Phase formation in iron silicide nanodots grown by reactive deposition epitaxy on Si(111)

The epitaxial growth by reactive deposition of $\epsilon \text{-FeSi}$ and $\beta {\text{-FeSi}}_2$ nanodots on Si(111) is studied as a function of Fe coverage. The nanodots density, size, and strain were analyzed by atomic force microscopy and x-ray diffraction. Almost single phase $\epsilon \text{-FeSi}$ and $\beta {\text{-FeSi}}_2$ were formed at low and high iron coverage, respectively. A $\epsilon \text{-FeSi}$ to $\beta {\text{-FeSi}}_2$ change in phase formation is observed at Fe coverage of 5.5 nm, which is coincident with the coalescence of the nanodots, the relaxation of the strain in both phases and a discontinuous increase of the grain size of the $\beta {\text{-FeSi}}_2$ phase. A direct comparison of the diffraction and microscopy data shows that nanodots of different phases also exhibit different shapes, being the $\epsilon \text{-FeSi}$ $\left(\beta {\text{-FeSi}}_2\right)$ nanodots smaller (larger) and with a low (high) aspect ratio.

Figure 1

(Color online) AFM images of samples with (a) ${\theta }_{\text{Fe}}=2.4$, (b) 3.8, and (c) 5.5 nm. Statistical analysis of the AFM images of the samples with (d) ${\theta }_{\text{Fe}}=2.4$, (e) 3.8, and (f) 5.5 nm. The nanodots with aspect ratio lower than 1 nm were denoted by open blue squares, while the nanodots with aspect ratio larger than 1 nm have been denoted by open red circles.

Figure 2

(Color online) Typical XRD difractograms of (a) low and (b) high iron coverage samples. The measurements were performed in reflection geometry and a small beam divergence of ${0.005}^0$ was used to avoid an overlap of the substrate signal. Therefore, all linewidths of peaks stemming from the nanodots can be inferred directly from the diffraction data, without the need of instrumental deconvolution.

Figure 3

(Color online) Analysis of the XRD data as a function of ${\theta }_{\text{Fe}}$: (a) relative concentration of the $\epsilon \text{-FeSi}$ and $\beta {\text{-FeSi}}_2$ phases, (b) strain along the growth direction, and (c) grain size along the growth direction.