Rich Oxide Layer Research Articles

Introducing dopants by diffusion to improve the conductivity of silicon quantum dot materials in 3rd generation photovoltaic devices

AbstractAn alternative method to introduce dopants in Silicon rich oxide (SRO) layers was investigated. A silicon dioxide (SiO2) layer containing phosphorous pentoxide (P2O5) or boron (B) was deposited on the quartz substrate. A bilayer structure was subsequently deposited using Si and SiO2. SRO layers were doped via diffusion during the high temperature annealing required to form the Silicon quantum dot (Si‐QD) materials. Two kinds of dopants (phosphorus and boron) were introduced as they are typical dopants for n‐type and p‐type Si respectively. The annealing time was also varied to achieve an optimum conductivity of the quantum dot material. Different Si concentrations in the SRO layers were investigated. A 5% change of Si concentration could change the conductivity of the Si‐QD material by more than one order of magnitude. Using circular transmission line measurement (TLM), the sheet resistivity of the Si‐QD material was extracted. It was demonstrated that this new method of introducing dopants by diffusion is successful. Moreover, it was found that the conductivity of Si‐QD materials decreased with increasing Si concentration in the SRO layers and was also influenced by the type of dopants. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evolution of crystalline domain size and epitaxial orientation of CdTe/Si(111) quantum dots

We have investigated the crystalline configuration of CdTe quantum dots (QDs) grown on hydrogen passivated Si(111) substrates by hot wall epitaxy. Coplanar and grazing incidence diffraction were used for determination of dot strain state and the vertical and lateral dimensions of the crystalline domain. A change in aspect ratio was observed as a function of dot size. X-ray diffraction (XRD) results show that despite a mismatch of almost 20% the islands grow with a fairly good epitaxial orientation with respect to the Si(111) substrate. The dot mosaicity was also determined and was found to decrease with island size from 7° to about 4° for the samples studied, indicating an improvement in epitaxial quality even before the island coalescence. Careful observation of CdTe(22¯0) reflections in an azimuthal scan showed that an additional ensemble of islands is responsible for low-intensity peaks with a 30° symmetry besides the expected 60° symmetry. Transmission electron microscopy results have shown good accordance with atomic force microscopy and XRD and revealed the presence of an amorphous Tellurium rich oxide layer at the CdTe/Si interface, which could explain the fully unstrained QD state observed.

Co,Ba,K/ZrO2 coated onto metallic foam (AISI 314) as a structured catalyst for soot combustion: Coating preparation and characterization

Abstract Co,Ba,K/ZrO 2 coating onto the wall surface of an AISI 314 (American Iron and Steel Institute) foam was obtained using a sequential process: (1) original foam calcination, (2) ZrO 2 layer formation and (3) active metal incorporation. After the different processes, the samples were characterized by X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Laser Raman Spectroscopy (LRS), Energy Dispersive X-ray Analysis (EDX) and Scanning Electron Microscope (SEM) and the coating adherence was confirmed by means of an ultrasound test. Their catalytic behavior was evaluated using the soot combustion (Temperature-programmed Oxidation (TPO) and isothermal reactions). The soot particles were added via immersion in a 600-ppm soot suspension in n-hexane. The sequential process used to produce catalytic coatings proved to be efficient. After calcination at 900 °C, the wall surface of the original foam was covered with a uniform rich Cr oxide layer. This layer presented an interesting roughness, which favored the anchorage of the ZrO 2 layer, obtained via washcoating. The latter layer was characterized by its mosaic-type morphology, an important fraction of the surface being rich in Zr (ZrO 2 flakes). In this fraction, ZrO 2 prevented the reaction between the active elements (Co, Ba, and K) and Cr 2 O 3 . The major components in the catalytic coating were Co 3 O 4 , BaCO 3 , KNO 3 and ZrO 2 . Each one of these components and the synergy between them generated a good combination for catalysts to be used in the abatement of soot and NO x in diesel exhausts. The whole catalytic system has an interesting mechanical and thermal stability and the apparent activation energies for soot conversions lower than 0.5 (92 kJ mol −1 ) are comparable to the global activation energy reported for the powder Co,Ba,K/ZrO 2 catalyst (82 kJ mol −1 ).

