Diamond nucleation experiments on Si wafers, precoated with Ni, Al, and Ni-Al duplex intermediate layers, have been conducted in a microwave plasma enhanced chemical vapor deposition reactor. The diamond nucleation density is dependent on the thickness of the single Ni interlayer and also the ratio of Ni/Al. The diamond nucleation density increases with the Ni thickness up to approximately 100 nm. Above 100 nm, decrease in the nucleation density is observed. The nondiamond carbon concentration increases when the Ni thickness increases from 40 to 200 nm, along with a simultaneous increase of nondiamond carbon accumulation on the Si substrate surface. The diamond grown on Si with an Al interlayer is of high purity but of low nucleation density. For the Ni-Al duplex interlayer, increase of Al fraction enhances both the purity and nucleation density of diamond, and markedly reduces the formation of nondiamond carbon on the Si substrate surfaces. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3490402]