Fabrication of Conductive Graphitic Layers in Diamond by Multispecies Focused Ion Beam

Ion beam irradiation of diamond can act on the crystal structure permanently modifying the phase of the irradiated surface: once a certain level of lattice damage has been reached, subsequent thermal annealing can convert the irradiated areas from diamond to graphite structure in an irreversible way. Since the graphitic layers are electrically conductive, the effect can be exploited to create integrated or buried conductive layers. These can support electrical driving of color centers or creating electrodes with good ohmic contact, especially if the ion irradiation can be carried out in designed areas like when using a focused ion beam system. In this work, a systematic study to produce controlled layers of graphite on “electronic grade” diamonds by focused ion beam implantation is reported in order to provide protocols for quantum technology applications. Two focused ion beam systems were tested: a liquid metal alloy ion source-based one allowed to select among Au++, Ge++, and Si++ species with implant energy of 70 keV; a plasma ion source system was instead tested to exploit a 30 keV Xe+ beam. The process window was defined for each ion species in terms of ion fluence and thermal annealing protocol, identifying the carbon phases by Raman spectroscopy. Exploiting a standard selective wet chemical etching, it was possible to measure the thickness of the produced graphitic films and the diamond layers involved in the phase transformation. Due to the ion mass difference, various graphite thicknesses were achieved, ranging from 40 to 125 nm. The comparison with the damage depth distributions expected from Monte Carlo simulations allowed to identify criteria to design the desired graphitic thickness. Finally, electrical resistivity was measured resulting in a similar value for graphitic layers produced with different ion species.