Hf Oxide Research Articles

Effect of Hf and Y alloy additions on aluminide coating performance

Abstract Iron- and Ni-base alloys, with and without Hf or Hf and Y alloy additions, were aluminized by chemical vapor deposition to study the potential for minor alloy additions to improve oxidation resistance of coated alloys. Compared to uncoated specimens, the coated specimens showed improved cyclic oxidation resistance at 1100° and 1150 °C. However, alumina scale spallation was observed at relatively short times and, particularly for the Ni-base alloy X, the aluminized lab-cast alloy with Hf tended to have poor coating performance compared to the commercial alloy without Hf. Internal oxidation of Hf at 1150 °C and rapid Al depletion in the relatively thin aluminide coatings contributed to the observed detrimental Hf effect. For the Ni-base alloys, the increased scale spallation could be attributed to much higher S contents (10–50 ppma) in the laboratory-cast alloys. Oxide scale spallation from the coating surface was minimized when Hf and Y were added to a casting and the [Y]/[S] content ratio was ∼ 1.

Quasiparticle calculations of the electronic properties ofZrO2andHfO2polymorphs and their interface with Si

Quasiparticle calculations are performed to investigate the electronic band structures of various polymorphs of Hf and Zr oxides. The corrections with respect to density-functional-theory results are found to depend only weakly on the crystal structure. Based on these bulk calculations as well as those for bulk Si, the effect of quasiparticle corrections is also investigated for the band offsets at the interface between these oxides and Si assuming that the lineup of the potential at the interface is reproduced correctly within density-functional theory. On the one hand, the valence-band offsets are practically unchanged with a correction of a few tenths of electron volts. On the other hand, conduction-band offsets are raised by 1.3--1.5 eV. When applied to existing calculations for the offsets at the density-functional-theory level, our quasiparticle corrections provide results in good agreement with the experiment.

Nano-regime Length Scales Extracted from the First Sharp Diffraction Peak in Non-crystalline SiO2 and Related Materials: Device Applications

This paper distinguishes between two different scales of medium range order, MRO, in non-crystalline SiO2: (1) the first is ~0.4 to 0.5 nm and is obtained from the position of the first sharp diffraction peak, FSDP, in the X-ray diffraction structure factor, S(Q), and (2) the second is ~1 nm and is calculated from the FSDP full-width-at-half-maximum FWHM. Many-electron calculations yield Si–O third- and O–O fourth-nearest-neighbor bonding distances in the same 0.4–0.5 nm MRO regime. These derive from the availability of empty Si dπ orbitals for back-donation from occupied O pπ orbitals yielding narrow symmetry determined distributions of third neighbor Si–O, and fourth neighbor O–O distances. These are segments of six member rings contributing to connected six-member rings with ~1 nm length scale within the MRO regime. The unique properties of non-crystalline SiO2 are explained by the encapsulation of six-member ring clusters by five- and seven-member rings on average in a compliant hard-soft nano-scaled inhomogeneous network. This network structure minimizes macroscopic strain, reducing intrinsic bonding defects as well as defect precursors. This inhomogeneous CRN is enabling for applications including thermally grown ~1.5 nm SiO2 layers for Si field effect transistor devices to optical components with centimeter dimensions. There are qualitatively similar length scales in nano-crystalline HfO2 and phase separated Hf silicates based on the primitive unit cell, rather than a ring structure. Hf oxide dielectrics have recently been used as replacement dielectrics for a new generation of Si and Si/Ge devices heralding a transition into nano-scale circuits and systems on a Si chip.

