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Structural and electronic properties of highly doped topological insulator Bi 2 Se 3 crystals
| Content Provider | Semantic Scholar |
|---|---|
| Author | Cao, Helin Xu, Suyang Miotkowski, Ireneusz Tian, Jifa Pandey, Deepak Hasan, Mohammad Z. Chen, Yong P. |
| Copyright Year | 2012 |
| Abstract | We present a study of the structural and electronic properties of highly doped topological insulator Bi2Se3 single crystals synthesized by the Bridgeman method. Lattice structural characterizations by X-ray diffraction, scanning tunneling microscopy, and Raman spectroscopy confirmed the high quality of the as-grown single crystals. The topological surface states in the electronic band structure were directly revealed by angle-resolved photoemission spectroscopy. Transport measurements showed the conduction was dominated by the bulk carriers and confirmed a previously observed bulk quantum Hall effect in such highly doped Bi2Se3 samples. We briefly discuss several possible strategies of reducing bulk conductance. Introduction The electronic properties of Bi2Se3, a semiconductor made of van der Waals coupled stacking “quintuple layers” (QL), were thought to be well understood since it had been studied for decades in light of its excellent thermoelectric properties. However, recently, it has been revealed that Bi2Se3 belongs to a new class of quantum materials: 3D topological insulators (TI). In a 3D TI, the bulk has a band gap (~0.3 eV for Bi2Se3), and the surface has non-trivial topologically protected surface states (SS), which give rise to 2D Dirac fermions with an odd number of Dirac cones (1 for Bi2Se3) and spin-momentum locking. Most as-grown Bi2Se3 samples have significant amount of (uncontrolled) Se vacancies that cause unintentional n-type bulk doping. Previous, welldocumented magnetotransport studies of Bi2Se3 with bulk carrier densities between ~ 10 and 10 cm show standard 3D transport behavior of a doped bulk semiconductor. Interestingly, recent measurements on Bi2Se3 in lower and higher carrier density regimes have both revealed novel magnetotransport behaviors. In very low bulk doping (with carrier density <10 cm) samples (synthesized with compensating dopants, such as Sb), quantum * Corresponding authors: e-mail: cao2@purdue.edu, Phone: +01 765 496 3059, Fax: +01 765 494 0706; e-mail: yongchen@purdue.edu, Phone: +01 765 494 0947, Fax: +01 765 494 0706. oscillations of the 2D Dirac fermions attributed to TI SS have been observed. In very high bulk doping (carrier density >~10 cm) samples, a bulk quantum Hall effect (QHE) with a 2D like transport behavior arising from parallel QLs in the 3D bulk has been observed. Here, we present comprehensive structural and electronic characterizations on such highly doped Bi2Se3. The structure of the crystals was analyzed by X-ray diffraction (XRD), scanning tunneling microscopy (STM), and Raman spectroscopy. The TI SS in our sample was directly revealed by angle-resolved photoemission spectroscopy (ARPES). On the other hand, our transport measurements show that the electronic conduction is dominated by the bulk. We present the temperature (T) dependent resistance data on samples with different thicknesses (from ~ 60 nm to 310 nm), and magnetotransport data on one of the sample (“A”) confirming the previously observed bulk QHE (up to higher magnetic field than that in Ref. 12). Our results underscore the challenge of accessing surface state electronic transport (due to high bulk conduction) in asgrown Bi2Se3 bulk crystals. At the end of this paper, we briefly discuss several possible strategies of reducing bulk doping or conduction. Figure 1 (a) Representative XRD pattern on single crystal Bi2Se3 (001) surface. The peaks are labeled with (hkl) Miller indices. (b) Atomic resolution STM image of a cleaved surface of Bi2Se3 crystal. (c) Representative Raman spectrum measured on a Bi2Se3 crystal. Three characteristic Raman peaks are labeled. Inset: optical image of the Bi2Se3 single crystals. (d) Highresolution ARPES energy-momentum dispersion band mapping along a pair of time-reversal invariant points M-Γ-M on Bi2Se3. Results and discussion High quality Bi2Se3 single crystals (optical images shown in the inset of Fig. 1c) were synthesized by the Bridgman technique (see Supplementary Information). Fig. 1a shows an XRD pattern measured on Bi2Se3 (001) surface. The XRD reflections are attributed to Bi2Se3 (0,0,6) (0,0,9) (0,0,12) (0,0,15) planes, confirming the sample’s single crystalline structure. From powder XRD data (not shown), we extract two lattice constants, a = 4.137 Å and c = 28.679 Å, which are consistent with the previously reported values. The atomic resolution STM image (Fig. 1b) of a cleaved Bi2Se3 (001) surface was measured in ultrahigh vacuum, showing a lattice constant a = 4.2 Å, confirming the XRD result. The lattice structure of our samples was further investigated by Raman spectroscopy. A representative Raman spectrum is shown in Fig. 1c. Three Raman peaks at proximately 72 cm, 131.8 cm, and 174.9 cm agree well with the characteristic lattice vibration modes A1g, Eg and A1g observed in previous study. The topological SS of our samples were directly revealed by ARPES as shown in Fig. 1d. The Fermi level is located at 150meV