Defect energy levels in HfO2 high-dielectric-constant gate oxide

Xiong, Ka; Robertson, John; Gibson, Michael C.; Clark, S. J.

doi:10.1063/1.2119425

Cited by 490 publications

(335 citation statements)

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“…Electron traps in HfO 2 are often associated with oxygen vacancy defects 10 on the basis of electron energy levels found in the energy range 1.2-1.8 eV below the conduction band (CB) bottom edge. 10,14 However, since theoretical calculations consistently predict that the O vacancy in HfO 2 can act also as a hole trap, [17][18][19][20][21] this assignment cannot be reconciled with the absence of hole trapping from the valence band (VB) inside the high-k layer: 22 The observed trapped hole charge in the SiO 2 /HfO 2 stacks appears to be insensitive to the HfO 2 layer thickness ranging from 5 to >100 nm. Rather, the strong electron trapping in HfO 2 films prepared from the nitrato precursor Hf(NO 3 ) 4 can be correlated with the presence of N-related defects revealed by electron spin resonance (ESR) 22,23 suggesting an impurity-related electron trapping.…”

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confidence: 99%

Intrinsic electron traps in atomic-layer deposited HfO2 insulators

Cerbu

Madia

Andreev

et al. 2016

Applied Physics Letters

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Analysis of photodepopulation of electron traps in HfO2 films grown by atomic layer deposition is shown to provide the trap energy distribution across the entire oxide bandgap. The presence is revealed of two kinds of deep electron traps energetically distributed at around Et ≈ 2.0 eV and Et ≈ 3.0 eV below the oxide conduction band. Comparison of the trapped electron energy distributions in HfO2 layers prepared using different precursors or subjected to thermal treatment suggests that these centers are intrinsic in origin. However, the common assumption that these would implicate O vacancies cannot explain the charging behavior of HfO2, suggesting that alternative defect models should be considered.

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confidence: 99%

Intrinsic electron traps in atomic-layer deposited HfO2 insulators

Cerbu

Madia

Andreev

et al. 2016

Applied Physics Letters

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“…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.…”

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

Gavartin

Ramo

Shluger

et al. 2006

Applied Physics Letters

338

191

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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...

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“…Recent theoretical calculations predicted polaronlike electron trapping near neutral oxygen vacancies in m-HfO 2 [11][12][13] and in the hypothetical amorphous HfO 2 [14]. Here we investigate whether electrons and/or holes could be also trapped intrinsically by the perfect lattice.…”

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confidence: 99%

“…The properties of the bulk HfO 2 are very similar to those of ZrO 2 , which has much wider abundance and range of applications. The primitive unit cell of m-HfO 2 (space group P2 1 =c) contains 12 atoms and two anion sublattices: in one oxygen ions are threefold coordinated (3C) and in the other -fourfold coordinated (4C).Recent theoretical calculations predicted polaronlike electron trapping near neutral oxygen vacancies in m-HfO 2 [11][12][13] and in the hypothetical amorphous HfO 2 [14]. Here we investigate whether electrons and/or holes could be also trapped intrinsically by the perfect lattice.…”

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confidence: 99%

Theoretical Prediction of Intrinsic Self-Trapping of Electrons and Holes in MonoclinicHfO2

Ramo

Shluger

Gavartin³

et al. 2007

Phys. Rev. Lett.

139

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We predict, by means of ab initio calculations, stable electron and hole polaron states in perfect monoclinic HfO 2 . Hole polarons are localized on oxygen atoms in the two oxygen sublattices. An electron polaron is localized on hafnium atoms. Small barriers for polaron hopping suggest relatively high mobility of trapped charges. The one-electron energy levels in the gap, optical transition energies and ESR g-tensor components are calculated. DOI: 10.1103/PhysRevLett.99.155504 PACS numbers: 61.72.Bb, 61.72.Ji, 71.15.Mb, 71.20.ÿb Control over carrier mobility in semiconductors and insulators under charge injection, photogeneration, or doping conditions is of enormous practical importance. In particular, the possibility of small electron and hole polaron formation in perfect deformable lattice (otherwise called self-trapping) has been considered in many studies (see for recent reviews). However, proving polaron self-trapping remains extremely challenging. Essentially, the subject of debate is whether the carrier trapping takes place at preexisting precursor states (shallow donor, acceptor, or defect states, disorder fluctuations) or whether the injected carriers self-trap in a perfect lattice, where the potential well is only created by the carrierinduced lattice polarization.Experimentally, distinguishing between trapping at defect sites and self-trapping in the perfect lattice is difficult due to crystal imperfection and generally small values of polaron self-trapping and migration energies. Reliable theoretical predictions, on the other hand, are rare, due to extreme sensitivity of the interplay between potential and kinetic energy of a polaron to a chosen Hamiltonian and boundary conditions (see, for example, Refs. [5,6]). The spectroscopic properties and diffusion barrier have been calculated for a number of systems, such as the selftrapped hole in alkali halides (V k center) [2,4], electron polaron in TiO 2 [7], and for a number of predominantly hole small polarons trapped at impurities (see, for example, Ref.[8]) and in amorphous silica [9].In this Letter we predict the self-trapping of both electrons and holes in the perfect monoclinic hafnium oxide (m-HfO 2 ) using static approach and density functional theory. HfO 2 has been in the spotlight of both scientists and engineers over the last ten years as a potential substitute for SiO 2 as a gate oxide in metal-oxidesemiconductor field-effect transistors (MOSFETs) [10]. The electron trapping in the dielectric layer in these devices may lead to degradation of their performance and reliability. The properties of the bulk HfO 2 are very similar to those of ZrO 2 , which has much wider abundance and range of applications. The primitive unit cell of m-HfO 2 (space group P2 1 =c) contains 12 atoms and two anion sublattices: in one oxygen ions are threefold coordinated (3C) and in the other -fourfold coordinated (4C).Recent theoretical calculations predicted polaronlike electron trapping near neutral oxygen vacancies in m-HfO 2 [11][12][13] and in the hypothet...

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Defect energy levels in HfO2 high-dielectric-constant gate oxide

Cited by 490 publications

References 28 publications

Intrinsic electron traps in atomic-layer deposited HfO2 insulators

Intrinsic electron traps in atomic-layer deposited HfO2 insulators

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

Theoretical Prediction of Intrinsic Self-Trapping of Electrons and Holes in MonoclinicHfO2

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