Negative oxygen vacancies in HfO2 as charge traps in high-k stacks

Abstract

We calculated the optical excitation and thermal ionization energies of oxygen vacancies in mHfO2 using atomic basis sets, a non-local density functional and periodic supercell. The thermal ionization energies of negatively charged V − and V 2− centres are consistent with values obtained by the electrical measurements. The results suggest that negative oxygen vacancies are the likely candidates for intrinsic electron traps in the hafnum-based gate stack devices.Hafnium based oxides are currently considered as a practical solution satisfying stringent criteria for integration of high-k materials in the devices in future technology nodes. However, high-k transistor performance is often affected by high and unstable threshold potential 1 , V t , and low carrier mobility 2 . These effects are usually attributed to a high concentration of charge traps and scattering centers in the bulk of the dielectric and/or at its interface with the silicon channel. Although the reported trap densities vary greatly with fabrication techniques, the majority of data point to existence of a specific intrinsic shallow electron centre common to all HfO 2 based stacks while some extrinsic defects, such as Zr substitution in HfO 2 , have also been considered 3 .Oxygen vacancies are dominating intrinsic defects in the bulk of many transition metal oxides including HfO 2 and ZrO 2 , and are thought to be also present in high concentrations in thin films. However, in spite of numerous experimental studies, evidence relating oxygen vacancies to measured characteristics of interface traps in high-k stacks is still mostly circumstantial. Therefore, accurate theoretical characterization of these defects is highly desirable.Previous theoretical calculations of oxygen vacancy in HfO 2 and ZrO 2 reported the ground state properties obtained within local or semilocal approximations to density functional theory (DFT) methods (see 4,5 for a review). This approach, however, significantly underestimates band gaps, which hampers determining energies of defect levels with respect to the band edges and precludes identifying shallow defect states 5,6,7 . As a result, most of the early local DFT calculations (except, perhaps, ref. 8 ) failed to predict unambiguosly negative charge states of oxygen vacancy in HfO 2 . Significant improvement was achieved by Robertson et al. 4,9,10 who used screened exchange approximation to predict vacancy energy levels including V − charge state. However, these calculations were performed using a small periodic supercell and therefore corresponded to extremely high vacancy concentrations. The quality of the functional used is also largely unknown and needs independent verification.In this work we used much bigger supercells and a nonlocal functional to calculate optical excitation and thermal ionization energies of oxygen vacancies in five charge states. To relate these energies to experimental data we distinguish optical absorption/reflection type measurements involving Frank-Condon type (vertical) excitations, and electric...

First principles investigation of defect energy levels at semiconductor-oxide interfaces: Oxygen vacancies and hydrogen interstitials in the Si–SiO2–HfO2 stack

“…The oxygen vacancy has been the object of extensive studies in literature 41,53,54 because it is suspected to be at the origin of the Fermi-level pinning in p-type polysilicon gate stacks. 55,56 The hydrogen interstitial is ubiquitous at such interfaces.…”

Oxygen vacancy levels and electron transport in Al2O3

“…This is also true for V O in HfO 2 . 18,33 However, the difference is that this trapped electron level much lies deeper in Al 2 O 3 at 1.8 eV compared to 0.4 eV in HfO 2 , 33 as summarized in Fig. 4.…”

A microscopic mechanism of dielectric breakdown in SiO2 films: An insight from multi-scale modeling

“…The defect thermal ionization energy, E t , is calculated as the total energy difference between the defect state and the state where the defect electron is delocalized in the conduction band and the system geometry is relaxed. 59 The defect relaxation energy, E rel , is calculated as the difference in total energies of the unrelaxed and relaxed defect state with extra electron(s). The parameters used for calculating the capture/ emission rates on both wide O-Si-O bond angle precursors and oxygen vacancies are reported in Table I.…”