Structure and electrical levels of point defects in monoclinic zirconia

Abstract

We performed plane wave density functional theory ͑DFT͒ calculations of formation energies, relaxed structures, and electrical levels of oxygen vacancies and interstitial oxygen atoms in monoclinic zirconia. The atomic structures of positively and negatively charged vacancies and interstitial oxygen atoms are also investigated. The ionization energies and electron affinities of interstitial oxygen atoms and oxygen vacancies in different charge states are calculated with respect to the bottom of the zirconia conduction band. Using the experimental band offset values at the interface of ZrO 2 films grown on silicon, we have found the positions of defect levels with respect to the bottom of silicon conduction band. The results demonstrate that interstitial oxygen atoms and positively charged oxygen vacancies can trap electrons from the bottom of the zirconia conduction band and from silicon. Neutral oxygen vacancy serves as a shallow hole trap for electrons injected from the silicon valence band. The calculations predict negative U for the O Ϫ center and stability of V ϩ centers with respect to disproportionation into V 2ϩ and V 0 in monoclinic zirconia.

Understanding Structure and Stability of Monoclinic Zirconia Surfaces from First-Principles Calculations

“…Table 2 summarizes the energies required for formation of a single oxygen vacancy depending on the location and coordination of the lattice site. In the m-ZrO 2 bulk, formation of an oxygen vacancy at the 4 O B site is more favorable than at the 3 O B site by approximately 0.1 eV, which is consistent with the reports by Syzgantseva et al [40] and Forster et al [7] This tendency withholds for the subsurface atomic layers of the m-ZrO 2 surfaces; however, the preference of the 4 O B sites becomes more clear with the differences of >0.15 eV compared to the 3 O B sites. In the surface layers of the m-ZrO 2 surfaces, formation of oxygen vacancies at the 3 O S sites is more preferable compared to the 2 O S .…”

“…Regardless of the vacancy location, the most stable (111) surface is the least vulnerable towards oxygen vacancies. lated band gap for the m-ZrO 2 bulk is 3.1 eV, which agrees with the previous computational [40,7] results and is slightly underestimated compared to the experimental values of 4.2-5.8 eV [38]. Formation of the oxygen vacancies leads to generation of additional electron states in the gap region.…”

“…Foster et al reported an evaluation of charged and neutral vacancies in the m-ZrO 2 bulk. [7] Syngantseva et al have reported a study on oxygen vacancies formed in the (111) and (101) m-ZrO 2 surfaces. [40] Nonetheless, the study has been limited by consideration of the topmost oxygen atoms alone, ignoring the possibility of vacancies to be present in the subsurface atomic layers.…”

Intrinsic point defects in aluminum antimonide

“…In fact, we observe that, qualitatively, the results obtained by applying no correction at all are more similar to the corrected results than what is obtained by assigning the band gap error fully to the conduction band. 43,44 Furthermore, a correction procedure often suggested in the literature 40 to simply shift the ionization energies of donor-like defects to track the conduction band minimum (implicitly leaving the ionization energies of acceptor-like defects tracking the valence band maximum), without accounting for the occupation of states, does not properly correct the errors in the formation energies. The scheme employed here results in an identical shift of the ionization energies, but also corrects the errors in the formation energies.However, we note that this scheme still neglects both level relaxations and changes in the double counting term.…”

Principles of doping ceria for the solar thermochemical redox splitting of H2O and CO2

“…From a bond breaking perspective we would expect that O-vacancies neighboring the Zr cation would be more unfavorable than those far from the Zr because Zr-O bonds are stronger than Ce-O bonds, based on the relative reduction energies of CeO 2 and ZrO 2 . 22,58 However, neither of these predictions is correct; therefore, the difference in Ovacancy formation energies arises from geometric changes in the material, as suggested by Wang et al 59 The formation of the O-vacancy in either position results in an expansion of the lattice from 11.63Å to 11.66Å. The similar expansion predicted for both O-vacancy congurations despite the large difference in reduction energy is contrary to the current explanation for the role of Zr, i.e.…”