Shape transitions and island nucleation for Si/Ge molecular beam epitaxy on stripe-patterned Si (001) substrate

Si and Ge growth on the stripe patterned Si (001) substrates is studied using scanning tunneling microscopy. During Si buffer growth, the stripe morphology rapidly evolves from multifaceted “U” to “V”-shaped forms. This involves successive transitions between different low energy $\left\{11n\right\}$ side facets, where $n$ continuously decreases from $n=3$ to 20. Ge growth on such stripes induces the formation of a pronounced side wall ripple structure when the Ge thickness exceeds three monolayers. This ripple structure consists of alternating {105} microfacets oriented perpendicularly to the stripes. Depending of the side wall geometry, Ge nanoislands subsequently nucleate either on the side walls or at the bottom of grooves. The latter only occurs for “V”-shaped stripes, where the side wall ripples extend all the way from the top to the bottom of the grooves, allowing efficient downward mass transport. For multifaceted “U” stripes, the side wall ripples are interrupted by steeper side wall segments such that mounds and subsequently, pyramids and domes grow on the side walls instead of at the bottom of the grooves. The island shapes strongly depend on their position on the pattern topography, which also affects the critical coverage for island nucleation as well as for the transition from pyramids to domes. The mechanisms for nucleation at different positions are clarified by detailed analysis and the role of kinetic as well as energetic factors identified.

Figure 1

(Color online) Top row: AFM images of stripe-patterned Si substrates after reactive ion etching showing stripes with two different filling factors $\delta =w/p=0.28$ and 0.5 for (a) and (b), respectively, but constant period $p=350\text{nm}$. Middle row: STM images of stripes after annealing at $750°\text{C}$ (c) and after subsequent 35 nm low temperature Si buffer growth at $450°\text{C}$ (d). The insert in (d) shows the $\left(2\times 1\right)$ reconstructed Si surface on the ridges on a magnified scale. Bottom: comparison of the cross-sectional stripe profiles after etching (bottom), after 5 min annealing at $750°\text{C}$ (middle) and after 35 nm low temperature Si buffer growth (top).

Figure 2

(Color online) Evolution of the stripe morphology during second $520°\text{C}$ Si buffer growth as revealed by STM images (left-hand side), surface orientation maps (SOM, center) and surface profiles (right hand side). (a) “U”-shaped stripes with {113} facets formed after 7 nm Si deposition, (b) shallower “U” stripes with flatter {114} facets after 20 nm Si deposition, (c) “V”-shaped stripes with shallow {11 10} facets formed after 20 nm Si on stripes with smaller filling factor $\delta =0.2$, (d) shallow “V” grooves with {11 20} facets formed after 2 nm Si growth at a higher temperature of $530°\text{C}$. The side wall angles of the grooves and the original stripe widths $w$ are indicated on the cross-sectional profiles on the right hand side. The facet spots in the SOMs are labeled by the symbols as depicted below. The change in stripe geometry is visualized by the 3D STM images depicted on the right hand side on equal scale.

Figure 3

(Color online) Top: high-resolution STM images of the side wall structure of the stripes with (a) multifaceted “U” geometry [cf. Fig. 2a] and (b) with a shallow {11 20} “V” geometry with $4°$ side wall inclination [cf. Fig. 2d]. Middle: (c) side view of the silicon crystal lattice perpendicular to the stripe direction. The dash-dotted lines indicate the high-indexed $\left(11n\right)$ facet surfaces observed on the side wall of the stripes. Bottom: (d) schematic illustration of the successive shape transformation of the stripes occurring with increasing Si buffer thickness.

Figure 4

(Color online) STM images of “V”-shaped stripes during initial Ge deposition recorded after 1.8 ML Ge at $520°\text{C}$ [top row (a)–(c)] and additional 0.9 ML Ge to 2.7 ML at $600°\text{C}$ [bottom row (d)–(f)]. The initial stripe geometry contained mainly {119} side wall faces as shown in Fig. 2c. The facet spots are indicated in the surface orientation maps shown as insets. Details of the surface reconstruction and step structures on the ridges as well as the side walls of the grooves are shown by the higher-resolution STM images depicted on the right hand side.

Figure 5

(Color online) STM images of “V”-shaped stripes after 3.6 ML Ge [top (a) to (c)] and 4.3 ML Ge deposition [bottom (d)–(e)] at $600°\text{C}$, showing the onset of {105} side wall ripple formation. This is also evidenced by the corresponding surface orientation maps shown as inserts. The zoomed-in STM images of the grooves and ridges are depicted on the right hand side, where the latter reveal the formation of dimer vacancy lines on the Ge wetting layer surface, as indicated by the dotted lines in the high-resolution STM inserts of (b) and (e).

