Bruce Clemens - US grants

This project explores the origin of magnetic anisotropy in ultrathin iron films deposited on single crystal tungsten substrates using ultra-high vacuum sputtering techniques. The films will be characterized in-situ during deposition and subsequent crystallization using grazing incidence X-ray scattering to monitor the crystallization process and to monitor lattice accommodation and the number and orientation of crystal dislocations generated. Magnetic anisotropy measurements will be made and correlated with microscopic structural features. A second part of the project is devoted to incorporation and study of the effects of rare earth atoms at the iron-tungsten interface. Rare earth atoms at the interface in dilute amounts have the potential to produce large magnetic anisotropy, and this effect will be studied. The dominating importance of interfacial effects in films which are only a few atomic layers thick can in principle be used to tune the magnetic anisotropy and magnetic moment of a given sample to a desired value. Since the mean free path of electrons in these multilayer media may be substantially longer than the layer thicknesses, remarkable electron transport properties, such as the giant magneto resistance effect have been observed. Many of the properties of multilayer ultrathin films are of interest for technological application, especially for data storage either magnetic or magneto optic.

The aim of the proposal is to explore the potential of multilayer structures by studying their synthesis by sputter deposition with in-situ characterization as well as measurement of their mechanical properties. Grazing incidence X-ray scattering is used to examine the surface and interfaces during deposition. The films are synthesized with alternating face-centered cubic and body-centered cubic metal pairs. These films offer the opportunity to study strengthening by finely spaced obstacles that inhibit dislocation motion and relating this strengthening to film epitaxy, film thickness and misfit strains. A strength in the program is the use of an in-situ analysis technique, which could lead to new understanding of film growth.

9408552 Clemens The purpose of this research is to investigate and optimize the strengthening mechanisms in epitaxial metal multilayers. A previous grant developed the capability for growing and structurally characterizing epitaxial (001) oriented FCC/BCC superlattices by sputter deposition. Hardnesses of these materials were investigated and enhancements nearly 300% found over the rule of mixtures. This remarkable phenomenon is the subject of this continued program. Using singly-oriented, epitaxial films, both the mismatch and relative thickness of layers are varied to deduce plastic flow mechanisms. Structural characterization involves high resolution cross-section transmission electron microscopy of as-deposited and deformed materials so that the atomic scale defects responsible for deformation can be identified. This information is incorporated into the modeling component of the program, which utilizes both continuum and atomistic approaches. %%% One result of this program will be metal films of unusually high hardness. These films could find applications in semiconductor, information storage, and coatings technologies. In addition, this program should develop fundamental understanding about plastic deformation in layered materials and ultra-thin films. ***

9700168 Wang Researchers will study spin dependent tunnelling junctions consisting of two ferromagnetic electrode layers separated by an insulating barrier layer. The growth, structure and performance of such junctions fabricated from Heusler alloys will be investigated. These devices have significant technological potential in magentic RAM, sensors and recording heads. ***

9710395 White Lithographically patterned thin film magnetic media offer the potential for extending the areal bit density of magnetic data storage media perhaps two orders of magnitude while preserving satisfactory signal-to-noise properties. Present magnetic media are formed of a continuous featureless thin film comprised of many very small independently acting single-domain grains. The size, shape, and position of the data bit is determined by the magnetic fields from the write head and each bit contains thousands of grains. As data densities get higher, and bit size smaller, the granularity of the medium has become troublesome, giving rise to unacceptable "media noise". In principle the noise could be reduced by decreasing grain size, but grains much smaller than are presently in use are unstable against thermally activated magnetization reversal. In this study, the magnetic structure and recording potential of lithographically patterned nanoparticle arrays will be studied. In such a medium, each nanoparticle is a single domain and a single data bit. The initial studies will be on epitaxial cobalt thin films, where the crystalline uniaxial anisotropy should allow the synthesis of single-domain bar shaped islands magnetized in plane and also parallel to the short axis of the bar. Such a nanoparticle array is compatible with present read-write technology. The single-domain character of isolated individual nanoparticles will be carefully examined using magnetic force microscopy and Lorentz microscopy. The stability of the magnetization pattern in an array will be determined, and the signal-to-noise characteristics of an array measured. To escape the restriction to a Cartesian or hexagonal geometry inherent in the epitaxial films, synthesis will be pursued of appropriately magnetized bar-shaped nanoparticles whose orientation on the substrate can be arbitrary (radial or circumferential on a disk, for instance). The required uniaxial anisotropy will be achieved using the anisotro pic strain relief in a bar-shaped nanoparticle, coupled with the magnetostriction of the material. The amorphous SmFe and SmCoFe family of highly magnetostrictive films will be explored for this purpose. Again, single domain character, array stability, and signal/noise characteristics will be evaluated. ***

