Heteroepitaxial nucleation of diamond on Si(100) via double bias-assisted hot filament chemical vapor deposition

A new process has been developed to obtain high density epitaxial diamond nucleation via a double bias-assisted hot filament chemical vapor deposition (HFCVD). In the process, a negative bias voltage is applied to the Si substrate and a positive bias voltage is applied to a steel grid placed on top of the hot filaments. With this arrangement, a stable plasma can be generated between the grid and the hot filaments. Ions in the plasma are then drawn to the substrate by a negative substrate bias voltage. The impinging rate of these ions can be easily controlled by adjusting the grid current, and the ion energy can be independently controlled by adjusting the substrate bias voltage. Hence, the energy and dosage of ion bombardment onto the Si(100) substrate can be controlled easily and independently. With the controlled ion bombardment, high density and heteroepitaxial nucleation can be achieved routinely. After the nucleation process, highly textured diamond films were grown by either the HFCVD or the microwave plasma chemical vapor deposition process (MPCVD).     

Characterization of diamond fluorinated by glow discharge plasma treatment

The surface fluorination of diamond by treatment in glow discharge plasmas of CF₄ for different times has been investigated. High quality diamond films were deposited onto silicon substrates using hot filament chemical vapor deposition (HFCVD). Subsequently, the films were exposed to a radiofrequency glow discharge plasma of CF₄ for times ranging from 5 min to 1 h. The effects of the plasma treatment on the surface morphology, diamond quality and elemental composition were investigated using atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Differences in film roughness caused by the plasma treatment were detected by AFM and confirmed by scanning electron microscopy (SEM). Raman spectroscopic analyses showed that the original diamond was of high quality and that the bulk of each film was unchanged by the plasma treatment. Analyses using XPS revealed increased surface fluorination of the films at longer treatment times. In addition, the density of free radicals in the films was probed using electron paramagnetic resonance spectroscopy (EPRS), revealing that untreated diamond possesses an appreciable density of free radicals (6×10¹² g⁻¹) which initially falls with treatment time in the CF₄ plasma but increases for long treatment times.     

Nanometer rough,sub-micrometer-thick and continuous diamond chemical vapor deposition film promoted by a synergetic ultrasonic effect

In this paper we report on a surface treatment to seed substrates for the promotion of diamond nucleation. This surface treatment consists of an ultrasonic abrasion process using poly-disperse slurry composed of a mixture of small diamond particles (<0.25 μm) and larger particles (>3 μm) which may consist of diamond, alumina, titanium, etc. Whereas ultrasonic abrasion with a mono-disperse diamond slurry results in a diamond nucleation density of ∼2–3×10⁸ particles/cm², treatment with poly-disperse slurries results in diamond nucleation density of values up to ∼5×10¹⁰ particles/cm². This effect was found to display a similar effectiveness on a variety of substrates such as silicon, sapphire, quartz, etc. The enhancement in diamond nucleation is interpreted by a ‘hammering’ effect whereby the larger particles insert very small diamond debris onto the treated surface, thus increasing the density of nuclei onto which diamond growth takes place during the chemical vapor deposition process. By increasing the nucleation density to values of ∼5×10¹⁰ particles/cm², continuous diamond films of thickness of less than ∼100 nm were grown after only 5 min of deposition. The roughness of continuous diamond films grown on substrates treated at optimum conditions obtains values of 15–20 nm. The effect of ultrasonic treatment on silicon substrates and the deposited films was investigated by atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.     

