Research Articles

Authors' Summary:
Quantum Spin Hall Insulator State in HgTe Quantum Wells

Markus König et al.

The discovery more than 25 years ago of the quantum Hall effect (1), in which the “Hall,” or “transverse electrical” conductance of a material is quantized, came as a total surprise to the physics community. This effect occurs in layered metals at high magnetic fields and results from the formation of conducting one-dimensional channels that develop at the edges of the sample. Each of these edge channels, in which the current moves only in one direction, exhibits a quantized conductance that is characteristic of one-dimensional transport. The number of edge channels in the sample is directly related to the value of the quantum Hall conductance. Moreover, the charge carriers in these channels are very resistant to scattering. Not only can the quantum Hall effect be observed in macroscopic samples for this reason, but within the channels, charge carriers can be transported without energy dissipation. Therefore, quantum Hall edge channels may be useful for applications in integrated circuit technology, where power dissipation is becoming more and more of a problem as devices become smaller. Of course, there are some formidable obstacles to overcome—the quantum Hall effect only occurs at low temperatures and high magnetic fields.

In the past few years, theoretical physicists have suggested that edge channel transport of current might be possible in the absence of a magnetic field. They predicted (2–4) that in insulators with suitable electronic structure, edge states would develop where—and this is different from the quantum Hall effect—the carriers with opposite spins move in opposite directions on a given edge, as shown schematically in the figure. This is the quantum spin Hall effect, and its observation has been hotly pursued in the field.

Resize Image

Schematic of the spin-polarized edge channels in a quantum spin Hall insulator.

Credit: C. Bickel/Science

Although there are many insulators in nature, most of them do not have the right structural properties to allow the quantum spin Hall effect to be observed. This is where HgTe comes in. Bulk HgTe is a II-VI semiconductor, but has a peculiar electronic structure: In most such materials, the conduction band usually derives from s-states located on the group II atoms, and the valence band from p-states at the VI atoms. In HgTe this order is inverted, however (5). Using molecular beam epitaxy, we can grow thin HgTe quantum wells, sandwiched between (Hg,Cd)Te barriers, that offer a unique way to tune the electronic structure of the material: When the quantum well is wide, the electronic structure in the well remains inverted. However, for narrow wells, it is possible to obtain a “normal” alignment of the quantum well states. Recently, Bernevig et al. (6) predicted theoretically that the electronic structure of inverted HgTe quantum wells exhibits the properties that should enable an observation of the quantum spin Hall insulator state. Our experimental observations confirm this.

These experiments only became possible after the development of quantum wells of sufficiently high carrier mobility, combined with the lithographic techniques needed to pattern the sample. The patterning is especially difficult because of the very high volatility of Hg. Moreover, we have developed a special low–deposition temperature Si-O-N gate insulator (7), which allows us to control the Fermi level (the energy level up to which all electronics states are filled) in the quantum well from the conduction band, through the insulating gap, and into the valence band. Using both electron beam and optical lithography, we have fabricated simple rectangular structures in various sizes from quantum wells of varying width and measured the conductance as a function of gate voltage.

We observe that samples made from narrow quantum wells with a “normal” electronic structure basically show zero conductance when the Fermi level is inside the gap. Quantum wells with an inverted electronic structure, by contrast, show a conductance close to what is expected for the edge channel transport in a quantum spin Hall insulator. This interpretation is further corroborated by magnetoresistance data. For example, high–magnetic field data on samples with an inverted electronic structure show a very unusual insulator-metal-insulator transition as a function of field, which we demonstrate is a direct consequence of the electronic structure.

The spin-polarized character of the edge channels still needs to be unequivocably demonstrated. For applications of the effect in actual microelectronic technology, this low-temperature effect (we observe it below 10 K) will have to be demonstrated at room temperature, which may be possible in wells with wider gaps.

Summary References

1. K. v. Klitzing, G. Dorda, M. Pepper, Phys. Rev. Lett. 45, 494 (1980).
2. S. Murakami, N. Nagaosa, S.-C. Zhang, Phys. Rev. Lett. 93, 156804 (2004).
3. C. L. Kane, E. J. Mele, Phys. Rev. Lett. 95, 146802 (2005).
4. B. A. Bernevig, S.-C. Zhang, Phys. Rev. Lett. 96, 106802 (2006).
5. A. Novik et al., Phys. Rev. B 72, 035321 (2005).
6. B. A. Bernevig, T. L. Hughes, S.-C. Zhang, Science 314, 1757 (2006).
7. J. Hinz et al., Semicond. Sci. Technol. 21, 501 (2006).

Did you find this summary useful? Let us know your thoughts by taking a brief survey.

Read the Full-Text Research Article