Columnar zinc oxide structures in the form of rods are synthesized via aerosol-assisted chemical vapor deposition without the use of pre-deposited catalyst-seed particles. This method is scalable and compatible with various substrates based on either silicon, quartz or polymers.
The overall goal of this procedure is to coat zinc oxide rods on silicon or silicon-based micro-machine platforms without using pre-deposited catalyst seeds as opposed to other state of the art methods that employ catalyst seeds to encourage the growth of columnar metal oxide structures. The metal presented here allows for aerosol-assisted chemical vapor deposition of single side rods via a known catalyzer vapor solid mechanist without the use of pre-deposited catalyst seeds. This metal disadvantages as it uses lower processing temperatures than previously reported chemical vapor deposition metals offering stability and compatibility with the various substrates like silicon, quartz and polymers.
To begin this procedure clean a 10 by 10 millimeter silicon substrate in isopropanol and rinse with deionized water. Blow dry the substrate with nitrogen to ensure good adherence of the film and uniform covering of the substrate. Place the substrate into a homemade stainless steel cylindrical reaction cell with an internal volume of approximately 7000 millimeters cubed, adapted to the dimensions of the silicon-based micro-machined platforms employed for the fabrication of gas sensors.
Place the silicon-based micro-machined platforms into the reaction cell and align with the shadow mask to confine the growth of material to the area of interest. Close the reaction cell making sure that the lid is properly sealed to avoid the leakage of reactive species. Next turn on the temperature control system consisting of resistant heaters integrated with the reaction cell, a thermocouple to sense the temperature of the substrate and a proportional integral derivative controller.
Set the temperature to 400 degrees Celsius and allow it to stabilize for approximately 30 minutes. Add 50 milligrams of zinc chloride. And 5 milliliters of ethanol to a 100 milliliter glass vial.
Once the solution is homogenous secure the vial to a vacuum trap using a glass joint clip and ensure the downpipe end sits 60 millimeters above the bottom of the vial and is not submerged in the solution. Now clamp the vial to a universal support. Adjust the height of the support to meet the bottom of the vial and the optimal focal point of an ultrasonic atomizer that operates at 1.6 megahertz and delivers an average aerosol droplet size of approximately three micrometers.
Next connect the inlet and the exhaust of the vacuum trap to the nitrogen pipe in the reaction cell respectively. Before starting the chemical vapor deposition process verify that the temperature in the reaction cell has reached steady state. Using a mass flow controller adjust the nitrogen flow to 200 centimeters cubed per minute and allow it to flow through the system.
Turn on the aerosol generator and keep the aerosol constant during the process until the solution containing the zinc precursor is completely delivered to the reaction cell. As soon as the solution has been fully delivered to the reaction cell turn off the aerosol generator in the temperature control system to cool down the reaction cell. When the temperature has dropped to room temperature stop the nitrogen flow and open the reaction cell then remove the sample that has a grayish matte color on the surface associated with the presence of columnar zinc oxide structures.
Aerosol assisted chemical vapor deposition of zinc chloride leads to the formation of grayish uniform in adherent films on bare silicon wafers. Scanning electron microscopy images show quasi-aligned hexagonal-shaped zinc oxide rods with a length and diameter of approximately 1, 600 nanometers and 380 nanometers respectively. Large errors in the set point temperature or the presence of temperature gradients along the substrate during the chemical vapor deposition may cause the deposition of other zinc oxide morphologies or films with non-uniform structures.
X-ray diffraction analysis of the rods show diffraction patterns associated with a hexagonal zinc oxide phase. Characterization of the rods by high resolution transmission electron microscopy shows marked planer spacing consistent with the internal lattice of the 002 plane of the hexagonal zinc oxide phase identified by XRD. Energy dispersive X-ray spectroscopy shows the presence of zinc with relatively low chlorine contamination.
An array of four micro machine gas sensors based on aerosol-assisted chemical vapor deposition rods is displayed here. These micro-systems are sensitive to relatively low concentrations of carbon monoxide with a maximum responses recorded when the sensors were operated at 360 degrees Celsius using the resistive micro-heaters integrated in the system. After watching this video you should have a good understanding of how to grow single sided rods on different surfaces without using pre-deposited catalyst seeds.
Once mastered this technique can be scaled out to larger surfaces if the system and parameters are rescaled properly. Following this procedure the selected deposition of single side morphologies other than those can also be achieved by changing the deposition temperature, the precursors and the carrier solvent. This procedure is compatible with a state of the art micro-fabrication process for silicon-based electronic devices and has the potential to be incorporated in process involving high heat resistant flexible materials due to relatively low temperatures for the aerosol-assisted chemical vapor deposition of the structures.