Study On Charge Trap Layers In Charge Trap Metal–Oxide–Semiconductor Field Effect Transistor

In this study, we have investigated the two types of charge trap devices in terms of performance and reliability. Either a silicon rich oxide (SRO) layer or a silicon rich oxynitride (SRON) trap layer was used as a charge trap layer. The trap layers were deposited by atomic layer deposition (ALD). The transistor with an SRO trap layer combined with a 3.5-nm-thick tunneling oxide layer yields the best electrical properties in terms of speed and retention. The device with an SRON trap layer yields slower programming/erasing behaviors and a lower retention owing to noncrystallites in the SRON thin film.

Transient charging current measurements and modelling in silicon nanocrystal floating gate devices

Nanocrystals-based memories are promising candidates for the scaling of Flash memories in the next decade. However, tools to specify where the carriers are actually stored are still needed in order to optimize the elaboration processes of the nanocrystals. This article presents a model which may be used to extract the physical parameters of the objects in which the carriers are charged. These parameters are extracted from transient current measurements which may be performed by most conventional static ammeters. The model is based on the study of an equivalent circuit followed by a finite elements analysis of the memory device. This technique has been used to study MOS capacitors containing silicon nanocrystals obtained by annealing LPCVD silicon rich oxide layers. Our model was found to predict all the transient features observed in the current. It quantitatively determined that in average, one electron is charged into 3–5 nm diameter nanocrystals having a density of 0.84–1.5 × 10 12 cm −2. These parameters are very close to the size and the density which have been targeted during the elaboration process.

Application of the phonon confinement model on the optical phonon mode of silicon nanoparticles

The Si-rich oxide (SiO x) thin films are prepared by the physical vapor deposition method (PVD). In order to induce the phase separation to SiO 2 and Si phases the samples were annealed at the temperatures 950–1100 °C. The phonon confinement model of the optical modes and the low frequency spherical acoustic vibration modes model were used to calculate the size of silicon nanoparticles from the recorded Raman spectra. In comparison to the high resolution transmission electron microscopy measurements, the both – acoustic and optical vibrational modes give ∼50% in average larger diameter of silicon nanoparticles. The results are interpreted by the formation of an amorphous silicon rich oxide layer that surrounds the silicon nanoparticle.

Fabrication and Characterization of Coplanar Waveguides on Silicon Using a Combination of SiO$_{2}$ and SRO$_{20}$

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this work, the use of silicon rich oxide (SRO) and chemical vapor deposition SiO<formula formulatype="inline"><tex Notation="TeX">$_{2}$</tex> </formula> double layers as passivation films of coplanar waveguides (CPW) on high resistivity silicon (HR-Si) with an <formula formulatype="inline"> <tex Notation="TeX">${\hbox{N}}^{+}$</tex></formula> backside is studied. The microwave performance of the fabricated CPWs is evaluated by computing the attenuation loss of the devices in the 0.045–50 GHz frequency range. Experimental results show that the <formula formulatype="inline"><tex Notation="TeX">${\hbox{N}}^{+}$</tex> </formula> layer can be used without affecting CPW performance. Also, using a combined dielectric layer (SRO<formula formulatype="inline"><tex Notation="TeX">$_{20}$</tex> </formula>/SiO<formula formulatype="inline"><tex Notation="TeX">$_{2}$</tex> </formula>), the attenuation losses are reduced compared to single dielectric layers. </para>

Absorption and transport properties of Si rich oxide layers annealed at various temperatures

Thin films of SiOx (x = 1.15, d = 1 and 2 µm), deposited by thermal vacuum evaporation of SiO on n- and p-type crystalline Si or quartz substrates, and then furnace annealed at 250, 700 and 1000 °C, are studied. Optical and infrared transmission measurements prove phase separation upon annealing at 700 and 1000 °C and growth of amorphous Si nanoparticles upon annealing at 700 °C, whose optical band gap is ∼2.6 eV. High-resolution electron microscopy data confirm growth of Si nanocrystals with average size ∼5 nm in the films annealed at 1000 °C. Both kinds of transmission data were used to estimate the nanoparticle volume fraction and values of 0.2–0.25 and 0.25–0.30 for the films annealed at 700 and 1000 °C, respectively, are determined. Current–voltage characteristics (at fields >5 × 104 V cm−1) are measured on metal/SiOx/c-Si/metal structures to explore carrier transport mechanisms in all kinds of samples. They are nearly symmetric, which indicates that in all samples carrier transport via structures is dominated by the transport in the SiOx layers. It is concluded that current transport is space-charge-limited for the layers annealed at 250 °C. In the films further annealed at 700 °C containing amorphous nanoparticles, Poole–Frenkel transport mechanism is reported while tunnelling is assumed for the films annealed at 1000 °C.

