Two-stage Chemical Vapor Deposition Research Articles

Growth of bilayer MoTe2 single crystals with strong non-linear Hall effect

The reduced symmetry in strong spin-orbit coupling materials such as transition metal ditellurides (TMDTs) gives rise to non-trivial topology, unique spin texture, and large charge-to-spin conversion efficiencies. Bilayer TMDTs are non-centrosymmetric and have unique topological properties compared to monolayer or trilayer, but a controllable way to prepare bilayer MoTe2 crystal has not been achieved to date. Herein, we achieve the layer-by-layer growth of large-area bilayer and trilayer 1T′ MoTe2 single crystals and centimetre-scale films by a two-stage chemical vapor deposition process. The as-grown bilayer MoTe2 shows out-of-plane ferroelectric polarization, whereas the monolayer and trilayer crystals are non-polar. In addition, we observed large in-plane nonlinear Hall (NLH) effect for the bilayer and trilayer Td phase MoTe2 under time reversal-symmetric conditions, while these vanish for thicker layers. For a fixed input current, bilayer Td MoTe2 produces the largest second harmonic output voltage among the thicker crystals tested. Our work therefore highlights the importance of thickness-dependent Berry curvature effects in TMDTs that are underscored by the ability to grow thickness-precise layers.

Growth of twisted bilayer graphene through two-stage chemical vapor deposition

We investigate growth of twisted bilayer graphene through two-stage chemical vapor deposition (CVD). Exploiting the synergetic nucleation and growth dynamics involving carbon sources from the residual carbon impurities in Cu bulk and gaseous CHx, sub-millimeter-sized single crystalline graphene grains with multiple merged adlayer grains formed underneath are grown on Cu substrate. The distribution of the twist angles is investigated through a computer algorithm utilizing spectral features from micro-Raman mapping. Besides the more thermodynamically stable AB-stacking (AB-BLG) or large angle (>15°) decoupled bilayer graphene (DC-BLG) configurations, there are some bilayer regions that contain specific twist angles (3–8°, 8–13°, and 11–15°) (termed as TBLG). The statistics show no TBLG formation for BLG with single nucleation center. The formation probability of TBLG is strongly dependent on the relative orientation of merging adlayer grains. Significant defects are found at the grain boundaries formed in AB-DC merging event without creating TBLG domain. The areal fraction of TBLG increases as H2/CH4 ratio increases. The growth mechanism of TBLG is discussed in light of the interactions between the second layer grains with consideration of strain generation during merging of adlayers.

Large-scale nanometer-thickness graphite films synthesized on polycrystalline Ni foils by two-stage chemical vapor deposition process

In this work, large-scale (120 × 120 mm) nanometer-thick graphite films (NGFs) were synthesized on polycrystalline Ni foils using a “two-stage” chemical vapor deposition (CVD) process. An intermediate cooling process was included in the CVD process; this process affected the isothermal graphene growth, which is explained using a growth model. We mainly studied a graphite film with the thickness of ∼40 nm and the good thickness uniformity was characterized by atomic force microscopy (AFM), Raman spectroscopy and extreme ultraviolet (EUV) mapping. The graphite formed on the Ni grain boundary was analyzed by transmission electron microscopy (TEM). The relationships between the thickness, optical and electrical properties of the NGF and growth temperature were also studied.

Chemical Vapor Deposition of High-Quality Large-Sized MoS2 Crystals on Silicon Dioxide Substrates.

Large-sized MoS2 crystals can be grown on SiO2/Si substrates via a two-stage chemical vapor deposition method. The maximum size of MoS2 crystals can be up to about 305 μm. The growth method can be used to grow other transition metal dichalcogenide crystals and lateral heterojunctions. The electron mobility of the MoS2 crystals can reach ≈30 cm2 V-1 s-1, which is comparable to those of exfoliated flakes.

An Investigation on the Formation of Carbon Nanotubes by Two-Stage Chemical Vapor Deposition

High density of carbon nanotubes (CNTs) has been synthesized from agricultural hydrocarbon: camphor oil using a one-hour synthesis time and a titanium dioxide sol gel catalyst. The pyrolysis temperature is studied in the range of 700–900°C at increments of 50°C. The synthesis process is done using a custom-made two-stage catalytic chemical vapor deposition apparatus. The CNT characteristics are investigated by field emission scanning electron microscopy and micro-Raman spectroscopy. The experimental results showed that structural properties of CNT are highly dependent on pyrolysis temperature changes.

