Evolution of SiO2/Ge/HfO2(Ge) multilayer structure during high temperature annealing

Use of germanium as a storage medium combined with a high-k dielectric tunneling oxide is of interest for non-volatile memory applications. The device structure consists of a thin HfO₂ tunneling oxide with a Ge layer either in the form of continuous layer or discrete nanocrystals and relatively thicker SiO₂ layer functioning as a control oxide. In this work, we studied interface properties and formation kinetics in SiO₂/Ge/HfO₂(Ge) multilayer structure during deposition and annealing. This material structure was fabricated by magnetron sputtering and studied by depth profiling with XPS and by Raman spectroscopy. It was observed that Ge atoms penetrate into HfO₂ layer during the deposition and segregate out with annealing. This is related to the low solubility of Ge in HfO₂ which is observed in other oxides as well. Therefore, Ge out diffusion might be an advantage in forming well controlled floating gate on top of HfO₂. In addition we observed the Ge oxidation at the interfaces, where HfSiO_x formation is also detected.     

Effect of Al2O3 concentration on zirconolite (Ca(Zr,Hf)Ti2O7) crystallization in (TiO2,ZrO2,HfO2)-rich SiO2–Al2O3–CaO–Na2O glasses

Glass-ceramic matrices containing zirconolite (nominally Ca(Zr,Hf)Ti₂O₇) crystals in their bulk that would incorporate high proportions of minor actinides (Np, Am, Cm) or plutonium could be envisaged for their immobilization. Zirconolite-based glass-ceramics can be prepared by controlled crystallization of zirconolite in glasses belonging to SiO₂–Al₂O₃–CaO–Na₂O–TiO₂–ZrO₂–HfO₂ system. In this study, neodymium was used as trivalent actinides surrogate. Increasing Al₂O₃ concentration in glass composition had a strong effect on the nucleation rate I _z of zirconolite crystals in the bulk, on the amount of neodymium incorporated in zirconolite phase and on the crystal growth rate of silicate phases (titanite + anorthite) from glass surface. These results could be explained by the existence of competition—in favor of aluminum—between Al³⁺ and (Ti⁴⁺, Zr⁴⁺, Hf⁴⁺) ions for their association with charge compensators cations to facilitate their incorporation in the glassy network. Differential thermal analysis (DTA) was used to study exothermal effects associated with bulk and surface crystallization. ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra showed that aluminum enters glasses network predominantly in 4-fold coordination. Neodymium optical absorption and fluorescence spectroscopies showed that the Al₂O₃ concentration changes performed in this study had not significant effect on Nd³⁺ ions environment in glasses.     

X-ray reflection spectroscopy of the HfO2/SiO2/Si system in the region of the O–K absorption edge

The paper shows the importance of soft X-ray reflection spectroscopy as a non-destructive in-depth characterization tool of the local atomic structure of high-k dielectric planar HfO₂/SiO₂/Si systems. The data obtained in the region of the O–K absorption edge demonstrate that the variation of the glancing angle enables the depth profilometry of the sample. By using the Kramers–Kronig analysis the reflection spectra are transformed into absorption spectra, from which the local physico-chemical environment of oxygen atoms is deduced, allowing also the knowledge of the local atomic structure and point defects associated with a thin superficial layer.     

SiO2 layer thickness effects on the (001) texture of FePtCu:SiO2 nanocomposite films

The influence of SiO₂ layer thickness of (Fe₅₂Pt₄₈)₈₈Cu₁₂:SiO₂ multilayer nanocomposite films on their structural and magnetic properties were investigated. The films were deposited on (001) textured FePt films, and then annealed at 873 K. The crystalline texture of (Fe₅₂Pt₄₈)₈₈Cu₁₂:SiO₂ films changes drastically with respect to the thickness of the SiO₂ layers. In the film with 50-Å thick SiO₂ layers, the (111) peak was strong although the (001) orientation is dominant, and self-organized spherical FePtCu particles were formed in the SiO₂ matrix. However, in the film with 19-Å thick SiO₂ layers, flat FePt grains with perfect (001) orientation were obtained. In addition, twins with different crystalline orientations were seen in the above films with different thicknesses of the SiO₂ layers. Accordingly, different perpendicular hysteresis loops were obtained.     

