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Scanning tunneling spectroscopy of semiconductor quantum-well structures

Abstract : Low-temperature scanning tunneling spectroscopy (STS) under ultrahigh vacuum was used to investigate In0.53 Ga0.47 As/In0.52 Al0.48 As quantum-well (QW) structures, grown by molecular beam epitaxy on lattice-matched InP(111)A substrates. In a first part, as a preliminary step, the (111)A epitaxial surface of n-type In0.53 Ga0.47 As was studied by STS. It was found that the surface Fermi level is located in the conduction band, close to the bulk Fermi level, and can be partially controlled by varying the n-type impurity density in the bulk. This result was confirmed by determining the conduction-band dispersion relation at the surface. Such partial unpinning of the surface Fermi level indicates a low density of acceptorlike surface states. It was proposed that these states originate from native point defects located at the surface. In a second part, based on the results of the first part, (111)A-oriented In0.53 Ga0.47 As surface QWs grown on top of In0.52 Al0.48 As barriers were studied by STS. The STS measurements were performed at the (111)A epitaxial surface of the In0.53 Ga0.47 As QW, in order to probe with nanometer-scale resolution the in-plane spatial distribution of electronic local density of states. It was confirmed that electron subbands are formed in the QW, and that the electron density in the QW can be varied owing to the partial unpinning of the surface Fermi level. It was found that a phenomenon of percolation of localized states occurs in each subband tail, due to the presence of a disorder potential in the QW. The percolation thresholds were determined by using a semiclassical model. The origin of the disorder potential was ascribed to the random distribution of the native point defects at the QW surface. It was also found that a bound state splits off from each subband minimum in the vicinity of a positively charged native point defect. Both the binding energy and the Bohr radius of the bound states could be directly determined. Moreover, it was shown that the binding energy and the Bohr radius are functions of the QW thickness, in quantitative agreement with variational calculations of hydrogenic impurity states.
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Contributor : Simon Perraud <>
Submitted on : Wednesday, July 6, 2011 - 11:26:41 PM
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  • HAL Id : tel-00606632, version 1


Simon Perraud. Scanning tunneling spectroscopy of semiconductor quantum-well structures. Physics [physics]. Université Pierre et Marie Curie - Paris VI, 2007. English. ⟨tel-00606632⟩



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