Engineering nitrogen- and hydrogen-related defects in ZnO nanowires using thermal annealing

Abstract

The chemical bath deposition (CBD) of ZnO nanowires (NWs) is of high interest, but their formation occurs in a growth medium containing many impurities including carbon, nitrogen, and hydrogen, rendering the accurate determination of predominant crystal defects as highly debated. In addition to the typical interstitial hydrogen in bond-centered sites $({\mathrm{H}}_{\mathrm{BC}})$ and zinc vacancy-hydrogen $({V}_{\mathrm{Zn}}\text{\ensuremath{-}}n\mathrm{H})$ complexes, we reveal that nitrogen-related defects play a significant role on the physical properties of unintentionally doped ZnO NWs. We show by density functional theory that the ${V}_{\mathrm{Zn}}\text{\ensuremath{-}}{\mathrm{N}}_{\mathrm{O}}\text{\ensuremath{-}}\mathrm{H}$ defect complex acts as a deep acceptor with a relatively low formation energy and exhibits a prominent Raman line at $3078\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}1}$ along with a red-orange emission energy of \ensuremath{\sim}1.82 eV in cathodoluminescence spectroscopy. The nature and concentration of the nitrogen- and hydrogen-related defects are found to be tunable using thermal annealing under oxygen atmosphere, but a rather complex, fine evolution including successive formation and dissociation processes is highlighted as a function of annealing temperature. ZnO NWs annealed at the moderate temperature of 300 \ifmmode^\circ\else\textdegree\fi{}C specifically exhibit one of the smallest free charge carrier densities of $5.6\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}3}$ along with a high mobility of $\ensuremath{\sim}60\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\mathrm{s}$ following the analysis of longitudinal optical phonon-plasmon coupling. These findings report a comprehensive diagram showing the complex interplay of each nitrogen- and hydrogen-related defect during thermal annealing and its dependence on annealing temperature. They further reveal that the engineering of the nitrogen- and hydrogen-related defects as the major source of crystal defects in ZnO NWs grown by CBD is capital to precisely control their electronic structure properties governing their electrical and optical properties in any devices.