This FRG/GOALI project is a collaborative effort between researchers at Stanford U., Lehigh U., and Applied Materials, Santa Clara, CA. The project addresses microstructural stability, micromechanics, and electrical properties of materials used in the fabrication of on-chip capacitors incorporating dielectrics such as BaxSr1-xTiO3 (BST) and PbZrxTi1-xO3 (PZT). The aim is to improve understanding of thin film micromechanics, on-chip capacitor electrical performance, and how both electrical and mechanical properties may be modified through thin film microstructural control. Emphasis is on development of a mechanistic understanding of stress relaxation, surface roughening, and film debonding processes occurring during processing of thin film electrodes and diffusion barriers for high-K capacitor applications. Degradation of interfacial contact resistivity and dielectric reliability using patterned capacitor test structures will also be studied. Thermal stress relaxation of electrodes, which affects both adhesion and roughness, will be modified by alloying electrode layers and producing two phase microstructures through internal oxidation. It is anticipated that the research will lead to new strategies for improving the reliability and processing stability of on-chip capacitors that use perovskite-structure high permittivity dielectrics. Research will be performed by a multi-disciplinary, multi-investigator team at Stanford University and Lehigh University. Students and faculty will collaborate with materials researchers and process-integration specialists at Applied Materials, Inc. Joint research activities with AMAT will include sharing of research resources and samples, joint planning of experiments and regular meetings to review progress, visits by students to Applied Materials' laboratories, and mentorship of students by AMAT personnel.
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The project addresses basic research issues in a topical area of materials science with high technological relevance. The basic knowledge and understanding gained from the research is expected to contribute to improving electronic materials performance in current and future device and circuit applications. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area. The multidisciplinary (materials science, electrical engineering) and industrially-connected nature of this FRG/GOALI program offers unique educational opportunities for students to experience a teamwork-oriented research environment from both academic and industrial perspectives. The project is co-funded by the Electronic Materials(EM) and Metals(MET) programs in DMR, the Mechanics And Structures of Materials program in ENG/CMS, and the ENG GOALI office.
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0930098
Bent

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).

Summary

Intellectual Merits: The development of sustainable energy solutions that will meet the needs of a growing world population, while reducing greenhouse gas emissions, will rely in large part on renewable energy sources. With a global radiation flux of 174,000 TW, solar energy is a renewable resource that has the potential to provide more than enough energy to power the world. However, current solar cell designs are too expensive to be adopted for large scale application. Hence, new materials and designs for photovoltaics are urgently needed. One design that is of growing interest is a nano- or microscale heterojunction design with interdigitated semiconductor layers, in which the light absorption path length can be decoupled from the carrier diffusion path to the device junction in order to maximize the device efficiency. However, the difficulty of making such nano- or microstructures using any currently available method will drive the cost of the solar cells up, likely negating any increase in efficiency. This project proposes to investigate a novel fabrication technique and materials system that will allow nanostructured or microstructured designs for solar energy conversion to be made at lower cost. Specifically, fundamental studies into a guided self-assembly process in which selected inorganic mixtures are induced to self-organize into the desired heterostructures will be conducted. A thin film of a multicomponent mixture will be deposited that will be converted during growth or through a second simple thermal or chemical process to a three-dimensional nanostructure via self-assembly. This study will investigate the choice of materials system that will form two semiconductors of opposite polarity with optimal band and interfacial properties and will explore methods to guide the assembly process into the desired photovoltaic structure. It is expected that fundamental advances to the understanding of guided self-assembly in materials science will be made. In addition, the research has significant potential for influencing not just the photovoltaic field but also a broad range of nanoscience studies.

