Day 2, Tuesday, March 25, 2008

SYMPOSIUM X

Organic Single Crystal Field-Effect Transistors

Alberto Morpurgo of the Kavli Institute of NanoScience, TU Delft, Netherlands, gave an excellent in-depth discussion on the current key issues that have slowed down progress in advancing organic electronics technology. He emphasized that even though organic electronics are much slower than Si based devices, they still have a wide field of applications where organics can compete very well where their traits such as extreme thinness and lightweight as well as low cost and capability for very large scale production is important and speed secondary. This feeds right into the energy forum topics where Prof. Martin Green and others emphasized that low cost solar technology is of utmost importance for rapid world wide implementation. Other commercial applications that are already being exploited are OLEDs, ultra-light and thin LCDs (Sony) and electronic paper (Lucent). Over the last 20 years, impressive progress in the quality of organic field-effect transistors (FETs) based on conjugated polymers and molecules has been made at a Moore’s law-like pace especially with respect to improvements in charge carrier mobility. It started in the mid eighties with mobilities as low as 10^-5 cm²/Vs and increased by more than five orders of magnitude to now 20 cm²/Vs. This progress was achieved by intensive materials development and purification and has reached a temporary optimum plateau. “Still missing”, emphasized Prof. Morpurgo, “is a good understanding on how organic thin film transistors operate." The materials quality is there, the complete device physics understanding is still lacking. This is best illustrated by the fact that identically prepared devices exhibit very different properties, often orders of magnitude different, without apparent reason. Key issues are charge carrier mobility and contact resistance. In other words, reproducibility is very low.

Progress has been made in two critical areas: The dielectric (gate) to organic channel interface and the interface to the source and drain metal contacts. A dependence was found of the carrier mobility to the relative dielectric constant of the gate oxide such that a low dielectric constant led to high mobilities and cross over from metallic to insulating like temperature behavior. The idea was borne that the dielectric changes the polarization cloud near the organic channel/dielectric interface. Higher polarisability means that when a charge carrier passes close to the dielectric, it induces stronger polarization of the opposite charge, which in turn slows down the charge carrier (Frölich Polarons) and was validated by theory. Therefore, the ideal gate would be vacuum, which can be established by recessing and placing the organic across the gap in form of a bridge.

The other key issue Prof. Morpurgo discussed in detail was contact resistance inconsistencies. Since the Schottky theory does not apply to OFETs, a concept needs to be developed to elucidate the underlying physics. Until recently, it was thought that a noble metal such as gold would be best, because it does not oxidize and therefore cannot form an insulating surface layer that could give rise to high contact resistance. It was found however that the contact resistance of gold contacts is very inconsistent, with values varying by three orders of magnitude for no apparent reason. Therefore, other less noble metals were investigated and it was found that nickel is most consistent and with the lowest contact resistance found to date of 100 Ωcm, two to four orders of magnitude lower than that of gold! In the case of Co, Ni, and Cu, quantitative analysis of the contact resistance in terms of Schotty diodes was now possible. This beginning understanding of the underlying physics of operation for OFETs will help to continue further rapid progress on the road to low cost and high performance organic electronics.

TECHNICAL TALKS

Symposium C: Advances in GaN, GaAs, SiC, and Related Alloys on Silicon Substrates

Jeffrey Laroche from Raytheon in symposium C presented the first demonstration of high performance InP-based HBT fabricated on a silicon wafer, opening the path to Si CMOS and III-V microelectronics/ optoelectronics functions hybrid integration. The HBTs demonstrated a peak current gain cutoff frequency ft of 170GHz at a nominal collector current density of 2mA/ìm². The starting material is based on a unique Germanium on oxide on silicon template wafer invented at MIT and fabricated by Soitec.

In a separate talk, Fabrice Letertre described the Soitec approach to combining wafer bonding and layer transfer via the use of ion implantation. Their process allows multiple high quality transfers of thin layers, from a single crystal donor wafer onto another substrate of a different nature, allowing the integration of dissimilar materials. This unique wafer technology enables placement of CS devices in arbitrary locations on the chip, while maintaining co-planarity with the CMOS for simple, high yield, monolithic integration.

Symposium F: Materials Science and Technology for Nonvolatile Memories
Symposium I: Synthesis and Metrology of Nanoscale Oxides and Thin Films

Many of the talks in Symposia I and F today focused on understanding and exploiting the influence of the substrate on ferroelectrics.  In symposium I, C. A. Vaz of Yale University presented results on the effect of a BiTiO₃ (BTO) substrate on Fe₃O₄.  Grown at the remarkably large strain of 5.2%, a sizable variation is seen in the magnetic anisotropy of the Fe₃O₄ as the BTO undergoes phase transformations with decreasing temperature.  Since it is a piezoelectric, controlling the BTO phase by an electric field holds the promise of accessing magnetoelectric coupling in this system. 

