Sculpting Semiconductor Heteroepitaxial Islands: From Dots to Rods

In the Ge on Si model heteroepitaxial system, metal patterns on the silicon surface provide unprecedented control over the morphology of highly ordered Ge islands. Island shape including nanorods and truncated pyramids is set by the metal species and substrate orientation. Analysis of island faceting elucidates the prominent role of the metal in promoting growth of preferred facet orientations while investigations of island composition and structure reveal the importance of Si-Ge intermixing in island evolution. These effects reflect a remarkable combination of metal-mediated growth phenomena that may be exploited to tailor the functionality of island arrays in heteroepitaxial systems.

Figure 1

AFM phase images show Ge islands grown on: (a) Au-patterned Si (110), (b) Au-patterned Si (001), (c) Au-patterned Si (111), (d) Si (110), (e) Si (001), and (f) Si (111) (image sizes: $750\times 750{\mathrm{nm}}^2$). Nominal Ge coverages are (a) and (d) 7.5 ML; (b) and (e) 4 ML; (c) and (f) 5 ML. The principle facets bounding islands on Au-patterned Si are labeled in (a)–(c). Island orientation is dependent only on substrate orientation and not the orientation of the evaporated metal pattern relative to the substrate.

Figure 2

Gray scale slope images (a), (c), (e), and (g) and their corresponding facet plots (FPs) (b), (d), (f), and (h) of Ge islands grown on metal-free and metal-patterned Si (110) with approximately 8 ML-Ge (slope image scale: $500\times 500{\mathrm{nm}}^2$; facet plot scale: 1.8). (a) Islands on metal-free Si (110) are composed of $\{105\}$, $\{15323\}$, $\{113\}$, and $\{111\}$ facets, as determined from the FP in (b). (c) Islands on Au-patterned Si (110) are bounded by these same facets, although the $\{111\}$-type facets form first as shown by the FP inset (corresponding to 4 ML-Ge) and compose the largest area of the nanorod island. (e) Islands on Sn-patterned Si (110) have a much more rounded shape and the FP in (f) shows that these islands have facets of the $\{113\}$, $\{15323\}$, and $\{111\}$ types. (g) Islands on Ag-patterned Si (110) are composed mainly of $\{113\}$ facets.

Figure 3

(a)–(f) Perspective view of AFM height images of islands formed on Au-patterned Si (001) and (110) before and after peroxide etching (image sizes: $450\times 450{\mathrm{nm}}^2$). (a) and (c) correspond to as-grown islands with 5 and 10 ML of Ge, respectively; island heights are approximately 36 and 48 nm, respectively, ($z=50\mathrm{nm}$). (b) and (d) show the same islands in (a) and (c) after ${\mathrm{H}}_2{\mathrm{O}}_2$ etching; islands heights are approximately 35 and 37 nm, respectively ($z=50\mathrm{nm}$). Islands formed by the deposition of (e) 4.5 ML and (f) 10 ML of Ge on Au-patterned Si (110) after peroxide etching ($z=25\mathrm{nm}$). Dotted lines have been included to mark the footprint of islands prior to etching. (g) Island volume change after ${\mathrm{H}}_2{\mathrm{O}}_2$ etching versus original island volume for islands grown on Au-patterned Si (001) (triangles) and Si (110) (squares). Having a slope of one and passing through the origin, the dashed line represents the case for complete island removal by peroxide etching. Volume measurements were performed on samples representing a range of Ge coverages.

Figure 4

Reciprocal-space maps from islands formed on Au-patterned Si (001) with Au-dot spacing of 800 nm. This spacing was chosen because it produced a high level of island ordering and uniformity in island shape and size. These maps were collected via x-ray microdiffraction near the Si (111) peak for Ge coverages of (a) 6 ML, (b) 9 ML, and (c) 50 ML. The maps are in the $H=K$ plane, with $L$ being the direction of the surface normal (reciprocal-space units are referenced to the Si lattice). Contours are spaced logarithmically. The dashed lines represent the position of diffraction peaks due to unstrained ${\mathrm{Si}}_{1\mathrm{\text{−}}x}{\mathrm{Ge}}_x$ terminating at the position of a peak due to pure Ge.