Fig. 1. (a-h) AFM images showing the surface morphology of pristine and F-doped Bi2Te3 samples with different plasma treatment time (tP = 0-70 s), respectively. Here, the analyzed sample area is 2 μm × 2 μm. (i) RMS roughness of those samples as a function of tP.

Fig. 2. (a) XRD patterns and (b) Raman spectra of pristine and F-doped Bi2Te3 samples (tP = 0-70 s).

Fig. 3. (a) XRF results showing the content of Bi, Te, and F elements in pristine and F-doped Bi2Te3 samples. (b) Magnified XRF profile of F dopants as a function of tP. (c) TOF-SIMS depth profiles of Bi and Te in a pristine sample. (d) TOF-SIMS depth profiles of F in pristine and F-doped samples.

Fig. 4. (a, b) High-resolution TEM images of Bi2Te3 samples compacted at 533 K, respectively. The inset images show FFT results recorded from the focused area (1 nm × 1 nm). (c, d) EDS data of pristine and F-doped samples (tP = 0 and 40 s), respectively.

Fig. 5. (a) Wide-scan XPS survey of compacted samples at 533 K with the use of Ar etching. Here, XPS peaks noted with numbers from 1 to 11 correspond to Te 3p, Te (MNN), Bi 4p, Te 3d, O 1 s, Bi 4d, Bi 4f, Te 4d, Bi 5d, F 1 s, and C 1 s, respectively. Gaussian-fitted components in XPS peaks: (b) C 1 s, (c) O 1 s, (d) F 1 s (Bi 4p), (e) Te 3d, and (f) Bi 4f of Bi2Te3 sample after Ar etching and F-doped-Bi2Te3 sample before and after Ar etching.

Fig. 6. DFT calculation results of undoped Bi2Te3 supercell. (a, b) Band structure and density of states (DOS) of undoped Bi2Te3 supercell, respectively, plotted using BURAI. The results show that the Fermi level (normalized to 0 eV) of the undoped Bi2Te3 is almost in the middle of the bandgap (~0.368 eV). Here, 13 k-points (Γ, L, B1, B, Z, Γ, X, Q, F, P1, Z, L, and P) were used for obtaining the band structures.

Fig. 7. DFT calculation results of F-doped Bi2Te3 supercell. (a, b) Band structure and DOS of F-doped Bi2Te3 supercell having one interstitial F dopant, respectively. (c, d) Band structure and DOS of F-doped supercell in which two interstitial F dopants bind to one Te atom, respectively. (e) DOS of F-doped supercell where two interstitial F dopants bind to one Te atom and one Bi atom (in a Bi-Te pair). Here, red arrows indicate the positions of F dopants.

Fig. 8. DFT calculation results of F-doped Bi2Te3 supercell. (a, b) Band structure and density of states (DOS), respectively, of F-doped Bi2Te3 supercell where F dopant substituted the 40th Te atom. (c, d) DOS of F-doped Bi2Te3 supercells where F dopant substituted the 5th and 30th Te atoms, respectively. Here, red arrows indicate the positions of F dopants in the supercell models. Here, red arrows indicate the positions of F dopants.

Fig. 9. (a) Hall coefficients of pristine and F-doped samples (tP = 0 and 40 s) showing that electron is their main charge carrier. (b) Bulk concentration, conductivity, and Hall carrier mobility of the pristine and F-doped samples measured at 273 and 345 K.

Fig. 10. Thermoelectric properties of Bi2Te3 samples with tP = 0-70 s. Their temperature-dependent changes in (a) electrical conductivity (σ), (b) Seebeck coefficient (α), (c) total thermal conductivity (kT), (d) electronic thermal conductivity (ke), (e) lattice thermal conductivity (kL), and (f) figures of merit (ZT).