Atom Probe Tomography Studies on the Cu(In,Ga)Se₂ Grain Boundaries

The overall goal of this procedure is to determine the correlation between chemical composition and structure of CIGS grain boundaries. This is accomplished by first lifting out a piece of the CIGS material and mounting a part of it on top of the sharp molybdenum pins of a transmission electron, microscopy half grid. The second step is to perform an electron back scattered diffraction measurement on the cross section of the CIGS piece, which was previously cleaned in a focused ion beam aim.

Next annular milling in the area of a selected grain boundary is performed. The aim of this step is to obtain at the end a very sharp tip of about 50 nanometers in diameter. The final step is to localize the precise position of the grain boundary within the tip by using the transmission electron microscope and to place this grain boundary close to the apex of the tip by using the focused ion beam tool.

Ultimately, atom probe tomography studies performed on the prepared tip showed the chemical composition of the selected grain boundary at the nanoscale. The main advantage of this technique over the existing standard lift out method is that we can directly correlate the chemical composition with the structural information such as the grain boundary type and the mis orientation. This method can help answer key questions related to the CHS vol dykes, such as reduction of the production cost at the enhancement of the efficiency.

Indeed, internal interfaces such as grand boundary can play a pivotal role as they can affect the recombination and the transport of the charge. Cos This approach is in principle applicable to all materials with grain sizes above 500 nanometers that can be characterized by APRO tomography. Generally, the present method is challenging for individuals new to the sphere because they need to be skilled at focused I and B electron back scatter fraction and transmission electron microscope techniques.

We first came up with the idea of developing this method when we found that the impurity concentration in particular sodium varies from one CIGS grain boundary to another Begin by creating a source of CIGS material for study. In this demonstration, 500 nanometers of molybdenum was sputtered onto a three millimeter thick soda, Lyme glass substrate, and two micrometers of CIGS was co evaporated onto the molybdenum. Next, create the structure that will be used to support a specimen.

Cut a maum transmission electron microscopy grid into two pieces, mount the half grid onto a specially designed holder electro polish the maum pins in 5%sodium hydroxide to obtain sharp posts with diameters smaller than two micrometers, working with a focused Dion Beam Mill. Two trenches in the CIGS film that undercut a region about 25 micrometers by two micrometers. Then use the beam to cut the left side of the region free.

Now deposit a platinum weld on the region and attach a microm manipulator with this in place. Make the final cut on the right side of the region to obtain a freestanding section of material. Cut the tips of the sharp pins of the half grid to create surfaces two to three micrometers in diameter as a good platform and joint for the extracted section.

Mount the extracted section of CIGS material on the pins using a platinum weld. Once the sample is attached, make a free cut to leave only about a two micrometer CIGS piece on top of the pin. Finally, fill the gap between the pin and the mounted piece with platinum.

Orient the grid with the pins facing upward. Set the focus Dion Beam to have an accelerating voltage of five kilovolts and to beam current of less than 50 picoamps. Use the beam to clean the cross section of the sample at the end.

Perform electron beam back scatter diffraction measurements on the cleaned cross section. Based on the data, choose a grain boundary of interest. Ideally choose a grain boundary that is perpendicular to the analysis direction of the atom probe.

Here the choice is shown by the schematized position of the atom probe tomography tip. Set up the focused ion beam to do annular milling to form a sharp tip near the chosen grain boundary. Perform the annular milling in the end.

The sharp tip should be suitable for further transmission electron microscopy. After the tip is formed, use transmission electron microscopy to locate the precise position of the grain boundary with respect to its apex. With the grain boundary located, return the sample to the focused ion beam operating at five kilovolts and less than 50 picoamps.

Continue to mill the sample to situate the grain boundary at a maximum of 200 nanometers. Below the apex of the tip, monitor the process with secondary electron microscopy. Now back at the transmission electron microscope, check the position of the grain boundary with respect to the tip using low magnifications and reduced exposure times to limit damage.

Make an overview image of the specimen to obtain knowledge of the grain boundary. Position the specimen diameter and the half shank angle. To begin the process, mount the prec charact sample in the atom probe tomography holder.

Then mount the holder in one of the three carousels available. Insert the carousel into the load lock and start the vacuum pump. When the pressure in the load lock is about 100 nano tour, insert the carousel into the buffer chamber.

Next, cool the system to below 60 kelvin to avoid diffusion of the atoms at the surface of the specimen during the analysis. Once the system is cooled, start the measurement after setting the apparatus to laser mode using a green laser with a wavelength of 532 nanometers and a pulse length of 12 picoseconds. Finally set the apparatus to the automatic mode for data analysis.

Use the integrated visualization and analysis software. The raw data from the measurements is contained in an RHIT file. To reconstruct the 3D map, verify that you have the correct file using the setup pane.

Perform the standard setup steps for Adam. Probe tomography data once set up is complete for reconstruction, choose the tip profile method. This will make use of the previously gathered transmission electron microscopy data.

Once the steps are completed, confirm the preview created in the reconstruction tab and save the analysis. Here is a three dimensional side view of A-C-I-G-S high angle grain boundary analyzed by the atom probe tomography technique. This grain boundary was selected using the site-specific preparation method.

Shown in this video, the misorientation angle of this green boundary is 28.5 degrees. These maps clearly show the co segregation of sodium, oxygen, and potassium respectively at the boundary. These impurities most likely diffused out of the soda lime glass substrate into the absorber layer during the deposition of the CIGS layer at 600 degrees Celsius.

For the same sample concentration depth profiles across the grain boundary for selenium, copper, indium, and gallium are shown on the left. Because of the different scales, the concentration depth profiles for sodium, oxygen and potassium are shown on the right. The concentrations at the grain boundary highlighted in gray are different compared with the green interior.

Both copper and gallium are depleted at this green boundary, whereas Indium is enriched. This is an agreement with abio density functional theory calculations. Furthermore, the sodium has the highest concentration at this boundary, 1.7 atomic percent followed by the oxygen and potassium with a concentration of 0.4 atomic percent and 0.035 atomic percent respectively Once muscle.

The site specific sample preparation can be done in 20 hours for six samples if it is performed properly. The application of this technique requires experience with four different techniques focused INB milling, transmission electron microscopy, electron back scattering diffraction, and of course apro tomography Following this method. Other technique such as luminance, can be performed in order to answer additional questions like recombination, activity of the charge chorea at the selected CHS grand boundaries After its development.

This technique paved the way for researchers in the field of material science to explore and understand physical phenomena at the nanoscale. After watching this video, you may have a good understanding of how to prepare a site-specific sample for the atom probe tomography technique, especially when internal interfaces, such as grand boundary, need to be analyzed.