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.     

Substrate pre-treatment by ultrasonication with diamond powder mixtures for nucleation enhancement in diamond film growth

Pre-treatment of silicon substrates by ultrasonic abrasion for nucleation enhancement in diamond film formation by hot-filament chemical vapour deposition is discussed. Scanning electron microscopy, atomic force microscopy and visible Raman spectroscopy were employed as analysis techniques. Ultrasonication was applied by suspensions of isopropanol with micro-or nanosized diamond powders, micro-sized metal and alumina particles and mixtures thereof. The root mean square roughness of the ultrasonically pre-treated samples varied from 0.2 to 12.0 nm depending on the applied powder mixture. All samples that were ultrasonically pre-treated had a larger diamond nucleation density than the untreated silicon wafer. As expected, for an effective increment of the diamond nucleation density by several orders of magnitude the application of diamond powder is necessary, since the generation of surface roughness alone is not sufficient to enhance the diamond nucleation kinetics satisfactorily. The simultaneous action of diamond powders and large alumina or titanium particles leads to an increase in diamond nucleation density up to a factor of 10⁶. When nano-diamond powder is used, the embedment of diamond fragments is best and in combination with titanium grains (50–75 µm) a diamond nucleation density of 8 × 10⁹ cm^− 2 is obtained. After 8 h of film growth, the diamond surface grains are significantly smaller for the samples that demonstrated higher nucleation densities, whereas the quality of the diamond layers is equal.     

Domain formation in diamond nucleation on iridium

Domain formation in epitaxial diamond nucleation on Ir(001) surfaces using the bias-enhanced nucleation (BEN) procedure has been studied. Bright areas of up to several microns lateral size with negligible topographic contrast are observed by scanning electron microscopy (SEM) after ion bombardment. When a growth step is applied after BEN, these domains develop into islands of identical shape consisting of epitaxial diamond with a high local area density of oriented grains. Outside the domains the nucleation density is either orders of magnitude lower or the grains are completely non-oriented. The diamond nuclei or precursors which are formed during the BEN step proved to be very stable: They still yielded oriented diamond islands when the samples were stored in air for 1 year before the growth step. Electron backscatter diffraction (EBSD) patterns taken from inside and outside the domains immediately after BEN did not show any significant difference. This allows the conclusion that the modification of the iridium crystal lattice accompanied with diamond nucleation is either very faint or only restricted to a very thin layer at the surface. Kelvin probe force microscopy (KPFM) measurements indicate a reduced work function within the domains.     

A new method for nucleation enhancement of diamond

Diamond nucleation of sufficient density to allow for the growth of continuous films requires substrate pretreatment either by abrasion with diamond particles or by in-situ biasing of the substrate, a method which applies only to electrically conductive materials. Even with the above methods, nucleation on materials which do not form carbides such as quartz is quite low. We report on a new method for increasing the abrasion effect using the ultrasonic cavitation method, through the use of slurry, which is composed of more than one component. Using this slurry, the nucleation density increases by up to several orders of magnitude, which shows the potential to enable the growth of continuous films on ceramic materials. A qualitative model to explain the mechanism of this nucleation enhancement phenomenon is suggested.     

Phase behavior study of PEB/PES blend by time resolved laser light scattering

The phase behavior of PEB/PES (poly(ethylene-co-styrene)/poly(ethylene-co-butene)) blend system was studied by time resolved laser light scattering with both wide angle and small angle laser light scattering. The phase separation kinetics show that the phase diagram of this system is a closed loop with both lower critical solution transition (LCST) and upper critical solution transition (UCST), which is inconsistent with the results obtained by optical microscope we reported before. All of the phase separation processes including early, middle and late stages have been studied. The coarsening behavior of the blend was characterized in terms of the time change of the maximum intensity I_m, and the magnitude of the scattering vector q_m, at which the intensity becomes maximum. The theory to interpret the phenomena has been studied by many researchers, and it is waiting for further theoretical systematic research.     

