FIGURA 4 LECHO DE SECADO DE ARENA

A range of Ge1-xSnx epilayer samples were investigated, all of a Ge1-xSnx/Ge/Si(001)

structure, with a range of Sn fractions, layer thickness and initial epilayer degrees of strain relaxation. These samples were also investigated in the preceding lab based thermal treatment with in-situ XRD study, allowing for direct comparison of the results between the two methods.

The samples were loaded into the high-temperature cryostat as described in chapter 3 of this work. The sample environment was then evacuated with a vacuum pump, leaving the samples is a moderate vacuum environment during measurements with the atmosphere being the helium cryogen. Hence, in contrast to the lab based study in the previous section, the samples investigated in the high temperature cryostat are in an oxygen free environment.

The diffractometer configuration was then calibrated at room temperature, with the sample Si substrate, Ge buffer and Ge1-xSnx epilayer symmetric (004) and

asymmetric (224) Bragg peak reflection angles identified. The sample was then cooled to base temperature of ~20 K, and the temperature was stabilised for approximately 1 hour. This long stabilization time at a temperature below the measurement temperature is to ensure the cryostat cold head is as cold as possible, as

this limits the maximum temperature which can be investigated, as the excessive cold head temperatures end the experiment.

The sample was then heated to 200 K and 300 K with measurements at each temperature. Subsequently, the sample was investigated at more regular temperature intervals, initially with 20 °C increments at lower temperature range followed by 5 °C increments at higher temperatures, again with measurements at each temperature. The 20 °C temperature increments were used at lower temperatures as the layers are known to be stable at these temperatures from previous investigations. The smaller temperature increments were used as sample temperature approached temperatures at which interesting behaviour had previously been observed for each sample.

At each temperature the Si, Ge and Ge1-xSnx Bragg peaks in symmetrical (004) and

asymmetrical (224) orientations were scanned over to measure main Bragg peak intensity, FWHM (indicative of crystallinity), omega position (for epilayer composition and thermal expansion).

5.3.3 Critical Temperature

The critical temperature is the threshold temperature beyond which sudden crystal degradation is observed, as detailed previously in this chapter. From previous investigations the expectation is for the Ge1-xSnx Bragg peak intensity to remain

essentially constant with temperatures below the critical temperature, with some slight loss of intensity due to the increased thermal disorder of the crystal lattice, but without the loss interaction volume due to epilayer oxidation as observed with the Anton Paar study.

Figure 5-17 shows the temperature variation of the Ge1-xSnx Bragg peak intensity for

samples with ~9 at. % Sn alloy epilayers of a range of thicknesses, each fully strained to the Ge buffer. A critical temperature is observed for 40 and 70 nm thick Ge0.909Sn0.091 epilayer samples, but not for the Ge0.908Sn0.092 50 nm epilayer sample.

The 40 nm epilayer has a critical temperature of 282 ± 5 °C, the 70 nm epilayer has a critical temperature of 307 ± 5 °C.

Similarly, Figure 5-18 shows the Ge1-xSnx Bragg peak intensities from several

samples with a range of Sn fractions Ge1-xSnx alloy epilayers fully strained to the Ge

buffer. The Ge0.942Sn0.058 70 nm epilayer sample does not display a critical

temperature. The Ge0.909Sn0.091 and Ge0.894Sn0.106 epilayers, both 40 nm thick, display

critical temperatures of 282 ± 5 °C and 288 ± 5 °C, respectively. The only significant difference in the material properties of these two samples is the Sn fraction, but the critical temperatures are similar. However, unlike the Ge0.909Sn0.091 sample the

Ge0.894Sn0.106 sample also shows a parasitic peak – as seen in the lab based study.

This peak appears at the same temperature as the critical temperature for the main Ge0.894Sn0.106 peak, shown by the blue line.

Shown in Figure 5-19 are the Ge1-xSnx Bragg peak intensity results of a fully strained

Ge0.894Sn0.106 epilayer sample and the partially relaxed Ge0.884Sn0.116 epilayer sample.

These show a critical temperature of 288 ± 5 °C and 315 ± 10 °C, respectively. The significant material properties between these samples is that the Ge0.884Sn0.116

epilayer sample has a thicker epilayer and has existing lattice defects at the epilayer- buffer interface. The combination of these two factors appears to increase the critical temperature of the partially relaxed epilayer sample. Both of these samples show a parasitic peak, appearing at the critical temperature for the main Ge1-xSnx peak. This

multiple samples when using the synchrotron set-up. This demonstrates that the parasitic peak, along with the critical temperature is not a oxidation driven process, as both a repeatedly seen in the absence of oxygen during the thermal treatments. Comparing these results to those obtained using the Anton Paar stage, the critical temperatures of samples under vacuum vary from those under atmosphere, critical temperature is observed to both increase and decrease for samples. This is indicative of other factors influencing the critical temperature. This is supported by some samples which do not show a critical temperature in atmosphere, do under vacuum, as for the 40 nm Ge0.916Sn0.084 epilayer. Conversely some samples which have a

critical temperature in atmosphere don’t under vacuum as for the 50 nm Ge0.918Sn0.082

Figure 5-17 GeSn epilayer Bragg peak intensity vs. sample temperature. (top) 40 nm thick Ge0.909Sn0.091 (centre) 50 nm Ge0.908Sn0.092 (bottom)

70 nm Ge0.908Sn0.092, with the parasitic peak shown as the blue line

appearing as the main Ge1-xSnx peak intensity has decreased. All epilayers

Figure 5-18 GeSn Bragg peak sum signal. (top) Ge0.942Sn0.058 70 nm (centre)

Ge0.909Sn0.091 40 nm (bottom) Ge0.894Sn0.106 40 nm, with the blue line

Figure 5-19 (top) fully strained Ge0.894Sn0.106 epilayer (bottom) Partially

relaxed, R=19% relative to the Ge buffer Ge0.884Sn0.116 epilayer. The blue

lines indicate the intensity of the parasitic peaks which appear at the critical temperature for both samples.