Diamond & Related Materials 20 (2011) 1287–1301

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Diamond & Related Materials

j ourna l homepage: www.e lsev ie r .com/ locate /d iamond

A review of diamond synthesis by CVD processes

Michael Schwander ⁎, Knut PartesBIAS Bremer Institut für angewandte Strahltechnik GmbH, Klagenfurter Str. 2, D-28359 Bremen, Germany

⁎ Corresponding author. Tel./fax: +49 421 2185036.E-mail address: michael.schwander@gmx.de (M. Sch

0925-9635/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.diamond.2011.08.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 September 2010Received in revised form 2 August 2011Accepted 13 August 2011Available online 27 August 2011

Keywords:CVDHot filamentPlasma CVDCathodic arc dischargeCombustion synthesis

Diamond has some of the most extreme mechanical, physical and chemical properties of all materials. Withinthe last 50 years, a wide variety of manufacturing methods have been developed to deposit diamond layersunder various conditions. The most common process for diamond growth is the chemical vapor deposition(CVD). Starting from the first publications until the latest results today, a range of different developmentscan be seen. Comparing the basic conditions and the process parameters of the CVD techniques, the technicallimitations are shown. Processes with increased pressure, flow rate and applied power are the generaltendency.

wander).

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© 2011 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12872. Applications of diamond coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12883. The chemical vapor deposition process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1288

3.1. Thermal induced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12903.1.1. Transport processes from hot filament . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12903.1.2. Techniques for synthesis of diamonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12913.1.3. Diamond coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1291

3.2. Chemical induced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12913.2.1. Exothermic combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12923.2.2. Techniques for synthesis of diamonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12923.2.3. Diamond coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1293

3.3. Electromagnetic excitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12933.3.1. Absorption of electromagnetic waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12933.3.2. Techniques for synthesis of diamonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12943.3.3. Diamond coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1296

3.4. Electrical induced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12963.4.1. Heating from apply voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12973.4.2. Techniques for synthesis of diamonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12973.4.3. Diamond coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1298

4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12995. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1300Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1300References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1300

1. Introduction

The range of industrial used tools reaches from single bulk material,coated and treated systems to multilayer devices. Depending on the

Fig. 1. Schematic diagram of the mechanism from CVD processes for diamond growth.

1288 M. Schwander, K. Partes / Diamond & Related Materials 20 (2011) 1287–1301

requirement of the system, different material compositions can be cho-sen. Because of its extreme properties, diamond is one of the most ver-satile usable materials for coated systems. Almost all diamondproperties are in a higher or lower limit of possible desirable behavior.Diamond is for example the hardest known material and at the sametime it has the lowest coefficient of thermal expansion. Other advan-tages are attributes like being chemically inert, highly wear-resistant,highly thermally conductive, electrically insulate and broadly optically-transparent from the ultraviolet (UV) to the far infrared (IR) [107].

Diamond consists of carbon atoms, which are bounded over tetra-hedral sp3 hybrid orbitals in a face centered cubic (fcc) crystal system.Furthermore, each primitive (Bravais) cell consists of two carbonatoms. The structure can be seen, as two face-centered cubic latticesinterpenetrating along the body diagonal (1/4 1/4 1/4) of the cubicunit cell [84].

The industrial use of diamond began with the first synthesis by“General Electrics” in 1955 [14]. High pressure and high temperature(HPHT) surrounding conditions were generated, which were similarto the conditions for natural diamond growth. The conversion of car-bon depended on the fact, that diamond is the densest allotrope formof carbon. With this technique, monocrystalline diamonds with highpurity can grow up to several millimeters. Most of these diamondsare used for grinding and cutting tools. Nevertheless, there are alsostrong disadvantages like the highly expensive equipment and thelimited size of the diamonds.

Since the middle of the 1950s, the interest in diamond increasedwith the possibility of growing carbon by using a wide variety ofchemical vapor deposition (CVD) techniques. These techniquesallow the deposition of four different types of carbon: amorphous car-bon with sp2-bonded atoms (a-C), tetrahedral bonded amorphouscarbon (ta-C), polycrystalline and monocrystalline diamonds [3].

This review focuses on the various deposition techniques by usingCVD for poly- and monocrystalline diamond growth and on the basicenergy transfer concepts. Special attention is given to the differentenergy supply, associated temperature and density distribution. Thereview concludes with a comparison of process parameters, whichare measured for all types of CVD apparatuses.

2. Applications of diamond coatings

Due to the notable properties, diamond coatings find use on a va-riety of applications including, as semiconductor, as an optical com-ponent, as heat sink and as wear-resistant coating.

The advantages of semiconductor are the wide band gap, the veryhigh electric breakdown and the thermal conductivity. For the appli-cations diamond can be doped by boron, which results in a p-typesemiconductor with a bandgap about 0.37 eV from the valence bandmaximum [76]. The phosphorus doping can result in an n-type semi-conductor and a donor level of 0.6 eV from the conduction band min-imum [32]. Without doping the wide bandgap at 300 K of 5.5 eV fromdiamond is used for photodetectors. These photodetectors are inter-esting for investigation of inflammation and explosion dynamic be-cause of the sensitivity in the range of 185 to 250 nm [36]. Oneadvantage is the transmission of near-UV (300–400 nm), visible andIR radiation, so that diamond can be used as solar-blind UV photode-tectors, even on a daylight background.

The features of hardness, highest bulk modulus, lowest compress-ibility, high wear resistance and a low friction coefficient against awide range of materials are used for diamond coatings onto cuttingstools. MMCs, aluminum–silicion alloys and Co-cemented tungstencarbide tools are used [2] as substrates. But Co-cemented tungstencarbide (WC–Co) is commonly considered the most suitable substrateto receive a diamond layer for tooling applications [81]. However, themajor drawback is a very good solvent (0.2–0.3 wt.%) of carbon intocobalt (Co) and the resulting poor adhesion which is the main techni-cal limit for diamond-coated tools [19]. Besides, the presence of

metallic cobalt suppresses the diamond nucleation through catalyz-ing the formation of graphite. Because of these facts, the Co binderis removing from the substrate surface by using Murakami reagentand acid etching. This can result in an optimally conditioned substratesurface for diamond coating [23].

The thermal, mechanical and optical properties of diamond areused in optical windows, especially for high-power IR lasers and air-borne IR sensor systems. The diamond windows combine withstandagain high power irradiation, high temperatures and aerodynamicload [112].

“The physical properties of CVD diamond can differ significantlyfrom their single-crystalline counterparts due to intentionally orunintentionally added impurities, grain boundaries and other extendeddefects. For any industrial application precise knowledge of the relevantmaterial is necessary” [112]. Therefore, this paper makes no statementabout the quality of diamond films, it only gives a brief summary ofthe produced layers in each chapter.

3. The chemical vapor deposition process

As its name implies, chemical vapour deposition (CVD) involveschemical reaction inside a gas-phase as well as deposition onto a sub-strate surface. An early work which deals with chemical processesthat is important for the diamond generation had been published in1993 by J. E. Butler et al. [15]. However, the processes have been de-veloped since this time. The process procedure with the wholerange of selectable process parameters is shown in Fig. 1. The sketchillustrates the various direct and indirect adjustable parameters. Thefirst group shows the different selectable process gases which canbe used for CVD. The second group reflects a selection of energysources for the activation of the chemical process, followed by ensur-ing parameters. Below that, there is the substrate with the growingdiamond layer.

However, in most cases a mixture of hydrogen and methane isused for diamond growth. It is generally accepted that atomic hydro-gen (or oxygen) is the most critical component in the gas phase mix-ture and methane or other hydrocarbon molecules are only neededfor the supply of carbon atoms. The primary function of hydrogen(or oxygen) is to terminate the dangling carbon bonds on the surfaceof the diamond layer or diamond nucleus. Beyond this, the hydrogenatoms can cleave the neutral hydrocarbons and create reactive radi-cals such as CH2. This excited hydrocarbon can bond on this exposedcarbon and form trigonal sp2 (a-C) graphite or tetrahedral sp3 (ta-C)bonded carbon [5]. Another purpose of hydrogen is to prevent thegrowth of graphite. This is possible due to the fact, that atomic

Fig. 3. Calculated solubility of carbon at constant pressure and different temperatures[82].

1289M. Schwander, K. Partes / Diamond & Related Materials 20 (2011) 1287–1301

hydrogen etches sp2 bonded graphite much faster than diamond likesp3 carbon. Therefore, diamond growth can be described as ‘five stepsforward, but four steps back’ [63]. In the same way as hydrogencleaves hydrocarbons it also suppresses the build-up of polymers orlarge ring structures which might deposit onto the growing surface.

In addition to the functional principle of the diamond growth, theCVD processes have several consistent settings and practices. This in-cludes the substrate selection, the pretreatment of the samples andthe temperature range. Mostly used substrates in current publicationsare molybdenum, silicon nitride and tungsten carbide. As pretreat-ment abrasion by mechanical polishing at diamond powder of 0.1–10 μm particle size or through an ultrasonically bath slurry with amixture of abrasive grit in a hydrocarbon medium is made.