In situ observation of nucleation by optical emission spectra in CVD diamond

Optical emission spectroscopy (OES) was used to in situ detect the intensity variation of C₂ radical with the deposition time in the boundary layer during homo-nucleation of CVD diamond by DC Arcjet Plasma. The obvious drop and fluctuation of the optical emission intensity were found during the early growth stage. The samples grown after selected deposition time were characterized by micro-structural probes (transmission electron microscope (TEM), high resolution electronic microscope (HREM), selected area diffraction (SAD) and electron energy loss spectra (EELS)), in order to determine the occurrence of the diamond nucleation. Based on the results of the OES and the micro-structural probes, it was revealed that the variation of the optical emission intensity corresponded to the diamond nucleation. The incubation period and the lasting time of nucleation were thus deduced as 6–8 min and 20–60 s depending on the concentration of CH₄ in H₂. The incubation period decreased and the lasting time of nucleation increased with the increase of the concentration of CH₄ in H₂.     

Analysis of diamond nucleation on molybdenum by biased hot filament chemical vapor deposition

The nucleation and initial growth of diamond on molybdenum using biased hot filament chemical vapor deposition were investigated by scanning electron microscopy, Raman spectroscopy and adhesion force tests. The studies showed that the negative biased pre-treatment greatly enhanced the nucleation density and adhesion force of diamond films on molybdenum. The experimental evidence was confirmed that there is large stress near the interface between the diamond and the Mo substrate, which were originated from the disordered graphite phases and molybdenum carbide near the interface. This may play an important role during nucleation stage. However, larger stress can cause the degradation of the adhesion force of diamond films on Mo substrate. However, the adhesion force was enhanced with increasing bias voltage. The theoretical relationship between the adhesion force and the bias voltage is given by theoretical calculation.     

Study on enhancement of diamond nucleation on fused silica substrate by ultrasonic pretreatment

Fused silica substrates were pretreated by the ultrasonic vibration in the diamond powder slurry (UVDS). The influence of UVDS parameters such as the grain size of diamond powder, the liquid medium used to form the slurry, the weight ratio of diamond powder to liquid medium and the pretreatment time on the diamond nucleation density (DND) were systemically investigated. The grain size of diamond powder greatly affected the DND, the larger the grain size the higher the DND in our experiment conditions. The DND was about the same using acetone or ethanol or hexane medium. The best weight ratio of diamond powder (grain size 20–40 μm) to liquid medium was ∼1/60. Under appropriate pretreatment and CVD conditions, the DND of ∼10¹⁰ cm⁻² was obtained on fused silica substrates. Continuous ultra-thin diamond films with uniform and smooth surface (diamond grain size: ∼150 nm and surface roughness: ∼6 nm) were synthesized in an improved hot filament chemical vapor deposition (HFCVD) system. Nano-damaged sites on the pretreated surface mainly enhanced the DND and shortened the incubation time of nucleation.     

First stages of diamond nucleation on iridium buffer layers

During the first stages of epitaxial diamond nucleation by the bias-enhanced nucleation procedure on monocrystalline iridium films the metal surface shows a characteristic roughening. Two different types of furrows and ridges along 〈100〉 and 〈110〉 develop. From atomic force microscopy images a typical structure height of 3 nm is deduced. Transmission electron microscopy indicates {111} faceting. In addition, after 45 min of nucleation 5–10 nm large structures are found with a typical distance of 100–300 nm showing a bright contrast in scanning electron micrographs. While a subsequent 30 min growth step results in a similar density of diamond grains with a size of approximately 100 nm, prolonging the biasing step does not increase the size of these nucleation centers. It is shown that under the present bias-enhanced nucleation conditions large diamond grains are etched. The fact that diamond can nucleate under conditions under which diamond cannot grow has strong implications on the nature of the observed nucleation centers and on theoretical models describing the nucleation process.     

