Ions In SiO2 Research Articles

Molecular Dynamics Simulation of Dipole Layer Formation at High-k/SiO2 Interfaces

Implementation of high-k/metal gate stack poses a challenge in controlling the threshold voltage (VTH) of MOSFET devices. Recent studies have shown that an electric dipole layer formed at high-k/SiO2 interface is responsible for anomalous VTH shifts. The origin of the dipole layer has been explained by several mechanisms, such as the areal density difference of O atoms [1], electronegativity difference [2], and contact induced gap state [3]. Among these models, the first model proposed by Kita and Toriumi [1] is noteworthy because of its simplicity and potential to be applicable to wide variety of high-k materials. The model states that the interface dipole is formed by the movement of negatively charged O ions from higher oxygen-density side to a lower one.Recently, we performed molecular dynamics (MD) simulations of high-k/SiO2 interfaces and succeeded in reproducing the O atom migration across the Al2O3/SiO2 interface [4]. The O ion moves one-sidedly from Al2O3 side to SiO2 one, but opposite movement never occurs. The simulation result implies that the O ion migration is induced by some drift forces, but not by a simple diffusion due to the O density gradient across the high-k/SiO2. In this paper, we discuss the driving force of the O ion migration across the high-k/SiO2 in terms of the multipole moment around oxide cation.The MD simulation was performed by a commercial simulation package SCIGRESS ME from Fujitsu Ltd. Al2O3/SiO2 interface model was constructed by sandwiching amorphous Al2O3 blocks in between amorphous SiO2 blocks as shown in Fig.1. The hetero-oxide structure was annealed by the isothermal-isobaric MD calculation for 100 ps, thermostated at 1000 K by a velocity scaling with keeping the pressure at atmospheric pressure. The Born-Mayer-Huggins potential is employed in the MD simulation.Figure 2(a) shows a charge density profile across the SiO2/Al2O3 interface. Electric dipole layer appears at the interface, and it is directed from the SiO2 side to Al2O3 side. Figure 2(b) shows the electric potential profile around the interface, which is obtained by solving one-dimensional Poisson’s equation. The built-in potential at SiO2/Al2O3 interface is about 0.5 V, which matches the experimental value of the flat-band voltage shift [1]. Thus the direction and magnitude of the dipole moment in our model agrees with the experimental result. Figure 3 shows the magnified image of the SiO2/Al2O3 interface, in which the O ions originated from Al2O3 are colored by green. It can be clearly seen that the O ion moves one-sidedly from Al2O3 side to SiO2 one.The simulation result indicates that the O ion is driven by some drift forces, but not by a simple diffusion due to the density difference at the interface. It is inferred that the O ions in Al2O3 are attracted to the strong positive charges of Si ions. The positive charges of Si ions must be canceled by the negative charges of O ions in SiO2, but at very close distance, there is an appreciable octupole moment around the Si ion which is located at the center of SiO4 tetrahedron (see Fig.4). On the other hand, there is an hexadecapole moment around the Al ion which located at the center of AlO6octahedron. The electrostatic field induced by the octupole moment around the Si ion is stronger than that of the hexadecapole moment around the Al ion, and thus O ions in the vicinity of the interface can be attracted to Si ions in specific directions. Opposite dipole formation at SiO2/Y2O3interface can also be explained by the difference in the multipole moments around Si and Y ions [4].The multipole moment around an oxide cation is relevant to its electronegativity of the O density of the oxide. Therefore, our “multipole moment induced O migration model” can be regarded as the unified model of the O density [1] model and the electronegativity model [2]. Acknowledgement This work is supported by JST-CREST, and a Grant-in-Aid for Scientific Research (B) from the MEXT Japan.

