Jennifer E. Gerbi

Bonding structure in nitrogen doped ultrananocrystalline diamond

The transport properties of diamond thin films are well known to be sensitive to the sp2/sp3-bonded carbon ratio, the presence of the grain boundaries and other defects, and to the presence of various impurities. In order to clarify the roles these factors play in the conduction mechanisms of nitrogen-doped ultrananocrystalline diamond (UNCD), Raman scattering, near edge x-ray absorption fine structure (NEXAFS), soft x-ray fluorescence (SXF), and secondary ion mass spectroscopy (SIMS) measurements were performed. Transmission electron microscopy analysis of nitrogen doped UNCD has previously indicated that the films are composed of crystalline diamond nano-grains with boundaries of amorphous carbon, and NEXAFS measurements reveal that the global amount of sp2-bonded carbon in these films increases slightly with nitrogen doping. The nitrogen content is quantified with high-resolution SIMS analysis, while NEXAFS and SXF indicates that the nitrogen exists primarily in tetrahedrally coordinated sites. These m...

Low temperature growth of ultrananocrystalline diamond

Ultrananocrystalline diamond (UNCD) films were prepared by microwave plasma chemical vapor deposition using argon-rich Ar∕CH4 plasmas at substrate temperatures from ∼400 to 800°C. Different seeding processes were employed to enhance the initial nucleation density for UNCD growth to about 1011sites∕cm2. High-resolution transmission electron microscopy, near-edge x-ray absorption fine structure, visible and ultraviolet Raman spectroscopy, and scanning electron microscopy were used to study the bonding structure as a function of growth temperature. The results showed that the growth of UNCD films is much less dependent on substrate temperature than for hydrogen-based CH4∕H2 plasmas. UNCD with nearly the same nanoscale structure as those characteristic of high-temperature deposition can be grown at temperatures as low as 400°C with growth rates of about 0.2μm∕hr. The average grain size increased to about 8nm from 3 to 5nm that is characteristic of high-temperature growth, but the relative amounts of sp3 and s...

Materials science and fabrication processes for a new MEMS technology based on ultrananocrystalline diamond thin films

Most MEMS devices are currently based on silicon because of the available surface machining technology. However, Si has poor mechanical and tribological properties which makes it difficult to produce high performance Si based MEMS devices that could work reliably, particularly in harsh environments; diamond, as a superhard material with high mechanical strength, exceptional chemical inertness, outstanding thermal stability and superior tribological performance, could be an ideal material for MEMS. A key challenge for diamond MEMS is the integration of diamond films with other materials. Conventional CVD thin film deposition methods produce diamond films with large grains, high internal stress, poor intergranular adhesion and very rough surfaces, and are consequently ill-suited for MEMS applications. Diamond-like films offer an alternative, but are deposited using physical vapour deposition methods unsuitable for conformal deposition on high aspect ratio features, and generally they do not exhibit the outstanding mechanical properties of diamond. We describe a new ultrananocrystalline diamond (UNCD) film technology based on a microwave plasma technique using argon plasma chemistries that produce UNCD films with morphological and mechanical properties that are ideally suited for producing reliable MEMS devices. We have developed lithographic techniques for the fabrication of UNCD MEMS components, including cantilevers and multilevel devices, acting as precursors to micro-bearings and gears, making UNCD a promising material for the development of high performance MEMS devices. We also review the mechanical, tribological, electronic transport, chemical and biocompatibility properties of UNCD, which make this an ideal material for reliable, long endurance MEMS device use.

n-type conductivity in ultrananocrystalline diamond films

Hall effect measurements have been carried out to determine the carrier density and mobilities in ultrananocrystalline diamond films grown with added nitrogen. The results show clear n-type conductivity with very low thermal activation energy. Mobility values of 1.5cm2V−1s−1 are found for a sheet carrier concentration of 2×1017cm−2. These measurements indicate that ultrananocrystalline films grown with high nitrogen levels in the growth gas mixture can have bulk carrier concentrations of up to 1021, which is very high for diamond films. The n-type nature of this material was also confirmed by Seebeck effect measurements.

