10 research outputs found
Scalable Graphite/Copper Bishell Composite for High-Performance Interconnects
We present the fabrication and characterizations of novel electrical interconnect test lines made of a Cu/graphite bishell composite with the graphite cap layer grown by electron cyclotron resonance chemical vapor deposition. Through this technique, conformal multilayer graphene can be formed on the predeposited Cu interconnects under CMOS-friendly conditions. The low-temperature (400 °C) deposition also renders the process unlimitedly scalable. The graphite layer can boost the current-carrying capacity of the composite structure to 108 A/cm2, more than an order of magnitude higher than that of bare metal lines, and reduces resistivity of fine test lines by ∼10%. Raman measurements reveal that physical breakdown occurs at ∼680–720 °C. Modeling the current vs voltage curves up to breakdown shows that the maximum current density of the composites is limited by self-heating of the graphite, suggesting the strong roles of phonon scattering at high fields and highlighting the significance of a metal counterpart for enhanced thermal dissipation
Ultrafast and Low Temperature Synthesis of Highly Crystalline and Patternable Few-Layers Tungsten Diselenide by Laser Irradiation Assisted Selenization Process
Recently, a few attempts to synthesize monolayers of transition metal dichalcogenides (TMDs) using the chemical vapor deposition (CVD) process had been demonstrated. However, the development of alternative processes to synthesize TMDs is an important step because of the time-consuming, required transfer and low thermal efficiency of the CVD process. Here, we demonstrate a method to achieve few-layers WSe<sub>2</sub> on an insulator <i>via</i> laser irradiation assisted selenization (LIAS) process directly, for which the amorphous WO<sub>3</sub> film undergoes a reduction process in the presence of selenium gaseous vapors to form WSe<sub>2</sub>, utilizing laser annealing as a heating source. Detailed growth parameters such as laser power and laser irradiation time were investigated. In addition, microstructures, optical and electrical properties were investigated. Furthermore, a patternable WSe<sub>2</sub> concept was demonstrated by patterning the WO<sub>3</sub> film followed by the laser irradiation. By combining the patternable process, the transfer-free WSe<sub>2</sub> back gate field effect transistor (FET) devices are realized on 300 nm-thick SiO<sub>2</sub>/P<sup>+</sup>Si substrate with extracted field effect mobility of ∼0.2 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>. Similarly, the reduction process by the laser irradiation can be also applied for the synthesis of other TMDs such as MoSe<sub>2</sub> from other metal oxides such as MO<sub>3</sub> film, suggesting that the process can be further extended to other TMDs. The method ensures one-step process to fabricate patternable TMDs, highlighting the uniqueness of the laser irradiation for the synthesis of different TMDs
Low Temperature Growth of Graphene on Glass by Carbon-Enclosed Chemical Vapor Deposition Process and Its Application as Transparent Electrode
A novel carbon-enclosed chemical
vapor deposition (CE-CVD) to grow
high quality monolayer graphene on Cu substrate at a low temperature
of 500 °C was demonstrated. The quality of the grown graphene
was investigated by Raman spectra, and the detailed growth mechanism
of high quality graphene by the CE-CVD process was investigated in
detail. In addition to growth of high quality monolayer graphene,
a transparent hybrid few-layer graphene/CuNi mesh electrode directly
synthesized by the CE-CVD process on a conventional glass substrate
at the temperature of 500 °C was demonstrated, showing excellent
electrical properties (∼5 Ω/□ @ 93.5% transparency)
and ready to be used for optical applications without further transfer
process. The few-layer graphene/CuNi mesh electrode shows no electrical
degradation even after 2 h annealing in pure oxygen at an elevated
temperature of ∼300 °C. Furthermore, the few-layer graphene/CuNi
mesh electrode delivers an excellent corrosion resistance in highly
corrosive solutions such as electroplating process and achieves a
good nucleation rate for the deposited film. Findings suggest that
the low temperature few-layer graphene/CuNi mesh electrode synthesized
by the CE-CVD process is an excellent candidate to replace indium
tin oxide (ITO) as transparent conductive material (TCM) in the next
generation
Plasma-Assisted Synthesis of High-Mobility Atomically Layered Violet Phosphorus
Two-dimensional layered materials
such as graphene, transition
metal dichalcogenides, and black phosphorus have demonstrated outstanding
properties due to electron confinement as the thickness is reduced
to atomic scale. Among the phosphorus allotropes, black phosphorus,
and violet phosphorus possess layer structure with the potential to
be scaled down to atomically thin film. For the first time, the plasma-assisted
synthesis of atomically layered violet phosphorus has been achieved.
