Thermal Chemical Vapor Deposition System Research Articles

Asymmetric contact-induced selective doping of CVD-grown bilayer WS2 and its application in high-performance photodetection with an ultralow dark current.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are excellent candidates for high-performance optoelectronics due to their high carrier mobility, air stability and strong optical absorption. However, photodetectors made with monolayer TMDs often exhibit a high dark current, and thus, there is a scope for further improvement. Herein, we developed a 2D bilayer tungsten disulfide (WS2) based photodetector (PD) with asymmetric contacts that exhibits an exceptionally low dark current and high specific detectivity. High-quality and large-area monolayer and bilayer WS2 flakes were synthesized using a thermal chemical vapor deposition system. Compared to conventional symmetric contact electrodes, utilizing metal electrodes with higher and lower work functions relative to bilayer WS2 aids in achieving asymmetric lateral doping in the WS2 flakes. This doping asymmetry was confirmed through the photoluminescence spectral profile and Raman mapping analysis. With the asymmetric contacts on bilayer WS2, we find evidence of selective doping of electrons and holes near the Ti and Au contacts, respectively, while the WS2 region away from the contacts remains intrinsic. When compared with the symmetric contact case, the dark current in the WS2 PD with asymmetric (Au, Ti) contact decreases by an order of magnitude under reverse bias with a concomitant increase in the photocurrent, resulting in an improved on/off ratio of ∼105 and overall improved device performance under identical illumination conditions. We explained this improved performance based on the energy band alignment showing a unidirectional charge flow under light illumination. Our results indicate that the planar device structure and compatibility with current nanofabrication technologies can facilitate its integration into advanced chips for futuristic low-power optoelectronic and nanophotonic applications.

Enhancement of Electromagnetic Wave Shielding Effectiveness by the Incorporation of Carbon Nanofibers-Carbon Microcoils Hybrid into Commercial Carbon Paste for Heating Films.

Carbon microcoils (CMCs) were formed on stainless steel substrates using C2H2 + SF6 gas flows in a thermal chemical vapor deposition (CVD) system. The manipulation of the SF6 gas flow rate and the SF6 gas flow injection time was carried out to obtain controllable CMC geometries. The change in CMC geometry, especially CMC diameter as a function of SF6 gas flow injection time, was remarkable. In addition, the incorporation of H2 gas into the C2H2 + SF6 gas flow system with cyclic SF6 gas flow caused the formation of the hybrid of carbon nanofibers-carbon microcoils (CNFs-CMCs). The hybrid of CNFs-CMCs was composed of numerous small-sized CNFs, which formed on the CMCs surfaces. The electromagnetic wave shielding effectiveness (SE) of the heating film, made by the hybrids of CNFs-CMCs incorporated carbon paste film, was investigated across operating frequencies in the 1.5-40 GHz range. It was compared to heating films made from commercial carbon paste or the controllable CMCs incorporated carbon paste. Although the electrical conductivity of the native commercial carbon paste was lowered by both the incorporation of the CMCs and the hybrids of CNFs-CMCs, the total SE values of the manufactured heating film increased following the incorporation of these materials. Considering the thickness of the heating film, the presently measured values rank highly among the previously reported total SE values. This dramatic improvement in the total SE values was mainly ascribed to the intrinsic characteristics of CMC and/or the hybrid of CNFs-CMCs contributing to the absorption shielding route of electromagnetic waves.

Fabrication of silicon PIN diode with SiGe junction for soft x-ray detector using low-temperature technology

In this paper, a low-temperature technology was introduced for the fabrication of PIN photodiode. A double dielectric oxide layers were prepared by nitric acid oxidation and PECVD, respectively. The selective epitaxial growth of SiGe thin layers, which were in-situ doped in a reactive thermal chemical vapor deposition system, were applied for the preparation of PN junction and high-low junction. A SiNx passivation layer combined with a post-annealing was adopted to lower the reverse current. The temperature was controlled below 450 °C for the whole fabrication process. Finally, the reverse current of 0.7 nA was achieved at room temperature for our 5 mm-diameter PIN detector and the energy resolution of 240 eV was obtained at 5.9 keV, −60 °C. The results indicated that the low-temperature technology is suitable for the fabrication of high quality PIN detector, and the cost can be reduced effectively.

