The overview of cromium-containing coatings obtained from diearens is presented in Table []
Dibenzenechromium Cr(η6-C6H6)2
Dibenzenechromium Cr(η6-C6H6)2 is a brown diamagnetic extremely air sensitive solid with melting point 284 °C (172-173°C?). It can be synthesized by following methods:
Fischer-Hafner Synthesis: CrCl3 + 2 /3 Al + 1/3 AlCl3 + 2 C6H6 → [(C6H6)2Cr]+ AlCl4-
[Cr(C6H6)2]+ + ½ S2O42- + OH- → Cr(η6-C6H6)2 + HSO3-
Co-condensation: Cr + 2 C6H6 → Cr(η6-C6H6)2
Thermodynamic calculations to simulate the MOCVD preparation of chromium and of chromium carbide films from Cr(C6H6)2 were performed; the conclusion was that simultaneously increasing the process pressure and decreasing the precursor molar fraction, the carbon content of the films can be decreased; slight reduction of carbon content can also be achieved by increasing temperature. A change of the composition of the reactive gas on the growing surface to a C6H6/Cr value of 0.2 was proposed in order to fit available experimental data. This surface composition at equilibrium is in excellent agreement with estimates made by using a model based on the control of deposition and consequently of carbon incorporation by surface adsorption of benzene. [[i]]
[i] C. Vahlas, F. Maury, L. Gueroudji, Chemical Vapor Deposition, Volume 4, Issue 2, pages 69–76, March 1998
Growth of pure Cr metal films having no carbon contamination from Cr(C6H6)2 by CVD at 300°C was reported. [i]
Chromium-based coatings containing significant amounts of carbon, were grown by CVD from dibenzenechromium at temperatures 300 to 500°C.[ii]
Dibenzenechromium produced Cr films containing Cr23C6 and C (0.5-10 wt.% C) at by CVD at 400-700°C [917]
Dibenzenechromium was applied for the growth of metallic Cr films on silicon and stainless steel at 500°C by direct liquid injection (DLI) atmospheric pressure MOCVD. Addition of 10% hexachlorbenzene C6Cl6 to the 0.03M Cr(η6-C6H6)2 / toluene precursor solution reduced carbon incorporation and led to deposition of metallic chromium (bcc-Cr) phase, having some C contamination (Cr0.87C0.13) at growth rate 6.0 nm/min. solutions, using N2 as carrier gas. Injection conditions were following: injection frequency 4-10 Hz, opening time 0.3-0.5ms; the molar fraction of dibenzene chromium in the mixture was 66-200ppm. Flash vaporization chamber was maintained at 200°C. [913]
[i] M. Tsutsui, Ann. N.Y. Acad. Sci., 137(1966)205.
[ii] B.D. Nash, T.T. Campbell, F.E. Block, US Bureau of Mines, Report Investigation 7112, Washington 1968
Cr(C6H5)2 for Cr carbides CVD
Dibenzenchromium is expected to produce a mixture of Cr7C3 and Cr3C2 by MOCVD at 450°C temperature, 10 mbar process pressure, and Cr(C6H6)2 molar ratio 10-3. [832]
Dibenzenechromium was applied for the growth of chromium carbide, nitride as well as CrCx/CrN nanostructured multilayer coatings with a bilayer period as low as 50 nm, on silicon and stainless steel at 500°C by direct liquid injection (DLI) atmospheric pressure MOCVD, 0.03M Cr(η6-C6H6)2 / toluene solutions, using N2 as carrier gas. Precursor vaporiser was kept at 200°C. [913, 914]
Cr(C6H6)2 for CrCx by MOCVD
The early investigation reported growth of the mixture of Cr23C6 and Cr7C3 (carbon content 1-8%) by CVD of dibenzenechromium at 500°C [833]
Chromium carbide coatings were deposited direct liquid injection (DLI) atmospheric pressure MOCVD from 0.03M Cr(η6-C6H6)2 / toluene solutions by, using N2 as carrier gas; carbon in the films was originating mainly from the decomposition of dibenzenechromium. Injection conditions were following: injection frequency 4-10 Hz, opening time 0.3-0.5ms; the molar fraction of dibenzene chromium in the mixture was 66-200ppm. Flash vaporization chamber was maintained at 200°C. Amorphous layers with composition Cr0.69C0.28O0.03 were deposited with growth rate 5.5 nm/min. [913]
Cr(C6H6)2 for CrN by MOCVD
