Transport in bent quantum wells in the quantum Hall regime
Final Report Abstract
With advances in technology shrinking down electronic circuits, it is important to investigate the properties of the smallest electrical connectors called one‐dimensional quantum wires ‐ which are effectively one‐electron in diameter. All one‐dimensional conductors share common behavior, so studying the properties of a one‐dimensional quantum Hall edge, for example, can elucidate the properties of a one‐dimensional carbon nanotube. The project described here centers on a new kind of structure, a bent quantum well, which can be thought of as a sheet of conducting electrons bent at a sharp 90 degree angle and residing inside a semiconductor lattice. When put in a large magnetic field at low temperatures, the sharp corner enters the quantum Hall regime and hosts a one‐ dimensional electron system along its corner with exotic properties which are to be characterized in this research. A quantum Hall edge is a special case of a one‐dimensional conductor since electrons only flow in one direction, because a large magnetic field causes the electrons to skip along the boundary like a top spinning against a wall. In the structures studied here, two quantum Hall edges can be lined up and joined together with a quantum wire to create a new kind of one‐dimensional boundary system which exists at the corner of a bent quantum well in a high magnetic field at low temperatures. The properties of this particular one‐dimensional system are interesting because such a varied behavior is not easily achieved in standard wire systems made by planar lithography, and the internal properties of the present one‐dimensional system can be tuned by simply changing the external magnetic field, thus effectively realizing many different systems in one device. There are few examples of non‐planar quantum Hall systems such as this, and these investigations can probe their properties and compare with simulations to see how well we understand their internal structure. Of the different kinds of behavior possible in generic one‐dimensional systems, we reported metallic, weakly insulating, and strongly insulating states in this one‐dimensional system as a function of the external magnetic field. We thoroughly characterized the insulating behavior to understand the conduction mechanism, and we observed that a variable range hopping model seems to be the best fit over a broad temperature range, and that electron‐electron Coulomb interactions do not play a strong role in enhancing backscattering. Initial work was done in a new magnetic field geometry where the field is perpendicular to one of the facets of the bent quantum well. This work also spurred many crystal growth and device processing advances. Transmission electron microscope pictures of the cross‐section of the bent quantum well were taken for the first time, and demonstrate that the radius of curvature is of order 3.5 nm or smaller. The smallness of this radius justifies the assumption that underlies the calculations for the underlying one‐dimensional states at the corner junction. Unexpected stripes of material appeared along the corners which were shown to consist of aluminum‐rich AlGaAs alloy which precipitates at the corner due to the lower surface mobility of the aluminum adatoms at the corner. To improve contacting for all range of electrical devices, a shadow‐modulated growth technique was invented which enables different layers, such as dopant layers or barriers, to be selectively patterned during crystal growth. New possibilities for measuring low‐density and p‐type systems were explored, by developing capacitive contacts. Using this latter technique, we were able to demonstrate 4‐point magnetotransport of a conducting layer without ohmic contacts to that layer.
Publications
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“Capacitive contacts: Making four‐point characterizations without ohmic contacts”. Int. J. Mod. Phys. B 21, 1435 (2007)
N. Isik, M. Bichler, S. Roth, M. Grayson
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“Metallic and insulating states in a bent quantum Hall junction“. Phys. Rev. B 76, 201304‐07(R) (2007)
M. Grayson, L. Steinke, D. Schuh, M. Bichler, L. Hoeppel, J. Smet, K. v. Klitzing, D. K. Maude, G. Abstreiter
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“Novel one‐dimensional states in a bent quantum Hall junction“. Int. J. Mod. Phys. B 21, 1207 (2007)
M. Grayson, L. Steinke, M. Huber, D. Schuh, M. Bichler, G. Abstreiter
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"Hopping conduction in strongly insulating states of a diffusive bent quantum Hall junction". Phys. Rev. B 77, 235219 (2008)
L. Steinke, D. Schuh, M. Bichler, G. Abstreiter, and M. Grayson
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"Nanometer scale sharpness in corner overgrown heterostructures". Appl. Phys. Lett. 193, 193117 (2008)
L. Steinke, P. Cantwell, D. Zakharov, E. Stach, N. J. Zaluzec, A. Fontcuberta i Morral, M. Bichler, G. Abstreiter, and M. Grayson
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"Shadow modulated two‐dimensional heterostructures using vertical pillars". Appl. Phys. Lett. 92, 173505 (2008)
N. Isik, M. Bichler, S. F. Roth, A. Fontcuberta i Morral, O Goktas, and M. Grayson
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"Sharp quantum Hall edges: Experimental realizations of edge states without incompressible strips". Phys. Status Solidi (b) 245, 356‐365 (2008)
M. Grayson, L. Steinke, M. Huber, D. Schuh, M. Bichler, and G. Abstreiter
