Controllable synthesis and electronic structure characterization of multiple phases of iron telluride thin films

We present an investigation on the controlled growth of epitaxial iron telluride films of multiple phases by molecular beam epitaxy. By optimizing the substrate temperature, we fabricate different phases of α-FeTe, β-FeTe, and $\mathrm{Fe}{\mathrm{Te}}_2$, respectively, whose crystalline morphologies are determined by means of in situ scanning tunneling microscopy/spectroscopy and ex situ scanning transmission electron microscopy. While both α- and β-FeTe films are metallic, we uncover a ∼185- and 65-mV semiconducting band gap for the (100) and (011) facets of $\mathrm{Fe}{\mathrm{Te}}_2$ film, respectively, with the former one being compatible with the first principles calculations. Moreover, for $\mathrm{Fe}{\mathrm{Te}}_2$, we observe reduced gaps with enhanced conductance in the vicinity of edge boundaries, which are dependent on the geometries of step orientation and possibly in correlation to the magnetic anisotropy with structural distortion. Our study provides insight into the controllable synthesis of Fe-Te compounds with variation of stoichiometries and surface terminations, which may generalize to other epitaxial Fe chalcogenide films.

Figure 1

α- and β-FeTe films grown on a $\mathrm{SrTi}{\mathrm{O}}_3$ substrate. (a),(b) 3D and top views of atomic structures for bulk α- and β-FeTe, respectively. (c),(d) High resolution STM images for α- and β-FeTe surface, respectively. Toy models are superposed onto both surfaces, with the unit cell marked as black rhombus and square, respectively. Scanning conditions: $3\times 3\mathrm{n}{\mathrm{m}}^2$, $V_b=+100\mathrm{mV}$, $I_t=50\mathrm{pA}$. (e) Comparison of tunneling spectra between α- and β-FeTe films. The $y$ axis is offset for clarity, with the blue horizontal bar marking the zero conductance ($V_b=+0.1\mathrm{V}$, $I_t=100\mathrm{pA}$, $V_{\mathrm{mod}}=+1.414\mathrm{m}{\mathrm{V}}_{\mathrm{rms}}$).

Figure 2

The (100) surface of orthorhombic $\mathrm{Fe}{\mathrm{Te}}_2$ films. (a) The crystallographic structure of $\mathrm{Fe}{\mathrm{Te}}_2$ in a perspective view. Two distinct planes of (100) and (011) are shaded as blue and pink sections, respectively. (b),(c) Top and side views of atomic structure for the (100) plane of $\mathrm{Fe}{\mathrm{Te}}_2$ lattice, respectively. The topmost Te atoms are slightly buckled. A rectangular unit cell of (0.39 ± 0.01) nm × (0.63 ± 0.01) nm is marked. (d) STM morphology of $\mathrm{Fe}{\mathrm{Te}}_2$ films oriented along the [100] direction ($200\times 100\mathrm{n}{\mathrm{m}}^2$, $V_b=+2.0\mathrm{V}$, $I_t=10\mathrm{pA}$). (e) The line profile across a step edge along the black line in (d), with a step height of 0.54 ± 0.01 nm. (f) The atomically resolved image of the (100) surface. Inset shows a toy model of Te atoms, with the 0.38 nm × 0.63 nm unit cell labeled as black rectangle ($3\times 2\mathrm{n}{\mathrm{m}}^2$, $V_b=+100\mathrm{mV}$, $I_t=50\mathrm{pA}$).

