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 pre cursor addition to the gas phase over a wide range, two regimes were found: in the low concentration regime the emission intensity from atomic and mol ecular hydrogen is independent of the Li concentration, thus facilitating t he use of the Li/H emission intensity ratio as a relative measure for the c oncentration of atomic Li in the plasma; in the high concentration regime t he Li addition feeds back on the absolute intensity of the atomic Balmer li nes, on the relative intensities of atomic and molecular hydrogen lines and on the activation of different Li transitions. This indicates a modificati on of the electron energy distribution and also the plasma chemistry. The i ncorporation of lithium in the deposited homoepitaxial diamond films as stu died by elastic recoil detection (ERD) measurements strongly varies with th e deposition temperature. It is favoured by low substrate temperatures. A r eduction 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 incorporat ion in the crystal lattice and not the grain boundaries. The potential role of other impurities is discussed. (C) 2000 Elsevier Science S.A. All right s reserved.