⇦Chinese Physics Letters, 2020, Vol. 37, No. 7, Article code 077501 Site Preference of Se and Te in Bi$_2$Se$_{3-x}$Te$_x$ Thin Films Yizhe Sun (孙逸哲)1, Moorthi Kanagaraj1, Qinwu Gao (高钦武)1, Yafei Zhao (赵亚飞)1, Jiai Ning (宁纪爱)1, Kunpeng Zhang (张昆鹏)2, Xianyang Lu (陆显扬)1,2, Liang He (何亮)1,3*, and Yongbing Xu (徐永兵)1,2,3* Affiliations 1National Laboratory of Solid State Microstructures and Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China 2York-Nanjing Joint Center (YNJC) for Spintronics and Nano-engineering, University of York, York YO105DD, United Kingdom 3Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China Received 20 March 2020; accepted 10 May 2020; published online 21 June 2020 Supported by the National Key Research and Development Program of China (Grant No. 2016YFA0300803), the National Natural Science Foundation of China (Grant Nos. 61474061, 61674079, and 61974061), and the Jiangsu ShuangChuang Program and the Natural Science Foundation of Jiangsu Province of China (Grant No. BK20140054).
*Corresponding author. Email: heliang@nju.edu.cn; ybxu@nju.edu.cn Citation Text: Sun Y Z, Kanagaraj M, Gao Q W, Zhao Y F and Ning J A et al. 2020 Chin. Phys. Lett. 37 077501    Abstract The ternary topological insulators Bi$_2$Se$_{3-x}$Te$_x$ have attracted a great deal of attention due to their exotic physical and chemical properties. While most of the studies focus on the properties of these ternary TIs, limited research was performed to investigate the dynamic atomic stack of its crystal structure. We prepared high-quality Bi$_2$Se$_{3-x}$Te$_x$ thin films on GaAs(111)B substrates using molecular beam epitaxy, characterized with Raman spectroscopy, x-ray diffraction and photoelectron spectroscopy. It is found that when Se is replaced by Te, the preferred substituting sites are the middle layer at $0 < x < 1$, and this is also valid for Se substituting Te at $2 < x < 3$. In the middle region, the substituting atoms prefer to go to the first and the fifth layer. DOI:10.1088/0256-307X/37/7/077501 PACS:75.78.-n, 75.47.Lx, 75.30.Ds © 2020 Chinese Physics Society Article Text Topological insulators (TIs) such as bismuth telluride and bismuth selenium have a "conventional" energy gap in the bulk materials, but the surface hold gapless Dirac-like states, which are protected against any time-invariant perturbations such as crystal imperfections and impurities.[1,2] Three-dimensional (3D) TIs have attracted great interests in the last few decades[3–6] and results were reported in diverse experimental fields,[7] such as the applications for electronic,[8] optoelectronic[9] and thermoelectric devices.[10] Recently, ternary TIs such as the Bi$_2$Se$_{3-x}$Te$_x$ compound has demonstrated higher mobility,[11,12] larger surface contribution and enhanced thermoelectric performance,[13,14] compared with binary TIs.[15,16] However, the preferred atomic sites of Te substitution in the place of Se have not been explored thoroughly.[17,18] Therefore, it is important to investigate their structural evolution during the growth of such ternary TIs, which may help to understand the ternary TI Bi$_2$Se$_{3-x}$Te$_x$ system more clearly. In this work, Bi$_2$Se$_{3-x}$Te$_x$ films of six quintuple layers (QLs) were grown on high resistivity ($\approx $ 10 M$\Omega$$\cdot$cm) GaAs(111)B substrates using molecular beam epitaxy (MBE).[19] The base pressure of the MBE chamber is below ${2 \times 10^{-10}}$ Torr. The GaAs(111)B substrates were cleaned before being loaded into the chamber.[20] Then the substrates were annealed at ${580}\,^\circ\!