Enhancement in Electromagnetic Wave Shielding Effectiveness through the Formation of Carbon Nanofiber Hybrids on Carbon-Based Nonwoven Fabrics.

The selective hybrid formation of numerous tiny carbon nanofibers (CNFs) in carbon-based nonwoven fabrics (c-NFs), namely CNFs formed only on the surfaces of individual carbon fibers (i-CFs) constituting c-NFs and not on the surfaces of carbon microcoils (CMCs), could be formed by the incorporation of H2 gas flow into the C2H2 + SF6 gas flow in a thermal chemical vapor deposition system. On the other hand, the nonselective hybrid formation of numerous tiny CNFs in c-NFs, that is, tiny CNFs formed on the surfaces of both i-CFs and CMCs, could be achieved by simply modulating the SF6 gas flow on and off in continuous cycles during the reaction. Detailed mechanisms are suggested for the selective or nonselective formation of tiny CNFs in c-NFs. Furthermore, the electromagnetic wave shielding effectiveness (SE) values of the samples were investigated across operating frequencies in the 8.0–12.0 GHz range. Compared with previously reported total SE values, the presently measured values rank in the top tier. Although hybrid formation reduced the electrical conductivity of the native c-NFs, the total SE values of the native c-NFs greatly increased following hybrid formation. This dramatic improvement in the total SE values is ascribed to the increased thickness of c-NFs after hybrid formation and the electromagnetic wave absorption enhancement caused by the intrinsic characteristics of CMCs and the numerous intersections of tiny CNFs.

Cellulose-derived flexible carbonized paper for high-performance electromagnetic interference shielding

In this study, we describe the fabrication of flexible carbonized paper (f-CP) from toilet paper samples using a sandwich-type installation technique in a thermal chemical vapor deposition system. During the reaction, the substrate was heated up to 1000 °C at a rate of ~20 °C/min and naturally cooled down to room temperature in an Ar gas environment without temperature holding. The f-CP sample thus obtained contained continuous carbon fibers similar to the native toilet paper sample while carbonized paper (CP) fabricated using the conventional installation technique contained discontinuous carbon fibers. Compared to CP, f-CP exhibited higher electrical conductivity, mechanical flexibility, and shielding effectiveness (SE) against electromagnetic radiation. Even after bending the f-CP sample several hundred times, its SE decreased only slightly. Multi-layered f-CP (mf-CP; 3 mm thickness and 0.053 g/cm3 density) fabricated by stacking f-CP samples showed a remarkably high SE of 58 dB in the operating frequency range of 8.2–12.4 GHz. Therefore, f-CP and mf-CP, which exhibit excellent electromagnetic shielding characteristics, may potentially be used in electronic devices.

Significant control of metal-insulator transition temperature through catalytic excessive oxygen doping in high-performance vanadium dioxide nanobeam channel

The strategy of a reliable transition temperature control of vanadium dioxide (VO2) is reported. Rectangular VO2 nanobeams were synthesized by a thermal chemical vapor deposition (TCVD) system. The metal-insulator transition (MIT) temperature increases to above 380 K when the TiO2 ratio of the source is 5 at.%, although the Ti source is not physically doped into VO2 nanobeams. The XPS spectra of the V 2p orbital reveal the excessive oxidation of V after the TCVD processes with a higher TiO2 ratio, indicating that the TiO2 precursor is important in the O-doping of the surface VO bonds when forming volatile Ti-O gas species. Thus, TiO2 reactants can be used as a VO2 surface chemical modifier to manipulate the MIT transition temperature and maintain a homogenous VO2 phase, which is useful for a Mott device application with a record on/off switching ratio > 104 and Mott transition temperature > 380 K.

