Plasma Enhanced Chemical Vapor Deposition Research Articles

Gas sensitivity of PECVD β-Ga2O3 films with large active surface

The gas-sensitive properties of Ga2O3 films, first synthesized by plasma-enhanced chemical vapor deposition (PECVD), with respect to gaseous species of environmental and industrial interest were investigated in detail. The addition of some N2 during the PECVD process made it possible to reduce the inherent high resistance of Ga2O3 material and increase its gas sensitivity. The PECVD-Ga2O3 films demonstrated high responses to H2, O2 and NH3 with maximum response temperatures of 600 °C, 700 °C and 350 °C, correspondingly. The responses of Ga2O3 films to 1 vol % of H2, 40 vol % of O2 and 1 vol % of NH3 at these temperatures were 400.73 %, 480.22 % and 335.35 %, correspondingly. The short response and recovery times of 7.6 s and 31.0 s have been achieved under H2 exposure. A plausible mechanism of the sensory effect of the PECVD-Ga2O3 films was suggested. Thus, the PECVD synthesized Ga2O3 films demonstrate a large potential for the development of high-speed performance H2 and O2 sensors working at high temperatures.

Evolution of hard carbon layers on Ti-45Al-2Nb-2Mn-1B by plasma enhanced chemical vapor deposition of hydrocarbons and hydrogen

Extension of pulsed DC plasma enhanced CVD (PECVD) of carbon from hydrocarbons resulted in thick hard carbon layers on Ti-45Al-2Nb-2Mn-1B. PECVD for less than 2 h led to the formation of surface interlayers of TiC and Ti2AlC compounds under the hard carbon layer. After 5 h until 20 h, the formation of carbons layers surpassed the growth of TiC and Ti2AlC layers and resulted in a thicker hard carbon layer. This can be explained with the deceleration of carbon diffusion into the base alloy that resulted in the accumulation of a hard structure of carbon on the outer surface. The thickness of the hard carbon layers reached up to ∼40 μm, which were revealed using SEM microcopy. The hardness on the outer surface of hard carbon layer was around 600–1500 HV0.5 (5.88–14.71 GPa). EDX analysis across the surface layers showed ∼50–100 at. % carbon on the outermost layers. Raman spectroscopy of carbon layers showed sp3 (D) and sp2 (G) peaks of carbon at ∼1330 and ∼1560 cm−1 and peaks of TiC at ∼200 cm−1 peaks.

Cotton fabric surface treatment by plasma-deposited hexamethyldisilazane thin films

In the realm of fabric finishing, achieving the best wettability in cotton fabric is often a desired outcome. To accomplish this, a plasma-enhanced chemical vapor deposition (PECVD) system was used to deposit Hexamethyldisilazane (HMDSN) thin films onto cotton fabric for two different durations: 15 and 30 minutes. Various fabric properties were evaluated to determine the effectiveness of the treatment, including wettability, surface morphology, chemical structure, the effect of deposition on fabric stiffness, and the film fastness on the abrasion and washing. These properties were assessed using SEM, FTIR, and Martindale tester. Results indicated that both treatment durations improved the fabric's wicking ability. However, the film did not have a good fastness on abrasion and washing.

Numerical characterization of capacitively coupled plasma driven by tailored frequency modulated radio frequency source

Abstract Capacitively coupled plasma (CCP) is widely used in plasma etching and deposition processes because of its low cost, simple structure, and easy generation of a uniform plasma in large areas. Conventional CCPs are operated under a fixed frequency power source; however, CCPs driven by a variable frequency power source are poorly understood. In this paper, numerical simulations of CCPs driven by frequency modulated (FM) radio frequency (RF) sources within the frequency range of 2 MHz to 18 MHz are carried out with a Particle-in-Cell/Monte Carlo collision model. Our research indicates that the CCP driven by an FM RF source can maintain a stable glow discharge and form a time-dependent plasma. Plasma density, electron and ion current, energy and heating rate, ion flux, and energy on the electrodes fluctuate consistently with the FM period. The electron and ion energy distribution function can also be modulated by the frequency variation of the FM source. A multi-peak structure that varies and shifts with frequency variation is observed in the ion energy distribution function. In addition, by fixing the chirp period while varying the start or end frequency of the chirp signal (start frequency from 0.4 to 6 MHz, or end frequency from 18 to 48 MHz), effective modulations can be produced on the electron density, electron energy, and the shape of the EEPF and IEDF.