Similar topics arose in symposium F where Ramamoorthy Ramesh, UC Berkeley, presented an overview of work on BiFeO₃, a candidate ferroelectric to replace lead-based PZT.  It shows great promise for use in non-volatile memory applications, with larger polarizations than our current workhorse ferroelectrics (PZT and SBT), very low fatigue and minimal imprint on switching.  The polar domain retention at high temperatures is excellent, showing no dramatic loss of domain orientation after 50 hours at 500° C.  Ferroelectric behavior is even seen in films on the order of a few unit cells thick, but Ramesh warned that such results should be taken with a grain of salt given the great difficulty of quantitative measurement of order parameters on these scales.  He identified the main challenges in the field as understanding imprint, fatigue and leakage before BFO can be technologically viable, but confessed to being himself distracted by the tantalizing prospect of multiferroic coupling in this material. 

Another approach to achieving this coupling is the use of multiphase heterostructures, as shown by A. Gupta, Unversity of Alabama, in symposium I. In this case, however, he sought to remove or decrease the influence of the substrate by using arrays of MgO nanowires, which exhibit minimal clamping to the substrate.  The nanowires are synthesized via a CVD process in which gold nanodots are first patterned onto an MgO substrate to seed the growth of vertical MgO nanowires.  The wires are cubic rods rather than columns and grow in a dense forest, pushing the gold dots up off the substrate as they grow.  Gupta also demonstrated manipulation of single wires, placing them across electrodes with a microprobe to measure their individual electric and magnetoelectric properties. 

Also in symposium I, Jason Hoffman, Yale University, described efforts to tune the magnetotransport properties in LSMO via electric fields.  Interfaces were again the name of the game and the LSMO was grown on ferroelectric PZT to exploit the interface polarization, which either depletes or concentrates carriers in the LSMO.  In so doing, he observes a shift of the metal/insulator transition Tc in LSMO on the order of 35 K. In the same session, Franklin Wong presented another take on the metal insulator transition, this time in LaTiO₃ under epitaxial strain.  He presented an elegant model for the effect using simple chemical bonding arguments to explain the bandwidth dependence on tetragonal distortion. 

Symposium I also enjoyed a second invited talk by Ramesh where he delved into the multiferroic properties of BiFeO₃ to which he had alluded in his earlier talk. Ramesh emphasized that there is a real need for deterministic control of magnetism with electric fields in order to realize functional multiferroics.  Here, the focus was on exchange-bias coupling at the interface of BFO and LSMO.  He presented compelling evidence for the existence of exchange bias in this system and demonstrated its dependence on temperature and film thickness, as well as a shift in the hysteresis with varying field cooling.  Though these effects are only seen at low temperatures, below 100 K, Ramesh stressed that there is still a lot to learn from this system and that it has technological potential.  He ended by defining the central questions remaining for BFO, namely, understanding the effect of strain and defects in the films, developing methods for probing the coupling, and investigating the interface electronic structure.

Symposium Z: Materials Structures--The Nabarro Legacy

It is hard to overstate the impact of Prof. Frank Nabarro’s contributions to modern-day materials science and engineering. His seminal research on dislocation theory has had a profound impact on numerous present-day materials and processes. The Peierls-Nabarro force on moving dislocations and the Herring-Nabarro creep theory will always keep his name in materials science textbooks for generations to come. He has made major contributions on the elastic theory of dislocations, theory of work hardening, effect of elastic energy on the shape of precipitates, diffusional creep, mechanism of rafting in superalloys, and, recently, dislocation patterning. This symposium was designed to honor Nabarro's legacy in the field and included an all-day series of invited talks by experts in various disparate areas touched by his discoveries. A poster session was also held in the evening.

Dislocations and Plasticity in Small Scale Metallic Structures Deformed in Tension
Gerhard Dehm (Montanuniversität Leoben, Leoben, Austria) in his presentation described dislocations and plasticity in the deformation of very small metallic structures. This is hardly an academic exercise since such metallic structures are found in various applications, for example in microelectronics and MEMS devices. Dehm first described straining of polycrystalline copper wires using a specially designed set-up including tweezers. In situ tensile testing of wires with diameters between 50µm and 0.5µm and of films with thicknesses down to 40 nm was carried out. He then discussed size effects in micro-compression testing indicating dislocation pileup at the flat punch interface, particularly at small aspect ratios, resulting in a significant increase in flow stress. This likely contributes to the size effect observed. Finally, Dehm described the in situ tensile testing of 40 nm thick single crystal Au films revealing dislocation driven plasticity. However, deformation was found to be carried by partial dislocations rather then perfect dislocations, in contrast to film thicknesses above ca. 100 nm.

