||Nabil Dawahre, Joseph Brewer, Gang Shen, Nicholas Harris, Soner Balci, William Baughman, Lee Butler, Shawn Wilbert, Richard Martens, Seongsin Margaret Kim, Patrick Kung
Zinc oxide (ZnO) has become an important wide bandgap semiconductor material because of its wide range of optical and electrical properties, including a large bandgap ~3.3 eV and exciton binding energy ~60 meV, its piezoelectricity, room temperature ferromagnetism and magneto-optic effects. ZnO nanostructures ranging from nanowires to nanobelts and nanoribbons have dramatically expanded the range of applications of this material, including light emitting diodes, lasers, solar cells, transparent electronics, chemical sensors, and mechanical energy harvesting devices. Atom probe tomography (APT) is an analytical technique that has the unique ability to identify and map the positions of individual atoms from a nanostructure with three-dimensional atomic resolution. This capability is becoming essential when trying to understand defect distribution (e.g. composition fluctuations or doping) in semiconductor materials, as they can significantly affect device characteristics. In this study, we present the first APT compositional analysis of individual ZnO nanowires and the challenges encountered in developing this new technique for ZnO. We have grown ZnO nanowires by thermal chemical vapor deposition on sapphire substrates. To promote the orientation and alignment of the nanowires, ZnO seeds were prepared by oxidizing zinc acetate in suspension in an alcohol solution. The synthesis was carried out at 900 °C. A mixture of ZnO and graphite in powder form was used as precursor, with the carbon acting as a reducing agent for the oxide. Various molar ratios of the ZnO to carbon were investigated. The resulting nanowires were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction, photoluminescence (PL), Raman spectroscopy, atom probe tomography, and Terahertz time domain spectroscopy (THz-TDS). THz-TDS is an emerging technique that is capable of probing the electrical conductivity of nanostructures in a non-destructive manner because it does not require making an electrical contact. SEM and x-ray diffraction showed the nanowire were 500 to 3000 nm long, with a diameter range from 50 to 150 nm, and were crystallographically orientated with their c-axis perpendicular to the substrate. Their density was on the order of 108 cm-2. TEM was used to analyze the atomistic structure of the nanowires. Raman spectroscopy revealed the expected phonon modes of wurtzite ZnO while photoluminescence exhibited near band edge emission near 380 nm and allowed the probing of the visible defect-related emission near 500 nm as a function of the growth parameters. These nanowires were subsequently prepared for APT. Measurements under various voltage and laser pulse conditions were investigated and the resulting data will be presented.