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Atmospheric Lidar Group
Atmospheric Spectroscopy Group
Center for Advanced Studies in Photonics Reseach (CASPR)
Condensed Matter Group
Joint Center for Astrophysics (JCA)
Joint Center for Earth Systems Technology (JCET)
Laboratory for Nonlinear Optical Studies of Macromolecular Photonic Materials
Quantum Information Group
Quantum Optics Group

Condensed Matter and Materials Physics

Defects in Semiconductor Devices

Understanding the role of deep levels and how to control them is essential in semiconductor device technology because deep states can have strong effect on the electrical performance of devices. Also deep levels associated with lattice defects introduced by particle irradiation are a major cause of device failure in satellites. We study deep levels using a technique called Fourier Transform Deep Level Transient Spectroscopy using a commercial digital system made by BioRad (DL 8060) with optical excitation, a fast pulse interface and a constant capacitance feature.
The group works closely with researchers at the Naval Research Laboratory in Washington, DC.
Group Members: Dr. Geoffrey P. Summers

Mechanical Alloying Group

Mechanical alloying is a high energy ball milling process that is used toprepare nanostructured materials, amorphous alloys, stable and metastable compounds, etc. As its application does not require high temperature, it is capable of combining components that are incompatible for most other bulk processing methods. High energy ball milling can also be used to induce chemical reactions. Termed mechanochemistry in this case, the method finds applications in a variety of areas: It is the basis of inorganic and organic preparation processes, activation of silicates (cement), the processing of minerals and ceramics, etc. Broader application of ball milling methods require better understanding and control of the process. Our interest is to study exactly what happens inside the milling container, how material properties and milling conditions influence the kinetics of the process and the structure and properties of the resulting products. An interesting situation is when ball milling induces a self-supporting thermal reaction (gasless combustion) in a highly exothermic system. For example, milling titanium and boron powders results in the combustive formation of titanium diboride, a refractory compound often used to enforce light high temperature alloys. Our investigations of the combustive ball milling process gave better understanding of the mechanism of mechanochemical reactions.

We are also interested in utilizing mechanical alloying to prepare materials with interesting properties, especially magnetic properties. Magnetic nanocomposites were prepared, some with the assistance of in situ chemical reactions. Many investigations were performed on the Fe-Zn-O system, with the purpose of preparing nanocomposites of Fe particles in ZnO.
Major Equipment: SPEX 8000 and Pulverisette-5 ball mills
X-Ray Diffractometer (Philips X'Pert)
Vibrating Sample Magnetometer (Lake Shore 7307)
High Temperature DTA (Shimadzu DTA-50)
Double Width Glove Box
Group Members: Laszlo Takacs Associate Professor
Valerii Soika Research Associate
Andrew H. Weber Undergraduate Assistant

Microscopy and Materials Characterization Group

Current research addresses the characterization of the morphology, composition, and magnetic and crystallographic structure of mechanically alloyed powders (e.g., Ta-S alloys ), thin films ( e.g. NiO ), and highly deformed metals ( steel sheet ). Techniques used include scanning electron microcopy (SEM), energy dispersive x-ray spectroscopy, electron backscattered diffraction (EBSD), and atomic force microscopy (STM/AFM).
Supporting techniques used on occasion are Mossbauer spectroscopy and positron annihilation.
Major Equipment: JEOL JSM-5600 SEM with Oxford Isis ( x-ray) and Opal ( EBSD ) attachments
Thermomicroscopes STM/AFM
Mossbauer spectrometers and a delayed coincidence gamma ray spectrometer
Sample Preparation Facilities
- high temperature vacuum annealing furnace
- sample polishing and sputtering apparatus
- optical metallography
Group Members: Dr. Robert C. Reno Associate Professor
Christa Shockey Graduate Research Assistant
Patrick Hughes Undergraduate Research Assistant

Surfaces & Interfaces Group

Our research is concerned with the effects of surfaces and interfaces on the magnetic properties of materials. We have studied surface effects on magnetic phase transitions in the antiferromagnet UO2 using resonant magnetic x-ray scattering. We found that the moments near the surface disordered at a lower temperature than the moments in the bulk. We are performing similar studies using low-energy electron diffraction to study the magnetic structure at the (001) surface of the antiferromagnet NiO. The surface magnetic structure of NiO is studied as a function of both temperature and film thickness.