Role of surface states in electrodeposition of Pb on n-Ge(111)

The influence of surface states on the kinetics of the initial stages of Pb electrodeposition on n-Ge(111) was studied by cyclic voltammetric and current transient measurements. The substrate surface was modified to n-Ge(111)H or n-Ge(111)OH applying appropriate experimental and polarization routines. In transient experiments, the driving force for nucleation, the supersaturation, was varied by both changing the electrode potential U at c_Pb²⁺=const and the concentration c_Pb²⁺ at U=const. The results obtained indicate that, under the experimental conditions used, the nucleation site density N₀ remains constant with the electrode potential and thus, does not influence the initial nucleation kinetics. The nucleation rate, the density of nucleation sites and the number of atoms N_crit in critical nuclei were obtained analyzing current transients on the basis of models for instantaneous and progressive nucleation. Nucleation kinetics were found to be more inhibited on n-Ge(111)OH. The experimental results show that the different nucleation behavior of H- and OH-terminated n-Ge(111) surfaces can be explained by different densities of Ge surface free radicals acting as nucleation sites. Problems connected with an application of electrodeposition for metallization of micro- and nanostructures are discussed.     

Quantitative study of epitaxial CVD diamond deposits: Correlations between nucleation parameters and experimental conditions

In the case of diamond deposit obtained by microwave plasma assisted chemical vapour deposition technique (MPCVD) where the bias enhanced nucleation (BEN) was used to initiate diamond islands on silicon substrate, we simultaneously studied nucleation parameters such as crystal density and epitaxial ratio according to main synthesis conditions. These ones were relative to in situ pretreatment steps occurring before diamond growth, i.e. plasma etching and bias sequences. The nucleation parameters were studied by the high resolution SEM associated to image analysis techniques on homogeneous 1 cm² samples.We observed that hydrogen etching duration clearly modified the epitaxial ratio without any change on the crystal density. So an optimal epitaxial ratio was reached for a moderate hydrogen etching while crystal density remained quite constant. The bias step was analysed in terms of duration and electrical behaviour (voltage and intensity) in relation to the plasma density that we were able to modify by physically confining the discharge. Bias duration clearly modified crystal density and epitaxial ratio. In later case, we observed a short optimal duration (between 30–90 s) for a 120 V bias voltage depending on the etching. We showed too that for a given bias duration the epitaxial ratio was all the more high as the voltage is low. The study of crystal density in relation to electrical characteristics of bias step showed that the more important parameter for nucleation is the electrical charge density (including intensity and time) and not the voltage, since nucleation density of 10⁸ cm^− 2 can be maintained for voltage close to 35 and 50 V respectively if the plasma power density during the bias is higher or if the BEN duration is longer.     

Nucleation and growth surface conductivity of H-terminated diamond films prepared by DC arc jet CVD

High-quality polycrystalline diamond film has been extremely attractive to many researchers, since the maximum transition frequency (f_T) and the maximum frequency of oscillation (f_max) of polycrystalline diamond electronic devices are comparable to those of single crystalline diamond devices. Besides large deposition area, DC arc jet CVD diamond films with high deposition rate and high quality are one choice for electronic device industrialization. Four inch free-standing diamond films were obtained by DC arc jet CVD using gas recycling mode with deposition rate of 14 μm/h. After treatment in hydrogen plasma under the same conditions for both the nucleation and growth sides, the conductivity difference between them was analyzed and clarified by characterizing the grain size, surface profile, crystalline quality and impurity content. The roughness of growth surface with the grain size about 400 nm increased from 0.869 nm to 8.406 nm after hydrogen plasma etching. As for the nucleation surface, the grain size was about 100 nm and the roughness increased from 0.31 nm to 3.739 nm. The XPS results showed that H-termination had been formed and energy band bent upwards. The nucleation and growth surfaces displayed the same magnitude of square resistance (R_s). The mobility and the sheet carrier concentration of the nucleation surface were 0.898 cm/V s and 10¹³/cm² order of magnitude, respectively; while for growth surface, they were 20.2 cm/V s and 9.97 × 10¹¹/cm², respectively. The small grain size and much non-diamond carbon at grain boundary resulted in lower carrier mobility on the nucleation surface. The high concentration of impurity nitrogen may explain the low sheet carrier concentration on the growth surface. The maximum drain current density and the maximum transconductance (g_m) for MESFET with gate length L_G of 2 μm on H-terminated diamond growth surface was 22.5 mA/mm and 4 mS/mm, respectively. The device performance can be further improved by using diamond films with larger grains and optimizing device fabrication techniques.     

