Photoelectron spectroscopic investigation of the bias-enhanced nucleation of polycrystalline diamond films

In the present work we describe an investigation of the nucleation mechanism of polycrystalline diamond films if the bias-enhanced-nucleation (BEN) method is used. Photoelectron spectroscopy with excitation energies in the ultraviolet [ultraviolet photoelectron spectroscopy (UPS)] and x-ray regime (x-ray photoelectron spectroscopy) as well as electron energy loss spectroscopy are employed to monitor the nucleation process and the subsequent diamond film growth. The deposition is performed in situ, thus avoiding surface contamination with oxygen or hydrocarbons. The observation of the temporal evolution of composition and structure of the deposited film and its interface with the underlying silicon substrate allow us to develop a qualitative model, which describes the nucleation process. The BEN pretreatment leads, through the irradiation with low-energy ions, to the codeposition of an amorphous carbon phase and the crystalline diamond phase. The presence of both phases is readily apparent in the UPS analysis, which will prove to be an indispensible tool in the structural characterization of the carbon phase present at the surface. There is no indication for the presence of graphite or large graphitic clusters. A deconvolution of the C 1s and Si 2p core-level peaks does confirm the presence of two carbon phases and the formation of a silicon carbide interface. With increasing deposition time the contribution of diamond to the carbon film increases and upon switching to diamond growth conditions the amorphous carbon phase is rapidly etched and only the diamond crystals remain and continue to grow. This removal of the amorphous phase leads to a decrease in the overall carbon concentration at the surface by 18-30% during the first 30 sec of the diamond growth period and was observed for a variety of pretreatment conditions. A silicon carbide interfacial layer is formed early on during the BEN pretreatment and its thickness is reduced considerably by etching during the diamond growth period. These results are summarized and discussed in the framework of a qualitative model for the nucleation process.