Encapsulation of atomic-scale Bi wires in epitaxial silicon without loss of structure

We demonstrate, using x-ray diffraction, that we have taken one-dimensional Bi nanolines fabricated on a Si(001) surface, and buried them in crystalline silicon while retaining both their one-dimensional characters and important aspects of their structure. In particular, after burial, the nanolines retain the two-by periodicity associated with their surface structure along their length. We have used density functional theory calculations to model a structure for these buried nanolines, whose minimum length can be estimated to be 100 nm from the coherence length of the x-ray measurements.

Figure 1

X-ray diffraction patterns from the Si epilayer cap/Bi lines/Si(001) substrate samples. The left figures are magnified patterns around the direct-beam positions. (a) Schematic diagram of experimental geometry, where the direction of the incident x-rays and the position of the x-ray imaging-plate detector with respect to the direction of Bi atomic lines before burial are indicated. (b) $\varphi =0°$, (c) $\varphi =45°$, (d) $\varphi =90°$. The surface unit vectors ${\mathbf{a}}_{s,1},{\mathbf{a}}_{s,2}$, and ${\mathbf{a}}_{s,3}$ are defined by $(1∕\sqrt{2}){\mathbf{a}}_1+(1∕\sqrt{2}){\mathbf{a}}_2,-(1∕\sqrt{2}){\mathbf{a}}_1+(1∕\sqrt{2}){\mathbf{a}}_2$, and ${\mathbf{a}}_3$, where ${\mathbf{a}}_1,{\mathbf{a}}_2$, and ${\mathbf{a}}_3$ are the unit vectors in bulk. Incident x rays were parallel (b), at 45° (c) and perpendicular (d) to ${\left[100\right]}_S$.

Figure 2

(Color online) (a) The haiku model for the Bi nanolines. Note the complex reconstruction beneath the Bi dimers consisting of five- and seven-membered rings. (b) Perspective view of our proposed model for the buried Bi lines, showing their one-dimensional nature. Note that the reconstruction normally associated with the Bi nanolines has not survived burial, and that the Bi dimers are now bonded to silicon atoms above them. In both images, Bi atoms are shown darker and Si atoms lighter.