Effect of bias enhanced nucleation on the nucleation density of diamond in microwave plasma CVD

The variation of diamond nucleation density as a function of the conditions of bias enhanced nucleation (BEN) were studied. The nucleation density increased with microwave power, but decreased with the substrate temperature. The nucleation density also increased with bias voltage above 60 V, and had a maximum around 100 V. The crystal growth of diamond took place when either the bias voltage was high or the deposition time was long. The shift of C_1s energy measured by X-ray photoelectron spectroscopy indicated that the ratio of carbon sp³ bonds in the amorphous carbon and/or SiC phases formed before the nucleation of diamond, increased around the bias voltage of 100 V, which seemed to be the reason for enhancement of diamond nucleation by bias voltage. A simple computer simulation was performed in order to understand the effect of BEN conditions on the nucleation of diamond. The simulation reproduced the experimentally observed changes of nucleation density and particle size.     

Structural investigation of polymers by neutron scattering

Abstract

Neutron scattering is a unique probe of the structural and dynamic properties of polymeric materials. Two properties of the neutron particle have played a major role in the development of the technique: (i) length and timescales accessed by neutron scattering simultaneously match those of typical molecular processes, and (ii) the capability to manipulate the contrast by deuteration. Starting from what could now be regarded as rather simple experiments on common polymeric systems, the technique has been developed into a tool used to elucidate structure and dynamics of increasingly complex, multi-component materials. Exploiting the high penetration power of the neutron particle, in situ measurements can be performed, so that structural changes can be followed mimicking processing conditions. With continuous improvements in neutron flux and instrumentation, the range of systems that can be investigated is continuously expanding, as well as the accessible spatial and time range instrumentally accessible. Recent years have seen not only advances in synthetic chemistry, but also in molecular modelling and neutron scattering has afforded a way to compare experimental data and computer simulations. With plans being made for next generation neutron sources, e.g. the second target station at ISIS and SNS in the USA, the future is looking very bright for neutron scattering.     

BEN-HFCVD diamond nucleation on Si(111) investigated by HRTEM and nanodiffraction

The initial stages of diamond formation on firstly thinned Si(111) samples have been studied in a hot filament CVD (HFCVD) reactor by means of bright-field imaging and nanodiffraction experiments. Diamond crystals not resolved in high-resolution imaging are detectable using nanodiffraction. Nucleation mechanisms taking place during an HFCVD growth preceded or not by a bias treatment have been compared. The HRTEM observations strengthen previous reports concerning the smallest size of observable diamond islands. Finally, the bias effects on the surface morphology, on the nucleation density of diamond and on the orientation of diamond crystals versus the substrate are discussed. After bias, a matching between five {220} diamond and four {220} β-SiC planes in the [2–20] direction has been proposed. The former relationship is compared with previous studies of the diamond/β-SiC interface.     

Raman scattering by defect-induced excitations in boron-doped diamond single crystals

Polarized Raman spectra of the oriented boron-doped diamond with a different content of boron (≤ 200 ppm) were obtained with 514.5 and 1064 nm excitations. The additional bands were found in the region below 1200 cm^− 1. Their intensity increased with doping. It was shown that in polarized spectra these bands were in agreement with the singularities of density of phonon states (DOS) of diamond for the A_1g, E_g and F_2g symmetries. It was assumed that the ~ 900 cm^− 1 band which does not coincide with any DOS peak and has the highest resonance character may be attributed to the localized mode of boron in a diamond lattice. The spectra were accompanied by continuum that had the same symmetry F_2g as optical phonon at 1333 cm^− 1.     

Bias enhanced nucleation of diamond on iridium buffer layers studied by a NanoAuger probe

The early stages of diamond BEN-HFCVD on iridium have been studied using a NanoAuger probe which is a particularly powerful technique to probe crystals with a lateral size lower than 150 nm. Indeed, the optimal experimental conditions provide an electron beam with a diameter of 20 nm. More, the KVV Auger transition has a high sensitivity to the carbon binding state. According to our results, the BEN step is absolutely required to promote diamond nucleation on iridium otherwise the formation of graphite layers takes place under the CVD environment. The expected mechanisms induced by the ion bombardment are further discussed.     

