Chemical vapor deposition of magnetic iron-cobalt alloy thin films: Use of ammonia to stabilize growth from carbonyl precursors

In previous work, it was demonstrated that Fe_xCo_(1−x) alloy thin films with near ideal magnetic properties can be grown by chemical vapor deposition (CVD) from the precursors Fe(CO)₅ and Co₂(CO)₈; previous attempts to grow such films by CVD, using these or other precursors, had not been able to afford high saturation magnetization. However, it was found that the morphology and composition were extremely sensitive to small variations in the deposition temperature and the precursor partial pressures. In a second work, it was showed that the CVD of pure iron films from Fe(CO)₅ is subject to a self-poisoning effect in which the growth surface accumulates carbon, which causes the growth rate to decline progressively to zero. Then it was shown that the poisoning effect can be eliminated by adding a coflow of NH₃ during CVD, which does not introduce measurable quantities of nitrogen into the film. In the current work, the authors return to the compositional instabilities in Fe_xCo_(1−x) alloy growth and show that, as seen for pure Fe growth, these instabilities can be as attributed to a surface poisoning effect involving dissociative chemisorption of carbon monoxide. It was found that a coflow of ammonia, which inhibits CO adsorption, enables the growth of Fe_xCo_(1−x) films over a wide temperature window with highly reproducible morphology and stoichiometry. Alloys that were grown under the NH₃ coflow with suitable compositions (x ∼ 0.6) achieve near ideal values of the saturation magnetization.