HfO2 SiO2 Research Articles

Plasma-enhanced atomic-layer-deposited HfO2–SiO2 nanolaminates for broadband antireflection coatings

The ever-increasing demand for broadband antireflection (AR) coatings has led to a growing interest in nanolaminates (NLs) with tunable properties. However, achieving excellent optical performance in NLs can be challenging due to multiple interfaces and a rather thin sublayer. In this study, we systematically investigated the factor that deteriorates the optical performance, focusing on the sublayer thickness deviation, sublayer n deviation, and interfacial diffusion, to correct the n-profiles of NLs. A pre-correction strategy for the AR coating design is proposed and experimentally demonstrated via plasma-enhanced atomic layer deposition. The excellent fit of the spectrum demonstrates that the proposed design strategy can benefit structure-sensitive NL-based coatings.

Hybrid-material-based mirror coatings for picosecond laser applications

Four hybrid materials (HfO2/Al2O3 nanolaminate, HfO2/SiO2 nanolaminate, HfO2–Al2O3 mixture, and HfO2–SiO2 mixture) and four hybrid-material-based mirror coatings are prepared via electron-beam evaporation. The refractive index (n), optical bandgaps, and microstructural properties of the hybrid materials are investigated and compared with those of pure materials. Experimental results show that the hybrid materials suppress crystallization, reduce surface roughness, and provide tunable bandgaps and optical constants. Among the four hybrid materials with similar n values, the HfO2–Al2O3 mixture shows the largest bandgap. All the hybrid-material-based mirror coatings exhibit a higher laser-induced damage threshold (LIDT) at 8 picosecond (ps) than the conventional pure-material-based mirror coating. The HfO2–Al2O3 mixture-based mirror coating with a relative high LIDT is used for further study of the pulse width dependence of the LIDT, and shows superior LIDT over the conventional mirror coating at 0.5 ps, 1 ps and 3 ps. Typical damage morphological features induced at different pulse widths show that the damage-initiation mechanism of the mirror coating changes around 3 ps.

Horizontally aligned ALD-SnO films grown on SiO2-passivated high-k HfO2 dielectrics for high-mobility and low-power P-channel thin-film transistor

The fabrication of SiO2-passivated HfO2 (HfO2/SiO2) as a high-k dielectric stack for high-mobility, low-power p-channel SnO thin-film transistors (TFTs) is reported herein. To achieve superior TFT performance, a high-k HfO2 film was introduced by replacing the thermally-grown SiO2. However, the atomic layer deposited (ALD)-SnO films grown on the HfO2 substrate exhibited a very rough surface and out-of-plane tilted SnO (001), resulting in a significantly degraded field-effect mobility (μFE) compared with that of SnO TFTs based on an SiO2 gate dielectric. Therefore, 1–10 nm-thick ALD-SiO2 passivation layers were deposited on high-k HfO2 to effectively improve the surface morphology of the ALD SnO. Furthermore, SiO2 passivation enabled the control of the crystallographic orientation of the ALD SnO. Through SiO2 passivation, the horizontally aligned SnO (00l) planes parallel to the substrate demonstrated significantly improved μFE characteristics. The optimized SnO TFTs fabricated with HfO2/SiO2 stacks exhibited excellent carrier transport and low-voltage operation characteristics, with μFE of 2.3 cm2/V·s, subthreshold swing of 0.5 V/dec, on-to-off drain current ratio of 1.4 × 104, and Vth of −1.2 V.

Study of short-wavelength pass dichroic laser mirror coatings with hafnia–silica mixture layers

Hafnia/silica (HfO2/SiO2) short-wavelength pass dichroic laser mirror (DLM) coatings are widely utilized in high-power laser systems. However, the half-wave hole effect resulting from the refractive index inhomogeneity of HfO2 severely restricts the spectra and potential applications of these coatings. To address this issue, an investigation was conducted on HfO2–SiO2 mixture coatings with different proportions, which were produced by electron beam co-evaporation. Compared to the inhomogeneous growth characteristics observed in pure HfO2 coatings, the introduction of 10 % or higher SiO2 admixing in HfO2–SiO2 mixture coatings results in an amorphous structure characterized by excellent refractive index homogeneity and improved surface smoothness. Leveraging this, different DLM coatings with the objective of achieving high transmittance across the 400 to 900 nm range were designed and prepared, while ensuring high reflectance at 1064 nm. The incorporation of HfO2–SiO2 mixture layers instead of pure HfO2 layers in the mirror coatings led to a fundamental inhibition of the half-wave hole effect throughout the entire broad transmission band, resulting in a substantial improvement in the optical performance of the coatings. Moreover, by incorporating several types of HfO2–SiO2 mixture layers with different proportions in the coating stacks, the transmittance of DLM coatings in the pass band can be further increased, and the transmission fluctuations at different wavelengths can be further compressed. Additionally, compared to traditional DLM coatings with HfO2 layers, DLM coatings incorporating HfO2–SiO2 mixture layers exhibit lower roughness, higher laser-induced damage threshold (LIDT) at 532 nm, and similar LIDT at 1064 nm. These results indicate a promising strategy for fabricating DLM coatings tailored for high-power laser systems.

