The impact of diamond grain size on hydrogen concentration, bonding configuration, and electron emission properties of polycrystalline-diamond films

In the present work we review our recent studies of the incorporation of hydrogen and its bonding configuration in diamond films composed of diamond grains of varying size. Polycrystalline-diamond films are deposited by three different methods; hot filament (HF), microwave (MW) and direct current glow discharge (DCGD)CVD. The size of the diamond grains which constitute the films varies in the following way; hundreds of nanometers in the case of HFCVD ("submicrometer size", similar to 300 nm), tens of nanometers in the case of MWCVD (3-30nm), and a few nanometers in the case of DCGDCVD ("ultra nanocrystalline diamond", similar to 5 nm). Raman spectroscopy (RS), secondary ion mass spectroscopy (SIMS), and high-resolution electron energy loss spectroscopy (HREELS) are applied to investigate the hydrogen trapping in the films. The hydrogen retention of the diamond films increases with decreasing grain size, indicating the likelihood that hydrogen is bonded and trapped in grain boundaries, as well as on the internal grain surfaces. RS and HREELS analyses show that at least part of this hydrogen is bonded to sp(2)- and sp(3)-hybridized carbon, thus giving rise to typical C-H vibration modes. Both vibrational spectroscopies show the increase of sp(2) C-H modes in transition from sub-micrometer to ultra nanocrystalline grain size. The impact of diamond grain size on the shape of the RS and HREELS hydrogenated diamond spectra is discussed. In addition, the dependence of electron emission properties on film thickness and diamond grain size is reported.