Measurement of the effect of pretreatment and adsorption on the electrical properties of ZnO powders using a microwave-Hall-effect technique.

Abstract

Microwave-Hall-effect (MHE) and electrical conductivity measurement techniques can now be used to obtain absolute values of the electrical properties of semiconductor powders under different controlled conditions. A commercial ESR spectrometer was modified to conduct MHE experiments and a network analyzer was utilized to tune the microwave cavity and obtain Q factors. A sample of ultrahigh-purity ZnO powder (30 ${\mathrm{m}}^{2}$/g) was prepared and studied. Evacuation of ZnO at 673 K decreased the electron Hall mobility but increased the conductivity and electron density at 300 K, and typical values for this high-surface-area powder were \ensuremath{\mu}=4 ${\mathrm{cm}}^{2}$/V s, \ensuremath{\sigma}=0.05 ${\mathrm{\ensuremath{\Omega}}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and ${\mathit{N}}_{\mathit{e}}$=2\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$/g. Oxygen chemisorption at 300 K increased the mobility but decreased the conductivity and electron density, and after this step typical values were \ensuremath{\mu}=50 ${\mathrm{cm}}^{2}$/V s, \ensuremath{\sigma}=0.001 ${\mathrm{\ensuremath{\Omega}}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and ${\mathit{N}}_{\mathit{e}}$=2\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$/g. These changes were reversible. The \ensuremath{\mu} values varied more than an order of magnitude; therefore, the assumption of constant mobilities cannot be made safely even at these low carrier densities. Three different ZnO powders were studied to observe the effect of surface area (i.e., particle size) on electrical properties.Lattice scattering was the dominant mechanism determining electron mobility for large particles, where the particle radius was much greater than the electron mean free path; however, as the ZnO particle size approached that of the estimated electron mean free path, defect scattering in the surface region became the dominant controlling mechanism. On an evacuated ZnO surface, ${\mathrm{CO}}_{2}$ adsorption had a significant effect on the electrical properties while CO adsorption had little effect, whereas on an O-covered ZnO surface CO adsorption had a noticeable effect but ${\mathrm{CO}}_{2}$ adsorption produced a much smaller change. Hydrogen adsorption on either surface gave little change in electrical properties.