Photocurrent study of electronic defects in nanocrystalline diamond

The broad optical spectral range (0.2–6 eV) photoionization spectra of the nominally undoped, carefully oxidized and hydrogenated nanocrystalline diamond (NCD) thin films grown on silicon and sapphire substrates were measured using the dual-beam photoconductivity method (under constant UV light bias). The novel amplitude modulated step scan Fourier transform photocurrent spectroscopy (AMFTPS) was applied in IR region. The wet chemistry etching was used to open 6 × 6 mm window in Si substrate and to create NCD membrane in the silicon frame. In photoionization spectra we found localized defect states with the threshold around 1 eV in both hydrogenated and oxidized NCD films. The threshold of the photoionization cross section shifts towards lower photon energies in the hydrogenated samples. The photosensitivity increases by several orders of magnitude with hydrogenation. We suggest that the main deep defect is related to the nanocrystalline grain boundaries and it can be passivated by the hydrogen. In hydrogenated samples we detect at low temperature shallow defect states with photoionization threshold energies well below 1 eV, probably related to the hydrogenated surface.