The temperature of the substrate is restricted to the range from1000 to 1400 K, since the deposition rates reduce at lower tempera-tures and the growth of graphite dominates at higher temperature.Depending on the energy supply, the pressure range and the gas tem-perature, the substrate has to be either cooled down or heated up.

Bachmann et al. were the first who demonstrated, that diamondsynthesis is only possible in a small area of the C/H/O-gas-phase com-positional diagram, shown in Fig. 2 [6]. Most of the present processesstraddle the line, represented by identical concentration of C and Oatoms. As a consequence it is possible to grow diamond films byusing oxygen or hydrogen free gas mixture with different methaneconcentrations. Prijaya et al. [82] calculated the solubility of carbonin the gas phase composition by a thermal equilibrium system,which consists of a solid graphite disk and hydrogen/oxygen gasesat constant pressure, shown in Fig. 3. According to Bachmann's re-sults, he found a similar composition in a temperature range from1400 to 3000 K, which corresponds to the temperature range of themost CVD processes. This shows that at the presence of temperatureequilibrium and a uniform distribution of the gas molecules, the de-position process is exclusively dependent on the carbon-to-hydrogen-to-oxygen ratio.

However, in real CVD processes the temperature distribution, sol-ubility and internal energy clearly depend on the type of excitation.Therefore, the significant difference of CVD processes for diamondsynthesis is based on the sort of energy supply. The activation canbe generated by microwave (MW), radiofrequency (RF), laser in-duced (LI), direct current (DC), hot filament (HF) and chemical acti-vation (CA). In the same surrounding condition like pressure, flow

Fig. 2. C–H–O phase diagram for diamond growth [6].

rate, size of the heated area and applied power, different energy sup-plies generates different mole fractions and temperature distribu-tions. On that account, this review mainly deals with the functionand construction of different types of energy supply. Nevertheless,according to Spitsyn et al. the CVD processes can be separated intofour different types shown in Fig. 4 [98]. Firstly, it is subdivided intoheated gas and ionization plasma. A further breakdown is made onthe nature of excitation through thermal, chemical, electrical or elec-tromagnetically activation. That processes are united in hybrid sys-tems which use more than one energy supply.

Through thermal and chemical activation, the gas phase can beheated up to 3500 K. At this temperature, the distribution and densitycan sometimes be estimated by equilibrium systems. However, due toelectrical and electromagnetically activation the gas-phase ionizesand produces a plasma ball or a jet formation. After the plasma activa-tion, the stable molecules convert into neutral and charged particles:C2H2, C2H4, CH3, C2H, CH2, C2, CH+, CH2

+, etc. proceeds. The fraction ofhydrogen molecules dissociates to H atoms and also to ions H+, H3

+,and H−. Inside the plasmas, there are different temperatures for elec-trons (Te) and molecules (Th). Depending on the interaction, the tem-peratures can be similar or vary in orders of magnitude.Thermodynamic equilibrium (LTE) means, that the temperatures aresimilar and non-local thermodynamic equilibrium (non-LTE) meansthat the temperatures are different [101]. In LTE plasma each kindof collision must be balanced by its inverse: excitation/deexcitationand ionization/recombination. Inelastic collisions between electronsand atoms create the plasma reactive species whereas elastic colli-sions heat the gas. The molecules' temperature is close to the electrontemperature. Tendero [101] for example, specifies the temperaturesTe and Th in the arc plasma core at round about 10,000 K. However,most plasmas, used for diamond deposition, deviate from LTE. Innon-LTE plasma, inelastic collision between electrons and heavy par-ticle induce the plasma chemistry, too. But due to the fact, that theheavy particles are slightly heated by only a few collisions, the tem-perature of electrons can increase from 10,000 to 100,000 K. Duringthis process the molecule's temperature itself can only rise from 300to 10,000 K. The temperature of non-LTE plasma can be estimatedby spectrometric-, Langmuir probe- and thermocouple measure-ments. In the majority of cases the temperature is unknown becauseof the high technique requirements.

Fig. 4. Diamond CVD techniques [98].

1290 M. Schwander, K. Partes / Diamond & Related Materials 20 (2011) 1287–1301

3.1. Thermal induced

In thermal induced CVD processes for diamond deposition the gasphase is activated by hot filaments or hot surfaces. In contrast to ex-citation with electromagnetic waves or by direct current, the gasreaches only temperatures of about 2300 to 2900 K [31]. Using thistemperature and the classic thermodynamic ideal gas equation itcan be shown, that the internal energy is factor 100 lower than theionization energy. For this reason it is only spoken about thermalheated gas.

3.1.1. Transport processes from hot filamentThe energy- and mass-transport in thermal heated gases can be

described by fluid mechanics and thermodynamic processes. Thesefundamentals are given by an example of a hot filament process illus-trated in Fig. 5. As shown, the process gas flows into the chamber andis heated up by a hot filament. Since the chemical and thermal reac-tion close to the wire is not well understood, the heated zone nearthe filament is described by an approximation, which assumes a con-stant temperature and chemical composition. As a clue for the ap-proximation, the temperature of the gas near the filament can bedetermined by thermocouples [38] or laser scattering [86] and isabout 2000 K. However, the filament temperature itself can only bemeasured through a pyrometer and amounts to about 2600 K. Thedistance between the filament and the substrate is normally 5 mm[64], 5 to 6 mm [16], or 7 mm [83]. In individual cases, this amountcan vary up to 20 mm [75]. Through this distance and a substratetemperature around 1000 and 1200 K, the temperature gradient and

Fig. 5. Schematic representation o

chemical conversion, which depends on convection and thermal con-ductivity of the gas, can be set.

To determine the chemical composition in the areas around thehot filament and between filament and substrate it is necessary toknow the temperature distribution and chemical ratio around the fil-ament. Therefore, Harris and Weiner determine the chemical ratioof CH4/C2H2 with on-line mass spectrometry as 1:1. AdditionallyGoodwin et al. calculated the chemical compounds of H2, C2H2, H,CH4 and C2H4 around the filament by the assumption of an interactiontime around 0.1 to 0.2 s. This is necessary for an inlet gas to generate aratio from CH4/C2H2 with 1:1 at a temperature of 2000 K.

Under the conditions of low pressure (typical 2600 Pa), a distanceof 5 mm, a gas velocity on the order of 0.5 cm/s and the above givenconditions of temperature and chemical compounds, the followingassumptions can be made [31,38,35]:

• The gas flow can be described as a continuum laminar flow withoutroll cells in the area of chemical conversion.

• The temperature profile can be assumed to be independent fromthe chemical gradient and only dominated by the conduction ofthe gas.

• The temperature profile can be assumed to be linear, as shown inFig. 5.

• The energy transport from the filament to the substrate is dominatedby conduction and transport of atomic hydrogen recombination.

• The mass flux is mainly generated by molecular diffusion.• Gas phase recombination of atomic hydrogen is significantly influ-enced by the reaction H+CH4_H2+CH3

f a hot filament CVD process.

Fig. 7. Homoepitaxial growth of diamond from an etched carbon source with a hot fil-ament and a heated graphite disk [98].

1291M. Schwander, K. Partes / Diamond & Related Materials 20 (2011) 1287–1301

3.1.2. Techniques for synthesis of diamondsMatsumoto et al. were the first who achieved the diamond depo-

sition on substrates other than diamond [61]. The standard experi-ment set-up and the process variable are shown in Fig. 5 and havebeen described in the previous chapter. The reasons for the wideuse of that technique are the simple process and the cheapequipment.

A recent hybrid system, which is mainly based on a hot filamentprocess, for silicon [60] and for WC/Co [45] is shown in Fig. 6. Theset-up is used for bias enhanced nucleation of diamond films withmethane as carbon source. Therefore, negative voltage is applied tothe substrate and positive voltage is applied to a grid placed on topof three tungsten filaments. A feature of this process is the stable plas-ma between grid and filaments. Due to a negative voltage on the sub-strate, ions from this plasma can diffuse to it.

Instead of using a carbon-containing gas source, it is also possibleto use a solid disk or rod, consisting of graphite as carbon source [85].In this case, the hot zone is created either by a hot filament, [95] or bya heated graphite disk [7]. During the deposition hydrogen is suppliedto the process. This process is called chemical transport reaction(CTR) and is shown in Fig. 7.

In the lower sketch of Fig. 7 an experiment set-up, which has al-ready been used in 1966 by Spitsyn et al. for epitaxial regrowth ofnatural diamond, is portrayed [98]. In this early study, diamondgrowth occurs by diffusion of carbon from the 2300 K hot graphitelayer to the overlying diamond seed. A major drawback of this pro-cess is the concentration of carbon in the gas phase, which cannotbe controlled independently by the excitation temperature.

The advantage of using a hot filament in contrast to a hot diskwithout methane or another gas carbon source, is the absence of car-bon around the filament and the high concentration on the substratesurface. In this way a higher gas flow up to 1 slpm can be realized[95]. The use of methane as carbon source would lead to a graphitegrowing on the filament, the vanishing of the catalytic effect of thetungsten filament and a degradation of the diamond quality. Typicalconditions and results for thermal induced diamond deposition aresummarized in Table 1.

