Thin AlN diffusion barrier for carbon nanotubes growth over stainless steel substrate

2017 edition

Luis Fernando Pantoja Suárez, Islam Alshaikh

Nowadays, there are diverse kinds of diffusion barriers (DB) to avoid the migration (diffusion) of catalyst atoms inside the substrate during the annealing process before starting the carbon nanotubes (CNTs) growth using PECVD process. Different oxides and nitrides can be used as a DB. We have compared aluminum oxide (Al2O3) and aluminum nitride (AlN), which act very well over different substrates. These materials have a high melting point, relative low cost, but also a relatively high electrical resistance. One approach to circumvent this disadvantage is the deposition of a thin layer of material that allows the tunneling of electrons. AlN layers of 1 nm of thickness were deposited over austenitic stainless steel 304 (SS304) using DC‑pulsed sputtering process with an Al target under a N2/(N2 + Ar) gas flow ratio of 40%, and 3 Pa processing pressure at room temperature. In order to study the effectiveness of the diffusion barriers, a thin layer of catalyst material was deposited on the AlN layer. Iron layers of 1, 2 and 3 nm thickness were deposited and then annealed up to 700 °C. The nucleation of Fe over AlN/Si substrate was optimized adopting the Box-Wilson experimental design. Si substrate was used instead of SS to study the nucleation behavior because it is difficult to observe the nano-islands over SS304 with a roughness of tens of microns. We have chosen the best nucleation conditions for each catalyst layer thickness deposited on our metallic substrate. Then, we started the PECVD process to check whether the CNTs grow or not. In other words, we determined if our diffusion barrier works properly.  Scanning electron microscope (SEM) images have been obtained to study the surface morphology of the SS304-AlN-CNTs layered system. The nano-islands formation of iron was successful. Depending on the Fe layer thickness, the diameter of the nano-islands ranges from 5 nm to 30 nm, which was concluded from the diameter of the CNTs observed by SEM.