BORON-DOPED DIAMOND FILMS - ELECTRICAL AND OPTICAL CHARACTERIZATION AND THE EFFECT OF COMPENSATING NITROGEN

Polycrystalline diamond films were prepared by microwave plasma assist ed chemical vapour deposition from CH4-H-2 mixtures. For p-type doping with boron, trimethylborate was added at concentrations between 50 pp b 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 x 10(18) cm(-3) and 2.1 x 10(20) cm(-3). A linear increas e in the boron content in the film was observed for boron concentratio ns in the reactant gas up to 10 ppm. The boron incorporation saturates for gas phase concentrations of more than 50 ppm. For non-contact cha racterization of the doped films, Raman and Fourier transform spectros copy were applied. With increasing boron concentration, the Raman spec tra show an increasing asymmetry in the lineshape of the 1332 cm(-1) z one centre optical phonon which is attributed to a Fano interference e ffect with the electronic continuum. Strong IR absorption around 3000 cm(-1) due to acceptor ionization was observed and related to the boro n concentration. Temperature dependent electrical measurements were ta ken to obtain information on the resistivity, mobility, activation ene rgy and carrier concentration. Nitrogen is an omnipresent impurity in diamond and, as a deep donor, is able to compensate accepters. In orde r to study the influence of nitrogen contamination, a series of diamon d films with constant boron content and various nitrogen concentration s was prepared and characterized. The data reveal the strong influence of nitrogen on the properties of boron-doped diamond films. In partic ular, the conductivity is decreased by one order of magnitude, highlig hting the compensating character of nitro en in p-type doped diamond.