3.1.3. Diamond coatingsThermal induced CVD-processes are used under gas mixtures of

H2, CH4 and trimethyl borate or similar boron-containing precursorsto deposited boron doped diamond layers. In this way carrier concen-tration up to 1022 cm−3 has been obtained [109]. Among other, theselayers are used as electrode layers with diamond resistivity of 0.01 Ω

Fig. 6. Schematic diagram of a double bias-assisted hot filament CVD method [60].

cm for nitrate elimination [20] and as electrode for a green process ofaluminum electrolysis [110].

Due to the good reproducibility and surface structure of thermalinduced CVD-processes as HFCVD, coatings have been depositedearly on cutting, milling and grinding tools. The research deals cur-rently with the increase of attachment to turning tools [16] and theinfluence of micro-, submicro and nanometric diamond crystallitesize on machining [2,29].

Other applications are thin transparent electrodes. These are usedin optical micro-electro-mechanical systems (MEMS) [13] and inMicro Electrode Arrays (MEAs) for biochemical application [30]. Suit-able substrates are transparent materials like sapphire, quartz andbio-glass.

3.2. Chemical induced

In 1988 Hirose et al. first described the chemical vapor depositionof diamond on using a combustion flame [42]. The temperature of thistype of combusting flame reached only values in the range of 2000 to3550 K [62]. As in the case of hot thermal induced CVD processes, thegenerated internal energy is about factor 100 lower than the ioniza-tion energy. Therefore, the major interest is the exothermic chemicalconversion of the process gases.

Table 1Parameter and operation range of thermal activated CVD processes for diamondsynthesis.

Thermal activated

Total gas flow [slpm] 0.1 to 0.5Typical process gas H2/CH4

Typical mixture range H2/CH4 Typical 99/1Pressure [Pa] 600 to 14,000Deposition area [mm2] Up to 100,000Common linear growth rate [μm/h] 1–10Advantages Large area at low pressure

Comprehensible process parameterSimple set-upLow-priced equipmentHigh quality of diamond layers

Drawbacks Low growth rateChamber is requiredDegradation of the filament

Table 2Abstract of chemical reactions in a combusting flame [41].

Reaction A n E(mol, cm³, s) (kJ/mol)

C2H3+H⇄C2H2+H2 4.0×1013

C2H3+OH⇄C2H2+H2O 2.0×1013

C2H3+O2⇄C2H2+HO2 1.12×108 0.913 0.83C2H3+O2⇄C2H3O+O 3.64×1011 0.27 0.423C2H3+O2⇄CHO+CH2O 4.6×1016 −1.39 4.25C2H2+CH2⇄C3H3+H 2.4×1013 27.71

Fig. 9. The simplest form of process set-up for premixed torch apparatus.

1292 M. Schwander, K. Partes / Diamond & Related Materials 20 (2011) 1287–1301

3.2.1. Exothermic combustionIn contrast to other CVD processes for diamond deposition, the

heating occurs not by an additional external energy source, but byan exothermic conversion of the process gas. Most studies have ap-plied acetylene and oxygen as fuel for the combustion [26,3], at a sub-strate temperature between 770 and 1470 K [71]. According toMurakava and Takeuci [73] the primary and secondary proceduresof chemical reaction are given through Eqs. (1) and (2).

O2 þ C2H4→2COþH2 þ 448kJmol

ð1Þ

2COþH2 þ32O2→2CO2 þH2Oþ 850

kJmol

: ð2Þ

On the supposition that the process occurs without any work, vol-ume change, heat transfer, changes in kinetic or potential energy anda given pressure, the adiabatic flame temperature arises. Thereby theupper limit of the gas temperature can be estimated to 3550 K [62].The complete gas-phase chemistry of a hydrocarbon flame typicallyconsists of 40 species and 200 reversible elementary chemical reac-tions [49]. It can be assumed, that each reaction proceeds accordingto the Arrhenius equation:

kf ¼ ATne−EART

� �: ð3Þ

In theses equation kf denotes the rate constant of chemical reac-tion; A the pre-exponential factor; T the absolute temperature; Rthe gas constant; EA, the activation energy; and n the temperaturedependent on the pre-exponential factor. By measurement or simula-tion of the temperature a declaration about the chemical compositionof the flame can be made [34]. An abstract of chemical reactions in acombusting flame for diamond deposition with the corresponding pa-rameter is shown in Table 2.

The structure of an acetylene–oxygen flame is illustrated onthe left side of Fig. 8. The flame can be split into three subdivision

Fig. 8. Structure of an acetylene–oxygen neutral flame and the

regions: (α) the dark flame core, with a temperature less than700 K consisting of acetylene and oxygen; (β) the hot and brightarea, called acetylene feather, with a high decay of acetylene in hy-drogen and carbon-monoxide by Eq. (1) at a temperature up to3550 K; and (γ) the outside flame with the secondary chemical reac-tion of Eq. (2), which produces carbon-dioxide and vaporizes water ata temperature of 1500 to 2700 K [26,41].

The second sketch in Fig. 8 shows the flame with three differentratios of O2/C2H2: the acetylene-rich flame (a) has an O2/C2H2 ratioof 0.7 to 0.98; the neutral flame (b), which was already described pre-viously, with a ratio of 1; and the oxygen-rich flame (c), with a ratiohigher than 1. Diamond is synthesized with a ratio from 0.7 to 0.98only in the area of acetylene feather [43].

3.2.2. Techniques for synthesis of diamondsThe simplest form of a process set-up for the premixed torch ap-

paratus is shown in Fig. 9. The flame is generated through a commer-cial oxyacetylene torch and is perpendicular to the substrate, which ismounting on a water cooled block.

Usually, the process is controlled by measuring the substrate tem-perature (pyrometer or thermocouples) and by adjusting the ratio ofO2/C2H2 through a regulated mass flow. To increase the depositionarea, the flame is scanned across the substrate with the use of a rail.A problem of this procedure can be the oxidation of the film duringthe scanning process.

A further technique to increase the deposition area is the usage ofmultiple torches. Tzent et al. have operated with a linear array of ninetorches aimed at a rotating substrate [104]. The process set-up ofMurakawa and Takeuchi is shown in Fig. 10. Based on the applicationof this multi flame deposition in a chamber, they determined threeadvantages:

• Lower heat transfer to the substrate by transferring the secondchemical reaction out of the chamber.

three different ratios of O2/C2H2 0.7 to 0.98/1/N1 [25,33].

Fig. 10. Process set-up developed from Murakawa and Takeuchi [73].

Table 3Data for chemical induced diamond deposition.

Chemical activated

Total gas flow [slpm] 2–10Typical process gas C2H2/O2

Typical mixture range C2H2/O2 0.9/1 to 1.3/1Pressure [Pa] 5000–100,000Deposition area [mm2] up to 5000Common linear growth rate [μm/h] 10–200Advantages High linear growth rates

No chamber is requiredSimple set-upLow-priced equipment

Drawbacks High heating of the samplesSmall deposition area without railFlash-back and/or blow-off occurs easily

1293M. Schwander, K. Partes / Diamond & Related Materials 20 (2011) 1287–1301

• Prevention of oxidation via the exclusion of air.• Larger deposition area by the use of multiple torches.

An alternative to multiple torches for an enlargement of the depo-sition area is a flat flame burner. Flat flame burners have achievedlarge uniform areas at atmospheric [72] and reduced pressure [52].An example of a flat flame at reduced pressure and enlarged deposi-tion area is portrayed in Fig. 11. Due to the low-pressure, the oxygenand methane flame expands after passing the nozzle and the temper-ature of the acetylene feather drops from 3500 K to 2290 K [33]. Adisadvantage of this low-temperature, low-pressure flame, is theuncompleted oxidation of the fuel in the primary flame front andthe associated substantial hydrocarbon concentration in the post-flame region. Typical data for chemical induced diamond depositionare summarized in Table 3.

3.2.3. Diamond coatingsIn addition to other CVD-processes the deposition of diamond

layers by use of combustion flame is not significant in application. De-spite the industrial advantages of open atmosphere deposition, mostpublications only deal with results of the success or failure of synthesisunder certain experimental conditions. The few application-orientedpublications deal with deposition on substrates of molybdenum [4]and tungsten carbide [25] to use as wear resistance by sliding againstAl alloys [90] or for use in electronic device as resistor with 1010–1013 Ωm [77].

Most of the recent publications, by the small community of re-searchers, deal with the combination of a combusting flame andCO2 laser radiation [54]. The aim of those studies is to determinethe effect of laser power, density and wavelength on growth rate,grain size, surface morphology and crystal orientation [113]. Somescientists assume that the improvement of the diamond growthduring the combination with CO2 laser radiation is given becauseof resonant excitation of C2H4 molecules and not only as a resultof localized heating [114].

