Photoluminescence spectroscopy of CVD diamond films

Abstract

Photoluminescence spectra from diamond films deposited from CHO and ArCHO gas mixtures were measured using the 488 nm excitation wavelength of an Ar laser in the energy range 1.6–2.5 eV (7587-328 cm −1). With this excitation energy only a fraction of the diamond band gap of 5.45 eV is detectable. However, our results suggest that even with this limitation, fundamental information about the defect structure, for example the incorporation of non-carbon atoms into the diamond films, can be obtained. The luminescence spectra are dominated by peaks from Raman excitations at 512 cm −1 (Si substrate), 1332 cm −1 (diamond), 1360–1600 cm −1 (graphite), 2170 cm −1 (second-order peak from graphite) and 6952 cm −1 (1.68 eV, luminescence peak). The 1.68 eV peak is related to the incorporation of silicon (from the quartz wall of our CVD reactor) into the diamond films. The peak intensity for films prepared under different gas compositions correlates well with plasma characterization by optical emission spectroscopy (OES) showing the existence of silicon in the plasma during diamond synthesis of which the concentration increases with increasing concentration of atomic hydrogen. As a consequence we find a correlation of the 1.68 eV peak in the luminescence spectrum with the concentration of atomic hydrogen in the plasma gas (determined using OES). A further correlation was detected between the broad luminescence emission and the quality of the diamond film (as determined from Raman peak intensities related to diamond and amorphous carbon). For example, for diamond films deposited from ArCH 4O z; gas mixtures the luminescence signal increases with step-like behaviour by about 30% when the molar fraction of carbon X c c+o is increased to greater than 0.5. This change reflects the change of carbon deposition from amorphous to diamond, which can be revealed by the Raman signal intensity of these species.