Characteristics and origin of the 1.681 eV luminescence center in chemical-vapor-deposited diamond films

Abstract

Employing photoluminescence (PL) spectroscopy, we have carried out a detailed study of the characteristics and origin of the 1.681 eV luminescence center in chemical-vapor-deposited (CVD) diamond films. In addition to the zero-phonon line (ZPL), the fine structure of the PL emission spectrum revealed vibronics corresponding to both phonon emission and phonon absorption. Most of the vibronics can be identified with known lattice phonons in a natural diamond, although a few of the peaks appear to be associated with a local vibrational mode. The emission intensity was found to be a strong function of the CH4/H2 ratio used to grow the diamond films. Decreasing the methane concentration from 2.0% to 0.25% resulted in an increase of about four orders of magnitude in the PL intensity. It has been found that residual stress on the order of 0.5 GPa in the diamond film quenches the PL emission intensity but has no measurable effects on the peak position and half-width of the 1.681 eV ZPL. Temperature has a marked influence on the PL emission characteristics. With increasing temperature, the emission intensity decreases, the peak position shifts to lower energy, and the half-width broadens. Some of these results provide further support to the conclusion arrived at recently by some researchers that the 1.681 eV center, characteristic of ‘‘high-quality,’’ CVD-grown diamond films, is not the GR1 center present in radiation-damaged bulk diamond. The plausibility of the Si-impurity model, proposed to explain the origin of this 1.681 eV center, was critically examined.