First principles study of the oxygen vacancy formation and the induced defect states in hafnium silicates

Abstract

Using first-principles density functional theory calculations, we have investigated the O vacancy formation and the relevant induced defect states in hafnium silicates over a wide range of compositions. The PBE0 hybrid density functional was employed for the analysis of the electronic properties and the charge transition levels of the O vacancy in crystalline HfSiO4 and in amorphous Hf-silicates, respectively. Based on the generated structure models, eight typical kinds of O coordination structures were identified in amorphous Hf-silicates. Our calculated results show that the positions of the induced defect energy levels in the band gap and the formation energies of O vacancy are largely determined by the local structures of the vacancy sites, which appear to be nearly independent of the composition of amorphous Hf-silicates. Our calculations also show that O vacancy can possess the negative-U behavior in crystalline HfSiO4 but not in amorphous Hf-silicates, where most of the O vacancies can simply exhibit the negative-U behavior as in the positive charge states. Given the measured band offset of 3.40 eV between Si and amorphous Hf-silicates, a considerable number of O vacancies were found to prefer to stay in the charge neutral state as the Fermi level lies within the band gap region of Si. Furthermore, due to its relatively higher formation energy, the concentration of O vacancy in Hf-silicates can be much lower than that in m-HfO2 when the Fermi level lies below the midgap region of Si. Accordingly, a significantly reduced flat band voltage shift and less transient threshold voltage instability can be found in Hf-silicates as compared with m-HfO2, which are in good agreement with the recent experimental findings.

Microstructure and optical properties of Pr3+-doped hafnium silicate films

“…To investigate this behavior in detail, we take interest in the strong background PL from 350 to 550 nm for the layers annealed at 800°C to 900°C in Figure 4c. This broad band may be ascribed to more than one kind of defect [5,6,27]. For the layers annealed at higher T A such as 1,000°C, the intensity of this PL band drops deeply while the Pr 3+ PL intensity increases notably.…”

Evolution of Defect-Associated Subband Energy States in Nanocrystalline TiO2 Films on Si and Ge Substrates

“…In extensive studies on subband states in TiO 2 , it was established that these subband states are primarily due to O-vacancies. 6,16) In single-crystal TiO 2 , O-vacancies are supposed to be distributed in the lattice sites, but in polycrystalline TiO 2 , O-vacancies are highly localized at the grain boundaries, where the severe local bonding distortion is mainly present. This difference in the physical origins of O-vacancies, either as bulk or grain boundary defects, appeared as the distinctive O-vacancyrelated subband spectra in the " 2 spectra of Fig.…”

“…Later calculations resolved the convergence issues of defect states in TM/RE oxides by taking into consideration the factors mentioned above. 6) In addition to the asymmetric local bonding structure of nanocrystalline TM/RE oxides, the channel/high-k-oxide interfacial chemistry is considered to be a crucial factor in controlling the device performance of MOS gate stacks because the chemical interaction between the semiconductor and the high-k oxide is typically strong enough to result in chemically intermixed interfacial layers. 7) This interfacial layer greatly affects the gate stack properties by altering the effective dielectric constant value, interfacial trap density, and effective channel mobility.…”

Correlation of luminescence measurements to the structural characterization of Pr3+-doped HfSiOx

“…Unlike the CL results reported above for the layer annealed at 800 • C, the corresponding PL spectrum exhibits only a very weak signal at 493 nm which may originates from the 3 P 0 → 3 H 4 transition of Pr 3+ ions. The broad PL band observed between 350 and 650 nm was already present in the PL spectrum of undoped sample and was attributed to the response of host material defects under the UV excitation wavelength [27,28]. As Pr 3+ ions do not present an absorption band at 285 nm, the excitation energy used here does not allow a direct excitation of Pr 3+ ions.…”