Low-temperature Plasma-enhanced Chemical Vapor Deposition Research Articles

Plasma-enhanced chemical vapor deposition a-SiOCN:H low-Z thin films for bulk acoustic wave resonators

The 5th generation (5G) wireless telecommunication standards with newly defined frequency bands up to 6 GHz are currently established around the world. While outperforming surface acoustic wave (SAW) filters above 1 GHz, bulk acoustic wave (BAW) resonators in multiplexers for radio-frequency front-end (RFFE) modules continuously face higher performance requirements. In contrast to free-standing bulk acoustic resonators (FBARs), solidly mounted resonator (SMR) technology uses an acoustic Bragg mirror, which has already been successfully applied for several GHz applications. In this work, we investigate the potential of amorphous hydrogenated silicon-oxycarbonitride (a-SiOCN:H) thin films synthesized with low-temperature plasma-enhanced chemical vapor deposition (PECVD) as a low acoustic impedance (low-Z) material. Compared to the state-of-the-art where in Bragg mirrors up to now SiO2 is used as standard, the acoustic impedance ratio against the high-Z material tungsten (W) is enhanced for a better device performance. To limit the expected increase in viscous loss when the acoustic impedance is reduced, to a minimum, predominantly the mass density was reduced while keeping the mechanical elasticity high. By doing so, acoustic impedance values as low as 7.1 MRayl were achieved, thereby increasing the impedance ratio of high-Z to low-Z materials from 8:1 up to 14:1.

Nucleation and growth of graphene at different temperatures by plasma enhanced chemical vapor deposition

Temperature plays an important role in the synthesis of graphene by chemical vapor deposition. In the low-temperature plasma-enhanced chemical vapor deposition (PECVD) process, although the cleavage of hydrocarbon precursor no longer depends on growth temperature, which can affect the nucleation and growth of graphene by the diffusion and adsorption of active species, the catalytic ability of substrate and other factors, and ultimately lead to various growth of graphene. In this paper, the nucleation and growth of graphene at different temperatures by PECVD were studied. We noted that the nucleation and growth of graphene varied greatly in different temperature zones. Graphene could not be grown at low temperatures (< 600 ℃). At the medium and low temperature (650 ∼ 800 ℃), high nucleation density of graphene with nanometer size were produced. In the middle and high temperature region (850 ∼ 900 ℃), larger grains of graphene grown, and in the high temperature region (> 950 ℃), the coexistence of large and small grains was finally formed. In addition, the models of the nucleation and growth of graphene in different temperature zones were proposed.

Modeling and simulation of coverage and film properties in deposition process on large-scale pattern using statistical ensemble method

This study modeled deposition processes using statistical ensemble and feature-scale voxel methods to predict the coverage and film properties on a large-scale pattern for the first time. Certain new concepts, such as the use of probabilities to express physical and chemical phenomena occurring on the surface, interaction among voxels, and super particles for fast calculations, were introduced. Further, the model was used to analyze the experimental characteristic variation between the morphology and film density of SiN in low-temperature plasma-enhanced chemical vapor deposition using a SiH4/NH3/N2 gas mixture with different SiH4 flow rates, which has not been found in SiO2 films. The simulation results demonstrated the effect of the precursor mass in the gas phase on surface migration and morphology formation. In addition, a short residence time was required to prevent the generation of large and heavy precursors to realize good SiN coverage and film properties at a low temperature.

Low-Temperature Direct Growth of Nanocrystalline Multilayer Graphene on Silver with Long-Term Surface Passivation.

