Consumer electronics such as TV’s and personal computers with flat-panel displays are part of a multi billion-dollar industry which is still growing. These electronics use transparent semiconductors to display images. Zinc oxide (ZnO) is an attractive transparent conductor that can potentially substitute the commercially used materials at a reduced cost. A good transparent conducting oxide is characterized by high-electrical conductivity and optical transparency. Since ZnO in its pure form is an insulator our aim was to improve its electrical properties and to understand the mechanism responsible for such a desirable effect. ZnO powders with various impurity levels and different grain sizes were prepared and exposed to reducing environments to improve their electrical properties. The electrical conductivity and Seebeck coefficient of the samples were measured before and after reduction. For some samples, the conductivity increased up to 1400 %. The atomic and micro-structural properties of these powders were determined by x-ray fluorescence, x-ray diffraction and scanning electron microscopy and correlated to the electrical properties. The impurities and grain size distribution in these zinc oxide materials had an effect on their electrical behavior. Studying the direct correlation between the atomic-scale structure and the desirable macroscopic electrical properties of zinc oxide can lead to a better understanding and a more effiient optimization of this material for commercial devices. Abstract DEPARTMENT OF PHYSICS Improving the Electrical Properties of Zinc Oxide Leonel Hernandez Advisor: Dr. Gonzalez Department of Physics Introduction Objective Materials and Methods Results Acknowledgements I would like to thank Dr. Gabriela Gonzalez-Aviles for giving me the opportunity to work on this research project from which I have gained a great amount of knowledge. I would also like to thank the following peers who also worked on this research project: Alexander Slawik, Tom McManus, and Jared Hennen. Transparent conducting oxides (TCOs) are used in many industrial applications. They are used because of their conductivity and transparency in the visible region. Zinc oxide (Zn0) has the potential to become a very good TCO, with further research. It is relatively cheaper than other TCO’s and is non toxic. Applications of TCO’s include LCD’s and solar cells. Zinc oxide’s atomic structure, as a crystal, is regular and repeating. This rigid pattern binds all of the material’s electrons to the atoms. Atomic defects or irregularities in this atomic structure may increase the electron concentration, and hence the material's ability to conduct. The objective is to explore the defect structure of ZnO, analyzing influential factors and synthesis procedures. Side view represent O 2- represent Zn 2+ ZnO oxide powder from different manufacturers was used; each with different purity levels and grain size. Two, labeled N1 and N2, have grain size on the scale of nanometers, while the others, B1, B2, and B3, have larger grain size. The manufacturers provided a purity level of above 99%. Each material was labeled based on purity level, with 1 being the most pure. Purity was experimentally measured and B3 and N1 had many impurities. Synthesis Since we sought to find the conductivity and thermopower of the materials, the powder had to be formed into a solid object. It was pressed into small pellets, of 6 mm diameter and about 0.16 grams. The press: used at just over 1000 lbs The powders had to be macerated with acetone and then dried prior to pressing for homogenization purposes. The same procedure was used to press all the pellets. A stainless steel die with an inner chamber of radius 3 mm was used to hold the powder. Annealing Once the pellets were pressed, in order to increase the particle-particle contact, they were placed in a high temperature furnace. This process, called annealing, increases their durability, allowing us to test their properties without damaging the pellets. The pellets were baked in the furnace for a couple of days at 1200 degrees Celsius. The maximum temperature achievable by the furnace is 1500 degrees. Forming Gas Reduction Once annealed, about half of the pellets were chosen to be reduced in a forming gas chamber. Forming gas is a mixture of 4% hydrogen in nitrogen, and is commonly used to dissipate moisture and oxygen. We reduced these pellets in order to determine the effect that forming gas reduction has on thermopower and conductivity. In theory, gas reduction will remove some oxygen molecules from ZnO, increasing the carrier concentration of electrons and thus conductivity. The reducing chamber in which our samples were placed. They were held at about 500 degrees Celcius under forming gas. Conclusions The results from the tests conducted indicate that forming gas reduction improved the conductivity in some cases up to more than 1400%.The purest samples had better electrical properties. More experiments are being carried out to reproduce these measurements, and similar results have been obtained. Recent efforts are focusing on optimizing the synthesis procedure and reduction conditions to further improve the electrical performance of zinc oxide After having annealed the different samples and testing them for conductivity, a few were chosen to be reduced. The reduced samples were retested for conductivity. Below is a graph showing the increase in conductivity, after annealing. From the table above we can see that there is a definite increase in conductivity after reduction. Samples from the B1 batch showed an increase of up to more than 1400%. The graph below shows the data for the thermopower before and after reduction. For the most part the samples had a very small change in their thermopower. The negative values indicate that the samples are n-type. The smaller magnitudes correspond to a higher electron concentration. Testing the Electrical Properties After the pellets were annealed they were tested for their conductivity using a four-point probe and for thermopower using the soldeing iron. The four-point probe setup with the probe on the far left and the current source and voltmeter in the center and right, respectively. By applying different currents to the two outer probes and using a voltmeter to measure the voltage across the two inner probes we were able to determine the pellet’s conductivity. Pellets were placed between the soldering iron and metal block with two thermocouple attachments. To test the thermopower we placed the pellet between two thermocouples. A temperature difference was created by placing the pellet on top of a metal block initially at room temperature and connecting a modified soldering iron to the top. This allows us to measure the carrier concentration of each pellet, which helped us determine the effect of the gas reduction process. SEM pictures of B2 samples. The picture above is before reduction at 20K x zoom and the picture to the right is after reduction at only 5K x zoom.