Condensed Matter Physics Seminar

The break-through concept of semiconductor superlattice heterostructures, first proposed by Esaki and Tsu in the 1970’s[i], can be described as the controlled construction of a nanoscale periodic potential modulation by designed combination of different materials[ii]. Traditional semiconductor superlattices periodically modulate the electronic potential either by composition variation[iii],[iv] or by dopant induced electrical field effect[v],[vi],[vii]. Such an artificial layering of multi-phase or doped semiconductors has engineered a variety of functional materials with novel optical and transport properties, leading to a variety of electronic and optoelectronic applications including resonant tunneling devices, lasers, photodiodes, and photodetectors[viii].

In this talk, a new form of low-dimensional semiconductor superlattices will be presented, i.e., orientation-modulated superlattice nanohelices[ix] using a single phase semiconductor material that has an anisotropic crystal structure. Experimental studies on this type of superlattice nanostructures have given rise to some anomalous physical properties including nanoscale electronic transport[x],[xi] and mechanical properties[xii]. Specific design and engineering of such type of new materials represent a new challenge in the crystal growth physics and chemistry, but at the same time could potentially open a new path for band structure interface engineering. It is envisioned that such a new form of superlattices in a single phase might have a great potential for future electronic and optoelectronic applications.

References:

[i] L. Esaki and R. Tsu, “Superlattice and negative differential conductivity in semiconductors”, IBM J. Res. Dev. 1970, 14, 61-65; L. Esaki, in “Synthetic modulated structures”, edited by L.L. Chang and B.C. Giessen (Academic, New York, 1985), chapter 1.

[ii] M.T. Bjork, B.J. Ohlsson, C. Thelander, A.I. Persson, K. Deppert, L.R. Wallenberg, and L. Samuelson, “Nanowire resonant tunneling diodes”, Appl. Phys. Lett. 2002, 81, 4458-4460.

[iii] R. Ludeke, L. Esaki, and L.L. Chang, “Ga_1–x Al_x As superlattices profiled by Auger electron spectroscopy”, Appl. Phys. Lett. 1974, 24, 417-419.

[iv] R. Dingle, W. Wiegmann, and C.H. Henry, "Quantum states of confined carriers in very thin Al_xGa_1-xAs-GaAs-Al_x Ga_1-xAs heterostructures", Phys. Rev. Letts. 1974, 33, 827-830.

[v] G.H. Dohler, Phys. Status Solidi 1971, 52(79), 533; “Doping superlattices”, J. Vac. Sci. Technol. 1979, 16, 851-856.

[vi] G.H. Dohler et.al, “Observation of tunable band gap and two-dimensional subbands in a novel GaAs superlattice”, Phys. Rev. Letts. 1981, 47, 864-867.

[vii] G.H. Dohler, “n-i-p-I doping superlattice-semiconductors with tunable electronic properties”, in: H.L. Grubin, K. Hess, G.J. Iafrate, D.K. Ferry (Eds), The physics of submicron structures, Plenum Press, New York, 1984.

[viii] F. Capasso, “Band-gap engineering: from physics and materials to new semiconductor devices”, Science 1987, 235, 172-176.

[ix] P.X. Gao, Y. Ding, W. J. Mai, W. L. Hughes, C. S. Lao and Z. L. Wang, “Conversion of Zinc Oxide Nanobelts into Superlattice Structured Nanohelices”, Science, 2005, 309, 1700.

[x] P.X. Gao, and Z.L. Wang, to be submitted, 2008.

[xi] P.X. Gao, J. Liu, J.L. Lee, and Z.L. Wang, “Bridged ZnO Nanowires Across Trenched Electrodes”, Appl. Phys. Letts, 2007, 91, 142108.

[xii] P.X. Gao, W.J. Mai, and Z.L. Wang, “Superelasticity and Nanofracture of ZnO Nanohelix”, Nano Lett. 2006, 6, 2536.