Defects density and carrier lifetime in nitrogen-doped ultrananocrystalline and polycrystalline diamond films

Abstract

The electrical activity of nitrogen related defects are investigated in ultra-nanocrystalline diamond (UNCD) films achieved using different N2% in the gas phase by transient photocurrent technique at λ = 193 nm, and by steady state photocurrent measurements in the photon energy range 1-6 eV. In undoped UNCD films, spectrally resolved photocurrent measurements reveal a threshold at about 1 eV, related to the absorption of non diamond carbon phases, followed by a monotonic increase by more than one order of magnitude up to about the diamond energy gap, where a steep rise occurs due to band to band transitions. In nitrogen doped UNCD films a clear onset of the spectral photocurrent signal is hardly detectable, although an apparent shift towards higher energies is evidenced, in agreement with a possible nitrogen induced Fermi level shift upward in the band gap. The main N-related feature of the spectra is however a sharp peak at about 4 eV, which is also observed in polycrystalline diamond films grown in a nitrogen rich gas mixture, particularly close to the boundary of the deposition area. On the other hand, photocurrent pulse shape analysis gives carrier lifetime values in the 6-10 ns range, almost independent of nitrogen content. Instead, N-related defects appear mainly responsible for trapping processes, which slow down carrier transport and give rise to long transit times. Such results are discussed in terms of photoionization of N-related defects formed in the non diamond carbon phase.