3.3. Electromagnetic excitation

Excitations of gases with electromagnetic waves for CVD processesare used with a large bandwidth of wavelengths. The publications aredominated by three different main focuses: radiofrequency (RF) [12],

Fig. 11. Flat flame bur

microwave (MW) [10,96] and laser induced plasma (LIP) [50,108]. RFrange is from 3 kHz (very low frequency) to 3 GHz (ultra high fre-quency), whereby it mostly means an inductive coupled wave witha frequency in the range up to several MHz. MW is in the rangefrom 300 MHz to 300 GHz. However, frequency of 2.45 GHz [24] isusually applied, because of its wavelength is often used in industrymicrowave heaters and therefore available with high power [102].The power distribution in a novel reactor has been presented bySilva in 2010 [97]. For LIP, different types of laser are used. Some ex-amples are Excimer (3000 THz), Nd:YAG (281 THz) [67] and CO2 laser(28.3 THz) [108].

3.3.1. Absorption of electromagnetic wavesThe two aggregate phases, gaseous and plasma, which are present

in CVD processes, can absorb electromagnetic waves in differentways. An abstract of the important absorption mechanisms is shownin Fig. 12.

The absorption of electromagnetic radiation is described by the in-teraction of electrons, photons, ions, atoms and molecules. This in-cludes a wide range of excitation and collision processes [65].However, in the beginning of most of the processes only the gasphase exists. Since the gas has no free carriers, the absorption of elec-tromagnetic waves can only emerge through electrically neutralatoms and molecules. The energy of the individual photons, in theused frequency range, is generally not high enough for absorptionby the “photoelectric effect” or “pair production”. Instead, the gas isheated by dielectric heating and multiphoton absorption.

3.3.1.1. Dielectric heating. Dielectric heating is the process in whichelectromagnetic waves heat the material through dipole rotation.The absorption is based on the ability of the electric field to polarizethe charge carrier in neutral material and the inability of this polariza-tion to follow extremely rapid reversals of the electric field [79]. Inaddition to the rotation, the dipoles of molecules are able to absorbelectromagnetic waves through vibration. The vibrational and rota-tional modes are associated with different amounts of energy. There-fore, different molecules absorb different frequencies of theelectromagnetic wave. A popular example is the absorption of solarradiation by the molecules of the atmosphere [56].

ner for CVD [34].

Fig. 12. Abstract of absorption mechanisms for electromagnetic waves.

1294 M. Schwander, K. Partes / Diamond & Related Materials 20 (2011) 1287–1301

The maximum absorption appears if the wavelength and reso-nance frequency of the molecules vibration or rotation states areequal. To prevent the absorption at the surface and to increase thepenetration depth of the electromagnetic wave, a frequency whichis a factor ten smaller than the resonance frequency is usually used.This setting is, for example, used in microwave heaters.

At a sufficiently high power density, the absorption can lead to an in-crease of internal energy and through scattering to an ionization of themolecules in the gas [87]. After the ionization of the gas, the absorptionof electromagnetic radiation is based largely on the charged particles.

3.3.1.2. Multiphoton absorption. The second absorption process for agas, shown in Fig. 12, is called multiphoton absorption [70]. It de-scribes the direct absorption of a large number of photons by atomsto cause photoionization without the necessity for interaction withfree electrons. Normally a single-photon can only be absorbed by anatom if the photon energy is higher or equal to the energy gap be-tween two different states. In the case of multiphoton absorption anelectron is excited into a virtual state, having the same energy ofthe incoming photon [70]. This has to be done in a certain time slot,given by the “uncertainty principle”. If a second photon is absorbedwithin this time slot, a higher virtual state can be achieved which cor-responds to the energy of two photons. By successively absorbingphotons, an atom can be ionized.

By the supply of charged particles in a gas, the absorption can in-crease and the required power density for a gas breakthrough can bereduced. As example, Schubnov et al. reduced the required energydensity of a CO2 laser-assisted plasma process, by the ejection of elec-trons from a briefly inserted titanium rod [92]. After ignition, the plas-ma generates more electrons through impact ionization. Anadditional supply of electrons is no longer necessary.

With the conversion of gas into plasma, the number of absorptionand interaction mechanisms increase [65]. Three important addition-al absorption mechanisms shown in Fig. 12 are: free–free absorption,stochastic plasma heating and ohmic plasma heating.

3.3.1.3. Free–free absorption. The free–free absorption, which is alsoknown as “Inverse Bremsstrahlung” describes the absorption of aphoton by an electron in the vicinity of a charged particle. From aclassical point of view, an electron in an alternating electric field canbe excited to oscillation [94]. If the electron does not interact withother particles in plasma, it returns the energy back to the electro-magnetic wave. However, for an energy transfer to the plasma, it isnecessary that the oscillating electron interacts with another particle.Hence, one important parameter in the plasma is the collision rateυeff, which is proportional to the free–free absorption. The collisionrate itself is a function of electron density, temperature and the inten-sity of the incoming electromagnetic field [47].

3.3.1.4. Ohmic heating. Since plasma is an electrical conductor, it canbe heated up by a current that passes through it. If the induced cur-rent density can be described by a local relationship between theelectromagnetic field and the conductivity of the plasma, the process

is called “ohmic heating” [50]. The absorption of the energy happensagain, as in the free–free absorption, through collisions of the acceler-ated electrons with other particles inside the plasma. By means of anincreasing temperature, the resistance of the plasma decreases andthe ohmic heating becomes less effective.

The direction of the electrical current for ohmic heating depends onthe type of excitation. It can be separated into capacitive-coupled plas-ma (CCP) and inductive-coupled plasma (ICP) [22]. CCP reactors usuallyconsist of two electrodes built like a capacitor with a power supply ofusually 13.56 MHz [12]. The induced electrical current oscillates per-pendicular to the electrodes in the direction of the displacement cur-rent. In the case of ICP, the reactor consists of an arrangement thatcorresponds to a transformer, where the second coil has been replacedby the conductive plasma. If a time-varying electric current is passedthrough the first coil, it creates a time varying magnetic field, which in-duces a circular electric current in the plasma [11].

3.3.1.5. Stochastic heating. The plasma can be separated into the quasi-neutral region, in which electrons are present and conduction currentis dominated and regions without electrons, where displacement cur-rent is dominated. The electron-less positive space charge region isdenoted as the “sheaths”, which cyclically expand and collapse. Theexpanding and collapsing depends on the electromagnetic field andthe polarity of the current changes. The narrow interface region be-tween the sheaths and the quasi-neutral plasma, which reflects inci-dent electrons, is called the “sheath edge”. The oscillation of thesheath edge can increase the electron energy during the reflectionand thereby heating the plasma. This process is called “stochasticheating” or “collisionless heating”. If the electron collision frequencyis less than the angular frequency of the induced current and the elec-tron mean free path is comparable to the maximum sheath width,stochastic heating is the dominated heating processes.

3.3.2. Techniques for synthesis of diamondsIt is evident, that pressure and wavelength have to be known, to

use electromagnetic waves (EM-CVD) for diamond deposition. There-fore, in most of the cases the process is characterized by these param-eters. For the deposition processes it is moreover necessary to knowthe chemical composition, ionization, molecule fraction and tempera-ture above the substrate. However, these parameters are unknown inmany cases, due to the highly technical requirements and the depen-dency on the layout. The simplest classification of plasma is the divi-sion into plasma-jet and plasma-ball formation.

3.3.2.1. Plasma-jet. The plasma-jet results from a point excitation, witha gas flow through it. The shape of the plasma-jet depends on severalfactors, such as the domination absorption mechanisms, the design ofthe nozzle and the waveguide. The used process arrangements aresimilar to one of the three examples shown in Figs. 13–15.

Fig. 13(a) depicts a type of “torche à Injection Axial” (TIA) [46]. Thereactor consists of two waveguides which are arranged perpendicularto each other. Thefield enhancement is generated through the “Beenak-ker cavity” which produces a standing wave pattern inside the cavity[68]. Depending on the nozzle structure, a thin converging cone witha tail flame on top of it can be created. Combined with the microwaveplasma torches (MPT), which exhibit a similar composition, it repre-sents the most frequently used process for diamond deposition by elec-tromagnetic induced plasma-jet formation. In most of the cases, a2.45 GHz generator is utilized to excite the process gas. The dominatingabsorptionmechanisms are themultiphoton absorption and the inverseBremsstrahlung [47]. The following influence parameters have a signif-icant impact on the process parameters:

• Nozzle geometry• Waveguide and conductor structure.

Fig. 13. Torche à Injection Axial (TIA) plasma-jet [46].

Fig. 15. Laser induced plasma (LIP) process [66].

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Fig. 14 shows a typical construction of inductive coupled plasma(ICP) [11]. The major driving force for ICP, is the availability of fullydeveloped equipment with an output of 40–80 KW. Due to this highavailable power, the gas flow can reach up to 100 slpm [8]. In contrastto TIA and MPT, the plasma excitation is not generated in a singlepoint, but an annular ohmic heating area is produced because of a cir-cular acceleration electron flow. The influence parameters with sig-nificant impact are:

• Convolution density, diameter and length• Electrical supply of the coil.

Fig. 15 illustrates a laser induced plasma (LIP) process [66]. Thelaser beam, which is typically located between 200 nm and 10 μm,is guided through mirrors into the operation range. A focusing mirroris used to produce a breakdown in the gas flow and to generate a localplasma [55]. The heating and ionization depends on multiphoton

Fig. 14. Inductive coupled plasma (ICP) process [11].

absorption, inverse Bremsstrahlung and electron oscillation of theplasma. The influence parameters are:

• Laser spot, wavelength• Power density, pulse length.