A wide variety of transition metals, including copper and gold, have been successfully used as substrates for graphene growth. On the other hand, it has been challenging to grow graphene on silver, so realistic applications by combining graphene and silver for improved electrode stability and enhanced surface plasmon resonance in organic light-emitting diodes and biosensing have not been realized to date. Here, we demonstrate the surface passivation of silver through the single-step rapid growth of nanocrystalline multilayer graphene on silver via low-temperature plasma-enhanced chemical vapor deposition (PECVD). The effect of the growth time on the graphene quality and the underlying silver characteristics is investigated by Raman spectroscopy, X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and cross-sectional annular dark-field scanning transmission electron microscopy (ADF-STEM). These results reveal nanocrystalline graphene structures with turbostratic layer stacking. Based on the XPS and ADF-STEM results, a PECVD growth mechanism of graphene on silver is proposed. The multilayer graphene also provides excellent long-term protection of the underlying silver surface from oxidation after 5 months of air exposure. This development thus paves the way toward realizing technological applications based on graphene-protected silver surfaces and electrodes as well as hybrid graphene-silver plasmonics.

Bandgap engineering of indium gallium nitride layers grown by plasma-enhanced chemical vapor deposition

This paper reports on the fabrication of InxGa1−xN (InGaN) layers with various compositions ranging from InN to GaN using a cost-effective low-temperature plasma-enhanced chemical vapor deposition (PECVD) method and analyzes the influence of deposition parameters on the resulting films. Single-phase nanocrystalline InGaN films with crystallite size up to 30 nm are produced with deposition temperatures in the range of 180–250 °C using the precursors trimethylgallium, trimethylindium, hydrogen, nitrogen, and ammonia in a parallel-plate type RF-PECVD reactor. It is found that growth rate is a primary determinant of crystallinity, with rates below 6 nm/min producing the most crystalline films across a range of several compositions. Increasing In content leads to a decrease in the optical bandgap, following Vegard’s law, with bowing being more pronounced at higher growth rates. Significant free-carrier absorption is observed in In-rich films, suggesting that the highly measured optical bandgap (about 1.7 eV) is due to the Burstein–Moss shift.

Low-Temperature PECVD Growth of Germanium for Mode-Locking of Er-Doped Fiber Laser.

A low-temperature plasma-enhanced chemical vapor deposition grown germanium (Ge) thin-film is employed as a nonlinear saturable absorber (SA). This Ge SA can passively mode-lock the erbium-doped fiber laser (EDFL) for soliton generation at a central wavelength of 1600 nm. The lift-off and transfer of the Ge film synthesized upon the SiO2/Si substrate are performed by buffered oxide etching and direct imprinting. The Ge film with a thickness of 200 nm exhibits its Raman peak at 297 cm−1, which both the nanocrystalline and polycrystalline Ge phases contribute to. In addition, the Ge thin-film is somewhat oxidized but still provides two primary crystal phases at the (111) and (311) orientations with corresponding diffraction ring radii of 0.317 and 0.173 nm, respectively. The nanocrystalline structure at (111) orientation with a corresponding d-spacing of 0.319 nm is also observed. The linear and nonlinear transmittances of the Ge thin-film are measured to show its self-amplitude modulation coefficient of 0.016. This is better than nano-scale charcoal and carbon-black SA particles for initiating the mode-locking at the first stage. After the Ge-based saturable absorber into the L-band EDFL system without using any polarized components, the narrowest pulsewidth and broadest linewidth of the soliton pulse are determined as 654.4 fs and 4.2 nm, respectively, with a corresponding time–bandwidth product of 0.32 under high pumping conditions.

High-Sensitivity Focus-Induced Photoresponse in Amorphous Silicon Photodiodes for Enhanced Three-Dimensional Imaging Sensors

