Growth of diamond thin films by chemical vapour deposition

Deposition of diamond thin films on non-diamond substrates at low pressures (<760 torr) and low temperatures (<2000°C) by chemical vapour deposition (CVD) has been the subject of intense research in the last few years. The structural and the electrical properties of CVD diamond films grown on p-type 〈111〉 and high-resistivity (>100 kΩ-cm) 〈100〉 oriented silicon substrates by hot filament chemical vapour deposition technique are described in this review paper.     

Formation of large diamond crystals in oxyacetylene flame chemical vapour deposition

Diamond coatings were deposited on diamond-polished molybdenum substrates from a premixed oxyacetylene flame for a long time (up to 4 h) at substrate temperatures between 700 and 950°C, acetylene-to-oxygen ratios 1.02–1.07 and total flow rates between 230 and 310 standard Lh^–1. The coatings contain, in addition to the densely populated octahedral crystals making a continuous layer of determined thickness, a number of individual large cuboctahedral crystals sticking out far above the layer. A large cuboctahedral crystal is formed from an octahedral one when the latter reaches a certain height at which its temperature becomes sufficiently high for the octahedron-to-cuboctahedron conversion to take place. This conversion was found to occur by a flattening of the octahedron pyramid tip whereby a {100} face perpendicular to the growth direction is formed. Both the height of the crystal and size of the {100} face increase upon further deposition, reaching up to 230 m above the octahedral crystals layer and up to about 200 m, respectively. The large crystals have smooth {100} faces, but otherwise often have an irregular shape which may be due to a high temperature favouring deposition of non-diamond carbon.     

Nucleation and growth mechanism of diamond during hot-filament chemical vapour deposition

High-resolution transmission electron microscopy (HRTEM) was employed to study the nucleation and subsequent growth mechanism of crystalline diamond grown on copper TEM grids by the hot-filament chemical vapour deposition process. The HRTEM revealed direct evidence for the formation of a diamond-like amorphous carbon layer 8–14 nm thick, in which small diamond microcrystallites about 2–5 nm across were embedded. These diamond microcrystallites were formed as a result of direct transformation of the diamond-like carbon into diamond. Large diamond crystallites were observed to grow from these microcrystallites. The diamond surface was found to be non-uniform. It is envisaged that the diamond microcrystallites present in the amorphous, diamond-like carbon layer provide nucleation sites on which the large diamond crystallites grew. A mechanism of diamond growth has been proposed, based on the experimental findings, and is consistent with available theoretical models and numerous experimental observations reported in the literature.     

The fundamentals of chemical vapour deposition

Fundamentals of the chemical vapour deposition process are described and examples given of its application. The most important aspects of the process are reviewed; these include deposit structure (with its relation to process parameters), process control through application of the principles of thermodynamics and reaction kinetics (with emphasis on deposit thickness uniformity, deposit composition control and deposit-substrate adherence) and basic design features of the equipment used.     

The effects of deposition variables on deposition rate in the chemical vapour deposition of silicon nitride

Amorphous Si₃N₄ was deposited by a chemical vapour deposition technique using an SiCl₄, NH₄₃ and H₂ gaseous mixture onto a silicon single crystal. The effects of deposition time, substrate temperature, gas flow rate, system pressure and partial pressure of reactant gases on the deposition rate were investigated. The metal-insulator-semiconductor (MIS) structure was fabricated by evaporating gold on Si₃N₄ to measure the charges stored in the interface between the insulator and silicon. The charges were determined for different deposition temperatures from capacitance-voltage plots. The experimental results indicate that the growth of Si₃N₄ is a thermally activated process in this experimental condition. It is calculated that the apparent activation energy is about 33 kcal mol^-1 below the deposition temperature of 1100 °C. The homogeneous reactions in the gas phase are promoted when the total pressure is increased. At pressures above 300 Torr, deposition rates are decreased because the reactant gases are depleted because of the homogeneous reaction. The charges stored in the MIS structure are found to be larger at lower deposition temperatures.     

Tuning the conditions for the deposition of nanocrystalline diamond by hot filament chemical vapour deposition

Although large focus has been placed into the deposition of nanocrystalline and ultra-nanocrystalline diamond films, most of this research uses microwave plasma assisted CVD systems. However, the growth conditions used in microwave systems cannot be directly used in hot-filament CVD systems. This paper, aims to enlarge the knowledge of the diamond film depositing process. H2/CH4/Ar gas mixtures have been used to deposit micro, nano and ultra-nanocrystalline diamond films by hot-filament CVD systems. Additionally, the distance between the filaments array and the substrate was varied, in order to observe its effect and consequently the effect of a lower substrate temperature in the nucleation density and deposition. All the samples were characterized for microstructure and quality, using scanning electron microscopy and Raman spectroscopy.     

