RESEARCH

 

Current Areas of Research
Flexible Electronics
Experimental Statistical Mechanics
Statistical Properties of Nanostructures
    Persistance, Electromigration, and Noise
Pattern Formation and Structure Evolution
Imaging Current Flow with MFM
Analytical Applications of PEEM

Collaborations

The Williams group uses the tools of experimental surface science to explore fundamental issues in statistical mechanics, and their practical applications in the growing field of nanotechnology.  Many of the research activities exploit the power of direct imaging techniques such as scanning tunneling microscopy (STM) for exploring previously inaccessible areas of science.

Current Areas of Research


Experimental Statistical Mechanics

From the ability to image surfaces with atomic scale resolution follows the ability to quantify the populations of different structural features. Of particular interest are the steps that bound crystalline layers. Because of the low coordination at step edges, thermal excitations and mass exchange are relatively easy. As a result, steps play an important role in chemical reactivity and structural rearrangements. 

The spatial wandering of a step and its temporal wandering under dynamic equilibrium can both be accessed using STM and other direct imaging tools. Using the tools of classical statistical mechanics, the measured step behavior is analyzed in terms of spatial and temporal correlation functions, yielding thermodynamic free energies and equilibrium time constants. Continuing research issues include the relationship between the true atomic potential energy surface and the thermodynamic parameters determined from the correlation functions, and the experimental signatures of systems with competing atomic-scale processes.

See:

“Steps on Surfaces: Experiment and Theory,” Surface Sci. Reports 34, 171-294, 1999,  (H.-C. Jeong and E.D. Williams)

Publications list


Pattern Formation and Structure Evolution

Nanoscale structures, by virtue of their relatively large surface area, will be particularly susceptible to thermal decay, and perturbation by environmental effects. Continuum models of mass transport will fail for such small structures, but fortunately we can use the properties of steps to predict nanostructure response. Basically, we can describe any type of crystalline nanostructure in terms of the steps that must bound its edges. Then, using the thermodynamic step parameters discussed above, each step can be assigned a local chemical potential. This in turn defines chemical potential gradients that will drive the evolution of the structure. The time constants for step fluctuations (also discussed above) then set the time scale for structure evolution.

Experimental applications of this approach to structure evolution on silicon surfaces, silicon nanostructures and lead crystallites have confirmed its utility down to length scales of nanometers.

See: Step Dynamics in Crystal Shape Relaxation
        Pattern Formation Under Electromigration 
        Publications list


Statistical Properties of Nanostructures: Persistence, Electromigration and Noise
For nanoscale structures and devices, thermal fluctuations and low probablility events such as activated nucleation or electromigration biased mass flow may introduce significant stochasticity into the properties of interest.  Such effects may be manifest as noise, or as discrete changes in a structural property such as connectedness.  In collaboration with the theory groups of Prof. Das Sarma and Prof. Rous, the Williams group is investigating the issues of persistence in step fluctuations and the role of electrical current in biased surface diffusion.  Correlations of noise with structural fluctuations are under study in collaboration with the experimental group of Prof. Fuhrer.
See: Persistence in Step Fluctuations

“Nanoscale Fluctuations at Solid Surfaces,” Physics Today 52 24-28 (1999) (Z. Torozckai and E.D. Williams). (Review Article).

"Experimental Persistence Probability for Fluctuating Steps,"  Physical Review Letters 89, 36144-7, 2002 (D.B. Dougherty, I. Lyubinetsky, E.D. Williams, M. Constantin, C. Dasgupta, and S. Das Sarma).
Publications list


Imaging Current Flow with Magnetic Force Microscopy

The small electromigration force that causes problems in integrated circuits may become a big problem in nanostructure devices, where atoms at defects and surfaces are a larger fraction of the atomic volume. One aspect of the electromigration problem is that the current flowing through a structure isn’t necessarily uniform: current “crowds” into higher density as it flows around a defect. At small (micron or less) scales, there was previously no way to measure this crowding effect. We have developed an application of magnetic force microscopy (MFM) that allows us to determine the distribution of current flowing around a defect by measuring the magnetic fields above the structure. The panels at the left show the measured and calculated current distributions in a 10-micron wide line with a FIB-fabricated defect. The current density at the tip of the defect is 4x as large as the uniform density far away from the defect! Using this technique, current variations at unknown structures can be determined – a capability that will be very useful in diagnostics of electrical connections to nanostructures.

see: 

MFM of Current Crowding
Current Flow around Defect Structures
R. Yongsunthon Ph.D. Dissertation: Magnetic Force Micrsocopy for Observation of Current Crowding in Electromigration Phenomena
Publications list

 


Analytical Applications of PEEM

This program developed from initial observations of interesting contrast effects in photo-electron emission microscoy (PEEM) made during studies of surface electromigration. Potential applications in which PEEM could be used to complement SEM in semiconductor metrology were noted, and development of this capability has been actively pursued under LPS support. We have used FIB implant patterning, and photo-lithography to create controlled model structures to characterize the nature of PEEM contrast. The effect of electric fields on patterned structures is particularly striking and allows buried structures to be detected under oxide layers.

see:

PEEM Contrast for Imaging Semiconductor Implants
PEEM contrast: Electric Field Effects
Precision Micropositioner for PEEM
K. Siegrist Ph.D Dissertation: Characterization of Contrast Mechanisms of the Photoelectron Emission Microscope 
Publications list


Collaborations

Our research program is strengthened by an extensive network of collaborations. Much of the collaborative research at the University of Maryland is carried out within the Nanostructures Group of the MRSEC. In addition, collaborations with colleagues at local federal laboratories and international collaborations are important in our research.

 

Collaborators

 

Sankar Das Sarma
Ted Einstein
John Weeks
Michael Fuhrer
Janice Reutt-Robey
Ray Phaneuf
Philip Rous (UMBC)

Vince Ballarotto, LPS
Rick Silver, NIST
Chandan Dasgupta, Indian Institute of Science, Bangalore
Lloyd Whitman, NRL
Hiroshi Iwaskaki, ISIR, Osaka University
Makio Uwaha, Nagoya University
Hans Bonzel, Julich
 

 

Photo Gallery

Ted Einstein
Physics Department
University of Maryland
Vince Ballarotto
Laboratory for Physical Science
 
Janice Reutt Robey
Chemistry Department
University of Maryland
Ray Phaneuf
Materials Science and Engineering Department
University of Maryland

John Weeks
Physics Department & IPST
University of Maryland

Sankar Das Sarma, Magdalena Constantin and Chandan DasGupta
Physics Department
University of Maryland

E. D. Williams, Min Ouyang and Michael Fuhrer
Physics Department
University of Maryland

 

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