Bond nature of active metal ions in SiO2-based electrochemical metallization memory cells

Electrochemical metallization cells are candidates for the next-generation non-volatile memory devices based on resistive switching. Despite the intensive studies in recent years a microscopic model of the processes in these nanoscale electrochemical systems is still missing and the physicochemical properties of the active metal ions have been rarely reported. We examined the bonding characteristics of Cu(z+) and Ag(+) ions in SiO(2)-based cells using soft X-ray absorption spectroscopy. Whereas the Ag/SiO(2) interfaces showed no chemical interaction of Ag ions, the Cu/SiO(2) showed clear signatures of partial oxidation into two ionic species of Cu(2+) and Cu(+). The analyses on the orbital hybridization strength evidently showed that the Cu(2+)-O(2-) bonds in SiO(2) are much weaker than the Cu(+)-O(2-) bonds, analogous to the case of bulk CuO and Cu(2)O. This suggests that the Cu(2+) ions should be more mobile and with a dominating role in the process of resistive switching.

Investigation of saturation and excitation behavior of 1.5 μm emission from Er3+ ions in SiO2 sensitized with Si nanocrystals

AbstractUsing time‐resolved photoluminescence spectroscopy we investigated the 1.5 μm emission from Er3+ ions embedded in SiO2 matrix sensitized with Si nanocrystals. Temporal characteristics are analyzed as the function of excitation powers and temperatures. We confirm the predominance of two excitation channels – a fast one, with nanosecond dynamics, and a slow one, taking place on the microsecond time scale. Upon increasing excitation power, the fast mechanism saturates and further increase of total photoluminescence intensity is facilitated by the slow energy transfer process. Consequences of this finding on the microscopic modeling of the excitation process are discussed (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photon cutting for excitation of Er3+ions in SiO2sensitized by Si quantum dots

We present evidence of a mechanism for the indirect excitation of Er${}^{3+}$ ions in a SiO${}_{2}$ matrix sensitized with Si quantum dots (SiQDs). The proposed process enables the simultaneous and rapid excitation of two proximal Er${}^{3+}$ ions upon absorption of a single high-energy photon in a SiQD. The experimental evidence leading to the identification of this energy-transfer path is obtained from investigations of the photoluminescence quantum yield of two Er-related emission bands at 1.54 and 0.98 $\ensuremath{\mu}$m in SiO${}_{2}$ layers doped with small ($\ensuremath{\sim}$2-nm-diam) SiQDs and a high concentration of Er${}^{3+}$ ions, prepared by sputtering on hot substrates. In contrast to the previously considered mechanisms for the indirect excitation of Er${}^{3+}$ ions in SiO${}_{2}$, this excitation process offers an advantage of efficient suppression of the most important channels of nonradiative deexcitation---the Auger energy transfer to free carriers as well as the so-called energy back transfer of excitation reversal. This feature revives hopes for the practical application of Er-doped SiO${}_{2}$ sensitized by SiQDs.

Redox processes in silicon dioxide thin films using copper microelectrodes

Although SiO2 is a typical insulator, we demonstrate an electrochemical characteristic of the Cu/Cu+ oxidation at the interface with 30 nm thick silicon dioxide thin films studied by cyclic voltammetry. This study reveals the process of anodic oxidation and subsequent reduction of oxidized Cu ions injected in the SiO2 layer with special attention to the kinetics of the redox process. We estimated the diffusion coefficient and the mobility of Cu ions in SiO2. The results gain deeper insight in the processes involved during resistive switching of Cu/SiO2 based nonvolatile memory devices.

Luminescence Dependence of Pr3+ Activated SiO2 Nanophosphor on Pr3+ Concentration, Temperature, and ZnO Incorporation

Green-emitting ZnO nanoparticles were successfully embedded in Pr3+-doped SiO2 by a sol–gel method resulting in a red-emitting ZnO·SiO2:Pr3+ nanocomposite phosphor. The particle morphology and luminescent properties of SiO2:Pr3+ phosphor powders, with or without ZnO nanoparticles, were, respectively, investigated by electron microscopy and luminescence spectroscopy. Luminescence of SiO2:Pr3+ was studied as a function of different Pr3+ concentrations and temperature. Both Pr3+ concentration and temperature were found to influence the 1D2 emission from Pr3+ strongly. Defects emission from ZnO nanoparticles was measured at 517 nm compared with the normal peak at 470 nm from micrometer-sized ZnO powders. Only red emission at 614 nm from Pr3+ ions in SiO2:Pr3+ and ZnO·SiO2:Pr3+ was observed. The green emission from ZnO in ZnO·SiO2:Pr3+ was quenched, and the red emission from Pr3+ was doubly enhanced compared with SiO2:Pr3+. The enhancement of the PL intensity from SiO2:Pr3+ with ZnO incorporation denotes the presence of energy transfer from ZnO to Pr3+ ions.