Transfer learning based 3D fuzzy multivariable control for an RTP system

Rapid thermal processing (RTP) is an important process in the fabrication of semiconductor devices. It is difficult to achieve temperature uniformity control of the wafer in RTP since the system is a highly nonlinear process with strong spatial distribution. In this study, a transfer learning-based three-dimensional (3D) fuzzy multivariable control scheme is proposed for the temperature uniformity control of an RTP system. In difference to the traditional expert-knowledge based design, a two-level framework of transfer learning methodology is constructed to design the 3D fuzzy multivariable controller (3D FMC) with the help of a multi-output support vector regression (M-SVR). The 3D FMC defines a qualitative spatial fuzzy structure that will be transferred to the M-SVR. On the other hand, the structure parameters of the M-SVR will be learned from data and transferred to design quantitative parameters of the 3D FMC. Under the framework of transfer learning, the control laws (e.g. human control experience) hidden in spatio-temporal data can be extracted and formulated back into multi-output 3D fuzzy rules. The proposed method provides an effective integration of the spatial fuzzy inference and the transfer learning for 3D FLC design. The newly developed method is applied to the temperature uniformity control of a rapid thermal chemical vapor deposition (RTCVD) system at the set temperature 1000K, and the maximum non-uniformity along the wafer radius is close to 1K.

Effect of CS<sub>2</sub> Flow Rate on the Formation of Aligned Carbon Microcoils

The formation of aligned carbon microcoils could be achieved using C2H2 as a source gas and CS2 as an incorporated additive gas under thermal chemical vapor deposition system. To elucidate the ratio of C2H2/CS2 for the formation of the aligned carbon microcoils, the CS2 flow rate was first manipulated under the identical C2H2 flow rate (500sccm) condition. The formation and the alignment of carbon microcoils could be only achieved under the ratio of C2H2/CS2 = 33.3 condition, namely the flow rates of CS2 = 15sccm and C2H2= 500sccm. The total flow rate of the used gases was varied under the identical C2H2/CS2 flow rate ratio (33.3) condition. The C2H2 flow rate was manipulated under the identical CS2 flow rate (15sccm) condition. It was found that the formation and the alignment of carbon microcoils could be only achieved under the condition of 15sccm of CS2 flow rate in the range of 200 ~ 500sccm of C2H2 flow rate, regardless of the flow rate ratio of C2H2/CS2 and the total flow rate. The crystal structure of the well-aligned CMCs reveals the increase in the (002) peak in XRD spectrum for the aligned carbon microcoils, indicating the existence of the more regular structure in the aligned carbon microcoils. Based on these results, the cause for the formation of the aligned carbon microcoils only in the case of the CS2 flow rate = 15sccm with the imaginary pictures for the flow rate ratio of C2H2/CS2 just above the substrate were proposed.

Growth and characterization of polycrystalline diamond films on silicon using sugarcane bagasse as carbon precursor at atmospheric pressure by thermal chemical vapor deposition

Here we demonstrate that by using sugarcane bagasse as a carbon precursor, highly crystalline diamond films possessing H3 [N-V-N] and [Si-V]− optical centers can be grown on Si (100) substrates using a simple thermal chemical vapor deposition (thermal-CVD) system at atmospheric pressure and reduced temperature (~900 °C). In this process, the rich chemistry of effluent gas species produced during the pyrolysis of sugarcane bagasse is found to play a vital role. Diamond films have also been characterized by using X-ray diffraction (XRD), scanning electron microscope (SEM), micro-Raman, and photoluminescence (PL) spectrometer. Presence of nitrogen and silicon related defect was probed by PL and H3, i.e. [N-V-N] at 505 nm and [Si-V]− at 736 nm optical centers have been confirmed. The other observed peaks at 445.7 nm, 468 nm, and 884 nm are assigned to nitrogen-containing defects.

