The overall goal of this procedure is to construct spatially confined complex oxide thin films for electron transport measurements. This is accomplished by first growing single crystal menonite thin films using pulse laser deposition. The second step is to use photo lithographic techniques to etch as grown films to desired size and geometry.
Next, prepare the devices for transport measurements by creating low resistance for probe contacts and attaching to the measurement system. The final step is to do resistivity measurements from 300 to five kelvin using various applied magnetic fields. Ultimately, the lithographically confined devices are used to show that transport can be dominated by isolating one or a few electronic domains, which forces the probing electrons to interact with multiple regions, as opposed a simple path of least resistance and the unconfined structures.
The main advantage of this technique over existing methods like electron beam lithography and folks are on beam meeting, is that a sample can be acted quickly and without ionic implantation damage. This method can help answer key questions such as how electronic phase competition and electronic correlation lengths drive emergent phenomenon strongly correlated materials. To begin clean a five millimeter by five millimeter by 0.5 millimeter strontium titanate single crystal substrate with a miscut angle less than 0.1 degrees with acetone and then water in an ultrasonic cleaner for 10 minutes.
Next, form a titanium dioxide termination on the strontium titanate by etching the substrate in 10%hydrogen fluoride for 30 seconds. Then rinse the substrate in water for one minute and an kneel at 1100 degrees Celsius for 10 hours. After a kneeling is completed and the substrate is cooled down to room temperature mounted on a heater suitable for ultra high vacuum conditions and place it in a vacuum chamber.
Next, open the chamber oxygen source to fill the chamber with one times 10 to the minus four tor oxygen. Raise the oven temperature to 800 degrees Celsius and allow the substrate to anil for 20 minutes. Monitor the temperature using a computer control parameter or a thermocouple couple.
To begin film deposition. Start an exer pulses laser using a laser fluence of one to two joules per square centimeter and a laser frequency of one or two hertz. The laser pulses will hit the target material and generate a plume flux.
The flux will penetrate through the oxygen environment and deposit onto the substrate. During film deposition. Use reflection high energy electron diffraction as described by HME and others to monitor unit cell growth and confirm surface quality.
This technique allows for very clear thickness monitoring when the film is of the desired thickness, turn off the laser and decrease the heater temperature at a rate of five degrees Celsius per per minute. Once the heater is cool to room temperature, turn off the oxygen source and remove the sample. Next, perform XC two A kneeling to remove oxygen deficiencies that may have built up.
So accomplish this. Place the sample in a tube furnace under one atmosphere flowing oxygen. Then raise the temperature from 20 degrees Celsius to 700 degrees Celsius at five degrees Celsius per minute, ane for two hours, and then decrease temperature from 700 degrees Celsius to 20 degrees Celsius at two degrees Celsius per minute.
It is important to never post a needle at higher temperatures than those used during film growth. When filling oxygen vacancies first ultrasonically clean the sample in acetone and then water for 10 minutes each. Use an optical microscope to check that the sample surface is clean of large particulates.
Next spinco to one micron thick layer of AZ 180 1. Three foot resist by spinning a single drop of it at 6, 000 RPM for 80 seconds, and then place a sample on a heat plate at 115 degrees Celsius for two minutes to cure the photoresist. Once the photoresist is cooled, check the quality of the coating under an optical microscope.
If the coating is uniform and has no bubbling, place a sample into a MAs aligner and expose it under a predefined lithography mask using UV light for nine seconds with an exposure dose of 90 millijoules per square centimeter, then heat the photo resistance sample on a heat plate at 115 degrees Celsius for 80 seconds. To further cure the exposed photo, resist next, transfer the sample into the developer solution for 25 to 35 seconds, and then rinse it in water for 30 seconds. The part of photo resist not covered by the mask is washed away while the part which was covered remains.
Then use plastic tweezers to rinse the sample in an etching solution for approximately 10 seconds. The unprotected part of the thin film is etched away during this step. After 10 seconds, immediately rinse the sample in pure water for 60 seconds.
Then use an optical microscope to see if the thin film has been properly etched. If not, etch for two to three more seconds and immediately rinse again with pure water. Once proper etching has been achieved, rinse the sample in acetone for 20 seconds.
To remove the remaining photoresist, check the quality of sample again with an optical microscope. Using an appropriate photo mask evaporate five nanometers of titanium and 100 nanometers of gold onto the sample. Using an electron beam evaporator at background pressure of one times 10 to the minus five, use an evaporation rate of two angstroms per second for titanium and 15 angstroms per second for gold.
Next, remove the photoresist by rinsing the sample and acetone for one minute. Then view the pads under a microscope to confirm their quality rinsing. An acetone will leave only the desired contact pad geometry.
Use GE varnish to mount the sample onto the sample puck. Allow 15 minutes for the sample to cure, then mount the puck on the wire bonder stage. Finally, use the wire bonder to connect the aluminum wires to the sample.
Now you are ready to perform electrical measurements. During this procedure, films can be easily over etched to destroying the sample in just a few seconds. On the left is a properly etched sample etched for 15 seconds.
At 21 seconds, the desired structure is damaged and at 25 seconds is mostly removed. The process shown here will produce a sample with electrical contacts for four probe transport. This makes creating wire connections from the resulting sample to items such as resistivity puck, easy to accomplish.
The development of this technique has allowed researchers have strongly correlated materials to explore electronic phase separation. After watching this video, we should have a good understanding of how to confine complex oxide films for investigation of the competing electronic faces residing within the.
