A research essay outlining the process of
Chemical Vapour Deposition, specifically in the
context of diamond synthesis. Submitted to the
Faculty of Science, University of the
Witwatersrand, Johannesburg, in partial
fulfilment of the requirements for the degree of
Bachelor of Science
Johannesburg, November 2022
CHEMICAL VAPOUR
DEPOSITION OF DIAMOND
Independent Research Essay
Zangothando Kubheka, 2127264
Abstract
Chemical Vapour deposition is categorized as an industrial coating process that leverages
thermally induced chemical reactions to produce a desired deposit material on a compatible
substrate. The process occurs through the substrate being exposed to the requisite gaseous reagents
that are thermally activated in the reaction. These activated reagents then either react with or
decompose onto the substrate, producing the target material deposit. Diamond is a material of
particular interest because of its combination of chemical and mechanical properties. Namely, its
material hardness, chemical inertness and high index of refraction for optical applications. As such,
a process where diamond can be synthesized is incredibly useful. There is a rich history of the
synthesis of diamond and a variety of processes that can be explored, but the low pressure process
proves to be the most effective in terms of quality. The chemical vapour deposition of diamond is
a process in which diamond is synthesized at relatively low pressures and temperatures through
utilizing thermally induced chemical reactions of hydrogen-rich reagent gases to produce a thin
film of diamond on an inert substrate. The diamond produced through this process can be used for
mechanical applications in masonry due to its hardness, or as a heat sink in electrical components
due to high thermal conductivity and electrically insulating properties. The applications of CVD
extend beyond just the synthesis of diamond as the process can be applied in the synthesis of other
materials. A particular material of interest being another allotrope of carbon, namely graphene.
and subsequently, carbon nanotubes and bucky-paper.
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Table of Contents
Abstract [i]
Table of Contents [ii]
1. Introduction [1]
2. Types of Diamond Synthesis [3]
2.1. High Temperature High Pressure (HPHT) [3]
2.2. Chemical Vapour Deposition (CVD) [5]
2.3. Detonation [6]
2.4. Cavitation [6]
3. Defining Chemical Vapour Deposition (CVD) [7]
4. CVD Synthesis of Diamond [9]
5. Applications of CVD Diamond [11]
6. Closing Remarks [12]
References [13]
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1. Introduction
Diamond is an extremely rare allotrope of arising naturally about 150-200 kilometers deep into
the Earth’s crust due to the high temperatures and pressures present at this depth. The
temperatures range from 900-1300˚C and pressures are in the region of 4,5-6 GPa. Under these
specific conditions, diamond’s tetrahedral crystalline structure becomes the most stable physical
nucleation of carbon atoms as opposed to the layered hexagonal structure of graphite, leading to
the formation of diamond crystals.
Figure 1 Allotropes of Carbon (taken from ScienceNotes.org)
Diamond is widely considered a super material throughout the scientific and engineering
community because of its rare combination of chemical and mechanical properties due to the
strength of the intermolecular bonds present in the diamond crystal.
Diamond is the hardest naturally occurring mineral known to man according to the Mohs
Hardness scale. This means it is completely resistant to abrasion caused by any other material
besides diamond. It also has one of the lowest compressibility factor of any solid material.
Pure diamond is an electrical insulator, but it has an incredibly high thermal conductivity. In
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excess of 2000 m∙K , conducting heat nearly five times better than copper. This thermal
conductivity is coupled with a high level of chemical inertness due to the stability of diamond’s
tetrahedral molecular structure. This means that diamond does not react with strong acids or
alkalis. It can; however, burn with high enough temperatures in the presence of sufficient
oxygen.
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Diamond has the highest refractive index of any naturally occurring transparent material. High
reflectance makes it highly coveted by the jewellery industry for its aesthetic appeal. Lastly,
diamond has a negative electron affinity which means that energy is released when the
diamond surface accepts an electron.
