Effect of oxygen on the bias-enhanced nucleation of diamond on silicon

The influence of traces of oxygen in the process gas on the bias-enhanced nucleation (BEN) of diamond on silicon has been studied in the present work. CO₂ in concentrations ranging from 0 to 3000 ppm was added during the nucleation procedure at U_bias=−200 V in microwave plasma chemical vapour deposition (MPCVD). A significant influence of CO₂ could already be detected for a concentration of 75 ppm, corresponding to a C:O ratio of 600:1. It resulted in a continuous reduction of the biasing current and a delay in the nucleation process accompanied by a decrease of the final diamond-covered substrate surface area with increasing CO₂ concentration. At 3000 ppm, the nucleation was completely suppressed. An etching of diamond nuclei by the oxygen could be excluded from in-situ growth rate measurements under bias. Instead, optical emission spectra of the H_β Balmer line indicated a decrease in electrical field strength in the plasma above the substrate. For all gas compositions allowing diamond nucleation, epitaxially aligned films could be obtained, provided that the duration of the biasing step was chosen appropriately. Thus, traces of oxygen do not completely suppress epitaxy. However, the in-plane orientation of the films as determined by X-ray diffraction measurements deteriorates with increasing oxygen concentration.     

Initial growth of heteroepitaxial diamond on Ir (0 0 1)/MgO (0 0 1) substrates using antenna-edge-type microwave plasma assisted chemical vapor deposition

Initial growth of heteroepitaxial diamond on Ir (0 0 1)/MgO (0 0 1) was investigated by scanning electron microscopy, reflection high-energy electron diffraction (RHEED) and atomic force microscopy. Bias-enhanced nucleation (BEN) was performed by antenna-edge-type microwave plasma assisted chemical vapor deposition. In BEN, diamond crystallites nucleated and grew along the [−1 1 0] and [1 1 0] directions of iridium. Diamond was likely to nucleate on protruded iridium areas. After BEN, in addition to the diamond diffraction spots, iridium bulk diffraction spots, which were not observed before BEN, were observed by RHEED. The iridium surface appeared to be protruded and changed by the high ion current density in BEN. Under [0 0 1] selective growth conditions, diamond crystallites, which were less than 10 nm in diameter, were etched by H₂ plasma. Diamond nucleated areas corresponded to the surface ridges of iridium along the [−1 1 0] and [1 1 0] directions at 10–40 nm intervals before BEN.     

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.     

Large area deposition of boron doped nano-crystalline diamond films at low temperatures using microwave plasma enhanced chemical vapour deposition with linear antenna delivery

We report on the preparation and characterisation of boron (B) doped nano-crystalline diamond (B-NCD) layers grown over large areas (up to 50 cm × 30 cm) and at low substrate temperatures (< 650 °C) using microwave plasma enhanced linear antenna chemical vapour deposition apparatus (MW-LA-PECVD). B-NCD layers were grown in H₂/CH₄/CO₂ and H₂/CH₄ gas mixtures with added trimethylboron (TMB). Layers with thicknesses of 150 nm to 1 μm have been prepared with B/C ratios up to 15000 ppm over a range of CO₂/CH₄ ratios to study the effect of oxygen (O) on the incorporation rate of B into the solid phase and the effect on the quality of the B-NCD with respect to sp³/sp² ratio. Experimental results show the reduction of boron acceptor concentration with increasing CO₂ concentration. Higher sp³/sp² ratios were measured by Raman spectroscopy with increasing TMB concentration in the gas phase without CO₂. Incorporation of high concentrations of B (up to 1.75 × 10²¹ cm³) in the solid is demonstrated as measured by neutron depth profiling, Hall effect and spectroscopic ellipsometry.     