Figure 3

The (011) surface of orthorhombic $\mathrm{Fe}{\mathrm{Te}}_2$ films. (a) STM morphology of $\mathrm{Fe}{\mathrm{Te}}_2$ films oriented along the [011] direction ($150\times 150\mathrm{n}{\mathrm{m}}^2$, $V_b=+2.0\mathrm{V}$, $I_t=10\mathrm{pA}$). Inset is a line profile across the step edge of 0.33 ± 0.01 nm. (b) Top and side views of atomic structure for the (011) plane of $\mathrm{Fe}{\mathrm{Te}}_2$ lattice, respectively. A rectangular 2 × 1 reconstruction of (0.74 ± 0.01) nm × (1.05 ± 0.01) nm is marked. The Te atoms bonding with neighboring Fe atoms are also marked as cyan ellipses. (c)–(e) The closeup STM images of the (011) surface at different bias voltages. The corresponding Fourier transformation of (a) is inserted in the top left corner. Toy models with 2 × 1 rectangle and lattice constants are superposed. In (d), the elongated Te atoms can be seen by cyan ellipses, corresponding to the bonding features of Te and Fe atoms in (b). Scanning conditions: $5\times 5\mathrm{n}{\mathrm{m}}^2$, $I_t=50\mathrm{pA}$, $V_b=+120\mathrm{mV}$ for (c), −200 mV for (d), and +1000 mV for (e). (f) The cross-sectional STEM image of a typical $\mathrm{Fe}{\mathrm{Te}}_2$ grown on $\mathrm{SrTi}{\mathrm{O}}_3$ substrate. The protective Te layer is also easy to resolve. (g) A zoomed-in ADF image from red square in (f). A toy model with unit cell and lattice constants is superposed. (h) Top view of atomic structure along the normal of (100) plane, reflecting the STEM results of cross section.

Figure 4

Electronic structures of $\mathrm{Fe}{\mathrm{Te}}_2$ films. (a) STS spectra recorded on the (100) and (011) terrace, showing a semiconducting gap of ∼185 and 65 meV, respectively, comparing to our theoretical calculations ($V_b=+0.2\mathrm{V}$, $I_t=100\mathrm{pA}$, $V_{\mathrm{mod}}=+1.414\mathrm{m}{\mathrm{V}}_{\mathrm{rms}}$). Each point spectrum is taken at the location of solid circle in Figs. 2 and 3 for the (100) and (011) surfaces, respectively. The energy range of calculated DOS is rescaled by shifting $E_{\mathrm{F}}$ to −80 meV for better comparison. All spectra are shifted along $y$ axis for clarity. The gray horizontal lines represent the corresponding zero differential conductance for each curve. (b) Calculated band structure and orbital characters of bulk $\mathrm{Fe}{\mathrm{Te}}_2$ by first-principles calculations. The band gap is indirect with the size of 191 meV, as amplified around the Fermi energy inset. The red, green, and blue symbols mark the contributions of $p_z$ orbitals of Te, ${d_z}^2$ and $d_{\mathrm{xz}}$ orbitals of Fe, respectively. The size of the symbols corresponds to the weight of the states. The Fermi level is set as zero with dashed cyan lines.

Figure 5

Edge states of $\mathrm{Fe}{\mathrm{Te}}_2$ films. (a) STM topography of the $\mathrm{Fe}{\mathrm{Te}}_2$(100) surface containing a step edge ($15\times 3\mathrm{n}{\mathrm{m}}^2$, $V_b=+0.1\mathrm{V}$, $I_t=100\mathrm{pA}$). (b) Series of dI/dV spectra recorded near the step edge of the (100) terrace along the magenta line in (a), showing the gradual reduction of band gap size. The corresponding location of each point spectrum is indicated by the solid circle in (a) with the same color. The spectra are shifted along $y$ axis for clarity. (c) 2D plot of tunneling spectra for (b) in spatial variation. The edge states are apparently seen when approaching the boundaries. Set points: $V_b=+0.2\mathrm{V}$, $I_t=100\mathrm{pA}$, $V_{\mathrm{mod}}=+2.828\mathrm{m}{\mathrm{V}}_{\mathrm{rms}}$. (d)–(f) The same as (a)–(c), but acquired near the $\mathrm{Fe}{\mathrm{Te}}_2$(011) step edge.