above the bulk conduction band minimum (CBM) in this sample. The Fermi wave vector and Fermi velocity of the SS are found to be kF = 0.1 Å and vF = 5×10 m/s, similar to previous APRES measurements on Bi2Se3 (see Supplementary Information for more analysis on the ARPES data). Fig. 2a shows the 4-terminal longitudinal resistance (Rxx) of a 150 nm-thick exfoliated flake (sample A) measured from room T down to 5K, displaying a metallic behavior, as qualitatively expected for the highly doped bulk. The T dependence of Rxx can be fitted to a simplified phenomenological model developed for doped Bi2Te3 bulk crystals (which shows generally similar transport properties as doped Bi2Se3) Rxx = R0+α×e+β×T (1) R0, a low T residual resistance, corresponds to the contributions of impurity scattering. Phonon scattering and electron-electron scattering give rise to the exponential and the quadratic terms respectively. We found R0 = 22.46 Ω, α = 13.5 Ω, θ = 217 K and β = 0.00009 Ω/K give the best fit (as shown by the red line in Fig. 2a) to the experimental data (round black circle in Fig. 2a). The T dependence is dominated by the phonon scattering and the fitting parameter θ corresponds to an effective phonon frequency ω = kBθ/ħ = 3.1×10 rad/s. The very small value of β indicates that e-e scattering effect is negligible in our sample. We measured T dependence resistivity on 6 samples with different thicknesses. As an indicator of the metallic behavior, we take the high T (270 K) resistance normalized by the respective low T (15 K) value, and plot the ratio against the sample thickness (Fig. 2b). Interestingly, the thinner samples appear to be “less metallic” as measured by this ratio. Fig. 2b suggests that thinning down the crystal thickness can be an effective means to reduce the metallic bulk conduction of Bi2Se3 (even for samples with high bulk doping) that may help bring out the SS transport signatures that are often overwhelmed by the bulk conduction. Figure 2 (a) Temperature dependence of four-terminal longitudinal resistance (Rxx) in sample A. The red line shows the fitting to Eq. 1. (b) R(T = 270 K)/R(T = 15 K) plotted against sample thicknesses. Fig. 3a shows Rxx and Hall resistance Rxy for sample A as functions of perpendicular magnetic field (B) applied along the c-axis at 340 mK. The carrier (n-type) density and mobility extracted from the low B measurements are 4.7×10 cm and ~400 cm/Vs respectively. At higher B, Rxx oscillates periodically in 1/B (with a period BF), which can be interpreted as SdH oscillations due to the formation of Landau levels (LL). The N minimum of Rxx, counting from B = BF (which defines N=1), corresponds to the N LL (labeled in Fig. 3a). We plot the assigned LL index N against the inverse of the magnetic field (B) positions of the observed minima in Rxx(B) in the inset of Fig 3a. The black solid line is a linear fit with N-axis intercept 0±0.02 and slope BF = 163T, corresponding to a bulk carrier Fermi wave vector kF = 0.07 Å. Furthermore, accompanying the minima in Rxx, Rxy shows developing quantized plateaus. In Fig. 3b, we plot normalized quantized Hall step size Δ(1/Rxy) and ΔRxy (difference between two adjacent plateaus in 1/Rxy and Rxy respectively, then normalized by their own values at N = 6) as functions of LL index N. It clearly shows Δ(1/Rxy) is largely independent with increasing LL index, while ΔRxy decreases. The approximately constant value of Δ(1/Rxy) for different LLs is ~1.2e/h per QL (the number of QLs is determined by sample’s thickness, where the scaling of Δ(1/Rxy) with the thickness as observed previously further confirms the transport is dominated by the bulk). The quantization in Rxy can be interpreted as a “bulk QHE” attributed to parallel 2D electron gas’s arising from the stacking QLs in highly doped Bi2Se3 crystals (carrier density > ~10 cm, where the Se vacancies may help reduce the electronic couplings between the QLs) , and not caused by SS or any other surface conduction channels. The quantized Hall step size in 1/Rxy was examined down to lower LLs compared to our previous study, and confirmed to remain approximately constant for different LLs. Our results (here and in Ref. 12), along with those from earlier experiments 11, 17-19, on less-doped Bi2Se3, demonstrate the rich physics in the magnetotransport of Bi2Se3 in different regimes of bulk carrier densities. Figure 3 (a) Hall resistance (Rxy) and Rxx of sample A as functions of magnetic field (B, perpendicular to the QLs) measured at T = 340 mK. The minima in Rxx corresponding to a series of Landau Level indices are labeled with arrows. The inset: LL fan diagram. (b) Normalized step size in 1/Rxy and Rxy plotted against LL index (N). Conclusion High quality Bi2Se3 single crystals were synthesized by Bridgman techniques. The lattice structure is characterized by XRD, STM and Raman spectroscopy that confirmed the excellent crystalline quality. Our asgrown crystals are found to have high bulk carrier density (>10 cm, presumabl |
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| Alternate Webpage(s) | http://www.physics.purdue.edu/quantum/files/Bi2Se3_PSS-RRL.pdf |
| Alternate Webpage(s) | http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2339&context=nanopub |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
