Conductive layers in diamond formed by hydrogen ion implantation and annealing

High conductivity is extremely difficult to obtain in diamond due to its wide band gap and low solubility of dopands. The goal of the investigation was to form a conductor inside HPHT synthetic diamond plates with initial high sheet resistivity rho(s) (similar to 10(12) Omega/sq) for 400 mu m thickness. We used metastable character of diamond structures relative to the graphitization of defective layers formed by 50 keV hydrogen molecular ions at high fluence Phi = (1-13) x 10(15) cm(-2) ion implantation. High temperature (HT) (500-1600 degrees C) and vacuum or high pressure (VP/HP) (3 x 10(-3)/4 x 10(9) Pa) thermal annealing were chosen to provide the annealing regimes where the graphitic carbon is the most stable phase. Sheet resistance, dropped down up to nine orders of magnitude (rho(s) similar to 10(3) Omega/sq), as well as Raman spectroscopy, and AFM measurements were used to determine electrical, optical and geometrical properties of multilayered heterostructures formed in the set of experiments. Temperature dependences of the conductivity show, that after highest fluencies and annealing temperatures the conductivity is quasimetallic and electronic system is above metal-insulator transition (MIT). At lower fluences and/or annealing temperatures the system is under MIT with the transport of charge carriers being well described by variable range hopping (VRH) mechanism with variable decay length of wave function for localized states. Two or three order of magnitude differences in the conductivity in VP and HP annealed samples are attributed with the higher dimensions of graphite nanocrystals in the case of vacuum annealing. This suggestion coincides with Raman spectra and optimum hopping length for carrier jumps in VRH model for conductivity in the buried layers. (C) 2011 Elsevier B.V. All rights reserved.