Excitation of erbium ions in SiO2 with Si nanocrystals via virtual Auger process

AbstractHere we propose a possible mechanism of fast Er3+ ions excitation in SiO2 matrix with Si nanocrystals. We show that the presence of Si nanocrystals allows for non‐resonant optical pumping of erbium ions by virtual Auger transition, which is the second order process via an intermediate virtual state: the first step is the optical transition inside the Si NC, and the second one is Auger excitation of Er3+ ion accompanied by intraband transition of the confined carrier. This mechanism of excitation can take place when the energy of photon absorbed is larger than the sum of the confined electron‐hole ground state energy and the excitation energy of Er3+ ion. We have calculated the excitation cross‐section as a function of the excitation energy for erbium ions situated both inside and outside the NC.We show that virtually all Er3+ ions inside NCs can be excited directly into the first excited state 4I13/2 (responsible for the 1.5 µm emission) by the laser pulse of duration 5–10 ns. The results obtained for ions located outside NCs demonstrate the efficiency of the virtual excitation Auger process for transition of erbium ions into the higher excited states. 1.5 µm PL appears in this case as a result of nonradiative relaxation of excited ions down to the 4I13/2 state. Correspondingly, the rise time of the 1.5 µm PL should be about several microseconds. The cross‐sections calculated demonstrate the efficiency of such Auger process (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Si nanoclusters coupled to Er 3+ ions in a SiO 2 matrix for optical amplifiers

In this work we will give an overview of the optical properties of Si nanoclusters coupled to Er 3+ ions in SiO 2 matrices produced by reactive magnetron co-sputtering. We have divided the work into two separate studies realised on the same samples, which are the result of a thorough optimisation work. The first one have been realised in order to get a clear picture of the interaction mechanism. On the second we will show a quantitative evaluation of the potential performances from a material point of view (determination of the whole optically active Er 3+ content, excitable by direct or indirect means) and actual performance in a waveguide device (determination of internal gain values).

Modeling SiO2 Ion Impurities Aging in Insulated Gate Power Devices Under Temperature and Voltage Stress

This paper presents a formal computational methodology to explain how the oxide in semiconductors degrades over time and the dependence of oxide degradation on voltage and temperature stresses. The effects of aging are modeled and quantified by modification of the gate-source capacitance value. The model output is validated using experimental results of a thermally aged power semiconductor device. *

Voltage‐assisted sodium ion incorporation and transport in thin silicon dioxide films

AbstractNa+ is a well‐known contaminant in metal‐oxide‐semiconductor field effect transistor devices, which may cause severe changes in their input characteristics. This work is addressed to investigate the incorporation and transport behavior of mobile Na+ ions in SiO2, representing a material of major importance in semiconductor technology. We design an appropriate model system, to deliberately incorporate Na+ ions into a thin SiO2 film, which is attached on an n‐doped Si wafer. A spin‐coated thin polymer film, doped with sodium triflate in a low concentration, serves as external Na+ host matrix. The incorporation is assisted by means of an external voltage. Time of flight–secondary ion mass spectrometry is chosen as the tool for determination of Na+ ion location and distribution in SiO2. Measurements show that once Na+ was incorporated, the enormous internal electrical field in the bulk SiO2 causes an immediate drift of these ions toward the SiO2/Si interface, where a pile up of Na+ occurs. The related diffusion part can therefore be neglected. However, this accumulation has a counteracting effect on the electrical field and thus influences the incorporation rate. A comparison between the detected number of Na+ ions from potentiostatic and spectrometric data shows good agreement. Copyright © 2010 John Wiley & Sons, Ltd.

