Randall M. Feenstra - US grants

0079416
Hanon

Low-Energy Electron Microscopy (LEEM) is used to generate real-time images of surfaces with a lateral resolution of better than 10 nanometer. Surfaces can be imaged at arbitrarily high temperatures, and during growth. Contrast in LEEM arises because of differences in electron reflectivity at the surface, which reflect variations in the structural, chemical and magnetic properties of the surface. This award will help establish a LEEM facility at Carnegie Mellon University for use by an interdisciplinary group of researchers spanning four University departments. Proposed research projects include investigations of phase transitions at surfaces, two-dimensional coarsening and growth, step and phase boundary fluctuations, GaN growth, wetting of organic films, surface magnetism, growth at chiral surfaces, and texture development in thin film growth.

Low-Energy Electron Microscopy (LEEM) is used to generate real-time images of surfaces with a lateral resolution of better than ten nanomters, during growth, and at arbitrarily high temperatures. These unique features allow growth at surfaces to be studied in unprecedented detail. A LEEM facility will be established at Carnegie Mellon University for use by an interdisciplinary group of researchers. LEEM will be applied to a wide range of growth problems, from fundamental investigations of the chemistry and physics of surfaces, to process optimization in the development of new magnetic media.

Development of Equipment for Fabrication of Quantum Cellular Automata

D.W. Greve
ECS-0079485

Current interest in nanostructures and nanoelectronics is in part motivated by the perceived limits to scaling of current electronic devices. One critical barrier to the development of future nanoelectric technology is the development of techniques for patterning and the integration of patterning with device fabrication. In this project we will develop equipment for fabrication of one of the highly promising structures for future nanoelectronics. Quantum cellular automata are organized arrays of quantum dots which implement interconnection lines and logical operations. We will develop equipment for fabrication of quantum dot arrays which implement all the essential features of a digital electronic circuit. Quantum dot arrays will be fabricated on silicon substrates using silicon-compatible materials. The arrays will have a quantum dot size suitable for demonstration of operation at cryogenic temperatures (4-77 K) and the techniques used will have the potential for scaling to quantum dot sizes which will permit operation at room temperature. The apparatus we will develop integrates quantum dot lithography, growth of quantum dots and an overlayer, and in situ characterization facilities. This equipment will make possible development of a broad research program directed at future digital electronics. It will provide a focus for future semiconductor device and technology research, with strong potential for interactions with researchers both within and outside the university. This research will be highly interdisciplinary, and will also impact the education of undergraduate students through involvement in research projects and also through the develoment of new course materials specifically related to our nanostructure research.

