Research ArticleMATERIALS SCIENCE

Persistent optical gating of a topological insulator

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Science Advances  09 Oct 2015:
Vol. 1, no. 9, e1500640
DOI: 10.1126/sciadv.1500640
  • Fig. 1 Persistent optical gating of a TI channel.

    (A) Longitudinal resistance as a function of time during UV and visible illumination. For t < 1900 s, a series of 30-s exposures to UV light (purple highlighting), followed by 120-s dark periods, illustrates the optical gating effect and its persistence as the sample’s chemical potential is tuned across the charge neutrality point. For t > 1900 s, a series of red light exposures (pink highlighting) reverses the effect. The exposure times were chosen for clarity given the differing kinetics of the two effects. The backside of the sample was held at 0 V for the duration of the experiment. (B) Schematic of measurement setup, showing Van der Pauw indices of the electrical contacts. (C) Schematic of the band structure of (Bi,Sb)2Te3, showing the effect of optical illumination on the chemical potential μ of the TI layer (dotted line and arrows).

  • Fig. 2 Charge carrier response to electrostatic and optical gating.

    (A) Longitudinal resistivity ρxx (blue circles) and Hall coefficient RH (red squares) as a function of electrostatic back-gating. The peak in resistivity and change in Hall coefficient sign show the ambipolar response of the TI channel. (B) Resistivity and Hall coefficient after a series of consecutive timed exposures to UV light, illustrating the optical gating effect. (C and D) 2D carrier concentration n2D (blue circles) and Hall mobility μH (red squares) as a function of optical and electrostatic gating calculated with a one-carrier model. A data point was omitted from (C) and (D) because n2D diverges as RH→0 in this model. Data in all plots were collected in the dark at least 60 s after any illumination had ceased.

  • Fig. 3 Optical and electrostatic tuning of WAL.

    (A and B) Magnetoconductance ΔG of a (Bi,Sb)2Te3/SrTiO3 heterostructure as a function of (A) electrostatic and (B) optical gating. A zero-field cusp develops when the sample is subjected to a positive back-gate voltage or after exposure to UV light. This is consistent with enhancement of WAL as the chemical potential rises above the top of the valence band and surface-bulk scattering is reduced. Qualitatively similar behavior is seen with both gating techniques. Data in all plots were collected in the dark at least 60 s after any illumination had ceased. Data are offset for clarity.

  • Fig. 4 Spectral and temperature dependence.

    (A) Spectral dependence of the optical gating effect. The relative change in longitudinal resistance ΔR (blue circles) is shown due to identically timed exposures to different energies of light. Before each exposure, red and UV light were used to reset the chemical potential to a similar starting position in the p-type regime such that ΔR maps roughly to chemical potential shift Δμ (see inset). The transmission spectrum (red squares) of an identically prepared SrTiO3 substrate is shown for comparison. Both measurements were conducted at 5.2 K. (B) Temperature dependence of the optical gating effect, showing its persistence to room temperature. Longitudinal resistance is plotted as a function of time at various temperatures before, during (purple highlighting), and after UV illumination. Traces are offset for clarity. The 295 K trace is multiplied by 10.

  • Fig. 5 Writing and imaging p-n junctions in a TI.

    (A) Scanning reflectance image of a (Bi,Sb)2Te3 channel. (B and C) Photocurrent images of the same region showing the longitudinal component of chemical potential gradients due to p-n junctions patterned with the optical gating technique. The channel edges are shown as gray lines for reference. In each image, the field of view was first initialized p-type by exposure to red light from a HeNe laser. Rectangular areas (dotted lines) were exposed to UV light before imaging, locally gating these regions n-type. UV exposure and photocurrent imaging were repeated, and the images were averaged to reduce noise. (D and E) Schematics generated by numerical integration of (B) and (C), depicting the chemical potential as a function of lateral position on the channel. The temperature was 5 K.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/9/e1500640/DC1

    § S1. Persistence of the optical gating effect for 16 hours.

    Fig. S1. Persistence of the optical gating effect for 16 hours.

    § S2. Superposition of electrostatic and optical gating.

    Fig. S2. Superposition of electrostatic and optical gating.

    § S3. Optical gating of ≈50-nm sputtered ZnO film on SrTiO3.

    Fig. S3. Optical gating of ≈50-nm sputtered ZnO film on SrTiO3.

    § S4. Mechanism of the optical gating effect.

    Fig. S4. Mechanism of the optical gating effect.

    § S5. Sample growth and characterization.

    Fig. S5. Sample growth and characterization.

    References (4245)

  • Supplementary Materials

    This PDF file includes:

    • S1. Persistence of the optical gating effect for 16 hours.
    • Fig. S1. Persistence of the optical gating effect for 16 hours.
    • S2. Superposition of electrostatic and optical gating.
    • Fig. S2. Superposition of electrostatic and optical gating.
    • S3. Optical gating of ≈50-nm sputtered ZnO film on SrTiO3.
    • Fig. S3. Optical gating of ≈50-nm sputtered ZnO film on SrTiO3.
    • S4. Mechanism of the optical gating effect.
    • Fig. S4. Mechanism of the optical gating effect.
    • S5. Sample growth and characterization.
    • Fig. S5. Sample growth and characterization.
    • References (42–45)

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