Hf Si Bonds Research Articles

Room temperature formation of Hf-silicate layer by pulsed laser deposition with Hf-Si-O ternary reaction control

We investigated the room temperature growth of HfO2 layers on Si substrates by pulsed laser deposition under ultra-high vacuum conditions. The laser fluence (LF) during HfO2 layer growth was varied as a growth parameter in the experiments. X-ray photoemission spectroscopy (XPS) was used to observe the interface chemical states of the HfO2/Si samples produced by various LFs. The XPS results indicated that an interface Hf-silicate layer formed, even at room temperature, and that the thickness of this layer increased with increasing pulsed LF. Additionally, Hf-Si bonds were increasingly formed at the interface when the LF was more than 2 J/cm2. This bond formation process was related to decomposition of HfO2 to its atomic states of Hf and O by multiphoton photochemical processes for bandgap excitation of the HfO2 polycrystalline target. However, the Hf-Si bond content of the interface Hf-silicate layer is controllable under high LF conditions. The results presented here represent a practical contribution to the development of room temperature processing of Hf-compound based devices.

Effect of RF power of post-deposition oxygen treatment on HfO2 gate dielectrics

The radio frequency (RF) power effect of post-deposition O2 plasma treatment on the physical, electrical, and reliability characteristics of high-k HfO2 dielectric films was comprehensively investigated in this study. The experimental results indicated that increasing the RF power of post-deposition O2 plasma treatment resulted in a stoichiometric HfO2 film, but led to a thinner interfacial layer and the formation of new HfSi bonds. Additionally, the electrical performance and reliability of HfO2 dielectric films were significantly impacted by the RF power of the post-deposition O2 plasma treatment. As the RF power is less than 30W, the leakage current density and time-to-breakdown of the O2 plasma-treated HfO2 films were improved in comparison with those of the as-deposited samples. However, further increasing RF power to exceed 50W would cause the continuous degradation in the electrical performance and reliability due to the plasma damage induced by oxygen active spices in a plasma environment. Therefore, performing a post-deposition O2 plasma treatment process on the as-deposited HfO2 dielectric films can effectively improve the dielectric's properties. However, the applied RF power is an essential controlling parameter, avoiding serious plasma damage occurrence.

RF Power Effect of Post-Deposition Oxygen Plasma Treatment on HfO2 Gate Dielectrics

The radio frequency (RF) power effect of post-deposition O2 plasma treatment on the physical, electrical, and reliability characteristics of high-k HfO2 dielectric films were comprehensively investigated in this study. The experimental results indicated that increasing the RF power of post-deposition O2 plasma treatment resulted in a stoichiometric HfO2 film, but led to a thinner interfacial layer and the formation of new Hf-Si bonds. Additionally, the electrical performance and reliability of HfO2 dielectric films were significantly impacted by the RF power of the post-deposition O2 plasma treatment. As RF power is less than 30 W, the leakage current density and time-to-breakdown of the O2 plasma-treated HfO2 films were improved in comparison with those of the as-deposited samples. However, further increasing RF power to exceed 50 W would cause the continuous degradation in the electrical performance and reliability due to the plasma damage induced by oxygen active spices in a plasma environment. Therefore, performing a post-deposition O2 plasma treatment process on the as-deposited HfO2 dielectric films can effectively improve the dielectric's properties. However, the applied RF power is an essential controlling parameter, avoiding serious plasma damage occurrence.

RF Power Effect of Post-Deposition Oxygen Plasma Treatment on HfO2 Gate Dielectrics

The RF power effect of post-deposition O2 plasma treatment on the physical, electrical characteristics, and reliability performance of high-k HfO2 dielectric films were investigated in this study. Increasing RF power in post-deposition O2 plasma treatment results in a stoichiometric HfO2 film, but leading to a thinner interfacial layer with a new Hf-Si bond. Additionally, the electrical performance and reliability of high-k HfO2 dielectric films were related to the RF power in post-deposition O2 plasma treatment. As RF power is less than 50 W, the leakage current density and time-to-breakdown were improved as compared to those of un-treated samples. However, these properties were continuously degraded with an increase of RF power due to plasma damage induced by oxygen active spices in a O2 plasma environment. Therefore, the post-deposition O2 plasma treatment is benefit in the improvement of HfO2 gate dielectric properties, but the plasma power is needed to be well controlled.

