Chromium (II) alkyls CrR2 are yellow unstable solids; more stable when bulky groups such as tBu or CH2tBu are present; can be stabilised in the form of CrR2(phosphane)2
Chromium (III) alkyls CrR3 are unstable solids (more stable, when intramolecular Cr-N bonds are present), fairly stable in adducts CrR3L3 (L = Lewis-base such as thf).
Chromium (IV) alkyls CrR4 are surprisingly stable volatile monomeric compounds (blue paramagnetic solids). They have been synthesized by methathesis of CrCl3THF)3 and alkyllithium in pentane at -78°C, or by reaction between tetrakis(alkoxychromium) and corresponding alkyllithium [941]
CrEt4 cannot be handled at room temperature because of rapid hydrogen β-elimination and is therefore unsuitable for CVD applications. [[i]] However, some of chromium (IV) alkyls where hydrogen β-elimination is hindered, such as CrMe4, CrtBu4, Cr(CH2tBu)4, are stable enough and have been applied as CVD precursors for the low-temperature (300 °C) growth of chromium carbide (f.e. Cr3C2) films. (Table)
[i] F. Maury, L. Gueroudji, C. Vahlas, Surf. Coat. Technology, vol. 86-87, Part 1, 1 December 1996, Pages 316-324 and refs. therein
Tetramethylchromium CrMe4
Tetramethylchromium CrMe4 was applied for the plasma CVD deposition of chromium carbide (thought to be Cr3C7) on tantalum substrate. CrMe4 precursor was cryogenically cooled to liquid state (at -20° C); it was supplied to the growth chamber with argon carrier gas in proportion ~ 1:100 on a volume basis (Ar flow ~600sccm); plasma was generated at 40.68 MHz frequency
Tetrakis(tert-butyl)chromium Cr(tBu)4
Tetrakis(tert-butyl)chromium Cr[C(CH3)3]4 is sufficiently volatile and thermally stable and was applied as a chromium source in OMCVD of thin films of chromium carbide in the exceptionally low temperature range 100–150 °C. The films were X-ray amorphous and the atomic ratio of carbon to chromium was in the range 1.2–1.4, even using hydrogen as carrier gas. Partial oxidation of the films was observed if stringent precautions were not used to avoid traces of oxygen and moisture. XPS revealed carbon incorporation both in carbidic form and free carbon form (about 50%); carbon contamination of films grown under dynamic vacuum was very low. It was suggested that the β-elimination mechanism initiates the decomposition but carbon incorporation may be due to side heterogeneous reactions involving carrier gas molecules. [942]
Tetrakis(neopentyl)chromium Cr(CH2tBu)4
Tetrakis(neopentyl)chromium Cr[CH2tBu]4 is dark red solid subliming at 70°C in high vacuum. It melts at 110°C ant starts to decompose at ~150°C (its heptane solution. decomposes at 150°C within 3 hours). It has been synthesized by methathesis of CrCl3(THF)3 and 0.5M tBuCH2Li in pentane at -78°C. (Table ).
Tetrakis(neopentyl)chromium Cr[CH2tBu]4 has been applied as OMCVD precursor for the growth of chromim carbide films.at low deposition temperatures of 100–150 °C. XRD: Ther prepared films were X-ray amorphous, at. ratio was C/Cr 1.2–1.4 even with H2 as carrier gas.. Carbon incorporation included both carbidic and free carbon (~50%) according to XPS; carbon contamination of the films grown under dynamic vacuum was very low. [942]
The earlier report from the same group described the growth of chromium carbide thin films from Cr[CH2tBu]4 in the higher temperature range 250–350 °C (under an inert or hydrogen atmosphere in a hot wall, low pressure chemical vapour deposition reactor). As a result of the high reactivity of this precursor towards oxygen, the films were partially oxidized; oxygen incorporation decreased on increasing the deposition temperature. Films had poor crystallinity; electron diffraction and XPS revealed that the layers consisted of a mixture of cubic chromium carbide CrC1−x phase, amorphous Cr metal, chromium oxides (mainly Cr2O3) and free carbon, whose proportions depended on the deposition conditions. [943]
As an example, amorphous films with Cr:C ratio 1.9:1 were grown at the deposition temperature 330 °C, which upon annealing at 600 °C/ 10-3 Torr for 18 h produced Cr3C2. As volatile byproducts in the thermolysis of Cr(CH2tBu)4 following species were formed: neopentane, isobutylene, propene, tetramethylhexene. The detailled decomposition mechanism is similar to that of Ti and Zr neopentyls M(CH2tBu)4 (M = Ti, Zr). [The Chemistry of Metal CVD, Lect. Prof. Dr. Heinrich Lang]
Healy et al. used even higher growth temperature range (520°C-570°C) and very low pressure (10-4 Torr) for the growth of cromium carbide thin films from tetrakis(neopentyl)chromium by OMCVD, no carrier gas was used. According to XPS, AES, XRD, SEM and ERD (elastic recoil detection), high quality films with low oxygen content, but with some residual amorphous carbon, were grown. The films grown at 570°C were crystalline with composition Cr7C3. The authors found that standard Schlenk tube and drybox techniques coupled with modest care (storage of precursor at -40°C and avoidance of light as much as possible) allowed to prepare low oxygen contaning films. [945]
The growth of amorphous chromium carbide phase from tetrakis(neopentyl)chromium by CVD has been also reported. Amorphous films deposited at 330 °C were much harder than the crystalline films grown at higher temperatures; the latter had lower resistivities. The films were mirror-bright and very smooth with no significant features observable by SEM. [958]
Chromium derivative of atraine (1-dimethyl-1-amino-2,8,9-trimethyl-2,8,9-triaza-5-aza-tricyclo[3.3.0] undecane)
Liquid chromium derivative of atraine (1-dimethyl-1-amino-2,8,9-trimethyl-2,8,9-triaza-5-aza-tricyclo[3.3.0] undecane) has been applied for the growth chromium nitride on tantalum substrates by plasma CVD. The liquid chromium atraine complex was sublimed at a temperature of 25° C and carried to the growth chamber by the argon carrier gas with the precursor / Ar volume ratio about 1:100.
A chromium-silicon atraine complex (i.e. chromium/silicon derivative of 1-dimethyl 1 amino -2,8,9-trimethyl2,8,9-triaza-5-aza-tricyclo [3.3.0] undecane was proposed as precursor for the growth of chromium silicide layers.