Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope

The optical, electrical, and structural properties of dislocations and of grain boundaries in semiconductor materials can be determined by experiments performed in a scanning electron microscope. Electron microscopy has been used to investigate cathodoluminescence, electron beam induced current, and diffraction of backscattered electrons.

Transcript

The overall goal of the methods presented here is to determine optical, electrical, and structural properties of extended defects, such as dislocations or grain boundaries in semiconductor materials using the scanning electron microscope. These methods can help on the key questions in the semiconductor field because extended defects have a strong influence on the performance of microelectronic devices and of solar-cell materials. The advantage of the use of the scanning electron microscope is that different physical properties of extended defects can be studied on one sample from room temperature down to very low temperatures.

The cathodoluminescence giving insight into the optical properties of extended defects in semiconductors can also be applied to studying materials that are only slightly luminescent such as minerals. In jury drills, which are new to electron backscatter diffraction for strain analysis may struggle because of issues regarding the fraction pattern quality and electron beam stability. To begin, mount the 60 degree pre-tilted sample holder onto a metallic socket.

Then, put a 0.5 millimeter thick piece of indium foil on the sample holder and place the clean sample on top. Next, place the socket onto a heating plate. Switch on the heating plate and heat up the socket to 150 degrees Celsius in order to make the indium foil ductile.

Once heated, press on the sample with a wooden toothpick for one second in order to fix the sample onto the indium foil. Then, switch off the heating plate and cool the system for about 30 minutes. First, move the light-collecting elliptical mirror from the parking position to the measuring position in the scanning electron microscope, or SEM.

Then, mount a test sample with a direct band gap transition onto the stage. Evacuate the chamber until the column chamber valve opens. During this time, set the imaging parameters as described in the accompanying text protocol.

Use the everhart-thornley detector for imaging with secondary electrons. Next, move the stage towards the pole piece until the electron beam can be focused on the sample surface at a working distance of 15 millimeters. Then, switch on the high-voltage power supply for the photomultiplier tube and the laptop with the cathodoluminescence control program.

In the cathodoluminescence control program, choose the measurement of the photomultiplier tube signal versus time and set the contrast to maximum and the brightness to 46%Next, adjust the light collecting mirror to maximize the integral cathodoluminescence intensity on the test sample by tilting and rotating the mirror. Record a test spectrum using the cathodoluminescence control program. Once set up, vent the sample chamber, remove the test sample, and mount the actual sample on indium foil onto the sample holder.

In addition, evacuate the SEM chamber and make the cryo-attachments to the SEM system as outlined in the accompanying text protocol. Additionally, insert the tubing for the liquid helium into the liquid helium dewar and connect the outlet of the helium transfer tube with the inlet for cryogenic gases of the cryo-stage. Next, set the electron beam parameters as shown here.

Then, move the stage towards the pole piece and use the everhart-thornley detector to focus the electron beam onto the sample surface at a working distance of 15 millimeters. Choose the area of interest on the sample surface and continuously scan this region during the entire cooling-down procedure. To start the cooldown procedure enter the lowest target temperature and the appropriate parameters for PID control into the temperature controller according to the technical manual.

Then, open the valve of the liquid helium transfer tube. Carefully monitor the temperature and pressure during the cooling-down procedure. After reaching the target temperature, re-establish the working distance of 15 millimeters for focused images.

In addition, correct the adjustment of the light-collecting mirror to achieve maximum integral cathodoluminescence intensity on the actual sample. Next, set the appropriate values for the grading and the spectral region. Also, set the step width to 5 nanometers, the time per measuring point to 5 seconds, and the slit width of 2 millimeters.

Record the cathodoluminescence spectra of the sample using the control software and save the files for later analysis. Next, choose the planar mirror in the monochromator for pancromatic cathodoluminescence imaging and a blaze grading at a particular wavelength for monochromatic cathodoluminescence imaging. Then, adjust the brightness and contrast values in a small window of the image, into the linear range of the dependence of the image grey values from the photomultiplier tube signal.

Finally, for a magnification between 201, 000 set the scan speed to the lowest speed of 14 combined with pixel averaging, or a higher speed of eight, combined with line average over 20 lines. Record the resulting images and save them for later analysis as an example for the comparison of the local distribution of the luminescence of the different D lines, shown here for D1 and D4.For cross correllation electron backscatter diffraction, mount the sample on a sample holder with the sample surface parallel to the holder. Then, insert the sample and evacuate the SEM chamber until the column chamber valve opens.

Using the imaging parameters shown here, focus the electron beam on the sample surface at a working distance of about 25 millimeters. Then, tilt the sample through 69 degrees around the X axis and set up a working distance of 18 millimeters. Next, switch the electron beam acceleration voltage and close the column chamber valve.

Then, turn on the power supply for the electron backscatter diffraction detector and move the detector from its parking position to its measuring position. Refocus the electron beam on a region of interest on the sample surface and then open the electron backscatter diffraction software and load the calibration file for the chosen geometry. Perform a background acquisition at low magnification while rotating the single crystalline sample.

Set up the measurement in the control software according to the operating manual. Then, read out the position of the pattern center and the detector distance for the chosen working distance from the control software. Following beam stabilization and a final refocusing of the electron beam, schedule line scans parallel to the tilt axis in the region of interest.

Using beam mappings with indexing disabled in order to speed up the measurements. Be sure to select save all images. Run the line scans until the last scan is finished providing slightly different diffraction images due to internal strains.

Then, switch off the electron beam acceleration voltage and close the column chamber valve. Finally, retract the electron backscatter difraction detector from its measuring position to its park position. Tilt the stage back to 0 degrees, vent the chamber, and remove the sample.

The image shown here is an example of the appropriate positioning of a silicon crystal on the indium foil. This guarantees good thermal contact to the cryosample holder in which the temperature is measured by the thermocouple. The cathodoluminescence spectra of a silicon single crystal at 4 Kelvin is shown with the sample in the virgin state, after plastic deformation, and after an additional annealing.

The characteristic peaks in the spectra are labelled with B-B for a band to band transition, and D1 to D4 for dislocation induced luminescence bands. In contrast, this image by backscattered electrons shows a silicon wafer with a track of recrystallized material, which appeared following treatment by a high energy electron beam. The differences in the cathodoluminescence spectra, measured at point one, two, and three, are caused by extended defects induced during recrystallization.

The three normal and the three shear strain components of the local strain tenser along the line scan which is in front of the recrystallization track were calculated from the cross-correlation electron backscatter diffraction investigations. After watching this video, you should have a good understanding how to perform cathodoluminescence investigations and cross-correlation electron backscatter diffraction on crystalline semi-conductor materials. After its development, the cross-correlation electron backscatter diffraction technique paved its way for repertrus to analyze very small strain in homogeneties and lattice rotations in crystalline materials.

Don't forget that working with cryogenic agents like liquid helium and liquid nitrogen can be extremely hazardous. And precautions such as wearing protective glasses and protective gloves should always be taken while performing these steps.

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Journal (Engineering)

Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope

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