Crystal structures of crown and aza-crown ether complexes with zirconium, hafnium, niobium, and tantalum fluorometallates

A systematic X-ray diffraction study of the interaction products of Zr(IV), Hf(IV), Nb(V), and Ta(V) oxides (fluorides) with crown-ethers (CEs) in aqueous solutions of hydrofluoric acid is performed. It is shown that oxygen-containing CEs form oxonium complexes with [NbF6]s- and [TaF6]s- hexafluorometallate anions. In two systems, [cis-syn-cis-DCH18C6-H3O][TaF6] and [B18C6·H3O][TaF6], the phenomenon of supramolecular isomerism is found, which is caused by a change in the conformation of the macrocycle or by a partial redistribution of intermolecular hydrogen bonds. The use of aza-crown ethers as extractants made it possible to extract unique hydrolytically unstable anions, the products of incomplete fluorine substitution for oxygen atoms in the starting oxides in the form of crystalline complexes with a composition of [(HA15C5)2][Ta2F10O] and [(HA18C6·H2O)(A18C6·H2O)] [(H2O)Nb2F9O]. In [(18C6)(H7O3)2×(Hf2F10·2H2O)], [(HA18C6)(M2F10·2H2O)·(H3O)·H2O], and [(H2DA18C6) (M2F10·2H2O)·2H2O] (M=Zr, Hf) complexes, the metals are extracted in the form of identical (M2F10·2H2O)2s- anions with a similar topology. The performed study demonstrates that macrocyclic complexones are undoubtedly promising to extract Zr(IV), Hf(IV), Nb(V), and Ta(V) from fluorine-containing aqueous solutions.

Atomic layer deposition of HfO 2: Growth initiation study on metallic underlayers

The initial stages of HfO 2 film growth by atomic layer deposition (ALD) on Co, Ta, and Pt thin films have been compared by x-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transform infrared spectroscopy. The initial tetrakis-dimethylamido(IV) hafnium (TDMAH) adsorption rate was highest on Co and increased with substrate temperature between 150 °C and 250 °C. The initial adsorption rate of TDMAH on Ta was less than on Co or Pt at the same substrate temperatures. The Ta surface also proved to be more stable with regard to TDMAH decomposition as fluctuations of adsorbed Hf and decomposition products were not observed during ALD cycling. Larger amounts of carbon were observed on Pt after TDMAH exposure compared to Co or Ta, suggestive of decomposition products. The XPS Hf(4f) peak measured after initial exposure of TDMAH on Pt showed evidence of two Hf oxidation states at the surface. It is postulated that the reaction of TDMAH and decomposition by-products with available oxygen or hydroxyl molecules at the surface of the various metal underlayers affected the initial growth of HfO 2.

In-situ Studies of High-κ Dielectrics for Graphene-Based Device

The deposition and subsequent oxidation of metallic Al and Hf on graphite has been studied as a route to the formation of a high-κ dielectric on graphene. We employ in-situ x-ray photoelectron spectroscopy (XPS), ex-situ atomic force microscopy (AFM) and Raman spectroscopy to study reactions with the substrate and dielectric film morphology. We show that Al2O3 can be formed on the freshly cleaved graphene surface by annealing the deposited metallic Al in ozone or oxygen. HfO2 is also formed in this manner, but we demonstrate that in the case of Hf, defects are created on the graphene surface unless physisorbed H2O is removed by vacuum annealing prior to deposition and the deposition performed under ultra high vacuum (UHV) conditions.

Oxygen Recovery in Hf Oxide Films Fabricated by Sputtering

The chemical structure of ultrathin Hf oxide films (< 10 nm) fabricated by a standard sputtering method is investigated using x-ray spectroscopy and Rutherford backscattering spectroscopy. According to the experiments, oxygen species are impacted to the HfO2/Si interface during the initial sputtering, and then released back to the upper HfO2 region driven by the oxygen concentration grads. A vacuum annealing can greatly enhance this recovery process. Additionally, significant SiO2 reduction in the interface is observed after the vacuum annealing for the thick HfO2 films in our experiment. It might be an effective method to confine the interfacial layer thickness by sputtering thick HfO2 in no-oxygen ambient.