During direct liquid injection (DLI) atmospheric pressure MOCVD using dibenzenechromium solution, the addition of NH3 to the reaction mixture (NH3 / Cr(η6-C6H6)2 ratio 280) resulted in chromium nitride layer growth. Crystalline layers (fcc-CrN phase) with composition Cr0.40N0.27O0.31C0.02 and mean cystallite size 12 nm were obtained at growth rate 4.2 nm/min. [913]
Cr(C6H6)2 for Cr(C,N) and CrN by MOCVD
Bis(benzene) chromium in combination with NH3 or N2H4 as nitrogen sources was applied for the preparation of chromium carbonitride and nitride hard coatings by OMCVD in the temperature range 350–550 °C. Amorphous films were obtained below 450 °C; above this temperature different phases - Cr7C3, Cr2(N,C) or CrN were obtained depending on the gas phase composition. Slight contamination of the films with free carbon was detected by electron probe microanalysis and XPS. Coating microhardness was comparable with that of similar material grown by other techniques. [915]
Cr(C6H6)2 for Cr-doped InP by MOVPE
Bis-benzene chromium proved to be efficient Cr precursor for the growth of chromium-doped semi-insulating InP films by MOVPE. Layers with resistivity 3 × 108 Ω cm having compensating deep donor concentration up to 3 × 1016 cm−3 were obtained. [908]
Bis(methylbenzene)chromium (Bis(toluene)chromium) Cr(MeC6H5)2
Bis(toluene)chromium Cr(MeC6H5)2 have been applied for the chromium carbide hard coatings preparation by CVD in the temperature range 300–600°C. Mass spectrometry fragmentation patterns were studied to explain trends in the deposition temperature and chemical composition of the obtained thin films. [916]
Chromium films with Cr23C6 and C admixture (0.5-10 % C by weight).were grown from bis(methylbenzene)chromium by CVD at 400-700°C [917]
Bis(1,4-dimethyl)chromium (Bis(p-xylene)chromium) (Cr(1,4-Me2C6H4)2
Bis(p-xylene)chromium has ben used as precrusor for the growth of chromium carbide coatings by CVD at 300-600°C. [916]
Bis(dimethylbenzene)chromium Cr(Me2C6H4)2 was reported to produce at 400-700°C chromium films with Cr23C6 and C contamination (carbon content was 0.5-10 % by weight). [917]
Bis(trimethylbenzene)chromium Cr(Me3C6H3)2
Cr(Me3C6H3)2 was applied as CVD precursor for the growth of Cr metal films containing admixture of Cr23C6 and C (carbon content 0.5-10 % (weight) at 400-700°C temperatures. [917]
Bis(ethylbenzene)chromium Cr(EtC6H5)2
Bis(ethylbenzene)chromium Cr(EtC6H5)2 has a sandiwch structure in which a Cr atom (oxidation state 0) is in between two η6-coordinated ethylbenzene ligands. The complex is air-sensitive, similar to other bis(arene)metal complexes. The melting point estimated by Travkin et al [920] is 322°C, while decomposition is starting at 330°C. The vapor pressure equation in the temperature range 150-250°C was estimated to be logP(Torr) = -4039/T (K) + 9.66. This vapor pressure curve is close to that determined by Devyatykh et al.[921] : logP(Torr) = -3610/T (K) + 8.81.
The structure and bondlengths of Cr(EtC6H5)2 was estimated by software Cerius [928]: the ring-ring distance is 322 pm, which is corresponding van-der.Waals distance between two π-systems, the C-C distance is 142 pm, in agreement with the literature data [[i]]. Low temperature XRD and electron diffraction studies reveal a D6h symmetry structure with identical bond lengths.
Bis(ethylbenzene)chromium preparation methods are:
1. Fischer-Hafner synthesis: CrCl3 + Al + AlC3 + 2 EtC6H5 , followed by dissolution in THF and precipitation from reaction micture by Et2O. After decomposition by methanol and water and reduction with Na2S2O4 in alkaline medium Cr(EtC6H5)2 is obtained (maximum yield 60%)
2.) Co-condesation of chromium vapor with ethylbenzene on cold chamber walls (Cr evaporated by electron beam at 10-5 Torr pressure): Cr + EtC6H5 → Cr(EtC6H5)2 . This is very energy-consuming process, but is suitable for the preparation of extremely pure bis(ethylbenzene)chromium on laboratory scale.