Rare earth emitters coupled to Si nanoclusters in thin films

The present article deals with the use of Si nanoclusters as efficient sensitizers towards Er 3+ or Nd 3+ ions. For this purpose, reactive magnetron co-sputtering process has been used for the fabrication of the doped layers. Depending on the deposition parameters, the photoluminescent properties of the rare earth have been analyzed. Thus, the effect of the hydrogen rate as a possible way to optimize the coupling rate between Er 3+ and Si nanoclusters has been evidenced. On the other part, we have shown that above a critical Nd concentration within the silicon rich silicon oxide layer (SRSO), a concentration quenching occurs and the photoluminescence of the Nd 3+ ions decreases dramatically. Two decay times have been determined linked to the surrounding of the Nd 3+ ions.

Corrosion mechanisms of austenitic stainless steels in nitric media used in reprocessing plants

Austenitic stainless steels type 304L, 316L and 310Nb are largely used as structural materials for equipments handling nitric acid media in reprocessing plants. In almost all nitric media, these materials, protected by a chromium(III) oxide rich layer, remain in their passive state. However, in some particular nitric media, their corrosion potential may be shifted towards their transpassive domain. In this domain, they can suffer intergranular corrosion, even though they are not sensitized owing to their very low carbon content. The corrosion potential of the steel depends greatly on the cathodic reaction involved in the oxido-reduction process between the elements Fe, Cr, Ni of the steel and the oxidizing species of the medium. Three cases of an increase in the corrosion potential can be found in reprocessing media: pure nitric acid–water solutions, in which the cathodic reaction is the reduction reaction of HNO 3; nitric acid media containing oxidizing species, in which the cathodic reaction is the reaction of reduction of the oxidizing species into the reduced one; nitric media containing metallic elements electrochemically more noble than the steels, causing galvanic coupling. In each case, the mechanism and the relevant situations we experimentally studied are described.

Electrochemical formation of highly pitting resistant passive films on a biomedical grade 316LVM stainless steel surface

The results discussed in the paper demonstrate that a significant improvement in pitting corrosion resistance of a biomedical grade 316LVM stainless steel can be achieved by electrochemically forming highly-protective passive oxide films on the material's surface, under cyclic potentiodynamic polarization conditions. The film formed in a sodium nitrate electrolyte is completely resistant to pitting corrosion in simulating physiological solutions even at high temperatures (60 °C), and after sterilization. The high pitting resistance of the electrochemically-formed films was explained on the basis of their semiconducting properties. Namely, the enrichment of the outer part of the electrochemically formed passive film with Cr(VI)-species results in a decrease in the density of oxygen vacancies, which act as pitting initiation sites, and their ‘replacement’ by metal vacancies formed by the electrochemical oxidation of Cr(III) to Cr(VI). In this configuration, the outer Cr(VI)-rich oxide layer behaves as cation selective, which results in the increased pitting corrosion resistance of the film. The simple electrochemical passivation technique discussed in the paper can be efficiently used to form highly pitting resistant passive films on 316LVM-built medical implant devices of any geometry.

Fe–Ti–Cr oxides from the Permian Xinjie mafic–ultramafic layered intrusion in the Emeishan large igneous province, SW China: Crystallization from Fe- and Ti-rich basaltic magmas