Spectroscopic characterization of natural chrysotile

Chrysotile, a variety of serpentine mineral was investigated by vibrational and magnetic resonance spectroscopic methods. The SEM photograph of the sample shows that chrysotile is made up of thin and flexible fibrils, classified as asbestos. The calculated lattice parameters were a = 5.1961 ± 0.0053 Å, b = 9.2365 ± 0.0107 Å, c = 14.6208 ± 0.0039 Å; β = 90.15 ± 0.19°; Z = 4, with space group C2/ m. Three bands observed in the highest wavenumber region (3689 and 3648 cm −1) originate from the stretching vibrations of the two crystallographically different OH groups. The Raman spectrum shows bending vibrations of the SiO 4 tetrahedra at 390 and 348 cm −1. Substitution of the Si ions in SiO 4 tetrahedra by Al ions leads to low-field shifts in the 29Si NMR spectrum and the resonance line corresponding to Q 3 (1 Al) was observed at −89 ppm. This result was supported by the resonance line at ≈57 ppm in 27Al NMR spectrum.

Optical properties of Yb3+ ions in SiO2–Al2O3–CaF2 glasses

The absorption and emission properties of Yb3+ doped oxy-fluoride-silicate glasses prepared by the melt-quenching method have been investigated. Yb3+ ions in the oxy-fluoride-silicate glasses show large absorption and emission cross-section and the fluorescence lifetimes are longer than 1.3 ms. The minimum pumping intensity, minimum fraction of excited ions, saturation pumping intensity, minimum pulse duration and peak power are also analysed to characterize the samples. The minimum pulse duration and peak power are 79 fs and 0.432 PW cm−2, which indicates the potential application in high power and ultrashort pulse lasers.

Optically active Er3+ ions in SiO2 codoped with Si nanoclusters

Optical properties of directly excited erbium (Er3+) ions have been studied in silicon rich silicon oxide materials codoped with Er3+. The spectral dependence of the direct excitation cross section (σdir) of the Er3+ atomic I415/2→I411/2 transition (around 0.98 μm) has been measured by time resolved μ-photoluminescence measurements. We have determined that σdir is 9.0±1.5×10−21 cm2 at 983 nm, at least twice larger than the value determined on a stoichiometric SiO2 matrix. This result, in combination with a measurement of the population of excited Er3+ as a function of the pumping flux, has allowed quantifying accurately the amount of optically active Er3+. This concentration is, in the best of the cases, 26% of the total Er population measured by secondary ion mass spectrometry, which means that only this percentage could provide optical gain in an eventual optical amplifier based on this material.

Sensitized luminescence through nanoscopic effects of ZnO encapsulated in SiO2:Tb3+ sol gel derived phosphor

Terbium (1 mol%) doped ZnO–SiO2 binary system was prepared by a sol–gel process. Nanoscopic effects of ZnO on the photoluminescence (PL) and the cathodoluminescence (CL) properties were studied. Defects emission from ZnO nanoparticles was measured at 560 nm and the line emission from Tb3+ ions in SiO2:Tb3+ and ZnO–SiO2:Tb3+ with a major peak at 542 nm was measured. The PL excitation wavelength for 542 nm Tb3+ emission was measured at ∼320 nm in both SiO2:Tb3+ and ZnO–SiO2:Tb3+. The CL data showed quenched luminescence of the ZnO nanoparticles at 560 nm from a composite of ZnO–SiO2:Tb3+ and a subsequent increase in 542 nm emission from the Tb3+ ions. This suggests that energy was transferred from the ZnO nanoparticles to enhance the green emission of the Tb3+ ions. The PL and CL properties of ZnO–SiO2:Tb3+ binary system and possible mechanism for energy transfer from the ZnO nanoparticles to Tb3+ ions are discussed.

Space-separated quantum cutting in differently prepared solid-state dispersions of Si nanocrystals and Er 3+ ions in SiO 2

We investigate the efficiency of Si nanocrystal-mediated photoluminescence of Er 3+ ions in SiO 2 as a function of quantum energy of incoming photons. We demonstrate that for identical nanocrystals the threshold energy of the recently reported space-separated quantum cutting effect does not change with the nanocrystal-to-Er distance. In contrast, the enhancement of excitation efficiency observed above the threshold, in the high-energy photon regime, diminishes at larger separations, thus exhibiting a clear nanocrystal-to-Er distance dependence.

