Dopants in nanoscale ZnO
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1174-V07-03
Dopants in nanoscale ZnO Matthew D. McCluskey,1 Win Maw H. Oo,1 and Samuel Teklemichael1 1
Washington State University, Pullman, WA 99164-2814
ABSTRACT Zinc oxide (ZnO) is a metal-oxide semiconductor that has attracted resurgent interest as an electronic material for a range of device applications. In our work, we have focused on how defect properties change as one goes from the bulk to the nanoscale. Infrared (IR) reflectance spectra of as-grown and hydrogen-annealed ZnO nanoparticles were measured at near-normal incidence. The as-grown particles were electrically semi-insulating, and show reflectance spectra characteristic of insulating ionic crystals. Samples annealed in hydrogen showed a significant increase in electrical conductivity and free-carrier absorption. A difference was observed in the reststrahlen line shape of the conductive sample compared to that of the as-grown sample. In addition to hydrogen doping, we successfully doped ZnO nanoparticles with Cu. To probe the electronic transitions of Cu2+ impurities in ZnO nanoparticles, IR transmission spectra were taken at liquid-helium temperatures. Two absorption peaks were observed at energies of 5781 and 5821 cm-1. Finally, we tentatively assign a series of IR spectral lines to Na acceptors.
INTRODUCTION Doping issues in nanocrystals are only beginning to be understood [1]. These issues are especially important in ZnO, a wide-bandgap metal-oxide semiconductor [2] that has attracted tremendous interest as a blue light emitting material [3], a buffer layer for GaN-based devices [4] and a transparent conductor [5] in solar cells [6]. Theoretical work has predicted ferromagnetism above room temperature for Mn-doped ZnO (given a large hole concentration) [7], an important requirement for spintronic devices. In addition to the potentially high Curie temperature (Tc), ZnO has numerous properties that are desirable for device applications, including low cost, environmental friendliness, and efficient light output. In bulk, single-crystal ZnO doped with hydrogen or deuterium, infrared (IR) spectroscopy and Hall-effect measurements show that hydrogen binds to a host oxygen atom and donates an electron to the conduction band. The IR-active complex is unstable, decaying over a few weeks at room temperature. A more stable hydrogen donor may be substitutional hydrogen, predicted by Janotti and Van de Walle [8]. The effect of hydrogen on ZnO nanoparticles is more dramatic than for bulk crystals. Due to the large surface-to-volume ratio, hydrogen increases the free-carrier concentration at relatively low diffusion temperatures (300°C).
EXPERIMENTAL DETAILS Nanoparticles were produced by the chemical reaction of zinc acetate dihydrate with sodium hydrogen carbonate, in an open-air furnace, at 200 °C for 3 hr. In prior work, x-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that the particles are 15-20 nm diameter with the hexagonal wurtzite structure [14]. The particles were pressed into 7 mm diameter pellets with a thickn
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