Improving ZnO for Use in Ultraviolet Light Emission and Sensor Applications

Larry Halliburton, West Virginia University, DMR 0508140

Zinc oxide (ZnO) is a promising material for electro-optic applications such as ultraviolet lasers, light-emitting diodes, and detectors. Point defects limit device performance. Thus far, it has proven difficult to produce p-type material. Native defects are suggested to be responsible for the observed optical and electrical behaviors of ZnO crystals, thin films, powders, and nanoparticles.

A comprehensive experimental study of defects in ZnO is in progress. Native defects are purposefully introduced, and observed using spectroscopic techniques. Optical and magnetic resonance “signatures” are established, and charge transfers between defects are monitored. EPR and ENDOR techniques allow for specific identification of a point defect or point defect complex. Optical absorption associated with the neutral oxygen vacancy has been identified and the ground state of this defect within the band gap has been determined. The mechanism by which hydrogen passivates zinc vacancies has been identified. Transition-metal-ions are shown to play an important role in charge trapping.

• Halliburton et al., Applied Physics Letters 87, 172108 (2005)

• Giles et al., Applied Physics Letters 89, 251906 (2006)

• Jiang, Giles, Halliburton, Journal of Applied Physics 101, 093706 (2007)

• Kappers et al., Nuclear Instrum. Methods B, in press (2007).

Modifications--thermal anneals in air, vacuum, Zn vapor, and hydrogen--electron-irradiations

Sample Growth--vapor transport--high-temperature melt--hydrothermal--powders

Characterizations--magnetic resonance (EPR, ENDOR)--photoluminescence (PL, PLE)--optical absorption (UV/Vis, FTIR)--transport measurements (Hall)

Point Defect Identification

Improving ZnO for Use in Ultraviolet Light Emission and Sensor Applications

Larry Halliburton, West Virginia University, DMR-0508140

BROADER IMPACTS:

A physics faculty member from William Jewell College (Liberty, Missouri) has made extended visits during the past year to the PI’s laboratory at West Virginia University. Two undergraduate physics majors from William Jewell also participated in several of these visits. This transfer of expertise in magnetic resonance is allowing a research program using EPR to be established as part of the undergraduate research program at the four-year college.

The PI and co-PI led graduate student recruiting efforts in the Physics Department at West Virginia University in 2007. This resulted in an entering class of 18 students in August of 2007 with the majority planning to pursue research in condensed matter/nano physics. Nine of these students are from regional colleges and universities (Ohio, Pennsylvania, and West Virginia).

The co-PI teaches the department’s undergraduate and graduate solid-state physics courses to physics and engineering students and uses examples of her research in the classroom. In Spring 2007, she gave a campus-wide public lecture describing optical properties of wide band gap semiconductors.

One of three Bruker EPR systems in the variable-temperature magnetic resonance facility at WVU Physics Department. In-situ illumination (lasers, lamp/monochromator) is used to produce changes in charge states of defect centers.

Two physics graduate students are supported by the project each year.