Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing

The method presented here allows for controlling the amount of oxygen vacancies in oxide thin films both during and after the deposition. The main advances of this approach is that electrical and magnetic properties can be tuned by modifying the amount of oxygen vacancies. Oxygen vacancies serve as functional defects in most oxide materials, and the properties of many oxides can therefore be controlled systematically by defect engineering using this approach.

Demonstrating the procedure will be Shinhee, Carlos, and Eric, a post-doc and two PhD students from our laboratory. To begin, purchase mixed-terminated strontium titanate substrates with a typical surface angle of 0.05 to 0.2 degrees with respect to the crystal planes. Clean the desired number of substrates by ultrasonication in acetone for five minutes.

Then ultrasonicate the substrates for 20 minutes at 70 degrees Celsius in clean water, which dissolves strontium oxide or forms strontium hydroxide complexes at surface domains terminated with strontium oxide, while leaving the chemically stable, titanium dioxide terminated domains unchanged. In the meantime, prepare aqua regia solution by slowly adding hydrochloric acid into water and then adding nitric acid into the solution. Next, ultrasonicate the substrates in an acidic solution containing hydrochloric acid, nitric acid, and water at 70 degrees Celsius for 20 minutes in a fume hood to selectively etch strontium oxide due to the basic nature of strontium oxide surface domains, the acidity of titanium dioxide, and the presence of the strontium hydroxide complexes.

Remove the residual acid from the substrates by ultrasonication in 100 milliliters of clean water for five minutes at room temperature in a fume hood. Then bake the substrates in an atmosphere of one bar of oxygen for one hour at 1, 000 degrees Celsius with a heating and cooling rate of 100 degrees Celsius per hour in a ceramic tube furnace to relax the substrate surface into a state with low energy. To deposit a thin film on the substrates, mount the substrates on a heater or chip carrier, depending on whether NC2 transport measurements are to be performed during the deposition.

Next, place the titanium dioxide terminated substrate 4.7 centimeters from the single crystalline alumina target for a typical deposition of gamma alumina on strontium titanate at room temperature. Prepare for ablating from a single crystalline alumina target in an oxygen pressure of 10 to the power minus five millibar. Tune the properties using the oxygen content by using an oxygen deposition pressure in the range of 10 to the power minus six to 0.1 millibars, or by varying other deposition parameters.

After incubation, observe the substrate for the desired thickness of gamma alumina deposition. Next, remove the sample from the deposition chamber, and stop any electrical measurements. Then store the sample in a vacuum.

The sample degradation is slowest when stored in a vacuum or nitrogen. Mount the sample on a chip carrier using silver paste. Then electrically connect the sample to the chip carrier using wedge wire bonding of aluminum wires in the Van der Pauw geometry.

Next, place the chip carrier with the sample in a closed furnace. Then using a connector and wires with thermally resistant insulation, electrically connect the chip carrier to the measurement equipment, and start the sheet resistance measurements. Then place the chip carrier equipped with the sample in a closed furnace and flush thoroughly with the gas used for the annealing while checking whether the sample resistance is sensitive to a change in the atmosphere.

Anneal the sample using the desired annealing profile, depending on the thickness of the top film and the desired rate of oxygen incorporation. Abort the annealing when a desired change in the sheet resistance has occurred. Using this setup, the development of the sheet resistance in oxide heterostructures such as gamma alumina strontium titanate and lanthanum aluminate strontium titanate can be monitored in-situ during pulsed laser deposition.

When the measurement environment is changed by measuring ex-situ or via in-situ oxygen flushing, significant changes in the sheet resistance of the strontium titanate based heterostructures can be observed. In samples where gamma alumina is deposited on strontium titanate, the electron mobility stays largely unchanged at room temperature, but changes dramatically at two Kelvin when the deposition pressure is varied. The properties of oxide heterostructures can also be tuned after deposition using annealing.

The final state is determined by the annealing time and annealing temperature and atmosphere. The sheet conductants of heterostructures composed of strontium titanate capped with gamma alumina or amorphous lanthanum aluminate are measured at various annealing temperatures. The fastest decrease in the conductance was observed for the amorphous lanthanum aluminate strontium titanate heterostructures.

For the strontium titanate heterostructures, the carrier density is controlled by controlling the annealing and oxygen. Consecutive annealing steps result in a steady decrease of carrier density and a transition from a metallic conducting to an insulating interface. Changing the conducting state in the strontium titanate heterostructure can enable different properties.

Here, attempts to write nanowires using a conducting atomic force microscope were not possible before annealing. However, after annealing, conducting lines can be written and erased at the interface. Using this approach, we can systematically change the magnetic and electronic properties of oxide heterostructures, and in this way, study the role of oxygen vacancies in determining these properties.