Our group is also interested in magnetic multilayers and the phenomena of giant magnetoresistance. We have grown magnetic tri-layers at NIST in collaboration with the Quantum Metrology Group. Experiments are underway to study the induced moments in the non-magnetic spacer layers of these samples as a function of distance from the ferromagnetic-nonmagnetic interface.
Major Equipment: ultra-high vacuum (UHV) surface chamber with high-resolution
low-energy electron diffraction (HR-LEED)
double-pass cylindrical-mirror analyzer, and e-beam evaporator
four-circle x-ray diffractometer for sample characterization

In the near future we will have a second UHV chamber with a photoemission-electron microscope for imaging of magnetic domains in thin films.

Our group is a member of the X22C PRT at the National Synchrotron Light Source at Brookhaven National Laboratory. Work has also been performed at the ESRF (European Synchrotron Radiation Facility) using beam line ID20.
Group Members: Dr. Gavin M. Watson Assistant Professor
Jason Cheng Research Assistant
Rundon Wan Research Assistant
Recent Graduates: Matthew Brown (Masters, 2000 Jan)
Brian Hinz (Masters, 2000 Jan)

Theoretical Surface Physics


Our research is based on the theory of electron scattering and transport at interfaces and the exploitation of such processes for surface modification and the manipulation of chemical dynamics at surfaces. The research is a collaborative effort involving both theoretical and experimental researchers at UMBC and elsewhere and the group offers an ideal environment for those interested in theoretical physics that has a direct connection to experimental research.
Currently, we have active research projects in the two areas described below:

Transient Electronic States At Surfaces and Surface Dynamics

A fundamental aspect of the microscopic description of surface dynamical processes is the understanding of transient electronic states of adsorbed atoms and molecules and how these excited states couple to inelastic decay channels such as vibrational excitation, dissociative attachment and desorption. At the most fundamental level, such transient electronic states are the most elementary steps in many surface chemical reactions.
Currently, we are developing new theoretical methods which will allow the computation of the lifetime of excited electronic states in adsorbed molecules, a quantity that can be regarded as a controlling parameter in many surface dynamical processes. We are trying to answer the question: Is it possible to control the lifetime of excited states by altering the scattering environment of the atom or molecule?. We are exploring an elegant analogy to be found in the classic quantum electrodynamical problem of the radiative decay of excited atomic states in micro-cavities. Our ultimate goal is to develop our fundamental understanding of the decay of resonant states to level where it becomes possible to design nanoscale structures at surfaces which would allow the control of the temporal evolution of excited states in adsorbates. From a technological viewpoint this goal has applications in the control of surface chemical reactions, molecular electronics and sensor technology.

Surface Electromigration

A fundamental question faced in development of new processes for surface modification is: how is it possible to influence, externally, the morphology of the surface of a material? Traditionally, surface modification has been achieved by utilizing specific chemical reactions, deposition and growth, and atomic manipulation using the scanning tunnelling microscope. We are currently exploring another route to the nano-structural engineering of materials, one which exploits the phenomenon of electromigration. This work is performed in collaboration with the NSF Materials Science and Enginneering Center based at the University of Maryland.

Surface electromigration describes mass transport at the surface of a solid that is driven by an electrical current. Electromigration is well-known phenomenon in bulk materials and it has been implicated as a cause of interconnect failure in integrated circuits where interfacial (grain boundary) migration is the primary failure mechanism in micron-scale metallic lines. By contrast, in nanoscale electronic devices the dominant electromigration pathway is the surface of the material.

We are developing quantum mechanical theory of the electromigration force at surfaces. Our theory can be tested directly by scanning tunneling microscopy studies of adatom migration in the presence of a surface current. This phenomenon could be exploited to enable the external manipulation of atomic and cluster diffusion at surfaces. Put simply, by controlling the electrical current flowing at the surface of a material it may be possible to change the magnitude and direction of the driving force causing electromigration.

The theory of electromigration has an important application to gas sensor technology based on metallic films. It is well known that atomic or molecular adsorption onto the surface of a conducting thin film changes its resistivity since the adsorbates act as scattering centers for carriers striking the surface. Therefore the contribution of an adsorbate to the surface resitivity of the film is directly related to the electromigration force acting upon the particle. Since this magnitude of this force is a strong function of the scattering properties of the adsorbate, the change in surface resistivity upon adsorption is chemically specific.


 

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