Effect of H2/Ar plasma on growth behavior of ultra-nanocrystalline diamond films: The TEM study

Incorporation of H₂ species into Ar plasma was observed to markedly alter the microstructure of diamond films. TEM examinations indicate that, while the Ar/CH₄ plasma produced the ultrananocrystalline diamond films with equi-axed grains (~ 5 nm), the addition of 20% H₂ in Ar resulted in grains with dendrite geometry and the incorporation of 80% H₂ in Ar led to micro-crystalline diamond with faceted grains (~ 800 nm). Optical emission spectroscopy suggests that small percentage of H₂-species (< 20%) in the plasma leads to partially etching of hydrocarbons adhered onto the diamond clusters, such that the C₂-species attach to diamond surface anisotropically, forming diamond flakes, which evolve into dendrite geometry. In contrast, high percentage of H₂-species in the plasma (80%) can efficiently etch away the hydrocarbons adhered onto the diamond clusters, such that the C₂-species can attach to diamond surface isotropically, resulting in large diamond grains with faceted geometry. The field needed to turn on the electron field emission for diamond films increases from E₀ = 22.1 V/μm (J_e = 0.48 mA/cm² at 50 V/μm applied field) for 0% H₂ samples to E₀ = 78.2 V/μm (J_e < 0.01 mA/cm² at 210 V/μm applied field) for 80% H₂ samples, as the grains grow, decreasing the proportion of grain boundaries.     

Mechanistic study of ion-induced diamond nucleation

The effect of low-energy ion bombardment of silicon on diamond nucleation was investigated. By bombarding 100 eV ions of methane and hydrogen on a silicon substrate prior to diamond growth by chemical vapor deposition, diamond nucleation can be immensely enhanced. The ion beam treatment deposited a layer of nano-crystalline graphitic carbon embedded with amorphous SiC. Diamond then nucleated on the graphite overlayer; the nucleation density increased with increasing ion dose. At 1×10¹⁹ ions cm⁻², a nuclei density of 4×10⁸ cm⁻² was obtained. These results show that ion bombardment of the substrate enhances diamond nucleation.     

High-quality diamond film deposition on a titanium substrate using the hot-filament chemical vapor deposition method

Diamond film on titanium substrate has become extremely attractive because of the combined properties of these two unique materials. Diamond film can effectively improve the properties of Ti for applications as aerospace and biomedical materials, as well as electrodes. This study focuses on the effects of process parameters, including gas composition, substrate temperature, gas flow rate and reactor pressure on diamond growth on Ti substrates using the hot-filament chemical vapor deposition (HFCVD) method. The nucleation density, nuclei size as well as the diamond purity and growth tendency indices were used to quantify these effects. The crystal morphology of the material was examined with scanning electron microscopy (SEM). Micro-Raman spectroscopy provided information on the quality of the diamond films. The growth tendency of TiC and diamond film was determined by X-ray diffraction analysis. The optimal conditions were found to be: CH₄:H₂ = 1%, gas flow rate = 300 sccm, substrate temperature T_sub = 750 °C, reaction pressure = 40 mbar. Under these conditions, high-quality diamond film was deposited on Ti with a growth rate of 0.4 μm/h and sp² carbon impurity content of 1.6%.     

Optical-quality diamond growth from CO2-containing gas chemistries

From interpretation of the Bachmann diagram, it is conceivable that there may be some advantage to be gained by moving up the H–CO tie line for optical-quality diamond deposition. A convenient system for achieving this are gas chemistries containing CO₂, which, when combined with gases such as CH₄ and C₂H₄ (ethylene), enables the diamond deposition region to be traversed and, with the addition of hydrogen, to move along the H–CO tie line. The fabrication of free-standing diamond wafers using combinations of these feed-stock gases with a high-pressure, 2.45-GHz microwave source reactor (HPMS) able to operate at up to 140 Torr and 6 kW has been investigated. The FWHM line width of the 1332 cm⁻¹ Raman peak is found to be predominately a function of the gas composition. The growth rate is also dependent on the input power and the deposition pressure. The deposition plasmas are bright green in colour, and optical emission spectroscopy (OES) of the plasma reveals distinctive C₂ and Hα peaks. In some cases, it is possible to correlate characteristics of the deposited diamond layer to features in the OES spectra.     