The Impact of F.R.N. Nabarro on the LEDS Theory of Work-hardening
Doris Kuhlmann-Wilsdorf of the University of Virginia knew Frank Nabarro well and worked with him for several years. Most of her talk was devoted to reminiscences of the circumstances leading to her working with Nabarro. She met him first in a conference just after World War II. It was clearly a difficult time to be a female German scientist and researcher at this time and Kuhlmann-Wilsdorf described how she and her work were often not taken seriously. She moved to Bristol, England, to work with Nabarro and actually stayed 3 months with Nabarro and his wife in a room sublet from them. She got married and moved to South Africa. Subsequently, Nabarro also moved to South Africa and became her Departmental Chair at the University of the Witwatersrand. He very successfully developed the Department. Kuhlmann-Wilsdorf eventually moved to the United States but continued to work in areas related to Nabarro's work.

Kuhlmann-Wilsdorf devoted the rest of her talk to describing the developmente of a Low-Energy Dislocation Structure (LEDS) theory that explains the work hardening behavior of materials. The LEDS theory asserts that while under stress, all parts of deformed materials are in mechanical equilibrium everywhere and have the lowest free energy achievable under the prevailing conditions of friction. Release of the stress then causes only minor rearrangements of the crystal defects. She indicated that the LEDS theory has already quantitatively accounted for the bulk of all important aspects of plasticity and their correlated dislocation structures, for both “planar” and “wavy glide” metals, in unidirectional strain and in fatigue, for single and poly-crystals.

Symposium FF: Molecular Motors, Nanomachines, and Active Nanostructures

The functioning of the world’s smallest rotary motor was outlined by George Oster from the University of California, Berkeley in Symposium FF. Indeed, the world’s smallest rotary motor is not an object that is nanofabricated by engineers but is ATP synthase, a protein devised by nature. The protein has two parts: the F0 part is embedded within a membrane, whereas the F1 part is soluble. The two parts are connected together by a shaft and operate according to distinct principles. Theoretical work by Oster and his colleagues along with input from experimentalists has allowed reconstruction of the approximate kinematic motion of the two segments. The protein pump is remarkably versatile and reversible. The flow of protons and sodium ions drives ATP synthesis, whereas the reverse, the driving of the ion pump by the hydrolysis motor is also possible. The key power stroke for the F1 motor involves the binding of ATP to the catalytic site, whereas the exhaust stroke involves the hydrolysis of inorganic phosphate from ATP. In the Fo motor, the flow of ions leads to the flashing of an electric field, which generates a torque to drive the motor. Oster emphasized that all motors depend on the fundamental property of asymmetry. The catalytic state, β-sheet, and shaft are asymmetric in the F1 motor, which leads to counterclockwise rotation and sequencing. Similarly, the input and output channels are asymmetrically positioned in the Fo motor.

Symposium JJ: Materials Research for Electrical Energy Storage

Symposium JJ featured invited talks by prominent researchers in the field of electrical energy storage. Andrew Gewirth from the University of Illinois at Urbana Champaign presented deliberations from a workshop on basic research needs for electrical energy storage organized by the Department of Energy. The participants at the workshop identified several principal research directions to develop electrical energy storage systems that combine safety, high storage capacity, long lifetime, and low cost. Firstly, Gewirth suggested that it is critical to probe energy storage chemistry and physics at all time and length scales and to develop novel techniques to study the dynamics and structure of these systems. Furthermore, understanding the efficacy of structure in energy storage is of great importance. Gewirth also emphasized the need to study charge transfer and transport in energy storage systems and to develop innovative electrical energy storage electrolytes based on either polymers or ionic liquids. The final principal research direction agreed upon in the workshop calls for emphasis on multiscale computation for electrical energy storage. “It is the best of times and the worst of times for electrical energy storage”, said Gewirth, adding that the primacy of electrochemistry in solving future energy needs has become clearly but the funding climate is cause for much concern.

Symposium LL: Energy Harvesting--From Fundamentals to Devices

In keeping with the strong focus on energy at the Spring meeting, Leif Hammarstrom from Uppsala University in Sweden discussed photobiological approaches to the production of energy from sunlight and water. Delivering an invited lecture in Symposium LL, Hammarstrom emphasized that photovoltaics alone does not constitute a solution to the energy crisis, a means of storing energy such as hydrogen is required. Indeed, the European energy roadmap calls for a shift to a hydrogen-oriented economy by the year 2050. The speaker discussed several approaches for hydrogen production based broadly on mimicking or harnessing photosynthetic processes. The first set of approaches involves the production of hydrogen by heterocyst cyanobacteria in an ambient oxygen atmosphere. The genetic modification of these bacteria by the introduction of foreign hydrogenases appears to be a promising approach to improve efficiency.

