Dependence of Annealing Temperature on the Conductivity Changes of ZnO and MgZnO Nanoparticle Thin Films from Annealing
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Dependence of Annealing Temperature on the Conductivity Changes of ZnO and MgZnO Nanoparticle Thin Films from Annealing in a Hydrogen Atmosphere at Mild Temperatures Christine Berven, Lorena Sanchez, Sirisha Chava, Hannah Marie Young, Joseph Dick, John L. Morrison, Jesse Huso, Leah Bergman Department of Physics, University of Idaho, Moscow, ID 83844-0903, USA ABSTRACT We report apparent robust doping of ZnO and MgxZn1-xO (x ~20%) nanoparticle films by annealing in hydrogen gas. The annealing was done at sequentially higher temperatures from about 20 ˚C to 140 ˚C. The effect of the annealing was determined by comparing current-voltage measurements of the samples at room-temperature and in vacuum after each annealing cycle. The nanoparticles were grown using an aqueous solution and heating process that created thinfilms of ZnO or MgZnO nanoparticles with diameters of about 30 nm. When exposed to hydrogen gas at room-temperature or after annealing at temperatures up to about 100 ˚C, no measureable changes to the room-temperature vacuum conductivity of the films was observed. However, when the samples were annealed at temperatures above 100 ˚C, an appreciable robust increase in the room-temperature conductance in vacuum occurred. Annealing at the maximum temperature (~135-140 ˚C) resulted in about a factor of about twenty increase in the conductivity. Furthermore, the ratio of the conductance of the ZnO and MgZnO nanoparticle films while being annealed to their conductance at room-temperature were found to increase and then decrease for increasing annealing temperatures. Maximum changes of about five-fold and seven-fold for the MgZnO and ZnO samples, respectively, were found to occur at temperatures just below the annealing temperature threshold for the onset of the robust hydrogen gas doping. Comparisons of these results to other work on bulk ZnO and MgZnO films and reasons for this behavior will be discussed. INTRODUCTION ZnO and MgZnO having a large band gap of 3.23 eV and 3.6 eV, have a wide range of potential applications such as in solar cells, gas sensors, chemical sensors, electrical devices, and luminescent devices. They have also been used for low voltage and short-wavelength electrooptical device applications due to their large exciton binding energies of ~60 meV.[1] ZnO and MgZnO nanoparticles have also recently been of great interest for the detection of pollutants and toxic gases.[2] The motivation for this work was to understand how the electrical properties of ZnO and MgZnO semiconducting nanoparticles can be modified through doping by exposure to hydrogen gas. To accomplish this, changes in the electrical properties of films of ZnO and MgZnO nanoparticles were studied as a function of annealing temperature in a hydrogen ambient. Similar work was done by this research team on ZnO nanowires, where a significant response to the presence of H2 gas at elevated temperatures was found.[3]
EXPERIMENTAL DETAILS The ZnO and MgZnO nanoparticles were synthesized by mixing magnesium acetate and zinc acetate in an
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