Tomography of AlSb islands

The conical, needle-shaped specimen is reconstructed from the aligned series of images with the WBP algorithm. The SIRT algorithm results in a detrimental blurring at the het-erostructure interface even after a small number of iterations. This effect is related to the

Si

GaSb 100 nm

[110]

Figure 4.12.The isosurface visualisation of the reconstructed volume is presented in two inclined views onto the GaSb layer (blue) and the Si substrate (red). The two colours correspond to different grey values, i.e.

the visualisation is a montage of two isosurfaces that illustrate differences in the roughness of the specimen surface.

high contrast artefact which occurs at abrupt, high differences in specimen mass density [12, 142, 186]. The reconstructed specimen is shown in figure 4.12 as a montage with two grey values represented in red and blue. The values display the Si and the GaSb surfaces, respectively. The two viewing directions convey a notion of the morphology. The FIB processing leads to an apparently smoother specimen on the Si substrate side. The GaSb exhibits an uneven surface. The very fine roughness on both sides is an artefact caused by noise in the tomogram. Beside physical aspects of specimen milling which explain material and orientation dependent effects of ion sputtering, a pronounced chemical sensitivity of GaSb to the Ga⁺-ion beam is assumed. Therefore, it is more difficult to obtain an equally smooth surface without the protective carbon layer on both the GaSb layer and the Si sub-strate. However, the different thickness of substrate and layer within the needle is attributed to the limited control of the milling position during FIB preparation at 3 kV acceleration voltage.

The tomography data reveals the morphological character of the AlSb islands. They ex-hibit an anisotropic, lateral elongation and indications for coalescence. Statements are based on slices through the reconstructed volume which are presented in figures 4.13 and 4.14.

The former figure presents a slice oriented parallel to the substrate surface and close to the interface, i.e. it is tilted away from the (001) plane by the amount of the substrate miscut.

The identified and numbered AlSb islands are schematically sketched in the right of the fig-ure along with the specimen silhouette. The predominant elongation of islands points along the [110] direction. It is remarked that this direction is perpendicular to the prevailing sur-face step direction. The lateral dimensions of AlSb islands range from 18 nm to 110 nm. In fact, their expansion amounts on average only to (36±3) nm and (30±3) nm in the [110]

and the [1¯10] directions, respectively, unless there is an indication of coalescence. Concave parts in the perimeter are treated as such indications. Dashed lines in the scheme of figure 4.13 mark sites of coalescence. The long chain of island 10 is an illustrative example for the connection of three islands. The consideration of h110i directions appears natural because edges directed along the [110] and the [1¯10] orientation predominate at the interface.

Further observations from the slice through the tomogram in figure 4.13 concern the

spec-100 nm

Figure 4.13.The left image presents a slice through the reconstructed volume parallel to the substrate surface.

The distribution, the lateral size and shape of nucleated islands is accessible. The upper right isosurface presentation depicts the imprint of the AlSb islands within the GaSb layer. It conveys the 3D notion of the reconstructed volume. The scheme on the bottom right outlines the numbered islands along with the outer specimen shape.

imen constitution due to FIB preparation. The black arrow points to a grey rim bordering the section through the specimen. It is associated to the amorphised damage layer caused during FIB sample preparation and must not be confused with the similar grey value of the AlSb islands. The damage layer is approximately 6 nm thick. Beyond, a bulge of the amor-phous layer into the specimen below island number 1 is outlined. The bright grey interspace represents GaSb which covers the islands as well as the substrate in between the AlSb is-lands. Bright features within the islands, e.g. in island number 6, are considered as artefact in the tomogram. Residual contrast from dislocation lines in the input tilt series for the tomographic reconstruction occurs along low indexed viewing directions (cf., for instance, [127] or section 2.2.2). This deviation from Z-contrast is assumed to cause this artefact. The isosurface in figure 4.13 illustrates AlSb island shapes as imprints into the GaSb layer. The isosurface presentation of the islands themselves and the subsequent morphological analysis is not possible with the reconstructed data.

Slices orthogonal to the heterostructure interface are extracted from the 3D volume and shown in figure 4.14. The (1¯10) slices depict the specimen cross-section perpendicular to the predominant direction of the FIB preparation or the tilt axis, respectively. The elliptical shape is a result of the different sputter rates of GaSb and Si. It clearly deviates from the intended circular cross-section. The sectioned AlSb islands are emphasized by dotted lines and labelled by the corresponding numbers introduced in figure 4.13. The interface of the III-Sb layer to the Si substrate appears smooth, i.e. a chemical reaction of Ga and Si and a subsequent roughening is not observed. A reshaping of AlSb during GaSb overgrowth is not observed either. Black arrows pointing downward mark notches in islands 2 and 10. They correspond to positions that are highlighted by dashed lines in the scheme of figure 4.13.

That is, they agree with the findings on coalescence sites of islands from the lateral analysis.

Island heights range from 11 nm to 16 nm. Most islands exhibit a flat top facet parallel to the (001) lattice plane. Islands 2, 6, 8 and 12 differ from this top morphology. They show

(110) slices

Figure 4.14.Slices parallel to the (1¯10) and (110) planes are extracted from the reconstructed volume. They provide information about side and top facets of the islands and their height along the growth direction.

Black arrows indicate sites of AlSb island coalescence. White arrows mark islands with unusual heights.

peaks exceeding the average height of (12±3) nm. Peaks are highlighted by white arrows pointing upward in figure 4.14. The side facets of the AlSb islands are always inclined with respect to the surface normal. {111} planes are expected in accordance with the predomi-natingh110iedges. Indeed, different inclinations are observed which implies the presence of side facets with higher indices. A quantitative determination of the higher indexation is beyond the scope of the data.