Soft X-ray XANES of N in ZnO:N - Why is doping so difficult?

Soft X-ray absorption near-edge experiments (XANES) were carried out at the N K-edge for both as-grown and rapid thermal annealed epitaxial N-doped ZnO samples grown on both { 1 1 over(2, ̄) 0 } sapphire substrates and the (0 0 0 1) face of ZnO single crystal substrates. Samples were grown by plasma-assisted MBE at a growth temperature of 450 °C using a solid Zn source and high-purity O₂ and N₂ RF radical sources. Calibrated secondary ion microscopy measurements demonstrated an as-grown N chemical concentration of approximately 10²⁰ cm^-3. The location in the ZnO lattice of N was determined in concert with first-principles real-space multiple scattering simulations. For as-grown samples, it was determined that N incorporates substitutionally on oxygen sites. It was found that a short 3 min rapid thermal anneal to 800 °C resulted in the unambiguous formation of molecular nitrogen underscoring the metastable nature of substitutional nitrogen. High resolution XANES scans clearly matched those of a N₂ gas standard. These results strongly suggest that while nitrogen can be incorporated in ZnO using metastable growth processes, the small activation barrier to the formation of molecular nitrogen will make it very difficult for nitrogen to be a dopant in actual device fabrication.