Defect analysis

Figure 4.23: HR-XTEM of sample 15-42; Ge buffer layer grown at 400°C. The average measured thickness of the islands 8.3nm.

Araldite

8.3nm +/- 0.5% Germanium

Straight through beam

Si(001) [110]

[001] Lomer dislocation

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Figure 4.24: HR-XTEM of sample 15-46. Stacking faults propagating from a single stair-rod dislocation, can be seen. The measured angle from either stacked section of the layer is 55° along [110] direction.

Figure 4.23 is a lattice resolved image taken of a single Ge island grown at 400°C to 8.3nm thickness. A Lomer dislocation is seen at the interface between the substrate and epilayer. 2D defects are predominantly seen for the Ge buffer layers grown at 350°C and 300°C. Figure 4.24 is a lattice resolved image of 6.4 nm Ge buffer layer showing opposing stacking faults that emanate from a stair rod dislocation. Stair rod dislocations can occur in islands where the shear stress is particularly high at the edges. Figure 4.26 is a plan view TEM image taken at the edge of a milled hole of a 350°C layer grown to 42nm thickness. 2D defects are seen, possibly emanating from stair rod dislocations and the measured angle is 125° on the (001) plane. If these are caused by stair rod dislocations, then the correct angle is 135° however since the sample is bent around the edges of the milled hole there is possibly a 10° discrepancy. When the layer is grown thicker, to 174nm thickness as seen in figure 4.27, plan view images show that the 2D defects have vanished which is expected because the facet islands form with the {111} planes as boundaries.

Germanium 6.4nm +/- 0.5%

Straight through beam Si(001) Araldite

[110]

[001] _{Stair rod dislocation} 55°

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Figure 4.25: Plan view TEM of sample 15-47 (300°C growth 78.9nm thickness) showing threading dislocations. TDD for this image is 𝟏. 𝟎𝟓 × 𝟏𝟎𝟏𝟏 _𝐜𝐦−𝟐_{. Average TDD for this sample is 𝟗. 𝟖𝟔 × 𝟏𝟎}𝟏𝟎 _𝐜𝐦−𝟐_.

Figure 4.26: Plan view TEM of sample 14-300 (350°C growth, 42nm thickness) showing 2D defects as indicated by the red dashed circles, possibly emanating from the stair rod dislocations. The angle measured between 2D defects is 125°.

Threading dislocations

125°

Dark field g = 220

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Figure 4.27: Plan view TEM of sample 15-43 (350°C growth 174nm thickness). TDD for this image is 𝟐. 𝟑𝟐 × 𝟏𝟎𝟏𝟎 _𝐜𝐦−𝟐_{. Average TDD for this sample is 𝟐. 𝟑𝟎 × 𝟏𝟎}𝟏𝟎 _𝐜𝐦−𝟐

Figure 4.28: Plan view TEM of sample 14-302 (350°C growth and 95nm thickness) showing Moiré fringes on the left and side of the image caused by the interference between the substrate diffraction vector and the thin partially relaxed epilayer diffraction vector. Average TDD for this sample is 𝟔. 𝟏𝟓 × 𝟏𝟎𝟏𝟎 _𝐜𝐦−𝟐_.

The amount of thin area available for imaging becomes drastically reduced for thin samples and where Moiré fringes occur. This effect occurs when there is a change in

diffraction vector |∆𝑔| between the partially relaxed epilayer and substrate and is more pronounced in particularly thin layers. The thinnest sample imaged in this investigation was sample 14-300 (350°C growth and 42nm thickness). Their presence makes it

Dark field g = 220

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difficult to clearly see the threading dislocations. Only the thickest (78.9 nm) 300°C grown sample was imaged as shown in figure 4.25.

Figure 4.29: Plan view TEM of sample 15-38 (400°C growth, 351nm thickness). TDD for this image is 𝟗. 𝟏𝟑 × 𝟏𝟎𝟗 _𝐜𝐦−𝟐_{. Average TDD for this sample is 𝟖. 𝟔𝟏 × 𝟏𝟎}𝟗 _𝐜𝐦−𝟐_.

Different materials have been known to have different relationships between thickness and TDD [107]. From figure 4.31 it is seen that as the layer thickness, h, increases for all growth temperatures, the TDD value decreases. Several power law functions were used to fit the thickness vs TDD data, however the function that gave the closest to a linear fit when taking log 10 on both axes (figure 4.32) was the following relationship:

TDD =530 h − 0.5

This relationship can be explained by the annihilation mechanism of > 108_cm−2 _TDD,

where closed loop meeting of anti-parallel Burgers vector is preferred with misfit- misfit dislocation interaction creating a kinetic limit. So far, annealing of 78nm thick 400°C layers at 650°C has shown the creation of uniform buffer layers, with a more ordered Lomer network at the substrate interface, as well as 2.1nm roughness and a transition to 0.036% tensile strain as opposed to 0.2% compressive strain. 1min annealing reduces the threading dislocation density by approximately x6 of an

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equivalent un-annealed sample. Annealing for a further 4 mins (5 mins total) at 650°C reduces the TDD but only by a further x1.1 (figure 4.30). This can be explained by the increased glide velocities made available through annealing for 60° misfits along {111} planes and Lomers along (001), however again a kinetic limit is present with annealing. Higher annealing temperatures could be investigated as well as longer anneal times at 650°C in future works but at the risk of de-stabilising the layer.

Figure 4.30: Plan view TEM of sample 15-61 (400°C growth + 650°C for 5 mins 78nm thickness). TDD for this image is 𝟔. 𝟏𝟖 × 𝟏𝟎𝟗 _𝐜𝐦−𝟐_{. Average TDD for this sample is 𝟕. 𝟓𝟕 × 𝟏𝟎}𝟗 _𝐜𝐦−𝟐_.

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Figure 4.31: Threading dislocation density vs thickness for LT-Ge buffer layers

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