Lithium addition during CVD diamond growth: influence on the optical emission of the plasma and properties of the films

Lithium-doped homoepitaxial CVD diamond films were grown on synthetic type Ib (100) single crystal diamonds by addition of lithium-t-butoxide (LiOC4H9) to the CH4/H-2 microwave plasma. Atomic lithium can easily be detected in the plasma by optical emission spectroscopy (OES). Varying the lithium precursor addition to the gas phase over a wide range, two regimes were found: in the low concentration regime the emission intensity from atomic and molecular hydrogen is independent of the Li concentration, thus facilitating the use of the Li/H emission intensity ratio as a relative measure for the concentration of atomic Li in the plasma; in the high concentration regime the Li addition feeds back on the absolute intensity of the atomic Balmer lines, on the relative intensities of atomic and molecular hydrogen lines and on the activation of different Li transitions. This indicates a modification of the electron energy distribution and also the plasma chemistry. The incorporation of lithium in the deposited homoepitaxial diamond films as studied by elastic recoil detection (ERD) measurements strongly varies with the deposition temperature. It is favoured by low substrate temperatures. A reduction from T-sub = 800 degrees C to T-sub = 620 degrees C increases the Li concentration from 8 ppm to 71 ppm. Maximum Li concentrations of 290 ppm were found. The high crystal quality of the films speaks for an incorporation in the crystal lattice and not the grain boundaries. The potential role of other impurities is discussed. (C) 2000 Elsevier Science S.A. All rights reserved.