X-ray emission and photoluminescence spectroscopy of nanostructured silica with implanted copper ions

Quartz glass samples and compacted SiO2 nanopowders have been studied by x-ray emission (CuL2, 3 transition 3d4s → 2p1/2, 3/2) and photoluminescence spectroscopy following pulsed Cu+ ion implantation (energy, 30 keV; pulse current up to 0.5 A; pulse duration, 400 μs; irradiation doses, 1015, 1016, and 2 × 1017 cm−2). It has been established that ion irradiation gives rise to the formation of glassy and compacted SiO2 samples of nanosized metallic and oxide phases in the structure. An analysis of CuL x-ray emission spectra has shown that copper nanoparticles are thermodynamically metastable and chemically active because ion beam bombardment transfers them readily to the oxide form. This results from the radiation-stimulated fracture of regular Si-O-Si bonds in amorphous SiO2 and the formation of defective Si-Si bonds, followed by capture of oxygen by copper atoms. The enhanced degree of oxidation of copper ions in SiO2 nanostructured pellets can be reduced by coimplantation and thermal annealing. Optical spectroscopy studies suggest that, in glasses and SiO2 nanostructured pellets, there exist metallic Cun0 nanoclusters, which at low temperatures exhibit quantum-confined photoluminescence with a characteristic stepped excitation spectrum.

EXAFS study of the Er3+ ion coordination in SiO2–TiO2–HfO2 sol–gel films

The local order around ion-implanted Er3+ ions in SiO2–TiO2–HfO2 thin films prepared by sol–gel, was studied by extended X-ray absorption fine structure at the Er-LIII edge. Both the first and second coordination shells of Er3+ were analyzed for different heat-treatments. While the first coordination shell always consisted of ∼6–7 oxygen atoms at distances varying between 2.23 and 2.27Å, the structure of the second shell was found to vary with the film composition and heat-treatment. Namely, whereas Si was found to be the only second neighbor of erbium in binary SiO2–TiO2 films, the addition of HfO2 caused a preferential replacement of Si by Hf. The post-implantation thermal treatments also played a fundamental role in determining the final environment of the erbium ions.

Excitation of Er3+ ions in SiO2 with Si nanocrystals

Probabilities of excitation of erbium ions via Coulomb interaction with carriers localized in silicon nanocrystals embedded in SiO2, in recombination and intraband relaxation of these carriers, have been calculated.

Electrochemical deposition of Te and electroless deposition of Se nanoparticles in etched tracks of Au + ions in SiO 2 layer on n-Si(1 0 0) wafers

The techniques for Te electrodeposition and Se electroless deposition into nanoporous SiO 2 layer on n-Si(1 0 0) substrate are developed. To produce a porous SiO 2 layer, scanned beams of 400 MeV Au + ions are employed for the bombardment of silicon oxide thermally grown on Si(1 0 0) substrate. Pores are formed by chemical etching of the irradiated SiO 2 layer in dilute HF. Etching results in the formation of the uniform pores randomly distributed over the surface and shaped like truncated cones with base diameters of 100 and 250 nm. Electrodeposition of Te into nanopores of SiO 2 layer takes place at potentials below the silicon flat band potential. Te electrodeposited selectively into the nanopores – deposition on insulating SiO 2 layer – is excluded. Electrodeposition of Se into nanopores takes no place due to Se reduction to H 2Se. Chemical interaction of H 2Se and H 2SeO 3 in the solution leads to the formation of roundish Se particles on SiO 2 surface near the pores. Precipitation of Se into nanopores is realized by electroless deposition from SeO 2 solution with addition of HF.

Theory of energy transfer between Er 3+ ions and carriers confined in Si nanocrystals in SiO 2 matrix

A theoretical model of energy transfer between Si nanocrystals and Er 3+ ions in SiO 2 matrix is presented. The mechanism, based on the Coulomb interaction between carriers confined in nanocrystal and f-electrons of erbium ions, is considered to be responsible for the excitation and quenching of the 1.55 μ m luminescence, and particularly for its recently discovered nanosecond dynamics. It has been shown that phonon-assisted intraband transition of a confined carrier can give rise to erbium transition into the first excited state with characteristic time being in the nanosecond range.