Ion acceleration in bias-enhanced nucleation of diamond at relatively high pressures

In order to explain the bias-enhanced nucleation (BEN) of diamond at pressures of approximately 6 kPa, the flux and energy of hydrogen ions to a negatively biased substrate in microwave plasma chemical vapor deposition (MWP-CVD) were investigated. Ion flux increased rapidly upon increasing the negative bias voltage to greater than 150 V when the pressure was higher than 3 kPa. This phenomenon was attributed to the additional ionization in the vicinity of the substrate due to biasing. Optical emission spectroscopy at 6 kPa clarified that the emission intensity of the Swan band of C₂ radicals was reduced, and intramolecular temperatures were enhanced in this bias region. A numerical simulation on the basis of the measured ion flux and density revealed that the most frequent energy was less than 2 eV under the conditions of 200 V and 6 kPa, and that the average number of collisions in the plasma sheath reached 60. We propose, on the basis of these results, that the substrate biasing in BEN at pressures above 3 kPa causes the additional ionization and activation of neutral species in the sheath, while the direct momentum transfer of ions to the substrate is strongly suppressed in BEN in this pressure range.     

Thermal stability and surface modifications of detonation diamond nanoparticles studied with X-ray photoelectron spectroscopy

Detonation nanodiamond (ND) particles were dispersed on silicon nitride (SiN_x) coated sc-Si substrates by spin-coating technique. Their surface density was in the 10¹⁰–10¹¹ cm^?2 range. Thermal stability and surface modifications of ND particles were studied by combined use of X-ray Photoelectron Spectroscopy (XPS) and Field Emission Gun Scanning Electron Microscopy (FEG SEM). Different oxygen-containing functional groups could be identified by XPS and their evolution versus UHV annealing temperature (400–1085 °C) could be monitored in situ. The increase of annealing temperature led to a decrease of oxygen bound to carbon. In particular, functional groups where carbon was bound to oxygen via one σ bond (C–OH, C–O–C) started decomposing first. At 970 °C carbon–oxygen components decreased further. However, the sp²/sp³ carbon ratio did not increase, thus confirming that the graphitization of ND requires higher temperatures. XPS analyses also revealed that no interaction of ND particles with the silicon nitride substrate occurred at temperatures up to about 1000 °C. However, at 1050 °C silicon nitride coated substrates started showing patch-like damaged areas attributable to interaction of silicon nitride with the underlying substrate. Nevertheless ND particles were preserved in undamaged areas, with surface densities exceeding 10¹⁰ cm^?2. These nanoparticles acted as sp³-carbon seeds in a subsequent 15 min Chemical Vapour Deposition run that allowed growing a 60–80 nm diamond film. Our previous study on Si(100) showed that detonation ND particles reacted with silicon between 800 and 900 °C and, as a consequence, no diamond film could be grown after Chemical Vapour Deposition (CVD). These findings demonstrated that the use of a thin silicon nitride buffer layer is preferable insofar as the growth of thin diamond films on silicon devices via nanoseeding is concerned.     

Nucleation of diamond films by ECR-enhanced microwave plasma chemical vapor deposition

A novel nucleation technique based on electron cyclotron resonance microwave plasma was developed to enhance the nucleation of diamond. By choosing a suitable experimental condition, a nucleation density higher than 10⁸ nuclei cm⁻² was achieved on an untreated, mirror-polished silicon substrate. Uniform diamond films were obtained by combining this nucleation method with subsequent growth by the common microwave plasma chemical vapor deposition. Furthermore, the possibility of this new nucleation method to generate heteroepitaxial diamond nuclei on (001) silicon substrates was explored.     

Conductive and resistive nanocrystalline diamond films studied by Raman spectroscopy

This paper shows what structural properties of amorphous non-diamond phases in nanocrystalline diamond films are responsible for the transition from resistive to conductive films. The films incorporated with nitrogen, oxygen, and hydrogen are prepared by microwave plasma chemical vapor deposition using Ar-rich gas mixtures. The amount, composition, and bonding properties of non-diamond phases are studied mainly by Raman spectroscopy and compared with the electrical resistivity of the films. The addition of N₂ gas decreases the resistivity down to the order of 10^− 2 Ω cm for deposition temperatures above a threshold of ∼ 1100 K. Non-diamond phases for high n-type conductivity are characterized by graphitic components with improved sp² bond angle order for trivalent carbon atoms in addition to CN bonds. The addition of O₂ or H₂ gas promotes incorporation of oxygen or hydrogen into the films, not preferential etching of non-diamond phases. The resistivity increases or decreases largely by oxygen or hydrogen incorporation, respectively, then inversely changes by thermal annealing due to the deoxidization and dehydrogenation.     

