CARBON CLUSTERING IN SI1-XCX FORMED BY ION-IMPLANTATION

Silicon-carbon alloys were formed by multiple energy implantation of C + ions in silicon and in Silicon on Sapphire (SOS), The ion fluence ra nged between 5 x 10(16) - 3 x 10(17) ions/cm(2) and the energy between 10-30 keV in order to obtain constant carbon concentration into a dep th of 100 nm. The carbon atomic fraction (x) was in the range 0.22-0.5 9 as tested by Rutherford backscattering spectrometry (RES). Thermal a nnealing of the implanted films induced a transition from amorphous to a polycrystalline structure at temperatures above 850 degrees C as de tected by Infrared spectrometry (IR) in the wavenumber range 600-900 c m(-1). The optical energy gap and the intensity of the infrared signal after annealing at 1000 degrees C depended on the film composition: t hey both increased linearly with carbon concentration reaching a maxim um at the stoichiometric composition (x = 0.5). At higher carbon conce ntration the IR intensity saturated and the optical energy gap decreas ed from the maximum value of 2.2 to 1.8 eV. The behaviour at the high carbon content has been related to the formation of graphitic clusters as detected by Raman spectroscopy.