Material characterization supports the formation of violet phosphorus/InN
over InP substrate where the layer structure of violet phosphorus
is clearly observed. The identification of the crystal structure and
lattice constant ratifies the formation of violet phosphorus indeed.
The critical concept of this synthesis method is the selective reaction
induced by different variations of Gibbs free energy (Δ<i>G</i>) of reactions. Besides, the Hall mobility of the violet
phosphorus on the InP substrate greatly increases over the theoretical
values of InP bulk material without much reduction in the carrier
concentration, suggesting that the mobility enhancement results from
the violet phosphorus layers. Furthermore, this study demonstrates
a low-cost technique with high compatibility to synthesize the high-mobility
atomically layered violet phosphorus and open the space for the study
of the fundamental properties of this intriguing material as a new
member of the fast growing family of 2D crystals
Transfer-Free Growth of Atomically Thin Transition Metal Disulfides Using a Solution Precursor by a Laser Irradiation Process and Their Application in Low-Power Photodetectors
Although chemical vapor deposition is the most common
method to synthesize transition metal dichalcogenides (TMDs), several
obstacles, such as the high annealing temperature restricting the
substrates used in the process and the required transfer causing the
formation of wrinkles and defects, must be resolved. Here, we present
a novel method to grow patternable two-dimensional (2D) transition
metal disulfides (MS<sub>2</sub>) directly underneath a protective
coating layer by spin-coating a liquid chalcogen precursor onto the
transition metal oxide layer, followed by a laser irradiation annealing
process. Two metal sulfides, molybdenum disulfide (MoS<sub>2</sub>) and tungsten disulfide (WS<sub>2</sub>), are investigated in this
work. Material characterization reveals the diffusion of sulfur into
the oxide layer prior to the formation of the MS<sub>2</sub>. By controlling
the sulfur diffusion, we are able to synthesize continuous MS<sub>2</sub> layers beneath the top oxide layer, creating a protective
coating layer for the newly formed TMD. Air-stable and low-power photosensing
devices fabricated on the synthesized 2D WS<sub>2</sub> without the
need for a further transfer process demonstrate the potential applicability
of TMDs generated via a laser irradiation process
Low-Temperature Chemical Synthesis of Three-Dimensional Hierarchical Ni(OH)<sub>2</sub>‑Coated Ni Microflowers for High-Performance Enzyme-Free Glucose Sensor
Since prevention methods of type-II
diabetes and knowledge of prediabetes
are lacking, the development of sensitive and accurate glucose sensors
with an ultralow detection limit is imperative. In this work, the
enzyme-free glucose sensor based on three-dimensional (3D) hierarchical
Ni microflowers with a NiÂ(OH)2 coating layer has been demonstrated
in a simple one-step chemical reaction at a low temperature of 80
°C. The as-synthesized materials were characterized by several
analytical and spectroscopic techniques. In addition, the thin NiÂ(OH)2 layer formed at the surface of the Ni microflower was evidenced
by Raman, HRTEM, and XPS, which is the key factor to achieve highly
sensitive enzyme-free glucose sensors based on low-cost materials
such as copper, nickel, and their oxide and hydroxide. Moreover, our
modified electrode exhibits an outstanding detection limit as low
as 2.4 nM with an ultrahigh sensitivity of 2392 μA mM–1 cm–2, which is attributed to not only the increased
surface area due to the controlled formation of spikes but also the
contribution of the NiÂ(OH)2 coating layer
Direct Synthesis of Graphene with Tunable Work Function on Insulators via In Situ Boron Doping by Nickel-Assisted Growth
Work
function engineering, a precise tuning of the work function, is essential
to achieve devices with the best performance. In this study, we demonstrate
a simple technique to deposit graphene on insulators with in situ
controlled boron doping to tune the work function. At a temperature
higher than 1000 °C, the boron atoms substitute carbon sites
in the graphene lattice with neighboring carbon atoms, leading to
the graphene with a p-type doping behavior. Interestingly, the involvement
of boron vapor into the system can effectively accelerate the reaction
between nickel vapor and methane, achieving a fast graphene deposition.