Broader Impacts: The research promises to introduce a new approach to making photovoltaics which, if successful, could have a significant worldwide impact. Our approach has the potential to provide high-efficiency solar cells at a cost comparable to cheap window glass coating, allowing solar electricity to be competitive with that produced from coal and leading to significant reduction in greenhouse gas emissions. In addition, the proposed program will achieve broader impacts in several key ways: (1) Teaching and training; (2) Broadening participation of underrepresented groups; (3) Disseminating research results broadly to enhance science and technological understanding; (4) Outreach to the public. The proposed project will continue building upon the important foundation of training both graduate and undergraduate students carried out by the PIs. Currently many of the students in their research groups are minority or female. Moreover, the PIs propose to engage a high school teacher from local minority-serving schools on this project in the laboratory for two summers through the Summer Program of Professional Development.

Technical abstract:

This project explores the dynamics of nanomaterials, employing atomic-scale spatial resolution, femtosecond temporal resolution x-ray techniques to answer a fundamental question: How do the dynamics of nanoscale materials differ from that of bulk materials? Snapshots of nanoscale phase transitions will be captured as these processes occur, providing a new window into the ultrafast atomic-scale rearrangements that fundamentally determine the efficiency, stability, and speed of functional devices. These experiments will elucidate the rules that govern nanoscale phase transitions in real time and lead to new ways of controlling these processes at the atomic-scale. An integrated educational outreach program involving visits to a nearby middle school will be carried out, in which students will be exposed to the results of this work through demonstrations meant to motivate and develop excitement about nanotechnology and science in general.

Non-technical abstract:

This project investigates how nanosized systems transform, using the equivalent of stop-action photography to visualize the pathways that nanoscale materials follow. Novel experimental techniques enabling atomic-scale resolution, real-time measurements are used to capture the first steps in processes fundamental to the efficiency and speed of technologically relevant devices. The output of this program will be a basic understanding that will potentially enable the engineering of new materials with unique and technologically useful properties. An integrated outreach program will be carried out as part of this project, in which the results of this work will be presented in a non-technical manner to students at an economically-disadvantaged middle school, with the goal of developing student interest in science.

Non-Technical Description:
The National Nanotechnology Coordinated Infrastructure site at Stanford University, nano@stanford, promotes nanoscience and engineering by making experimental resources and the know-how to use them available to all. At the core of nano@stanford are four advanced research facilities that are open for use by any researcher, from other universities, industry, or government: the Stanford Nano Shared Facilities (SNSF), the Stanford Nanofabrication Facility (SNF), the Stanford Microchemical Analysis Facility (MAF), and the Stanford Isotope and Geochemical Measurement and Analysis Facility (SIGMA). These facilities are staffed with technical experts dedicated to supporting the progress of science and together span the full range of fabrication and characterization methods to serve the broad user community. The site welcomes all disciplines; researchers use the facilities to solve real world problems in energy, environment, medicine, and beyond. The site also hosts artists and teachers, as its mission is to train and educate, not only the researchers in the facilities, but anyone anywhere wanting to learn about experimental nanoscience and technology. nano@stanford cultivates a library of just-in-time educational materials aimed at building foundational knowledge for the newest researchers and is available to everyone everywhere. nano@stanford has developed and will expand programs in workforce development, teacher training, and K-12 outreach. Through its partners in the NNCI network, nano@stanford will continue to expand these efforts to educate beyond the classroom and beyond the lab.

Technical Description:
nano@stanford offers a comprehensive array of nanofabrication and nanocharacterization equipment and expertise, housed in facilities that encompass ~30,000 ft2 of lab space, including 16,000 ft2 of cleanrooms, 6,000 ft2 of which is low vibration. Fabrication capabilities are anchored by a full electronics device fabrication cleanroom and a nanopatterning laboratory that are supplemented by a dozen lab spaces providing specialized and flexible processing systems. Characterization capabilities encompass the full suite of tools for imaging and chemical/physical property identification of materials. nano@stanford offers advanced capabilities not normally available to the research community at large. These specialized capabilities include: MOCVD for growing crystalline films of III-V materials; Electron-Beam Lithography for wafers up to 200 mm; NanoSIMS for isotope analysis at high lateral resolution; scanning SQUID for high resolution mapping of surface magnetic fields. Experienced, technical staff support all researchers, who have used the facilities to develop and characterize advanced structures, such as photonic crystals, photodetectors, optical MEMS, inertial sensors, optical/electronic biosensors, cantilever probes, nano-FETs, new memories, batteries, and photovoltaics. nano@stanford welcomes researchers in non-traditional areas of science and engineering, such as the life sciences and medicine, earth and environmental sciences, and offers personal consultations, seed grants, fabrication and characterization services, seminars and webinars, to the nano-curious.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.