SiO2 Overlayer Research Articles

Influence of CoSi Oxidation and Passivation During Silicide Selective Etching on Junction Leakage: Applications to Schottky Diodes Devices

For new analogic microelectronic circuits development based on non-linear devices such as Schottky diodes formed in Si active regions, new Co-silicide integrations are required to reduce junction leakages. To gather targeted device requirements, precise Co silicide/Si interface optimization and a limited silicide formation at the active edges is needed. The selective etching during the “Salicide” process plays a real role in the oxidation and/or passivation of the silicide layer. Here, we propose a systematic study including a very large spectrum of experiments around the main parameters of CoSi selective etching. The main conclusions are (1) diode leakages are directly linked to SiO2 layer thickness formed during the SC1 dispense or by air exposure over the CoSi layer, (2) significant effect of dispense flow on SiO2 formation is measured through X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry characterizations; (3) optimized diode leakages together with contact resistances are then demonstrated for low SC1 delivery flow and long dispense time; and (4) major changes in final CoSi2 layer morphology and silicide/silicon interface are observed by transmission electron microscopy-energy-dispersive X-ray analyses for different selective etching processes, which are potentially explained by enrichment in Co atoms at CoSi/Si during SiO2 overlayer growth.

Pd/SiO2 as an active and durable CH4 oxidation catalyst for vehicle applications

The removal of CH4 is critical to address environmental concerns for developing natural gas vehicles (NGVs), because CH4 has a 20-fold-greater contribution to the greenhouse effect than CO2. Alumina supported Pd catalysts are widely used for CH4 oxidation due to their superior catalytic activity and durability compared to other CH4 oxidation catalysts. However, the continuous deactivation of Pd-based catalysts during vehicle applications needs further development of active and durable catalysts. Here, we report that Pd/SiO2 can be active and durable catalysts for CH4 oxidation in practically relevant condition via a simple reductive regeneration. CH4 oxidation light-off curves of freshly prepared Pd/SiO2 (air, 550°C) present higher activity than those of Pd/Al2O3, but severe deactivation was observed after hydrothermal aging (HTA, air with 10% H2O) at 850°C. However, X-ray diffraction (XRD), scanning transmission electron microscopy (STEM) and volumetric CO adsorption suggest that Pd/SiO2 has similar particle sizes as Pd/Al2O3 even after HTA, indicating that Pd/PdO particle sintering is not the origin of the deactivation of Pd/SiO2. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) shows that the CO was not adsorbed on Pd/SiO2 after HTA, whereas it was adsorbed on Pd/Al2O3 after HTA. These results demonstrate that the deactivation of Pd/SiO2 originates mainly from the blockage of the Pd/PdO surface by the SiO2 overlayer formed during HTA. The Pd/PdO surfaces are re-exposed by a simple H2 treatment at 500°C, resulting in CO adsorption on the Pd/PdO surface and regeneration for CH4 oxidation activity, which suggests that the Pd/PdO surface was re-exposed by a reduction treatment. This work also demonstrates that SiO2-supported Pd catalysts can be a good candidate for active and durable CH4 oxidation catalysts by using the proper regeneration protocols.

Controlled oxide interlayer for improving reliability of SiO2/GaN MOS devices

AbstarctThe impact of controlling Ga-oxide (GaOx) interlayers in SiO2/GaOx/GaN gate stacks is investigated by means of physical and electrical characterizations. Direct deposition of SiO2 insulators produces thin GaOx interlayers, and subsequent oxidation treatment attains high-quality insulator/GaN interface. However, the Ga diffusion into the SiO2 layers severely degrades the breakdown characteristics of GaN-MOS devices. To improve reliability of such devices, we proposed a two-step procedure with the initial SiO2 deposition conducted under nitrogen-rich ambient, followed by thick SiO2 capping. We found that this two-step procedure enables nitrogen incorporation in the insulator/GaN interface to stabilize GaN surface. Consequently, the Ga diffusion into the SiO2 overlayer during the oxidation annealing is effectively suppressed. The proposed method allows us to achieve a SiO2/GaOx/GaN stacked structure of superior electrical property with improved Weibull distribution of an oxide breakdown field and with interface state density below 1010 cm−2 eV−1.

