The basic concept of this technique is to use electron channeling and back scatter phenomena within a scanning electron microscope as a mode for diffraction based imaging of crystalline defects. Using the electron channeling pattern for guidance, the sample is oriented within the microscope to establish Bragg diffraction conditions between the electron beam and crystal planes. This results in electron channeling and thus low back scatter yield.
When in diffraction conditions, crystal defects create perturbations in the lattice planes that scatter electrons out of the channeled beam, creating localized regions of increased back scatter signal. These defect features can be effectively imaged by scanning the electron beam across the sample surface and mapping the electron back scatter signal. The resulting images are equivalent to those obtained via TEM, but are produced in only a fraction of the time, making this a powerful rapid characterization technique with applications in a wide range of crystalline materials research areas, The characterization of crystalline defects and microstructure is extremely important to just about every field of materials research.
This is especially true for the photovoltaic and optical electronic materials that our group investigates. The standard method used for this kind of analysis is TEM, but because of the often lengthy sample preparation that's needed, which includes extensive sample thinning, this often leads to routine characterization, bottlenecks, or, or slowdowns, as well as potential issues related to sample damage. The main advantage of EI is that it's performed in a scanning electron microscope or SEMS scs are typically more accessible instruments and also allow for the use of much larger samples and thus larger analysis areas.
Using this method for our epic taxal materials, we can typically use as grown samples with no other preparation needed. This has the benefit of not only ensuring sample integrity and thus analytical accuracy, but it also provides for much higher throughput, which saves time and resources. Cleve out a 50 nanometer thick sample of gallium phosphate grown on a 0 0 1 oriented silicon substrate for the instrument used here.
An FEI Syrian field emission, SEM. An approximately five millimeter by five millimeter sample size is sufficient. Next, attach the small chip to the SEM sample mount using a clip or conductive adhesive bent the SEM chamber and then insert the sample mount into the specimen holder.
Verify that the height of the sample will not interfere with the back scatter electron detector Before closing the SEM door. Next, pump down the SEM chamber and wait until the system indicates that the pressure is low enough to start the measurements. Switch to the secondary electron detector through the detector menu.
Then turn on the electron beam by clicking the associated button in the beam control area and set the accelerating voltage to 25 kilovolts. Next, set the beam current to five, which corresponds here to approximately 2.4 nano amps. Using the secondary electron detector, focus on a small particle or feature and adjust the image focus by right clicking and dragging the mouse horizontally on the software interface.
Then hold the shift key and drag vertically and horizontally to adjust the ion. Next, move the sample into the vertical working distance. By incrementally changing the Z position of the stage and adjusting the focus and is needed, adjust the Z position until a working distance of five millimeters is achieved.
Switch into back scatter electron mode through the detector's dropdown menu in the software interface. Then decrease magnification to its slowest setting, which is about 27 x here. Next, adjust the image, contrast and brightness to enhance the visibility of the electron channeling pattern.
Being careful not to oversaturate the image, adjust the sample rotation and tilt using the r and t entries in the stage control area to help make features of the channeling pattern more apparent. Depending on the sample and instrument being used, the channeling pattern can be somewhat difficult to see. Making small sample tilt and rotation adjustments which result in the relative motion of the channeling pattern can help provide something to catch the user's eye.
Enable the optic axis marker by clicking the target button, adjust the samples rotation and tilt to align the target CCI bandage with the SEM optic axis to set the specific diffraction conditions to be used in the imaging. This is the key step for achieving diffraction based contrast imaging. By aligning to the cchi band edge rather than the cocci line or band itself, a specific electron diffraction condition is established that will provide imaging contrast for features with both dark and bright contrast levels.
Once the desired diffraction condition is achieved, increase the magnification around 14, 000 X is used here. Center the image on the specific defect or feature being examined, and then adjust the image for clarity, including contrast, brightness, focus, and stagnation. Next, optimize the diffraction condition for maximum imaging.
Contrast by making small adjustments to the sample tilt orthogonally to the chosen kaci band of no more than plus or minus 0.2 degrees, adjust as needed to achieve maximum contrast. Note that moving toward the inside of the Kaci band will typically reduce the relative contrast of bright features while moving toward the outside of the band. Toward the cocci line will typically reduce the relative contrast of dark features, adjust the scan rate to a slow or average scan in order to provide an image with a low signal to noise ratio.
Shown here is an electron channeling pattern image montage from the gallium phosphate silicon sample. As indicated, the relative position of the SEM optic axis on the channeling pattern determines the contrast of any crystalline defects that are observed. In this case, the misfit dislocation network at the gallium phosphide silicon interface can be seen under a range of different diffraction conditions.
These are example electron channel. In contrast, images captured from various thicknesses of gallium phosphide grown on silicon misfit dislocations are observed starting with the 50 nanometer thick sample, indicating that the critical thickness for this sample type is somewhere between 30 nanometers and 50 nanometers. Here are two additional types of defects that are highlighted using electron channel.
In contrast imaging on the left are examples of surface penetrating, threading dislocations, and on the right is an example of a stacking fault. Once mastered electron channeling, contrast imaging can be performed in a fraction of the time as comparable. TEM work with routine micro structural characterization possible in as little as an hour.
We believe this technique has the potential to become a powerful tool for materials research, providing rapid access to vital data that is otherwise difficult to obtain.