This unique combination of properties means that diamond holds a host of mechanical and
chemical applications. Its material hardness makes it useful for industrial applications such as
coating high wear surfaces on tools or creating abrasives. Its thermal conductivity makes it
ideal for creating heat sinks in electrical components. Lastly, its chemical inertness makes it a
suitable material for creating robust medical implants that will not interfere with the patient’s
physio-chemistry.
The rarity of natural diamond means it holds tremendous economic value. The cost of
production associated with its immense mining operations leads to it being ranked as one of
the most valuable minerals in the world. As such, it would simply be too expensive to use
natural diamond for its mechanical and chemical properties on a substantial scale. This
necessitated the inception of processes that could be employed to synthesize diamond. The
process that is of particular interest to us in this essay is the low pressure process of Chemical
Vapour Deposition (CVD) which is categorized by gaseous reagents being deposited onto a
designated substrate to form a thin, crystalline film of the desired deposit material atom by
atom and layer by layer to ultimately form a solid crystal.
Through this essay, I hope to highlight the important properties of diamond as a material, then
establish a holistic and fundamental understanding of the chemical vapour deposition process,
assessing the presently available types of diamond synthesis, with the goal of ultimately
developing an understanding of the process of chemical vapour deposition in the specific
context of diamond by breaking down the scientific principles and methodology behind the
topic material into their simplest form and reconstructing them in a palatable and intellectually
accessible way. I then hope to bring attention to the potential applications of not only synthetic
diamond, but of the CVD process as a whole.
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2. Types of Diamond Synthesis
Following the confirmation of Lavoisier’s theory, conceived in 1772, that diamond is
comprised purely of carbon in the year 1796 by English chemist Smithson Tennant, the notion
that diamond could be synthetically produced from ignited the imagination of the scientific
community and spurred the pursuit of a process that artificially creates diamond.
2.1. High Pressure High Temperature (HPHT)
The earliest conceived process to achieve this goal came about in 1954. The American
company General Electric sanctioned a project that was codenamed “Project Super Pressure’.
The company enlisted the services of physical chemist and researcher, Howard Tracy Hall to
design a process that artificially synthesizes diamond. Hall designed a machine, a belt press,
that simulates the high pressures and temperatures under which natural diamond forms. A
variety of temperatures, pressures and methods were investigated in order to find the optimal
conditions to promote diamond growth. Through trial and error, Hall and his team found that
exposing graphite dissolved in a metal medium to temperatures around 2000C and pressures
exceeding 10 GPa would yield a resultant material that scored a 10 on the Mohs scale. Upon
closer inspection, it was discovered that the team had successfully created the first ever
synthesized diamond. This process was called the High Pressure High Temperature (HPHT)
process.
Figure 2 H. Tracy Hall and his HPHT tetrahedral press machine (courtesy of the H. Tracy Hall Foundation and Legacy Books)
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Figure 3 Micrograph of diamonds produced using HPHT machine (courtesy of the H. Tracy Hall Foundation and Legacy Books)
The diamonds this process would yield were incredibly small and impure, far from gem
quality, and were thus used in an industrial context to create abrasives and cutting tools. The
process was; however, revised, reiterated and improved to result in General Electric
successfully producing gem quality diamonds by the year 1971. The process involved adding
tremendous pressure and heat to a graphite seed enclosed in a tube, which yielded a massive
increase in the quality and size of the diamonds produced, but this process was not without its
faults. The resultant stones were often yellow, which was later discovered to be attributed to
an excess of nitrogen present during the process. This nitrogen would become incorporated in
the crystal lattice structure of the carbon, absorbing the blue part of the light spectrum and
leaving the diamond with a yellowish tint. It was also economically not viable as it was
incredibly expensive to produce the requisite energy for the process to take place and the
quality of the diamond produced did not justify the cost. The process was continually revised
throughout the decades, with particular focus being placed on the removal of process nitrogen.
At present, this process yields diamonds that rival, and sometimes even exceed, natural stones
in size and clarity, but remains incredibly expensive and unfeasible.
