Hydrogen in red ZnO – defects or lattice expansion

Abstract

The effect of hydrogen on ZnO while annealing at 1100 °C with Zn vapour or Ti metal is studied. ZnO turns red only when hydrogen is present in combination with oxygen deficiency. The coloration is reversible at 1100 °C by removing hydrogen either by evacuating or by gettering hydrogen with Ti at low temperature while annealing the ZnO at >950 °C. The latter is a simple method to lower the hydrogen impurity concentration. The annealing data are consistent with an activation energy >1.5 eV to reverse the coloration. Hall data on red ZnO reveal only a shallow donor level around 45 meV, too low for the colour change. Temperature dependent X-ray diffraction reveals a lattice expansion due to the presence of hydrogen which is sufficient to explain the colour change.