Ka Xiong

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

This letter presents calculations of the energy levels of the oxygen vacancy and oxygen interstitial defects in HfO2 using density functional methods that do not need an empirical band gap correction. The levels are aligned to those of the Si channel using the known band offsets. The oxygen vacancy gives an energy level nearer the HfO2 conduction band and just above the Si gap, depending on its charge state. It is identified as the main electron trap in HfO2. The oxygen interstitial gives levels just above the oxide valence band.

Passivation of oxygen vacancy states in HfO2 by nitrogen

Nitrogen is known to reduce leakage currents and charge trapping in high-dielectric-constant gate oxides such as HfO2. We show that this occurs because nitrogen, substituting for oxygen atoms next to oxygen vacancy sites, repels the occupied gap states due to the neutral and positively charged oxygen vacancies out of the band gap into its conduction band. The state of the negatively charge vacancy is also repelled upwards but remains as a shallow gap state. This occurs because the vacancy becomes effectively positively charged; the adjacent Hf ions relax outwards from the vacancy and shift its states upwards. We show this using ab initio calculation methods which do not require an empirical correction to the band gap.

Defect states in the high-dielectric-constant gate oxide HfSiO4

Hafnium silicate has a high dielectric constant and is a leading candidate to act as a gate dielectric. The defect energy levels have been calculated. The oxygen vacancy is found to give rise to Si-like levels which lie within the band gap of Si. The vacancy states are very localized and are localized on the neighboring Si sites. A second defect level high in the oxide gap is localized on the Hf sites. The behavior of ZrSiO4 is similar.

Defect states in the high-dielectric-constant gate oxide LaAlO3

We present calculations of the energy levels of the oxygen vacancy, AlLa antisite, and oxygen interstitial defects in LaAlO3 using density functional methods that do not need an empirical band gap correction. The levels are aligned to those of the Si channel using the known band offsets. The oxygen vacancy gives an energy level near the LaAlO3 conduction band and above the Si gap. It is identified as the main electron trap and the cause of instability. The AlLa antisite gives a state near midgap, neutral when empty, which would be an important trap, with no counterpart in HfO2.

Fermi level pinning and Hf–Si bonds at HfO2: Polycrystalline silicon gate electrode interfaces

Doped polycrystalline Si (poly-Si) gate electrodes on HfO2 films on Si substrates are found not to cause as large shifts in the flat band voltage as those of SiO2 on Si. This effect has been attributed to a weak pinning of the Fermi level at the top poly-Si HfO2 interface. The effect is shown to be consistent with the formation of Hf–Si bonds at an otherwise O-terminated oxide interface. Vacancies, divacancies, and substitutional Si atoms are introduced into models of oxygen-terminated Si–HfO2 (100) interfaces and the resulting Hf–Si bonds are found to create a metallic interface with the Fermi level pinned at about 0.3 eV below the Si conduction band-edge.

Atomic structure, electronic structure, and band offsets at Ge:GeO:GeO2 interfaces

The band gaps of GeO2 and GeO are calculated to be 6.1 and 4.0 eV, respectively, using hybrid density functionals that do not require band gap corrections. The conduction band offsets for Ge:GeO2 and Ge:GeO interfaces are calculated to be 0.8 and 0.4 eV, respectively, relatively small and similar to those found by photoemission. The atomic structure of solid GeO is found to have threefold coordinated Ge and O sites with planar oxygen sites.

Point defects in ZrO/sub 2/ high-/spl kappa/ gate oxide

This paper presents calculations of the electrical energy levels of the main point defects in ZrO/sub 2/, the oxygen vacancy and the oxygen interstitial. The levels are aligned to those of the Si channel using the known band offsets. The oxygen vacancy gives an energy level in the Si gap or just above the gap, depending on its charge state. This is the main electrically active defect and trap in ZrO/sub 2/ films. The oxygen interstitial gives levels just above the oxide valence band, and the neutral interstitial also gives a level near the Si conduction band.

Low Resistance Ohmic Contacts to Bi2Te3 Using Ni and Co Metallization

A detailed study of the impact of surface preparation and postdeposition annealing on contact resistivity for sputtered Ni and Co contacts to thin-film Bi 2 Te 3 is presented. The specific contact resistivity is obtained using the transfer length method. It is observed that in situ sputter cleaning using Ar bombardment before metal deposition gives a surface free of oxides and other contaminants. This surface treatment reduces the contact resistivity by more than 10 times for both Ni and Co contacts. Postdeposition annealing at 100°C on samples that were sputter-cleaned further reduces the contact resistivity to < 10 ―7 Ω cm 2 for both Ni and Co contacts to Bi 2 Te 3 . Co as a suitable contact metal to Bi 2 Te 3 is reported. Co provided similar contact resistance values as Ni, but had better adhesion and less diffusion into the thermoelectric material, making it a suitable candidate for contact metallization to Bi 2 Te 3 based devices.

Hydrogen-induced defects and degradation in oxide ferroelectrics

We investigate hydrogen-induced defects in BaTiO3, PbTiO3, PbZrO3, and strontium bismuth tantalate (SBT). We find that interstitial hydrogen produces a shallow level and OH− ions leading to loss of switchable polarization if the oxide’s band gap is under about 4.2eV, but hydrogen is deep in wide gap compounds like BaZrO3. In SBT, hydrogen is more stable in the Bi–O layer.

Impact of incorporated Al on the TiN/HfO2 interface effective work function

First principles calculations of the impact of Al incorporation on the effective work function of a TiN/HfO2 interface are presented. The undoped interface has a midgap effective work function. We find that Al in the metal and Al substituting for O in the dielectric make the effective work function more n-type. More importantly, Al substituting for Hf in the oxide near the interface—the energetically stable position for most growth conditions—increases the effective work function, making it more p-type. Furthermore, the shift of the work function increases with increasing the Al concentration at the interface. The calculated results are consistent with experimental data.