Y. Avigal

Growth of aligned carbon nanotubes by biasing during growth

Well aligned multiwalled carbon nanotube(CNT) growth was achieved by positively biasing the substrate during growth. Growth was performed in a flowing mixture of 7% CH 4 in Ar onto Co covered Si held at 800 °C with and without the presence of an electric field. High resolution scanning electron microscopy shows that the tube alignment occurs only when a positive bias is applied to the substrate whereas no aligned growth occurs under a negative bias and no tube growth is observed, under the presently applied conditions, with no field. This finding may open up the possibility of realizing cold electron emitting devices based on CNTs with a large electric field enhancement. In particular, it may be possible to utilize the same gate which is needed to turn the device on also to obtain field assisted aligned carbon nanotube growth into the desired regions. Alternatively, due to the fact that no CNTs grow under the conditions of this experiment without bias, selected area biasing may permit selected area growth of vertically aligned carbon nanotubes, a process that may find many applications.

Absolute quantum photoyield of diamond thin films: Dependence on surface preparation and stability under ambient conditions

Absolute quantum photoyield (QPY) measurements (140–210 nm) of chemical vapor deposited (CVD) diamond films are reported. The dependence of the QPY on hydrogenation by exposure to a hydrogen microwave (MW) plasma and oxidation by a mixture of acids or on exposure to air under ambient conditions have been studied. Films deposited by MWCVD display a higher QPY than those grown by hot filament (HF) CVD. The QPY values are found to depend on the state of the surface. Hydrogen-terminated films exhibit values above 12% at 140 nm, whereas even small amounts of oxygen strongly degrade the QPY. B-doping, at the level of 1500 ppm, has no apparent effect on the photoemission properties. Exposure of the hydrogenated films to ambient conditions results in oxygen adsorption, leading to degradation of the photoemission properties. Analysis of the data within the three-step model of photoemission clearly shows that the state of the surface is a dominant factor determining the QPY.

Nucleation and growth of diamond on cemented carbides by hot-filament chemical vapor deposition

The experiments were performed on cemented carbide substrates (WC-6%Co). The depositions were carried out using hot-filament chemical vapor deposition and a CH4H2 1% gas mixture. The nucleation process was studied on polished and fractured surfaces. We investigated the effects of ultrasonic pretreatment, annealing in molecular hydrogen and deposition temperature on the nucleation rate. The deposition experiments performed on fractured substrates suggest that the main role in enhancing nucleation is played by surface contamination. It was found that when heating at 940°C in a CH4H2 1% activated gas mixture, there is a strong etching effect of cobalt. The growth of diamond films was studied on ground substrates both etched and unetched. The different growth kinetics on etched and unetched substrates in the early stage of deposition are explained.

Investigation of low-pressure diamond deposition on cemented carbides

Abstract The main problem of diamond deposition on cemented carbides is the presence of cobalt, a large amount of which on the substrate surface can disturb diamond nucleation and growth. Our research is focused on understanding the mechanism of cobalt influence on diamond coatings grown on cemented carbide substrates. The deposition was carried out using the hot-filament chemical vapor deposition method and a CH 4 H 2 1% gas mixture. The substrates had the following composition: 91.7% WC, 2.25% TaC, 0.25% NbC and 5.8% Co. In order to study the cobalt effect on diamond film deposition, etched and unetched diamond-ground samples were used. The etching was made to remove cobalt from the surface of the substrates. The substrate surface temperature, which affects cobalt mobility is an important factor in the coating of cemented carbides. It was found that at relatively high temperatures, cobalt strongly attacks diamond crystals.

[100]-Textured diamond films for tribological applications

Abstract Application of CVD diamond films as hard coating on tools may offer, in addition to their mechanical properties, also low friction coefficient. In this sense [100]-textured films are potentially advantageous over non-textured films due to their lower roughness. In the present work we demonstrate the capability to deposit [100]-textured diamond films on steel substrates coated with a nitrided chromium thin interlayer, and assess their tribological properties. The [100]-textured diamond films were deposited by MW-PACVD using a methane-hydrogen gas mixture with the addition of minute amounts of nitrogen in the ppm level. Continuous, textured films of good adhesion, and improved tribological properties were obtained. Scanning electron microscopy (SEM), Raman spectroscopy and friction coefficient measurements were used for characterization of the deposited films.

A NEW METHOD FOR NUCLEATION ENHANCEMENT OF DIAMOND

Abstract 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.