SiO2 and Al2O3/SiO2 coatings for increasing emissivity of Cu(In, Ga)Se2 thin-film solar cells for space applications

In this study, optical coatings were investigated as substitutes for the coverglass on flexible thin-film space solar cells. The inherent low emissivity of copper-indium-gallium-diselenide (CIGS) thin-film solar cells was increased using optical coatings for thermal balance in space. Evaporated silicon dioxide (SiO₂) and an additional aluminum oxide (Al₂O₃) coating on the CIGS solar cell increased the emissivity from 0.18 to 0.77. Higher emissivity was realized with the Al₂O₃/SiO₂ double-layer coating than with the SiO₂ single-layer coating. The straightforward double-layer coating gives the CIGS solar cells appropriate radiative properties for keeping the cell within a permissible temperature range in space.     

Determination of the optical constants of HfO2–SiO2 composite thin films through reverse fitting of transmission spectra

Composite thin films of HfO₂:SiO₂ with wide range of relative composition from 100:0 (pure HfO₂) to 10:90 have been deposited on fused silica substrates by co-evaporation technique and the optical properties of the films have been studied by measuring the transmission spectra of the samples by spectrophotometer. Different important optical parameters viz., band gap, refractive index and absorption coefficients of the samples have been obtained by fitting the measured optical spectra with theoretically generated spectra and the variation of the optical constants as a function of SiO₂ content in the films have been obtained. Two different dispersion models viz., the single effective oscillator model and the Tauc–Lorentz model have been used to generate the theoretical spectra in the above fitting procedure. X-ray reflectivity (XRR) measurement technique has been used to find the densities of the films in order to explain the observed variation in optical properties of the films with increase in SiO₂ content.     

Resistive switching characteristics of multilayered (HfO2/Al2O3)n n = 19 thin film

A transparent resistive random access memory used as Indium Tin Oxide (ITO) electrode, ITO/HfO₂/Al₂O₃/…/HfO₂/Al₂O₃/ITO capacitor structure is fabricated on glass substrate by atomic layer deposition. The unipolar resistive switching characteristics can be performed by applying the positive- or negative-bias through top electrode, however, the differences of switching and stability in the two different operations can be observed. The diversities of electrical property are attributed to different oxide/ITO interface materials, which influence the current flow of the injected electrons.     

Tunneling induced decomposition of Mo(CO)6 onto TiO2(110) surface

Tunneling induced decomposition of Mo(CO)₆ from the gas phase was studied on TiO₂(110) surface by scanning tunneling microscopy (STM) and spectroscopy (STS). The efficiency of the procedure was followed by measuring the dot volume as a proportional indicator of the amount of the decomposed precursor. It was found that below 1 × 10⁻⁵ Pa background pressure of Mo(CO)₆, there is no measurable effect and above 1 × 10⁻⁴ Pa, the nanodot size is too large compared to the curvature of the tip (20-40 nm). A threshold bias of +3.1(±0.1) V on the sample was measured for the decomposition of Mo(CO)₆ in gas ambient. In the absence of the precursor, dot formation was observed only above +3.7(±0.2) V, in good agreement with the results reported in our earlier work about nanolithography on clean TiO₂(110) substrate (E. Kriván, A. Berkó: J. Vac. Sci. & Tech. B 15(1) (1997)25). By applying voltages in the range of 3.1-3.5 V, a systematic enlargement of the created nanodots was found in the range of 2-20 s of duration and 0.01-1.0 nA of tunneling current. The I-V curves detected on the top of the nanodots have shown that the created features are of insulator character. This observation indicates that the decomposition of Mo(CO)₆ is also accompanied by oxidation of the deposited Mo species.     

Crystal data on ytterbium chlorosilicate,Yb3(SiO4)2Cl

Ytterbium chlorosilicate, Yb₃(SiO₄)₂Cl, was obtained in the form of single crystals up to 1.5 × 1.5 × 0.5 mm³ as by-product of the synthesis of ytterbium oxychloride. The compound has orthorhombic symmetry (space group Pnma - D_2h¹⁶). The unit cell parameters are: $\text{a}\text{= 6.753}\text{A}\text{?}\text{, b}\text{= 17.583}\text{A}\text{?}\text{, c}\text{= 6.137}\text{A}\text{?}$. The substance is characterized by means of morphology and X-ray powder data.     

