In Situ Surface Photovoltage Spectroscopy of ZnO Nanopowders Processed by Remote Plasma
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In Situ Surface Photovoltage Spectroscopy of ZnO Nanopowders Processed by Remote Plasma Raul M. Peters1, Stephen P. Glancy2, J. Antonio Paramo1, and Yuri M. Strzhemechny1 1 Department of Physics and Astronomy, Texas Christian University, Fort Worth, TX, USA 2 Department of Engineering Science, Trinity University, San Antonio, TX, USA ABSTRACT In many instances the quality of the surface in ZnO nanoscale systems is a key performance-defining parameter. The surface itself could be a very significant source of lattice defects as well as contaminating impurities, and this influence may extend into the sub-surface vicinity. In our work, key element of the surface analysis is the surface photovoltage (SPV) spectroscopy known for its advantages, such as: identification of conduction vs. valence band nature of the defect-related transitions and the defect level positions within the band gap, ability to measure relatively low densities of surface defects as well as their cross sections. Additional information can be obtained from the SPV transient measurements. In our system, SPV characterization is run in high vacuum, complemented by in situ remote plasma treatment. This combination of surface-sensitive and surface-specific tools is well-suited for studying surface properties with a high degree of reliability since there is no exposure to common air contaminants between processing and characterization cycles. We employed O/He remote plasma treatments of ZnO nanocrystalline surfaces. In situ SPV spectra and transient measurements of the as-received and processed samples revealed, on the one hand, a number of common spectral features in different ZnO nanopowder specimens, and, on the other hand, a noticeable plasma-driven changes in the surface defect properties, as well as in the overall electronic and optical surface characteristics. INTRODUCTION Market-driven technological miniaturization imposes new demands on the device geometry and performance: as the volume of the crystals is shrinking, the relative significance of the crystal surfaces and their properties is increasing. In nanostructures therefore one deals with the domination of surface-enhanced phenomena, many of which are mediated by defects. Thus, understanding and control of the specifically surface as well as near-surface defects comes to the forefront. Major thrust in the studies of defects in bulk ZnO crystals has been recently focused primarily on overcoming the difficulties with producing a high-quality reliably p-type material [1]. Such material is sought because of great potential for spintronic and optoelectronic applications. As of today, efforts to obtain a device-grade p-type ZnO have not resulted in a material suitable for mass production. Remarkably, nanoscale ZnO structures are offering numerous applications that do not require resolving the challenge of p-type conductivity. Recent reports demonstrated that nanosize ZnO could be employed in such systems as nanotransducers [2], random lasers [3], dye-sensitized solar cells [4], nanosenso
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