Thin films are basic building blocks for devices. As device size shrinks, quantum effects become important, and traditional thinking in terms of electron flow must be revised. The figure shows electronic structure (dispersion curves) for 8, 8.6, and 9 atomic layers (AL). At 8.6 AL, the dispersion curves split (circled area) because of the presence of both 8 and 9 atomic-layer thicknesses in the film. At 8 AL, the film thickness is perfectly uniform; each dispersion curve shows a kink (squared area) at the band edge of Ge due to electronic coupling to the substrate. This unexpected observation leads to a better understanding and more accurate description of electronics in the quantum regime. Quantum Behavior of Electrons in Ultra Thin Silver Films on Germanium Tai C. Chiang, U of Illinois at Urbana-Champaign, DMR and DMR Phys. Rev. Lett. 93, (2004)

Atomically uniform films of Ag have been successfully prepared on Ge. This has significant implication for nanoscale electronics, which are expected to replace the current device and information processing technology in the near future. Ag is particularly interesting as it has the lowest room temperature electrical resistance of all metals (even better than copper, which is currently the choice for chip interconnects – the copper interconnect technology was pioneered by IBM). Ge, like Si, is an important substrate material with a mature technology base. (While not shown in this work, Ag on Si can also be prepared with a similar level of perfection.) The issue of interest is the detailed electronic structure of ultra thin films. Unlike traditional discussion of device physics based on semi-classical ideas, description of the electronic structure of ultra thin films must use quantum mechanics. A standard model is based on quantum wells, namely, the electrons in the film are taken to be trapped in a potential well and form discrete energy levels very much like the electronic levels in an atom. In a sense, a thin film is like an artificial atom (or molecule), except that one can change the dimension of this artificial atom (thickness of the film) at will. The quantum well description is widely employed in the literature and discussed in textbooks. A fundamental premise in this model is that each electron can be treated independently of the other electrons. Of course, electrons do interact among themselves, but in most cases the interactions can be averaged out and one can still treat each electron independently. This makes the quantum well model so successful. However, our recent results show that this model has its limits. As our experiment reveals, an energy level in the film can split when it gets close to the substrate band edge. This splitting violates the fundamental premise that an electron is a fundamental, undividable particle. The observed peculiar behavior can be well explained if one takes into full account of the electron- electron interactions (or many-body interactions). The measurements were carried out at the Synchrotron Radiation Center (NSF DMR ). Quantum Behavior of Electrons in Ultra Thin Silver Films on Germanium Tai C. Chiang, U of Illinois at Urbana-Champaign, DMR and DMR

Education: The group working on thin film quantum electronics includes postdoctoral research associate Shu- Jung Tang with a Ph.D. from Ward Plummer’s group at Tennessee, Mary Upton, who just graduated with a Ph.D. and will join Brookhaven National Laboratory, a new graduate student Ellen Keister with a BS from the College of William and Mary, and Professor Tom Miller. The work is carried out at the Synchrotron Radiation Center in Stoughton, Wisconsin, which is a national research facility supported by the National Science Foundation (DMR ). Societal Impact: Ultra thin films are the future for electronics. Atomic layer uniformity, as demonstrated here, is critical for precision measurements and fundamental studies. Such investigations pave the way for applications ranging from nanoscale electronics, supersensitive detectors, and components for quantum computing. Quantum Behavior of Electrons in Ultra Thin Silver Films on Germanium Tai C. Chiang, U of Illinois at Urbana-Champaign, DMR Dr. Shu-Jung Tang explained his poster during the 2004 User’s Meeting at the Synchrotron Radiation Center. He won the best-poster prize.