ScienceWeek

APPLIED PHYSICS: ON IMAGING ELECTRON FLOW

The following points are made by M.A. Topinka et al (Physics Today December 2003):

1) Semiconductor heterostructures have revolutionized solid-state physics and its applications. Most of us use the fruits of this revolution every day in CD and DVD recorders and players, cellular telephones, laser-based telecommunications, satellite television, and much more. The technology, based on atomic layer-by-layer growth using molecular beam epitaxy (MBE), is sophisticated, remarkable, and marketable.

2) One class of semiconductor heterostructures, the two-dimensional electron gas (2DEG), has been a focal point for theorists and experimentalists and a wellspring of new physics. A 2DEG can be produced at low temperatures at an interface of two distinct layers (a so-called heterojunction) doped nearby with atoms that donate electrons. The electrons at such a junction are confined to the lowest quantum state in the direction normal to the interface; by charging gate electrodes on the top surface of the heterostructure some distance away to repel them, the electrons can be further confined in the other directions to make dots, wires, resonators, and other shapes. That technology has led to celebrated discoveries including the integer and fractional quantum Hall effect (QHE),(1) the Coulomb blockade and single-electron transistors, and conductance quantization in quantum point contacts (QPCs).(2)

3) The potential for exploiting these and many other quantum effects is spawning new fields of single electronics(3) and spintronics(4) -- new approaches to logic that use single electron charges and spins to represent bits of data -- and the new area of quantum information processing, based on the coherent interaction of quantum mechanical qubits.(5)

4) Despite all the beautiful experiments already performed on 2DEGs and all that is riding on the new science and phenomena made possible by them, researchers have been blind until recently as to how electrons actually move through them. Most of the knowledge of electron flow in 2DEGs is indirect, based on electron-transport measurements of macroscopically averaged quantities. To be sure, many of the statistical properties are known, such as the electron mean free path. But macroscopically averaged parameters do not reveal the details of the fascinating behavior to be found on the nanoscale. For that, imaging is needed.

5) Imaging a system is essential to understanding its fundamental properties and developing new electronic and magnetic devices. Imagine the difficulty of designing and fabricating an integrated circuit from a silicon crystal without the use of an optical or electron microscope. As device sizes continue to decrease, quantum behavior becomes important and offers new research and application opportunities. To understand the fundamental behavior of electrons in this quantum regime and to make functioning devices based on this behavior, one must develop ways to visualize the flow of electron charges and spins through semiconductors. The invention of the scanning tunneling microscope (STM) allowed researchers to directly view the pattern of atoms on a material's surface. Additional methods are needed to image the flow of electrons beneath the surface.

6) Obtaining images of 2DEGs inside semiconductors is no easy matter, because the electrons are buried beneath the surface and because the samples must be cooled to low temperatures to show quantum behavior. Nonetheless, a number of groups have recently developed liquid-helium-cooled scanning probe microscopes (SPM) for this purpose.

References (abridged):

1. T. Chakraborty, P. Pietilainen, eds., The Quantum Hall Effects: Integral and Fractional, 2nd ed., Springer-Verlag, New York (1995)

2. L. L. Sohn, L. P. Kouwenhoven, G. Schoen, Mesoscopic Electron Transport in Semiconductor Nanostructures, Kluwer Academic, New York (1997)

3. K. K. Likharev, Proc. IEEE 87, 606 (1999)

4. S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molnar, M. L. Roukes, A. Y. Chtchelkanova, D. M. Treger, Science 294, 1488 (2001)

5. C. H. Bennett, D. P. DiVincenzo, Nature 404, 247 (2000)

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