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2.2. Chemical Vapour Deposition (CVD)
A simple definition for CVD is that “chemical vapour deposition is a coating process that uses
thermally induced chemical reactions at the surface of a heated substrate, with reagents
supplied in gaseous form. These reactions may involve the substrate material itself, but often
do not,” (TWI: 2020), but in the context of diamond, CVD is categorized as the “synthesis [of
diamond] from a gas phase in the conditions of thermodynamic metastability of diamond at
low pressures” (BV Spitsyn: 2000) The CVD process arose independently from the HPHT
process and the first patent for the CVD synthesis of diamond was issued in 1954, with the
inception of research taking place simultaneously in the USA and in Russia. A year before
General Electric announced the success of the HPHT process, but the method was not adopted
until the late 1980s, by which time through decades of research, scientists had developed a
fundamental understanding of the nature of chemical vapour deposition as well as the
relationship between atomic hydrogen and its interaction with carbon bonds. Through this
knowledge, they were able to devise a process in which a mixture of a hydrocarbonous gas
(most commonly methane) and dissociated, atomic hydrogen is heated in a vacuum chamber
to deposit a film of diamond onto a substrate in the chamber. This process will be expanded
on in subsequent sections of this research essay.
The advantage that CVD synthesis holds over the HPHT process is that it takes place at
significantly lower temperatures and pressure than the HPHT process and thus requires a
substantial amount of less energy to sustain, making it cheaper to undertake and simpler to
maintain. Both processes are capable of yielding high grade diamonds, but comparatively,
CVD diamonds prove to hold a higher chemical purity than their HPHT counterparts. Because
the reaction takes place in an evacuated chamber, provided the optical windows of the chamber
do not degrade and become part of the reaction, the resultant crystal is comprised entirely of
carbon, whereas in HPHT processes, nitrogen and boron often infiltrate the structure of the
resultant diamond.
Figure 4 Cut and polished CVD diamond (courtesy of AliCat Scientific)
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2.3. Detonation
Detonation is a process in which an oxygen deficient, carbon rich explosive mixture of
trinitrotoluene (TNT) and hexogen is detonated in a closed, reinforced chamber. The chamber
is then rapidly cooled yielding a graphitic ash that is rich in nano-diamonds (diamonds with a
circumference in the range of 5nm). The rapid cooling is employed to prevent the newly formed
diamond from reverting back to graphite. This process was initially discovered in 1963 by
Soviet Union scientists K.V. Volkov, V.V. Danilenko, and V.I. Elin. The flash of high
temperature and pressure from the explosion forces tiny nucleations of the carbon present in
the explosive to adopt a diamond crystal lattice structure. The lack of oxygen in the reaction
prevents the carbon from burning and forming carbon dioxide. In order to extract the nanodiamond from the ash, the ash is boiled in nitric acid for a prolonged period of time (1-2 days)
at a temperature of around 250˚C. This is done to dissolve the graphite and remove metal
contaminants from the detonation chamber, resulting in a high concentration nano-diamond
powder that holds a variety of applications in industry. This process differs from the HPHT
process in that the high pressure and temperature of the HPHT process is static whereas in
detonation, it is dynamic.
Figure 5 Electron Micrograph of Detonation Nano-Diamond (courtesy of Materialscientist)
2.4. Cavitation
This relatively new process uses the ultrasonic cavitation of a fluid comprised of hexagonal
graphite suspended in a variety of organic liquid media at ambient temperature and
atmospheric pressure to produce diamonds in the range of 6-9μm ± 0.5μm with a yield of up
to 10% diamond by mass. The quality of the cubic crystal lattice of the product is significantly
worse than that of HPHT diamonds and this process, due to it being in its infancy, presently
has no industrial applications. The process continues to be investigated and optimized.