Localized corrosion of nitrogen-doped diamond-like carbon films on the surface of 304 stainless steel

Nitrogen-doped diamond-like carbon (N-DLC) films with different nitrogen content were prepared on the surface of 304 stainless steel (SS) using plasma-enhanced chemical vapour deposition technology. Localized failure mechanisms of the N-DLC films under long-term exposure to sodium chloride solutions have been investigated. The results demonstrated that nitrogen doping significantly affected the failure behavior of the N-DLC films. When undoped, the number of pore defects in the film is high, providing a channel for the corrosive medium to attack the substrate metal. Meanwhile, due to the high internal stress within the undoped film, extensive exfoliation of the film occurred under the effect of the growth of oxides at the film-substrate interface. The DLC film on 304 SS is subject to stress corrosion cracking failure during long-term service in sodium chloride solution. However, when nitrogen doping is 10 sccm, the pH of the micro-zone solution was highly variable due to the dissolution of nitrogen in the film and the processes of formation and hydrolysis of NH4+. A large number of residual metal cations dissolved from the substrate combine with oxygen at the corrosion pits mouth to form oxides. This can aggravate the occlusion effect of the pits and severe pitting corrosion of 304 SS occurs under autocatalytic effects. The N-DLC film with 5 sccm nitrogen addition showed the excellent durability properties. The evolution mechanism of oxidation and dissolution behaviors in the presence of variations in local hydrochemistry is also discussed.

Formation of SiO2 thin films through plasma- enhanced chemical vapor deposition using SiH4/Ar/N2O

SiO2 films and their formation technologies are critical for the fabrication of Si-based integrated circuits (ICs). SiO2 films fabricated using plasma-enhanced chemical vapor deposition (PECVD) have become increasingly important because of the increasing trend of three-dimensional (3D) architectures in ICs. SiO2 films formed by PECVD have been widely studied for over 40 years; however, owing to the complex plasma reactions and the differences in system conditions, the relationship between the experimental parameters and film characteristics are disputed. In this study, focusing on the two important characteristics (surface morphology and deposition rate) of SiO2 films for 3 D ICs manufacturing, SiO2 film growth was systematically investigated via SiH4/Ar/N2O in PECVD; a theoretical reaction process model (including gas-phase reaction, chemical adsorption, and surface reaction) bridging the experimental parameters (chamber pressure, gas ratio, radio frequency power, and substrate temperature) and characteristics (surface morphology and deposition rate) of SiO2 films was established. This study contributes to the precise formation of sub-100 nanometer SiO2 films using PECVD for manufacturing 3 D ICs.

Highly Transparent Oxygen‐Doped Poly‐Si with In‐situ N2O Oxidant for Poly‐Si Passivating Contacts in Perovskite/Silicon Tandem Solar Cells

Tunnel oxide passivated contact (TOPCon) silicon solar cells have been developed and transferred into industrial mass production, which is beneficial to the future production of perovskite/silicon tandem solar cells (TSCs) in a large scale. However, the doped polycrystalline silicon (poly‐Si) layer in the poly‐Si‐based passivating contact structure yields a profound optical loss from reflection and parasitic absorption, which obstacles the efficiency promotion of TSCs. In this work, the optical property of poly‐Si is improved by in‐situ oxygen incorporation using plasma enhanced chemical vapor deposition (PECVD) with nitrous oxide (N2O) as the oxygen source. The p‐type oxygen‐incorporated poly‐Si (poly‐SiOx) shows a reduced refractive index and extinction coefficient over 700–1200 nm wavelength, leading to a reduced reflection, a lower parasitic absorption and a higher transmission. After applying an optimized p‐type oxygen‐incorporated poly‐Si in the front‐side poly‐Si passivating contact structure of c‐Si bottom cell, the short‐circuit current density and efficiency of a perovskite/c‐Si tandem cell increases by ≈0.32 mA cm−2 and ≈0.8%, respectively, leading to 25.12% tandem cell efficiency.

