Abstract
The magnetic and electronic properties of the hydrogenated highly conductive zinc oxide (ZnO) microparticles were investigated by electron paramagnetic resonance (EPR) and contactless microwave (MW) conductivity techniques in the wide temperature range. The EPR spectra simulation allowed us to resolve four overlapping EPR signals in ZnO microparticles. The Lorentzian EPR line with isotropic g-factor 1.9623(5) was related to the singly ionized oxygen vacancy. Another Lorentzian line with g|| = 1.9581(5), g⊥ = 1.9562(5) was attributed to the zinc interstitial shallow donor center, while EPR signal with g|| = 1.9567(5), g⊥ = 1.9556(5) and Gaussian lineshape was assigned to the hydrogen interstitial shallow effective-mass-like donor. The EPR signal with g|| = 1.9538(5), g⊥ = 1.9556(5) and Lorentzian lineshape was tentatively attributed to the shallow donor center. The charge transport properties in ZnO microparticles have been investigated by the contactless MW conductivity technique at T = 5–296 K. Two conduction mechanisms, including ionization of electrons from the shallow donors to the conduction band and hopping conduction process, have been distinguished. The hopping conduction process follows Mott’s variable-range hopping T−1/4 law at T = 10–100 K. The evaluated values of the average hopping distance (15.86 Å), and hopping energy (1.822 meV at 40 K) enable us to estimate the donor concentration in the investigated ZnO microparticles as ~ 1018 cm−3.
Highlights
Zinc oxide (ZnO) is a wide bandgap (3.37 eV) semiconductor material with excellent electrical and optical characteristics such as high n-type conductivity and optical transparency in the visible region
Shallow donors related to the charge transport process that occurred in hydrogenated zinc oxide (ZnO) microparticles have been studied by electron paramagnetic resonance (EPR) spectroscopy
This assignment is supported by the fact that Zni and V·O centers simultaneously appeared in the EPR spectra of ZnO micropowders at T = 80 K, while EPR signals from Hi and shallow donor centers appeared at T < 60 K
Summary
Zinc oxide (ZnO) is a wide bandgap (3.37 eV) semiconductor material with excellent electrical and optical characteristics such as high n-type conductivity and optical transparency in the visible region. These properties make ZnO the promising material for the production of the optoelectronic devices, ultraviolet and white light-emitting diodes, photodetectors as well as for the preparation of the transparent electrodes for the thin-film amorphous silicon solar cells and different display p anels[1,2,3]. The other set of the EPR signals of isotropic and axial symmetry with g ~ 1.99–2.00 was assigned with surface defects attributed to the Zni, negatively charged Zn vacancies ( V−Zn ) and in some reports to the positively charged V·O observed in electron-irradiated ZnO and under p hotoexcitation[15,16]
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