Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model

The overall goal of these experiments is to create high-quality vanadium dioxide films by atomic layer deposition and characterize the optical properties through the metal-insulator transition to produce a model describing vanadium dioxide as a tunable refractive index material. This method can help answer key questions in the fields of atomic layer deposition and phase change materials such as ways to promote different stoichiometries of transition metal oxides. The main advantage of this technique is that it enables fabrication of heterogeneously integrated phase change materials that are highly conformal and uniform in composition and thickness across large areas.

Generally individuals new to this method struggle because determining the narrow experimental parameter space for each step is key to achieving correct film properties. First, sonicate a double-sided-polished, c-plane sapphire substrate in acetone at 40 degrees Celsius for five minutes. Transfer the substrate to isopropyl alcohol heated to 40 degrees Celsius and sonicate for another five minutes.

Rinse the substrate in flowing, deionized water for two minutes and dry the substrate with a stream of nitrogen gas. Store the clean, dry substrate in a wafer container. Next, confirm that the atomic layer deposition reactor chamber is at 150 degrees Celsius.

Vent the reactor with ultra-high purity nitrogen gas. Once the reactor is ready, load the substrate into the reactor, close the reactor, and pump down the reactor to less than 17 pascals or 0.128 torr. Wait at least 300 seconds to allow the substrate to reach 150 degrees Celsius.

Then, begin flowing UHP nitrogen gas into the chamber at 20 sccm, ensuring that the base pressure does not exceed 36 pascal or 0.270 torr. Pulse ozone for 15 saturated cycles, where each cycle is a 0.5-second pulse following by a 15-second purge. Then, to grow amorphous vanadium dioxide, pulse TEMAV for 0.03 seconds, purge for 30 seconds, pulse ozone for 0.075 seconds, and purge for 30 seconds.

Repeat the pulse and purge cycle until the film has reached the desired thickness. Afterwards vent the reactor chamber with UHP nitrogen gas. Transfer the sample from the reactor to a metal plane to cool.

Close and evacuate the reactor. Ensure that the sample sled is in the load lock of an ultra-high vacuum annealing chamber. Vent and open the load lock.

Place the vanadium dioxide thin film sample on the sled and close the load lock. Use the roughing pump to reduce the load lock pressure to about 0.1 pascal. Switch to the turbo pump and reduce the load lock pressure to less than 10 to the minus fourth pascal.

Open the gate valve and transfer the sled to the annealing chamber. Pump down the annealing chamber to below 10 to the minus fifth pascals, then flow UHP oxygen gas into the annealing chamber at 1.5 sccm. Heat the sled to 560 degrees Celsius at 20 degrees Celsius per minute.

Hold the sample at 560 degrees Celsius for one to three hours, depending on the film thickness. Afterwards turn off the heater and move the sled back to the load lock to quench the sample. Keep the sample in the oxygen environment overnight or until the sample temperature is below 150 degrees Celsius.

Then turn off the flow of oxygen and close the gate valve. Vent the load lock with UHP nitrogen gas. Once the sample temperature is below 50 degrees Celsius, transfer the sample from the load lock to a metal plate to cool to room temperature.

Close and pump down the load lock when finished. Place a vanadium dioxide thin film sample on the sample stage of a Raman microscope with a 532-nanometer laser excitation source. Focus the microscope on the sample.

In the instrument software, set the laser power to four milliwatts, the exposure time to 0.125 seconds, the number of scans to 10, and the preview size to 40 micrometers. Click Live Spectrum to observe the spectrum. Optimize the microscope focus, laser power, exposure time, and number of scans for maximum signal-to-noise ratio.

Save the spectrum when the optimal image is obtained. Evaluate the peaks to determine the crystallinity, phase, and strain of the film. Load a vanadium dioxide thin film sample into an XPS sample holder and vent the instrument load lock.

Insert the sample holder into the load lock and pump down the load lock to below four times 10 to the minus fifth pascals or three times 10 to the negative seventh torrs. Transfer the sample holder into the main chamber and verify that the pressure is below seven times 10 to the negative sixth pascal or 5.25 times 10 to the negative eighth torr. Create or load an experiment sequence.

Start the X-ray gun with a 400-micrometer spot size and turn on the flood gun. Define a point for a survey measurement and points for high-resolution scans of carbon, nitrogen, vanadium, and oxygen. Set the survey scan pass energy and number of scans to 200 electron-volts and two respectively.

Set the high-resolution scan pass energy and number of scans to 20 electron-volts and 15 respectively. Place point measurement crosshairs at the desired locations on the sample. Then run the experiment.

Once data collection has finished, use the Survey ID tool to identify and analyze the elements in the film. Evaluate the peak locations and integrated intensities in the high-resolution scans to analyze the bonding and stoichiometry of the film. Unload the samples by standard procedures when finished.

Load a vanadium dioxide thin film sample into an AFM set to tapping mode and move the sample under the AFM's scan head. Select Tip Reflection and lower the scan head to the sample surface until the reflection on the tip of the surface is in focus. Then click the Sample button to change the focus to the sample.

Close the AFM hood and check the experiment parameters. Ensure that the scan size is set to less than one micrometer, the scan speed is at 3.92 hertz, and that the number of samples per line is set to 512. Engage the parameters and wait 20 seconds.

Then set the scan size to three micrometers, adjust the drive amplitude, the amplitude set point, and the integral and proportional gains as needed to optimize the AFM image. Once the image is of the desired quality, click Frame Down to restart the scan at the top of the frame and click Capture to capture the new image. Withdraw the sample head when the scan is finished.

Open the AFM image in analysis software and evaluate the morphology, surface roughness, depth histogram, and mean grain size. Afterwards unload the sample by standard procedures. XPS of an as-deposited amorphous vanadium dioxide film showed that the surface was primarily composed of vanadium oxide while the bulk was the expected vanadium oxide form.

Annealing the amorphous film in a low-pressure oxygen environment resulted in the surface stabilizing as vanadium dioxide. The overall zero-two-zero orientation aligned with the peak of the sapphire substrate. Narrow peaks were observed by Raman spectroscopy, indicating high crystalline quality.

The differences in peak energy between the as-grown and annealed vanadium dioxide suggested the introduction of tensile strain in the crystalline filaments. AFM showed that the as-grown and annealed films both had crystal grain sizes on the order of 20 to 40 nanometers. The Root Mean Square roughness increased slightly from 1.4 nanometers for the as-grown film to 2.6 nanometers for the annealed film.

Transmittance and reflectance data collected and absorptance data calculated from vanadium oxide in its insulating and metallic phases were used to design an oscillator model for the temperature-and wavelength-dependent dielectric permittivity and refractive index of vanadium oxide. The optimized model accurately predicted the optical behavior of vanadium oxide as it transitioned from insulator to metal. After its development, these techniques paved the way for researchers in the field of thin film growth to explore fabrication of optical devices with a tunable refractive index.