Synchrotron X-ray sources function by accelerating electrons with variable electric fields in a large circle, avoiding the limiting factors of lab based sources. Synchrotrons produce an extremely high intensity beam of X-ray radiation, and the wavelength can be selected. X-ray beam optics are used to select a narrow band of monochromatic beam with a small beamspot.
Synchrotron facilities allow for extremely high intensity beams, several orders of magnitude greater than possible with lab sources, to be achieved. This allows some measurements which normally require a significant scan time to be done in comparatively short time, which can facilitate the ability to examine time dependant phenomena not possible with lower intensity sources [116].
However, the high beam intensity can cause significant sample heating, which can be mitigated by sample cooling [81]. Despite this cooling, the high beam intensity increases the probability of beam damage to the sample compared to standard lab based diffractometers.
Though not part of this work, these incredibly intense beams facilitate the use of Fresnel lens and other methods of producing very small beam width, ~1 μm, allowing the isolation of signal from sample features on a micro and sub-micron scale [117].
As the X-ray beam wavelength is not limited to metal target emission lines, users can freely and reliably alter the x-ray photon energy such that x-rays can be used in techniques other than diffraction, such as X-ray absorption spectroscopy (XAS) and
extended X-ray absorption fine structure (EXAFS) which can probe the local bonding structure around specific species of atoms [118,119]. Though synchrotrons have many advantages over standard lab diffractometers, synchrotron facilities are very expensive to construct and maintain thus users typically have a limited measurement time.
For this work the synchrotron used was the Diamond Light Source, near Didcot, Oxfordshire, UK. This is a 3rd generation Synchrotron, with a 3 GeV electron beam (medium energy). Specifically, beamline I16 was used.
3.6.1 Sample Mounting and Temperature Control
During Synchrotron measurements the sample temperature was controlled by a high temperature cryostat, capable of producing sample temperatures of 20 K to 700 K. The cryostat was attached to a 6-circle kappa goniometer. The cryostat cryogen and vacuum shields each had a beryllium dome at the base, where the sample was mounted, as beryllium is a low atomic number element the X-ray beam transmits through with minimal attenuation, shown in Figure 3-13. The detector used was a 2- dimensional 100K 172 × 172 μm pixel detector. A standard focus setup was used, with an X-ray beam spot size of 30 × 200 μm, therefore results will be an average of this sample area. As samples are homogeneous at this scale, such averaging is reasonable.
The sample environment during measurement was a vacuum, ~10-6 mbar. A second shield was used to maintain a vacuum between itself and the first to provide thermal insulation from the environment, Figure 3-13.
Samples of Ge1-xSnx/Ge/Si were cleaved ~1 × 1 cm squares which were mounted via
a thermally conductive adhesive silver paste onto a copper stub. Prior to mounting in the cryostat the sample and stubs were cured at 100 °C for 1 hr, this is to ensure the adhesive stability of the silver paste and prevent out-gassing. After curing, the copper stub was mounted on the temperature controlled cryostat sample stage, as shown in Figure 3-12.
During each experiment run the sample was first cooled down to the base temperature of ~20 K. This cooling is to minimize the temperature of the cryostat cold head, which requires protection from high temperatures. Several temperature sensors were used to determine sample temperature in-situ.
Figure 3-12 A GeSn/Ge/Si sample mounted on copper stub via silver paste and mounted onto the high temperature cryostat sample cradle. To the right
3.6.2 Measurement Attributes
In synchrotron XRD experiments, a huge quantity of information are gathered very quickly, all of the information that is attainable in lab based sources, discussed in the previous section, can be obtained but significantly more rapidly due to the much higher beam brilliance. However, available measurement time is limited and requires applying in months advance, thus such experiments are not ideal for quick turn- around between experiment design and execution. This diffraction method still cannot be used to directly measure layer thickness. Because of the wide range of experiments conducted at synchrotrons, and because each beamline set-up is unique, and each experiment produces potentially huge amounts of data, the data analysis itself is not standardized, increasing its complexity and time intensity. Figure 3-14 shows an unprocessed tiff image with a Bragg peak visible, this is the raw data from which the synchrotron results are extracted.
Figure 3-13 The high temperature cryostat with (left) the inner cryogen shield mounted, to contain the cryogen (right) the outer vacuum shield mounted, which is used to maintain a vacuum for thermal insulation.