Figure 6

(Color online) (a) Surface profiles of across the side wall ripples of the “V”-shaped stripe with {11 10} side walls of Fig. 5f after 4.3 ML Ge deposition. As indicated by the arrows, the ripple amplitude is around 1 nm and the period in the range of 12–18 nm. (b) Schematic illustration of the {105} microfaceted geometry of side wall ripples. As indicated, the $\left[\overline{5}\overline{5}1\right]$ intersection lines between adjacent {105} microfacets are inclined by the $8.05°$ with respect to the (001) surface. This is exactly equal the inclination angle of the original {11 10} side walls surfaces formed after Si buffer growth. Under this condition, the microfacets can extend without interruption from the top to the bottom of the grooves.

Figure 7

(Color online) STM images of Ge island nucleation on “V” stripes with {11 10} side walls after 4.6 ML (a,b), 4.9 ML (c,d), and 5.2 ML (e,f) Ge deposition at $600°\text{C}$. Overview images $\left(0.5\times 0.5\mu {\text{m}}^2\right)$ of the stripes are displayed in the upper panel, details of Ge island structure and their transformation from mounds to pyramids and domes are shown in the lower panel on a magnified scale. The transformation of the island shapes is also manifested by the appearance of additional facet spots in the surface orientation maps shown as inserts.

Figure 8

(Color online) Surface evolution and Ge island growth for shallow “U” stripes with {119} and {115} side wall segments. The Ge thicknesses increases from 2ML at $520°\text{C}$ to 4.3, 5.1, and 5.5 ML at $600°\text{C}$ from (a) to (f), respectively. Surface orientation maps are presented as inserts. At 2 ML coverage [(a) and (b)] the sidewall surfaces are still smooth, but 2D islands are formed on the {115} side wall surfaces as indicated by the arrow. At 4.3 ML coverage (c), pronounced side wall ripples are formed on the shallow {119} areas, whereas on the {115} faces mounds are formed as precursors for 3D islands. Increasing the coverage to 5.1 ML [(d) and (e)], the mounds are transformed to small asymmetric pyramids, which subsequently transform into domes at higher coverage (f), while the side wall ripples remain nearly unchanged. Image sizes: $0.5\times 0.5\mu {\text{m}}^2$ except in (b) and (d) of $0.2\times 0.2\mu {\text{m}}^2$.

Figure 9

(Color online) Surface evolution and Ge island growth on steeper “U” stripes defined by {114} and {119} side wall segments. The Ge coverage increases from 2.7, to 4.3, 5.1, 5.9 to 6.9 ML from (a) to (f), respectively, at $600°\text{C}$. At 2.7 ML coverage, the side wall surfaces are still smooth, whereas beyond 4 ML coverage ripples are formed on the {119} side wall faces. At 4.4 ML (b), these ripples drastically coarsen such that large {105} faceted prisms are formed on the lower part of the side walls as shown in (c) and (d) for 5.1 ML coverage. At 5.9 ML coverage (e), predome islands start to nucleate on top of the prisms, which transform to the usual Ge domes at coverages beyond 6.5 ML (f). This transition is shown in more detail in Fig. 11. Note that the small spots seen in the STM images are due to growth defects due to long sample storage in UHV. Surface orientation maps are shown as insert and the STM image size is $0.4\times 0.4\mu {\text{m}}^2$, except for (d) with $0.2\times 0.2\mu {\text{m}}^2$.

Figure 10

(Color online) STM profiles along “U”-shaped stripes with {114} side wall faces at Ge different coverages of 4.4, 5.1, 5.9, and 6.9 ML from bottom to top, respectively. The surface profiles were measured along the lower {119} side walls of the stripes as indicated by the dashed line in Fig. 9b. The surface profile over the side wall ripples of the “V” shaped stripes of Fig. 5d at 4.6 ML Ge coverage is shown for comparison as lowest curve on equal scale.

Figure 11

Details of the Ge island formation process on “U” stripes with {114} side facets for Ge coverages increasing from 2.7, to 4.4, 5.1, 5.9, and 6.9 ML from (a) to (f), respectively (image size $0.2\times 0.2\mu {\text{m}}^2$). At 3 ML coverage (a), the side wall surfaces are still very smooth, whereas beyond 4 ML coverage, ripples are formed on the upper and lower {119} side wall faces. With increasing Ge coverage, the ripples drastically coarsen (b) until large {105} faceted prisms (c) are formed on the lower side wall areas, which are fed by step-flow Ge growth from the upper {119} side faces at the top of the grooves as indicated by the dashed arrow. Beyond 5.7 ML, nucleation of additional Ge 2D islands at the roof of the prisms sets in (d), which leads to the nucleation of predomes and domes on top of the prisms as shown in (e) and (f), respectively. While the topography on the {119} are drastically changes with increasing Ge coverage, the {114} steeper sidewall segments remain nearly unchanged.

Figure 12

Schematic illustration of the size ranges for mounds (m), pyramids, transitional domes (TD) and domes for Ge growth on either a 2D (001) Si surface, “V”-shaped stripes with {11 10} sides faces, shallow “U” stripes with {115} facets and steeper “U” stripes with {114} facets from top to bottom, respectively. For the latter, the surface evolves from ripples, preprisms, prisms, and transitional domes to completed domes with increasing size and increasing Ge coverage. Ge growth was performed under the same conditions for all cases at a substrate temperature of $600°\text{C}$.