Spiral Growth without Dislocations: Molecular Beam Epitaxy of the Topological Insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ on Epitaxial Graphene/SiC(0001)

We report a new mechanism that does not require the formation of interfacial dislocations to mediate spiral growth during molecular beam epitaxy of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Based on in situ scanning tunneling microscopy observations, we find that ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ growth on epitaxial graphene/SiC(0001) initiates with two-dimensional (2D) nucleation, and that the spiral growth ensues with the pinning of the 2D growth fronts at jagged steps of the substrate or at domain boundaries created during the coalescence of the 2D islands. Winding of the as-created growth fronts around these pinning centers leads to spirals. The mechanism can be broadly applied to the growth of other van der Waals materials on weakly interacting substrates. We further confirm, using scanning tunneling spectroscopy, that the one-dimensional helical mode of a line defect is not supported in strong topological insulators such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3$.

Figure 1

(a) STM image of a 30 QL ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ film grown on epitaxial graphene/SiC taken at room temperature, showing the formation of spirals ($I_t=0.79\mathrm{nA}$, $V_s=-0.047\mathrm{V}$). (b) Close-up view of a spiral, and line profile across $AB$ ($I_t=1.17\mathrm{nA}$, $V_s=-1.70\mathrm{V}$). (c) Atomic resolution image of the spiral core taken at 78 K, showing a step originated from the spiral core in the center of the image ($I_t=1.30\mathrm{nA}$, $V_s=1.10\mathrm{V}$). (d) X-ray diffraction of a 30 QL ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ film using Cu $K_{{\alpha }_1}$ emission, with the high intensity SiC peak removed. Additional small peaks are due to additional diffractions from extraneous x-ray emissions of the Cu source, e.g., the peak at 17.41° is a side peak of (006) due to Cu $K_{{\beta }_1}$ ($\lambda =0.1392\mathrm{nm}$). Inset: $\theta –2\theta$ rocking curve. (e) Raman spectra of a 30 QL ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ film, showing two characteristic peaks at 129.5 and $171.7{\mathrm{cm}}^{-1}$.

Figure 2

(a) STM image of a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ island 2 QLs high with the arrow marking a SiC step ($I_t=0.10\mathrm{nA}$, $V_s=-0.48\mathrm{V}$). (b) STM image of two spirals originating at a SiC step edge ($I_t=0.28\mathrm{nA}$, $V_s=-0.27\mathrm{V}$), with line profile $AB$ crossing a 1 QL ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ island overlaid on top of a double bilayer SiC step. (c) STM image of a 6 QL ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ film at the intermediate stages of spiral growth ($I_t=0.10\mathrm{nA}$, $V_s=-0.70\mathrm{V}$). Images shown in (a)–(c) are taken at room temperature. (d) Atomic resolution image taken at 78 K, showing the atomic structure of a grain boundary on ${\mathrm{Bi}}_2{\mathrm{Se}}_3(111)$ observed on the 6 QL film ($I_t=1.10\mathrm{nA}$, $V_s=0.30\mathrm{V}$). The two straight lines indicate the 3° angle between the two neighboring domains.

Figure 3

(a) Ball-and-stick models of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ along the [111] direction, and a 2D island with type $A$ [$\overline{1}2\overline{1}$] steps with two dangling bonds per edge atom and type $B$ [$\overline{2}11$] steps with one dangling bond. (b) Schematic diagrams showing a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ island leaping over a straight upper SiC step. (c) Schematic diagrams (side and top views) illustrating the formation a clockwise spiral.

Figure 4

(a) $dI/dV$ spectrum taken on the ${\mathrm{Bi}}_2{\mathrm{Se}}_3(111)$ surface at 78 K with the Dirac point marked. (b) STM image and (c) $dI/dV$ map of the same area obtained simultaneously at 78 K, with three spiral cores labeled $1–3$ ($I_t=0.50\mathrm{nA}$, $V_s=-0.15\mathrm{V}$). (d) Energy-dependent $dI/dV$ images of the boxed area in (c).