Boron doped diamond films: Electrical and optical characterization and the effect of compensating nitrogen

Abstract

Polycrystalline diamond films were prepared by microwave plasma assisted chemical vapour deposition from CH 4H 2 mixtures. For p-type doping with boron, trimethylborate was added at concentrations between 50 ppb and 500 ppm referred to the total gas phase. The boron concentration in the film, as determined by secondary ion mass spectrometry, varied between 1.1 × 10 18 cm −3 and 2.1 × 10 20 cm −3. A linear increase in the boron content in the film was observed for boron concentrations in the reactant gas up to 10 ppm. The boron incorporation saturates for gas phase concentrations of more than 50 ppm. For non-contact characterization of the doped films, Raman and Fourier transform spectroscopy were applied. With increasing boron concentration, the Raman spectra show an increasing asymmetry in the lineshape of the 1332 cm −1 zone centre optical phonon which is attributed to a Fano interference effect with the electronic continuum. Strong IR absorption around 3000 cm −1 due to acceptor ionization was observed and related to the boron concentration. Temperature dependent electrical measurements were taken to obtain information on the resistivity, mobility, activation energy and carrier concentration. Nitrogen is an omnipresent impurity in diamond and, as a deep donor, is able to compensate acceptors. In order to study the influence of nitrogen contamination, a series of diamond films with constant boron content and various nitrogen concentrations was prepared and characterized. The data reveal the strong influence of nitrogen on the properties of boron-doped diamond films. In particular, the conductivity is decreased by one order of magnitude, highlighting the compensating character of nitrogen in p-type doped diamond.