A modified TDEAT (tetrakisdiethylamino titanium) based MOCVD precursor for deposition of thin amorphous TiN:Si diffusion barrier layers. The TDEAT is doped with 10 at % Si using TDMAS (trisdimethlyaminosilane); the two liquids are found to form a stable solution when mixed together. Deposition occurs via pyrolysis of the vaporised precursor and NH.sub.3 on a heated substrate surface. Experimental results show that we have modified the precursor in such a way to reduce gas phase component of the deposition when compared to the unmodified TDEAT-NH3 reaction. Deposition temperatures were the range of 250-450.degree. C. and under a range of process conditions the modified precursor shows improvements in coating conformality, a reduction in resistivity and an amorphous structure, as shown by TEM and XRD analysis. SIMS and scanning AES have shown that the film is essentially stoichiometric in Ti:N ratio and contains low levels of C (.about.0.4 at %) and trace levels of incorporated Si (0.01