Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides

This procedure can help answer key questions in the functional oxide community, such as how chemical and physical disorder directly affect long-range magnetism. The main advantage of this technique is that allows for a large degree of tunability in the material chemistry and can be applied to a variety of ESO compositions. Demonstrating the procedure is Peter Meisenheimer, a graduate student in my lab.

First, grind the required oxide powders and use a die press to compress the powder into a pellet to form the target. Sinter the target in air at 1, 100 degrees Celsius for 24 hours. Then, while still at 1, 100 degrees Celsius, remove the crucible containing the target from the furnace.

Use tongs to place the target on the same heat-tolerant surface. Wait for the target to stop glowing and quickly quench the target in room-temperature water. When the target is no longer sputtering, remove the target from the water and set it aside to air-dry.

Once the target is cool and dry, calculate the target density and the percent theoretical density. If the measured density is insufficient for pulsed-laser deposition, regrind and resinter the target. Prepare additional targets in this way as desired.

Polish each target in a circular motion using progressively finer grits of silicon-carbide paper until the surface is reflective and uniform. Store the polished targets in a desiccator until ready to begin the pulsed-laser deposition procedure. Ensure that a 248-nanometer krypton-fluoride pulsed excimer laser with a pulsed width of about 20 nanoseconds is ready to use.

Place the polished targets on the rotating carousel in the deposition chamber. Place a two by two-centimeter piece of burn paper on the final target in the beam path. Pulse the laser once and measure the resulting burn mark across both axes.

Adjust the focusing lens until the beam pulse produces a 0.27 by 0.24-centimeter ellipse. Then remove the burn paper and close the chamber door. Evacuate the chamber to 0.5 torr or 67 pascals with a dry-scroll roughing pump.

Then spin up the turbo pump to 1, 000 hertz and pump down the deposition chamber to a base pressure of at least 10 to the negative seventh torr or 1.3 times 10 to the negative fifth pascal, as measured by an ionization gauge. Once that pressure is achieved, reduce the turbo pump to 200 hertz. Next, sonicate a single crystalline one-side-polished magnesium oxide substrate for two minutes each in semiconductor-grade trichloroethylene, semiconductor-grade acetone, and high purity isopropyl alcohol.

Dry the substrate with ultra-dry compressed nitrogen gas. Use a small amount of thermally-conductive silver paint to fix the substrate on a substrate platen. Heat the substrate and platen on a hot plate for 100 degrees Celsius for 10 minutes to cure the paint.

Use the external transfer tool to place the substrate platen on the transfer arm in the load lock of the PLD. Close and pump down the load lock to at least 10 to the seventh torr or 1.3 times 10 to the negative fifth pascal. Then open the gate valve between the load lock and the deposition chamber and use the transfer arm to mount the platen on the heater assembly.

Retract the transfer arm into the load lock and close the gate valve. Lower the heater to achieve a substrate target distance of seven centimeters. Next place an energy meter in the beam path just before the chamber.

Irradiate the photodiode on the meter with 50 laser pulses at a rate of two hertz and determine the mean energy. Adjust the laser excitation voltage to attain an average pulse energy of 310 millijoules, with a stability of plus or minus 10 millijoules. Remove the energy meter when finished.

Heat the substrate at 1, 000 degrees Celsius at 30 degrees Celsius per minute under vacuum. Hold the substrate at that temperature for 30 minutes to dehydroxylate the magnesium oxide crystal’s surface. Afterwards, cool the substrate to 300 degrees Celsius at 30 degrees Celsius per minute and allow the substrate to equilibrate at that temperature for 10 minutes.

Once the oxygen partial pressure is stable, set the desired target to raster and rotate and ensure that the substrate shutter is closed. Ablate the target for 2, 000 pulses at a rate of five hertz. Then open the substrate shutter, pulse the laser 10, 000 times at six hertz to deposit an approximately 80-nanometer thick entropy-stabilized oxide film on the substrate.

After deposition has finished, increase the oxygen partial pressure in the chamber to one torr or 133 pascals to inhibit the formation of oxygen vacancies. Cool the sample to 40 degrees Celsius at 10 degrees Celsius per minute. Then close the flow of oxygen gas and allow the chamber pressure to stabilize.

Open the gate valve, raise the heater, and use the transfer arm to move the substrate platen into the load lock. Close the gate valve and vent the load lock to atmospheric pressure. Remove the substrate platen using the external transfer tool.

Use a razor blade to separate the sample and the platen. Polish the platen to remove silver paint and deposited material when finished. Two theta-omega X-ray diffraction of cobalt variant and copper variant entropy-stabilized oxides as bulk ceramics showed that the synthesized samples were the rock salt structure with no secondary phases.

The deposited films were single crystalline and epitaxial to the zero-zero-one oriented magnesium oxide substrate as shown by only the zero-zero-two and zero-zero-four film peaks being observed. Laue fringes were observed around these peaks, indicating that the films were of high crystalline quality with smooth interfaces. The oscillation period was consistent with a film thickness of approximately 80 nanometers.

X-ray photoelectron spectroscopy showed that their constituent cations were in the two plus oxidation state and, if applicable, were high spin. The compositions calculated from these spectra matched the nominal compositions with less than 1%error. Energy dispersive X-ray spectroscopy maps were consistent with the nominal compositions and indicated that the films were chemically homogenous.

Atomic force microscopy showed that the thin films were flat across a five micrometer by five micrometer scan range with subunit cell root mean square roughness values. Low-angle two-theta omega XRD data agreed with these roughness numbers. The peak-to-peak roughness was approximately 3.3 angstroms for all films.

Entropy-stabilized oxides are a nascent field of study with many desirable properties. After watching this video, you should have a good understanding of how to synthesize bulk and thin film entropy-stabilized oxide materials. PLD is an ideal method for the deposition of high-quality single crystalline thin films to investigate exotic functionalities and thus is the best technique to facilitate the study of thin film entropy-stabilized oxides.