Synergistic effects in MOS capacitors with an Au/HfO2–SiO2/Si structure irradiated with neutron and gamma ray

The properties of oxide trapped charges and interface state density in the metal oxide semiconductor (MOS) capacitors with an Au/HfO2–SiO2/Si structure were investigated under irradiation of 14 MeV neutron and 60Co gamma-ray. In the mixed neutron and gamma irradiation environment, the formation of the oxide trapped charges in the HfO2–SiO2 layer is determined by the total deposited ionization energy, i.e. the sum of ionization energy deposition of the neutrons and the accompanying gamma rays, while the influence of the displacement damage caused by 14 MeV neutrons can be ignored. The interface state density depends not only on the ionizing energy loss (IEL) but also the non-IEL (NIEL), and NIEL plays a major role below the critical neutron fluence of 4.5 × 1012 n cm−2. The synergistic effect of the interface state is observed increases with energy deposition in the oxide at lower fluences, while decreasing above the critical fluence. These results confirm the existence of the synergistic effect of neutron and gamma irradiation in damaging HfO2 MOS devices.

Microstructure of High Temperature Oxidation Resistant Hf6B10Si31C2N50 and Hf7B10Si32C2N44 Films

High-temperature oxidation resistant amorphous Hf6B10Si31C2N50 and Hf7B10Si32C2N44 films were deposited by reactive pulsed dc magnetron sputtering. To investigate the oxidation mechanism, the films were annealed up to 1500 °C in air. The evolved microstructures were studied by X-ray diffraction and transmission electron microscopy. A three-layered microstructure was developed upon exposure to high temperature. An oxidized layer formed at the top surface for both films consisting of monoclinic and/or orthorhombic m-/o-HfO2 nanoparticles embedded in an amorphous SiOx-based matrix. The as-deposited bottom layer of the films remained amorphous (Hf6B10Si31C2N50) or partially recrystallized (Hf7B10Si32C2N44) exhibiting a h-Si3N4 and HfCxN1−x distribution along with formation of t-HfO2 at its top section. The two layers were separated by a partially oxidized transition layer composed of nanocrystalline h-Si3N4 and tetragonal t-HfO2. The oxidation process initiates at the bottom/transition layer interface with oxidation of Hf-rich domains either in the amorphous structure or in HfCxN1−x nanoparticles resulting in t-HfO2 separated by Si3N4 domains. The second stage occurs at the oxidized/transition layer interface characterized by densely packed HfO2, Si3N4 and quartz SiO2 nanostructures that can act as a barrier for oxygen diffusion. The small t-HfO2 nanoparticles merge and transform into large m-/o-HfO2 while h-Si3N4 forms amorphous SiOx matrix. A similar oxidation mechanism was observed in both films despite the different microstructures developed.

Hafnium silicate formation during the reaction of β‐cristobalite SiO2 and monoclinic HfO2 particles

AbstractHafnium silicate (HfSiO4; hafnon) is under consideration as an environmental barrier coating material for high‐temperature applications. However, its rate of formation from mixtures of monoclinic HfO2 and crystalline (β‐cristobalite) SiO2 powders is unknown. Here it has been synthesized and its formation rate measured during their solid‐state reaction at temperatures from 1250°C to 1400°C. Rietveld refinement of X‐ray diffraction patterns indicates that at 1250°C the hafnon phase fraction increases linearly with time, while at the highest reaction temperature, the hafnon phase fraction exhibited a parabolic dependence upon time. Between these two limiting temperatures, a region of linear behavior preceded a transition to parabolic kinetics, with the transition occurring at an earlier time as the reaction temperature increased. Arrhenius relations fitted the kinetics of hafnium silicate formation in both the linear and parabolic regimes. Scanning electron microscopy indicated that the reaction proceeded by diffusion of SiO2 into HfO2, similar to the mechanism by which zirconium silicate has been formed from vitreous SiO2 and tetragonal ZrO2. The initial linear rate of reaction is consistent with the growth of the contact area between the SiO2 and HfO2 particles combined with rapid permeation of the Si4+ and O2− through the initial, incompletely formed hafnon. After a thin hafnon layer had formed between the reactants, the rate of hafnium silicate growth slowed and further growth was governed by the rate of diffusion of Si4+ and O2− through the reaction product consistent with the observed parabolic dependence of the phase fraction upon time.