Effect of plasma conditions on fabrication of multi-walled carbon nanotubes grown perpendicularly on Hastelloy C276®

We prepared multi-walled carbon nanotubes (MWCNTs) on Hastelloy C276® by a direct current (dc) plasma-assisted hot filament chemical vapor deposition in CH₄/N₂ atmosphere. Two-step deposition method, consisting of nucleation step and the subsequent CNT growth step, was employed and MWCNTs were grown by the catalytic growth with metal particles, which were precipitated from Hastelloy C276® and mainly consisted of Ni. The plasma generated at the current density of 30 mA/cm² was necessary to form nuclei for CNT growth, while the reducing of the plasma current density to the range between 10 and 20 mA/cm² was needed to grow MWCNTs. Based on these results, we proposed the possible mechanisms of the nucleus formation and MWCNT growth.     

Mechanism of diamond nucleation enhancement by electron emission via hot filament chemical vapor deposition

The mechanism of diamond nucleation enhancement by electron emission in the hot filament chemical vapor deposition process has been investigated by scanning electron microscopy, Raman spectroscopy and infrared (IR) absorption spectroscopy. The maximum value of the nucleation density was found to be 10¹¹ cm⁻² with a −300 V and 250 mA bias. The electron emission from the diamond coating on the electrode excites a plasma, and greatly increases the chemical species, as we have seen by in situ IR absorption. The experimental studies showed that the diamond and chemical species were transported and scattered from the diamond coating on the electrode and through the plasma towards the substrate surface, where they caused enhanced nucleation.     

In situ study of the initial stages of diamond deposition on 3C–SiC (100) surfaces: Towards the mechanisms of diamond nucleation

The mechanisms involved in the diamond nucleation on 3C–SiC surfaces have been investigated using a sequential in situ approach using electron spectroscopies (XPS, XAES and ELS). Moreover, diamond crystals have been studied by HRSEM. The in situ nucleation treatment allows a high diamond nucleation density close to 4 × 10¹⁰ cm^− 2. During the in situ enhanced nucleation treatment under plasma, a negative bias was applied to the sample. The formation of an amorphous carbon phase and the roughening of the 3C–SiC surface have been observed. The part of these competing mechanisms in diamond nucleation is discussed.     

Efficiency improvement of GaN-based ultraviolet light-emitting diodes with reactive plasma deposited AlN nucleation layer on patterned sapphire substrate

The flip chip ultraviolet light-emitting diodes (FC UV-LEDs) with a wavelength of 365 nm are developed with the ex situ reactive plasma deposited (RPD) AlN nucleation layer on patterned sapphire substrate (PSS) by an atmospheric pressure metal-organic chemical vapor deposition (AP MOCVD). The ex situ RPD AlN nucleation layer can significantly reduce dislocation density and thus improve the crystal quality of the GaN epitaxial layers. Utilizing high-resolution X-ray diffraction, the full width at half maximum of the rocking curve shows that the crystalline quality of the epitaxial layer with the (RPD) AlN nucleation layer is better than that with the low-temperature GaN (LT-GaN) nucleation layer. The threading dislocation density (TDD) is estimated by transmission electron microscopy (TEM), which shows the reduction from 6.8 × 10⁷ cm⁻² to 2.6 × 10⁷ cm⁻². Furthermore, the light output power (LOP) of the LEDs with the RPD AlN nucleation layer has been improved up to 30 % at a forward current of 350 mA compared to that of the LEDs grown on PSS with conventional LT-GaN nucleation layer.     