Water chemistry on two-dimensional silicates studied by density functional theory and temperature-programmed desorption

The interactions of water with two-dimensional (2D) silica and aluminosilicate bilayers were studied using density functional theory (DFT) and temperature-programmed desorption (TPD). The DFT calculations encompassed a systematic investigation of the impacts of silanol groups, Si:Al ratio, and the proximity of a Pd(111) growth substrate on water adsorption, permeation, and dissociation, as well as the thermal stability of charge balancing protons at the 2D aluminosilicate surface. Water adsorption on pure, defect-free 2D SiO2 was weak; however, the barrier for water penetration to the Pd(111) substrate was small. Water adsorption at the 2D SiO2/Pd(111) surface was slightly weaker than bare Pd(111) but once at the metal surface water dissociation energetics were scarcely affected by the SiO2 overlayer. Silanol groups at 2D SiO2 edges substantially strengthened water adsorption but by only roughly half as much as hydroxyls bridging Al and Si atoms in 2D aluminosilicates. The metal substrate did not fundamentally alter the surface chemistry of the outer 2D aluminosilicate surface; water adsorption energies on the bridging hydroxyls differed by only a few meV for freestanding versus Pd-supported bilayers. Removal of the charge balancing proton via a dehydration process that produces an oxygen vacancy was favored over a dehydrogenation pathway irrespective of the presence of the metal. The dehydration pathway is uphill by at least 3.37 eV suggesting that temperatures in excess of 1100 K would be required to remove charge balancing protons as neutral species. In contrast, chemical bonds between the 2D aluminosilicate and the metal substrate make it difficult to protonate the metal side of the 2D aluminosilicate bilayer via water dissociation. The TPD experiments for pure 2D SiO2/Pd(111) revealed desorption curves consistent with adsorption at the metal surface, supporting weak adsorption on a defect-free bilayer and permeation through and around the silica overlayer to the metal. Incorporating Al into the bilayer shifted the water TPD peak to higher temperatures but not as much as would be expected based on the computed adsorption strength at bridging hydroxyls on 2D aluminosilicate surfaces. No water or hydrogen desorption consistent with removal of charging balancing protons from 2D aluminosilicate surfaces via dehydration or dehydrogenation was observed. The results suggest thermally robust Brønsted acid sites on the surface of 2D aluminosilicates supported on Pd(111).

Improved performance of thermochromic VO2/SiO2 coatings prepared by low-temperature pulsed reactive magnetron sputtering: Prediction and experimental verification

The paper deals with thermochromic VO2/SiO2 coatings prepared by low-temperature pulsed reactive magnetron sputtering on conventional soda-lime glass substrates without any substrate bias and without any interlayer. Thermochromic VO2 layers were deposited using reactive high-power impulse magnetron sputtering with a pulsed O2 flow control at a substrate surface temperature of 300 °C. Antireflection SiO2 layers were deposited using mid-frequency bipolar dual magnetron sputtering onto the top of VO2 layers at a surface temperature below 35 °C in order to improve the optical and mechanical performance. We focus on the dependence of the luminous transmittance (Tlum) and the modulation of the solar transmittance (ΔTsol) on the SiO2 layer thickness. The measured dependencies are in good agreement with those predicted using properties of pure VO2 layers measured by spectroscopic ellipsometry. Two different VO2 layer thicknesses (30 and 88 nm) have been used to demonstrate the tradeoff between Tlum and ΔTsol. We show an improvement due to the SiO2 overlayer of up to 16% (from 40.3% to 56.3%) for Tlum measured at 25 °C and up to 2.6% (from 7.7% to 10.3%) for ΔTsol. The results are important for the design and low-temperature fabrication of high-performance durable thermochromic VO2-based coatings for smart window applications.

Enhanced antioxidation and microwave absorbing properties of SiO2-coated flaky carbonyl iron particles

SiO2 was successfully coated on the surface of flaky carbonyl iron particles using a chemical bath deposition method in the presence of 3-aminopropyl triethoxysilane (APTES). The morphologies, composition, valence states of elements, as well asantioxidation and electromagnetic properties of the samples were characterized by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetric (TG) and microwave network analyzer. TG curve shows the obvious weight gain of carbonyl iron was deferred to 360°C after SiO2-coated, which can be ascribed to the exits of SiO2 overlayer. Compared with the raw carbonyl iron, SiO2-coated sample shows good wave absorption performance due to its impedance matching. The electromagnetic properties of raw and SiO2-coated carbonyl iron particles were characterized in X band before and after heat treatment at 250°C for 10h. It was established that SiO2-coated carbonyl iron demonstrate good thermal stability, indicating SiO2-coating is useful in the usage of microwave absorbers operating at temperature up to 250°C.

Shallow Heavily Doped n++ Germanium by Organo-Antimony Monolayer Doping.