Effects of oxygen conditions during deposition on memory performance of metal/HfO2/SiO2/Si structured charge trapping memory

In this work, we compare the storage performance of two Al–HfO2–SiO2–Si structured charge trapping memory (CTM) devices, where the charge trapping layer (HfO2) is grown under an anaerobic or aerobic condition, respectively. The device based on the HfO2 film deposited under an anaerobic condition shows a large memory window of 4.1 V at a scan voltage of ±11 V. Moreover, it has better retention characteristics and has a charge loss rate as low as 1.1% after being retained for 104 s. The HfO2 film has a higher oxygen vacancy defect density and is capable of trapping charges more efficiently. We characterize the elemental composition of HfO2 thin films in single-layer CTM devices by XPS. It can be concluded that there are more oxygen defects in the HfO2 films deposited under an anaerobic condition. In addition, the defect states of HfO2 films deposited under both anaerobic and aerobic conditions are investigated by photoluminescence and photoluminescence excitation measurements. It is found that the defect states are distributed in the deep levels from 2.3 eV to 2.9 eV below the conduction band. We provide further insights into the appropriate oxygen condition for the production of CTM devices based on the HfO2 thin films.

Study on Brewster angle thin film polarizer using hafnia–silica mixture as high-refractive-index material

Two kinds of polarizer coatings were prepared by electron beam evaporation, using HfO2–SiO2 mixture and HfO2 as the high-refractive-index materials, respectively. The HfO2–SiO2 mixture layer was implemented by coevaporating SiO2 and metal Hf, the materials were deposited at an oxygen atmosphere to achieve stoichiometric coatings. The certain HfO2 and SiO2 content ratio is controlled by adjusting the deposition rate of HfO2 and SiO2 using individual quartz crystal monitor. The spectral performance, surface and interfacial properties, as well as the laser-induced damage performance were studied and compared. Comparing with polarizer coating using HfO2 as high-refractive-index material, the polarizer coating using HfO2–SiO2 mixture as high-refractive-index material shows better performance with broader polarizing bandwidth, lower surface roughness, better interfacial property while maintaining high laser-induced damage threshold.

Laser ablation behavior of Cf/SiHfBCN ceramic matrix composites

The behavior of Cf/SiHfBCN composites upon laser ablation was investigated. The evolution of microstructure and phase composition of the composite was studied by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It is found that the ablation mechanism and the microstructure evolution are mainly dependent on the local temperature which directly relies on the distance from the laser spot and the thermal conduction of the carbon fibers. Three regions with different ablation behavior are identified. In the center region, bubble-like structures consisting of HfO2, and SiHfxOy-based layers, cover the ends of the carbon fibers which retain their original shape despite their corrosion. Sheets consisting of HfCxOy and SiO2 as well as porous HfO2 and liquid SiO2 are generated in the transition region. The carbon fibers are well protected from oxidation in the two regions. SiO2 is produced and suppresses the further oxidation of the matrix in the fringe region.

Glass–water interaction: Effect of high-valence cations on glass structure and chemical durability