Formation of Cu/Pd bimetallic crystals by electrochemical deposition

The early stages of the palladium electrodeposition process onto a vitreous carbon (VC) substrate as well as the deposition of Cu on such Pd/VC modified surface were investigated using classical electrochemical techniques, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Within the potential range considered the kinetics of the Pd electrodeposition from a PdCl₂ acid solution can be described by a model involving progressive nucleation on active sites and diffusion-controlled 3D growth. The nucleation rate constant, A₀, and the number of active sites of the substrate, N₀, were determined from the analysis of potentiostatic current transients on the basis of an existing theoretical model. The AFM images corroborated the progressive nucleation mechanism showing irregular palladium crystals randomly distributed over the VC surface, with different sizes and 3D morphological characteristics. The electrodeposition of Cu was carried out onto the characterized Pd/VC modified surface from a Cu²⁺ containing solution using a well defined polarization routine. The SEM/EDX images confirmed the formation of Cu/Pd bimetallic crystals uniformly distributed on the VC surface and the in situ AFM images obtained during this process corroborated that Cu formed a core-shell structure with the Pd crystals. Nevertheless, the subsequent anodic stripping produced only a partial dissolution of the Cu deposits, and therefore, the formation of a Cu/Pd alloy could be inferred.     

Nucleation behavior in electroless displacement deposition of metals on silicon from hydrofluoric acid solutions

We investigate the nucleation behavior in the electroless displacement deposition of metal particles (Pt, Rh, Pd, Cu, Ag, and Au) onto n-Si wafers from a metal-salt solution containing HF. The particle density of metals varies widely from 10⁶ (Pt) to 10¹¹ (Au) cm⁻², depending on the kind of metal. Deposited metals can be classified into two types of nucleation behavior. One consists of the platinum group elements, including Pt, Rh, and Pd, which display lower particle densities than elements of the other group and depend on the type of pretreatment of the n-Si wafer, and thus the surface conditions of Si. The second group consists of the copper group elements, including Cu, Ag, and Au, which display higher particle density than the first group and are independent of pretreatment. The size of deposited particles decreases from hundreds nm to tens nm as the particle density increases. Moreover, the displacement deposition of the Pt and Ag particles onto n-Si are in progressive and instantaneous nucleation modes, respectively.     

On the influence of the nucleation overpotential on island growth in electrodeposition

Electrochemical deposition of a metal onto a foreign substrate usually occurs by an island growth mechanism. A key feature of island growth for a material M on a foreign substrate S is that the onset potential for deposition is shifted negative from the equilibrium potential for the metal ion couple. The nucleation overpotential, defined as η_n(M⁺/S) = |U_n(M⁺/S) − U_eq(M⁺/M)|, influences key aspects of deposition of a metal on a foreign substrate. Here we discuss how the nucleation overpotential influences the kinetics of island growth, the implications of the nucleation overpotential on island shape and orientation, and the consequences of the coupling between the island density (applied potential) and the island size at coalescence (grain size). We then discuss the kinetics of island growth in terms of the contributions to vertical and lateral growth. Finally, we present examples of experimental methods to manipulate the nucleation overpotential and overcome some of the limitations imposed by the nucleation overpotential.     

Facile fabrication of spherical architecture of Ni/Al layered double hydroxide based on in situ transformation mechanism

Spherical architectures of nickel–aluminum layered double hydroxide (NiAl‐LDH) with hydrotalcite‐like nanoflakes as building blocks were facilely fabricated by precipitation reaction in aqueous solution without any surfactants and organic solvents. Growth of such unique structure undergoes preorganization of primary nanospheres of colloidal amorphous aluminum hydroxide (AAH) in solution, followed by nucleation and crystallizaion of LDH from exterior to interior of AAH spheres by an in situ transformation mechanism. The structure and morphology of LDH spheres depend on both starting raw materials and synthetic parameters including reaction time, reaction temperature, and aqueous ammonia dosage. NiAl‐LDH sphere as positive electrode material delivers improved rechargeable and discharge capacity, with the highest discharge capacity of 173 mAh g^?1 at a current density of 30 mA g^?1 within a potential range from ?0.1 to 0.45 V in 10 mol L^?1 KOH solution, due to the faster diffusion processes in the spherical architecture than the powder sample. © 2014 American Institute of Chemical Engineers AIChE J 60: 4027–4036, 2014     