In this work, we report on the bias-tunable and highly sensitive irradiance-dependent focus-induced photoresponse (FIP) in amorphous silicon p-i-n diodes for enhanced three-dimensional imaging applications. We identify a nonlinear current breakdown at irradiance down to at least $0.6\phantom{\rule{0.25em}{0ex}}\ensuremath{\mu}\mathrm{W}/\mathrm{m}{\mathrm{m}}^{2}$ due to defect-induced electrical field screening, surpassing state-of-the-art device sensitivities by at least a factor of 16 in the visible range. We further demonstrate the tunability of FIP by systematic device and field engineering to enable both precise and long-range high-resolution distance measurements in just one device architecture. We evaluate distance measurements up to at least 15 m and achieve a depth resolution of 2.1 mm at 3.3 m. The reproducible low-temperature (300 \ifmmode^\circ\else\textdegree\fi{}C or lower) plasma-enhanced chemical vapor-deposition technology enables systematic tailoring of the FIP for three-dimensional imaging applications and allows for back-end sensor integration on top of silicon or flexible electronics with fill factors up to 100%.

Surface treatment of biopolymer films Polylactic acid and Polyhydroxybutyrat with angular changing oxygen plasma ‒ More than just gradual purification

Bio-based and biodegradable polymers such as polylactic acid (PLA) and polyhydroxybutyrat (PHB) are developing into increasingly important raw materials. Especially when it comes to replacing conventional polymers in order to save resources and reduce environmental impact. Such biopolymers often conform to mechanical requirements of industry, but inappropriate surface characteristics such as poor barrier properties and increased degradation limit their application. Appropriate surface modification, such as the deposition of thin amorphous hydrogenated carbon films (a-C:H) produced by low-temperature mediated radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) can provide a remedy here. Prior to this, the surfaces are mostly cleaned/activated with oxygen plasma to enable better deposition of the subsequent a-C:H layer. It has been noticed that the O2 treatment, strongly changes the surface properties. It is now to be determined how a change in the O2 plasma treatment geometry affects the surface properties and whether this can be used accordingly. Surface morphology was analyzed by AFM, surface free energy (SFE) was determined using a contact angle goniometer, and changed barrier properties were investigated via the water vapor transmission rate. Top surfaces were studied by synchrotron supported X-ray spectroscopy, revealing angle-dependent changes in C and O content and binding conditions. The present angle-dependent O2 treatment not only changed the surface roughness and its chemical composition, but also, contrary to expectations, improved the barrier properties of PLA up to 18% at highest value, while the transmittance of PHB increased by more than 10%. A similar change in the mobility of the contact angle hysteresis is observed for both materials.

Graphene on Nanoscale-Thick Au Films: Implications for Anticorrosion in Smart Wearable Electronics

Gold is normally considered inert to chemical reaction. Nevertheless, as a common electrode material, it would suffer from corrosion when exposed to certain solutions such as sweat and body fluids. Here, we report low-temperature plasma-enhanced chemical vapor deposition (PECVD) of graphene on gold and demonstrate its feasibility for anticorrosion application. The effects of hydrogen-to-methane ratio and the underlying gold substrate on the graphene growth are investigated, and the growth mechanism of PECVD graphene on gold is proposed. When immersed in an oxygenated saline solution, the PECVD-grown graphene-covered gold surface is found to remain intact after an acceleration soaking test at 90 °C for 24 h, which is in contrast to the degradation of bare gold surface subject to the same test. Our findings suggest that consumer/medical wearables and implantable devices with exposed gold can benefit from the protection of a direct, low-temperature PECVD-grown graphene layer for anticorrosion, thereby prolonging the efficacy and reliability of gold electrode-based biosensors.

Ultrathin Ge epilayers on Si produced by low-temperature PECVD acting as virtual substrates for III-V / c-Si tandem solar cells

