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Grain Boundary Properties


used to reduce the sample roughness. In this technique, the samples were ion-milled at a glancing angle to the surface. Figure 5 shows SEM images of a CIGS surface before and after ion milling. T e goal of the ion-milling step was to remove a minimum amount of material (~100 nm) that would improve the EBSD pattern quality and still allow information on the grain size and orientation to be obtained near the original surface. T is depth represents ~ 10% of the total CIGS fi lms thickness (~ 1.2 µm). Ion-milling was done at a glancing angle for several minutes over a width of 50 µm to provide a suffi cient area for EBSD analysis. AFM measurements indicated that the surface roughness decreased substantially aſt er ion-milling; however, typically ~200–250 nm was removed from the surface by ion-milling in order to provide a smooth surface. T e depth of the CIGS removed during the ion-milling step was estimated by using AFM measurements of the median height diff erence between the ion-polished and as-received regions as shown in Figure 6 .


Figure 7 shows inverse pole figures after ion-polishing for Devices 1–3. In general, we observed that the quality (confidence index) of the EBSD patterns collected from ion-polished surfaces was lower compared to as-received surfaces. This was attributed to surface damage from Ga ion-implantation during ion-milling; however, the quality of EBSD patterns collected from ion-milled surfaces was sufficient for indexing of the grains without additional surface treatment (for exmaple etching or Ar ion milling to remove surface damage). It is interesting to note that while porosity was observed in all devices (based on both SEM imaging and EBSD data), it does not appear to degrade device performance dramat- ically at this efficiency level since all devices had 10–11% efficiency. Regions with a low confidence index (pores) are denoted in black. The EBSD patterns were collected over regions 12 μ m × 12 μ m in size for each sample in order to collect a sufficient number of grains for statistical analysis, but a smaller region (7 μ m × 7 μ m) is shown in Figure 7 for clarity.


Grain size measurements . The EBSD data were used to determine the grain size distribution for Devices 1–3 ( Figure 8 ). The average grain size for Device 1 was ~ 0.6 µm, which was significantly larger than the average size ~ 0.4 µm for Devices


36


Figure 5 : Ion-beam polishing. (a) Secondary electron SEM (plan view) image of the CIGS surface before (right side) and after (left side) the ion-milling polishing procedure for Device 1. Image width = 52 µm. Image (b) is fi ve times the magnifi cation of image (a). Image width = 10.4 µm. Samples are oriented so that the stainless steel rolling direction is aligned with the long direction of the page. Some enlargement of the pores may occur as an artifact during the ion-milling process.


2 and 3. The CIGS grain size diameter can be reasonably represented by a log-normal distribution as shown in Figure 8 .


The performance levels of Devices 1–3 were all similar, and therefore the average grain size did not correlate with device performance at this efficiency level of 10–11%. This result is consistent with previous work, which showed that the grain size has a limited impact on device performance, and other factors such as recombination at the hetero- junction interface, internal potential fields, or recombination at the back contact may play more significant roles [ 3 , 8 ]. In addition, the role of the relative grain orientation or porosity may also be a limiting factor to attainment of higher efficiencies. It is interesting to note that Device 2 was annealed for additional time (40 minutes) at 600 o C in an Se reactor to promote grain growth compared to Device 1, but this anneal did not have a significant effect on grain growth. Grain boundary type .


The distribution of low- and high-angle grain boundaries was also investigated.


Figure 6 : (a) AFM scan of 50 µm × 50 µm region across both as-deposited CIGS and ion-polished regions. (b) Histogram showing the height distribution for scanned area ( x -axis in µm). The as-deposited fi lm shows a broad distribution of heights (over an area of ~ 25 µm × 50 µm) due to a combination of the thin fi lm roughness plus a non-uniform stainless steel substrate. Conversely, the ion-polished area shows a relatively narrow height distri- bution. The depth removed during the ion-milling step was estimated by comparing the median height of the two regions. The difference in median heights for this area was ~ 234 nm.


www.microscopy-today.com • 2018 May


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