Comparative study of the metal organic chemical vapor deposition of β-coga thin films from dialkylgallium tetracarbonylcobaltate single-source precursors

The volatile heterodinuclear organometallic compounds L(CO) ₃Co-GaR₂(Do) and L(CO)₃-Co-Ga[(CH ₂)₃NR¹₂](R) (R = H, CH₃, C₂H₅, CH₂^tBu, CH ₂SiMe₃; Do = THF, NMe₃, NC₇H ₁₃; L = CO, PMe₃, PPh₃; R¹ = CH ₃; C₂H₅; 1-10 were studied as single molecule precursors for the deposition of binary Co/Ga alloy thin films by MOCVD using a horizontal hot-walled reactor in the absence of carrier gases in vacuo. The metal concentrations of the thin films were found to depend on the substrate temperature and the type of substituents at the gallium atom. Cobalt-rich films were typically deposited below 250°C. The 1:1 ratio of the metals in the precursor compounds is retained above 300-350°C. Typical growth rates were between 0.1 and 1 μm h^-1 at ∼1 Pa total pressure. The best results were obtained with the precursor compound (CO)₄Co-GaEt ₂(NMe₃) (5a). The grown films showed impurity levels of C, N, and O close to the detection limit of the used analytical methods (≤0.5 at. % by AES). The thin-film resistivities were around 150(±30) μΩ cm in these cases. At substrate temperatures below 300°C, alkyl-transfer reactions are important, which are likely to be surface driven. These processes generate volatile and thermally stable gallium alkyls GaR ₃, which pass through the reaction zone. This mechanism explains the gallium deficiency of the Co₁Ga_1-x films grown at low substrate temperatures. Other byproducts were mainly unsaturated hydrocarbons (e.g., ethene or H₂C=CHCH₂NMe₂) and nonfragmented donor ligands Do (e.g., THF, NMe₃). The films were routinely examined ex situ by SEM-EDX and AUGER electron spectroscopy. Films grown on various substrates (quartz, GaAs, silicon) were structurally characterized by X-ray diffraction showing the cubic β-CoGa as the only detectable crystalline phase in the case of GaAs(100) and quartz substrates (ca. 300°C substrate temperature). At higher deposition temperatures (350-380°C for GaAs and 400-450°C for silicon) interfacial solid-state reactions occurred to give various other phases such as α-Co _1-δGa_δ, β-CoGa, CoGa₃, CoAs, and Co₂Si.