Investigation of cobalt behaviour during diamond deposition on cemented carbides

Abstract Obtaining high quality diamond depositions on cemented carbides is hindered by the presence of cobalt. This study deals with the movement of cobalt and its interaction with diamond crystals. The deposition was carried out using a hot-filament chemical vapour deposition method and a CH 4 -H 2 1% gas mixture. Diamond-ground cemented carbide substrates (WC-6%Co) were used. The diamond coatings were investigated by transmission electron microscopy, scanning electron microscopy, Auger electron spectroscopy and Raman spectroscopy. It was found that the interaction between diamond and cobalt results in zones which contain small cobalt particles and disordered graphite. The disordered graphite obtained as a result of the presence of cobalt promotes nucleation of diamond.

Diffusion of hydrogen in undoped, p-type and n-type doped diamonds

Hydrogen is a key impurity in diamond since it is unintentionally incorporated in all chemical vapor deposition (CVD) grown diamond layers.Its presence in the material can grossly affect its electrical and optical properties.Theoretically, hydrogen has been predicted to be present in diamond in one of the three charge states, H , H and H .Moreover it may form complexes q 0 y with impurities, native defects or with other hydrogen atoms.This paper is comprised of two parts: (a) a review of previous results of studies investigating different aspects of the diffusion of hydrogen (deuterium) in various kinds of diamonds.The diamonds studied are: undoped type IIa diamonds, undoped CVD diamond layers containing growth defects only, p-type B-doped homoepitaxially CVD grown diamond layers or B ion implanted type IIa diamonds and n-type P doped homoepitaxially CVD grown diamond or N-doped type Ib natural diamonds.Hydrogen is introduced in diamond by exposing the diamond surface to hydrogen plasma or by using hydrogen ion implantation.The following issues are discussed: (1) the influence of the interaction between H and the dopants and defects on the hydrogen diffusion. (2) The kinetic of (B, H), (P, H ) and (N, H) pair formation and dissociation. (3) The modification of the optical and electrical properties as a result of hydrogen incorporation and annealing. It is found that, under certain conditions, H diffuses into the B containing layer and it passivates B acceptors.In contrast, no H diffusion could be observed in n-type diamonds, up to 1000 8C. (b) Recent results of our group regarding other aspects related to the diffusion of H in diamond are presented.These include results on: (i) the influence of ion implantation related defects on the diffusion of deuterium.For this study type IIa samples implanted with B or non-dopant ions are used. (ii) The determination of the charge state of H or Hydefects complex as a function of diamond type.For that, annealing under bias is applied to deuterated diamond layers.We show that the presence of implantation defects retards the deuterium diffusion in a B-implantation doped diamond, demonstrating that D strongly interacts with defects, thus inhibiting diffusion.The new-formed complexes deteriorate the electrical properties of the diamonds and are very stable up to high temperatures.We confirm that, as expected, in highly B-doped CVD diamond layers, H diffuses as a positive ion.In lightly B-doped homoepitaxial layers, however, D is incorporated in complexes which seem to be negatively charged. � 2003 Elsevier Science B.V. All rights reserved.

The chemical nature of the carbon precursor in bias-enhanced nucleation of CVD diamond

Abstract The effect of sample biasing on CVD diamond nucleation on Si(100) substrates by the MW method has been previously addressed; however, the question as to the changes a silicon substrate undergoes under bias treatment is still open. In the present work we address this question by investigating the chemical reactivity of the material deposited during substrate bias. The different etching behavior in a hydrogen plasma of the possible precursor layer formed on silicon during the bias treatment is utilized. Silicon substrates were subjected to a three-step sequential process: (i) bias pretreatment, (ii) exposure to a pure hydrogen plasma and (iii) deposition under normal conditions. The characterization of the deposited films following each of these stages was carried out by Raman, AES and AFM. It was found that the carbon precursor to diamond growth, formed during the biasing stage, is stable under the etching process. This result suggests that the deposited material at this stage does not consist solely of a carbon network etchable by the hydrogen plasma, such as amorphous carbon (sp2 or sp3 bonded) and microcrystalline graphite or silicon carbide. It may therefore, be suggested that the precursor to diamond growth deposited during the bias process consists of non-etchable nanocrystallite diamond particles.

The effect of silicon surface preparation on the nucleation of diamond by chemical vapor deposition

Abstract The effect of surface impurities left on silicon following surface abrasion with diamond paste, on diamond nucleation in filament-assisted CVD was studied. Organic material left on the silicon surface following abrasion was found to be responsible for strongly enhancing nucleation of diamond crystallites. Dissolution of this organic material was found dramatically to decrease nucleation density, and total elimination of diamond nucleation results from effectively etching graphite from the scratches. This suggests that graphite left in the scratches plays an essential role in diamond nucleation, while additional organic matter on the surface further enhances nucleation.