Thermal decomposition of Cr(EtC6H5)2in the temperature range 300-500°C was investigated. Main decomposition products were C6H6, EtC6H5, MeEtC6H4, Et2C6H4, Et3C6H3 as well as H2 and CH4. It was deterrmined that introduction of ethyl substituent to the benzene ring has only small influence on the reactivity of bis(arene) precursors. [918, 919]
[i] C. Elschenbroich, A. Salzer, Organometallchemie, B.G. Teubner, Stutgart, 1986, p.499
Cr(EtC6H5)2 for Cr metal CVD
Cr(EtC6H5)2 was applied for preparation of Cr metal by OMCVD with addition of chlorinated hydrocarbons (optimum 5%) catalysing thermal decomposition of the Cr(EtC6H5)2 and reducing carbon contamination in the deposited films [923]. Carbon content in the grown layers could be as well increased from 3.5 to 9 % by addition of aromatic hydrocarbons like benzene or ethylbenzene; it was determined that C contamination was coming not from the additives, but from Cr(EtC6H5)2 precursor itself. [[i]]
Reaction mechanism and kinetics of chromium deposition from Cr(EtC6H5)2 was studied. The activation energy of Cr deposition at 300-400°C was found to be 145 kJ/mol. The reactor rate constant was found to be k = 2.3·104·exp(-34600/RT) [923]. Devyatykh et al. [921] calculated activation energy of 186 kJ/mol at temperatures <400°C. 3 temperature-dependent regions were distinguished for chromium carbide deposition from bis(arene) compounds at 1 mbar process pressure [929]; maximum growth rate 150-180 µm/h was achieved.
Chromium films contaminated with Cr23C6 and C (0.7-6 wt.% C).were grown from bis(ethylbenzene)chromium by CVD at 400-700°C [917]
Carbon impurity in the Cr layers grown at 480°C using Cr(EtC6H5)2 on steels increases cohesions trength and plasticity of the layers, but decreases hardness. In order to obtain Cr layers with high hardness and good adhesion, carbon content was reduced by controlled addition of chlorohydrocarbons [926]
Pure chromium films (carbon contamination 0.1 wt. %) were grown from Cr(EtC6H5)2 by CVD at 300-475°C; carbon content in the film was reduced by addition of inhibitors in the gas phase [925]
The adhesion od chromium layers produced from bis(arene)cpomounds by MOCVD at 425°C, on various substrates (Al, Cu, Ni, steel (X12CrNi18-10), Ti, Nb, Zr, Mo, Si, W) was studied. It was observed that on Ti, Nb, Zr, Mo substrates having similar to chromium thermal expansion coefficient (α ~6.1-6.6·10-6 K-1) the deposited chromium films have lower internal strains, so film deformation happens only at thicknesses higher than 25µm. On the other substrates such as steel, Al, Ni and Cu having significantly higher thermal expansion coefficient (α > 12·10-6 K-1), deformations (caused by compressive strain on cooldown) were appearing already at 15µm thickness, whch later led to film peel off. [[ii]]
[i] M.R. Leonov, G.V. Solov’eva, T.V. Bukina, Zhurnal Prikladnoi Khimii, 52(8), 1848-1853, 1979
[ii] V.A. Kostenkov et al., Fiz. Khim. Obrab. Mater., 2: 109-113, 1979
Cr(EtC6H5)2 for Cr carbides CVD
Bis(ethylbenzene)chromium has been applied for the growth of Cr7C3 coatings on graphite at 400-550°C. It was concluded that carbon is originating not from the aliphatic substituent, but from the secondary aromatic hydrocarbons (produced via dehydration) which are adsorbing on the surface. [927] Before decomposition of the metallorganyl, the side chains are decomposing [918] The activation energy of the chromium carbides growth from bis(ethylbenzene)chromium is ~150 kJ/mol; at 450°C deposition temperature and reduced pressure, several carbide phases are expected to be obtained.[928]
Cr-C non-porous shiny coatings have been prepared by MOCVD at 430–470°C temperature and 0.1–3 Torr pressure. aver. Films were amorphous by XRD, having high hardness (1800–2000 HVN), good adhesion and corrosion- and wear resistance. Average grwoth rates were 0.5–1.5 µm/min; maximum growth rate 150-180 µm/h was achieved.[929]
The mechanism of CVD growth of chromium carbides from bis(ethylbenzene)chromium at 300-500°C was studied [[i], 919].