$C in the growth chamber until the streak patterns appeared as monitored by the real-time reflection high-energy electron diffraction (RHEED).[21] High-purity Bi($99.9999\%$), Se($99.9999\%$) and Te($99.9999\%$) were evaporated by conventional effusion cells. During the growth, Bi effusion cell was kept at ${525}\,^\circ\!$C, Se and Te effusion cells were kept at different temperatures with a VI/V vapor rate ratio of 10, where VI stands for Se and Te, V refers to Bi. For Bi$_2$Se$_3$, Se cell was kept at ${135}\,^\circ\!$C; for Bi$_2$Se$_{2.1}$Te$_{0.9}$, Se and Te cells were kept at ${125}\,^\circ\!$C and ${230}\,^\circ\!$C, respectively; for Bi$_2$Se$_{1.4}$Te$_{1.6}$, ${115}\,^\circ\!$C and ${240}\,^\circ\!$C were used, respectively; for Bi$_2$Se$_{0.7}$Te$_{2.3}$, ${105}\,^\circ\!$C and ${250}\,^\circ\!$C were used, respectively; for Bi$_2$Te$_3$, Te cell was kept at ${260}\,^\circ\!$C. The GaAs(111)B substrates were set at ${165}\,^\circ\!$C for all the samples. Figure 1(a) shows the representative RHEED patterns of all Bi$_2$Se$_{3-x}$Te$_x$ films. Sharp streaky patterns were observed on all Bi$_2$Se$_{3-x}$Te$_x$ films, which indicate flat surface morphology. Figure 1(b) shows a typical AFM (NT-MDT, Integra Spectra II, probe aperture size $\sim{100}$ nm) image of an as-grown Bi$_2$Se$_3$ film with a thickness of ${6}$ QLs. Characteristically triangle-shaped terraces were observed with a size of $\sim$200 nm, reflecting the hexagonal crystal structure inside the (0001) plane.[22] RHEED intensity oscillations were observed from the first layer of the growth, as shown in Fig. 1(c). It demonstrates a layer-by-layer growth mode,[21,23] and the growth rate is found to be ${127}$ s/QL. Figure 1(d) illustrates the evolution of the $d$-spacing of Bi$_2$Se$_{3-x}$Te$_x$ films during growth. The horizontal black dashed line indicates the $d$-spacing of the GaAs(111)B substrate, the vertical green dashed line stands for the beginning of the growth. In addition, the finishing time of the first QL is shown by blue dashed line. This suggests that after the growth of the first layer, the films have completely released the stress and reached their intrinsic lattices of Bi$_2$Se$_{3-x}$Te$_x$ thin films.[6] In our experimental case, the stoichiometries were determined by the $d$-spacing of the RHEED, which is inversely proportional to the in-plane lattice constant. Meanwhile, the in-plane lattice constants $a$ were calculated from $d$-spacing, which is shown in Fig. 1(e). It increases with the increasing Te concentration, which agrees well with the result reported.[24]
cpl-37-7-077501-fig1.png
Fig. 1. (a) RHEED patterns of GaAs(111)B, Bi$_2$Se$_3$, Bi$_2$Se$_{2.1}$Te$_{0.9}$, Bi$_2$Se$_{1.4}$Te$_{1.6}$, Bi$_2$Se$_{0.7}$Te$_{2.3}$, Bi$_2$Te$_3$, respectively. The sharp diffraction streak pattern indicates the two-dimensional morphology and high quality of the film. The double-headed arrow between the two first-order stripes represents the $d$-spacing, which is inversely proportional to the in-plane lattice constant. (b) A typical AFM image of the as-grown Bi$_2$Se$_3$ film with thickness of 6 QLs. (c) RHEED oscillations demonstrate a layer-by-layer growth mode, indicating a growth rate of ${127}$ s/QL. (d) The time evolution of the $d$-spacing of the films during growth. The horizontal black dashed line indicates the $d$-spacing of the GaAs(111)B substrate, the vertical green dashed line stands for the beginning of the growth, the blue dashed line shows the finishing time of the first QL. (e) The calculated lattice constant $a$ of Bi$_2$Se$_{3-x}$Te$_x$ with various Te concentrations.