Local oxidation nanolithography on Hf thin films using atomic force microscopy (AFM)

Well controlled Hf oxide patterns have been grown on a flat Hf thin film surface produced by the dc magnetron sputtering method on Si and SiOx substrates. These patterns have been created by using the technique of semi-contact scanning probe lithography (SC-SPL). The thickness and width of the oxide patterns have been measured as a function of applied voltage, duration and relative humidity. There is a threshold voltage even at 87% humidity, due to insufficient energy required to start the oxide growth process for a measurable oxide protrusion. Electrical characterization was also performed via the I–V curves of Hf and HfOx structures, and the resistivity of HfOx was found to be 4.284 × 109 Ω cm. In addition to the I–V curves, electric force microscopy and spreading surface resistance images of Hf and HfOx were obtained.

Significant reduction of Positive Bias Temperature Instability in high-k/metal-gate nFETs by incorporation of rare earth metals

A significant reduction of Positive Bias Temperature Instability (PBTI) in both planar and multiple-gate n-channel FET devices is reported after La and Dy incorporation into the HfSiO/TiN gate stack. Gate current reduction at low gate voltages is also seen in the La-passivated stack. Both observations are consistent with the theoretical prediction of rare earths passivating negatively charging oxygen vacancies in Hf oxide. Overcompensation of PBTI, i.e., the creation of positively charged defects is observed and is ascribed to an excessive amount of the rare earth element in the high-k layer.

Application of non-linear optical second harmonic generation and X-ray absorption and spectroscopies to defect related properties of Hf silicate and Hf Si oxynitride gate dielectrics

Three different Hf oxide based dielectrics have emerged as viable candidates for applications in advanced ULSI devices. This article focuses on two of these: (i) phase separated Hf silicates with (i) 70–85% nano-crystalline HfO 2 with a nano-grain size <2 nm, and 15–30% ∼2 nm non-crystalline SiO 2 inclusions, and (ii) Hf Si oxynitride alloys, the most promising of which has a composition, (HfO 2) 0.3(SiO 2) 0.3(Si 3N 4) 0.4 designated as 3/3/4 Hf SiON. X-ray absorption spectroscopy has been applied to identification of defect associated with vacancy structures in phase separated silicates, and network disruption defects in the Hf Si oxynitrides. Optical second harmonic generation is introduced in this article for the first time as a non-invasive approach for detecting macroscopic strain, that is shown to be absent in these low defect density dielectrics, the phase separated Hf silicates, and Hf Si oxynitrides, but present in HfO 2 films, and Hf silicates with lower HfO 2 content, e.g., the 40% HfO 2 film of this article.

High-throughput study of the anodic oxidation of Hf–Ti thin films

A comprehensive study of the anodic oxide formation on Hf–Ti alloys over the entire range of composition was conducted. Combinatorial thin film libraries were prepared by co-sputtering. HRSEM and XRD were used to characterize the thin films and to confirm that a continuous change of the composition was obtained for this fully miscible system. Electrochemical investigations were performed by means of an automatic scanning droplet cell with a droplet radius of 100 μm. Subsequent potentiodynamic scans with intermittent impedance measurements allowed a quantitative determination of film formation factor and dielectric constant for each composition with a resolution of 0.5 at.%. Mott–Schottky analysis of the oxides was used to evaluate the relationship between parent metal composition and oxide properties, namely flat band potential and donor density. The structure–property and composition–property relationships are discussed in detail.

Hf O 2 ∕ Si interface formation in atomic layer deposition films: An in situ investigation

The authors have studied the initial stages of the atomic layer deposition (ALD) of HfO2 onto Si by means of x-ray photoelectron spectroscopy using synchrotron radiation. The ALD was obtained using HfCl4 and H2O as precursors. The investigation was carried out in situ giving the possibility to determine the properties of the grown film after each ALD cycle. The Si 2p, O 1s, and Hf 4d+Cl 2p spectra show the growth of HfO2 in a smooth way until the complete formation of two oxide layers. The averaged growth rate is found to be 0.33 (one layer after three cycles) in accordance with previous works but, within the formation of one oxide layer, each ALD cycle behaves in a distinct way: the oxidation step in the various cycles shows a different efficiency leading to the inclusion of Cl impurities into the Hf oxide. In relation to the experimental results we discuss the origin of the Cl contamination proposing a mechanism based on the adsorption geometry of HfCl4 onto the–OH terminated substrate.