This project addresses nanoscale geometric and electronic structure of semiconductor surfaces and interfaces. Particular focus is placed on the formation and properties of semiconductor alloys, including both ternary, e.g. In x Ga 1-x N, and quaternary, e.g. In x Ga 1-x As y P 1-y , materials. Incorporation of alloy constituents during growth will be studied, together with the phenomena of atomic ordering, short-range clustering, and long-range alloy compositional fluctuations (lateral composition modulation). The effect of external perturbations such as ion implantation and annealing on the compositional variations will also be examined. Heterostructures, containing layers of material with varying lattice constant, will be used to impose strain on the material. The effect of this strain on the resultant alloy composition and surface morphology will be observed. The intent of the research is to elucidate the underlying microscopic mechanisms responsible for the observed structural or compositional variations, with the aim of designing improved growth and/or processing techniques by which these mechanisms can be controlled.
The major technique to be used for these studies is scanning tunneling microscopy (STM), which provides nanoscale information on the atomic arrangement on a surface. Spectroscopic studies with the tunneling microscope will be used to probe electronic states at the surface. Studies will be performed both in a plan-view mode, probing in situ the growth surface of films, as well as in cross-sectional mode in which a heterostructure is cleaved in situ.
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The project addresses basic research issues in a topical area of materials science having high potential technological relevance. The research will contribute basic materials science knowledge at a fundamental level to new understanding and capabilities in electronic/photonic devices. A variety of fundamental issues are to be addressed in these investigations. 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.
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This project aims to advance fundamental knowledge of structures and transport properties of semiconductor surfaces, the former with respect to greater understanding and optimization of growth of GaN and other III-nitrides, in particular, and the latter as exploration of an area with potential impact on nanoscale electronic devices. The structural and electronic properties of wide-band-gap semiconductor surfaces will be studied with a combination of atomic-scale microscopic and spectroscopic probes. Bilayer-thick metallic Ga layers that naturally terminate GaN will be studied by low-energy electron diffraction (LEED) at temperatures of about 750 C. The morphology and coverage of these layers will be observed as a function of incident Ga flux. The behavior of alternate terminating layers such as indium will also be investigated. Similar experiments will be performed for AlN. The electronic states of terminating layers on GaN will also be investigated by scanning tunneling spectroscopy (STS). The surface electronic structure will be probed with STS measurements on in-situ prepared GaN surfaces, comparing clean surfaces to ones that have been exposed to oxygen or other adsorbates. High- dynamic-range STS measurements, using carefully prepared (metallic) probe-tips, will be used to observe evolution of surface state-density as a function of exposure. LEEM observation will be used to observe overall morphological effects of adsorbates. Transport properties of surface electronic bands will be studied by STS, including measurements over a wide range of tunnel currents and at temperatures from 300 K down to 7 K. The observation of spectral shifts, as a function of current, will be used to deduce transport parameters of surface bands. Spectral shifts will be compared with those expected from a 3-dimensional solution of Poisson's equation for the tip-semiconductor geometry. Inclusion of electrostatic effects in the theory arising from surface accumulation or depletion of charge due to limited transport in the semiconductor is expected to permit identification and evaluation of limiting transport mechanisms.
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The project addresses basic research issues in a topical area of materials science with high technological relevance. Experimental tools are now available which allow atomic level observation of elementary surface processes which when better understood allow advances in fundamental science and technology. Broader impact of the work lies in its application to semiconductor thin film growth (for nitrides) and for its potential in future nanoscale electronics. 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.
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Technical. This project addresses synthesis/processing of graphene; a range of studies involving variation in growth parameters and detailed characterization of film properties will be conducted to identify and understand growth mechanisms. Initially, reactive gases will be introduced as a background environment during annealing. Hydrogen is expected to perform some etching (perhaps of defects) during the annealing, whereas oxygen may act to getter impurities and/or assist in the removal of silicon from the surface. Additionally, carbon-containing gases such as propylene will be used to assess their potential for deposition of graphene on SiC. The resulting films will be characterized by atomic force microscopy, low-energy electron diffraction, and Auger electron spectroscopy, with optical and electrical characterization to be performed by collaborating researchers at Sarnoff Corp., Argonne National Lab, MIT Lincoln Labs, and Notre Dame University. Complementary studies on the detailed structure of the graphene/SiC interface will be performed by scanning tunneling microscopy and scanning tunneling spectroscopy. Modeling of those results using first-principles theory will be performed by collaborators at Ohio University.
Non-Technical. The project addresses fundamental research issues in a topical area of electronic/photonic materials science having technological relevance. Societal benefits of the proposed research are potentially very large since graphene is a strong candidate material for extending electronics capabilities beyond the limits of silicon technology. The project will also provide interdisciplinary training to graduate students in the nanotechnology field. Additionally, the PI presents annual lectures to middle and high school students on Nanotechnology, as well as teaching a course on Nanoscience and Nanotechnology. The graphene-related research of this proposal will be used to enhance the content of these lectures, providing students with an example of a current research topic in Nanotechnology. The course includes a laboratory component, and development of a laboratory exercise dealing with graphene exfoliation and observation is planned.

