Nivel de ingresos Tabla No 3

In this work strained Ge1-xSnx epilayers grown onto Ge-buffered Si(001) substrates

were investigated, the epilayers were produced at a range of growth temperatures, producing a wide range of Sn fractions and layer thicknesses. Fully strained epilayers are layers thin enough to not exceed the critical thickness of relaxation and are lattice matched to the Ge lattice. Strained layers are of interest due to their low defect density and the effects of bandstructure of lattice strain. All Ge1-xSnx layers

investigated in this work are compressively strained, making the Ge1-xSnx bandgap

L-Γ separation larger than a similar composition layer which is strain relaxed.

In the asymmetric XRD (224) RSM of strained epilayer samples, as example shown in Figure 4-9, the Ge1-xSnx epilayer Bragg peak is observed directly under the Ge

peak, i.e. the epilayer and buffer have an identical qx, indicating the epilayer real

space in-plane lattice parameter is matched to the underlying Ge buffer. In the symmetric (004) RSM, example in Figure 4-9, the Ge1-xSnx epilayer Bragg peak is

observed directly under the Ge Bragg peak, indicating that the epilayer is not tilted relative to the Ge-buffer. Note that the Ge buffer is under slight tensile strain due to the thermal coefficient mismatch between Ge and Si. In XRD rocking curves of strained epilayer samples, thickness fringes are observed in satellite positons either side of the Ge1-xSnx epilayer Bragg peak. The observation of thickness fringes

indicates the epilayer surface is smooth and the epilayer/buffer interface has a low concentration of lattice defects.

A fully strained epilayer TEM image aligned along the (004) plan in dark field condition is shown in Figure 4-15. In this sample no strain relieving lattice dislocations are observable at the GeSn/Ge interface. The interface is observed as an uninterrupted straight continuous line, the epilayer surface is also smooth and there is no discernible Sn segregation.

Figure 4-9 Various XRD scans from Ge0.91Sn0.09/Ge/Si (Upper left)

Asymmetric (224) RSM, Note the GeSn peak is directly under the Ge peak and thickness fringes are visible. (Upper right) Symmetric (004) RSM

The surface morphology of strained epilayer samples is relatively smooth, with a surface roughness ~3 nm, with the dominant surface feature being surface pits. These surface pits can be attributed to material etching during growth due to the production of HCl as a by-product during growth; this process is observed when using the SnCl4

precursor to produce SnO2 [129]. A representative sample of AFM scans of strained

layers, with a range of epilayer alloy compositions is given in Figure 4-10. Figure 4-10 50 × 50 μm AFM scans of fully strained epilayer samples with a

range of compositions (left) Ge0.942Sn0.058 (centre) Ge0.908Sn0.092 (right)

Ge0.896Sn0.104. All samples have an RMS roughness of 3 nm. All samples

have surface pit features, but no indication of crosshatching or other significant features.

Figure 4-11 Raman spectra from several strained Ge1-xSnx epilayers with Sn fractions

of 1 (black), 6 (red), 8 (violet) and 11 at. % (pink). The lower plot highlights the shift in Ge-Ge mode, note the Raman peak shifts from 1 at. % Sn to 6 at. % Sn but the peak shift is slight. From 6 at. % to 11 at. % Sn fraction no additional shift in the Ge-

Figure 4-11 Raman spectra from several strained Ge1-xSnx epilayers with Sn

fractions of 1 (black), 6 (red), 8 (violet) and 11 at. % (pink). The lower plot highlights the shift in Ge-Ge mode, note the Raman peak shifts from 1 at. %

Sn to 6 at. % Sn but the peak shift is slight. From 6 at. % to 11 at. % Sn fraction no additional shift in the Ge-Ge mode is observed. This is attributed to the effect on Ge-Ge Raman mode position of increased layer compressive

strain compensating for the increased Sn fraction. Sample intensities are offset in intensity (y-axis) for clarity.

Ge mode is observed. This is attributed to the effect on Ge-Ge Raman mode position of increased layer compressive strain compensating for the increased Sn fraction. Sample intensities are offset in intensity (y-axis) for clarity. Figure 4-11 shows Raman spectra from strained Ge1-xSnx epilayer samples for a range of compositions.

The presence of the Ge-Sn Raman peak at ~260 cm-1 confirms the incorporation of Sn into the Ge lattice [121]. The Ge-Ge Raman mode, at ~300 cm-1, of fully strained Ge1-xSnx epilayers is shown not to shift significantly with increased Sn fraction. It is

expected that that this Ge-Ge Raman peak would

shift to lower wavenumber with increasing Sn fraction. The unexpected observation of a fairly constant Ge-Ge Raman shift value for all epilayer alloy compositions can be attributed to a balance between the tendency to shift to higher wavenumbers because of increased strain, from the increasing Sn fraction increasing the bulk lattice parameter, and the shift to lower wavenumbers because of the direct change in Sn fraction. This coupling of strain and alloy composition effects makes Raman spectroscopy unsuitable for determining the composition of strained Ge1-xSnx

epilayers.

The maximum thickness observed by TEM of fully strained Ge1-xSnx epilayer

samples for a range of compositions investigated in this work is shown in Table 4. These values place lower bounds on the critical thickness of plastic relaxation under these growth conditions.

Sn fraction

(at. %) 1 6 8.5 10.6

Epilayer

Thickness (nm) 300 75 50 40

Table 4 The maximum fully strained epilayer thickness obsereved in this work at a range of alloy Sn fractions

High crystal quality fully lattice strained Ge1-xSnx epilayers have been grown onto

Ge-buffered Si(001) substrates with a range of epilayer thickness and Sn fractions. The material properties of the epilayers were controlled by modifying the growth temperatures and precursor mixture. For all growth temperatures investigated no evidence of polycrystallinity or Sn segregation is observed.