Epitaxial twin coupled microstructure in GeSn films prepared by remote plasma enhanced chemical vapor deposition

Growth of GeSn films directly on Si substrates is desirable for integrated photonics applications since the absence of an intervening buffer layer simplifies device fabrication. Here, we analyze the microstructure of two GeSn films grown directly on (001) Si by remote plasma-enhanced chemical vapor deposition (RPECVD): a 1000 nm thick film containing 3% Sn and a 600 nm thick, 10% Sn film. Both samples consist of an epitaxial layer with nano twins below a composite layer containing nanocrystalline and amorphous. The epilayer has uniform composition, while the nanocrystalline material has higher levels of Sn than the surrounding amorphous matrix. These two layers are separated by an interface with a distinct, hilly morphology. The transition between the two layers is facilitated by formation of densely populated (111)-coupled nano twins. The 10% Sn sample exhibits a significantly thinner epilayer than the one with 3% Sn. The in-plane lattice mismatch between GeSn and Si induces a quasi-periodic misfit dislocation network along the interface. Film growth initiates at the interface through formation of an atomic-scale interlayer with reduced Sn content, followed by the higher Sn content epitaxial layer. A corrugated surface containing a high density of twins with elevated levels of Sn at the peaks begins forming at a critical thickness. Subsequent epitaxial breakdown at the peaks produces a composite containing high levels of Sn nanocrystalline embedded in lower level of Sn amorphous. The observed microstructure and film evolution provide valuable insight into the growth mechanism that can be used to tune the RPECVD process for improved film quality.

Protection of Si Nanowires against Aβ Toxicity by the Inhibition of Aβ Aggregation.

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by the accumulation of amyloid beta (Aβ) plaques in the brain. Aβ1-42 is the main component of Aβ plaque, which is toxic to neuronal cells. Si nanowires (Si NWs) have the advantages of small particle size, high specific surface area, and good biocompatibility, and have potential application prospects in suppressing Aβ aggregation. In this study, we employed the vapor-liquid-solid (VLS) growth mechanism to grow Si NWs using Au nanoparticles as catalysts in a plasma-enhanced chemical vapor deposition (PECVD) system. Subsequently, these Si NWs were transferred to a phosphoric acid buffer solution (PBS). We found that Si NWs significantly reduced cell death in PC12 cells (rat adrenal pheochromocytoma cells) induced by Aβ1-42 oligomers via double staining with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and fluorescein diacetate/propyl iodide (FDA/PI). Most importantly, pre-incubated Si NWs largely prevented Aβ1-42 oligomer-induced PC12 cell death, suggesting that Si NWs exerts an anti-Aβ neuroprotective effect by inhibiting Aβ aggregation. The analysis of Fourier Transform Infrared (FTIR) results demonstrates that Si NWs reduce the toxicity of fibrils and oligomers by intervening in the formation of β-sheet structures, thereby protecting the viability of nerve cells. Our findings suggest that Si NWs may be a potential therapeutic agent for AD by protecting neuronal cells from the toxicity of Aβ1-42.

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.

Hole and positron interaction with vacancies and p-type dopants in epitaxially grown silicon

The concentration of vacancies and impurities in semiconductors plays a crucial role in determining their electrical, optical, and thermal properties. This study aims to clarify the nature of the interaction between positrons and ionized p-type impurities, emphasizing the similarities they share with the interaction between holes and this type of impurity. An overall strategy for investigating defects in semiconductor crystals that exhibit a combination of vacancies and p-type impurities is presented. By using positron annihilation spectroscopy, in particular, Doppler broadening of the annihilation radiation, we quantify the concentration of vacancies in epitaxial Si crystals grown by low-energy plasma-enhanced chemical vapor deposition. The vacancy number densities that we find are (1.2 ± 1.0) × 1017 cm−3 and (3.2 ± 1.5) × 1020 cm−3 for growth rates of 0.27 and 4.9 nm/s, respectively. Subsequent extended annealing of the Si samples effectively reduces the vacancy density below the sensitivity threshold of the positron technique. Secondary ion mass spectrometry indicates that the boron doping remains unaffected during the annealing treatment intended for vacancy removal. This study provides valuable insights into the intricate interplay between vacancies and ionized impurities with positrons in semiconductor crystals. The obtained results contribute to advance the control and understanding of material properties in heterostructures by emphasizing the significance of managing vacancy and dopant concentrations.

