Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

The aim of the following experiment is to tune morphology and structure of aluminum doped zinc oxide grown by pulse laser deposition for photovoltaic applications. This is achieved by ablating, a zinc oxide, aluminum oxide target by nanosecond laser pulses to produce a plume of atomic species ejected from the target as a second step. The background gas pressure during ablation is controlled to tune the deposition process.

Next, compact and Lumina or nano porous hierarchical aluminum dope zinc oxide films are grown depending on the gas pressure results are obtained that show high transparency for compact structures and highlight scattering for hierarchical ones. The main advantage of this technique is in the versatility, in the realization of different structure morphologies from compact films to nano pasal forest like systems. And this procedure can also be applied to a variety of metals and oxides.

This method can help to produce novel materials of potential interest in photo vol tanks, such as transparent electrodes with strong light scattering capability. Visual demonstration of this method is critical as the deposition steps are difficult to learn due to different tissues related with vacuum technology. And laser technology Begin by preparing the substrates onto which deposition will take place.

Cut several one centimeter by one centimeter squares each from a silicon wafer from one millimeter thick soda lime glass, and from a polymer sample. Here et fe. Clean the substrates by sonicate them an isopropanol for five to 10 minutes.

Rinse an isopropanol and dry using a nitrogen flow as a first step. Warm the neodymium YAG laser. Use a fourth harmonic generator composed of two second harmonic generators in cascade to produce light at 266 nanometer wavelength.

Next mount a two inch diameter circular target consisting of 2%by weight, aluminum oxide in zinc oxide on the target manipulator of the deposition apparatus. Point the laser at the center of the target with an incidence angle of 40 degrees to 45 degrees. Start the rotation and translation of the target.

This is always done to ensure uniform ablation. Set the vertical range of the motion so that the laser spot does not touch the external steel ring used to hold the target. Now select the laser repetition rate and pulse energy.

In this demonstration, there are 10 hertz and 75 millijoules respectively. Block the beam and monitor the laser stability with a power meter. For the next step, attach a piece of light sensitive paper to the target.

To measure the spot size, move the focusing lens to a distance from the target that yields the laser fluence of about one joule per square centimeter. Test this by firing one to five laser shots onto the paper. At each candidate lens position, remove the paper.

When done first mount a two inch diameter circular paper sheet in the substrate holder for alignment tests. Move the substrate holder to a target to substrate distance of 50 millimeters. Start pumping down the chamber until the vacuum level reaches one hundreds of a pascal.

Once the vacuum level is established, select a gas type and adjust. Its pumping speed and gas flow to have the proper gas pressure. Oxygen at 160 pascals is used in this experiment.

Adjust the XY position of the substrate holder of axis with respect to the plume center. This ensures a uniform film thickness over a circular corona when the sample holder is rotated. Next, begin ablation by removing the power meter and starting target motion.

This pre ablation may be necessary to clean the target for later determination of plasma plume length. Begin taking pictures of the plasma with an accumulation time of one half to one second when a deposit can be seen on the paper from a view port. Stop the ablation with the beam stopper to calibrate the film thickness.

Move the empty substrate holder out of the way to at least 100 millimeters from the target. Next, move a quartz micro balance to the experiment's target to substrate position of 50 millimeters from the target. Begin ablation and deliver 1000 laser shots to the target.

For this experiment, it will take about 100 seconds. Use the quartz micro balance output to determine the deposited mass on the surface. Then move the quartz micro balance away with the system back at atmospheric pressure.

Mount a silicone substrate in the sample holder, pumped down the system as before, and introduced the desired gas type at the desired pressure. Now depositor test sample generated from about 18, 000 laser shots. Once done, vent the system to retrieve the sample.

Use cross-sectional scanning electron microscopy images to calibrate the deposition rate to deposit compact A ZO films mount one of the prepared substrates. Once the substrate is properly aligned, move the substrate holder to a target to substrate distance of 50 millimeters. Again, pump the system down to 100th of a pascal.

When that pressure is reached, start the substrate rotation switch on the ion gun, set the ion energy to 100 electron volts and the radio frequency power to 75 to 100 watts. Start the Argonne gas at a flux to 70 cubic centimeters per minute. Clean the substrate with the argonne gun for five to 10 minutes.

After the cleaning, close the gas inlet and pump down the chamber to remove the argon. When the pressure has returned to 100th of a pascal, introduce oxygen gas. Adjust the pumping speed and flux to have oxygen pressure at two pascal.

Start the ablation and deliver 18, 000 shots with the laser during ablation. Check that the plume length is the same as was found previously at the same pressure. Finally stop the ablation and close the gas inlet.

Pump down the chamber and then vent it to remove the samples. The procedure for hierarchically structured films is very similar. Mount and position a substrate as before.

Pump the system to 100th of a pascal and start the targets and substrates motion. At this point, introduce oxygen gas and adjust its pumping speed and gas flow. To have a pressure of 160 pascals, begin the ablation and deliver 18, 000 laser shots.

Verify that the plume length is consistent at the end of ablation, close the gas inlet and pump down the chamber. Again, vent the chamber to remove the sample. The pulse laser deposition of aluminum zinc oxide in an oxygen atmosphere is affected by the oxygen pressure at pressures below 10 Pascals.

A compact transparent film is produced as seen here on the left for an oxygen pressure of two pascals above 10 pascals. The deposition process yields a meso porous forest like structure shown on the right for an oxygen pressure of 160 pascals. The light transmission and electrical properties of the aluminum sync oxide films vary with the oxygen background pressure of the deposition environment.

In the top plot, the optical transmittance is seen to increase with increasing background pressure, eventually 90%This plot also shows that 400 to 700 nanometer haze factor. The ratio of scattered to transmitted photons increases the oxygen background pressure as well. The electrical resistivity also varies with the background pressure, mainly due to the different connectivity of the films produced as seen in the plot.

The compact regular films created below 10 Pascal pressure show a low resistivity. The irregular forest like films have a resistivity that increases with background pressure reaching 10 to the six ohm centimeters at 100 pascals Once mastered. The deposition of one films can be done in about one hour, one hour and depth, depending on the thickness.

While attempting this procedure, it's important to control all the deposition parameters during film growth with particular attention to the visible plume length. After watching this video, you should have a good understanding of how to tune the morphology of post laser deposited a ZO films, in particular, the proper settings of the deposition parameters for the deposition of compact and nano powerous films. Don't forget that working with high energy laser can be extremely ardo and precautions such as protected goggles should always be taken while performing this procedure.