Band offset and interface chemistry of HfO2/Si (001) prepared by electron‐beam evaporation in ultrahigh vacuum using atomic oxygen

AbstractThe band offset and interface chemistry of HfO2 films prepared by ultrahigh‐vacuum electron‐beam evaporation was investigated by synchrotron radiation photon electron spectroscopy (SRPES). A relatively large valence band (VB) offset of the HfO2 film with Si was determined to be 3.56 eV. Both the Hf‐silicate and a small number of HfSi bonds were formed at the interface of HfO2/Si. This HfSi bond, which cannot be prevented completely in an ultrahigh‐vacuum preparation system using the Hf metallic source despite the atomic oxygen source adopted, may be mainly responsible for this large value of VB offset of HfO2/Si(001). This result also suggests that metal–silicon bonds at the interface of high k oxides on Si may generally lead to a larger VB offset than that with all silicon atoms at the interface bonding directly with oxygen.

Fermi-Level Pinning at the Poly-Si/HfO2 Interface

The high threshold voltage in metal-oxide-semiconductor field effect transistor (MOSFET) devices adopting hafnia for the gate oxide material is a serious concern for alternative high-κ gate oxides. The Fermi-level pinning at the highly-doped poly-Si/metal oxide interface has been reported as a cause because the interfacial metal-Si bonds can induce pinning of the Fermi level. In this work, the electronic structure and the chemical bonding states at the interface of n type and p type polySi/HfO2 were measured by using transmission electron microscopy (TEM) and X-ray photoemission spectroscopy (XPS) in order to find the reason for the high threshold voltage. The obtained Hf 4f core-level spectrum shows no Hf-Si state at the interface. Therefore, metallic Hf-Si bond formation at the interface may not be the reason for the high threshold voltage.

Density functional calculations on atomic and electronic structures of amorphous HfO2/Si(001) interface

The interface properties of amorphous hafnium dioxide (a-HfO2) in contact with silicon have been investigated by using the projector augmented wave method within the generalized gradient approximation. The a-HfO2 model structure of the interface is generated by ab initio molecular dynamics simulations in a melt-and-quench scheme. Calculations indicate that the simulated a-HfO2 essentially shows the characteristics of the experimental a-HfO2 structure. The results on a-HfO2/Si interface suggest that atomic coordination of interface Si atoms would significantly affect the interface electronic properties, e.g., the Hf–Si bond formed at the interface could result in metallic behavior. With band lineup of the core level, the valence band offset of a-HfO2/Si interface is determined to be 2.62±0.35 eV, in good agreement with recent experimental data.

Chemical structure and electrical properties of sputtered HfO 2 films on Si substrates annealed by rapid thermal annealing

The chemical structure and electrical properties of HfO 2 thin film grown by rf reactive magnetron sputtering after rapid thermal annealing (RTA) were investigated. The chemical composition of HfO 2 films and interfacial chemical structure of HfO 2/Si in relation to the RTA process were examined by X-ray photoelectron spectroscopy. Hf 4 f and O 1 s core level spectra suggest that the as-deposited HfO 2 film is nonstoichiometric and the peaks shift towards lower binding energy after RTA. The Hf–Si bonds at the HfO 2/Si interface can be broken after RTA to form Hf–Si–O bonds. The electrical characteristics of HfO 2 films were determined by capacitance–voltage ( C–V) and current density–voltage ( J–V) measurements. The results showed that the density of fixed charge and leakage current density of HfO 2 film were decreased after the RTA process in N 2 atmosphere.

Chemical Structure of HfO2/Si Interface with Angle-Resolved Synchrotron Radiation Photoemission Spectroscopy

Interfacial chemical structure of HfO2/Si (100) is investigated using angle-resolved synchrotron radiation photoemission spectroscopy (ARPES). The chemical states of Hf show that the Hf 4f binding energy changes with the probing depth and confirms the existence of Hf-Si-O and Hf-Si bonds. The Si 2p spectra are taken to make sure that the interfacial structure includes the Hf silicates, Hf silicides and SiOx. The metallic characteristic of the Hf-Si bonds is confirmed by the valence band spectra. The depth distribution model of this interface is established.

Hydrogen shuttling near Hf-defect complexes in Si∕SiO2∕HfO2 structures

We propose that a defect complex comprising a suboxide Hf–Si bond and an interfacial dangling bond is responsible for the stress-induced buildup of interface traps in Si∕SiO2∕HfO2 capacitors. With the aid of first-principles calculations, we show that these defects possess a symmetric double-well energy minimum with a moderate intervening barrier. The calculated activation energies suggest a relatively easy hopping of H atoms between the two energy minima (a field-aided shuttling mechanism). This mechanism can explain the experimentally measured oscillations of interface-trap densities during switched-bias conditions following x-ray irradiation or constant-voltage stress.