Thermal expansion coefficient of Cr3C2 layers is 10.3·10-6 1/K leading to compressive strain in the films when steels are used as substrates. From one side, strains increase the hardness of the coating, from the other side, they can worsen coating adhesion..[928]
[i] a) G. A. Razuvarev, G. G. Petukhov and A. N. Artemov, Zh. Obshch. Khim., 39 (1969) 2494.
b) Y. M. Kolesko, B. S. Reznikov and E. A. Utkina, Lw. Akad. Nauk SSSR, Neorg. Mater., 15, (1979) 782.
(Ethylbenzene)(diethylbenzene)chromium
Cr(EtC6H5) (Et2C6H4)
The kinetics of MOCVD using (ethylbenzene)(diethylbenzene)chromium Cr(EtC6H5) (Et2C6H4) at 280-400°C temperatures was investigated in [924]. Precursor decomposition mechanism at 280-400°C was studied in [930]
Bis(isopropylbenzene)chromium (Dicumenechromium) Cr(η6-iPrC6H5)2 (Cr(cumene)2)
Decomposition of dicumenechromium has been studied and compared to decomposition of bis(ethylbenzene)chromium. It was determined that in case of change of alkyl substituent of the aromatic ring, to iPr the decompositon products are significantly changing comppared to Cr(EtC6H5)2: besides benzene and toluene, methylisopropylbezene and ethylisopropylbenzene have been detected. [928]
Cr(η6-iPrC6H5)2 for Cr metal CVD
Metallic Cr films have been obtained by decomposition of dicumene chromium Cr(η6-iPrC6H5)2 at 320–545°C. However, some carbon or hydrogen inclusion in the deposit was observed [1, 931] For example, for the coatings growth at 325-400°C carbon content 11% (wt.) was reported []
The growth of metal chromium films by MOCVD at 300-325°C was reported in [933].
Chromium films containing carbon admixture were grown by MOCVD using dicumene chromium at 350-520°C [934, 935] Chromium films supersatureted with carbon (7.5-12%) were obtained in [936]
Cr(η6-iPrC6H5)2 for Cr carbide CVD
Cr(cumene)2 formed in a coldwall CVD reactor chromium carbide deposits between 350 – 520 °C, deposition rate was 4.5 – 6.5 nm/min (reaching maximum at 450°C), films having resistivity 100 μΩcm were grown at 550 °C. [Prof. Lang, lectures Chemnitz]
Dicumene chromium has been applied for the growth of chromium carbide films by hot-wall low pressure CVD at 300 - 550°C. Low growth temperature range (300-500 °C) resulted in amorphous chromium carbide layers, whereas above 500 °C a textured crystalline Cr7C3 phase was obtained. Carbon excess of ~30% compared with the Cr7C3 stoichiometry was independent of the growth temperature; it was related to ~30%-40% of free C according to ESCA ( for both amorphous and crystalline coatings). As-deposited coatings had a constant carbon composition (13 wt.%) over the wide temperature range 300—550 °C [937]
Chromium carbide coatings were prepared on SAE 4135 steel substrates by hot-wall CVD at 300-550°C, a low temperature amorphous chromium carbide buffer to improve layer adhesion was used. Films grown at 510 °C were crystalline and exhibited strong internal compressive stresses whereas in the substrate tensile stresses were found. [938]
Several chromium carbide phases were grown by MOCVD at temperatures 300-500°C, close to 475°C only chromium was obtained; between 450-700°C in addition to chromium and carbon, a Cr23C6 carbide phase was present. [948]
Bis(naphthalene)chromium Cr(C6H5C6H5)2
Bis(naphthalene)chromium Cr(C6H5C6H5)2 was applied as CVD precursor for the growth of chromium films at 400-700°C; they were contaminated with Cr23C6 and C (0.5-10 wt.% C). [917]