Borosilicate glass is a durable solid, but it dissolves when in contact with aqueous fluids. The dissolution mechanism, which involves a variety of sequential reactions that occur at the solid–fluid interface, has important implications for the corrosion resistance of industrial and nuclear waste glasses. In this study, spectroscopic measurements, dissolution experiments, and Monte Carlo simulations were performed to investigate the effect of high-valence cations (HVC) on the mechanisms of glass dissolution under dilute and near-saturated conditions. Raman and NMR spectroscopy were used to determine the structural changes that occur in glass, specifically network formers (e.g., Al, Si, and B), with the addition of the HVC element hafnium in the Na2O–Al2O3–B2O3–HfO2–SiO2 system (e.g., Na/[Al+B]=1.0 and HfO2/SiO2 from 0.0 to 0.42). Spectroscopic measurements revealed that increasing hafnium content decreases N4 (tetrahedral boron/total boron) and increases the amount of Si–O–Hf moieties in the glass. Results from flow-through experiments conducted under dilute and near-saturated conditions show a decrease of approximately 100× or more in the dissolution rate over the series from 0 to 20mol% HfO2. Comparing the average steady-state rates obtained under dilute conditions to the rates obtained for near-saturated conditions reveals a divergence in the magnitude between the average steady state rates measured in these different conditions. The reason for this divergence was investigated more thoroughly using Monte Carlo simulations. Simulations indicate that the divergence in glass dissolution behavior under dilute and near-saturated conditions result from the stronger binding of Si sites that deposit on the surface from the influent when Hf is present in the glass. As a result, the residence time at the glass surface of these newly-formed Si sites is longer in the presence of Hf, which increases the density of anchor sites from which altered layers with higher Si densities can form. These results illustrate the importance of understanding solid–water/solid–fluid interactions by linking macroscopic reaction kinetics to nanometer scale interfacial processes.

Variation of coating transmission under intense laser irradiation

A model that considers both thermal expansion and thermo-optical effects is developed to investigate the transmission variation of optical coatings when they are exposed to an intense laser beam. Our results indicate that a higher gradient of the transmission spectrum curve at a certain wavelength leads to a more evident variation of the coating transmission. Three customized HfO2–SiO2 multilayer coatings with different transmission spectra are used to measure the transmitted power under the irradiation of a 1080 nm continuous-fiber laser. Excellent agreement is found between the experimental result and the theoretical prediction. Our result is helpful for the improvement of such devices in the application of high-power laser systems.

Erasing Characteristics of Stacked-Gate Flash Memories with High-k Dielectrics

By 2-D MEDICI simulation, flash memories with high-k IPDs clearly exhibit significant improvement in SFN erasing speed over those with conventional ONO IPD. Choosing HfO2 as the IPD, erasing voltage can be reduced larger than 48% at a typical 0.1ms erasing time due to significantly improved gate coupling ratio. However, effects of high-k TDs on erase performance of the stacked-gate flash memories with fixed HfO2 IPD is quite contrary with respect to the improvement observed for the flash memory with high-k IPDs. Due to reduced gate coupling ratio, erasing speed of the stacked-gate flash memories with high-k TDs using SFN tunneling is helpless. Compared to the flash memory with HfO2 IPD and SiO2 TD, erasing voltage for the flash memory with both HfO2 IPD and TD will degrade erasing voltage larger than 41% at a 0.1ms erasing time. On the other hand, increased electric field on HfO2 IPD would produce excess charge loss and narrow the operation window between programmed and erased state.

Sol–gel derived silica-based organic–inorganic hybrid materials as “composite precursors” for the synthesis of highly homogeneous nanostructured mixed oxides: an overview

The present contribution reports on our results concerning the synthesis of different binary and ternary oxide systems by using hybrid materials as “composite” precursors. In the last years, we have developed and explored a valuable strategy to yield a very homogeneous dispersion of nanoparticles of early metal transition oxide, MO2 (M = Zr, Hf) inside a silica matrix. This route is based on the use of the sol–gel process to obtain organic–inorganic hybrid silica-based materials embedding the oxide precursors (Zr and/or Hf oxoclusters), which are then calcined at high (T > 500 °C) temperatures to give the desired oxides. The “precursor” hybrid materials are prepared by a modified sol–gel process, involving the copolymerisation of the organically modified oxozirconium or oxohafnium clusters (M4O2(OMc)12 (M = Zr, Hf and OMc = methacrylate) with (methacryloxymethyl)triethoxysilane (MAMTES) or (methacryloxypropyl)trimethoxysilane (MAPTMS). Free radical copolymerisation of the 12 methacrylate groups of the oxoclusters with the methacrylate-functionalised siloxanes allows a stable anchoring of the oxoclusters to the silica network formed by the hydrolysis and condensation of the alkoxy groups. The sol–gel reactions of the two methacrylate-modified silanes methacryloxymethyltriethoxysilane and methacryloxypropyltrimethoxysilane were followed by using two independent time-resolved spectroscopic methods, viz., IR ATR and NMR with the aim to optimise their pre-hydrolysis times and consequently their use as precursors for hybrid materials. As mentioned, thermal treatment at high temperature of the hybrid yields a very homogeneous dispersion of ZrO2 and/or HfO2 nanoparticles in the silica matrix, since the molecular homogeneity of the starting hybrid is retained in the final mixed oxide. This route was successfully applied both to the synthesis of bulk materials and thin films characterised by different compositions (in term of M/Si molar ratios and M nature), heating route (conventional or microwave-assisted) and final temperature of annealing (from RT to 1,100 °C). The first example of the ZrO2–HfO2–SiO2 ternary oxide system was also prepared by this approach. The prepared systems, both in the form of hybrid materials as well as in the final form of binary or ternary oxides, were thoroughly characterised by a wide variety of analytical tools from a compositional, structural, morphological point of view. Moreover, in the case of the binary ZrO2–SiO2 bulk materials, also the evolution under heating was followed by different methods. In particular, the composition of the hybrid as well as of the final oxidic materials was determined by X-Ray Photoelectron Spectroscopy and elemental analysis, whereas FT-IR and multinuclear solid-state NMR spectroscopies shed light on the changes occurring in the composition upon thermal heating and the degree of condensation of the silica network. The morphology and the microstructure of the hybrids and of the oxides were studied by nitrogen sorption and Scanning Electron Microscopy. X-Ray Diffraction, Transmission Electron Microscopy and X-ray Absorption Fine Structure Spectroscopy X-ray Absorption Fine Structure Spectroscopy were used to follow the conversion of the amorphous oxides to the final materials consisting of crystalline zirconia or hafnia dispersed in amorphous silica. On selected systems, functional properties (surface reactivity, dielectric properties) were furthermore investigated. The obtained binary oxides were also used as substrates for functionalisation experiments with (1) dialkycarbamates and (2) long alkyl chains to produce functional materials for catalysis and HPLC applications, respectively.