The Effect of Various Catalyst on In-situ Synthesis of Carbon Nanotubes on the Glass Mat Using Thermal Chemical Vapor Deposition Method

The main aim of this research work is in situ synthesis of carbon nano tube (CNT) on nonwoven glass mat. Different types of catalyst were used and the effect of them on growth of carbon nanotubes on glass mat was investigated. The process was considered as variable catalysts to achieve optimal conditions for CNT deposition on glass mat. In this research, iron, cobalt, copper and nickel were used as catalysts and acetylene gas was used as the hydrocarbon source. Also, argon was used as a carrier gas. In first step, samples were coated by mentioned catalyst using plasma sputtering method. The thickness of the catalyst on the surface of samples was fixed at 100 nm. In second part, thermal chemical vapor deposition (TCVD) system was used for growth of carbon nano tubes. Scanning electron microscopy (SEM) and raman spectroscopy, thermogravimetric analysis (TGA-DTG) and transmission electron microscopy (TEM) were used for investigating the morphology and quality of produced CNT. Electrical conductivity was observed and compared on both untreated and coated glass mat. The results show that, by pre sputtering of samples with nickel, the quality of CNT on the surface of glass mat will be improved and the resistivity of samples reach to 610 Ω.

Enhanced Formation of the Carbon Microcoils by the Stepwise Type Manipulation of CS<sub>2</sub> Flow Injection

Carbon coils were synthesized using C2H2 as source gas and CS2 as an incorporated additive gas under the thermal chemical vapor deposition system. The flow rate of CS2 varied according to the different reaction processes. Geometries of as-grown carbon nanomaterials were developed from vine-type into coil-type with increasing CS2 flow rate from 5 to 20sccm. Above 20sccm of CS2 flow rate, indeed, most of them were appeared as the tiny-sized wavelike nanocoil type geometries. To develop the double helix-type microcoils, namely the carbon microcoils, the injection of CS2 flow was manipulated as the stepwise type on/off-cycle manner. Under the specific condition of the on/off-cycle number, the density of the carbon microcoils was enhanced. The cause for the enhanced formation density of the carbon microcoils by the stepwise type manipulation of CS2 flow injection was suggested and discussed.

As-pyrolyzed sugarcane bagasse possessing exotic field emission properties

The present study aims to demonstrate the application of sugarcane bagasse as an excellent field emitter. Field emission property of as-pyrolyzed sugarcane bagasse (p-SBg) before and after the plasma treatment has been investigated. It has been observed that electronic nature of p-SBg transformed from semiconducting to metallic after plasma treatment. Maximum current and turn-on field defined at 10 μA/cm2 was found to be 800 μA/cm2 and 2.2 V/μm for as-pyrolyzed sugarcane bagasse (p-SBg) and 25 μA/cm2 and 8.4 V/μm for H2-plasma treated p-SBg. These values are found to be better than the reported values for graphene and activated carbon. In this report, pyrolysis of bagasse has been carried in a thermal chemical vapor deposition (Th-CVD) system in inert argon atmosphere. Scanning electron microscopy (SEM), X-ray Diffraction (XRD), High-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) have been used to study the structure of both pre and post plasma-treated p-SBg bagasse’s sample. HRTEM study reveals that carbonaceous structures such as 3D-nanographene oxide (3D-NGO), graphite nanodots (GNDs), carbon nanotubes (CNTs), and carbon onions are present in both pre-treated and plasma-treated p-SBg. Hence, we envision that the performed study will be a forwarding step to facilitate the application of p-SBg in display devices.