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3. Defining Chemical Vapour Deposition (CVD)
Simply put, CVD is an industrial coating process that leverages thermally induced chemical
reactions to produce the desired deposit on a substrate. The process is not exclusively used in
coating. It can be used to grow crystals and other materials from gas deposits, but the key detail
in this process is that reagents or reactants are delivered in the gaseous phase and during the
reaction, the pyrolytic decomposition of gaseous reagent compounds provides a coating of a
solid reaction product. Essentially, the reagents condense and nucleate on the surface of the
substrate, forming a crystalline film product, layer by layer. Volatile by-products are often
produced in the reactions, but these are removed from the reaction chamber through an exhaust
and the mechanism of gaseous flow.
The fact that the reactants used are gases provides a key advantage of the CVD, allowing the
process to take advantage of the diffusion mechanism through which gases permeate. A
consequence of this is that CVD processes can be used to coat restricted, hard to access surfaces
as the gas dispersal that takes place through the mechanism of diffusion is even in three
dimensions, thus CVD is not a line-of-sight process like most other coating processes Another
advantage is that CVD can employ a wide range of coating materials based on the substrate
and the coating material used. The resultant film can be deposited with very low porosity levels
and with relatively high purity. The process proves to efficient and cost effective in that it can
be applied on a large scale in the context of industrial coating.
Figure 6 Schematic Diagram of CVD Process (Chemical Route derived Bismuth Ferrite Thin films and Nanomaterials:
Zhang, Qi)
The CVD process typically employs temperatures in the range of 600 - 1100°C. These high
temperatures may have a significant thermal effects on the substrate material, depending on
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properties of the substrate such as its thermal expansion and its thermodynamic properties.
This high heat may compromise the properties of substrates such as steels, that enter the
austenite phase region in this temperature range. Manufactures may need to reinforce the
desired properties of the substrate using suitable heat treatment processes.
A very commonly used method of thermal activation used in the CVD coating industry is
plasma assisted CVD (PACVD). In this process, an electrical discharge in a sub atmospheric
pressure gas is used to energize the gaseous reagent compounds and advance the CVD reaction,
which can serve to significantly lower the reaction temperatures needed for the process to take
place. This method, alongside that of microwave plasma activation, is also commonly used in
the vapour deposition of diamond. Other compounds that can deposited using the PACVD
technique include quartz, silicon, silicon nitride and titanium nitride.
Figure 7 Microwave Plasma Activation CVD (courtesy of nandne.com: 5 processes to make CVD diamond or synthetic
diamond)
CVD categorized by the pressure the reaction takes place at is divided into three main branches.
Namely: Atmospheric pressure CVD, Low-pressure CVD (LPCVD, which is also known as
sub-atmospheric CVD) and Ultrahigh vacuum CVD (UHVCVD)
Other denominations of CVD include the temperature of the reaction chamber. There are two
main branches of this subdivision. Namely, Hot wall CVD, in which the chamber temperature
is raised by heating it with an external power source and the substrate is heated through the
radiation emitted by the heated chamber walls. Alternatively, we have Cold wall CVD,
whereby only the substrate itself is directly heated, either by induction, heating the substrate
itself through physical contact with a heater or through running an electrical current through
the. The chamber walls maintain ambient temperatures.
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4. CVD Synthesis of Diamond
The key point of knowledge in the search for alternative methods of diamond synthesis was
the chemical constitution and structure of the diamond, which is essentially large molecules
constructed from sp3- covalently bonded atoms of carbon.
At ambient conditions, this structure is not thermodynamically stable, but the nature of organic
chemistry dictates that the synthesis of metastable molecules is highly permissible. Diamond,
being metastable at ambient temperatures and pressures in comparison to graphite, maintains
its high energy structure at atmospheric pressures and temperature up to and more than 1000oC,
provided the surrounding gases are oxygen deficient in order to prevent burning. It was
discovered that in ultra-high vacuum conditions or in an atmospheric pressures comprised of
ultra-pure hydrogen surrounding gases, diamond retains the tetrahedral crystal lattice structure
up to temperatures in the range of 1300-1400˚ C. This inspired the idea to take advantage of
this relative stability of diamond in these conditions to promote its artificial synthesis using
carbon rich gases at these low pressures. This notion is the foundation of diamond CVD
synthesis.