3.3.2.2. Plasma-ball. To generate a plasma-ball formation, the incom-ing wave has to be absorbed in a larger area then at LIP or MPT.This can be achieved by the modulation of a resonator chamber andthe adjustment of the absorption conditions resulting into a standingwave inside the chamber. Here, care has to be taken, that the penetra-tion depth of the incoming electromagnetic wave into the plasma is inthe order of the chamber. If the absorption coefficient of the plasma istoo high, the plasma formation might be too close to the quartz win-dow, where the radiation is coupled into the chamber. If the absorp-tion coefficient is too low, the energy transfer might be not highenough to keep the plasma ball excited and the etching of the reactorwall can cause severe contamination of the growing film. The absorp-tion coefficient depends on the pressure. Hence, the pressure has tostay in a certain range to keep the process running. However, theplasma-ball allows a larger and homogeneous diamond deposition.Three different types of resonators, which generate a plasma-ball for-mation, are shown in Figs. 16–18.

One of the first used plasma-ball reactors is the tubular reactor(TR) [8]. An early version of this reactor from 1982, which was actu-ally developed by Kamo et al. [48], is shown in Fig. 16. The position ofthe plasma-ball is adjusted in the center of the deposition chamber.The electromagnetic wave is radiated perpendicular to the reactortube, where the substrate is placed.

In Fig. 17 the circumferential antenna plasma (CAP) reactor isdepicted in cross section and 3-D view. The reactor, which was devel-oped by Pleuler et al. [80] in 2002, has been optimized for a homoge-neous flat plasma distribution. The microwave is guided via a coaxialwaveguide and coupled into the chamber via a circumferential ring-shaped quartz window, embedded in the metallic wall of the cham-ber [21]. Through the uniform amplitude and phase distribution, a ro-tational symmetry plasma-ball is formed directly on the substrate.

An improved bell-jar reactor, developed in 2009 at the MichiganState University [96] is illustrated in Fig. 18. The reactor has an ellip-tical shape and owns a quartz bell in the range of plasma excitation.The reactor is optimized for high density plasmas and a homogeneousdiamond deposition on up to 80,000 mm2.

In contrast to plasma-jet constrictions, the influence parametersare the chosen resonance mode, the coupling technique and theshape and location of the quartz window [100]. The skills are the lo-cally enhance field in front of the substrate with the possibility oflarge and uniform deposition areas. Typical conditions and results

Fig. 16. Tubular reactor (TR) for plasma-ball formation [48].Fig. 18. Bell-jar reactor for plasma-ball formation [96].

Table 4Data electromagnetic activated diamond deposition.

Electromagnetic activated

Plasma-jetTotal gas flow [slpm] 0007–150

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for electromagnetic wave induced diamond deposition are listed inTable 4.

3.3.3. Diamond coatingsIn addition to the thermal excitation, excitation by electromagnetic

radiation has the largest applications area. By supplying solid orgaseous precursor, boron [37,1], beryllium and phosphorus dopedsemiconductors are produced. The resulting band gaps are 0.37 eVfor boron [76], 0.6 eV for phosphorus [32] and 4.760 eV for beryllium[105].

In the domain of tool coating, deposition by electromagnetic radi-ation excitation is also used for industrial research. In the majority ofcases, the researchers do no longer deal with the process, but with theapplication of layers for drilling [51], machining [58] and cutting[106]. The studies are focused on the wear of the films at differentmechanical loads in tribological systems.

Depending on the high homogeneity, the fast growth rates and de-position areas with diameter up to 160 mm [96], a recipient of appli-cation is the production of freestanding large undoped and doped

Fig. 17. Circumferential antenna plasma (CAP) for plasma-ball formation [80].

single crystal diamond films. To remove the layers from the coatedsubstrates, mostly a lift-off process is used. The optical transparencyof undoped films in the wavelength range of 220 to 2500 nm is ashigh as the transparency of HPHT type-IIa diamond [103,69]. Byusing partially stabilized zirconia substrates, it was also shown thatthe growth of free-standing boron doped CVD diamond films [27] ispossible.

3.4. Electrical induced

Within this chapter all processes, which are based on a direct cur-rent (DC) plasma discharge between two electrodes for diamond de-position, are combined. The two used applications of glow discharge

Typical process gas Ar/H2/CH4

Typical mixture range H2/CH4 100/1 to 3/1Pressure [Pa] 500–100,000Deposition area [mm2] 20–2000Common linear growth rate [μm/h] 1–70Advantages High growth rate at large deposition area

No chamber is requiredHigh power available

Drawbacks High power and gas consumptionProcess control difficultExpensive equipment

Plasma-ballTotal gas flow [slpm] 0.1–1.5Typical process gas Ar/H2/CH4

Typical mixture range H2/CH4 50/1 to 2/1Pressure chamber [Pa] 40–30,000Deposition area [mm2] Up to 80,000Common linear growth rate [μm/h] 0.1–34Advantages Excellent quality of diamond layers

Stable deposition parameterVery large deposition area

Drawbacks Chamber is requiredSimulation of chamber is required3D-deposition difficultLow growth rates

Fig. 20. Direct current (DC) plasma process at low pressure used 1986 by Sawabe andK. Suzuki [99].

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and arc discharge are mainly influenced by the applied voltage, thecurrent flow, the electrode arrangement and the prevailing pressure.

3.4.1. Heating from apply voltageThrough the appliance of voltage over a gas flooded chamber,

charge carrier can be accelerated and by impact ionization the gascan be ionized. If the voltage is high enough, the ionization may beenhanced through an electron avalanche. The steady increase of theelectrical power results in a current–voltage curve, shown in Fig. 19.The curve can be divided into four areas [50]:

• By applying voltage, a very small amount of free electrons acceler-ate. After exceeding the breakdown voltage, the electrons can ionizethe gas. This leads to an electron avalanche which is described bythe Townsend coefficient. This range is normally not used for dia-mond deposition.

• After increasing the voltage over the requirement of glow dis-charge, an increase of power leads to an expansion of the plasma re-gion. This happens until the whole electrode is capped. ThroughCVD processes, a sample can be deposited on one of the electrodes.Therefore, by the expansion of the electrodes, this range is mostlyused for depositing large and uniform diamond layers at lowpressure.

• When the whole electrode is covered, the current can only increaseby an increase of the voltage. In the case of plate capacitor, thiscurve can be described by the Child–Langmuir Law.

• With a further increase of the current, the surface of the electrodeheats up. Thereby, electrons can pass out by thermal emission andthe transition until an arc discharge occurs. The arc discharge ischaracterized by high current and low voltage. In the range of 10to 100 kW applied power, a plasma core, which can be describedon the basis of a local thermal equilibrium (LTE), is generated.This discharge is used to generate a plasma-jet for diamonddeposition.

3.4.2. Techniques for synthesis of diamondsThe simplest way of direct current (DC) plasma discharge at low

pressure is shown in Figs. 20 and 21. The two examples have been de-veloped in 1986 and 1987 by Sawabe and Suzuki [99,88]. The reactorsconsist of two electrodes which are arranged parallel to each other. InFig. 20 the cathode depends on a grid over which the process gas issupplied. In contrast, the process gas in Fig. 21 is fed via a separate en-trance. Depending on the plasma density and the temperature, thedesired substrate temperature of 1100 K can be achieved by using acooled or heated water-flooded anode. Through a given pressure,the maximal power input into a hydrogen discharge can always bedefined sharply. Due to the characteristics that the voltage for stableplasma discharge is higher for lower pressures and the depositionarea gets smaller at higher pressure and lower power, it has to be

Fig. 19. Characteristic curve of a direct current (DC) plasma discharge [50].

decided between a high deposition rate and the possibility of largeareas up to 70 cm² [48].

For diamond deposition the plasma size is generally similar to thecathode size. Since the deposition area is determined by the plasmasize, it can be increased by using larger sized cathodes. The mainproblem is the generation of arcs between the electrodes and the sta-bility of the plasma. A frequently used method to avoid the formationof arc discharge [93] and to enlarge the deposition area [9], is the useof pulsed DC [39,40]. Another possibility is to stabilize the process bya magnetic field [74].

A further increase of pressure and power leads to an arc discharge.The setup of conventional high power industrial arc is shown inFig. 22. The cathode, which is not illustrated in this Figure, is normallymade of W–Ce and the anode is usually made of oxygen-free copper.In early works of diamond deposition, the setup was simply copiedfrom a plasma thermal spray, which is basically very similar. Withinthe depicted schematic diagram, the arc rotates with a proper speedaround the anode. The carbon source is above the nozzle and formsa stream around the hot plasma arc. After the flow through the arc,a uniform plasma jet is passing through the orifice. Due to the limitedexpansion shape of the plasma jet, only a small deposition area withvarying quality and thickness can be deposited. Furthermore, theplasma jet is not as uniform as desired, because of the use of conven-tional plasma torches, which consist of a cathode rod and a cylindricalanode.