Electrical contacts to ultrananocrystalline diamond

The contact behavior of various metals on n-type nitrogen-doped ultrananocrystalline diamond (UNCD) thin films has been investigated. The influences of the following parameters on the current-voltage characteristics of the contacts are presented: (1) electronegativity and work function of various metals, (2) an oxidizing acid surface cleaning step, and (3) oxide formation at the film/contact interface. Near-ideal ohmic contacts are formed in every case, while Schottky barrier contacts prove more elusive. These results counter most work discussed to date on thin diamond films, and are discussed in the context of the unique grain-boundary conductivity mechanism of the nitrogen-doped UNCD.

Investigating the role of hydrogen in ultra-nanocrystalline diamond thin film growth

Hydrogen has long been known to be critical for the growth of high-quality microcrystalline diamond thin films as well as homoepitaxial single-crystal diamond. A hydrogen-poor growth process that results in ultra-nanocrystalline diamond thin films has also been developed, and it has been theorized that diamond growth with this gas chemistry can occur in the absence of hydrogen. This study investigates the role of hydrogen in the growth of ultra-nanocrystalline diamond thin films in two different regimes. First, we add hydrogen to the gas phase during growth, and observe that there seems to be a competitive growth process occurring between microcrystalline diamond and ultra-nanocrystalline diamond, rather than a simple increase in the grain size of ultra-nanocrystalline diamond. Second, we remove hydrogen from the plasma by changing the hydrocarbon precursor from methane to acetylene and observe that there does seem to be some sort of lower limit to the amount of hydrogen that can sustain ultra-nanocrystalline diamond growth. We speculate that this is due to the amount of hydrogen needed to stabilize the surface of the growing diamond nanocrystals.

Mechanical Properties of Ultrananocrystalline Diamond Thin Films for MEMS Applications

Microcantilever deflection and the membrane deflection experiment (MDE) were used to examine the elastic and fracture properties of ultrananocrystalline diamond (UNCD) thin films in relation to their application to microelectromechanical systems (MEMS). Freestanding microcantilevers and membranes were fabricated using standard MEMS fabrication techniques adapted to our UNCD film technology. Elastic moduli measured by both methods described above are in agreement, with the values being in the range 930 and 970 GPa with both techniques showing good reproducibility. The MDE test showed fracture strength to vary from 3.95 to 5.03 GPa when seeding was performed with ultrasonic agitation of nanosized particles.

Preparation and Electrochemical Characterization of DNA-modified Nanocrystalline Diamond Films

Nanocrystalline diamond thin films of sub-micron thickness have been covalently modified with DNA oligonucleotides. Quantitative studies of hybridization of surface-bound oligonucleotides with fluorescently tagged complementary and non-complementary oligonucleotides were performed. The results show no detectable nonspecific adsorption, with extremely good selectivity between matched and mismatched sequences. Impedance spectroscopy measurements were made of DNA-modified boron-doped nanocrystalline diamond films. The results show that exposure to non-complementary sequences induce only small changes in impedance, while complementary DNA sequences produce a pronounced decrease in impedance. The combination of high stability, selectivity, and the ability to directly detect DNA hybridization via electrical means suggest that diamond may be an ideal substrate for continuously-monitoring biological sensors.

Publisher’s Note: “n-type conductivity in ultrananocrystalline diamond films” [Appl. Phys. Lett.85, 1680 (2004)]

Oliver A. Williams, Stephane Curat, Jennifer E. Gerbi, Dieter M. Gruen, and Richard B. Jackman Citation: Appl. Phys. Lett. 85, 5106 (2004); doi: 10.1063/1.1829716 View online: http://dx.doi.org/10.1063/1.1829716 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v85/i21 Published by the AIP Publishing LLC.