Dielectric relaxation and space charge limited transport in polycrystalline CVD diamond

The role played by grain boundaries in determining the electronic performance of polycrystalline, diamond films is investigated as a function of the film thickness. Ohmic and space charge limited transport have been evidenced by performing DC current-voltage measurements as a function of the applied voltage up to 1000 V and changing the temperature between 300 and 650 K. On the basis of a two phases material constituted by crystalline diamond grain and disordered grain boundaries, the nonlinear trend observed on thicker samples in the high voltage regime has been used to evaluate the distribution of electronic states within the forbidden band-gap of each phase. Increasing the film thickness, the amount of grain boundary materials reduces and conductivity activation energy increases from 0.3 +/- 0.1 eV of the thinner sample studied up to 1.5 +/- 0.1 eV estimated on the thicker ones. Small amplitude impedance spectroscopy analysis, aimed to infer the most relevant material parameters and the conductivity relaxation processes, have been carried out on a wide frequency and temperature range. Hopping transport among free energy barriers is the dominant transport mechanism at the grain boundary and the related distribution of conductivity relaxation times has been estimated from electric modulus spectra. The corresponding Debye-like peak decreases from 3 to 1.6 decades with the film thickness increasing, approaching the theoretical 1.14 decades value expected for a pure single crystal material. (c) 2005 Elsevier B.V. All rights reserved.