The phase purity and crystal structure of Bi$_2$Se$_{3-x}$Te$_x$ thin films have been identified by x-ray diffraction (Bruker D8 Discover single crystal diffractometer, $\lambda={1.5406}$ Å) and the spectra are shown in Fig. 2(a). Compared with the Joint Committee on Powder Diffraction Standards (JCPDS) data of Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$, these films have been found to exhibit rhombohedral crystal geometry (space group $R\bar{3}m$ ($D^{5}_{3d}$)) with no other detectable phases.[25] Overall, all the peaks shift to lower angles, as more Te substituted Se. Figure 2(b) shows a zoom-in view of the (006) peak. In Fig. 2(c), the lattice parameters $c$ and the unit cell volume were calculated according to the position of the XRD peaks, the lattice parameter $c$ (red) and unit cell volume (blue) increase monotonically, consistent with the previous reports.[26] Combined with the RHEED results, this demonstrates that the unit cells of Bi$_2$Se$_{3-x}$Te$_x$ thin films are expanded along the in-plane and out-of-plane directions while larger Te atoms replace Se atoms.[27] We have further characterized the elemental chemical states of Bi$_2$Se$_{3-x}$Te$_x$ films using monochromatic x-ray photoelectron spectroscopy (PHI 5000 Versa Probe system Al $k\alpha$) measurement at room temperature. The spectra are shown in Fig. S1 in the supplementary material. From the figure, Bi $4f$, Te $3d$ and Se $3d$ states can be observed,[28,29] suggesting the existence of Bi$_2$Se$_{3-x}$Te$_x$.
cpl-37-7-077501-fig2.png
Fig. 2. (a) XRD patterns of Bi$_2$Se$_{3-x}$Te$_x$ ($x=0, 0.9, 1.6, 2.3, 3$). (b) Expanded view showing a systematic shift of the (006) peak for Bi$_2$Se$_{3-x}$Te$_x$. (c) The calculated lattice parameters $c$ (red) and unit cell volume $V$ (blue) of Bi$_2$Se$_{3-x}$Te$_x$.
To investigate the vibrational properties and electron-phonon coupling in these layered compounds, Raman spectroscopy (Horiba Jobin-Yvon HR800) measurements were performed to understand the local structural changes for different Se:Te ratios.[30] It is known that the topological insulators were layered rhombohedral crystal, as shown in Fig. 3(a). There are five mono-atomic planes (–$A^1$–$B$–$A^2$–$B$–$A^1$–) which are covalently bonded to form five-atom thick layers, often referred to as one QL,[31,32] where $A$ refers to either Se or Te, and $B$ stands for Bi. On the other hand, these quintuple layers are weakly bonded to each other by the van der Waals force. Figure 3(b) shows the three Raman vibration modes of Bi$_2$Se$_{3-x}$Te$_x$ samples, $A^1_{1g}$, $E^2_{g}$ and $A^2_{1g}$.[23] The $A^1_{1g}$ mode is attributed to the synchronized movement of Bi and Se$^1$ (Te$^1$) along the out-of-plane direction, while $A^2_{1g}$ and $E^2_{g}$ correspond to the opposite movement of Bi and Se$^1$ (Te$^1$) along the in-plane and out-of-plane directions, respectively. For all of them, Se$^2$ (Te$^2$) is stationary. Thus, the Raman vibrations are sensitive to Te (Se) substituting at Se$^1$ (Te$^1$) position, but insensitive to Te (Se) substituting at Se$^2$ (Te$^2$) position.[33] The Raman spectra of the Bi$_2$Se$_{3-x}$Te$_x$ samples with different Te concentrations in the wavenumber range 50–200 cm$^{-1}$ measured at room temperature are shown in Fig. 3(c). Within the scanned frequency range, there are three characteristic peaks of ${71}$ cm$^{-1}$, ${132}$ cm$^{-1}$ and ${173}$ cm$^{-1}$, which correspond to $A^1_{1g}$, $E^2_{g}$ and $A^2_{1g}$ vibrational modes of Bi$_2$Se$_3$, respectively.[34] The peaks are shifted to the lower frequency with more Te incorporated. This can be attributed to the elongation of the chemical bonds between Bi and Te.[35] since Te has larger atomic size and weaker electro-negativity compared with Se. It is interesting to note that Raman shift seems to demonstrate three regions (red, white and blue) for all the three peaks, as shown in Fig. 3(d).[36] This may give a hint of the site preference of Te substituting Se and vice versa.
