Comprehensive defects study of Sb doped ZnO films : the insights of donors and acceptors

Abstract

To achieve p-type conductivity, Sb doped ZnO has been studied for several decades. But there are still some bottlenecks to obtain stable and reproducible p-type conductivity, which is limiting the widespread use of Sb doped ZnO material. The shallow acceptors that donate holes are not determined till now, and it is essential to study what is favorable condition to form the shallow acceptors In this present study, Sb-doped ZnO films were fabricated on c-plane sapphire using the pulsed laser deposition method and characterized by the Hall effect measurement, X-ray photoelectron spectroscopy, X-ray diffraction, Raman technique and positron annihilation spectroscopy. The effects of the Sb composition (weight ratio of 0.5%, 1%, 2% and 3%) and post-growth annealing (up to 900 oC) were systematically studied. All the as-grown Sb-doped ZnO samples with Sb=0.5% to 2% are n+ conductive having electron concentration larger than ~1020 cm-3, although further increasing the Sb composition to 3% turns the sample to highly resistive (n~4×1014 cm-3). As the undoped ZnO sample has n~1×1018 cm-3, it is thus plausible to say that the shallow donor responsible for the n+ conductivity is associated with the doping of Sb. The H concentration showed correlated increase with increasing Sb doping composition, and moreover had value very close to that of the Sb concentration. It is plausible to attribute the shallow donor to the SbZn or its complex defect probably containing the H. The ionization energy of donor in as-grown Sb doped ZnO film is around 5 meV, which is fitted by using temperature dependent hall measurement. For the low Sb composition ≤2%, Sb occupies the Zn sites and forms the donors, the electron concentration of the as-grown ZnO:Sb samples increases with the Sb doping composition and is effectively equal to the Sb concentration. Annealing these relatively lowly Sb-doped would lead to the formation of the SbZn-2VZn shallow acceptor with the ionization energy of 192 meV, which subsequently compensated the free carrier. It is found that Sb5+ states were more favorable to form the SbZn-2VZn defects in compared with Sb3+ states. For samples with Sb doping exceeding the threshold (3%), the Sb dopant occupies the O site rather than the Zn site and forms the deep acceptors, the yielded samples are highly resistive. Several additional modes, located at 235 cm-1, 511 cm-1, 534 cm-1, were observed in the Sb-doped ZnO thin films in addition to the host phonons in undoped ZnO sample. The intensity of these additional peaks correlated with the electron concentration and Fermi level. It is plausible that these additional modes are related with the donor defects in Sb doped ZnO samples.