“. It was supposed that, under the action of directional activity of surface forces, the bonds
between the atoms at the diamond surface and carbon atoms from decomposing carbonaceous
substances will rebuild diamond, if exclusively sp3-bonding formation was provided. Thus, it
was required to find conditions at which mainly and with high enough rate only sp3-type
carbon-carbon bonds are formed and the formation of other types of bonding between carbon
atoms at the surface of growing diamond are completely excluded.” (Alexenko: 2000)
In the CVD synthesis of diamond, atomic hydrogen is vital. This form of hydrogen is obtained
through the dissociation of hydrogen (H2) molecules. Thus, for diamond CVD, a hydrogen rich
(>90%) process gas is essential. In order to spur the hydrogen’s dissociation, it needs to be
energized to break up the hydrogen molecules. This can be achieved through the use of a
plasma activation (electrical or microwave), or a thermal activation using a hot filament. The
atomic hydrogen obtained serves the purpose of selectively etching graphite and breaking up
carbonic double bonds thus converting graphitic bonds into diamond bonds through the
removal of non-diamond phases leaving gaps for the sp3-type carbon-carbon bonds.to form.
The CVD process essentially produces diamond from a low pressure, thermally activated
mixture of a hydrocarbonaceous, gas (usually methane) and atomic hydrogen in a vacuum
chamber. Conventionally, heating this carbonaceous mixture at such low pressures would yield
graphite. This is where atomic hydrogen intervenes, promoting diamond formation since
diamond is more stable in this hydrogen rich environment. The graphite reacts with the atomic
hydrogen and evaporates in a newly formed gas phase and the remaining atomic hydrogen
forms a highly volatile hydrocarbon species that then decomposes, releasing the hydrogen to
form diamond from the remaining, pure carbon.
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Figure 8 Schematic Diagram of Diamond CVD (courtesy of the Journal of Semiconductors)
Figure 9 Carbon/hydrogen/oxygen balance required for diamonds to grow (courtesy of AliCat Scientific)
Diamond is usually deposited at growth rates in the range of 0.1 and 10 microns/hour for large
areas (>100 cm2) making it is an incredibly slow process, but much higher growth rates (>100
micron/hour) have been demonstrated for small areas (<1 cm2).
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5. Applications of CVD Diamond
Dopants and other impurities can be incorporated into the diamond produced during the CVD
synthesis process, meaning the optical and electronic properties of the resultant diamond can
be altered.
Diamond coated tools, virtually no wear and unrivalled hardness very effective for masonry
especially High heat conductivity makes the ideal heat spreader or heat sink in components,
coupled with good electrical insulation make it a desirable packaging material for
semiconductors Diamond’s unique multispectral transparency combined with its mechanical
strength, chemical inertness and abrasion resistance makes it a desirable material for optical
windows Boron-doped diamond exhibits electrical conductivity (acts as a semiconductor) and
has a large electrochemical window. The chemical inertness of these diamond electrodes has
great electrochemical application
Figure 10 Applications of CVD Diamond (courtesy of Roland Haubner)
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6. Closing remarks
Aside from being used as an advanced technique of industrial coating or to produce synthetic
diamond, which has an incredible host of applications, CVD can be used to ultimately grow,
graphene which, in proportion to its thickness, is the strongest material known, is an
extraordinary conductor of both heat and electricity, and is nearly 100% transparent to light,
elastic, flexible. it’s a super material with a host of properties carbon nanotubes which are
essentially just rolled sheets of graphene, but have amazing applications due to their atomic
configuration in turn can produce bucky-paper, a macroscopic aggregate of carbon nanotubes
used to create a thin macroscopic sheet of CNTs. can’t be made 100% the van der Waals force's
interaction between the nanotube surface and the surfactants used in the synthesis of the bucky
paper can often be mechanically strong and quite stable of CNTs all super materials based on
a hexagonal lattice formation of carbon and stronger than diamond (cubic crystal lattice) and
these form the foundation of the field of nanotechnology.
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References
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Diamond and Related Materials. Volume 49, pp. 77-86
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