cpl-37-7-077501-fig3.png
Fig. 3. (a) Crystal structure of rhombohedra Bi$_{2}$Se$_{3}$. (b) Schematic diagrams for the three Raman vibrational modes $A^1_{1g}$, $E^2_{g}$ and $A^2_{1g}$. (c) Raman spectra of Bi$_2$Se$_{3-x}$Te$_x$ thin films excited by a ${514}$ nm laser. The graphs have been shifted vertically and equally for clarity, thus the peak position shifts to lower wavenumbers as Te concentration increases. (d) The dependence of Raman peak position on stoichiometric ratio. It can be observed that the Raman shifts are small in the blue ($x < 1$) and red ($x>2$) regions, while it is much larger in the white ($1 < x < 2$) region for all the three peaks. (e) The relative intensity of $E^2_{g}$ to $A^2_{1g}$ with the Te concentration. It changes dramatically from $\sim$1 in the blue region to $\sim$3.6 in the red region.
As mentioned above, there are two atomic sites of Se in Bi$_{2}$Se$_{3}$, assigned as Se$^1$ and Se$^2$, as shown in Fig. 3(a).[37] The chemical bonding between Bi and Se$^2$ is of a pure covalent nature,[14] while it is a mixture of ionic and covalent bond between Bi and Se$^1$. Thus the binding energy of Bi–Se$^1$ is stronger than that of Bi–Se$^2$. Therefore, on the formation of Bi$_2$Se$_{3-x}$Te$_x$ compounds ($0 < x < 1$), Te atoms will preferentially replace Se at Se$^2$ sites first.[38] As discussed above, the three acoustic vibrations are insensitive to Te substituting in Se$^2$ sites.[39] Thus the peak positions remain relative constants, as shown by the blue region in Fig. 3(d). The slightly changes here are attributed to the larger bond length of Bi–Te$^2$ compared with Bi–Se$^2$. This argument is also suitable for the case that Se substitutes Te$^2$ site in Bi$_2$Se$_{3-x}$Te$_x$ compounds ($2 < x < 3$), corresponding to the red region shown in Fig. 3(d). When the Te concentration ($x$) lies between 1 and 2, a mixture of Se$^1$ and Te$^1$ exists in the films.[40] Thus the peak positions smoothly change from Bi$_2$Se$_2$Te to Bi$_2$SeTe$_2$, as shown by the white region in Fig. 3(d).[41] Previous works have found the similar results that there is a discontinuity in the revolution of Raman peaks from Bi$_2$Se$_3$ to Bi$_2$Te$_3$, of which the results provide useful insights in the realization of composition engineering and the understanding optical properties of Bi–Se–Te-based catalysts.[42] Here we have proposed a possible explanation to this slope change of the evolution of Raman peaks by the preferred substituting sites, which may further help to understand the precise structure of the ternary compound of Bi$_2$Se$_{3-x}$Te$_x$. Furthermore, the relative intensity of $E^2_{g}$ and $A^2_{1g}$ with the increasing Te concentration has also been investigated, as shown in Fig. 3(e). Similarly, it demonstrates a step-like feature. When the Te concentration $x$ is within 0–1, the relative intensity stays near 1. For Se substituting Te$^2$ in Bi$_2$Se$_{3-x}$Te$_x$ compounds ($2 < x < 3$), the $E^2_{g}$ to $A^2_{1g}$ ratio stays near 3.6. And it rapidly changes in the middle region, suggesting that Te (Se) are substituting Se$^1$(Te$^1$) here.[43]
