Physics at UIUC
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Yang Liu
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Longxiang Zhang
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The University of Illinois at Urbana-Champaign


Research

Our group studies semiconductor and metal surfaces and interfaces. We are interested in multiple facets of these systems including their electronic properties, growth morphology (sub-monolayer adsorption to thin film growth), quantum-well structures and interfacial atomic structure. Our primary techniques for investigating these systems are photoemission and x-ray diffraction (XRD).


X-Ray Diffraction

The X-Ray Scattering Research Team performs Diffraction experiments on metal, insulator (C60) and semiconductor (III-V's and group IV) surfaces and interfaces. The interest in surfaces and interfaces lies not in merely determining the atomic coordinates, but in relating that structural information to the physical properties of that surface or interface. There are a number of excellent and complementary methods for obtaining this structural information from a surface; where we use the term surface to refer to a specific type of interface, specifically a vacuum-crystal interface. However, the same strengths that make these techniques well suited for surface studies impede them in interface investigations. For example, electrons used in low-energy electron diffraction (LEED) strongly interact with atoms limiting their penetration depth. This results in only a couple of techniques that can probe and determine the structure of a buried interface.

X-rays are particularly well suited for these investigations. Their characteristic wavelengths, on the order of an angstrom, make them a nearly ideal diffraction probe of atomic dimensions. In addition, their weak interaction with matter not only contributes to their unmat ched penetration power, but also allows straightforward analysis using single scattering kinematic theory. Unfortunately, this strength is also their greatest drawback for use in surface studies. However, augmentation of the low surface signal rates attributable to the x-rays' weak interaction with the "small" number of surface atoms can be achieved by using a glancing incidence geometry and/or with the higher intensity beams available at x-ray synchrotrons.

We primarily use synchrotron radiation to perform our scattering experiments. There are many advantages to using synchrotron x-rays over conventional laboratory based sources. The availability of high intensity, tunable monochromatic x-rays or broad spectrum "white" radiation permit numerous experiments once technically formidable or infeasible to become routine. These include extended x-ray absorption fine structure (EXAFS), anomalous or resonant scattering and protein crystallography. The advent of x-ray sync hrotron sources has revolutionized materials research using x-rays.

The majority of our experiments to date fall into three main categories:

  • . . . Noble-metal semiconductor interfaces;
  • . . . C60 encapsulated surfaces; and,
  • . . . Transmission Diffraction and X-ray "RHEED"


Photoemission

Photoemission spectroscopy (PES) is a surface sensitive technique which has been applied to the study of many systems. Electrons are excited from the sample by an incoming beam of photons and subsequently analyzed by an electron energy analyzer. This information can be utilized to provide information about the chemical composition, local bonding arrangement, and electronic structure of the sample.

Photoemission spectroscopy has many inherent advantages as a surface science technique. The electrons probed typically arise from the first few layers of the sample. Electrons can be easily focused and tuned for analysis and are easily counted. The photoexcited electrons can also provide information regarding their initial state energy and momentum. Another advantage over other spectroscopic techniques using ions or atoms is that the electrons vanish after they have been detected. Finally, the impinging photon beam leaves the sample relatively unscathed, so that PES is a non-destructive technique.

The use of synchrotrons as photon sources has greatly advanced the capabilities of PES. Previously, monochromatic photons were only available from the characteristic emission lines of specific elements (e.g. Al K = 1486.6 eV or He I = 21.22 eV). Not only do synchrotrons provide access to photons of a continuously tunable energy, the intensity and energetic resolution provided by synchrotrons is unmatched. The photons available from synchrotrons arise from, as one might suspect, synchrotron radiation due to electrons held in a storage ring. A broad range of photons is produced as the electrons are bent to travel about the ring. Charged particles traveling in a nonlinear path will emit electromagnetic radiation. The various beamlines provide access to a selected energy range of these photons. They also act to focus the incoming light into a small spot onto the sample to provide maximum intensity and angular resolution.

 


Publications.

Recent Publications:

Complete List (click here)

R. Xu, J. Wong, P. Zschack , H. Hong, and T.-C. Chiang, "Soft phonons in delta-phase plutonium near the delta-alpha transition" EuroPhys. Lett. 82, 26001 (2008).

Y. Liu, N. J. Speer, S.-J. Tang, T. Miller, and T.-C. Chiang, "Interface induced complex electronic interference structures in Ag films on Ge(111)" Phys. Rev. B 78, 035443 (2008).

T.-C. Chiang, "Quantum physics of thin metal films" Bulletin of AAPPS, 81, No.2, 2-10 (2008).

S.-J. Tang, Wen-Kai Chang, Yu-mei Chiu, Hsin-Yi Chen , Cheng-Maw Cheng, Ku-Ding Tsuei, T. Miller, and T.-C. Chiang, "Enhancement of subband effective mass in Ag/Ge(111) thin film quantum wells" Phys. Rev. B 78, 245407f (2008).

R. Xu, H. Hong, P. Zschack, and T.-C. Chiang, "Direct mapping of phonon dispersion relations in copper by momentum-resolved x-ray calorimetry" Phys. Rev. Lett. 101, 085504 (2008).

H. Hong, R. Xu, A. Alatas, M. Holt, and T.-C. Chiang, "Central peak and narrow component in x-ray scattering near the displacive phase transition in SrTiO3" Phys. Rev. B 78, 104121 (2008).

Y. Liu, J. J. Paggel, M. H. Upton, T. Miller, and T.-C. Chiang, "Quantized electronic structure and growth of Pb films on highly oriented pyrolytic graphite" Phys. Rev. B 78, 235437 (2008).

K. Wang, X. Zhang, M. Loy, T.-C. Chiang, and X. Xiao, "Pseudogap mediated by quantum-size effects in lead islands" Phys. Rev. Lett 102, 076801 (2009).


Graduates

Nathan Speer (2008)
Intel Coorporation
Dom Ricci (2006)
HSBC(New York)
Mary Upton (2005)
Argonne National Laboratory
Peter Czoschke (2005)
Seagate Technology
Leo Basile (2005)
Escuela Politecnica Nacional
Martin V. Holt (2002)
Argonne National Laboratory
Tim E. Kidd (2002)
Northern Iowa University
Paul J. E. Reese (2001)
Intel Corporation
Dah-An Luh (2000)
National Central University
Evan D. Hansen (1998)
Brigham Young Unversity-Idaho
James M. Roesler (1997)
Intel Corporation
Richard D. Aburano (1997)
Infineon
Matthew T. Sieger (1996)
Intel Corporation
William E. McMahon (1996)
National Renewable Energy Laboratory
Eric S. Hirschorn (1994)
Hughes
Deng-Sung "Randy" Lin (1994)
National Chiao-Tung University
John A. Carlisle (1993)
Argonne National Laboratory
G. E. Franklin (1992)
Sandia National Laboratory
F. M Leibsle (1991)
U. of Missouri-Kansas City
M. A. Mueller (1990)
Tencor
A. Samsavar (1990)
Tencor
D. H. Rich (1989)
Ben-Gurion University of the Negev
P. John (1988)
U. of Dayton Res. Inst. & Wright Patterson AFB
T. C. Hsieh (1987)
Polaroid
A. L. Wachs (1987)
Oak Ridge National Laboritory
A. P. Shapiro (1987)
NASA
T. Miller (1986)
Uinversity of Illinois


 
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