Nitrogen diffusion in zinc oxide

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Nitrogen migration in ZnO was investigated by nitrogen isotope diffusion. The samples were deposited using plasma-assisted pulsed laser deposition. Nitrogen concentration depth profiles were obtained from secondary-ion-mass spectrometry measurements, and in gas effusion measurements, the molecular nitrogen flux was measured as a function of the heating rate. Measurements performed on sample stacks that were doped with isotopically enriched 15N and 14N in the top and bottom half of the samples, respectively, clearly demonstrate that nitrogen diffusion is governed by atomic diffusion and molecules are formed primarily at the sample surface. At high nitrogen concentrations, the diffusion coefficient, D, is thermally activated, while for low concentration diffusion, D is independent of temperature. The data can be described by a model, where N diffusion occurs between minimum energy positions by surmounting the barrier between sites at a saddle point. Separated in energy from the transport sites are deep levels with a concentration of ≈1018 cm−3. For high-concentration diffusion, the N chemical potential, μN, resides at ≈1.36 eV below the migration saddle point. For low concentration diffusion, μN shifts deeper in energy with a rate of ≈2.8 meV/K as the temperature increases. From N effusion data, the nitrogen density-of-states is derived. For high N concentration diffusion, two peaks are observed at ES–μN = −0.93 and −1.26 eV, while for low N concentration diffusion, a prominent peak at ES–μN = −1.63 eV occurs. Applying density functional theory calculations, different microscopic diffusion mechanisms are evaluated, and the corresponding transition states are derived.