The Low Temperature Epitaxy of Strained GeSn Layers Using RTCVD System

We have investigated the low temperature (LT) growth of GeSn–Ge–Si structures using rapid thermal chemical vapor deposition system utilizing Ge2H6 and SnCl4 as the reactive precursors. Due to inappropriate phenomena, such as, Ge etch and Sn segregation, it was hard to achieve high quality GeSn epitaxy at the temperature > 350 °C. On the contrary, we found that the SnCl4 promoted the reaction of Ge2H6 precursors in a certain process condition of LT, 240–360 °C. In return, we could perform the growth of GeSn epi layer with 7.7% of Sn and its remaining compressive strain of 71.7%. The surface propagated defects were increased with increasing the Sn content in the GeSn layer confirmed by TEM analysis. And we could calculate the activation energies at lower GeSn growth temperature regime using by Ge2H6 and SnCl4 precursors about 0.43 eV.

Geometry-Controlled Carbon Coils by SF6 Flow Injection Time with Reaction Temperature

Carbon nanocoils and/or microcoils were synthesized using C2H2 as the source gas along with the injection of SF6 as an incorporated additive gas under the thermal chemical vapor deposition (TCVD) system. To control the geometries of the carbon coils, we varied the SF6 flow injection time at different reaction temperature ranges. At the lowest reaction temperature (550°C), carbon microcoils were dominantly formed within a relatively short initial SF6 flow injection time (less than 5 min). By increasing the SF6 flow injection time, carbon nanocoils could be well developed on the entire surface of the sample. At 750°C, the formation of carbon microcoils dominated over the entire sample surface, irrespective of the SF6 flow injection time. Based on these results, the growth mechanism for the dominantly formed carbon coils was suggested and discussed. In addition, the causes for the dominant formation of carbon nanocoils and/or microcoils according to the SF6 flow injection times with the different reaction temperatures were analyzed.

Enhancement of shielding effectiveness for electromagnetic wave radiation using carbon nanocoil-carbon microcoil hybrid materials

Carbon nanocoil-carbon microcoil (CNC-CMC) hybrid materials were prepared via the formation of numerous carbon nanocoils (CNCs) on the surface of carbon microcoils (CMCs), using C2H2 as the source gas and CS2 as the additive gas in a thermal chemical vapor deposition system. During the reaction, CS2 was injected into the reactor in modulated on/off cycles. The density of CNCs formed on the surface of CMCs increased with an increase in the number of on/off cycle numbers. In the as-grown state, the CNC-CMC hybrid materials showed higher electrical conductivity than that of the materials composed only of CMCs. Composites of CNC-CMC hybrid materials in polyurethane (CNC-CMC@PU) and CMCs in polyurethane (CMC@PU) were fabricated. The CNC-CMC@PU composites showed higher shielding effectiveness than the CMC@PU composites, irrespective of the mixing ratios of carbon nanomaterials in PU. Based on the shielding effectiveness and electrical conductivity, we conclude that the formation of CNC-CMC hybrid materials can enhance shielding effectiveness through a reflection-based mechanism as well as an absorption-based mechanism.

Dominant Formation of the Carbon Nanocoils at Low Temperature

The single helix type carbon nanocoils were formed under the low temperature (550°C) condition. For the formation of the carbon nanocoils, C2H2 was used as a source gas and SF6 was used as an additive gas under the thermal chemical vapor deposition system. The morphologies of as-grown carbon materials at the low temperature (550°C) and the mass ratios of product/catalyst were investigated according to the ratio of C2H2/ SF6 flow and the injection times of SF6 and C2H2flows. The conversion efficiency from the carbon source gas (C2H2) to the as-grown carbon materials was estimated using the mass ratios of product/catalyst. The sample having the ratio of C2H2/SF6 flow = 50 had the highest conversion efficiency. For the dominant formation of the single helix type CNCs, the optimal ratio of C2H2/SF6 flow was determined to be around 10. As the growth aspect of the single helix type CNCs under the optimal ratio of C2H2/SF6 flow, the formation of the CNCs was understood to be initiated within the reaction time of 5min after the formation of the carbon nanofilaments. The detailed growth mode of the CNCs was proposed.

