The AFM & Quantum Transport measurements
are aimed at shedding light on some fundamental questions about electrical transport
in two-dimensional electron gasses. Shown below is a sketch illustrating the
basic idea behind these experiments:
The term "two dimensional electron gas" (2DEG) refers to the thin sheet of highly mobile conduction electrons which forms just beneath the surface of specially grown wafers. In this lab we use GaAs/AlGaAs heterostructures with 2DEGs located approximately 50nm beneath the surface. The combination of extremely long mean free path and long coherence times found in these systems at low temperature presents a unique opportunity to study coherent, ballistic electron flow through nano-scale devices.
The picture above illustrates one way that the AFM can be used to probe the transport through a typical 2DEG device, in this case a point contact. By simultaneously scanning the negatively charged AFM tip over the surface and measuring the conductance, (or other electrical characteristics) of the sample, one can image the current flow in the 2DEG. The negative charge on the cantilever tip causes electrons in the 2DEG immediately underneath the tip to be "pushed away", causing a local depletion region directly below the tip. Electrons flowing in the 2DEG are backscattered by this depletion region back through the Quantum Point Contact (QPC) reducing the conductance. When the tip moves over regions of high electron flow there is a large change in the conductance. When the tip is over regions of relatively low electron flow, there is no chance in conductance. By scanning the tip over the sample, and simulatenously measuring the change in conductance, the two-dimensional current density in a device is imaged.
For 2DEG devices larger than both the mean free path and the phase coherence length, electron transport is well described by classical, diffusive models. Current commercial semiconductor devices are usually in this limit.For smaller electronic devices, however, quantum mechanics plays an increasingly dominant role. This regime, often referred to as mesoscopic physics, (between microscopic and macroscopic), has been an area of active research for the past decade or so, and a wide variety of novel, interesting physical phenomena have been discovered and investigated. These include the Quantum Hall Effect, Quantum Chaos, Tunable Point Contacts (with resulting Conductance Quantization), and Quantum Dots (which illustrate Coulumb Blockade). Throughout all of these experiments, however, the actual charge and current distributionis only indirectly measurable. We are interested in directly spatially imaging the charge and current flow through devices like these.
Papers & Review Articles
COHERENT BRANCHED FLOW IN A TWO-DIMENSIONAL ELECTRON GAS, M.A. Topinka, B.J. LeRoy, R.M. Westervelt, S.E.J. Shaw, R. Fleischmann, E.J. Heller, K.D. Maranowski, A.C. Gossard, Nature 410, 183 (2001).
IMAGING COHERENT ELECTRON FLOW FROM A QUANTUM POINT CONTACT, M.A. Topinka, B.J. LeRoy, S.E.J. Shaw, E.J. Heller, R.M. Westervelt, K.D. Maranowski, A.C. Gossard, Science 289, 2323 (2000).
CRYOGENIC SCANNING PROBE CHARACTERIZATION OF SEMICONDUCTOR NANOSTRUCTURES, M.A.Eriksson, R.G.Beck, M.Topinka, J.A.Katine, R.M.Westervelt, K.L.Campman, A.C.Gossard, Appl. Phys. Lett. 69, 671 (1996).
QUANTUM TRANSPORT IN SEMICONDUCTOR NANOSTRUCTURES, C.W.J.Beenakker,H.van Houten, Solid State Physics 44, 1-228 (1991).