Functionalization of Ge surfaces with the aim of incorporating specific dopant atoms to form high-quality junctions is of particular importance for the development of solid-state devices. In this study, we report the shallow doping of Ge wafers with a monolayer doping strategy that is based on the controlled grafting of Sb precursors and the subsequent diffusion of Sb into the wafer upon annealing. We also highlight the key role of citric acid in passivating the surface before its reaction with the Sb precursors and the benefit of a protective SiO2 overlayer that enables an efficient incorporation of Sb dopants with a concentration higher than 1020 cm-3. Microscopic four-point probe measurements and photoconductivity experiments show the full electrical activation of the Sb dopants, giving rise to the formation of an n++ Sb-doped layer and an enhanced local field-effect passivation at the surface of the Ge wafer.

Dual-channel extraordinary ultraviolet transmission through an aluminum nanohole array

Ultraviolet (UV) surface plasmon (SP) has distinct applications in UV filters, high-density optical storage, spectral enhancement, optical detectors, and nanolithography, which are closely related to plasmon-induced extraordinary optical transmission (EOT). However, such EOT in the UV region has not been the subject of detailed research. We report UV transmission based on theoretical research using the finite-difference time-domain method, by modulating the Al thickness, hole size, array periodicity, and SiO2 overlayer thickness. It is notable that we can obtain dual-channel UV transmission peaks with excellent qualities such as high transmissivity, zero cross-talk, narrow bandwidth, and perfect symmetry, by optimizing the parameters. The UV transmission peaks have been discovered to non-monotonously shift with increasing hole size. Although array periodicity has great influence on the transmission peak position, the peak energy in the UV region is much less than the value predicted by the well-known periodicity-related surface plasmon polariton (SPP) wavelength equation; the energy discrepancy in the UV region can reach above 20%, which is much larger than the value (typically 4%) in the visible-infrared region. Furthermore, the SiO2 overlayer may significantly modify the transmission properties. The Al nanohole arrays have also been found to exhibit distinct multi-band UV electric field enhancement properties with special interface effect and size effect. Such extraordinary dual-channel UV transmission with zero cross-talk, based on a very simple Al nanohole array, has promising application in dual-channel UV filters, high-density optical storage, and plasmon-enhanced fluorescence/Raman spectroscopy, which generally involves two wavebands (writing/reading storage or exciting/emission wavelengths). This study is expected to broaden our fundamental understanding of the UV EOT phenomenon, and provide references for experimental research and application of deep-UV and near-UV-related dual-band plasmonic devices.

Optical characterization of randomly microrough surfaces covered with very thin overlayers using effective medium approximation and Rayleigh–Rice theory

The modification of the effective medium approximation for randomly microrough surfaces covered by very thin overlayers based on inhomogeneous fictitious layers is formulated. The numerical analysis of this modification is performed using simulated ellipsometric data calculated using the Rayleigh–Rice theory. The system used to perform this numerical analysis consists of a randomly microrough silicon single crystal surface covered with a SiO2 overlayer. A comparison to the effective medium approximation based on homogeneous fictitious layers is carried out within this numerical analysis. For ellipsometry of the system mentioned above the possibilities and limitations of both the effective medium approximation approaches are discussed. The results obtained by means of the numerical analysis are confirmed by the ellipsometric characterization of two randomly microrough silicon single crystal substrates covered with native oxide overlayers. It is shown that the effective medium approximation approaches for this system exhibit strong deficiencies compared to the Rayleigh–Rice theory. The practical consequences implied by these results are presented. The results concerning the random microroughness are verified by means of measurements performed using atomic force microscopy.

Factors Influencing the Stability of Au-Incorporated Metal-Oxide Supported Thin Films for Optical Gas Sensing

There is interest in using Au-nanoparticle incorporated oxide films as functional sensor layers for high-temperature applications in optical-based sensors for measurements in both highly-oxidizing and highly-reducing atmospheres at temperatures approaching 900°C–1000°C because of a relatively high melting temperature combined with the inert nature of Au nanoparticles. This study includes a systematic series of experiments and theoretical calculations targeted at further understanding stability of Au-nanoparticle incorporated TiO2 films as archetype sensing materials. A combination of thermodynamic modeling and long-term exposure tests were utilized to unambiguously determine that gas stream composition-dependent reactive evaporation of Au (to form predominately Au(g) or AuH(g), depending upon the environment) at the surface of the nanoparticles is the dominant mechanism for mass loss of Au. Primary factors dictating the rate of reactive evaporation, and hence the associated film stability, were determined to be the gas stream temperature and the concentration of H2, with the former playing a more significant role over the ranges of temperatures (700°C–800°C) and H2 concentrations (1% to 29% H2 by volume) explored. The mitigation of Au-mass loss through reactive evaporation was also successfully demonstrated by depositing a SiO2 overlayer on the Au-nanoparticle embedded films to prevent direct Au-nanoparticle/vapor-phase contact.