Hammarstrom stressed that photosynthesis as a whole is not a very efficient process but each of the primary steps is very efficient. Thus, an alternative approach involves the use of biomimetic complexes that are optimized for single-step energy conversion. The speaker and his colleagues have designed several such complexes based on manganese centers with tyrosine-like ligands to oxidize water. These complexes are additionally equipped with ruthenium-based senisitizers to harvest light. These bio-inspired synthetic reaction centers offer an interesting alternative to conventional heterogenous catalysts.

POSTER AWARDS

	F3.6 Polarization of Strained SrTiO3 Films Grown on Si (001) and the Ferroelectric FET Fred Walker1, Yaron Segal1, Jim Reiner1, Zhan Zhang2 and Charles Ahn1; 1Applied Physics and Center for Research on Interface Structures and Phenomena, Yale University, New Haven, Connecticut; 2Advanced Photon Source, Argonne National Laboratory, Argonne,, Illinois.
	G3.15 Formation of Sub 20 nm Cluster of Ge 2 Sb 2 Te 5 in Ge 2 Sb 2 Te 5 -TiO 2 Alloy for the Reduction of Reset Current in Phase Change Random Access Memory Dongbok Lee, Sung-Soo Yim, Min-Ho Kwon, Dongmin Kang, Hyun-Goo Jun, Sung-Wook Nam and Ki-Bum Kim; Department of Materials Science and Engineering, Seoul National Univ., Seoul, South Korea.
	O4.13 Ferromagnetic Semiconducting Fe1-xCoxSi Alloy Nanowires: Building Blocks for Silicon Based Spintronics Andrew L Schmitt, Jeremy M Higgins and Song Jin; Chemistry, University of Wisconsin-Madison, Madison, Wisconsin.

GOVERNMENT FUNDING SEMINARS

NSF Emphasizes Broadening Participation and International Activities in Materials Research
The National Science Foundation invests more than half a billion dollars annually in materials research and education. The Division of Materials Research (DMR), directed by Zayka Kafafi, invested $257.3 million in fiscal year (FY) 2007, and is expected to invest $260.2 million in FY 2008 and $324.6 million in FY 2009 in supporting people and their ideas, research groups and centers, enabling tools and instrumentation, and national facilities. As materials research and education are becoming increasingly global, Kafafi, in her presentation during the NSF funding seminar, focused on international programs as well as on broadening participation.

One of the goals of DMR is to mentor young investigators in order to increase their number of grant proposals as well as the quality of the proposals. In this way, young investigators will be able to turn in proposals that are more competitive with those of established researchers. Furthermore, DMR launched a pilot ACI [American Competitiveness Initiative] fellows program in 2008. DMR is also holding workshops in 2008 with the purpose of broadening the diversity of participants, including workshops on gender equity and scientists with disabilities. In regards to international collaborations, DMR has numerous Materials World Networks established with central and south America and western Europe, but little participation with Africa and Asia. This year, the division is pursuing more collaborations with China and Japan. Kafafi also described new research-funding opportunities and directions within DMR, including cyber-enabled innovation and discovery in materials research and education, transformative and innovative materials research, materials research at the interface between the physical and life sciences, biologically inspired materials, and green materials for the sustainability and advancement of humankind.

The website for all NSF DMR related information is www.nsf.gov/materials.

New Directions and Vision for Funding at the Department of Energy
Harriet Kung, Director of the Division of Materials Science and Engineering, Basic Energy Sciences at the Department of Energy gave an overview of the funding opportunities available in her agency. She gave a broad introduction to the structure of the organization, the major research themes and the personnel, meaning program directors, responsible for specific areas of research. She showed the research portfolio of interest. She then described the overall process and the methodology used to fund research. Detailed information is available at www.science.doe.gov/grants.

However, the FY08 budget for the organization saw severe reductions, significantly affecting the ability of DMS&E to fund proposals and affecting every aspect of the program. In fact, of the 700 proposals submitted for a 2007 solicitation, only 40 could be awarded. This is a major loss of momentum and opportunites. However, the picture appears to be brighter for 2009. A fairly significant increase is being proposed. Kung described a February 2003 report and 12 subsequent workshops that have identified clear vision and challenges in specific areas. In addition, a grand challenge subcommittee report released in January of 2008 has identified five major grand challenges. The whole field has to move from an observation science (20th century) to a control and manipulation science (21st century) said Kung. She directed people to visit the organization websites for details and also to sign up for e-mail alerts. She concluded by saying the "The community needs to make the case for science and its benefits to the nation, congress and the public. Funding is not an entitlement."

© Materials Research Society, 2008