Modeling the silicon–hafnia interface

We report first-principles calculations of the structure and electronic properties of several different silicon–hafnia interfaces. The structures have been obtained by growing HfO x layers of different stoichiometry on Si(1 0 0) and by repeated annealing of the system using molecular dynamics. The interfaces are characterised via their electronic and geometric properties. Moreover, electronic transport through the interfaces has been calculated using finite-element-based Green's function methods. We find that oxygen always diffuses towards the interface to form a silicon dioxide layer. This results in the formation of dangling Hf bonds in the oxide, saturated by either Hf diffusion or formation of Hf–Si bonds. The generally poor performance of the interfaces suggests that it is important to stabilise the system with respect to oxygen lattice diffusion.

Threshold Voltage Instability and Low Frequency Noise in Hafnium-Based Gate Dielectrics

This paper focus on two of the main reliability problems observed in Hafnium-based gate dielectrics: threshold voltage instability and low frequency noise. Both issues are ascribed to the Hf-Si bonds generated at the HfO2/polysilicon interface. These defects act as charge capture centers whose trapping probability decreases with the inverse of the number of injected charges. In the case of HfO2/polysilicon gate stack the drain low frequency noise increases of two orders of magnitude and the gate low frequency noise increases of three orders of magnitude with respect to SiO2/polysilicon gate stack. By using HfSiON as gate dielectrics or metal as gate electrode both reliability problems are strongly reduced.

Interfacial oxide growth at silicon∕high-k oxide interfaces: First principles modeling of the Si–HfO2 interface

We have performed first principles calculations to investigate the structure and electronic properties of several different Si–HfOx interfaces. The atomic structure has been obtained by growing HfOx layer by layer on top of the Si(100) surface and repeatedly annealing the structure using ab initio molecular dynamics. The interfaces are characterized via their geometric and electronic properties, and also using electron transport calculations implementing a finite element based Green’s function method. We find that in all interfaces, oxygen diffuses towards the interface to form a silicon dioxide layer. This results in the formation of dangling Hf bonds in the oxide, which are saturated either by hafnium diffusion or Hf–Si bonds. The generally poor performance of these interfaces suggests that it is important to stabilize the system with respect to lattice oxygen diffusion.

Physical origin of threshold voltage problems in polycrystalline silicon/HfO2 gate stacks

Based on theoretical calculations, we find that at p+ polycrystalline silicon (poly-Si)∕HfO2 gates, Si interstitials are easily migrated from the electrode, forming Hf–Si bonds with a charge transfer to the electrode, and the resulting interface dipole raises the Fermi level of poly-Si toward the pinning level, causing high flat band voltage shifts. In O-rich grown HfO2 on Si substrates, the Si atoms substitute for the Hf sites, leading to the formation of Hf-silicate layers, while under O-poor conditions, they remain as interstitial defects, binding with the Hf atoms, and behave as a negative-U trap, which causes the threshold voltage instability.

First-principles calculations of the electrical properties ofLaAlO3and its interface with Si

$\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ is one of the potential candidates to replace $\mathrm{Si}{\mathrm{O}}_{2}$ as a high permittivity dielectric for future generations of metal-oxide-semiconductor field effect transistors. Using first-principles plane-wave calculations within density functional theory, its bulk and surface electronic properties and the relative stability of cubic $c\text{\ensuremath{-}}\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}(001)∕\mathrm{Si}(001)$ interfaces are investigated. In agreement with experiment, our study shows that the dielectric constant of crystalline $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ $(\ensuremath{\sim}30)$ is comparable to that of hexagonal ${\mathrm{La}}_{2}{\mathrm{O}}_{3}$. To accurately calculate the $c\text{\ensuremath{-}}\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}(001)$ surface energy, several ways of eliminating the surface dipole moment of the polar surface are presented, with the transfer of an oxygen anion from one boundary surface to the other being identified as the energetically most favorable mechanism. We have found that lanthanum-terminated $c\text{\ensuremath{-}}\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}(001)∕\mathrm{Si}(001)$ interfaces are in general more stable than aluminum-terminated interfaces for both the oxidized and nonoxidized Si(001) surfaces. We have also identified a significant reduction of the $c\text{\ensuremath{-}}\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}(001)∕\mathrm{Si}(001)$ valence band offset due to the creation of interface dipoles for O-rich interfaces. Analysis of the density of interface states shows that La-Si bonds at the $c\text{\ensuremath{-}}\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}(001)∕\mathrm{Si}(001)$ interface do not create interface states in the silicon band gap, in contrast to Hf-Si bonds in $m\text{\ensuremath{-}}\mathrm{Hf}{\mathrm{O}}_{2}(001)∕\mathrm{Si}(001)$ interfaces studied previously.