Comparison between SiO2 films deposited by atomic layer deposition with SiH2[N(CH3)2]2 and SiH[N(CH3)2]3 precursors

Thin SiO₂ layers were deposited by atomic layer deposition (ALD) using either Bis-dimethylamino-silane (BDMAS: SiH₂(N(CH₃)₂)₂) or Tris-dimethylamino-silane (TDMAS: SiH(N(CH₃)₂)₃) precursors. The purpose of this study is to evaluate these precursors for their suitability for ALD of hafnium (Hf)-silicate gate dielectrics. The advantages of these precursors are that the melting points and vapor pressures are moderate. The thickness of SiO₂ deposited using ALD process is controlled by the number of growth cycles and the growth rate was different for each precursor, that for BDMAS being 1.5 times that for TDMAS at the same reactor pressure. The carbon impurity in the SiO₂ film deposited using BDMAS was about half an order of magnitude less than that using for TDMAS. Furthermore, the carbon impurity was reduced to about the detection limit of secondary ion mass spectrometry after high temperature annealing at 1000 °C during 5 s.     

Crystallization of CaHf1? x Zr x Ti2O7 (0?≤ x ≤?1) zirconolite in SiO2–Al2O3–CaO–Na2O–TiO2–HfO2–ZrO2–Nd2O3 glasses

Glass-ceramics containing (Hf,Zr)-zirconolite crystals (nominally CaHf_1−x Zr _x Ti₂O₇ with 0 ≤ x ≤ 1) were envisaged to immobilize minor actinides and plutonium. Such materials were prepared in this study by controlled crystallization of glasses belonging to the SiO₂–Al₂O₃–CaO–Na₂O–TiO₂–HfO₂–ZrO₂–Nd₂O₃ system. Neodymium was used as trivalent actinides surrogate. The effect of total or partial substitution of ZrO₂ by HfO₂ (neutron poison for fission reactions) on glass crystallization in the bulk and near the surface is presented. It appeared that Hf/Zr substitution had not significant effect on nature, structure, and composition of crystals formed both on glass surface (titanite + anorthite) and in the bulk (zirconolite). This result can be explained by the close properties of Zr⁴⁺ and Hf⁴⁺ ions and by their similar structural role in glass structure. However, strong differences were observed between the nucleation rate I_Z of zirconolite crystals in glasses containing only HfO₂ and in glasses containing only ZrO₂. Hf-zirconolite (CaHfTi₂O₇) crystals were shown to nucleate only very slowly in comparison with Zr-zirconolite (CaZrTi₂O₇) crystals. Composition changes - by increasing either HfO₂ or Al₂O₃ concentration or by introducing ZrO₂ in parent glass - were performed to increase I_Z in hafnium-rich glasses. The proportion of Nd³⁺ ions incorporated in the zirconolite phase was estimated using ESR.     

The effects of O2/Ar ratio on the structure and properties of hafnium dioxide (HfO2) films

HfO₂ films at various O₂/Ar flow ratios were prepared by reactive dc magnetron sputtering. The effects of O₂/Ar ratio on the structure and properties of HfO₂ films were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and UV-Visible spectroscopy. The results showed that the HfO₂ films were amorphous at different O₂/Ar ratios, and the atomic ratio of O/Hf in the HfO₂ films at high O₂/Ar ratio was nearly to 2:1. The peaks of Hf4f and O1s shifted to higher binding energy with increasing the oxygen flow proportion. The HfO₂ films at high O₂/Ar ratio had high transmissivity at the range of 400-1100 nm.     

Novel Dy-doped Ca2Gd8(SiO4)6O2 white light phosphors for Hg-free lamps application

The luminescent properties of Ca₂Gd_8(1−x)(SiO₄)₆O₂:xDy³⁺ (1% ≤ x ≤ 5%) powder crystals with oxyapatite structure were investigated under vacuum ultraviolet excitation. In the excitation spectrum, the peaks at 166 nm and 191 nm of the vacuum ultraviolet region can be assigned to the O²⁻ → Gd³⁺, and O²⁻ → Dy³⁺ charge transfer band respectively, which is consistent with the theoretical calculated value using Jφrgensen's empirical formula. While the peaks at 183 nm and 289 nm are attributed to the f-d spin-allowed transitions and the f-d spin-forbidden transitions of Dy³⁺ in the host lattice with Dorenbos's expression. According to the emission spectra, all the samples exhibited excellent white emission under 172 nm excitation and the best calculated chromaticity coordinate was 0.335, 0.338, which indicates that the Ca₂Gd₈(SiO₄)₆O₂:Dy³⁺ phosphor could be considered as a potential candidate for Hg-free lamps application.     