For a further increase of power, the arc plasma has to be stabilizedby some changes in the setup. One possibility is shown in Fig. 23. TheDC arc plasma torch, developed by Zhang et al., owns a comparative

Fig. 21. Direct current (DC) plasma process at low pressure used 1987 by Sawabe andK. Suzuki [88].

Fig. 22. Schematic diagram of an arc discharge [59].

Fig. 24. Arc-jet stabilized by four DC plasma torches [44].

1298 M. Schwander, K. Partes / Diamond & Related Materials 20 (2011) 1287–1301

long arc discharge and an external magnetic field [59]. Withinthis field the electrons follow the magnetic field lines which enablea better heat transfer. The process reaches a power of about 100 kWand allows a growth rate of 40 μm/h.

Another possibility to stabilize the plasma jet at high power hasbeen developed by Hirata et al. and is portrayed in Fig. 24 [44].The electrode configuration is made of four DC plasma torches. Oneplasma torch is mounted perpendicular to the three shown plasmatorches in the schematic diagram. The three changeable torches areattached in the plane along their axes. Arcs are struck betweenthe cathode and the three anodes, so that the discharge area can bewidened in radial direction of the plasma jet. Another unique featureis the high process pressure of up to 26.7 kPa. Furthermore, the car-bon source gas is induced into the plasma jet and not induced intothe generator.

Up to the present day, the highest deposition rate of 930 μm/hwith a DC plasma jet, is achieved by Ohtake et al. [78]. The usedsetup is shown in the schematic diagram of Fig. 25. In this apparatusthe plasma jet consists only of Ar and H2. The carbon carrier gas is notinjected directly into the generator but instead mixed into the plasmajet. The cathode is made of pure tungsten and the anode is made ofpure copper. Particularly, there are also three independent gas inletsof the cathode system. The process parameters for diamond

Fig. 23. Magnetic stabilized arc for high power deposition [59].

deposition are 25 kPa with 9.45 kW applied power. The achievedlayer thickness is 3 mm with a 10 10 mm2 large molybdenumsubstrate.

Another electrical induced deposition is the pulsed arc discharge,which is characterized by the repetition of short current pulses upto several 1000 A. Since the deposited layers only consist of amor-phous carbon, they are not interesting at this point. Typical data forglow discharge and arc discharge deposition of diamonds is given inTable 5.

3.4.3. Diamond coatingsAs for electromagnetic excitation, DC processes are used as a source

for plasma-jet and for homogeneous plasmadistribution. Therefore, it isnot surprising that also by the addition of boron-containing gases,boron doped semiconductors are produced by direct current plasmachemical vapor deposition (DC-PCVD)[28]. Although it was shownthat at comparable ratios of PH3/CH4 the P content is approximatelyten times greater by using DC rather than microwave excitation [89],there are only a few publications which use DC as a source for dopingwith phosphorus.

The major advantage of DC processes is the production of largefree-standing diamond layers with a pronounced diamond structure.By using DC arc plasma-jet and destroyable Ti interlayer on graphite,crack-free free-standing diamond films with thickness of 300 μm and

Fig. 25. DC arc-jet setup with the highest published diamond growth rate [78].

Table 5Data electrical activated diamond deposition.

Electrical activated

Glow dischargeTotal gas flow [slpm] 0.1 to 0.5Typical process gas H2/CH4

Typical mixture range H2/CH4 100/1 to 10/1Pressure [Pa] 6000 toDeposition area [mm2] 70,000Common linear growth rate [μm/h] 10–80Advantages Large area at low pressure

Simple set-upLow-priced equipmentHigh quality diamond layers

Drawbacks Small deposition area at high pressureLow growth rates at low pressureChamber is required

Arc dischargeTotal gas flow [slpm] 7.8 to 500Typical process gas Ar/H2/CH4

Typical mixture range H2/CH4 100/1 to 20/1Pressure [Pa] 7 to 101,330Deposition area [mm2] 11,000Common linear growth rate [μm/h] 10 to 930Advantages Highest linear growth rate

High diamond qualityDrawbacks Small deposition areas

Process control difficultExpensive equipmentHigh power and gas consumptionContamination through electrodedegradationChamber is required

Fig. 26. Pressure [Pa] and absolute flow [slpm] out of more than 100 highly regardedpublications for different CVD processes.

Fig. 27. Statistic view from the pressure field of applications.

1299M. Schwander, K. Partes / Diamond & Related Materials 20 (2011) 1287–1301

diameter of 60 mm are published [57]. Under the condition of10 μm/h they achieved grain size of about 1×1×1 mm3 [18].

Even larger free-standing diamond layers, with diameters up to203.2 mm (8-inch), are produced with the diode-type electrode con-figuration [17]. The advantages of these layers which grow at 7 to9 μm/h are the excellent uniformity in thickness, crystallinity andthermal conductivity. Up to now, coatings with larger surfaces(500×1000 mm) were only made by HFCVD method. However, thegrowth rates and purities are inferior to the DC and microwave exci-tation processes [91]. Therefore HFCVD is not used for free-standingdiamond layers.

As for other plasma-based processes, electrical induced synthesisis also used for coatings on WC–Co hard metal [23], SiC [111] or fur-ther materials, which are usable for tools. However, since tools mostlyneed a small coating area, only the plasma jet process is used for de-position [23].

4. Discussion

Today, different equipment for diamond synthesis by CVD isavailable. Although these techniques differ in energy supply, temper-ature range, pressure range and chemical composition, they have topossess the same requirements of desired diamond quality, growthrate and layer size. For industrial applications, gas consumption, eco-nomic efficiency and velocity of the process are also important.Therefore, each technique has a field of application with differentprocess parameters. By the analysis of more than 100 related publica-tions, a comparison of pressure and absolute flow is given in Fig. 26.The total gas flow consists of process gases like methane and hydro-gen and the additives gases such as argon and nitrogen. The exceptionis the chemical combustion, in which acetylene and oxygen are usedas process gases. Due to the fact, that not every publication showsthe details of pressure, gas flow and growth, the displayed valuesare decoupled from each other. Therefore, the number of measure-ment points varies slightly for the two axes.

However, unlike the gas flow, the pressure owns a maximumaround atmosphere pressure at 105 Pa. This particular limit is notgiven because it would not be feasible, but the use of hydrogen or ox-ygen in a pressure reactor is associated with a risk of explosion andtherefore with a high technical effort. Furthermore, it can be seen,that the process windows for electric and electromagnetic excitationare much larger than for thermal and chemical excitation. The restric-tion of chemical processes in the range of 104 to 105 Pa, is conditionedby thermal quenching of the flame at low pressure [53]. In contrast,thermal heating is mostly used in the range from 103 to 104 Pa. Therestriction of thermal heating is generally the maximum temperatureof the heated filament or layer, which results in a maximum flow andpressure.

In contrast to this, plasma generated processes can be used overthe range of five magnitudes from 101 to 105 Pa for diamond deposi-tion. In Figs. 27 and 31 the values of pressure and gas flow are com-bined in a statistic view with the following characteristics:maximum and minimum values are given by the star; whiskersgone from 5 to 95% probability; diamond formed boxes shown 25,50 and 75% probability; the square represent the arithmetic mean.

The growth rate in comparison with the absolute flow rate can beseen in Fig. 28. As expected, the growth rate's tendency increases tohigher values with higher absolute flow at different processes. Thiscan particularly be seen on the comparison of the statistic diagramsin Figs. 30 and 31.

For free standing diamond layers thermal induced CVD diamondcoating techniques are not suitable. This can also be concluded fromthe growth rate comparison. Thermal induced CVD techniques showthe lowest possible gas flows and hence the lowest growth rates.However, the growth rate is not sufficient information in order toconclude what application is possible. For example, chemical CVD

Fig. 28. Growth rate [μm/h] and absolute flow rate [slpm] out of more than 100 highlyregarded publications for different CVD processes.

Fig. 29. Growth rate [μm/h] and flow rate [slpm] from methane, hydrogen, acetyleneand oxygen for different processes.

Fig. 31. Statistic view of absolute flow [slpm] for different CVD processes.

1300 M. Schwander, K. Partes / Diamond & Related Materials 20 (2011) 1287–1301

techniques are also not used for the generation of large free standinglayers which is caused by the process geometry condition. However,it must be taken into account that the diagrams do not distinguish be-tween processes where only methane and hydrogen are used andprocesses with a major share of argon and nitrogen, which is oftenthe case in plasma-based processes. Additionally, it is consideredthat the deposition rate was specified for different substrate sizesand the specification in volume growth would be more accurate. Fur-thermore, the power of the used energy supply is neglected, as it isassumed that the efficiency of the processes do not deviate stronglyfrom each other and the experiments are running at best settings,

Fig. 30. Statistic view of growth rate for different CVD processes.

as described in the first chapter. Despite these simplifications, the in-terrelationship between the gas flow and the growth rate for the dif-ferent processes shown in Figs. 30 and 31 behaves very similarly.