Synthesis of diamond using ultra-nanocrystalline diamonds as seeding layer and their electron field emission properties

A modified nucleation and growth process was adopted so as to improve the electron field emission (EFE) properties of diamonds films. In this process, a thin layer of ultra-nanocrystalline diamonds (UNCD), instead of bias-enhanced-nuclei, were used as nucleation layer for growing diamond films in H₂-plasma. The morphology of the grains changes profoundly due to such a modified CVD process. The geometry of the grains transform from faceted to roundish and the surface of grains changes from clear to spotty. The Raman spectroscopies and SEM micrographs imply that such a modified diamond films consist of UNCD clusters (~ 10–20 nm in size) on top of sp³-bonded diamond grains (~ 100 nm in size). Increasing the total pressure in CVD chamber deteriorated the Raman structure and hence degraded the EFE properties of the films, whereas either increasing the methane content in the H₂-based plasma or prolonged the growth time improved markedly the Raman structure and thereafter enhanced the EFE properties of diamond films. The EFE properties for the modified diamond films can be turned on at E₀ = 11.1 V/μm, achieving EFE current density as large as (J_e) = 0.7 mA/cm² at 25 V/μm applied field.     

Gas-phase epoxidation of propylene through radicals generated by silica-supported molybdenum oxide

It was found that silica-supported molybdenum oxide was high effective for the epoxidation of propylene among various silica-supported metal oxides. The post-catalytic bed volume played an important role in its formation. On a MoO_x/SiO₂ with 0.255 mmol/g-SiO₂, a propylene conversion of 17.6% and a PO selectivity of 43.6% were obtained at 5 atm, 573 K and flow rates of C₃H₆/O₂/He = 10/5/10 cm³ min⁻¹. The characterization studies indicated that crystalline MoO₃ nano-particle species was more effective for propylene epoxidation to PO than molecularly dispersed Mo oxide species. The reaction mechanism of propylene epoxidation on MoO_x/SiO₂ catalysts is hypothesized to involve gas-phase radicals generated at relatively low temperature by the dispersed molybdenum oxide species. These radicals participated in homogeneous reactions with molecular oxygen to produce propylene oxide.     

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.     

Plasma catalysis over lanthanum substituted perovskites

The removal of CH₄ (3600 ppm) with O₂ (3 × 10⁴ ppm) in mixtures with Ar or N₂ as carrier gas has been studied in a plasma-catalyst system. The plasma yields CO plus H₂O as majority products. A small extra oxidation to CO₂ is found at 338 K when a catalyst (SiO₂ or La_1−xSr_xCoO_3−d (x = 0.5) perovskite) is placed in the glow zone of the plasma. With the perovskite, the oxidation efficiency to CO₂ increased with temperature up to 90% at 453 K. This result supports that this lanthanum substituted cobaltite further activates the plasma species producing a synergetic effect where the specific surface area is not a critical factor as previously reported in the literature.     

n-type phosphorus-doped polycrystalline diamond on silicon substrates

The microwave plasma-assisted deposition of reproducible and homogeneously n-type phosphorus-doped polycrystalline (microcrystalline) diamond films on silicon substrates is described. The phosphorus incorporation is obtained by adding gaseous phosphine (PH₃) to the gas mixture during growth. The low CH₄/H₂ ratio (0.15%) and the use of the same growth parameters as for homoepitaxial {111} films, led to a good crystalline quality of the continuous polycrystalline diamond layers, confirmed by SEM images and Raman spectroscopy measurements.Secondary-ion mass spectrometry (SIMS) analysis measured a phosphorus concentration [P] of at least 7 × 10¹⁷ cm^− 3. Cathodoluminescence spectroscopy in our P-doped polycrystalline films shows a phosphorus bound exciton (BE^TO_P) peak between 5.142 and 5.181 eV. Cathodoluminescence and Raman-effect spectroscopy confirmed the improvement of the crystalline quality of our films as well as a decrease in the intensity of the internal strain when the grain size was decreased. Cathodoluminescence imaging and SIMS depth profile of phosphorus demonstrated a very good homogeneity of phosphorus incorporation in the films.     