Influence of CF4 addition for HFCVD diamond growth on silicon nitride substrates

This work presents a study of CVD diamond growth on silicon nitride-based ceramics with the addition of carbon tetrafluoride (CF₄) in a hot filament-assisted reactor (HFCVD). Silicon nitride substrates were hot pressed under a nitrogen atmosphere for 90 min at 1750°C, giving specimens of very high density and good mechanical properties. The CF₄ addition is known to bring several advantages to diamond growth and, in particular, in this work, an important interaction of the CF₄-containing gas phase with the silicon nitride (Si₃N₄) substrates has been proven to be very beneficial for nucleation, growth and adherence of the diamond films. A basic gas mixture of H₂/1.5 vol.% CH₄/0.5 vol.% CF₄ was used in the growth experiments. The nucleation study reveals a strong interaction of the halogen-containing gas phase with the vitreous phase on the substrate surface. A strong erosion of the surface has been observed, which induced a high nucleation density (N_d) of the order of 10⁸ particles cm⁻², without any surface pre-treatment. Silicon nitride surface analysis was performed with Raman and infrared specular reflectance spectroscopy. Results suggest the erosion of the vitreous phase, mainly the silica (SiO₂) component, and the formation of silicon carbide, prior to diamond growth. Raman spectra and scanning electron microscopy (SEM) show better quality film grown with CF₄ addition. Indentation tests with a Rockwell C tip, at variable charge, show a better film adherence if grown with CF₄ addition.     

Influence of nucleation density on film quality,growth rate and morphology of thick CVD diamond films

The optimum growth parameters of our 5 kW microwave plasma CVD reactor were obtained using CH₄/H₂/O₂ plasma and high quality transparent films can be produced reproducibly. Among the films prepared in this system, the film of best quality has very smooth crystalline facets free of second nucleation and the full width at half maximum (FWHM) of the diamond Raman peak is 2.2 cm⁻¹, as narrow as that of IIa natural diamond. For this study, diamond films were grown on silicon substrates with low (10⁴–10⁵ cm⁻²) and high nucleation densities (>10¹⁰ cm⁻²), respectively. From the same growth run, a highly 〈110〉 textured 300 μm thick white diamond film with a growth rate of 2.4 μm/h was obtained from high nucleation densities (>10¹⁰ cm⁻²), and a white diamond film of 370 μm in thickness with a higher growth rate of 3 μm/h was obtained from low nucleation densities (5×10⁴–10⁵ cm⁻²) too. The effect of nucleation density on film quality, growth rate, texture and morphology was studied and the mechanism was discussed. Our results suggest that under suitable growth conditions, nucleation density has little effect on film quality and low nucleation density results in higher growth rate than high nucleation density due to less intense grain growth competition.     

N/S-Dual doped MnO modified spore-based ellipsoidal porous carbons for supercapacitors

Owing to the significance and requirement of renewable energy resources, in this study, ellipsoidal porous carbons with yolk-shell structures assembled using MnO and Ganoderma lucidum spores are fabricated for application prospects in energy storage systems; they exhibit excellent ion transfer capability. However, the surface of carbon nanomaterials is naturally hydrophobic, resulting in a lower energy density. Herein, heteroatom doping and O₂/Ar plasma surface treatment are utilized to obtain high specific capacitance and fast charging. Surface functionalization increases the surface roughness and oxygen-containing functional groups of the material. The specific capacitance of the best sample MnO/GSC-O–NS–10 was 568.9 F g⁻¹ when the current density was 0.5 A g⁻¹. The performance test was carried out for 10000 cycles at a current density of 10 A g⁻¹ and the capacitance retention rate was 75.11%. The assembled two-electrode capacitor exhibited a specific capacitance of 240.4 F g⁻¹ and an energy density of 33.4 Wh kg⁻¹ at a power density of 407 W kg ⁻¹. These findings provide sufficient theoretical guidance for the development of high-performance supercapacitors.     