Technical Description: The research performed in this project aims to understand and improve the formation of graphene on the (000-1) surface of SiC (the C-face). Prior work has identified a new interface structure for graphene on the C-face, occurring when the graphene is formed in a low-pressure disilane environment. Further research of this graphene formation mechanism is performed in this project. Decoupling of the C-face interface layer from the SiC is also studied, using hydrogen or oxygen gases. The electronic properties of the graphene are characterized, in collaborative studies with other scientists. Insulating layers such as boron nitride are epitaxially deposited on the graphene, as a first step towards the formation of graphene-insulator-graphene junction. Methods are explored for further deposition of a second layer of graphene on the boron nitride.

Non-technical Description: Graphene, a two-dimensional monolayer of carbon, has been intensively studied for the past decade because of the unique transport properties of electrons in the material. For large-area production of graphene (as needed for electronic devices and circuits), a leading method is the formation on silicon carbide (SiC). This research project focuses on formation mechanisms for graphene on SiC, particularly on its C-face. The project also provides broad multi-disciplinary training to graduate students in the nanotechnology field. Additionally, the PI presents annual lectures to middle and high school students on nanotechnology, as well as teaching an undergraduate course on Nanoscience and Nanotechnology. The graphene-related research of this project is used to enhance the content of these lectures, providing the students with an apt example of a current topic in nanoscience and nanotechnology.

Non-technical Abstract
Five professors at Carnegie Mellon University will acquire a low-temperature scanning tunneling microscope (LT-STM), through the NSF MRI program. This instrument permits the mapping of atomic arrangements on surfaces, at low temperatures and under high magnetic fields. Studies will focus on "two-dimensional (2D) materials", that is, materials that are only one layer of atoms thick. In such materials, electrons are confined to move within the single atomic layer, and they thereby acquire certain novel properties that do not occur for regular, three-dimensional materials. Additionally, in the proposed work, different types of 2D layers will be stacked on top of another. Such combinations of 2D materials possess properties that, again, are unlike any found in regular, three-dimensional materials. For example, electrons are found to move much faster in 2D materials than in 3D materials, permitting the fabrication of novel types of electronic devices (useful for computers that are faster and require less power). Additionally, the magnetic properties of electrons in 2D are unlike anything that occurs in 3D, which also has potential for new types of computing devices. The LT-STM will have impact not only for the researchers at Carnegie Mellon University, but also more broadly for the "Pittsburgh Quantum Institute", which includes about 50 faculty from University of Pittsburgh, CMU, and Dusquesne University. The proposed LT-STM will serve as a powerful characterization tool for research projects undertaken by members of this Institute.

Technical Abstract
Five investigators from Carnegie Mellon University (CMU) propose to acquire a low-temperature scanning tunneling microscope (LT-STM), including magnetic field capability. Two-dimensional (2D) materials and heterostructures will be studied. The 2D materials, which are only one or a few atomic layers thick, are formed by "exfoliation" from bulk crystals, that is, peeling off one or a few atomic layers from a bulk crystal and depositing those layer(s) on a suitable inert substrate. Such 2D layers exhibit a host of exotic properties including massless fermions, topologically protected states, superconductivity, and ferromagnetic phases, all of which will be probed in the LT-STM. Additionally vertical heterostructures will be formed by transferring one atomic layer atop the other; a state-of-the-art facility for performing such fabrication exists at CMU. Properties of the materials can be controlled in such heterostructures, since the presence of one layer in proximity to another yields collective behavior that differs from that of the individual layers.

All of the investigators are active in directing graduate and undergraduate research, and the proposed LT-STM instrument will significantly enhance those activities. Additionally, the facility will impact theoretical studies presently performed at CMU related to the experimental work of the investigators. The instrument is also expected to have significant impact on the "Center for 2D Materials and Devices for Energy-Efficient Computing" at CMU, which four of the PIs are members of. An operating plan for the LT-STM has been formulated that will permit external users to have access to it. Four of the investigators are members of the "Pittsburgh Quantum Institute", which includes about 50 faculty from University of Pittsburgh, CMU, and Dusquesne University. The proposed LT-STM will serve as a powerful characterization tool for research projects undertaken by members of this Institute.