Comparative study of CNT, silicon nanowire and fullerene embedded multilayer high-k gate dielectric MOS memory devices

Here, we present a comparative theoretical study on stacked (multilayer) gate dielectric MOS memory devices, having a metallic/semiconducting carbon nanotube (CNT), silicon nanowire (Si NW) and fullerene (C60) embedded nitride layer acting as a floating gate. Two types of devices, one with HfO2–SiO2 stack (stack-1) and the other with La2O3–SiO2 stack (stack-2) as the tunnel oxide were compared. We evaluated the effective barrier height, the dielectric constant and the effective electron mobility in the composite gate dielectric with the Maxwell–Garnett effective medium theory. Thereafter applying the WKB approximation, we simulated the Fowler–Nordheim (F–N) tunnelling/writing current and the direct tunnelling/leakage current in these devices. We evaluated the I–V characteristics, the charge decay and also the impact of CNT/Si NW aspect ratio and the volume fraction on the effective barrier height and the write voltage, respectively. We also simulated the write time, retention time and the erase time of these MOS devices. Based on the simulation results, it was concluded that the metallic CNT embedded stack-1 device offered the best performance in terms of higher F–N tunnelling current, lower direct tunnelling current and lesser write voltage and write time compared with the other devices. In case of direct tunnelling leakage and retention time it was found that the met CNT embedded stack-2 device showed better characteristics. For erasing, however, the C60 embedded stack-1 device showed the smallest erase time. When compared with earlier reports, it was seen that CNT, C60 and Si NW embedded devices all performed better than nanocrystalline Si embedded MOS non-volatile memories.

O-vacancies in (i) nanocrystalline HfO2 and (i) noncrystalline SiO2 and Si3N4 studied by x-ray absorption spectroscopy

Performance and reliability of semiconductor-oxide-metal devices with high-κ gate dielectrics are limited by electronically active O-atom vacancy defects. Synchrotron x-ray spectroscopy defect features have been interpreted using two-electron multiplet theory. This approach quantifies conduction band edge and pre-edge features assigned to intrinsic bonding effects. Theoretical studies based on density functional theory have identified these defects as neutral and negatively-charged O-atom vacancies in transition metal (TM) oxides including HfO2 and ZrO2. However, agreement between calculated electronic states and experiment has been less than satisfactory. In this paper the O-vacancy electronic structure is addressed using a theoretical approach extended from multiplet theory traditionally applied to occupied intrinsic, and alloy and impurity atom d states in TM oxides. An equivalent d2 model based occupation of two d orbitals of the TM atoms that border the neutral vacancy site has been used to determine...