Photoconductivity of Hf-based binary metal oxide systems

To explore the possibility of bandgap engineering in binary systems of oxide insulators we studied photoconductivity of nanometer-thin Hf oxide layers containing different concentrations of cations of different sorts (Si, Al, Sr, or Ce) deposited on (100)Si. The lowest bandgap of the Hf:Al oxide is close to the value 6–6.2 eV of elemental amorphous Al2O3 and insensitive to the Al content for concentrations of Al exceeding 36%. This result suggests that the Al oxide subnetwork with the largest bandgap preserves this energy width while development of a narrower gap of HfO2 is prevented possibly by dilution of the second cation subnetwork. When Ce is admixed to HfO2 an intermediate bandgap value (between the CeO2 and HfO2 bandgap widths) of 5.3+0.1 eV is observed for all concentrations of Ce, suggesting that the electronic structure of both elemental oxide subnetworks which form the binary metal oxide system, is affected. In Hf:Si oxide samples photoconductivity thresholds of 5.6–5.9 eV corresponding to the bandgap of HfO2 are observed for all studied Si concentrations, suggesting phase separation to occur. The photoconductivity of SrHfO3 exhibits two thresholds at 4.4 and 5.7 eV, which are close to the bandgaps of elemental SrO and HfO2, respectively, indicating, again, phase separation. Through this work we have illustrated photoconductivity as a feasible method to trace phase separation in nanometer-thin layers of binary systems of metal oxides.

High-K Gate Dielectric Structures by Atomic Layer Deposition for the 32nm and Beyond Nodes

In this paper we will review some of our previous work on ALD deposited high-K dielectric stacks as well as present some of our more recent advances. We have shown that ALD can be used successfully to deposit high quality high-K gate dielectrics of Hf and Zr oxide. In addition, because of the inherent conformality of ALD processes, these dielectrics can be easily migrated to advanced device structures such as FINFETs. We have also been successful in developing ALD processes to deposit rare earth oxides (REOs), such as Y2O3 and La2O3. These processes are useful in creating layered dielectric structures where the REO functions as a threshold voltage adjustment layer. We use physical and electrical characterization of the films as deposited by ALD, including spectroscopic ellipsometry, corona oxide semiconductor charge measurement, TXRF, AES, angle resolved XPS, HRTEM, AFM, MEIS, and RBS, to provide information on the film growth mechanism and the primary parameters that influence the ALD growth of high-K oxides. We further correlate these findings with basic electrical results obtained from MOSFET and MOSCAP devices fabricated using our ALD processes to understand how process changes affect both the as-deposited films as well as the resulting devices.

Photoconductivity of Hf-based binary metal oxides

In order to explore the possibility of bandgap engineering in binary oxide insulators we studied photoconductivity of nanometer-thin Hf oxide layers containing different fractions of cations of another sort (Si, Al, Sr, or Ce) deposited on (1 0 0)Si. The smallest bandgaps of the Hf:Al and Hf:Ce oxides are close to the values found in elemental Al 2O 3 (6–6.2 eV) and HfO 2 (5.6 eV), respectively, and show little sensitivity to the concentration of Al or Ce. This result suggests that the oxide sub-network with the largest bandgap preserves its gap energy, while development of a narrower gap is prevented, likely, by dilution of the second cation sub-network. In Hf:Si oxide samples photoconductivity thresholds of 5.6–5.9 eV, corresponding to the bandgap of HfO 2, were observed for all studied Si concentrations, suggesting phase separation to occur during deposition. Photoconductivity of SrHfO 3 exhibits two thresholds, at 4.4 and 5.7 eV, which are close to the bandgaps of elemental SrO 2 and HfO 2, respectively. These gap values indicate the phase separation also to occur in this binary oxide. Through this work photoconductivity is demonstrated to be a feasible method to trace phase separation in binary oxides, even in nanometer-thin layers.