Filled-carbon nanotubes: 1 D nanomagnets possessing uniaxial magnetization axis and reversal magnetization switching

The present study aims to control the direction of magnetization in Fe3C, Co and Ni nanorods filled inside carbon nanotube (CNT). This control has been achieved during growth by modifying thermal chemical vapor deposition (CVD) system. As-grown in situ filled-CNTs were found to exhibit permanent magnetization. These CNTs have been characterized by using scanning electron microscopy (SEM), X-ray diffraction, Raman spectroscopy and transmission electron microscopy (TEM). Afterwards, direction of magnetization in Fe3C, Co or Ni nanorod filled inside CNT has been further probed by using magnetic force microscopy (MFM). MFM measurements reveal that nanorod exhibits single domain behavior and direction of magnetization, instead of being controlled either by shape or magneto crystalline anisotropy, has been found to be influenced by magnetic field gradient, produced in modified thermal CVD system. Direction of magnetization has been found either along tube axis in vertical grown CNTs or in radial direction i.e. perpendicular to the tube axis in randomly grown CNTs. Besides investigated structural and magnetic properties, plausible growth model of in situ filling as well as mechanism to understand unique magnetization behavior has been proposed.

Low Temperature Synthesis of the Carbon Micro Materials

The formation of the carbon microcoils could be achieved under the low temperature (550°C). Ni powders were used as the catalyst on the alumina substrate. C2H2 was used as a source gas and a few amount of SF6 gas were used as an additive gas under the thermal chemical vapor deposition system. The surface morphologies of as-grown carbon materials at the low temperature (550°C) were also investigated according to the total pressure and the injection time of SF6 flow. At 80Torr and 10min injection time of SF6 flow, the carbon microcoils were highly developed on the entire surface of the sample. The lower or higher total pressure and the longer injection time of SF6 flow deteriorated the formation of CMCs. The head area of the carbon microcoils was closely investigated and the growth mode for the initiation of the carbon microcoils at the low temperature was suggested.

A Hierarchical Intelligent Methodology for Spatiotemporal Control of Wafer Temperature in Rapid Thermal Processing

Rapid thermal processing (RTP) is very important for semiconductor manufacturing. The RTP is actually a spatially distributed dynamical system (SDDS) with multiple control sources. Due to its highly nonlinear and spatiotemporal dynamics, it would be difficult to maintain the temperature uniformity inside the RTP. In this paper, a hierarchical intelligent methodology is proposed for the spatiotemporal control of RTP. The control scheme consists of three different levels. At the lower level, the decomposition strategy is taken to divide the SDDS into several relatively simple subsystems; each of them is handled by one control source. At the mid-level, 3-D fuzzy logic controllers (3-D-FLCs) are designed, which have inherent capability to process spatiotemporal dynamics, to maintain the temperature uniformity of each subsystem. At the upper level, a local coordination strategy will be designed to adjust 3-D-FLCs to overcome interactions between the adjacent subsystems. The proposed scheme is applied to the temperature uniformity control of a rapid thermal chemical vapor deposition system, and better temperature uniformity can be achieved.

Enhanced Formation of Carbon Microcoils with Different-Sized Ni Catalyst by Different Injection Gas Sequence

Carbon microcoils were synthesized using C2H2 flow as a source gas along with an initial short time SF6 flow as an incorporated additive gas under the thermal chemical vapor deposition (CVD) system. According to the different reaction processes, different injection sequences of the gases flows were investigated. Indeed, the injection sequence of C2H2 or SF6 flow was manipulated with the different-sized Ni catalysts, namely small (~0.1mm) or large (~100mm) size in diameter. In the case of the small-sized Ni catalyst, the formation of the carbon microcoils could be well-developed under the injection sequence of SF6 → C2H2 flow. In the case of the large-sized Ni catalyst, on the other hand, the injection of C2H2 flow prior to the injection of SF6 flow promotes the formation of carbon microcoils. Based on these results we revealed the role of C2H2 flow injection or SF6 flow injection with the different-sized Ni catalyst for the enhanced formation of the carbon microcoils