Nitrogen-Doped Ultrananocrystalline Diamond Films Exhibit High Ambient Temperature n-type Conductivity

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The use of diamond as an electronic material has remained elusive for many years. The problem lies in the difficulty of finding a way to dope diamond so that itŐs ambient temperature conductivity and carrier mobility are sufficiently high to make diamond-based devices work at room temperature. Traditional doping with nitrogen does not work, since nitrogen forms a deep donor level 1.7 eV below the conduction band, and thus is not thermally activated at room temperature. This is due to the fact that nitrogen is very reluctant to insert into the diamond lattice, and all efforts to dope microcrystalline diamond with electrically active nitrogen have to date met with very limited success.

Researchers at Argonne National Laboratory (ANL) may have found a unique way around this problem. The ANL team has worked for several years developing the use of microwave plasma enhanced chemical vapor deposition (MPCVD) to produce ultrananocrystalline diamond (UNCD) thin films. These films are grown using argon-rich plasmas rather than the traditional hydrogen-rich plasmas, which are routinely used to grow microcrystalline diamond films. The grain size of the UNCD films ranges between 2 and 5 nm, and the grain boundaries are almost atomically abrupt (~0.5 nm). The ANL team has demonstrated that UNCD films exhibit exceptional field electron emission, electrochemical, mechanical, and tribological properties, the latter particularly applicable to the development of a new microelectromechanical system (MEMS) technology based on UNCD. When nitrogen gas is added to the normal Ar/CH4 gas mixture used to grow undoped UNCD, the conductivity of the films increases by roughly five orders of magnitude, as reported by the ANL team in the September 3 issue of Applied Physics Letters

The films were grown on Si and SiO2 using gas mixtures of Ar/CH4(1%)/N2(1-20%) at total pressures of 100 Torr and 800 W of microwave power, while the substrates were maintained at 800ˇC. The number densities of the C2 and CN radicals formed in the plasma increase proportional to the nitrogen content up to 5% added nitrogen, as measured by absorption spectroscopy. Secondary ion mass spectroscopy (SIMS) data show that the content of nitrogen in the film saturates at 2«1020 atoms/cm3 (~0.2% total nitrogen content in the film) when the nitrogen concentration in the plasma is 5%. The conductivity at room temperature increases dramatically with nitrogen concentration, from 0.016 (1% N2) to 143 W-1cm-1 (20% N2). This is to be compared with the best values reported previously: 10-6 W-1cm-1 for nitrogen-doped microcrystalline diamond and 0.33 W-1cm-1 for phosphorous-doped microcrystalline diamond films

Temperature dependent conductivity and Hall measurements are both indicative of multiple, thermally activated conduction mechanisms with effective activation energies of <0.1 eV. This behavior is very similar to highly-boron-doped microcrystalline diamond. However, the ANL team does not feel that nitrogen is acting in the manner boron does. The researchers have proposed instead that conduction occurs via the grain boundaries and not the grains. Tight-binding molecular dynamics simulations have shown that nitrogen incorporation into the high-angle grain boundaries is favored by 3-5 eV over substitution into the bulk. Nitrogen increases the amount of three-coordinated carbon atoms in the grain boundary and leads to additional electronic states near the Fermi level. We propose that GB conduction involving carbon p-states in the GB is responsible for the high conductivities. On-going theoretical work at ANL has shown that many of these states near the Fermi-level are delocalized over several carbon nearest neighbors. The team is currently attempting to study quantitatively electronic transport mechanisms in this material. Fabrication of Ohmic and Schottky contacts has been accomplished as a prelude to the construction of UNCD MESFET devices. The group hopes to report the first results of this work at the Spring 02 MRS meeting.