Control over many of the unique properties of ultrananocrystalline diamond (UNCD) has been achieved by the modification of the microwave plasma chemical vapor deposition (MPCVD) process. UNCD thin films are synthesized using argon-rich plasmas instead of hydrogen-rich plasmas normally used to deposit microcrystalline diamond (>1 mm grain size). The use of small amounts of carbon source gases (C60, CH4, C2H2) with argon leads to the formation of C2-dimers, which are the growth species for all UNCD thin films. UNCD grown from C2 precursors consists of ultra-small (2-5 nm) grains and atomically abrupt grain boundaries. These films are superior in many ways to traditional microcrystalline diamond films: they are smooth, dense, pinhole free, and phase-pure, and can be conformally coated on a wide variety of materials and high-aspect-ratio structures.
More subtle control of the properties of UNCD films can be accomplished via the addition of supplementary gasses to the plasma (N2, H2, B2H6, PH3) and growth conditions (biasing, power). For instance, the addition of hydrogen leads to films with larger grains with columnar morphologies and are highly insulating. The addition of nitrogen, however, yields films that are much more electrically conductive than UNCD made with pure CH4/Ar plasmas. The added nitrogen leads to the formation of CN in addition to C2 in the plasma. The presence of CN results in decreased renucleation rates during growth, which leads to larger grains and grain boundary widths. The electrical conductivity of the nitrogen-doped UNCD films increases by five orders of magnitude (up to 143 W-1xcm-1) with increasing nitrogen content. Conductivity and Hall measurements made as a function of film temperature down to 4.2 K indicate that these films have the highest n-type conductivity and carrier concentration demonstrated to date for phase-pure diamond thin films. Further in-situ doping of UNCD will be performed with the addition of dilute diborane and phosphene to the process gasses, enabling UNCD to be doped both n- and p- type.
UNCD is finding a wide range of industrial applications: in microelctromechancial systems (MEMS), as tribo-coatings for rotating shaft pump seals, as photonic switches in optical cross-connects, as field emission cathodes, as electrochemical electrodes, and as hermetic coatings on bio-implants. With the addition of the ability to independently tailor both the film structure and electronic properties, UNCD can be optimized for these and a myriad of other individual applications. Most excitingly, the ability to electronically dope the material both n- and p- type opens the door to the next generation of novel high-speed, high-temperature, and even biocompatible electronics, produced with a technologically compatible, low-cost, scalable growth technique.
J.A. Carlisle, D.M. Gruen, L.A. Curtiss, O. Auciello; Argonne National Laboratory