Barrierless Formation and Faceting of SiGe Islands on Si(001)

The initial stages of the formation of SiGe islands on Si(001) pose a long-standing puzzle. We show that the behavior can be consistently explained by one simple assumption—that for strained SiGe, (001) is a stable orientation but not a facet orientation. Calculations of energy and morphology reproduce the key features of “prepyramid” and “pyramid” islands, and explain the initial formation and subsequent shape transition. Scanning tunneling microscopy measurements confirm the key assumptions and predictions of the model.

Figure 1

Equilibrium island shapes, calculated for islands of sizes 0.0035, 0.0074 ($V_1$), 0.0094 ($V_2$), 0.0144 ($V_3$), 0.0200, and 0.0401 $L^2$, from bottom to top respectively. Shapes for different size $V$ are offset vertically by $0.02L$. When there are two (meta)stable shapes for the same $V$, shapes are superposed, and facet edges on the right are indicated by open circles. $V_1$ is the size of the smallest metastable faceted island, $V_3$ is the size of the largest metastable smooth island, and both shapes are stable and degenerate in energy at $V_2$. The substrate is not shown; island edges meet the substrate at zero contact angle.

Figure 2

Island energy $E$ and chemical potential (per volume) $\mu$, relative to a planar film, vs volume $V$. The dotted line represents unstable solutions connecting smooth and faceted branches.

Figure 3

(a) STM image ($114\times 75{\mathrm{n}\mathrm{m}}^2$) of a small island obtained by depositing ${\mathrm{S}\mathrm{i}}_{0.5}{\mathrm{G}\mathrm{e}}_{0.5}$ on Si(001). (b) Line scan through island (dots) and cosine fit (line). Scan extends beyond the STM image, and a double arrow indicates the range of the image. Scale is in nm.

Figure 4

(a) STM image ($108\times 108{\mathrm{n}\mathrm{m}}^2$) of a faceted island obtained by depositing ${\mathrm{S}\mathrm{i}}_{0.6}{\mathrm{G}\mathrm{e}}_{0.4}$ on Si(001). (b) Cross section of a region near the top of island, from an average of 30 line scans taken from left to right close to the middle of the island. Scale is in nm.