GOALI: Growth-dependent identification and control of defects and dopants in ZnO – DMR 0513968

L. J. Brillson and D. C. Look • A major objective of this research is to understand the complex relationships between growth, processing, intrinsic defects and extrinsic doping within ZnO and at its interfaces in order to control doping, interface states and barrier formation. Key to Schottky barrier formation are the localized electronic states that form at the metal-semiconductor interface. Using low energy electron-excited luminescence spectroscopy to excite electron-hole recombination at this interface, we observed optical transitions involving chemically-induced states deep in the ZnO band gap.

• Figure 1(a) shows the Au-ZnO contact, the probe depth, and the nanometer-scale interface reaction zone. Annealing above the 642ºC Au-Zn eutectic temperature promotes interdiffusion of Au and Zn and formation of Zn vacancies near the interface.

• Figure 1(b) shows the 10 K, 5 keV luminescence spectrum of the Au-ZnO interface as a function of annealing temperature in Ar for 1 hour. A dramatic increase in 2 eV emission occurs only after a 650ºC anneal, corresponding to formation of Zn vacancies or their complexes. In contrast, reactive metals such as Al, Ta, and Ir that form oxides increase 2.5 and 3 eV emissions (not shown) that correspond to O vacancies or their complexes.

• The nature of point defects in ZnO has been hotly debated for decades. Chemical reactions that extract semiconductor atoms at their interface provides a new avenue to identify native defects and their electronic properties.

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Fig.1. Au interacts with ZnO at the Au-ZnO interface to form a Au-Zn eutectic, producing Zn vacancies (a) and luminescence (b) at 2 eV. Metals that form oxides produce O vacancies and 2.5 & 3 eV peaks.

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REU Physics undergraduate Christine Zgrabik adjusting monochromator resolution for ZnO interface spectroscopy

Growth dependent identification and control of defects and dopants in ZnO – DMR 0513968

L.J. Brillson and D.C. Look

CSG high school junior Amber Swain assembling UHV chamber for ZnO growth

Education and Outreach:• During summer 2006, Columbus School for Girls (CSG) high school student Amber Swain worked in the PI’s laboratory on a project based on nanoscale science and how it will enable future electronics. During her ten weeks on the Ohio State campus, she worked with REU OSU physics undergraduate Christine Zgrabik to learn optical measurement techniques such as photoluminescence and cathodoluminescence spectroscopy, which they used to measure the native electronic defects formed by chemical reactions at metal-ZnO interfaces. We plan to submit their results for publication in Applied Physics Letters. Amber and Christine also measured current-voltage properties of diodes prepared from these interfaces and assisted in the assembly of a new growth chamber for growing oxide semiconductors. Based on the NSF high school student support, the OSU College of Engineering provided additional funds to support three other CSG juniors working in Physics, Biomedical Engineering, and Mechanical Engineering’s Center for Automotive Research.1 Each girl presented her results in a formal lecture at summer’s end. This summer research program provided opportunities for hands-on laboratory experience, a chance to do original research, and a glimpse of the exciting opportunities in scientific research.