Low‐temperature sputtered mixtures of high‐κ and high bandgap dielectrics for GIZO TFTs

Abstract— This paper discusses the properties of sputtered multicomponent amorphous dielectrics based on mixtures of high‐κ and high‐bandgap materials and their integration in oxide TFTs, with processing temperatures not exceeding 150°C. Even if Ta2O5 films are already amorphous, multicomponent materials such as Ta2O5—SiO2 and Ta2O5—Al2O3 allow an increase in the bandgap and the smoothness of the films, reducing their leakage current and improving (in the case of Ta2O5—SiO2) the dielectric/semiconductor interface properties when these dielectrics are integrated in TFTs. For HfO2‐ based dielectrics, the advantages of multicomponent materials are even clearer: while HfO2 films present a polycrystalline structure and a rough surface, HfO2—SiO2 films exhibit an amorphous structure and a very smooth surface. The integration of the multicomponent dielectrics in GIZO TFTs allows remarkable performance, comparable with that of GIZO TFTs using SiO2 deposited at 400°C by PECVD. For instance, with Ta2O5—SiO2 as the dielectric layer, field‐effect mobility of 35 cm2/(V‐sec), close to 0 V turn‐on voltage, an on/off ratio higher than 106, a subthreshold slope of 0.24 V/dec, and a small/recoverable threshold voltage shifts under constant current (ID= 10 μA) stress during 24 hours are achieved. Initial results with multilayers of SiO2/HfO2—SiO2/SiO2 are also shown, allowing a lower leakage current with lower thickness and excellent device performance.

Correlation between local structure and refractive index of e-beam evaporated (HfO2–SiO2) composite thin films

In the present work we have reported the results of investigations on local structures of e-beam evaporated (HfO2–SiO2) composite thin films by synchrotron based extended x-ray absorption fine structure measurements. It has been observed that for the composite film with 10% SiO2 content, both Hf–O and Hf–Hf bond lengths are less than their values in pure HfO2 film. However the bond lengths subsequently increase to higher values as the SiO2 content in the composite films is increased further. It has also been observed that at the same composition of 10% SiO2 content, the films have smallest grain sizes (as obtained from atomic force microscopy measurements) and highest refractive index (as obtained from spectroscopic ellipsometry (SE) measurements) which suggests that the e-beam evaporated HfO2–SiO2 composite films with 10% SiO2 content leads to the most compact amorphous thin film structure.

Complete band offset characterization of the HfO2/SiO2/Si stack using charge corrected x-ray photoelectron spectroscopy

The HfO2–Si valence and conduction band offsets (VBO and CBO, respectively) of technologically relevant HfO2/SiO2/Si film stacks have been measured by several methods, with several groups reporting values within a range of ∼1 eV for both quantities. In this study we have used a combination of x-ray photoemission spectroscopy (XPS) and spectroscopic ellipsometry to measure the HfO2–Si VBO and CBO of both as-deposited and annealed stacks. Unlike previous XPS based measurements of the HfO2–Si VBO, we have corrected for the effect of charging in the XPS measurement. We find that after correction for charging, the HfO2–Si VBOs are decreased from their typical XPS-measured values, and agree better with values measured by UV photoemission spectroscopy and internal photoemission. We also report values for the rarely reported HfO2–SiO2 and SiO2–Si VBOs and CBOs in HfO2/SiO2/Si stacks. In addition to the band offsets, XPS was used to measure the band bending in the Si substrate of HfO2/SiO2/Si film stacks. Unannealed HfO2 stacks showed downward Si band bending of 0.4–0.5 eV, while annealed HfO2 stacks showed negligible band bending. Finally, we investigated the composition of the SiO2 layer in SiO2/Si and HfO2/SiO2/Si. By decomposing the Si 2p spectra into the spin orbit partner lines of its five oxidation states we observed that the growth of the HfO2 films resulted in the growth of the SiO2 underlayer and an increase by a factor of ∼2.3 in the density of suboxide species of SiO2. Based on the relatively high binding energy of the Si 2p4+ level with respect to the Si 2p0 level and a survey of results from literature, we conclude that the SiO2 layer in the HfO2/SiO2/Si samples we measured does not undergo significant intermixing with HfO2.

Atomic mechanism of flat-band voltage shifts by La2O3 and Al2O3 in gate stacks

The origin of the flat-band voltage shifts for La- and Al-based oxide capping layers in high k metal gate stacks is studied by ab initio calculations on atomic models. Substitutional La, Al, Sr, and Nb at the HfO2–SiO2 interface create dipoles, which shift the flat band voltage in the experimentally observed direction, negative for La and Sr and positive for Al and Nb. The shift does not correlate with the metal’s valence, being opposite for La and Al, which rules out a vacancy model. The shift does correlate with the metal electronegativity/work function. It does not correlate with oxygen ion density, as this is not varied.