cpl-37-7-077501-fig4.png
Fig. 4. (a) Schematic structures of Bi$_{2}$Se$_{3}$, Bi$_2$Se$_2$Te, Bi$_2$SeTe$_2$, Bi$_{2}$Te$_{3}$. (b) The occupational ratio of Se$^1$ (red line) and Te$^1$ (blue line) with the Te concentration. The occupational ratio of Se$^1$ stays at 2(0) and Te$^1$ stays at 0(2) when $0 < x < 1$ ($2 < x < 3$), as shown in the blue (red) region. In the middle region, Se$^1$ linearly decreases from 2 to 0 and Te$^1$ increases from 0 to 2. They are equal to 1 when $x=1.5$. (c) The occupational ratio of Se$^2$ (red line) and Te$^2$ (blue line) with the Te concentration. The occupational ratio of Se$^2$ decreases from 1 to 0, while Te$^2$ increases from 0 to 1 when $0 < x < 1$. This reverses in $2 < x < 3$, as Se substitutes Te in Bi$_{2}$Te$_{3}$.
Figure 4(a) shows the schematic structures of Bi$_{2}$Se$_{3}$, Bi$_2$Se$_2$Te, Bi$_2$SeTe$_2$, Bi$_{2}$Te$_{3}$. The quintuple layer stackings of Bi$_2$Se$_2$Te and Bi$_2$SeTe$_2$ are Se$^1$–Bi–Te$^2$–Bi–Se$^1$ and Te$^1$–Bi–Se$^2$–Bi–Te$^1$, respectively.[44] The occupational ratios of Se$^1$, Te$^1$, Se$^2$ and Te$^2$ are shown in Figs. 4(b) and 4(c). When the Te concentration is within 0–1, the occupational ratio of Se$^1$ stays at 2 while Te$^1$ remains at 0. The occupational ratio of Se$^2$ decreases from 1 to 0 as Te gradually substitutes Se at Se$^2$ sites, which is shown in the blue region. When the Te concentration is between 1 and 2, Te at Se$^1$ site increases from 0 to 2 while Te at Se$^2$ decreases from 1 to 0. When $x=1.5$, the ratio of Se to Te is 1 for each layer. These results are also in compliance with Se substituting Te$^2$ in Bi$_2$Se$_{3-x}$Te$_x$ compounds ($2 < x < 3$), which is shown in the red region. In summary, we have investigated the structural evolution and Raman spectra in high-quality Bi$_2$Se$_{3-x}$Te$_x$ thin films grown by MBE. While Se and Te continuously substitutes each other, a smooth change of lattice constant have been observed. Both analysis suggests that the Bi$_2$Se$_{3-x}$Te$_x$ or Bi$_2$Te$_{3-x}$Se$_x$ ternary compounds have the tendency to stay in the binary form of Bi$_{2}$Se$_{3}$ or Bi$_{2}$Te$_{3}$ when the substituting ratio $x$ is lower than 1. Thus the preferred substituting sites are the middle layer of Se$^2$ or Te$^2$. As a result, the shifts of the Raman peaks demonstrate a step-like feature, and it changes dramatically in the middle concentration region ($1 < x < 2$). Our findings may help to understand the precise structure of the ternary compound of Bi$_2$Se$_{3-x}$Te$_x$, which may become a theoretical basis of promising device applications. One can see the supplemental material for details of RHEED patterns, XRD peaks calculations, x-ray photoelectron spectroscopy characterization and the observed weak anti-localization effect by magnetoresistance measurements. 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