Imaging Percolation of Localized States in a Semiconductor Quantum Well


Simon Perraud1,2, Kiyoshi Kanisawa1, Zhao-Zhong Wang2, and Toshimasa Fujisawa1
1Physical Science Laboratory, 2LPN-CNRS

  One of the advantages of two-dimensional electron system (2DES) formed in semiconductor heterostructures is the possibility to control the main system parameters, including electron confinement tuned by varying the thickness of the grown layers and electron density tuned by applying an external electric field. Recently, we found that the Fermi level was mostly unpinned at the (111)A clean surface of n-type In0.53Ga0.47As [1]. Therefore, it could be possible to vary the electron density in the (111)A-oriented In0.53Ga0.47As surface quantum well (QW) to access 2DES by using scanning tunneling spectroscopy (STS) measurements in the ultra-high vacuum (UHV). This allows us to perform studies of electron phenomena in disordered 2DES at the nanometer-scale spatial resolution. The effect of disorder is very important to understand various electron behaviors, especially the many-body phenomena in the semiconductor structures.
  An In0.53Ga0.47As/In0.52Al0.48As multi-subband surface QW was grown by molecular beam epitaxy on lattice-matched InP(111)A substrate, and the electronic local density of states (LDOS) in this QW was measured at 5 K by low-temperature STS in UHV. The LDOS in the conduction band has a clear step-like energy dependence, revealing that 2DES subbands are formed in the QW (Fig. 1). At a given energy, the LDOS shows strong spatial fluctuations in the QW plane due to the presence of a disorder potential. The formation of the localized states is due to quantum-mechanical interference between electron waves that have undergone multiple scatterings by the disorder potential. Percolation of localized states with increasing energy is observed in each subband tail. This percolation is explained by using a semiclassical model. The origin of the disorder potential is ascribed to a random distribution of native point defects located at the QW surface [2].
  This work was partly supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science, JSPS KAKENHI (16206003).

[1] S. Perraud, et al., Appl. Phys. Lett. 89 (2006) 192110.
[2] S. Perraud, et al., Phys. Rev. B 76 (2007) 195333.

Fig. 1. Measured dI/dU spectrum averaged over the whole area (214 nm × 214 nm) of the InGaAs surface QW at 5 K in UHV. Each arrow indicates the percolation threshold of each subband calculated by the semiclassical model. Observed dI/dU spatial maps at several values of U covering the transition (a) from the band gap to the first subband, (b) from the first to the second subband, and (c) from the second to the third subband.

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