Nanostructured Thin Films

Theme

The Nanostructured Thin Films program is focused on the synthesis, characterization, and modeling of dimensionally constrained materials systems in which a nano-scale trait of the material (e.g. grain size, film thickness, interfacial boundary, etc.) fundamentally determines its structure-property relationships. The work performed in this program falls primarily into two areas: (1) studies of thin-film growth phenomena and film properties, with emphasis on diamond and multicomponent oxides; and (2) first principles quantum-mechanical calculations that model thin film growth processes and electronic structure. Frequently, the experimental and theoretical efforts are coordinated on common scientific issues in a particular material system. Current research is devoted to (a) growth processes and structure-property relationships in doped and undoped ultrananocrystalline diamond thin films, with emphasis on understanding their morphological, mechanical, tribological, electronic, electron emission, electrochemical, and transport properties; (b) measurement of mechanical and tribological properties of diamond thin films using both conventional instrumentation and diamond-based microelectromechanical systems, and (c) growth and segregation phenomena in multicomponent oxide thin film heterostructures. Computational quantum chemical methods are used to model growth mechanisms of diamond thin films, and the electronic structure properties of nanocrystalline diamond grain boundaries, and to study other complex systems via computational chemistry.
This program is concerned with the fundamental science of nanostructured thin film and surface systems. Program activities fall into three major thrust areas:

• Ultrananocrystalline diamond (UNCD) thin films encompasses experimental studies of diamond thin film growth processes, film morphology, grain boundary geometric and electronic structure, and their relation to structural, electronic, electrochemical, and electronic and thermal transport properties. This thrust is also focused on basic material science issues related to the development of diamond microfabrication techniques and the measurement of mechanical and tribological properties of diamond thin films using both conventional instrumentation and diamond-based microelectromechanical systems. The UNCD thin films are grown using a new microwave plasma chemical vapor deposition (PECVD) technique developed in this program. The incorporation of nitrogen into the argon-methane plasma and the subsequence incorporation of nitrogen as an electrically active dopant in UNCD is of high current interest.
• Complex oxide thin films and heterostructures includes studies of oxide film-substrate interactions; segregation barrier materials; hydrogen/oxygen annealing effects and gas diffusion barrier materials. The growth of high dielectric constant oxides and interface formation in oxide thin films on silicon substrates is also of particular interest. This thrust involves the use of the time-of-flight ion scattering angle resolved spectroscopy (TOF-ISARS) technique developed by this program to monitor and control oxide thin film growth in real-time, and will also be used for in-situ tribology studies on UNCD thin films in the near future.
• Quantum chemical methods play a crucial role this program, and frequently theory and experiment are coordinated on common scientific problems in a material system. For instance, current theory work is focused on the new observation from this program that nitrogen impurities enhance the conductivity of UNCD and change the morphology. The effects of nitrogen impurities in the grain boundaries of UNCD and the mechanisms for growth of UNCD thin films when CN is present in the plasma are of particular current interest. Various state-of-the-art electronic structure methods are being used, including ab initio molecular orbital theory and density functional theory. Over the past year a tight-binding density functional molecular dynamics self-consistent charge (TB-DFT-MD-SCC) method has been added to our simulation capabilities. The electronic structure theory used in this program also utilizes new quantum chemical methods that are being developed in the Molecular Materials effort (FWP 58510).

Highlights

• Ultrananocrystalline Diamond: A New Allotrope of Carbon Ultrananocrystalline diamond (UNCD) is grown using a new plasma deposition process developed at ANL, and consists of ultra-small (2-5 nm) grains and atomically abrupt grain boundaries. UNCD films overcome most of the drawbacks of traditional, microcrystalline (1 mm grains) 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. UNCD has been found to have unique mechanical (high hardness and fracture strength), tribological (extremely low friction) transport (tunable electrical conductivity, high thermal conductivity), electrochemical (wide potential window), and electron emission (low threshold voltage) properties. In addition, UNCD-based microfabrication techniques have also been developed, allowing the development of micro- and nano-scale instrumentation for the measurement of material properties (mechanical, electronic and thermal transport) of nanostructured materials. UNCD is finding a wide range of industrial applications: in microelectromechanical 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 bioimplants. The 2000 MRS Award was presented to Dieter Gruen for the development of UNCD by Gruen and co-workers at ANL over the past ten years.
• Ultrananocrystalline diamond (UNCD) is grown using microwave plasma chemical vapor deposition (MPCVD) and consists of ultra-small (2-5 nm) grains and atomically abrupt grain boundaries. UNCD films are phase-pure, and can be conformally coated on high aspect ratio structures on a variety of materials. UNCD has been found to have many unique materials properties, several of which are tunable via the precise control of plasma chemistry.
• High-conductivity UNCD thin films have been produced via the addition of nitrogen gas to the plasma. Conductivity and Hall voltage measurements as a function of film temperature revealed that the UNCD films have the highest n-type conduction and carrier concentration demonstrated up to now for diamond films with nitrogen incorporation. We have proposed a novel grain-boundary conduction mechanism to explain this remarkable behavior.
• UNCD coatings exhibited hardness (~ 97 GP) and YoungÕs modulus (~960 Gpa) values that are very similar to single crystal diamond. Preliminary tribological tests revealed that UNCD coatings also exhibit a very low friction coefficient (~ 0.01). These results indicate that UNCD has great potential for application in microelectromechanical systems (MEMS).
• Studies of electron field emission from UNCD coated flat substrates and microtip arrays have yielded consistently low threshold fields (1Ð2 V/µm), high total emission currents (up to 10 mA), and stable emission during long-duration testing (up to 14 days). These studies have lead to a deeper understanding of field electron emission as well as potential applications of UNCD-based cold-cathode electron sources.
• UNCD electrodes exhibit a wide working potential window, a low background current, and high degree of electrochemical activity for redox systems such as Fe(CN)6-3/-4, Ru(NH3)6+3/+2, IrCl6-2/-3, and methyl viologen (MV+2/+). These results, in combination with the biocompatibility properties of UNCD, could lead to the applications of UNCD electrodes for nerve stimulation.
• In situ mass spectroscopy of recoil surface analysis and X-ray photoeloectron spectroscopy were used to study oxidation processes in amorphous and crystalline TiAl and TiAlN layers. XPS revealed the formation of TiO2 on the surface of TiAlN at 500-600oC and on TiAl at 600-700 ¡C. making the latter m ore resistant to oxidation. Electrical characterization of PZT capacitors demonstrated that the TiAL provides a good diffusion barrier layer for integration of PZT capacitors with Si substrates.
• Density-functional based tight-binding molecular dynamics calculations of high-energy high-angle twist (100) diamond grain boundaries with and without nitrogen impurities have been performed. We found that about one-half of the carbons in the grain boundary are threefold coordinated and are responsible for states introduced into the band gap. Based on these findings, we proposed that GB conduction involving carbon p-states in the GB is responsible for the high electrical conductivities in nitrogen-doped UNCD thin films.

Impact

Over the past three years over 50 publications, over 20 invited talks, and three plenary talks have resulted from this work. In addition, UNCD is finding many potential applications as cold-cathode electron sources, tribomechanical coatings, biochemical electrodes, and microelctromechancial systems (MEMS). The 2000 MRS Award was presented to D.M. Gruen for the development of UNCD at ANL over the past ten years, and the 2000 Award of Integrated Ferroelectrics was presented to O. Auciello for work on in situ studies of perovskite film growth and interface processes.
INTERACTIONS

Morales R. Karelis

CI 18089995

ESS Secc 2