Oxygen Vacancy in Hafnia as a Blue Luminescence Center and a Trap of Charge Carriers

Abstract

The electronic properties of HfO2, in particular, luminescence and charge transport, are determined by defects and traps. The origin of luminescence centers and traps of charge carriers in a HfO2 crystal was studied using luminescence spectroscopy, charge transport, and quantum-chemical calculation. The 2.7 eV luminescence band and 5.2 eV absorption/luminescence excitation band are associated with an oxygen vacancy. The thermal activation energy 1.25 eV, estimated from the charge transport and thermoluminescence experiment, is equal to half of the Stokes shift in photoluminescence spectra. Hence, oxygen vacancies are supposed to operate as electron traps in HfO2, and the charge transport is described by phonon-assisted tunneling between traps.

First principles calculations of optical properties for oxygen vacancies in binary metal oxides

“…Furthermore, another absorption/emission line-a 3.66 eV PL excited at 4.4 eV and 5.4 eV-has also been detected. 27 The predicted optical absorption energy for the neutral vacancy is approximately 3.2 eV. 26 This corresponds to an electron being excited out of a doubly occupied vacancy state [which has its Kohn-Sham (KS) level in the middle of the bandgap] into an unoccupied state at the bottom of CB (type V excitation in Fig.…”

The Bright X‐Ray Stimulated Luminescence of HfO2 Nanocrystals Activated by Ti Ions

“…Spectroscopic investigations and computational calculations by DFT have evidenced that oxygen vacancies play a major role as defect centers responsible for the optical emissions in hafnia. In fact, different variants of oxygen vacancies have been related to additional electronic levels within the bandgap of HfO 2 . Nevertheless, in nominally pure HfO 2 NPs, the intrinsic defect‐related band centered at around 2.50 eV overlaps with the blue emission due to accidental Ti impurities in the matrix.…”

Intrinsic Defect Engineering in Eu3+ Doped ZnWO4 for Annealing Temperature Tunable Photoluminescence

“…Due to the large difference between the bandgap of ZnWO 4 (about 4 eV) and its emission energy (around 2.6 eV), we can exclude the possibility of band edge recombination as the candidate of the broadband emissions of ZnWO 4 grains. In actual cases, defect emissions often dominate the PL properties in diverse metal oxides such as HfO 2 [35], Zn 5 Mo 2 O 11 [28], SrAl 2 O 4 [36,37], BaAl 2 O 4 [33], and ZnMO 4 [38]. This also holds true for ZnWO 4 grains, where coordinatively unsaturated vacancies are active sites for luminescence.…”

“…The two defect energy levels are clearly marked in red, as shown Figure 12b. Since it is positively charged, V O can act as electron trap sites as well as luminescence center as they do in HfO 2 [35].…”