Additional to the decoupled measurement points, Fig. 29 showsthe direct correlation of growth rate and gas flow. Due to the require-ment of both specifications in a publication, the number of measure-ment points is reduced. Furthermore, the gas flow only consists of theprocess gases methane, hydrogen, acetylene and oxygen without ad-ditives gases such as argon and nitrogen. As also shown in Figs. 26 and28, the process window for electrical and electromagnetic excitationdiffers from thermal and chemical excitation. Moreover, it can beseen that the growth rate is rather dependent on the gas flow thanon the process itself. Hence it can be concluded that the upper limitof the growth rate is mainly depending on the upper limit of the gasflow rate. However this is only valid if arbitrary amounts of powerare available.

5. Conclusion

As shown in this review, the technical environment for diamonddeposition goes from a simple single excitation at low pressure tolarge deposition areas with multi-flame and enlarged plasmas at at-mosphere pressure with increased power. Until today, the depositionrate has risen further as a result of the increasing power and gas flow,without a physical limitation. The only known limitation is the in-crease to the atmosphere pressure. Hence the growth rate is techni-cally limited by the energy supply, which also limits thetemperature and the maximum gas flow. Finally, there have to bemore investigation on the technical and physical growth rate in thefuture.

Acknowledgments

This work was supported by the Deutsche Forschungsgemeinschaft(DFG) under contract no. VO 530/18-2 which the authors gratefullyacknowledge.

References

[1] A. Aleksov, A. Vescan, M. Kunze, P. Gluche, W. Ebert, E. Kohn, A. Bergmaier, G.Dollinger, Diam. Relat. Mater. 8 (1999) 941.

[2] F.A. Almeida, M. Amaral, F.J. Oliveira, R.F. Silva, Diam. Relat. Mater. 15 (2006)2029.

[3] Y. Ando, S. Tobe, T. Saito, J.-i. Sakurai, H. Tahara, T. Yoshikawa, Thin Solid Films457 (2004) 217.

[4] Y. Ando, S. Tobe, H. Tahara, Vacuum 83 (2009) 102.[5] M.N.R. Ashfold, P.W. May, C.A. Rego, Chem. Soc. Rev. 23 (1994) 21.[6] P.K. Bachmann, D. Leers, H. Lydtin, Diam. Relat. Mater. 1 (1991) 1.[7] P.K. Bachmann, in: Gordon Davies (Ed.), Properties and Growth of Diamond,

Institution of Electrical Engineers, London, 1994, p. 349.[8] P.K. Bachmann, in: Davies Gordon (Ed.), Properties and Growth of Diamond,

Institution of Electrical Engineers, London, 1994, p. 354.

1301M. Schwander, K. Partes / Diamond & Related Materials 20 (2011) 1287–1301

[9] Y.J. Baik, J.K. Lee, W.S. Lee, K.Y. Eun, Thin Solid Films 341 (1999) 202.[10] L. Bárdos, H. Baránková, Yu.A. Lebedev, T. Nyberg, S. Berg, Diam. Relat. Mater. 6

(1997) 224.[11] J.O. Berghaus, J.L. Meunier, F. Gitzhofer, Int. J. Refract. Met. Hard Mater. 16

(1998) 201.[12] H. Berthou, C. Faure, W. Hänni, A. Perret, Diam. Relat. Mater. 8 (1999) 636.[13] H. Björkman, P. Rangsten, K. Hjort, Sens. Actuators 78 (1999) 41.[14] F.P. Bundy, H.T. Hall, H.M. Strong, R.H. Wentorf, Nature 176 (1955) 51.[15] J.E. Butler, R.L. Woodin, L.M. Brown, P. Fallon, Philos. Trans. Phys. Sci. Eng. 342

(1993) 1664.[16] A. Chattopadhyay, S.K. Sarangi, A.K. Chattopadhyay, Appl. Surf. Sci. 255 (2008)

1661.[17] K.W. Chae, Y.J. Baik, J.K. Park, W.S. Lee, Diam. Relat. Mater. 19 (2010) 1168.[18] G.C. Chen, B. Li, H. Li, H. Lan, F.W. Dai, Q.J. Xue, X.Q. Han, L.F. Hei, J.H. Song, C.M.

Li, W.Z. Tang, F.X. Lu, Diam. Relat. Mater. 19 (2010) 1078.[19] M.S. Raghuveer, S.N. Yoganand, K. Jagannadham, R.L. Lemaster, J. Bailey, Wear

253 (2002) 1194.[20] C. Lévy-Clément, N.A. Ndao, A. Katty, M. Bernard, A. Deneuville, C. Comninellis,

A. Fujishima, Diam. Relat. Mater. 12 (2003) 606.[21] M.L. McConnell, D.P. Dowling, C. Pope, K. Donnelly, A.G. Ryder, G.M. O'Connor,

Diam. Relat. Mater. 11 (2002) 1036.[22] I.B. Denysenko, S. Xu, J.D. Long, P.P. Rutkevych, N.A. Azarenkov, K. Ostrikov,

J. Appl. Phys. 95 (2004) 2713.[23] F. Deuerlera, H. Grunerb, M. Pohlc, L. Tikanac, Surf. Coat. Technol. 142–144

(2001) 674.[24] K. Donnelly, D.P. Dowling, M.L. McConnell, R.V. Flood, O. Berkefelt, J. Svennebrink,

Diam. Relat. Mater. 9 (2000) 693.[25] J.B. Donnet, H. Oulanti, T. Le Huu, M. Schmitt, Carbon 44 (2006) 374.[26] R.J.H. Klein-Douwel, J.J. ter Meulen, J. Appl. Phys. 83 (1998) 4734.[27] L.E.V.S. Brandaoa, R.F. Pires, N.M. Balzaretti, Vib. Spectrosc. 54 (2010) 84.[28] Y. Feng, J. Lv, J. Liu, N. Gao, H. Peng, Y. Chen, Appl. Surf. Sci. 257 (2011) 3433.[29] J. Gäbler, S. Pleger, Int. J. Mach. Tools Manuf. 50 (2010) 420.[30] Z. Gao, V. Carabelli, E. Carbone, E. Colombo, F. Demaria, M. Dipalo, S. Gosso, C.

Manfredotti, A. Pasquarelli, S. Rossi, Y. Xu, E. Vittone, E. Kohn, Diam. Relat.Mater. 19 (2010) 1021.

[31] R. Gat, J.C. Angus, in: Gordon Davies (Ed.), Properties and Growth of Diamond,Institution of Electrical Engineers, London, 1994, p. 325.

[32] E. Gheeraert, S. Koizumi, T. Teraji, H. Kanda, M. Nesladek, Diam. Relat. Mater. 9(2000) 948.

[33] N.G. Glumac, D.G. Goodwin, Thin Solid Films 212 (1992) 122.[34] N.G. Glumac, D.G. Goodwin, Combust. Flame 105 (1996) 321.[35] D.G. Goodwin, G.G. Gavillet, J. Appl. Phys. 68 (1990) 6393.[36] E.V. Gorokhov, A.N. Magunov, V.S. Feshchenko, A.A. Altukhov, Instrum. Exp.

Tech. 51 (2) (2008) 280.[37] M.C. Granger, M. Witek, J. Xu, J. Wang, M. Hupert, A. Hanks, M.D. Koppang, J.E.

Butler, G. Lucazeau, M. Mermoux, J.W. Strojek, G.M. Swain, Anal. Chem. 72(2000) 3793.

[38] S.J. Harris, A.M. Weiner, T.A. Perry, Appl. Phys. Lett. 53 (1988) 1605.[39] P. Hartmann, R. Haubner, B. Lux, Diam. Relat. Mater. 5 (1996) 850.[40] P. Hartmann, R. Haubner, B. Lux, Int. J. Refract. Met. Hard Mater. 16 (1998) 207.[41] L.M. Hanssen, W.A. Carrington, J.E. Butler, K.A. Snail, Mater. Lett. 7 (1988) 289.[42] Y. Hirose, N. Kondo, Extended Abstracts of 35th Spring Meeting of the Japanese

Society, Applied Physics and Society, Tokyo, 1988, p. 343.[43] Y. Hirose, S. Amanuma, K. Komaki, J. Appl. Phys. 68 (1990) 6401.[44] A. Hirata, M. Yoshikawa, Diam. Relat. Mater. 4 (1995) 1363.[45] T. Izák, M. Marton, M. Varga, M. Vojs, M. Veselý, R. Redhammer, M. Michalka,

Vacuum 84 (2009) 49.[46] J. Jonkers, J.M. de Regt, J.A.M. van der Mullen, H.P.C. Vos, F.P.J. de Groote, E.A.H.

Timmermans, Spectrochim. Acta, Part B 51 (1996) 1385.[47] C.M. Jost, Untersuchung optischer-feldionisierter Wasserstoff- und Deuterium-

Plasmenmittels Thomson-gestreuten Laserlichtes, Ludwig-Maximilians-Universität,München, 2001.