Ultrathin (20 nm) epitaxial films of germanium are deposited on crystalline silicon wafers, to act as virtual substrates for the growth of III-V materials, opening a low-cost approach to tandem solar cells. Such ultrathin layers allow for material cost reduction, along with the possibility of using the silicon wafer as the bottom cell in tandem devices. A simple plasma-enhanced chemical vapor deposition (PECVD) process at 175 °C has been optimized to deposit these heteroepitaxial germanium films, which grow directly on the silicon wafers without any intermediate silicon-germanium alloy. Thanks to an in-situ plasma cleaning step prior to Ge epitaxy, the films can sustain high-temperature annealing in vacuum (up to 800 °C) without any delamination. The suitability of the germanium heteroepitaxial films as virtual substrates is analyzed by depositing III-V layers on them by conventional growth methods like chemical beam epitaxy (CBE) and metalorganic chemical vapor deposition (MOCVD). The properties of the GaAs films deposited on the virtual substrates are comparable in terms of roughness, microstructure, and crystallinity to these of the III-V layers co-deposited on c-Ge wafers, pointing at the effectiveness of the ultrathin c-Ge epitaxial layers to act as virtual substrates for III-V epitaxial growth. Moreover, growing the c-Ge layers on c-Si substrates with 5° miscut avoids the formation of antiphase domains. These substrates are finally used to demonstrate proof of concept tandem solar cells, proving the suitability of our low temperature and ultrathin virtual substrate approach.

Growth of Nanostructured Diamond Films on Glass Substrates by Low-Temperature Microwave Plasma-Enhanced Chemical Vapor Deposition for Applications in Nanotribology

Low-temperature fabrication of superior nano-/microcrystalline diamond (NCD/MCD) films is a major thrust in diamond technology. The widely used seeding techniques applied on the substrate surface are usually harsh, lead to defect formation, and suffer from the lack of reproducibility in maintaining homogeneity and uniformity over nano-/microdimensions. Low-temperature growth of a significant diamond phase on purely untreated glass substrates is indeed a challenging task. Using CO2 as the supplementary gas-phase constituent included in the (CH4 + H2)-plasma and a specific shadow-mask assembly to generate the diffuse plasma environment above the growth zone, the present experiment succeeds in the spontaneous growth of nanocrystalline diamond (NCD), including microcrystalline diamond (MCD) of an average crystal size of ∼0.9–1.2 μm at ∼300 °C on glass substrates, without any seeding pretreatment. In the plasma, the shadow mask establishes a virtual remote plasma configuration in setting the diffuse plasma, protects the growth surface from a direct bombardment of energetic ions, generates negative self-bias on the substrate, and, importantly, promotes significant gas-phase nucleation dominant over typical solid-state nucleation. The incubation of diamond embryo clusters onto the untreated glass substrates under the shadow-mask structure and its growth evolution through the efficient transport of the preferential carbonaceous precursors proceed within the diffuse plasma. In the course of network modification under the diffuse plasma, atomic O remains instrumental for efficient etching of the a-C overlayer on a diamond embryo and atomic H, with its small physical dimension, conveniently penetrates within the diamond network and promotes crystallization via transferring its moderate chemical energy to the diamond lattice. The low-temperature-grown nanostructured diamond films are up-and-coming for applications in nanotribology.

Hydrogen-assisted low-temperature plasma-enhanced chemical vapor deposition of thin film encapsulation layers for top-emission organic light-emitting diodes

In this work, we developed a single high-performance SiNx encapsulation layer that can be directly integrated into organic devices by low-temperature plasma-enhanced chemical vapor deposition (PECVD). We investigated a hydrogen-assisted low-temperature PECVD process at a temperature of 80 °C. The thin film density improved with an increased hydrogen gas ratio, and the moisture permeability was less than 5 × 10−5 g/m2·day. To verify the stability of the PECVD process, we applied the SiNx encapsulation layer directly to top-emitting organic light-emitting diodes. The results showed minor changes in the current-density–voltage characteristics after the PECVD process, as well as high reliability after a water dipping test.

Low-noise Kerr frequency comb generation with low temperature deuterated silicon nitride waveguides.