Synthesis of highly oriented CVD diamond films by ultra short bias enhanced nucleation step

Highly oriented diamond films have been deposited on silicon substrate by the MPCVD technique (microwave plasma assisted chemical vapour deposition) using an ultra short bias enhanced nucleation process (so called USBEN). We focus our attention on two points: the homogeneity of the deposit in order to perform a precise characterisation whatever surface location (on 1×1 cm² of single silicon substrate); and the simplification of the successive steps usually performed in the BEN process. This is carried out by optimising the microwave cavity and the d.c. discharge extension and by keeping the pretreatments just necessary to obtain high nucleation density with an acceptable epitaxial ratio and a good homogeneity. This leads to a drastic reduction of the bias time of only 30 s for low bias voltage. As we obtain a highly oriented diamond film with a short bias pretreatment without preliminary carburation step, we discuss the substrate transformation under a weak bombardment duration of ions having a quite low energy. We think that the bias step probably consists to a slight modification of the substrate surface.     

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.     

A MIS transistor using the nucleation surface of polycrystalline diamond

In this work, the nucleation surface of a polycrystalline diamond film was used for the first time to fabricate a MISFET structure using standard photolithographic procedures, with a channel length of 100 μm. The resulting structure works as an enhancement-type p-type MOSFET. The I_ON/I_OFF ratio is about three orders of magnitude. The saturation of the current is clearly observed, with I_DS currents of about 20 nA for V_DS of 20 V. The smoothness of the nucleation surface allows a higher control of the electrodes, as well as their size decrease. The results show that, even though in an early stage, this investigation opens the door for a new generation of devices built on free-standing diamond films.     

Heteroepitaxial diamond nucleation and growth on silicon by microwave plasma-enhanced chemical vapor deposition synthesis

Heteroepitaxial diamond films were successfully nucleated and deposited on 1-inch diameter Si(001) substrates by microwave plasma-enhanced chemical vapor deposition (MPECVD). The precursor gases for the synthesis were methane and hydrogen. Before the application of a negative d.c. bias to the substrate, an in-situ carburization pre-treatment on the silicon was found to be an indispensable step towards the heteroepitaxial diamond on the silicon. Morphologies of the films were characterized by scanning electron microscopy (SEM). Interface observations based on the cross-sectional HRTEM directly reveal the heteroepitaxial diamond nucleation phenomena in detail. No interlayers of silicon carbide and/or amorphous carbon phases were observed. Tilt and azimuthal misorientation angles between the heteroepitaxial diamond crystals and the substrate were determined by combining the Ewald sphere construction in the reciprocal lattice space and the selected area diffraction (SAD) patterns taken across the interface.     

激光光散射法研究紫外光引发聚合

采用激光光散射法跟踪,发现紫外光引发聚合体系中有关助剂及紫外光强度对体系的光固化的诱导期和聚合反应速度具有明显的影响,而过程中使用硫醇类光固化促进剂具有最佳效果.同时发现,体系中表观聚合速度与紫外光强的1.2次方成正比.     

The effect of ion bombardment on the nucleation of CVD diamond

The nucleation effect of CVD diamond by ion bombardment was studied by a two-step process. In the first step, hydrocarbon and hydrogen ion bombardment was used to induce nucleation on mirror-polished (001) Si substrates. In the second step, diamond films were subsequently deposited on the ion-bombarded substrates by a conventional hot filament chemical vapor deposition. It was found that after the ion bombardment, an amorphous layer embedded with nano-crystalline diamond particles formed on the Si substrate. These nano-crystalline diamond particles were proposed to serve as the nucleation centers for the growth in the second step. The nucleation density depended strongly on the ion dosage and a nucleation density of up to 2×10⁹ cm⁻² could be achieved under optimized conditions.