Optical and electrical characterization of boron-doped diamond films

Abstract

Diamond films synthesized from CH4H2 mixtures using microwave-assisted plasma deposition were prepared on Si substrates under different growth conditions resulting in 〈100〉 and 〈110〉/〈111〉 mixed textured diamond films. p-Type doping with boron was achieved by adding trimethylborate 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 films. For gas phase concentrations of less than 1 ppm, the boron incorporation rate in 〈100〉 textured films was found to be two orders of magnitude lower than for 〈110〉/〈111〉 textured films. Higher boron concentrations led to a deterioration of the 〈100〉 texture, whereas the structure and morphology of the 〈110〉/〈111〉 textured films remained almost unaffected even by the highest boron concentrations. The boron concentration of 〈110〉/〈111〉 textured films varied linearly with gas phase concentrations below 50 ppm. The boron incorporation saturates for gas phase concentrations exceeding 50 ppm, resulting in a maximum boron concentration of 2.1 × 1020cm−3 in the diamond film. Temperature-dependent Hall measurements were performed to obtain information on the carrier concentration and activation energy. The data for the highest doped samples reveal electrical properties which can be attributed to films 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 1332cm−1 zone centre optical phonon line with increasing boron concentration, which is assigned to a Fano-type interference. The shape of the phonon line can be fitted by a superposition of lorentzian and Fano lineshapes which can be explained by a growth sectordependent boron incorporation. The Fano asymmetry, increasing in strength with increasing dopant concentration, was also found in the Raman spectra of homoepitaxial boron-doped 13C diamond films which were grown on (111) natural diamond substrates. For the 〈110〉/〈111〉 textured films, strong IR absorption around 3000 cm−1 was observed due to acceptor ionization, the strength of which correlates with the boron concentration.