We report very low-loss deuterated silicon nitride (SiNx:D) micro-ring resonators fabricated by back-end CMOS compatible low-temperature plasma-enhanced chemical vapor deposition (PECVD) without annealing. Strong confinement micro-ring resonators with a quality factor of > 2 million are achieved, corresponding to a propagation loss in the 1460-1610 nm wavelength range of ∼ 0.17 dB/cm. We further report the generation of low-noise coherent Kerr microcomb states including different perfect soliton crystals (PSC) in PECVD SiNx:D micro-ring resonators. These results manifest the promising potential of the back-end CMOS compatible SiNx:D platform for linear and nonlinear photonic circuits that can be co-integrated with electronics.

Polymer-Compatible Low-Temperature Plasma-Enhanced Chemical Vapor Deposition of Graphene on Electroplated Cu for Flexible Hybrid Electronics.

Flexible hybrid electronics and fan-out redistribution layers rely on electroplating Cu on polymers. In this work, direct low-temperature plasma-enhanced chemical vapor deposition (PECVD) of graphene on electroplated Cu over polyimide substrates is demonstrated, and the deposition of graphene is found to passivate and strengthen the electroplated Cu circuit. The effect of the H2/CH4 ratio on the PECVD graphene growth is also investigated, which is shown to affect not only the quality of graphene but also the durability of Cu. 100,000 cycles of folding with a bending radius of 2.5 mm and the corresponding resistance tests are carried out, revealing that Cu circuits covered by graphene grown with a higher H2/CH4 ratio can sustain many more bending cycles. Additionally, graphene coverage is shown to suppress the formation of copper oxides in ambient environment for at least 8 weeks after the PECVD process.

Engineering CsPbBr3 quantum dots with efficient luminescence and stability by damage-free encapsulation with a-SiCx:H

Although encapsulation is an effective technique to improve the stability of perovskite quantum dots (PQDs), they are prone to photoluminescence (PL) degradation during encapsulation. Herein, a damage-free plasma-based encapsulation technique in combination with real-time in situ diagnosis is designed for CsPbBr3 QDs films. The CH4/SiH4 plasma has little destructive effects on the CsPbBr3 QDs. The a-SiCx:H films prepared with the CH4/SiH4 plasma by low-temperature plasma-enhanced chemical vapor deposition protect the CsPbBr3 QDs from surface damage during encapsulation, so that the intrinsic PL efficiency is maintained. Furthermore, the PL intensity is improved by increasing the carbon concentration in a-SiCx:H and the CsPbBr3 QDs films encapsulated by a-SiCx:H show long term stability under harsh conditions such as prolonged air exposure, UV light illumination for more than 60 days, immersion in water, as well as thermal treatment at 205 oC.

Impact of the Deposition Temperature on the Structural and Electrical Properties of InN Films Grown on Self-Standing Diamond Substrates by Low-Temperature ECR-MOCVD

The progress of InN semiconductors is still in its infancy compared to GaN-based devices and materials. Herein, InN thin films were grown on self-standing diamond substrates using low-temperature electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition (ECR-PEMOCVD) with inert N2 used as a nitrogen source. The thermal conductivity of diamond substrates makes the as-grown InN films especially attractive for various optoelectronic applications. Structural and electrical properties which depend on deposition temperature were systematically investigated by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and Hall effect measurement. The results indicated that the quality and properties of InN films were significantly influenced by the deposition temperature, and InN films with highly c-axis preferential orientation and surface morphology were obtained at optimized temperatures of 400 °C. Moreover, their electrical properties with deposition temperature were studied, and their tendency was correlated with the dependence on micro- structure and morphology.

Annealing of Boron-Doped Hydrogenated Crystalline Silicon Grown at Low Temperature by PECVD.