Enhanced electromagnetic shielding with ultrathin VGNs-Metal hybrid structures

Designing and preparing lightweight electromagnetic shielding materials at the micron level is crucial for aerospace and military applications. Vertical graphene nanowalls (VGNs) offer high efficiency for electromagnetic shielding due to attributes such as high carrier mobility, porosity, and low density. This study presents the preparation of ultra-thin VGNs/metal stacked films via plasma-enhanced chemical vapor deposition (PECVD) for VGNs, and combined electron beam evaporation (EBE) with cyclic PECVD for metal layers. The PECVD technique ensures homogeneity and controllable thickness of the VGNs. The experimental results demonstrate that the interface between the VGNs and the metal layer exhibits good interfacial contact, resulting in high carrier mobility. Furthermore, incorporating a metal layer with a thickness ratio of 10 % significantly enhances conductivity by an order of magnitude. The hybrid structures exhibit remarkable electromagnetic shielding effectiveness per unit thickness (∼5300 dB mm−1) evaluated in the X-band range (8–12 GHz). Stacked films as thin as 0.01 mm achieve up to 53.4 dB of electromagnetic shielding. Finite element simulations indicate that the presence of opposing electric fields at the graphene-metal interface contributes to higher ohmic loss. The coupling effect at the interface significantly contributes to the material's excellent electromagnetic shielding effectiveness.

A perspective of recent advances in PECVD-grown graphene thin films for scientific research and technological applications

The purpose of this review is to provide a perspective on some of the recent advances in applying graphene thin films, including monolayer and multilayer graphene sheets synthesized by a scalable approach based on plasma-enhanced chemical vapor deposition (PECVD), to scientific research and technologies. The technological applications present here include the use of graphene in flexible electronics, for anticorrosion of metals, and for Si-based technologies including superlubricity and optoelectronics. In the case of fundamental scientific research, we demonstrate that periodic patterns of nanoscale strain distributions in monolayer graphene can lead to local giant pseudomagnetic fields as well as global modifications to the electronic properties of monolayer graphene, including strain-induced valley Hall signals and quantized Landau levels in the absence of external magnetic fields, edge states, as well as increasing electronic correlation with increasing strain. These findings suggest new approaches towards developing emerging quantum states with tunable electronic correlation based on graphene straintronics.

Effects of the In Situ Growth of CNTs on Ti-Coated Diamond Surfaces on the Mechanical Properties of Diamond/Aluminum Composites.

Diamond/aluminum composites have attracted significant attention as novel thermal management materials, with their interfacial bonding state and configuration playing a crucial role in determining their thermal conductivity and mechanical properties. The present work aims to evaluate the bending strength and thermal conductivity of CNT-modified Ti-coated diamond/aluminum composites with multi-scale structures. The Fe catalyst was encapsulated on the surface of Ti-coated diamond particles using the solution impregnation method, and CNTs were grown in situ on the surface of Ti-coated diamond particles using the plasma-enhanced chemical vapor deposition (PECVD) method. We investigated the influence of interface structure on the thermal conductivity and mechanical properties of diamond/aluminum composites. The results show that the CNT-modified Ti-coated diamond/aluminum composite exhibits excellent bending strength, reaching up to 281 MPa, compared to uncoated diamond/aluminum composites and Ti-coated diamond/aluminum composites. The selective bonding between diamond and aluminum was improved by the interfacial reaction between Ti and diamond particles, as well as between CNT and Al. This led to the enhanced mechanical properties of Ti-coated diamond/aluminum composites while maintaining acceptable thermal conductivity. This work provides insights into the interface's configuration design and the performance optimization of diamond/metal composites for thermal management.