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.

Interface bonding structure of hafnium oxide prepared by direct sputtering of hafnium in oxygen

The interface properties of the hafnium gate oxide films prepared by direct sputtering of hafnium in oxygen with rapid thermal annealing have been investigated in detail. X-ray photoelectron spectroscopy reveals that the interface silicate layer is a random mixture of Hf–O, Si–O, Hf–Si, and excess Hf and Si atoms. The contributions of these bonds to the composition of silicate layer are governed by the Si/Hf ratio. At low Si/Hf ratio (<2), the silicate layer is a mixture of SiO4 and HfO4 phases. At higher Si/Hf ratio (2–5), the contribution of the HfO4 phase decreases and Hf–Si (silicide) bonds become important. At very high Si/Hf ratio (>9) and close to the substrate, Hf–Si dominates and the high strain Hf–Si bonds govern the electrical properties of the interface. These results explain the observed high interface trap density at the HfO2/Si interface and the soft breakdown behavior which is different from the silicon oxide film.

Two-step interfacial reaction of HfO 2 high- k gate dielectric thin films on Si

HfO 2 thin films have been deposited on p-type (1 0 0) Si substrates by pulsed laser deposition. Transmission electron microscopy observation illustrated that the HfO 2 films are in polycrystalline structure and the interface with Si substrate is free from amorphous layer. The rough feature of the film/Si interface suggested interfacial reaction and diffusion. Depth profile X-ray photoelectron spectroscopy (XPS) analysis results revealed the formation of Hf silicate and Hf silicide at the interface. A two-step reaction model was proposed to interpret the interfacial reaction. At initial stage of film growth, insufficient oxidation of Hf atoms on bare Si surface resulted in reaction of Hf with Si leading to silicidation of Hf, at least HfSi bonds formation. During the following film growth, the preformed Hf silicide was further oxidized leading to Hf silicate formation. This two-step reaction model suggests a gradient distribution of the film stack, that is, HfO 2/Hf silicate/Hf silicide/Si. It was found that with sufficient oxygen during film growth, the Hf silicide formation was suppressed.

Epitaxial growth of yttrium-stabilized HfO2 high-k gate dielectric thin films on Si

Epitaxial yttrium-stabilized HfO2 thin films were deposited on p-type (100) Si substrates by pulsed laser deposition at a relatively lower substrate temperature of 550 °C. Transmission electron microscopy observation revealed a fixed orientation relationship between the epitaxial film and Si; that is, (100)Si//(100)HfO2 and [001]Si//[001]HfO2. The film/Si interface is not atomically flat, suggesting possible interfacial reaction and diffusion. X-ray photoelectron spectrum analysis also revealed the interfacial reaction and diffusion evidenced by Hf silicate and Hf–Si bond formation at the interface. The epitaxial growth of the yttrium stabilized HfO2 thin film on bare Si is via a direct growth mechanism without involving the reaction between Hf atoms and SiO2 layer. High-frequency capacitance–voltage measurement on an as-grown 40-Å yttrium-stabilized HfO2 epitaxial film yielded an effective dielectric constant of about 14 and equivalent oxide thickness to SiO2 of 12 Å. The leakage current density is 7.0×10−2 A/cm2 at 1 V gate bias voltage.

Plasma-enhanced chemical vapor deposition and characterization of high-permittivity hafnium and zirconium silicate films

Deposition of hafnium silicate films with various hafnium contents was tried by plasma-enhanced chemical vapor deposition using tetraethoxysilane and a hafnium alkoxide. From x-ray photoelectron spectroscopy, the deposited films are confirmed to be silicate with Hf–O–Si bonds but without any Hf–Si bonds. The permittivity calculated from the capacitance of the accumulation layer increases monotonically with an increase in the hafnium content, whereas the optical band gap energy estimated from vacuum ultraviolet absorption spectra decreases. Similar results were obtained from zirconium silicate films deposited using tetraethoxysilane and a zirconium alkoxide. If we compare the films with the same hafnium or zirconium content, the hafnium silicate exhibits a higher permittivity and a larger band gap energy than the zirconium silicate.