Molecular beam epitaxy (MBE) was employed to prepare Bi₂Se₃ thin film on Al₂O₃ (001) substrate. Before film growth, the substrate was cleaned in UHV at 300°C for 15 min. Bi was sourced from high purity bismuth (Bi, 99.999%), and Se from selenium (Se, 99.999%). In order to obtain Bi and Se vapour, Bi and Se were put into the effusion tank for co-evaporation. The beam flux ratio of Bi and Se was 1:20, and the growth rate was ∼0.4 quintuple layer (QL) per minute. The total pressure was kept constant at 30 Pa. To enhance the crystalline property of the film, we adopted a two-step growth method. This method can increase the nucleation density of crystal grains on the substrate without causing tex-turing of the film (Park et al., 2016). In the first step, the substrate was heated to 170°C, and then deposited the initial 3–4 QLs of Bi₂Se₃ thin film. Then the obtained ultra-thin film was annealed at 300°C in H₂ atmosphere for 30 min. In the second step, the substrate temperature was raised to 350°C at a rate of 4°C per minute. Then continue to grow to the predetermined thickness of the film. To assess the stability and antioxidant capacity of the Bi₂Se₃ film, all the tests were carried out after 2 weeks of air exposure.

Conventional X-ray diffraction (XRD) technique was employed to represent the crystallographic orientation of the Bi₂Se₃ film on a Bruker D8 Advance X, Pert diffractometer (Cu-Ka: λ = 1.540 A). A scanning speed of 8° per min was chosen, ranging from 10° to 90°. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was used to determine the valence states of the elements. To detect the surface morphology of the film, feld emission scanning electron microscope (FE-SEM JSM-7500F) and atomic force microscope instrument (AFM Veeco DI-3100) were used. To further investigate the atomic arrangement of the film, a high-resolution transmission electron microscope (HRTEM, TEM 2010F) was used. To measure the optical properties of the film, such as transmission and absorption, a UV-vis-NIR spectrophotometer (Shimazu UV-3600PC) was used working in the wavelength range of 250–3,000 nm, plus a Fourier transform infrared spectrometer (FTIR) working in the range of 2.5–12 μm. To examine the electrical properties, a Hall-effect measurement system (ACCENT HL55OOPC) was introduced, the test range was between 90 and 300 K.

Figure 1A shows the XRD patterns of the resulting Bi₂Se₃ film. The only peaks observed in the scanned range are clearly defined and high intensity reflections [(003), (006), (009), (0015), (0021)]. The appearance of only (001) diffraction peaks indicates that the film is preferentially oriented along the c-axis perpendicular to the substrate surface. Further, the layered structures of Bi and Se atoms in the form of Se(1)-Bi-Se (2)-Bi-Se(1) are normal to the c-axis. The width of the half-maximum (FWHM) of the (0015) rocking curve (Figure 1B) is about 0.39°, indicating good crystallinity of the film. From the d-spacing’s of the (0015) peak, the lattice parameters of the Bi₂Se₃ film were determined as a = b = 4.138 (3) Å and c = 28.666 (2) Å , which conforms well to the reported standard card data file (No. 01-089-2008).

Figure 2 shows the X-ray photoelectron spectra (XPS) of Bi-4f and Se-3d of Bi₂Se₃ thin film. In the Bi-4f spectrum, two distinct peaks Bi-4f_7/2 = 158.5 eV and Bi-4f_5/2 = 164 eV are observed, both are intense and with binding energy. This is consistent with the Bi₂O₃ (Bi³⁺) phase, which leads to the conclusion that the Bi ions are in the tervalent state. Figure 2B reveals two adjacent sharp peaks in the Se-3d_5/2 spectrum at 53.6 and 54.3 eV on 3d_3/2 peaks, respectively. Compare with the X-ray Photoelectron Spectroscopy Database of the National Institute of Standards and Technology (NIST), and it can be inferred that the valence state of the Se ions is −2. XPS analysis reveals the valence states of Bi and Se to be +3 and −2 in the Bi₂Se₃ film, which is conducive to the formation of a pure Bi₂Se₃ phase. The atomic content of Bi and Se on the surface of the film is 0.47 and 0.53 respectively. The deviation of the surface stoichiometric ratio is due to the lower vapor pressure of the Se element, so Se is prone to volatilization during the growth process, especially on the surface of the film.

Figure 3A shows the film’s surface morphology atomic force microscope (AFM) image. It shows that the nano-particles is uniform and arrange orderly with an average size of 120 nm. As shown in Figure 3B, a higher-magnification view of the surface revealed triangular crystal facets and terrace structures of the Bi₂Se₃ thin film. The root-mean-square (RMS) roughness value for the 2 μm × 2 μm area is 0.9 nm. The thickness of Bi₂Se₃ film is shown in Figure 3C. The image indicates that the Bi₂Se₃ film thickness is uniform and tightly bonded to the substrate. No lattice defects such as threading dislocations are observed at the interface. In Figure 3D, the interplanar d spacing between lattice fringes is measured to be 0.2 nm, in conformance with the d-spacing of the (11 2 ¯ 0) planes in the pdf file No. 01-089- 2008. Selected area electron diffraction pattern (SAED) of Bi₂Se₃ film shows three different diffraction spacings ([11 2 ¯ 0] [ 1 ¯ 2 1 ¯ 0] and [ 2 ¯ 110]) indexed as a 6-fold symmetric [001] zone axis pattern, conforming to the layered structure along the c-axis orientation. The above conclusions indicate that the Bi₂Se₃ film is a single crystal film, and it preferentially grows along the c-axis direction.

