U₂O₅ Film Preparation via UO₂ Deposition by Direct Current Sputtering and Successive Oxidation and Reduction with Atomic Oxygen and Atomic Hydrogen

So, for the first time, we have been able to make thin films of U2O5. This material is very difficult to make because it sits right in between two very stable uranium oxides:Uo2, Uo3. And, this is why previous attempts to make it failed.

In this laboratory, we succeeded in making it, as thin films of about 200 monolayers thickness. This is very little, but enough to study its physical chemical properties, and, in particular, the interaction of the surface with the environment. The films are deposited from very small amounts of starting materials, and under highly controlled conditions.

In this way, the surface composition can be adjusted, and the results controlled, by high resolution X-ray for the electron spectroscopy. In addition to oxides, nitrides, carbides, and other compounds can be prepared and studied in C2.The experiments are performed using the lab station at the Joint Research Center Carlsruhe. The lab station allows preparation of films and analysis of sample surfaces in C2, without exposure to the atmosphere.

A sense of the lab station is provided by this schematic. There are chambers for loading a sample, and for storing it, labeled C1"to C3"When loaded, the sample is on a sample holder, and placed on a transport wagon that can move on a linear transport chamber. The wagon and sample can be transferred between different preparation chambers, labeled B1"to B3"and analysis chambers, labeled A1"to A4"Transfer rods at each chamber allow conveyance of the sample holder to and from the linear transfer chamber.

The entire system is kept under dynamic ultra-high vacuum. Control of the system is through computers set up in the laboratory. Take a clean gold-foil substrate, prepared for deposition of Uo2, to the loading chamber.

The gold substrate is spot welded to a stainless steel sample holder, with tantalum wire. Start loading by isolating the loading chamber, then open the nitrogen valve and wait until the chamber reaches atmospheric pressure. When ready, open the chamber door, and move the sample carriage into position for loading.

Next, place the sample holder and sample onto the carriage. Return the carriage to the loading chamber, and close the chamber door. Open the valve for the primary vacuum.

When the pressure is about 1 millibar, close the valve. Then, open the valve to the ultra-high vacuum pump. Using the loading chamber's transfer rod, manipulate the sample holder, and move it to the wagon in the intermediate chamber.

Return the transfer rod to the loading chamber, and close the valve between the loading and intermediate chambers. Open the valve between the intermediate and linear transfer chamber. Position the wagon in the transfer chamber, and connect it to the driving magnet before closing the valve.

Return to the linear transfer control program. To move it from its current position, select the conditioning chamber as the wagon's destination. Then, press start to begin the motion.

The wagon arrives at the base of the conditioning chamber from where it will be loaded. Open the valve from the conditioning chamber and use the transfer rod to move the sample holder inside. With the chamber isolated, orient the sample holder surface to face the ion gun before cleaning using argon sputtering for ten minutes.

When done, bring the thermocouple in contact with the sample holder, and anneal the sample for five minutes. After the sample has cooled, return the sample holder to the transfer chamber, before closing the conditioning chamber valve. Use the control software to move the wagon to the next chamber for a high-resolution X-ray photo-electron spectroscopy.

Transfer the sample into the chamber, and make preparations for the measurement. With the sample in the chamber, turn to the acquisition software to position it for measurement. When the sample is ready, use the acquisition software to obtain an overview spectrum to check the surface.

The absence of a Carbon-1 S peak at about 285 electron volts is a sign that the surface is clean. Next, acquire a gold-4 F core-level spectrum for later use. It will be compared to the spectrum of Uo2 on gold to determine the film thickness.

After the analysis, transfer the sample back to the linear transfer chamber using the control rod. Return to the linear transfer control software, and transfer the wagon with the sample holder to the DC sputtering chamber. Bring the sample holder under the sputterer source in the middle of the chamber.

With the sputter source shutter closed, open the oxygen valve and adjust the oxygen partial pressure. Next, open the argon gas valve until its target partial pressure is reached. Go to the sputtering program to set the parameters for the process.

Then, open the shutter of the sputter source and sputter for 300 seconds. Stop the sputtering, and close the argon and oxygen gas valves. Proceed to transfer the sample holder back to the linear transfer chamber.

With the linear transfer control software, move the sample back to the conditioning chamber. Isolate the sample there, and switch on the E-beam heater to set to a temperature of 573 Kelvin to anneal the sample. After setting the sample up for analysis, measure an overview spectrum.

Move the sample back to the chamber used for high resolution X-ray photo electron spectroscopy. This spectrum enables monitoring the quality of the Uo2 film. Next, acquire a gold-4 F core level spectrum.

Proceed to acquire a uranium-4 F spectrum. In addition, acquire an oxygen 1-second spectrum by changing these parameters to the stated values. Obtain a valence band spectrum by using these parameter values in the software.

From the linear track, move the sample to the atomic source chamber, which can be used both for oxidation and reduction by activating oxygen and hydrogen. Once it is there, isolate the sample in position, and heat it at 573 Kelvin for 5 minutes. After waiting, open the oxygen valve and set the partial pressure.

Turn the atom source on, and set the current to 30 milli-amps. Wait 20 minutes to achieve complete oxidation before switching off the source and closing the oxygen valve. Return the sample via the linear translation chamber to the X-ray photoelectron spectroscopy chamber.

Once the sample is isolated in the XPS chamber, acquire the uranium-4 F oxygen-1 S and valence band spectra as before. If the reduction time is too short, the spectra will have characteristics of incomplete oxidation. In particular, note the peak structure in the uranium-4 F and valence band spectra.

Begin with the sample back and isolated in the atomic source chamber. Open the hydrogen valve, and set the partial pressure. Start the atomic source, and switch it on and set the current to 30 milli-amps.

After 60 seconds of reduction time, switch off the atomic source. For the final steps, return the sample to the X-ray photoemission spectroscopy chamber. Analyze the sample, and characterize the reduction by acquiring the uranium-4 F, oxygen-1 S, and valence band spectra.

As with these plots, the spectra will reveal if the reduction time is too long, and U2O5 has reduced to Uo2. These are uranium 4-F core level X-ray photoemission spectra for uranium-4 and Uo2, uranium-5 and U2O5, and uranium 6 and Uo3. The spectrum for uranium metal is for comparison.

The data are from films of about 20 monolayers. The energy of the uranium-5 satellite allows the oxidation state to be easily identified. In this plot, the spectra have their uranium-4 F five-halves mainline peaks shifted to coincide.

The relative position of the satellite with respect to the peak is different for each oxidation state. This different provides another identifier for uranium oxidation states. This analysis is only possible with high resolution spectroscopy, due to the satellite peak's low intensity, and small binding energy difference from the main peak.

Producing U2O5 thin films is possible, but stopping the reduction process at its exact composition can be challenging, and even difficult to observe without high resolution spectroscopy. So, this is a new compound, and there will be plenty of properties to investigate. We will start using grazing angle X-ray diffraction to investigate the structural properties.

And then, we will move onto the status of magnetic properties of this compound, some of the electrical transport properties, and we will compliment the electronic structure investigations using techniques available, like synchrotron light sources, like an elastic X-Ray scattering. Physical chemical properties of this rather unusual oxidation states are investigated. The experimental data can be compared to theoretical predictions.

By this way, our experiment serves as a benchmark for theoretical models.