Atomic size effects studied by transport in single silicide nanowires

Ultrathin metallic silicide nanowires with extremely high aspect ratios can be easily grown, e.g., by deposition of rare earth elements on semiconducting surfaces. These wires play a pivotal role in fundamental research and open intriguing perspectives for CMOS applications. However, the electronic properties of these one-dimensional systems are extremely sensitive to atomic-sized defects, which easily alter the transport characteristics. In this study, we characterized comprehensively ${\mathrm{TbSi}}_2$ wires grown on Si(100) and correlated details of the atomic structure with their electrical resistivities. Scanning tunneling microscopy (STM) as well as all transport experiments were performed in situ using a four-tip STM system. The measurements are complemented by local spectroscopy and density functional theory revealing that the silicide wires are electronically decoupled from the Si template. On the basis of a quasiclassical transport model, the size effect found for the resistivity is quantitatively explained in terms of bulk and surface transport channels considering details of atomic-scale roughness. Regarding future applications the full wealth of these robust nanostructures will emerge only if wires with truly atomically sharp interfaces can be reliably grown.

Figure 1

SEM micrographs (plan-view) of ${\mathrm{TbSi}}_2$ wires grown on a Si(001)-$\left[110\right]4^{\circ }$ for different flash annealing cycles of the Si sample. (a) Three different nanostructure types are initially visible: ${\mathrm{TbSi}}_2$ wires aligned along the $\left[\overline{1}10\right]$ direction (highlighted in red, blue), most likely Tb silicide islands (highlighted in yellow), and apparently bigger ${\mathrm{TbSi}}_2$ wires sometimes rotated by $90{}^{\circ }$ (in green). (b),(c) Further flash annealing of the Si(001) substrate (up to 100 cycles) generally results in samples that only show the thinnest wires [highlighted in blue in (a)].

Figure 2

STM overview image ($V_{\mathrm{B}}=2V$; $I_{\mathrm{T}}=100pA$) of ${\mathrm{TbSi}}_2$ wires grown on $2\times 1$ Si(001) substrate after many ($>50$) flash-annealing cycles. The insets (a) and (b) show detailed STM images of the two ${\mathrm{TbSi}}_2$ wire bundles labeled with $a$ and $b$ in the figure.

Figure 3

(a) Two schematics to illustrate different correlation lengths $L$ and roughness $\mathrm{\Delta }$. (b) High resolution STM image ($V_{\mathrm{B}}=2V$; $I_{\mathrm{T}}=100pA$) showing the formation of additional nanoislands on top of the wires. This is displayed in detail in the upper inset and analyzed quantitatively using the height contour shown in the lower inset. (c) Derived HHCF plot along the wire (black) with fit curve according to equation mentioned in the text.

Figure 4

(a) Experimental $I\left(V\right)$ (left) and normalized $(dI/dV)/(I/V)$ spectra (right) taken at respective color-coded positions indicated in the STM image in the inset. The color code of the spectra corresponds to the one shown in the inset. The metallic behavior of the ${\mathrm{TbSi}}_2$ wires is obvious from both the steep $I\left(V\right)$ characteristics (see magnification in inset) and the finite $(dI/dV)/(I/V)$ signal at the Fermi energy. (b) The theoretical LDOS in the different space regions of the surface, labeled as substrate, transition, and wire in the inset. The LDOS for a fully H-saturated surface structure (right panel) is in excellent agreement with the experiment. For further details see text.

Figure 5

Sequence of plan-view SEM micrographs depicting the contacting procedure of a single $5\mu \mathrm{m}$ long ${\mathrm{TbSi}}_2$ wire with two STM probes. (a)–(c) First, the two tips are placed at the ends of the wire, (d)–(e) then keeping fixed the position of one tip, the second one is moved step-by-step towards the first, performing at each position an $I\left(V\right)$ measurement in order to determine the wire resistance. (f) Despite the adopted extremely gentle measurement procedure, white spots (marked by arrows) are appearing on the wire and indicate a slight local damage.

Figure 6

Two-point-probe $I\left(V\right)$ curves of the same ${\mathrm{TbSi}}_2$ wire shown in Fig. 5 for various probe spacings. The different slopes reflect the linear decrease of resistance as the two probes get closer. For sake of comparison, inset (a) shows an $I\left(V\right)$ curve when one of the tip accidently contacts the substrate, while inset (b) shows the wire-desorption induced by Joule heating after applying an extremely high voltage of 20 V.

Figure 7

Two-point-probe resistance vs probe spacing of a ${\mathrm{TbSi}}_2$ nanowire. The resistance increases linearly with the probe distance as expected for diffusive 1D transport. Moreover, the sudden change of the slope at around $3.5\mu \mathrm{m}$ can be attributed to a reduction of wire size (structural inhomogeneity) indicated in the inset.

Figure 8

Resistance determined by a two-point-probe geometry for several wires of different lengths and sizes. Three wire classes, named w1, w2, and w3 with a resistance per length around 1, 0.2, and $0.03\mathrm{M}\mathrm{\Omega }/\mu \mathrm{m}$, are highlighted in blue, red, and green, respectively.

Figure 9

SEM (a) and STM (b) micrographs showing the same surface area. The schematic drawing in (a) shows how both transport studies as well as STM images of the same wire were obtained. The STM image in (b) ($V_{\mathrm{B}}=2V$; $I_{\mathrm{T}}=100pA$) is composed by two micrographs connected at the black dashed line. The areas marked by dashed white ellipses show defects induced by contacting procedure for transport measurements. Red and blue framed squares highlight the area shown at higher magnification in (c) and (d), respectively. Red lines mark where line profiles were taken shown at the right.

Figure 10

(a) STM topography ($V_B=2\text{V};I_T=100\text{pA}$) of an apparently bigger wire. (b) SEM image showing the two-point-probe setup for such a wire. The color code corresponds to the one used in Fig. 8. (c) Height profile across the wire taken along the red line shown in (a).

Figure 11

Calculated resistivity vs the apparent height of the ${\mathrm{TbSi}}_2$ nanowires revealing a strong decrease with increasing height. The solid and dashed lines are deduced from classical and quantum theories. For details see text.