Structure of diamond and diamond like carbon thin films grown by hot-filament chemical vapour deposition technique

Micro-crystalline diamond (MCD) and diamond like carbon (DLC) thin films were deposited on silicon (100) substrates by hot-filament CVD process using a mixture of CH₄ and H₂ gases at substrate temperature between 400–800°C. The microstructure of the films were studied by X-ray diffraction and scanning electron microscopy. The low temperature deposited films were found to have a mixture of amorphous and crystalline phases. At high temperatures (> 750°C) only crystalline diamond phase was obtained. Scanning electron micrographs showed faceted microcrystals of sizes up to 2μm with fairly uniform size distribution. The structure of DLC films was studied by spectroscopic ellipsometry technique. An estimate of the amount of carbon bonds existing insp ² andsp ³ form was obtained by a specially developed modelling technique. The typical values ofsp ³/sp ² ratio in our films are between 1·88–8·02. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

High thermal conductivity diamond coated graphite by microwave plasma chemical vapour deposition

Diamond film was grown on high thermal conductivity graphite substrate using microwave plasma chemical vapour deposition method. Nanodiamond particles were uniformly seeded on the substrate to generate high nucleation density by a spray gun. The continuous and high purity diamond film was obtained, and growth rate was up to 2.7 μm h^??1. The thickness, surface morphology, quality and composite phase of the film were analysed by SEM, Raman and X-ray diffraction. It was shown that graphite coated with diamond presented a higher thermal conductivity (520?W?m^??1 k^??1) than copper. Furthermore, this coated material with high thermal conductivity, good strength and non-conductive surface will make it possible to be widely used in thermal management field.     

Laser chemical vapour deposition

The special role of lasers in material processing is outlined in this article. In the background of the various chemical vapour deposition processes, the laser-induced chemical vapour deposition processes have been described. The unique aspects of pyrolytic and photolytic laser chemical vapour deposition have been stressed. Some of the recent experimental results on thin film deposition by laser have been reviewed. The problems and future of laser deposition processes have also been mentioned. Based on the talk given at the Winter School of Laser Material Processing, November 16–21, 1987, Pune.     

Ion irradiation-induced modifications of diamond nanorods synthesised by microwave plasma chemical vapour deposition

Diamond nanorods (DNRs) synthesised by the high methane content in argon rich microwave plasma chemical vapour deposition (MPCVD) have been implanted with nitrogen ions. The nanorods were characterised by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The DNRs consist of single-crystalline diamond cores of 3–5?nm in diameter and several tens of nanometres in length. For purification from non-diamond contents, hydrogen plasma etching of DNRs was performed. Structural modifications of etched DNRs were studied after irradiating with 50?keV nitrogen ions under the fluence of 5?×?10¹⁴, 1?×?10¹⁵, 5?×?10¹⁵ and 1?×?10¹⁶?ions?cm^?2. Nitrogen-ion implantation changes the carbon–carbon bonding and structural state of the nanocrystalline diamond (NCD). Raman spectroscopy was used to study the structure before and after ion irradiation, indicating the coexistence of diamond and graphite in the samples. The results indicated the increase in graphitic and sp²-related content, at the expense of decrease in diamond crystallinity, for ion implantation dose of 5?×?10^15?cm^?2 and higher. The method proves valuable for the formation of hybrid nanostructures with controlled fractions of sp³–sp² bonding.     

Nucleation behaviour of diamond particles on silicon substrates in a hot-filament chemical vapour deposition

Diamond films and particles have been deposited on a silicon substrate using a hot-filament chemical vapour deposition (CVD) method in order to study the effect of hydrogen on the behaviour of diamond nucleation. The nucleation density of diamond was affected by both hydrogen treatment prior to deposition and filament temperature,T _f. The nucleation density was decreased markedly with increasing hydrogen-treatment time. The nucleation density also changed with increasingT _f, which increased initially and then reached a maximum at 2100°C and decreased thereafter. Etching of the substrate surface was observed and enhanced with both increasing hydrogen-treatment time and increasingT _f. The changes in nucleation behaviour were related closely to the etching of substrate surface. These results are explained in terms of the etching of nucleation sites.     

A mechanism for crystal twinning in the growth of diamond by chemical vapour deposition

A model for the formation of crystal twins in chemical vapour deposited diamond materials is presented. The twinning mechanism originates from the formation of a hydrogen-terminated four carbon atom cluster on a local {111} surface morphology, which also serves as a nucleus to the next layer of growth. Subsequent growth proceeds by reaction at the step edges with one and two carbon atom-containing species. The model also provides an explanation for the high defect concentration observed in 111 growth sectors, the formation of penetration and contact twins, and the dramatic enhancement in polycrystalline diamond growth rates and morphology changes when small amounts of nitrogen are added to the plasma-assisted growth environments.