Towards homogeneous and reproducible highly oriented diamond films

Diamond films have been deposited by the microwave plasma assisted chemical vapor deposition technique using an ultra short bias enhanced nucleation step to synthesize highly oriented diamond films on single silicon substrate. Firstly in this paper, we focus on the bias enhanced nucleation process to obtain homogeneous and reproducible diamond deposits. By optimizing the process, we obtained a crystal density value of 10⁹ cm⁻² on the whole substrate surface for a reduced polarization time of 60 s. Then, using scanning electron microscopy and image analysis, we report cartographies of crystal densities, covering rate and average radius on the whole sample surface. Next, we analyze a local area of the surface to produce a size distribution of the particles versus their type. Lastly, we present a discussion on the ratio of epitaxial crystals.     

Transmission electron microscopy study of diamond nucleation and growth on smooth silicon surfaces coated with a thin amorphous carbon film

Amorphous carbon (a-C) films with high contents of tetrahedral carbon bonding (sp³) were synthesized on smooth Si(100) surfaces by cathodic arc deposition. Before diamond growth, the a-C films were pretreated with a low-temperature methane-rich hydrogen plasma in a microwave plasma-enhanced chemical vapor deposition system. The evolution of the morphology and microstructure of the a-C films during the pretreatment and subsequent diamond nucleation and initial growth stages was investigated by high-resolution transmission electron microscopy (TEM). Carbon-rich clusters with a density of ∼10¹⁰ cm⁻² were found on pretreated a-C film surfaces. The clusters comprised an a-C phase rich in sp³ carbon bonds with a high density of randomly oriented nanocrystallites and exhibited a high etching resistance to hydrogen plasma. Selected area diffraction patterns and associated dark-field TEM images of the residual clusters revealed diamond fingerprints in the nanocrystallites, which played the role of diamond nucleation sites. The presence of non-diamond fingerprints indicated the formation of Si–C-rich species at C/Si interfaces. The predominantly spherulitic growth of the clusters without apparent changes in density yielded numerous high surface free energy diamond nucleation sites. The rapid evolution of crystallographic facets in the clusters observed under diamond growth conditions suggested that the enhancement of diamond nucleation and growth resulted from the existing nanocrystallites and the crystallization of the a-C phase caused by the stabilization of sp³ carbon bonds by atomic hydrogen. The significant increase of the diamond nucleation density and growth is interpreted in terms of a simple three-step process which is in accord with the experimental observations.     

Surface conductivity of nitrogen-doped diamond

Synthetic type Ib diamond crystals with (001) surfaces that expose growth sectors of different nitrogen content have been used to study the phenomenon of p-type surface conductivity upon plasma hydrogenation and upon overgrowth with thin epitaxial CVD diamond layers. We found that an unbiased microwave-driven hydrogen plasma leads to surface conductivity only on well-defined regions on the substrates that correlate with growth sectors of low nitrogen content; whereas no conductive layer is found on top of growth sectors with higher nitrogen concentrations in the range of 200 ppm. After growing a homoepitaxial intrinsic diamond layer of only 20 nm on top of the nitrogen doped diamond, these differences are no longer observed and surface conductivity is established homogeneously over the whole sample. The same effect can be achieved by exposing the Ib substrates to a pure hydrogen plasma provided the sample is biased with an additional DC voltage of −250 V. Both results can be understood in the framework of the surface transfer doping model suggested earlier by Maier and colleagues when the compensation of nitrogen donors by surface acceptors and their passivation by hydrogen is taken into account. The quantitative discussion shows that the doping capability of the surface acceptors is exhausted at lateral concentrations of approximately 1×10¹³ cm⁻², which also corresponds to the maximum hole concentration usually observed in hydrogen-induced p-type conductive layers.