FORMATION OF OHMIC CONTACTS TO P-TYPE DIAMOND USING CARBIDE FORMING METALS

The measurement of the specific contact resistance, rho(C), and micros tructural analysis at the metal/diamond interface were carried out for diamond with various acceptor concentrations, N(A), in order to under stand the carrier transport mechanism at the metal/diamond interface. The rho(C) measurements were carried out for polycrystalline boron-dop ed semiconducting diamonds which were prepared by the microwave plasma chemical vapor deposition. The acceptor concentrations, estimated by the boron concentrations measured by secondary ion mass spectroscopy, ranged from 3 x 10(18) to 3 x 10(20) cm-3. Ti and Mo films, which form carbides with diamond, were deposited on the diamonds using the elect ron-beam evaporation technique. The rho(C) values were measured by the circular transmission line method before and after annealing at tempe ratures in the range of 400-600-degrees-C. The dependence of the rho(C ) values on the acceptor concentrations suggested that the dominant tr ansport mechanism was the field-emission for the diamond with NA aroun d 10(20) cm-3 and the thermionic-field-emission for the diamond with N (A) from 3 x 10(18) cm-3 to 4 x 10(19) cm-3. The rho(C) values of the Ti contacts were observed to decrease upon annealing, whereas those of the Mo contacts decreased gradually with increasing annealing tempera ture. However, the pc values of both the Ti and Mo contacts reached at the same value of approximately 1 x 10(-6) OMEGA cm2 after annealing at 600-degrees-C for the diamonds with N(A) higher than 10(20) cm-3. N ote that the rho(C) values of the Mo contact were extremely stable at high temperatures: the rho(C) values did not deteriorate after anneali ng at 600-degrees-C for more than 3 h. The thermally stable molybdenum carbide (alpha-Mo2C) and amorphous layers were observed at the Mo/dia mond interface after annealing at 600-degrees-C by cross-sectional tra nsmission electron microscopy and x-ray diffraction.