The production of carbon nanotubes using two-stage chemical vapor deposition and their potential use in protein purification

Carbon nanotubes (CNTs) were produced by gas phase two-stage chemical vapor deposition using ferrocene as a catalyst and acetylene (C 2H 2) and hydrogen (H 2) as precursor gases. The CNTs were characterized using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and by thermogravimetric analysis (TGA). Process parameters such as reaction temperature, reaction time and gas flow rates of C 2H 2 and H 2 were optimized using Design-Expert software v.6.0.8. The statistical analysis reveals that the optimum conditions for the CNT production were reaction temperature of 850 °C, reaction time of 60 min and gases flow rates of 40 and 150 ml/min for C 2H 2 and H 2, respectively. The optimum CNTs prepared were purified and functionalized using acid treatment. Both functionalized and nonfunctionalized CNTs were used as column chromatographic media for skim latex protein purification. Skim latex protein was used as the model protein for the recovery of useful proteins and enzymes. During purification, process parameters such as pH and ionic strength of the running buffer were optimized to enhance protein purification. Results revealed that CNTs behave efficiently as a hydrophobic interaction chromatographic (HIC) media.

Microstructure of SiC fiber fabricated by two-stage chemical vapor deposition on tungsten filament

Continuous silicon carbide (SiC) fiber with a diameter of about 100 μm was fabricated by two-stage chemical vapor deposition on tungsten filament. Microstructure of the fiber was investigated by means of scanning electron microscopy, X-ray diffraction, transmission electron microscopy and Raman spectrometry. The results showed that the fiber consists of tungsten core, a W/SiC interfacial reaction zone with the reaction products of W 5Si 3 and WC, and a predominant β-SiC layer. The W/SiC interface can be described as W/W 5Si 3/WC/SiC. The SiC originates at the surface of the WC with a buffer layer, in which β-SiC crystallites nucleate and grow with their preferred (1 1 1) orientation, exhibiting strong 〈1 1 1〉 fiber texture. Raman spectra revealed that the SiC in the fiber is stoichiometric, which is composed of β-SiC and amorphous SiC. Furthermore, the formation mechanism of W/SiC interfacial reaction zone and structural evolution of SiC are discussed.

Texture and surface morphology improvement of Al by two-stage chemical vapor deposition and its integration in an Al plug-interconnect scheme for sub 0.25 μm metallization

A two-stage deposition with successive seed and bulk deposition steps was developed to improve the morphology and texture of chemical vapor deposited (CVD) aluminum on titanium. Dimethylaluminumhydride (DMAH) was used as the precursor. Typically, CVD Al deposited using a single deposition stage showed highly granular structure with surface “defects” resulting in films that become rough with increase in thickness creating integration problems with photolithography and etch. Here, a two-stage deposition process for CVD Al is described that significantly improves the morphology and texture of Al on titanium. In this process, the wafer surface is preconditioned with a short burst of DMAH before stabilizing gas flows or pressure. Such a treatment in the very first step results in a seed layer upon which proceeds the bulk film deposition in a subsequent step after stabilizing pressure, gas flows and equilibrating temperature. The two-stage deposition resulted in reflectivity improvement of CVD Al on Ti from ⩽ 160% to >210% (%Si, 480 nm) for a 3.5 kÅ thick film with complete elimination of surface defects. X-ray rocking curves show a decrease in the full width at half maximum of the Al(111) peak from 5.1° to 2.6°, indicating significantly improved texture. Secondary ion mass spectroscopy analysis of the CVD Al shows a decrease in carbon content using the two-stage deposition. The two-stage deposition for CVD Al was integrated with AlCu sputter process at temperatures below 380 °C, without a vacuum break between the CVD Al and PVD AlCu, to develop sub-0.25 μm technology via fill capability.

Nucleation Enhancement and Adherence of Diamond Film Coated on Tungsten Wire by Two-Stage Microwave Plasma CVD

Fine-grained and adherent diamond coating was obtained by the two-stage microwave plasma chemical vapor deposition (CVD) in the CO-H2 system on a tungsten wire substrate mounted horizontally 2mm upward on pyrophyllite susceptor. The susceptor was placed parallel to the irradiation direction of microwave. The nucleation density of diamond was found to increase with increasing the microwave power up to 1000W. The first stage CVD conditions for fine-grained diamond coating on tungsten wire were determined as microwave power: 900W, pressure. 2kPa and CO concentration: 5 vol%. On the other hand, the second stage CVD conditions for higher growth rate of diamond film were determined as microwave power: 550W, pressure: 4kPa and CO concentration: 10 vol%. Homogeneous and adherent diamond coating with a thickness of 16μm was prepared by the two-stage CVD process for a total treatment time of 10h (the first stage of 3h and the second stage of 7h) on tungsten wire of 1.0-mm diameter. A WC interlayer was formed during the CVD process.