OPTICAL AND ELECTRICAL CHARACTERIZATION OF BORON-DOPED DIAMOND FILMS

Diamond films synthesized from CH4/H-2 mixtures using microwave-assist ed plasma deposition were prepared on Si substrates under different gr owth conditions resulting in [100] and [110]/[111] mixed textured diam ond films. p-Type doping with boron was achieved by adding trimethylbo rate to the process gas at concentrations ranging from 0.05 to 500 ppm . The boron concentration in the film, as determined by secondary ion mass spectroscopy (SIMS), depended strongly on the texture of the film s. For gas phase concentrations of less than 1 ppm, the boron incorpor ation rate in [100] textured films was found to be two orders of magni tude lower than for [110]:[111] textured films. Higher boron concentra tions led to a deterioration of the [100] texture, whereas the structu re and morphology of the [110]/[111] textured films remained almost un affected even by the highest boron concentrations. The boron concentra tion of [110]/[111] textured films varied linearly with gas phase conc entrations below 50 ppm. The boron incorporation saturates for gas pha se concentrations exceeding 50 ppm, resulting in a maximum boron conce ntration of 2.1 x 10(20) cm(-3) in the diamond film. Temperature-depen dent Hall measurements were performed to obtain information on the car rier concentration and activation energy. The data for the highest dop ed samples reveal electrical properties which can be attributed to fil ms with doping levels near the metal-insulator Mott transition. Raman spectra of the more heavily doped [110]/[111] textured samples show an increasing asymmetry of the 1332 cm(-1) zone centre optical phonon li ne with increasing boron concentration, which is assigned to a Fano-ty pe interference. The shape of the phonon line can be fitted by a super position of lorentzian and Fano lineshapes which can be explained by a growth sector-dependent boron incorporation. The Fano asymmetry, incr easing in strength with increasing dopant concentration, was also foun d in the Raman spectra of homoepitaxial boron-doped C-13 diamond films which were grown on (111) natural diamond substrates. For the [110]/[ 111] textured films, strong IR absorption around 3000 cm(-1) was obser ved due to acceptor ionization, the strength of which correlates with the boron concentration.