Structural features of diamond layers photo-emitting at sub-band gap energies

The photoemission behaviour of a series of diamond-based polycrystalline films irradiated by the second (2.3 eV), third (3.5 eV) and fourth (4.7 eV) harmonics produced by a Q-switched-mode-locked Nd: Yag laser has been investigated and related to the structural and compositional characteristics of the layers. The materials were polycrystalline undoped diamond films as well as Nd- and N-containing diamond films grown by CVD techniques, diamond-like and amorphous carbon layers. The morphological and structural characteristics of the films were investigated by electron microscopy, Raman spectroscopy and electron diffraction. The analysis of the photoemission curves does not evidence any improvement of the emission efficiency in the case of Nd-containing films nor for the diamond films grown in the presence of N-2 The results evidence conversely a strong correlation between the characteristics of the photoemission process at sub-band gap energies and the presence of amorphous sp(2)-C patches located at the diamond film surfaces. The photoemitting properties of our samples are discussed and rationalized by considering charge emission occurring at the sp(2)-diamond-vacuum border and the emission process governed by the ratio of amorphous sp(2)-C to crystalline sp(2)-C. The rather high values of quantum efficiency measured in the course of the present research at 3.5 and 4.7 eV suggest that a proper distribution of amorphous carbon onto a good quality diamond surface is the key factor for the preparation of efficient and stable photocathode materials.