Aligned carbon nanotubes were obtained at 370 degrees C in a microwave plasma-enhanced chemical vapor deposition system using hydrogen-etched Fe-Si thin films as the catalyst. Effects of the catalyst characteristics on the growth of carbon nanotubes (CNTs) have been investigated. The surface of a hydrogen-etched Fe-Si thin-film catalyst consists of particles having a core-shell structure. There may also be an amorphous smooth layer beneath the particle layer. Although it is often reported that the growth and the diameter of CNTs correlate to the size of the particles on the surface of an etched catalyst film, we found that it is the smooth layer but not the particles that is critical to the CNT growth. Depending on the characteristics of the smooth layer, different amounts of CNTs were obtained at different growth rates. Optimal characteristics of the smooth layer were determined. (C) 2008 The Electrochemical Society.