Nonuniform current distribution in metal/diamond/metal vertical structures

The role of grain boundaries (GBs) in undoped polycrystalline diamond films has been investigated by dc and ac electrical measurements in a wide temperature range. Hopping transport along GB percolating paths and field-assisted thermal ionization of trapped charges are observed at low and high electric field strength, respectively. The temperature dependence of the ac conductivity, which reduces to a universal curve according to a random free-energy barrier model, suggests that, at low field strengths, current flow is mainly confined into GB domains in a wide temperature range. Only in the high-temperature and high-field ranges, do crystalline diamond grains become involved in current transport. ©2003 American Institute of Physics.