Residual stress development in Pb(Zr,Ti)O3/ZrO2/SiO2 stacks for piezoelectric microactuators

The residual stress of multilayers in piezoelectric microelectromechanical systems structures influences their electromechanical properties and performance. This paper describes the development of residual stress in 1.6 μm Pb(Zr_0.52,Ti_0.48)O₃ (PZT)/0.3 μm ZrO₂/0.5 μm SiO₂ stacks for microactuator applications. The residual stresses were characterized by wafer curvature or load-deflection measurements. PZT and zirconia films were deposited on 4-in. (100) silicon wafers with 0.5 μm thick thermally grown SiO₂ by sol–gel processes. After the final film deposition, the obtained residual stress of PZT, ZrO₂, and SiO₂ were 100–150, 230–270, and − 147 MPa, respectively. The average stress in the stack was  80 MPa. These residual stresses are explained in terms of the thermal expansion mismatch between the layers and the substrate. Load-deflection measurements were conducted to evaluate localized residual stresses using released circular diaphragms. The load-deflection results were consistent with the average stress value from the wafer curvature measurements. It was found that more reasonable estimates of the stack stresses could be obtained when mid-point vertical deflection data below 6 μm were used, for diaphragms 0.8–1.375 mm in diameter.     

Time dependent preferential sputtering in the HfO2 layer on Si(100)

The time dependent preferential sputtering in the HfO₂ layer on Si(100) has been investigated in-situ with X-ray photoelectron spectroscopy during Ar ion sputtering. Hf4f, O1s, and Si2p spectra show that three bonding environments (Hf⁰⁺ from the Hf metal, Hf²⁺ from HfO, and Hf⁴⁺ from HfO₂) co-exist inside the HfO₂ layer during sputtering. The Hf⁴⁺ doublet decreases with sputtering time in an exponential-like function. Both Hf⁰⁺ and Hf²⁺ doublets increase with sputtering time in opposite ways. Two concurrent sputtering mechanisms characterizing the formation of HfO and Hf due to preferential sputtering of oxygen within the HfO₂ layer can well explain the detailed bond breaking and re-formation process. The Hf metal is the final product and the HfO is an intermediate product during sputtering under vacuum. The HfO cannot be removed and acts as a residual component in the HfO₂ layer.     

Dynamic deformation and fracture of mullite (3Al2O3·2SiO2) ceramics under hypervelocity impact

Shock-recovery experiments have been performed on mullite ceramics to clarify the effect of a phase transition on the microstructural change and deformation mechanism under shock loading. The recovered samples have been examined using the X-ray diffraction method and transmission electron microscope observation. In the samples shocked above the phase-transition pressure, an amorphization of mullite occurs. Mullite nano-crystals with grain sizes less than 10 nm are dispersed in the amorphous phase, indicating that the relatively large starting mullite crystals become nano-crystals accompanied with the amorphization. Mullite bumper-shield experiments have also been performed to examine the influence of shock-induced microstructural changes to ultimate fracture under hypervelocity impact. The results suggest that the phase transition of mullite has an effect on protection against high-velocity impact.     

Temperature dependence of critical stress and phase diagram of ferroelastic Pb3 (PO4)2 Pb3 (VO4)2

The temperature dependence of the critical stress in ferroelastic Pb₃ (PO₄)₂ reveals a Curie-Weiß law ($\text{ß =}\text{1}\text{2}$) up to 145°C. Between 160°C and the transition point at 180°C a crossover to a $\text{ß =}\text{1}\text{3}$ regime was found. For mixed crystals Pb₃ (PO₄)₂ ? Pb₃ (VO₄)₂ a phase diagram is suggested from optical, dielectric and Raman spectroscopical experiments.