We investigate low-temperature (<200 °C) plasma-enhanced chemical vapor deposition (PECVD) for the formation of p–n junctions. Compared to the standard diffusion or implantation processes, silicon growth at low temperature by PECVD ensures a lower thermal budget and a better control of the doping profile. We previously demonstrated the successful growth of boron-doped epitaxial silicon layers (p+ epi-Si) at 180 °C. In this paper, we study the activation of boron during annealing via dark conductivity measurements of p+ epi-Si layers grown on silicon-on-insulator (SOI) substrates. Secondary Ion Mass Spectroscopy (SIMS) profiles of the samples, carried out to analyze the elemental composition of the p+ epi-Si layers, showed a high concentration of impurities. Finally, we have characterized the p+ epi-Si layers by low-temperature photoluminescence (PL). Results revealed the presence of a broad defect band around 0.9 eV. In addition, we observed an evolution of the PL spectrum of the sample annealed at 200 °C, suggesting that additional defects might appear upon annealing.

Low residual stress in hydrogenated amorphous silicon-carbon films deposited by low-temperature PECVD

Low residual stress in hydrogenated amorphous silicon-carbon (a-SixC1-x:H) films prepared by plasma-enhanced chemical vapor deposition (PECVD) at temperature range of 100–200 °C was obtained. Profilometry, Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM) measurements were carried out to characterize the films. The residual stress of each deposited film was calculated using profilometry measurements and the Stoney equation. The results showed that the residual stress decreases as the power density is reduced, or the temperature or the silane/methane ratio are increased. There is a deposition pressure at around 750 mTorr at which low residual stress is promoted. The residual stress showed a correlation with the carbon incorporation in the form of C–Hn molecules. The residual stress depends on the deposition regime: assisted either by silane radicals (also known as “silane starving plasma” (SSP)) or by both silane and methane radicals. Considering that the carbon incorporation under SSP regime is more controlled, there is a higher probability of having low residual stress in this regime. In agreement with the characterization, the most favorable PECVD parameters were selected to obtain a-SixC1-x:H films with low residual stress (below 100 MPa) within the temperature range (100–200 °C). These results are useful in areas such as flexible electronic devices, implantable devices, microfluidic systems, and microelectromechanical systems, among others, in which the materials and the parameters of fabrication are degraded or modified by temperature above 200 °C.

Investigation of an anti-reflection silicon nitride layer on a superconducting NbxSi1-xfilm absorber for inductive superconducting transition edge detectors

We report research on superconducting transition temperature (Tc) tuning and the antireflective layer of NbxSi1-x films that were used as the absorbing layer in inductive superconducting transition edge detectors (ISTED). The Tc of NbxSi1-x film absorber should be tuned to be to the operating temperature range of the readout nano superconducting quantum interference devices. The composition ratio of Nb/Si was controlled by the co-sputtering powers of the Nb and Si target to adjust the Tc. To improve the detection efficiency, a 30 nm Nb95.7Si4.3 film with Tc 6.3 K was chosen to demonstrate the effect of the antireflective layer SiNx made by low temperature plasma enhanced chemical vapor deposition (LT-PECVD). According to the spectral refractive indexes and extinction coefficients of the Nb95.7Si4.3 and SiNx films, the structure parameters for 633 nm incident light were designed and the optical properties were calculated. The reflectivity measurements showed that the reflectivity was effectively reduced, which was consistent with the calculation.

Low-temperature deposition of TiO2by atmospheric pressure PECVD towards photoanode elaboration for perovskite and solid-state dye-sensitized solar cells

An original low-temperature atmospheric pressure plasma-enhanced chemical vapor deposition process was used to deposit titanium dioxide thin films. The parametric study in dynamic mode deposition aimed at growing an ideal columnar film composed of aligned anatase monocrystals as solar cell photoanode, previously obtained on silicon wafers in static mode deposition. A process parameters optimization was necessary to deposit onto thermally sensitive glass/FTO substrates. In this paper, the morphology, crystallinity and optical transmission of the coatings have been studied. The coatings display a columnar cauliflower-like structure, composed of TiO2amorphous particles assembly. After deposition, the light transmission properties of the substrate were reduced. As a solution, an ultrasound bath cleaning was set up to enhance the transmitted light through the photoanode.