The ∼ 260 Ma Xinjie mafic–ultramafic intrusion in the Emeishan large igneous province, SW China, is a sill-like body hosting Fe–Ti–Cr oxide mineralization. The intrusion comprises, from the base upward, a marginal zone and three cyclic zones. Fe–Ti oxides (magnetite and ilmenite) are disseminated throughout the intrusion and the oxide mineralization includes several Fe–Ti oxide-rich layers. The oxides in the lower parts of cyclic zones I and II are associated with variable amounts of ferrichromite with abundant ilmenite lamellae, indicating crystallization from Fe- and Ti-rich silicate magma. The Fe–Ti oxides contain relatively high Cr 2O 3 contents, which decrease upward in the sequence, consistent with formation by crystallization from silicate magma in a closed system. In the lower parts of Zones I and II, chromite grains are enclosed in olivine, whereas magnetite and ilmenite grains are enclosed in either clinopyroxene or plagioclase. Magnetite and ilmenite are also interstitial to olivine. The textural relationships of the minerals suggest that the Fe–Ti oxides may have crystallized slightly later than chromite and olivine but slightly earlier than clinopyroxene and plagioclase. Therefore, two parallel crystallization series may have developed at the same time: chromite → Fe–Ti solid solution and olivine → clinopyroxene → plagioclase. In this way Fe–Ti oxide grains can be both interstitial to early formed olivine and enclosed in later-formed clinopyroxene and plagioclase. This explains the unusual association of chromite with Fe–Ti oxides in the lower part of the sequence.

High temperature corrosion of a nickel base alloy by helium impurities

High temperature corrosion properties of Haynes 230 were investigated in a purposely-designed facility under a typical very high temperature reactor (VHTR) impure helium medium. The study was focused on the surface oxide scale formation and its stability at about 1223K. The alloy developed a Mn/Cr rich oxide layer on its surface under impure helium at 1173K. Nevertheless, a deleterious reaction destructing the chromium oxide was evidenced above a critical temperature, TA. Reagents and products of this last reaction were investigated.

Photoluminescence of Silicon Nanocrystals in Silicon Oxide

Recent results on the photoluminescence properties of silicon nanocrystals embedded in silicon oxide are reviewed and discussed. The attention is focused on Si nanocrystals produced by high-temperature annealing of silicon rich oxide layers deposited by plasma-enhanced chemical vapor deposition. The influence of deposition parameters and layer thickness is analyzed in detail. The nanocrystal size can be roughly controlled by means of Si content and annealing temperature and time. Unfortunately, a technique for independently fine tuning the emission efficiency and the size is still lacking; thus, only middle size nanocrystals have high emission efficiency. Interestingly, the layer thickness affects the nucleation and growth kinetics so changing the luminescence efficiency.

Development of zinc oxide-rich inner layers in hot-dip zinc coating for barrier protection

The formation of ZnO composite-rich inner layers and its impact on the galvanization process and characteristics of hot-dip zinc coating were explored in the present study. Steel substrates were initially coated with a fragmented layer of ZnO and then subsequently galvanized. The presence of ZnO-rich inner layer influenced the galvanizing process substantially. The process resulted in the formation of more adherent layers with lower coating thickness. The presence of the inner oxide layer caused a modification of the inner alloy structure and the coating composition. The physical, mechanical as well as galvanic performance of the coating were also improved by the presence of the ZnO-rich inner alloy layers.

Corrosion of Steels in Lead-Bismuth Flow

For the development of lead-bismuth (Pb-Bi) cooled fast breeder reactors, the corrosion characteristics of various steels in a flowing Pb-Bi were investigated by means of steel corrosion test performed in the flowing Pb-Bi for 1,000 h. It was found that liquid metal corrosion was inhibited by single or multiple oxide layers formed on the steel surfaces. The oxide layers can be classified to an inner layer and an outer one. The existence of the inner oxide layer might be more important for the corrosion resistance since the inner one was compact and stuck to the steel surface while the outer one was easily broken into small pieces with cracks and peeled from the substrate. The effect of Cr content in steels on corrosion resistance was investigated. Stable Cr rich oxide layers were easily formed on the high Cr steels. The weight losses were lower in the steels with higher Cr contents. The content of Cr in the steel was effective to form the thin and compact Cr-rich oxide layer and suppressed the corrosion. In the case of Si rich steel, the weight loss was lower than the trends of weight loss with Cr content.