[48] M. Kamo, Y. Sato, S. Matsumoto, N. Setaka, J. Cryst. Growth 62 (1983) 642.[49] R.J. Kee, J.A. Miller, Physica D 12 (1984) 198.[50] A. von Keudell, Plasma-Oberflächen-Wechselwirkung, Ruhr-Universität Bochum,

Unpublished Results (2008).[51] Y.K. Chou, R.G. Thompson, A. Kumar, Thin Solid Films 518 (2010) 7487.[52] J.S. Kim, M.A. Cappelli, Surf. Coat. Technol. 76–77 (1995) 791.[53] N.I. Kim, T. Kataoka, S. Maruyama, K. Maruta, Combust. Flame 141 (2005) 78.[54] T. McKindra, M.J. O'Keefe, Z. Xie, Y. Lu, Mater. Charact. 61 (2010) 661.[55] V.I. Konov, A.M. Prokhorov, S.A. Uglov, A.P. Bolshakov, I.A. Leontiev, F. Dausinger,

H. Huegel, B. Angstenberger, G. Sepold, S. Metev, Appl. Phys. A Mater. Sci. Pro-cess. 66 (1998) 575.

[56] S. Kshudiram, The Earth's Atmosphere, Book, Spirnger-Verlag, Berlin Heidelberg978-3-540-78426-5, 2008.

[57] C. Li, L. Wang, L. Chen, Z. Liu, L. Hei, F. Lu, Diam. Relat. Mater. 18 (2009) 1348.[58] Q. Liang, Y.K. Vohra, R. Thompson, Diam. Relat. Mater. 17 (2008) 2041.[59] F.X. Lu, G.F. Zhong, J.G. Sun, Y.L. Fu, W.Z. Tang, J.J. Wang, G.H. Li, J.M. Zang, C.H.

Pan, C.X. Tang, T.L. Lo, Y.G. Zhang, Diam. Relat. Mater. 7 (1998) 737.

[60] V. Malcher, A. Mrska, A. Kromka, A. Satka, J. Janík, Curr. Appl. Phys. 2 (2002) 201.[61] S. Matsumoto, J. Mater. Sci. 17 (1982) 3106.[62] K.J. Matthes, E. Richter, Schweißtechnik. Schweißen von metallischen

Konstruktionswerkstoffen, Carl Hanser Verlag, München, 2008.[63] P.W. May, Philos. Trans. Math., Phys. Eng. Sci. 358 (2000) 473.[64] P.W. May, J.A. Smith, Yu.A. Mankelevich, Diam. Relat. Mater. 15 (2006) 345.[65] D. Meschede, Gerthsen Physik, Book, Spirnger-Verlag, Berlin Heidelberg, 2005,

p. 23, ISBN-10 3-540-254221-8.Auflage.[66] S. Metev, A. Stephen, J. Schwarz, C. Wochnowski, Focused on Laser Precision

Microfabrication (LPM 2002), 2003.[67] P. Mistry, M.C. Turchan, G.O. Granse, T. Baurmann, Mater. Res. Innov. 1 (1997)

149.[68] M. Moisan, G. Sauve, Z. Zakrzewski, J. Hubert, Plasma Sources Sci. Technol. 3

(1994) 584.[69] Y. Mokuno, A. Chayahara, H. Yamada, N. Tsubouchi, Diam. Relat. Mater. 18

(2009) 1258.[70] C.G. Morgan, Rep. Prog. Phys. 28 (1975) 621.[71] P.W. Morrison Jr., J.T. Glass, in: Gordon Davies (Ed.), Properties and Growth of

Diamond, Institution of Electrical Engineers, London, 1994, p. 368.[72] M.K. Murayama, S. Kojima, K. Uchida, J. Appl. Phys. 69 (1991) 7924.[73] M. Murakawa, S. Takeuchi, Surf. Coat. Technol. 54–55 (1992) 403.[74] M. Nesládek, Diam. Relat. Mater. 2 (1993) 357.[75] C. Nørgård, A. Matthews, Diam. Relat. Mater. 5 (1996) 332.[76] H. Okazaki, R. Yoshida, T. Muro, T. Nakamura, T. Wakita, Y. Muraoka, M. Hirai, H.

Kato, S. Yamasaki, Y. Takano, S. Ishii, T. Oguchi, T. Yokoya, Appl. Phys. Lett. 98(2011) 082107.

[77] Y. Okumura, K. Kanayama, K. Shogaki, Combust. Flame 157 (2010) 1052.[78] N. Ohtake, M. Yoshikawa, J. Electrochem. Soc. 137 (1990) 717.[79] P. Peregrinus, A.C. Metaxas, R.J. Meredith, Industrial Microwave Heating, Book,

Institution of electrical engineers0-906048-89-3, 1993.[80] E. Pleuler, C. Wild, M. Füner, P. Koidl, Diam. Relat. Mater. 11 (2002) 467.[81] R. Polini, M. Barletta, G. Cristofanilli, Thin Solid Films 519 (2010) 1629.[82] N.A. Prijaya, J.C. Angus, P.K. Bachmann, Diam. Relat. Mater. 3 (1994) 129.[83] S.A. Rakha, Z. Xintai, D. Zhu, Y. Guojun, Curr. Appl. Phys. 10 (2010) 171.[84] A.K. Ramdas, in: Gordon Davies (Ed.), Properties and Growth of Diamond,

Institution of Electrical Engineers, London, 1994, p. 13.[85] L.L. Regel, W.R. Wilcox, Acta Astronaut. 48 (2001) 129.[86] J. Reiche, Nichtresonante Laserstreudiagnostik an Hochdruckgasentladungen,

Universität Hannover, Unpublished results (2003)[87] B. Rosenkranz, J. Bettmer, TrAC, Trends Anal. Chem. 19 (2000) 138.[88] A. Sawabe, H. Yasuda, T. Inuzuka, K. Suzuki, Appl. Surf. Sci. 33–34 (1988) 539.[89] S.N. Schauer, J.R. Flemish, R. Wittstruck, M.I. Landstrass, M.A. Plano, Appl. Phys.

Lett. 64 (9) (1994) 28.[90] M. Schmitt, D. Paulmier, Tribol. Int. 37 (2004) 317.[91] L. Schäfer, M. Höfer, R. Kröger, Thin Solid Films 515 (2006) 1017.[92] A. Schubnov, F. Vollertsen, Vak. Forsch. Prax. 21 (2) (2009) 24.[93] S. Sciortino, S. Lagomarsino, F. Pieralli, E. Borchi, E. Galvanetto, Diam. Relat.

Mater. 11 (2002) 573.[94] J.F. Seely, E.G. Harris, Phys. Rev. A 7 (1973) 1064.[95] S.D. Shin, N.M. Hwang, D.Y. Kim, Diam. Relat. Mater. 11 (2002) 1337.[96] F. Silva, K. Hassouni, X. Bonnin, A. Gicquel, J. Phys. Condens. Matter 21 (2009)

364202.1.[97] F. Silva, X. Bonnin, J. Scharpf, A. Pasquarelli, Diam. Relat. Mater. 19 (2010) 397.[98] B.V. Spitsyn, L.L. Bouilov, A.E. Alexenko, Braz. J. Phys. 30 (2000) 471.[99] K. Suzuki, A. Sawabe, T. Inuzuka, Jpn. J. Appl. Phys. 29 (1990) 153.

[100] W. Tan, T.A. Grotjohn, Diam. Relat. Mater. 4 (1995) 1145.[101] C. Tendero, C. Tixier, P. Tristant, J. Desmaison, P. Leprince, Spectrochim. Acta,

Part B 61 (2006) 2.[102] E.A.H. Timmermans, Atomic and Molecular Excitation Processes in Microwave

Induced Plasmas, Eindhoven University of Technology, Unpublished Results(1999)

[103] N. Tsubouchi, Y. Mokuno, A. Chayahara, S. Shikata, Diam. Relat. Mater. 19 (2010)1259.

[104] Y. Tzeng, R. Phillips, C. Cutshaw, T. Srivinyunon, B.H. Loo, P. Wang, Appl. Phys.Lett. 58 (1991) 2645.

[105] K. Ueda, M. Kasu, Diam. Relat. Mater. 18 (2009) 121.[106] V.V.S.S. Srikanth, X. Jiang, A. Köpf, Surf. Coat. Technol. 204 (2010) 2362.[107] Verein Deutsche Ingenieure, Kohlenstoffschichten — Grundlagen, Schichttypen

und Eigenschaften, Beuth Verlag GmbH, Berlin, 2005.[108] F. Vollertsen, K. Partes, A. Schubnov, Prod. Eng. 4 (2010) 9.[109] Z.L. Wang, C. Lu, J.J. Li, C.Z. Gu, Appl. Surf. Sci. 255 (2009) 9522.[110] S. Wang, G. Chen, F. Yang, Thin Solid Films 517 (2009) 3559.[111] T. Wang, L. Xiang, W. Shi, X. Jiang, Surf. Coat. Technol. 205 (2011) 3027.[112] M. Werner, R. Locher, Rep. Prog. Phys. 61 (1998) 1665.[113] Z. Xie, Y. Zhou, X. He, Y. Gao, J. Park, H. Ling, L. Jiang, Y. Lu, Cryst. Growth Des. 10

(2010) 1762.[114] Z.Q. Xie, X.N. He, W. Hu, T. Guillemet, J.B. Park, Y.S. Zhou, J. Bai, Y. Gao, X.C. Zeng,

L. Jiang, Y.F. Lu, Cryst. Growth Des. 10 (2010) 4928.