Sample preparation

Small pieces of mc-Si wafers were provided by Dr. Winfried Seifert from BTU Cottbus. Two different etching routines have been applied to account for the different surface requirements of scanning electron microscopy (SEM), EBSD, and EBIC. For EBSD analysis it is essential to have a clean and smooth sample surface to achieve highest quality of the Kikuchi pattern. However, GBs are not visible in SEM on a perfectly smooth mc-Si sample, making navigation on the sample surface in SEM very difficult. Therefore a short etching of the mc-Si wafer with a mixture of nitric acid (HNO3) : hydrofluoric acid (HF) : acetic acid (CH3COOH) 2:1:1 was applied for a few seconds prior to SEM/EBSD analysis. This preferential etching creates a slight topology on the sample surface depending on the grain orientation and thus renders GBs visible in the SEM.

EBIC analysis requires a thin Al film on top of the mc-Si sample, as described later in 4.2.2. In order to improve the quality of this Schottky contact, it is essential to have a clean Si surface with a homogenous SiO interlayer on top prior to the Al deposition. This is achieved by alternating etching the sample in HF and piranha solution (1:1 mixture of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2)): 5 min piranha solution -> 30 s HF -> 5 min piranha solution. The sample is rinsed with deionized water subsequently and coated with a thin Al layer <20 nm by thermal evaporation or electron beam evaporation.

4.1.2 Focused ion beam

As described before, analyzing GBs with highest accuracy at atomic resolution places high demands on the sample preparation for APT and HR-STEM. APT specimens are prepared as needle-shaped specimens with a radius of curvature of ~50 nm and TEM specimens are generally prepared as a thin foil with a final thickness of 10-100 nm. In both cases, the sample preparation has to be site specific, to study a specific GB portion or a set of specific GB portions. At the moment, these requirements can only be met by FIB milling. In order to correlate the atomic structure of a GB with the 3D atomic distribution of impurities, it would be desirable to analyze the same specimen in HR-STEM and APT. Due to the different geometric

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dimensions and requirements, this approach is not reasonable. Therefore, an advanced site-specific lift-out technique was developed in the scope of this work, which enables the sample preparation of APT and TEM specimens from the same GB with a minimum lateral distance to each other. The developed method is a combination of the plan-view FIB out for TEM sample preparation [91] with the site-specific lift-out technique for APT specimens [92,93]. This idea is illustrated in Figure 4.1, where 3 different grains and thus 3 different GB interfaces are drawn. By preparing a TEM lamella from the middle part of the volume around the triple junction (TJ), it is possible to study the GB structure of all 3 present interfaces. In addition, the adjacent volumes at the right and the left side can be used to study the chemistry by APT.

Depending on the number of APT specimens and the size of the TEM lamella, the volume lifted out in the experiment for sample preparation has the dimension of ~30x3x3 µm³. The surface of the final TEM lamella is parallel to the mc-Si sample surface and the GBs are oriented perpendicular to the analysis direction in the APT specimen. This has two main advantages: due to the parallelism of TEM lamella and sample surface, the same plane is analyzed in EBSD, EBIC, and TEM, enabling a correlation of the different results. Second, having the GB plane perpendicular to the analysis direction in APT reduces the influence of possible artefacts like local magnification effect [94,95].

Figure 4.1: Combined sample preparation of TEM and APT specimens (schematic). The sample volume in the vicinity of a triple junction is used to prepare a TEM lamella from the triple junction and 2x3 APT specimens from the abutting GBs.

All positions selected for APT and HR-STEM analysis were pre-characterized by EBIC and EBSD. The latter is especially important in case of HR-STEM specimens, because all present grains have to be aligned along a common zone axis at a later stage in the STEM to achieve atomic resolution across the GB. Therefore it is indispensable to carefully investigate the orientation of the present grains in the EBSD data prior to the FIB preparation. In the present study the areas for HR-STEM investigations were selected on the basis of the EBSD maps, where all involved grains can be aligned along a common <110> zone axis by a rotation

<15° to avoid handling problems at a later stage in the STEM.

All samples for the present study were prepared using FEI Helios NanoLab 600/600i dual-beam FIB. The single steps of the sample preparation can be summarized as follows:

(1) To reduce the damage by Ga⁺ ions, the region of interest (~30x3 µm²) is protected by deposition of a thin layer of Pt on the surface by the gas injection system utilizing the electron beam (Figure 4.2 (a)).

(2) Markers are added by Pt deposition to visually separate volumes designated for APT and TEM analysis (only necessary if special features like TJs are present, see Figure 4.2 (b)).

4.1 Sample preparation

(3) A “U-shaped” trench with a depth of ≥8 µm is milled around the ROI using 30 kV FIB at a stage tilt of 52°. Subsequently the FIB is used at 0° stage tilt to perform a thin cut below the surface of the ROI resulting in a thick bar, which is only attached at one side to the bulk material (Figure 4.2 (b)).

(4) The micromanipulator is welded to the free side of this bar using Pt deposition at 0° stage tilt.

(5) The bar is cut free using the FIB and lifted out with the micromanipulator.

(6) The bar is welded to a suitable supporting post (halved and electropolished Mo grid, which is in a horizontal orientation parallel to the stage) using Pt deposition and a small part of 2 µm length is extracted by FIB milling. This step is repeated usually 3 times to create 3 individual wedges for shaping 3 individual APT specimens (Figure 4.2 (c)).

(7) In the same way a longer part of 5 µm length is extracted to a standard omniprobe TEM grid, which is oriented horizontally (parallel to the stage) in the SEM.

(8) Step (5) is repeated again 3 times for the second GB.

(9) The system is vented and the APT specimens are flipped manually upside down to weld the samples from the back side by Pt deposition using electron beam and/or FIB.

(10) The system is vented again and both grids are flipped manually from horizontal to vertical orientation (Figure 4.2 (d)).

(11) The TEM lamella is now milled following the procedure described in [91]. Final milling of the lamella is performed at low kV (usually 2 kV in the present study) to reduce damage by the Ga⁺ ions.

(12) Needle-shaped specimens for APT analysis are prepared individually using the annular milling at reduced voltages (Figure 4.2 (d)). Final milling is performed at 5 kV or 2 kV to reduce beam damage.

(13) TKD is used in between the milling steps to ensure that the desired GBs are contained in the single specimens and to control the distance of the GB to the apex of the APT specimens (Figure 4.2 (e-f)). A large step-size is used to speed up the process and to avoid contamination induced by the electron beam.

(14) Final imaging in SEM is performed to capture the exact tip geometry for APT reconstruction (Figure 4.2 (g-h)).

(15) Final tip shape and exact GB position are imaged for later correlation using the Jeol 2200 FS TEM operated at 200 kV (GB position and GB shape can be determined with much higher accuracy in TEM).

(16) In case of extensive TEM investigations the APT specimens should be cleaned once again at low-kV in the FIB to remove surface contamination prior to the APT analysis.

This advanced sample preparation is more time-consuming in comparison to the standard techniques, but a lot of additional information can be extracted from the measurements which is essential for a deeper understanding of the structure-chemistry relationship at the GBs.

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Figure 4.2. Overview of the lift-out process by focused ion beam (FIB): (a) Pt protection layer, (b) trenches around the lift-out volume and Pt markers separating GB area and triple junction area, (c) extraction of small volumes to support grid using a micro manipulator, (d) APT lift-out before annular milling, (e) Transmission Kikuchi diffraction (TKD) on APT specimen during annular milling, (f) TKD on TEM specimen during FIB milling, (g) final APT specimen (h) side-view of final TEM specimen.