Corrosion of Steels in Lead-Bismuth Flow

For the development of lead-bismuth (Pb-Bi) cooled fast breeder reactors, the corrosion characteristics of various steels in a flowing Pb-Bi were investigated by means of steel corrosion test performed in the flowing Pb-Bi for 1,000 h. It was found that liquid metal corrosion was inhibited by single or multiple oxide layers formed on the steel surfaces. The oxide layers can be classified to an inner layer and an outer one. The existence of the inner oxide layer might be more important for the corrosion resistance since the inner one was compact and stuck to the steel surface while the outer one was easily broken into small pieces with cracks and peeled from the substrate. The effect of Cr content in steels on corrosion resistance was investigated. Stable Cr rich oxide layers were easily formed on the high Cr steels. The weight losses were lower in the steels with higher Cr contents. The content of Cr in the steel was effective to form the thin and compact Cr-rich oxide layer and suppressed the corrosion. In the case of Si rich steel, the weight loss was lower than the trends of weight loss with Cr content.

Influence of oxide films on primary water stress corrosion cracking initiation of alloy 600

In the present study alloy 600 was tested in simulated pressurised water reactor (PWR) primary water, at 360 °C, under an hydrogen partial pressure of 30 kPa. These testing conditions correspond to the maximum sensitivity of alloy 600 to crack initiation. The resulting oxidised structures (corrosion scale and underlying metal) were characterised. A chromium rich oxide layer was revealed, the underlying metal being chromium depleted. In addition, analysis of the chemical composition of the metal close to the oxide scale had allowed to detect oxygen under the oxide scale and particularly in a triple grain boundary. Implication of such a finding on the crack initiation of alloy 600 is discussed. Significant diminution of the crack initiation time was observed for sample oxidised before stress corrosion tests. In view of these results, a mechanism for stress corrosion crack initiation of alloy 600 in PWR primary water was proposed.

Influence of heat treatments on residual tritium amount in tritiated stainless steel waste

In the framework of waste management strategy for future fusion reactor, it has been demonstrated that one of the major issue is to reduce the high level waste. The tritium activity strongly contributes to the waste classification. A better knowledge of the mechanisms involved in tritium trapping and desorption will allow to choose the most appropriate procedure to reduce the tritium constraint. This paper deals with the management of tritiated steels and the way to reduce tritium transfer. This experimental work concerns steel obtained by remelting under vacuum of tritiated austenitic stainless steel wastes. The goal of this study is to develop heat treatments in solid phase to lower the residual tritium concentration and its desorption kinetics at room temperature. The surface activity, the desorption of residual tritium at room temperature and the residual concentration have been measured for each condition (temperature, time, atmosphere, etc.). For reheating temperatures below 600 °C, the surface activity increases by one or two orders of magnitude; this is the consequence of the formation of a tritium rich oxide layer. After a 20 h annealing above 300 °C and removing of the tritium rich oxide layer, the surface activity is strongly reduced (80%) when compared with the value measured before annealing. For such annealing conditions, the amount of desorbed tritium is reduced by a factor 25 and the desorption kinetics yield a reduction factor of 40–50 after annealing at 400 °C for 20 h.

Effects of heat treatments on near surface elemental profiles of Fe–15Cr polycrystalline alloy

For the development of optimised surface modification of Fe–Cr alloys, the surface chemistry of an Fe–15 at.-%Cr alloy specimen during the initial stage of oxidation was investigated as well as the effects of hydrogen during annealing and following oxidation. Samples were exposed to various heat treatment atmospheres at 800°C: annealing in N2–H2 gas mixtures, oxidation in air and annealing then oxidation. After the heat treatments, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterise the morphology, elemental depth profiles and the chemical states of the elements. In the initial stage of oxidation, the chemistry of the oxide layer on the surface transformed in the following sequence: Fe rich oxide → Duplex oxide layer (outer Fe rich and inner Cr rich) → Cr rich mono-phase layer (M2O3). In the transformation, the reaction rate for the formation of the Cr rich oxide layer was controlled by the diffusion of cations. Annealing in H2-containing atmospheres increased the Cr content at the surface. An increase in hydrogen content in the atmosphere further increased both the Cr to Fe ratio in the near surface region, and the thickness of the layer affected by the heat treatment. Selective oxidation of chromium occurred as internal Cr2O3 formation, as a function of the Cr content rather than the oxygen partial pressure. Hydrogen facilitated the diffusion of chromium, probably by cleaning of fast diffusion paths. The addition of hydrogen to the annealing atmosphere before the oxidation was beneficial to obtain a Cr rich oxide layer on the surface.