the doped region was moved away from the interface. The original characteristics of the device were almost completely restored once the doped region was moved more than 15 nm away from the interface, and when moved further away the output characteristics portrayed a shift in threshold voltage while preserving all other electrical characteristics. Various doping concentrations were implemented in regions both near and far away from the interface to gain a better understanding of the phenomena. The experimental results given here imply that the geographical position of doping is as important as selecting a dopant material in the device optimization of TFTs.

The impact of atomic layer deposited SiO2 passivation for high-k Ta1−xZrxO on the InP substrate

Metal–oxide-semiconductor (MOS) capacitors with an amorphous Ta1−xZrxO composite gate dielectric film and a SiO2 passivation layer were fabricated on an indium phosphide (InP) substrate. To investigate the impact of the passivation layer, the interfacial chemical, physical and electrical properties of the Ta1−xZrxO/InP and Ta1−xZrxO/SiO2/InP MOS structures were studied in detail. Electrical conductivity measurements combined with chemical bonding analysis using X-ray photoelectron spectroscopy (XPS) and electron dispersive spectroscopy (EDS) were conducted in order to evaluate the suitability of a Ta1−xZrxO alloy as a gate dielectric film for an InP substrate. XPS results showed that the Ta1−xZrxO film retained its insulating characteristics and was thermally stable even after annealing at 500 °C. However, Fermi-level pinning and significant diffusion of indium through the Ta1−xZrxO were observed. The diffusion of In was remarkably reduced after introducing the SiO2 passivation layer, which resulted in an overall reduction in interfacial layer thickness. Parallel conductance contour measurements showed that the SiO2 passivation layer resulted in unpinning of the Fermi-level. The introduction of a SiO2 passivation layer with the Ta1−xZrxO composite gate dielectric film was found to provide remarkably improved dielectric performance, which was mainly attributed to reduced In diffusion and the passivation of interfacial and bulk dielectric defects.