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As was stated earlier (see Section 2.4), the bc8/r8 mixed structure formed using indentation is known to form in a similar manner from both a-Si and dc-Si starting materials. For the results in this present study, an a-Si precursor was chosen for the samples prepared for XRD measurements. This was done for two main reasons. Firstly, crystalline defects do not occur in a-Si, thus removing an added layer of complexity that may impact the final result. Secondly, the signal from the underlying substrate will also be collected in the XRD data set. An amorphous structure will appear as a broad ring while a crystalline structure will appear as discrete spots. This is important when a background image is removed to minimise the impact of the surrounding material. Removing an a-Si background image will not cause errors due to differences in rotation between the background and the image, as the broad rings from a-Si are rotationally homogeneous. This is an advantage that a crystalline substrate does not share. To form the a-Si precursor, a dc-Si sample was self-implanted with Si ions to form a 2 µm thick layer of a-Si, which was then relaxed by annealing at 450◦C for 2 hours. This

step was performed as relaxed a-Si has been reported to phase transform more readily than as-implanted a-Si [113].

The relaxed a-Si was indented using a TI 950 Triboindenter fitted with a ∼60 µm diameter spherical tip. A schematic of the sample is shown in Fig. 4.2 with the incident X-ray beam direction also indicated. Each indent was made to a maximum load of 750 mN, with a load- ing/unloading rate of 10 mN/s. These conditions have been previously reported to form the bc8/r8 mixed structure [55] and the presence of the mixed structure was confirmed using Ra- man microspectroscopy. Indents were made in an 80 x 10 array near the edge of the sample, with a 10µm separation between each indent. As each residual impression was∼10µm wide, this created a 800 µm x 100 µm area of the bc8/r8 structure that can be approximated as a “thin film” (which is indicated in blue in the schematic). As the a-Si phase transforms under indentation pressure in a manner similar to dc-Si, the thickness of the film can be approxi- mated as∼500 nm [191]. The sample is then polished down along the edge perpendicular to the long edge of the bc8/r8 film (indicated in Fig. 4.2as the polished edge). That is, from the perspective of the incident beam, the material behind the transformed film is removed to min- imise the amount of a-Si within the measured region. The schematic also defines a co-ordinate system, with the x-axis running parallel to the beam direction and the z-axis running parallel to the indentation direction (out of the page in the top view).

Fig. 4.2: A schematic of the bc8/r8 “thin film” samples used in this section for XRD measure- ments. The orange line/circle indicates the relative size of the X-ray beam, the blue region indicates the transformed bc8/r8 mixed structure, and the light grey region indicates the a-Si layer.

Figure 4.3(a) shows the load/unload curve from one the indents made into the a-Si. The presence of a pop-out event has been reported to be indicative of transformation to the bc8/r8 mixed structure. The pop-in events observed during loading are most likely due to plastic flow of the sample from under the indenter tip, as damage to the crystalline structure is not expected to occur due to the a-Si layer. Regardless of the cause of these pop-ins, they were not found to prevent phase transformation in subsequent indentations. The Raman spectra in Fig. 4.3(b) taken from the transformed region have prominent peaks associated to r8- and bc8-Si (such as the peaks at∼350 cm−1 _{and ∼430 cm}−1_{), which confirms the presence of the}

bc8/r8 structure.

Fig. 4.3: (a) An indentation load/unload curve to 750 mN. The pop-out event that is generally associated with transformation to the bc8/r8 mixed structure is indicated. (b) Raman spectra taken from the transformed region. The main peaks associated with the bc8- (437 cm−1) and r8-Si (352 cm−1) phases are labelled.

XRD measurements on this sample were performed at the 34-ID-E beamline at the Advanced Photon Source (APS). The incident X-ray beam had an energy of 25 keV and a spot size of ∼1 µm in diameter. Due to this small spot size, it was possible to graze the sample surface such that XRD data was collected only from the bc8/r8 film and the underlying a-Si.

The resulting XRD images were processed using the software package Dioptas [147]. Firstly, the background was removed by subtracting an a-Si image from the total image. That is, an image collected from purely a-Si can be removed from an image collected from both a- Si and the bc8/r8 structure to emphasise the bc8/r8 structure. Secondly, using the “cake” function, the images were converted from cylindrical coordinates to Cartesian coordinates. The advantage of this is that all reflections from a given latticed-spacing will form a vertical line. This was used to determine if there wasd-spacing variance within a ring of reflections. Finally, images were integrated to produce XRD profiles.

System (GSAS-II) [148]. While many different parameters can be refined using GSAS-II, the parameters used within this work can be roughly divided into five types. First, experimental parameters are those that are unique to the experimental set up (e.g. Caglioti formula param- eters U, V, W [192], sample thickness, crystallite size terms LX, GP, etc.) and primarily refine peak broadness. Second, unit cell parameters (e.g. unit cell length, unit cell angle) refine the position of the peaks. The final three all refine peak intensity. They are the atomic location (e.g. x, y, z, Uiso), preferred orientation (e.g. March-Dollase), and phase fraction parameters.