Scanning Gate Microscopy

Conventional transport measurements can provide deep insight into the properties of mesoscopic systems. When an appropriate measurement configuration is chosen, one can, for example, extract information how electrons flow and organize themselves. However, those measurements are typically blind to direct spatial information. Scanning Gate Microscopy (SGM) provides access to the properties of semiconducting samples on a local scale (<100 nm). For example, one can map the current flow in a device or, more generally, study the effect of a localized potential perturbation on the charge transport through the device.

In Scanning Gate Microscopy, a biased metallic tip is scanned above the sample of interest. A potential perturbation is locally induced due to the capacitive coupling between tip and device, and the resulting modulation of the conductance through the device is recorded. Because of the purely capacitive coupling between probe and device, SGM can also be used to study semiconductor heterostructures with a buried conducting layer.

Over the past years, we have built and continuously improved our SGM system. The scanning system is mounted on a top-loading probe for a 3He cryostat which allows for measurements at low temperatures (down to ~400 mK) and high magnetic fields (up to 8T). We use slip-stick coarse positioners to align the tip to the region of interest on the sample and a piezoelectric tube for the actual scans. The SGM tips sit on piezoresistive cantilevers which allow for in-situ AFM measurements. Our cantilevers are fabricated [1] in the group of Prof. Beth Pruitt in the Department of Mechanical Engineering at Stanford (http://microsystems.stanford.edu). In the past, we used Scanning Gate Microscopy to study the electron flow emerging from a quantum point contact (QPC) into a two-dimensional electron gas in GaAs/AlGaAs heterostructures [2].


Fig. 1: The SGM system is mounted at the head of a top-loading probe for a 3He cryostat. The two stages allow for precise alignment of the tip to the sample (not shown in image, mounted “face-up” below the tip).

The current focus of our SGM experiments is to study the edge-state transport of devices in the Quantum Spin Hall (QSH) state, the two-dimensional realization of a topological insulator (TI). This state is characterized by transport in helical edge states (in 3D TIs: surface transport), while the bulk is insulating. The opposite spin polarization of the counter-propagating edge states results in a suppression of backscattering as long as time-reversal symmetry is preserved. Nonetheless, earlier transport measurements [3] have shown that the expected dissipationless transport can only be observed if the sample length does not exceed approximately 1 micron. However, there is no clear understanding of the mechanism behind the observed backscattering yet.

Our devices are fabricated from HgTe quantum well structures grown in the group of Prof. Laurens Molenkamp at the University of Würzburg (Germany) (http://www.physik.uni-wuerzburg.de/EP3/). Due to a very strong spin-orbit coupling, HgTe as a bulk material has an inverted band structure. In quantum wells with a thickness above a critical thickness of approximately 6 nm, the inversion of the band structure is preserved, resulting in the QSH state when the Fermi level is located in the bulk gap.


Fig. 2: Scanning Gate Microscopy can be used to probe the QSH edge states on a local scale. The measured modulation of the conductance provides insight into the induced backscattering.

In our experiments, Scanning Gate Microscopy is used to locally perturb the QSH edge states. The induced potential perturbation is expected to result in a suppression of the edge-state conductance. The first goal of the experiments is to unequivocally demonstrate the presence of the edge states and to gain information about their spatial properties, e.g., their extension into the bulk. We then plan to measure tip-induced conductance changes as a function of the strength, shape, and location of the induced potential perturbation, to shed light on the detailed scattering mechanisms that suppress the otherwise non-dissipative transport. The role of time-reversal symmetry with respect to the backscattering of the edge states can be explored in further experiments in a finite magnetic field.

References

  1. Tip fabrication and characterization:
  2. Earlier work on current flow in GaAs-AlGaAs two-dimensional electron gas
  3. Transport experiments in the Quantum Spin Hall state:
    • M. König, S. Wiedmann, C. Brüne, A. Roth, H. Buhmann, L.W. Molenkamp, X.L. Qi, and S.C. Zhang: "Quantum Spin Hall Insulator State in HgTe Quantum Wells," Science 318, 766 (2007) (http://www.sciencemag.org/content/318/5851/766.full)
    • M. König, H. Buhmann, L.W. Molenkamp, T.L. Hughes, C.X. Liu, X.L. Qi, and S.C. Zhang: "The Quantum Spin Hall Effect: Theory and Experiment," Journal of the Physical Society of Japan 77, 031007 (2008) (http://jpsj.ipap.jp/link?JPSJ/77/031007/)
Contact M. König (markusk@), Matthias Baenninger (mbaennin@), or Reyes Calvo (rcalvo@) for more information.