Growth of diamond by DC Arcjet Plasma CVD: From nano-sized poly-crystal films to millimeter-sized single crystal grain

A 30 kW-powered DC Arcjet Plasma enhanced chemical-vapor deposition (CVD) system was applied to grow diamonds which included the nano-crystal free-standing film, the nano-/micro-crystal layered free-standing film, the gradient micro-crystal free-standing film and the millimeter-sized grain. The free-standing film quality, such as the roughness, the sp² content, the residual stress and the grain morphology, was studied by an atomic force microscope (AFM), Raman spectra, a scanning electron microscope (SEM) and a high resolution electron microscope (HREM). In large-sized grain deposition, as-grown deposit was obtained about 1 × 1 × 1 mm³ in size under the condition of 10 μm/h of the substrate moving speed without Nitrogen enhancement. Characterized by Raman spectra and Laue back reflection X-ray diffraction, the deposit was proven to be single crystal diamond with small grains coving its surfaces. The growth rate was about 30 μm/h. Optical emission spectrum (OES) was utilized to characterize gas phases in the plasma for diamond deposition. The mean electron temperature (Te) in the plasma was calculated based on the value of the emission intensity ratio of I_Hγ/I_Hβ. Te varied from 0.33 eV to 0.5 eV depending on the concentration of CH₄ in H₂ from 1.0% to 25%. C₂ radical was found to be the dominant carbon source compared with CH radical. The influence of the radical on the morphology of diamond was discussed. It was found that the nano-crystal could be grown when the ratio of the emission intensity, I_C2/I_CH, was larger than 8.     

Nucleation of nanocrystalline diamond on masked/unmasked Si3N4 ceramics with different mechanical pretreatments

Results are presented concerning different mechanical pretreatments performed on silicon nitride substrates and their influence on the nucleation and growth of nanocrystalline diamond (NCD). All substrates were equally sintered and finished, but differently pretreated. Then, they were diamond coated in a microwave chemical vapor deposition system (MPCVD) for relatively short periods, using Ar/H₂/CH₄ gas mixtures. The main objective was to identify the best pretreatment among those proposed, while verifying how it correlates with film uniformity and surface roughness after post-growth. The effect of a molybdenum mask during growth is investigated.The top surface analysis revealed major differences in the nucleation morphology of diamond nuclei on the pretreated samples, two different nucleation types having been identified. For all pretreatments, samples exhibited a very smooth and uniform underlayer of very fine grain particles before the formation of larger aggregates, suggesting a bi-phase nucleation mechanism. When no mask is used considerable changes in the nucleation concentration are found, the resulting films showing grain enlargement near the edges, where the morphology assumes microcrystalline nature. This effect is suppressed by the use of a mask that allowed obtaining very uniform smooth films (R_rms∼ 30 nm, thickness ∼ 1.3 μm, MUS pretreatment), indicating a strong edge effect for the unmasked case. This fact can be attributed both to increased local temperature, plasma density and gas turbulence.     

Quality of diamond wafers grown by microwave plasma CVD: effects of gas flow rate

We found a strong impact of gas flow rate on diamond growth process in a 5 kW microwave plasma chemical vapour deposition reactor operated on CH₄-H₂ gas mixtures. Diamond films of 0.1–1.2 mm thickness and 2.25 in. in diameter were produced at H₂ flow rates varied systematically from 60 sccm to 1000 sccm at 2.5% CH₄. The highest growth rate, 5 μm h⁻¹, was observed at intermediate F values (≈300 sccm). Carbon conversion coefficient (the number of C atoms going from gas to diamond) increases monotonically up to 57% with flow rate decrease, however, this is accompanied with a degradation of diamond quality revealed from Raman spectra, thermal properties and surface morphology. High flow rates were necessary to produce uniform films with thermal conductivity >18 W cm⁻¹ K⁻¹. Diamond disks with very low optical absorption (loss tangent tgδ<10⁻⁵) in millimetre wave range (170 GHz) have been grown at optimized deposition conditions for use as windows for high-power gyrotrons.