Local crystal structural modifications in pulsed laser deposited high- k dielectric thin films on silicon and germanium

The thin film growth conditions are correlated with the local structures formed in Hf x Zr 1− x O 2 ( x=0.0–1.0) high- k dielectric thin films on Si and Ge substrates during deposition. Pulsed laser deposition (PLD) technique has been used in the synthesis of the thin films with systematic variations of substrate temperature, Zr content of the targets and substrate selection. The local structural information acquired from extended X-ray absorption spectroscopy (EXAFS) is correlated with the thin film growth conditions. The response of the local structure around Hf and Zr atoms to growth parameters was investigated by EXAFS experiments performed at the National Synchrotron Light Source of Brookhaven National Laboratory. The competing crystal phases of oxides of Hf were identified and the intricate relation between the stabilized phase and the parameters as: the substrate temperature; Hf to Zr ratio; have been revealed. Specifically, HfO 2 thin films on Si(1 0 0) exhibit a tetragonal to monoclinic phase transformation upon increase in the substrate temperature during deposition whereas, HfO 2 PLD films on Ge(1 0 0) substrates remain in tetragonal symmetry regardless of the substrate temperature.

Wet Etching of Heat Treated Atomic Layer Chemical Vapor Deposited Zirconium Oxide in HF Based Solutions

Alternative materials are being considered to replace silicon dioxide as gate dielectric material. Of these, the oxides of hafnium and zirconium show the most promise. However, integrating these new high-k materials into the existing complementary metal–oxide–semiconductor (CMOS) process remains a challenge. One particular area of concern is the wet etching of heat treated high-k dielectrics. In this paper, work done on the wet etching of heat treated atomic layer chemical vapor deposited (ALCVD) zirconium oxide in HF based solutions is presented. It was found that heat treated material, while refractory to wet etching at room temperature, is more amenable to etching at higher temperatures when methane sulfonic acid is added to dilute HF solutions. Selectivity over SiO2 is still a concern.

Gas-phase study of the reactivity of optical coating materials with hydrocarbons by use of a desktop-size extreme-ultraviolet laser

The reactivity of prospective capping-layer extreme-ultraviolet (EUV) mirror materials with hydrocarbons is studied in the gas phase by use of mass spectroscopy of metal-oxide clusters. We report the results of chemistry studies for Sim, Tim, Hfm, and ZrmOn metal-oxide clusters in which the reaction products were ionized with little or no fragmentation by 26.5 eV photons from a desktop-size 46.9 nm Ne-like Ar laser. Hf and Zr oxides are found to be much less reactive than Si or Ti oxides in the presence of EUV light. The results are relevant to the design of EUV mirror capping layers that are resistant to carbon contamination.

Oxygen‐Release/Storage Properties of Ce0.5M0.5O2 (M: Zr, Hf) Oxides: Interplay of Crystal Chemistry and Electronic Structure.

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Highly Dispersed Mixed Zirconia and Hafnia Nanoparticles in a Silica Matrix: First Example of a ZrO2–HfO2–SiO2 Ternary Oxide System

AbstractZrO2 and HfO2 nanoparticles are homogeneously dispersed in SiO2 matrices (supported film and bulk powders) by copolymerization of two oxozirconium and oxohafnium clusters (M4O2(OMc)12, M = Zr, Hf; OMc = OC(O)–C(CH3)CH2) with (methacryloxypropyl)trimethoxysilane (MAPTMS, (CH2C(CH3)C(O)O)–(CH2)3Si(OCH3)3). After calcination (at a temperature ≥800 °C), a silica matrix with homogeneously distributed MO2 nanocrystallites is obtained. This route yields a spatially homogeneous dispersion of the metal precursors inside the silica matrix, which is maintained during calcination. The composition of the films and the powders is studied before and after calcination by using Fourier transform infrared (FTIR) analysis, X‐ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS). The local environment of the metal atoms in one of the calcined samples is investigated by using X‐ray Absorption Fine Structure (XAFS) spectroscopy. Through X‐ray diffraction (XRD) the crystallization of Hf and Zr oxides is seen at temperatures higher than those expected for the pure oxides, and transmission electron microscopy (TEM) shows the presence of well‐distributed and isolated crystalline oxide nanoparticles (5–10 nm).