Structural optimization and growth of intrinsic hydrogenated amorphous silicon films by HWCVD

Although the silicon heterojunction (SHJ) solar cell is the crystalline silicon solar cell with highest conversion efficiency at present, its higher cost of production line has been a factor restricting its industrial development. The use of hot wire chemical vapor deposition (HWCVD) technology instead of the mainstream plasma enhanced chemical vapor deposition (PECVD) technology for the deposition of amorphous silicon films can effectively reduce the cost of equipment and processing, and has a bright future. In recent years, i-a-Si:H films grown by PECVD have obtained great improvement in passivation quality, whereas HWCVD technology has been neglected in this field. This has significantly limited the development and application of HWCVD technology in SHJ cell production. In this work, we exploited the differences in the films properties and microstructures deposited by various hot-wire temperatures and successfully developed a structure for the high-passivation-quality i-a-Si:H film grown by HWCVD, consisting of a buffer layer and a double-layer bulk stack. By introducing the buffer layer grown with the 1650 °C hot-wire temperature and pure silane, the effective minority carrier lifetime was improved from 2.3 ms to 7.5 ms, and a cell efficiency enhancement of 0.4%abs was obtained. By depositing the bulk layer sequentially with hot-wire temperatures of 1800 °C and 1900 °C, the passivation quality and the conductance were both improved. An effective minority carrier lifetime of 8 ms and a further cell efficiency enhancement of 0.15%abs were obtained. Finally, SHJ solar cell efficiency of 24.35% was obtained with a home-made HWCVD-based pilot SHJ cell line.

Growth of Boron-doped Diamond-like-Carbon films from a low-pressure high-density plasma in Inductively Coupled Chemical Vapour Deposition

The main objective of this work was the preparation and optimization of boron-doped diamond-like carbon (DLC) films having a good crystalline structure, and electronic and optical properties, at an extremely low deposition pressure and relatively low growth temperature, by varying the flow rates of precursor gases (CH4, B2H6, and Ar) in the plasma controlled at 800 W of RF power. Planar inductively coupled plasma-enhanced chemical vapour deposition technique was used to enable deposition at ∼5.33 Pa and 450 °C, at a constant negative substrate bias of -40 V. The flow rate of the dopant gas (B2H6) was changed to obtain a set of samples, while the precursor gas (CH4) flow rate was kept fixed. The doped sample, prepared with B2H6/CH4 ratio (r) = 1%, was found to possess a minimum ID/IG (intensity ratio of D peak with that of G peak) of ∼0.819 and a maximum IDia/IG ratio (relative intensity of the Diamond peak to that of the G peak) of ∼1.149. A high sp3 content of ∼50% was revealed from the X-ray photoelectron spectroscopy analysis and the transmission electron microscopy images also indicated the presence of prominent <111>, <220>, and <311> crystallographic planes. The B-doped DLC film possessed enhanced electrical conductivity (σRT) of ∼3.02 × 10−5 S cm−1 relative to intrinsic DLC films, a corresponding minimum of activation energy (ΔEH) (as calculated in the above room temperature regime) of ∼170 meV, an optical band gap (Eg) of ∼3.35 eV, and the root mean square roughness of ∼48.12 nm.

Nitrogen admixture effects on growth characteristics and properties of carbon nanowalls

