Chromium dichloride CrCl2, and chromium difluoride CrF2 have been used as CVD/ALD precursors for the growth of Cr-containing thin layers.
Chromium difluoride CrF2 is a blue-green solid with melting point 894 °C and rutil structure (coordination no.6).
CrF2 has been applied for the growth of high purity metallic chromium films at substrate temperature 950-1150°C using hydrogen carrier gas.
Chromium dichloride CrCl2
Chromium dichloride CrCl2 is a colorless hygroscopic solid with melting point 815 °C and boiling point 1120 °C; it has rutil structure in crystal (coordination no. 6).
It can be synthesiszed either by reaction of hydrogen chloride with chromium metal at 1000°C: Cr + 2 HCl → CrCl2 + H2 , or reduction of chromium trichloride by hydrogen at 600°C: CrCl3 + ½ H2 → CrCl2 + HCl
Chromium dichloride CrCl2 has been applied for the growth of pure (>99-%) chromium metal films on steel, nickel, niobium alloy, however, usually high deposition temperatures of 900-1050°C were required. Hydrogen was used as carrier gas; deposition rates of 100-200 nm/min on steel and 600-5000 nm/min on Nb alloy was achieved.
Thus, CrCl2 was applied as the reactive gas for CVD of chromium metal; partial transformation of chromium into chromium carbide on carbon-containing substrates such as tool steels was observed. The substrate composition was found to have an important influence on the nature and properties of the chromium layer, because in the early stages of deposition the reaction is controlled by the exchange of Fe from the substrate for Cr. Growth rate was increasing nearly linearly with deposition time, because H2 reduction process plays increasingly important role in the further stages of deposition. The composition, structure, thickness, hardness, roughness, corrosion resistance and tribological properties of the preapred Cr coatings were investigated [885]
Chromium diiodide CrI2
Chromium diiodide CrI2 has been used for the preparation of chromium or Thorium –containing thorium by CVD. A controlled amount of CrI2 vapor was reduced by hydrogen in the presence of the entrained ThO2 particles. Chromium powder (20-40nm diameter) with ThO2 adhered to the surface were produced; particle diameter increased to 1-5µm upon annealing in hydrogen to remove residual CrI2.[ 886]
Chromium trichloride CrCl3
Chromium trichloride CrCl3 was applied for the CVD deposition of chromium phosphide coatings using PCl3 as co-reactant and H2/Ar as carrier gas. Single phase CrP layers were obtained below 800°C irrespective of the PCl3/CrCl3 gas flow ratio, whereas for Cr3P film growth PCl3/CrCl3 ratio over 8 and temperatures above 1100°C were needed. The microhardness of CrP and Cr3P layers was 1160–1370 and 1420 kg/mm2. The oxidation of CrP and Cr3P at a temperatures above 600°C produced CrPO4 and a mixture of CrPO4 and Cr2O3, respectively. [887]
CrCl3 with n-C4H10 as co-reactant was applied for the growth of thin Cr3C2 whiskers by CVD at 900–1200°C temperatures, using H2 / Ar as carrier gas. Optimum conditions for thin whiskers growth were 1000°C temperature, C4H10/CrCl3 gas flow ratio 0.5 Pillar-like whiskers were grown above 1000°C, while whiskers obtained below 1000°C were consisting of a bundle of many submicron whiskers. As an example, 3.5 mm long, 5 μm thick Cr3C2 whiskers were prepared at 1100°C and 90 min growth time. [888]
Anhydrous CrCl3, together with anhydrous LaCl3 as co-reactant, MgCl2 as dopant and O2 as oxidant, was used as precursor for the electrochemical vapor deposition (ECVD) of Mg-doped LaCrO3 films at 1200-1300°C on porous Ca-stabilised zirconia (CSZ) substrates. Metal chloride were produced in situ by corresponding metal chlorination with HCl. LaCl3 supply had to be sufficient otherwise admixture of Cr2O3 was co-deposited. Mg content in the LaCrO3 was very low (<2%), despite excess of MgCl2 in the reactant stream. Film growth showed parabolic behaviour (with parabolic growth rate constant 8.2-11·10-11 cm2/s at 1200-1250°C), indicating that the deposition was controlled by the solid state diffusion. Similarly, Sr-doped LaCrO3 were grown on gastight YSZ substrates at 1140°C. However, the presence of Sr was not confirmed by the EDX analysis, indicating that Sr incoroporation is very low. Parabolic film growth behaviour was observed for >2µm film thicknesses on 200-600µm YSZ substrates at 1137°C, indicating that the reaction was solely determined by the solid state diffusion through the growing film; the parabolic growth rate constant was 6.5·10-11 cm2/s. [889]
CrCl3 was used as a precursor for the CVD growth of chalcogenide spinel FeCr2S4 and CoCr2S4 single crystals. Resistivity, Hall effect and thermoelectric power were measured for p-type FeCr2S4 and n-type FeCr2S4 substituted by Cu. FeCr2S4 surface microstructure was studied by interference contrast and Michelson interference microscopies. The (111) face of FeCr2S4 single crystal was covered with a group of 7.6±0.7 Ǻ steps (half-elementary steps in the <111> direction), giving evidence of spiral growth mechanism of the crystals according to the Burton-Cabrera-Frank theory. [890]
Chromium tribromide CrBr3
Chromium tribromide CrBr3 has been applied for the growth of Cr metal films by low pressure CVD at substrate temperature 100°C and reactor pressure 0.5Torr,. Together with CrBr3 as metal precursor, F2 flow was used; He was used a carrier gas. 100nm chromium film with resistance 1·10-4 Ωcm was grown. [891].
Chromium triiodide CrI3, Chromium tetraiodide CrI4, chromyl iodide CrOI2; Chromium tetrabromide CrBr4, chromyl bromide CrOBr2;
Higher chromium iodides (CrI3, CrI4) or chromyl iodide CrOI2 have been so far not reported as CVD precursors. However, Cr2O3 chemical transport have been performed by addition of CrI3 to the Cr2O3/I2 system (no transport is observed without CrI3 addition); gaseous CrI4 was considered as the transport agent for Cr2O3. (by the reaction Cr2O3 (s) + CrI4 (s) -> CrOI2 (g) + CrI4 (g).).Analogous chemical transport in the Cr2O3/Br2 system in presence of CrBr3 has been reported; CrOBr2 and CrBr4 have been considered as transport agents [[i]] The volatility of these compounds indicates their potential suitability as CVD precursors for the deposition of chromium-containing layers.
[i] K. Nocker, R. Gruehn, Zeitschrift anorg. Allgem. Chem, Vol. 620, Iss. 11, p1953–1964, 1994