Figure 4A gives the optical transmission spectrum of the Bi₂Se₃ film in visible light-near infrared band. All the measured data have been deducted from the influence of the substrate. The film exhibits a transmittance of 50–70% in the entire visible light band, and a higher transmittance of 70–85% in the near-infrared band. Figure 4B gives the absorption spectrum of Bi₂Se₃ thin film. At 3,000 nm wavelength stands a sharp absorption edge. It is the fundamental absorption which energy corresponds to the energy required for the electron to transition from the top of the valence band to the bottom of the conduction band. Therefore, this absorption edge has been widely adopted to determine the forbidden band width. The optical absorption coefficient (a) and photon energy (hv) has a relation as such: α h ν = A ( h ν − E g ) m (1)

Where E _g is the optical bandgap of the materials, A is a constant, and m hinges on the type of transition: m = 1/2 when band transition is indirect, and m = 2 when band transition is direct (Deng et al., 2013). Figure 4C delineates the linear relationship between (ahv)² and hv, it indicates that the Bi₂Se₃ thin film has a direct bandgap structure. Extend the straight portion of the curve and the bandgap can be estimated as 0.47 eV. This value is a bit smaller than previously reported in the literature of 0.5 eV. Generally, the relationship between the forbidden band width and the carrier concentration can be expressed as such a linear relationship ΔE = ΔE (0) ‒ kn ^1/3, where n is carrier concentration. During the growth of Bi₂Se₃ thin film, high-temperature annealing will cause the absence of Se atoms, which will create Se vacancies in the film, forming a natural n-type semiconductor. The bulk carrier concentration n increases, so the actual measured band gap is lower than the theoretical value. Figure 4D shows the infrared transmission spectrum of the Bi₂Se₃ thin film. The total transmittance of Bi₂Se₃ film in the wavelength range of 3.2 (3,125 wavenumber) −7.2 (1,400 wavenumber) μm is as high as 85% with no obvious characteristic absorption peak. The high transmittance (or low absorptivity), especially in the infrared band, is primarily attributed to the free carrier plasma edge around the far-infrared frequency. Due to the collective oscillation of conduction band electrons, called plasma oscillation, the transmittance of the film decreases at 7.2 μm suddenly. According to Drude’s free electron theory, the plasma oscillation frequency ω_p of the material determines the upper limit of the transmission wavelength. When the incident light frequency ω < ω_p, the film exhibits strong reflectivity; when ω > ω_p, the film exhibits transmittance. Thus, ω_p sets the low cutoff frequency of the transmission band.

In order to further analyze the electrical characteristics and transmission characteristics of Bi₂Se₃ thin films. Multiple temperature Hall effect measurement were carried out for the low temperature range. The influences of temperature on the resistivity, carrier density and mobility were investigated. As well as high transmittance in infrared band, the Bi₂Se₃ thin film also exhibits excellent electrical properties. The graphics of temperature dependent carrier concentration, Hall mobility and resistivity are shown in Figures 5A–C. The electron concentration of the film increases as the temperature rises, and the bulk electron concentration of the film at room temperature is about 1.15×10¹⁷cm⁻³. The Hall mobility decreases as the temperature rises, and the Hall mobility at room temperature is as high as 1,015 cm²/Vs, which is two orders of magnitude higher than the copper-iron ore series p-type infrared transparent conductive film we prepared previously. The reason for the high mobility is that Bi₂Se₃ belongs to a three-dimensional topological insulator structure. The basic feature of the structure is that there are four time-reversed symmetry points in the Brillouin zone of its surface state with Kramers degenerate phenomenon, which form the Dirac Cone. The apex of the Dirac cone is called the Dirac point, and the dispersion relationship between energy and momentum near the apex is linear. Due to the spin-coupling effect, the spin direction of the surface state is always perpendicular to the direction of momentum, so that electrons travel on the surface with low loss (or lossless) at a speed similar to the photon. The Hall coefficient of Bi₂Se₃ film is −7.8 cm³C⁻¹ at room temperature, which indicates characteristic of n-type conduction. The room temperature conductivity of the film is 7 × 10⁻⁴ Ωcm. As shown in Figure 5D, Bi₂Se₃ film shows thermal activation behavior at room temperature, because the plots of lnρ ∼1,000/T show a linear relation. The conductivity can be expressed by 1 ρ = A e x p [ − E a / ( k B T ) ] , where A is constant, Ea is the thermal activation energy, and kB is Boltzmann constant. As temperature increases, electrical conductivity of the material increases, confirmative of the semiconductor nature of Bi₂Se₃ thin film in our study. The estimated E _a is 34 meV, which is less than 10% of the optical bandgap (E_g ≈ 0.47 eV) of Bi₂Se₃, indicating the electronic transport is thermally activated by a donor in the conduction band.

In order demonstrate the application of our thin film, we made N-Bi₂Se₃/P-CuScO₂ infrared transparent heterojunction diode, the schematic diagram of which is shown in Figure 6A. The schematic diagram of this diode is shown in Figure 6A. First, we use the polymer-assisted deposition method to grow the CuScO₂ film with a thickness of about 200 nm on a sapphire substrate, and then deposit a layer of Bi₂Se₃ thin film with a thickness of 150 nm by MBE. Finally, we evaporate In on the surface of the device as the electrode. Figure 6B shows the I–V curve of the heterojunction diode. It can be easily found the rectifying characteristic through the curve, accompanied by a threshold voltage of 3.3 V, which is consistent with the forbidden band width of CuScO₂ (3.3–3.5 eV). Thus, hetero-epitaxial growth at the CuScO₂ (lower part) and Bi₂Se₃ (upper part) interface is demonstrated.