The changes in surface potential and work function at different doping
levels were verified by Kelvin probe force microscopy, for which the
work function at different doping levels was shifted between 20 and
180 meV. Finally, the transport mechanism followed by the Mott variable-range
hopping model was found due to the strong disorder nature of the system
with localized charge-carrier states
Selection Role of Metal Oxides into Transition Metal Dichalcogenide Monolayers by a Direct Selenization Process
Direct reduction
of metal oxides into a few transition metal dichalcogenide (TMDCs)
monolayers has been recently explored as an alternative method for
large area and uniform deposition. However, not many studies have
addressed the characteristics and requirement of the metal oxides
into TMDCs by the selenization/sulfurization processes, yielding a
wide range of outstanding properties to poor electrical characteristics
with nonuniform films. The large difference implies that the process
is yet not fully understood. In particular, the selenization/sulfurization
at low temperature leads to poor crystallinity films with poor electrical
performance, hindering its practical development. A common approach
to improve the quality of the selenized/sulfurized films is by further
increasing the process temperature, thus requiring additional transfer
in order to explore the electrical properties. Here, we show that
by finely tuning the quality of the predeposited oxide the selenization/sulfurization
temperature can be largely decreased, avoiding major substrate damage
and allowing direct device fabrication. The direct relationship between
the role of selecting different metal oxides prepared by e-beam evaporation
and reactive sputtering and their oxygen deficiency/vacancy leading
to quality influence of TMDCs was investigated in detail. Because
of its outstanding physical properties, the formation of tungsten
diselenide (WSe2) from the reduction of tungsten oxide
(WOx) was chosen as a model for proof
of concept. By optimizing the process parameters and the selection
of metal oxides, layered WSe2 films with controlled atomic
thickness can be demonstrated. Interestingly, the domain size and
electrical properties of the layered WSe2 films are highly
affected by the quality of the metal oxides, for which the layered
WSe2 film with small domains exhibits a metallic behavior
and the layered WSe2 films with larger domains provides
clear semiconducting behavior. Finally, an 8′′ wafer
scale-layered WSe2 film was demonstrated, giving a step
forward in the development of 2D TMDC electronics in the industry
Wafer-Scale Growth of WSe<sub>2</sub> Monolayers Toward Phase-Engineered Hybrid WO<sub><i>x</i></sub>/WSe<sub>2</sub> Films with Sub-ppb NO<sub><i>x</i></sub> Gas Sensing by a Low-Temperature Plasma-Assisted Selenization Process
An
inductively coupled plasma (ICP) process was used to synthesize
transition metal dichalcogenides (TMDs) through a plasma-assisted
selenization process of metal oxide (MO<sub><i>x</i></sub>) at a temperature as low as 250 °C. In comparison with other
CVD processes, the use of ICP facilitates the decomposition of the
precursors at low temperatures. Therefore, the temperature required
for the formation of TMDs can be drastically reduced. WSe<sub>2</sub> was chosen as a model material system due to its technological importance
as a p-type inorganic semiconductor with an excellent hole mobility.
Large-area synthesis of WSe<sub>2</sub> on polyimide (30 × 40
cm<sup>2</sup>) flexible substrates and 8 in. silicon wafers with
good uniformity was demonstrated at the formation temperature of 250
°C confirmed by Raman and X-ray photoelectron (XPS) spectroscopy.
Furthermore, by controlling different H<sub>2</sub>/N<sub>2</sub> ratios,
hybrid WO<sub><i>x</i></sub>/WSe<sub>2</sub> films can be
formed at the formation temperature of 250 °C confirmed by TEM
and XPS. Remarkably, hybrid films composed of partially reduced WO<sub><i>x</i></sub> and small domains of WSe<sub>2</sub> with
a thickness of ∼5 nm show a sensitivity of 20% at 25 ppb at
room temperature, and an estimated detection limit of 0.3 ppb with
a <i>S</i>/<i>N</i> > 10 for the potential
development
of a low-cost plastic/wearable sensor with high sensitivity