Oxygen vacancies: The origin ofn-type conductivity in ZnO

Abstract

Oxygen vacancy (VO) is a common native point defects that plays crucial roles in determining the physical and chemical properties of metal oxides such as ZnO. However, fundamental understanding of VO is still very sparse. Specifically, whether VO is mainly responsible for the n-type conductivity in ZnO has been still unsettled in the past fifty years. Here we report on a study of oxygen self-diffusion by conceiving and growing oxygen-isotope ZnO heterostructures with delicately-controlled chemical potential and Fermi level. The diffusion process is found to be predominantly mediated by VO. We further demonstrate that, in contrast to the general belief of their neutral attribute, the oxygen vacancies in ZnO are actually +2 charged and thus responsible for the unintentional n-type conductivity as well as the non-stoichiometry of ZnO. The methodology can be extended to study oxygen-related point defects and their energetics in other technologically important oxide materials.

Deep vs shallow nature of oxygen vacancies and consequentn-type carrier concentrations in transparent conducting oxides

“…The computed value can be less negative than the experimental value by as much as 1 eV, hence the discrepancy between our calculated formation energy in O-poor conditions and the others shown in Table III. There are experimental reports of formation energies in reducing conditions of 1 eV or above [36,88], but the O 2 partial pressure which these [51,55], as expected given that the modHSE06 results included 37.5% exact exchange. With BB1k, however, we find that (2 + /0), while still a deep level, is significantly shallower (at 0.25 eV below the CBM) than that obtained using either PBE0 or B97-2, or those obtained in previous studies, none of which employed a functional of this kind.…”

“…III B 1, due to our slightly lower formation energies and usage of the experimental value of H (ZnO), our calculated E f (V O ) in O-poor conditions are significantly lower than those of other DFT studies, at about 0.05 eV for PBE0 and B97-2 and 0.19 eV for BB1k (see Table III). Such a low formation energy would indicate high nonstoichiometry in extreme reducing conditions, for which there is some experimental evidence [183][184][185], but measurements to investigate the intrinsic electrical conductivity properties of ZnO tend to be performed under less extreme conditions [36,88,186]. From such a low formation energy, we determine very high [V O ], of the order of 10 22 cm −3 for T > 400 K, using the PBE0 and B97-2 functionals.…”

“…Some experiments [81,82] have shown a variation in conductivity and mobility with P O 2 more indicative of the presence of interstitials or cation vacancies (which have also been shown to be present using positron annihilation spectroscopy) [83][84][85][86][87], but oxygen vacancies have been proposed to explain the intrinsic n-type conductivity [36,78,88] and persistent photoconductivity [37,76], as well as other experimental results [89][90][91][92]. Similar to SnO 2 , computational studies tend to indicate that the oxygen vacancy is deep, with the (2+/0) transition occurring at about 1 eV below the CBM [46][47][48][49][50][51][52][53].…”

Activity Tuning of Cobalt Ferrite Nanoparticles Anchored on N‐Doped Reduced Graphene Oxide as a Potential Oxygen Reduction Electrocatalyst by Zn Substitution in the Spinel Matrix

“…Divalent Zn 2+ ion when doped in the place of the trivalent Fe 3+ ion as evident from the XPS analysis, an oxygen vacancy (Vo) can be created in the crystal structure . As more Zn 2+ goes to the Fe 3+ site, concomitantly more the oxygen vacancies will be created to maintain the crystal neutrality ,. A nice correlation between the XPS and ORR features could be arrived based on the state of the compositions as indicated in the XPS data and the performance indicators deduced from the electrochemical experiments.…”

High-resistivity metal-oxide films through an interlayer of graphene grown directly on copper electrodes

“…This can help not only in maintaining the oxygen stoichiometry of the as-deposited oxide channel, but also in mitigating intermixing of the contact metal and channel during sputtering (fabrication) [26]. Suppressing out-diffusion of oxygen reduces oxygen vacancy concentration in the channel and allows preserving the bulk resistivity to that of the intrinsic oxide channel [27,28].…”