The plasma functionalization of carbon nanowalls (CNWs) is of great interest due to their potential applications in electron field emission and energy storage devices. In this study, the effects of nitrogen on the growth and properties of CNWs are thoroughly investigated by varying a wide range of the N2 concentrations in the CH4/H2/N2 discharge plasma. Nitrogen-doped carbon nanowalls (N-CNWs) were synthesized in a single-step growth using the hydrogen radical injection plasma-enhanced chemical vapor deposition technique. The formation of dense N-CNWs was found to be enhanced by nitrogen and was three times higher in the sample with a 150 sccm N2 flow rate than in the sample without N2 addition (N-0). As the N2 concentration increased from 0 to 200 sccm, the morphology of CNWs changed from relatively straight to curly-like nanowall structures. A high-resolution optical emission spectroscopic observation shows that as N2 flow increased, CN radicals became the dominant species, suppressing the formation of C2 and CH. Photoelectron spectroscopy analysis revealed the nitrogen doping effect on graphene layers at high N2 concentrations. According to Raman and Transmission Electron Microscopy analyses, the N-CNWs had more branched and thicker nanowalls with higher defect concentrations than the N-0 sample. The wettability of N-CNWs was improved, particularly for the sample with a N2 flow rate of 150 sccm.

Fabrication of a hybrid device for the integration of light-triggered proton pumps

Biological ion pumps, such as bacteriorhodopsin (bR), utilize photons to move ions against concentration gradients, offering energy harvesting and spatiotemporal control of chemical gradients. This capability goes far beyond the capabilities of today's synthetic devices, suggesting a hybrid approach to embed bRs in synthetic devices in order to direct the proton flow towards useful system applications. In this study, a hybrid silicon-based nanochannel network with integrated purple membranes (PM) containing bR was fabricated. The fabrication method combines thermal scanning probe lithography, etching techniques, atomic layer deposition, plasma-enhanced chemical vapor deposition, and photolithography to create devices with buried nanochannels on silicon substrates. PM patches were deposited onto specified sites by a tunable nanofluidic confinement apparatus. The resulting device holds the potential for locally controlling directed ion transport in micrometer scale devices, a first step towards applications, such as locally affected proton catalyzed chemical reaction networks. Furthermore, this fabrication strategy, employing a maskless overlay, is a tool for constructing intricate nanofluidic network designs which are mechanically robust and straightforward to fabricate.

Low-temperature growth at 225 °C and characterization of carbon nanowalls synthesized by radical injection plasma-enhanced chemical vapor deposition

The synthesis of carbon nanowalls (CNWs) necessitated a substrate heating temperature above 600 °C. This study describes a low-temperature growth at 225 °C and characterization of CNWs synthesized by the radical injection plasma-enhanced chemical vapor deposition. To investigate the effect of temperature on the growth process, CNWs were synthesized at various temperatures ranging from 200 °C to 700 °C. The morphology of the CNWs observed with a scanning electron microscope shows that wall density increases as substrate temperature decreases. The Raman spectroscopy analysis indicates that CNWs grown at 225 °C have higher defect levels than those grown at higher temperatures, such as 700 °C. Transmission electron microscopy confirmed the presence of multiple graphene layers in the CNWs grown at 225 °C. The water contact angle results revealed that CNWs grown at 225 °C had higher hydrophobicity than those grown at higher temperatures, opening up the potential for CNW applications.

Temperature-dependent morphology and composition of ultra-thin GeSn epilayers prepared by remote plasma enhanced chemical vapor deposition

Two distinct ultra-thin Ge1−xSnx (x ≤ 0.1) epilayers were deposited on (001) Si substrates at 457 and 313 °C through remote plasma-enhanced chemical vapor deposition. These films are considered potential initiation layers for synthesizing thick epitaxial GeSn films. The GeSn film deposited at 313 °C has a thickness of 10 nm and exhibits a highly epitaxial continuous structure with its lattice being compressed along the interface plane to coherently match Si without mismatch dislocations. The GeSn film deposited at 457 °C exhibits a discrete epitaxial island-like morphology with a peak height of ∼30 nm and full-width half maximum (FWHM) varying from 20 to 100 nm. GeSn islands with an FWHM smaller than 20 nm are defect free, whereas those exceeding 25 nm encompass nanotwins and/or stacking faults. The GeSn islands form two-dimensional modulated superlattice structures at the interface with Si. The GeSn film deposited at 457 °C possesses a lower Sn content compared to the one deposited at lower temperature. The potential impact of using these two distinct ultra-thin layers as initiation layers for the direct growth of thicker GeSn epitaxial films on (001) Si substrates is discussed.