硬 X 線光電子分光による酸化膜の評価

We report the applications of a hard X-ray photoelectron spectroscopy to the characterization of SiO2/Si(001) systems. Large escape depth of high-energy photoelectron enables us to probe buried layers and their interfaces in multilayer structures. Estimation of SiO2 overlayer thicknesses up to 25 nm by angle resolved XPS was possible in SiO2/Si(001) samples. Determination of the thickness profile of a wedged shape SiO2 buried layer was successfully done in Ir (8 nm)/HfO2 (2.2 nm)/thickness graded-SiO2 (0-10 nm)/Si(100). The Si 1s core level showed a SiO2 thickness dependent shift, which was ascribed to fixed charge at the SiO2-Si interface. Energy distribution of interface states at ultrathin thermal oxide/Si(100) interfaces were determined by Si 1s core level shift by applying gate bias in metal-oxide-semiconductor (MOS) structure with 5 nm Au gate electrodes.

Surface cleaning for enhanced adhesion to packaging surfaces: Effect of oxygen and ammonia plasma

The effects of direct plasma chemistries on carbon removal from silicon nitride (SiNx) and oxynitride (SiOxNy) surfaces have been studied by in-situ x-ray photoelectron spectroscopy (XPS) and ex-situ contact angle measurements. The data indicate that O2 and NH3 capacitively coupled plasmas are effective at removing adventitious carbon from silicon nitride (SiNx) and Si oxynitride (SiOxNy) surfaces. O2 plasma treatment results in the formation of a silica overlayer. In contrast, the exposure to NH3 plasma results in negligible additional oxidation of the SiNx or SiOxNy surface. Ex-situ contact angle measurements show that SiNx and SiOxNy surfaces exposed to oxygen plasma are initially more hydrophilic than surfaces exposed to NH3 plasma, indicating that the O2 plasma-induced SiO2 overlayer is highly reactive toward ambient. At longer ambient exposures (≳10 h), however, surfaces treated by either O2 or NH3 plasma exhibit similar steady state contact angles, correlated with rapid uptake of adventitious carbon, as determined by XPS. Surface passivation by exposure to molecular hydrogen prior to ambient exposure significantly retards the increase in contact angle upon exposure to ambient. The results suggest a practical route to enhancing the time available for effective bonding to surfaces in microelectronics packaging applications.

Thermal-Induced Formation of a Three-Dimensional Nanoplasmonic Sensor from Ag Nanocubes with High Stability and Reusability

Nanoplasmonic sensors based on the localized surface plasmon resonance (LSPR) of noble metal nanoparticles have many advantages, such as real-time detection, no need for reagent labelling, and no use of complicated equipment. However, the nanoplasmonic sensors with two dimensional structures usually suffer from a low LSPR signal and thus low sensitivity due to the low density of the nanoparticles. In addition, complicated surface functionalization is always required to suppress the non-specific binding of the analyst to the substrate of the sensor, because the two types of surface, that is, metal and substrate surfaces, are simultaneously exposed to the reaction medium. To overcome these problems, an innovative thermal-induced method has been proposed in the present work, to construct three dimensional (3D) nanostructure of Ag nanocubes on both surfaces of the substrate by using the unique amphiphilic property of 2-diethylaminoethanethiol. The prepared nanoplasmonic sensor is highly sensitive due to the high density of 3D structure of the nanoparticles and the low non-specific binding since only one type of surface is exposed. To enhance the stability of the sensor, a thin SiO2 overlayer was formed on the surface without using an additional coupling agent. Furthermore, the Ni(II)-nitriloacetic acid (Ni(II)-NTA) complex was covalently bound on the surface, so that the regeneration and reuse of the sensor becomes easy. Therefore, the easy fabrication, high stability, and good reusability of this 3D LSPR sensor makes our method competitive for the development of nanoplasmonic sensors.

Tailoring mesoscopically structured H-ZSM5 zeolites for toluene methylation

Mesoscopically structured zeolites based on H-ZSM5 were designed and synthesized as highly active and shape selective catalysts for methylation of toluene by tuning diffusion and acid site concentration of the catalysts. This was achieved by combining desilication, subsequent dealumination and chemical deposition of a mesoporous SiO2 overlayer of several nanometer thickness. The decreasing effective diffusion length in zeolite crystals achieved by desilication and dealumination increased the turnover rate of toluene by favoring activation of methanol and facilitating desorption of the produced xylenes, albeit with some loss in p-xylene selectivity. The presence of the SiO2 overlayer increased the p-xylene selectivity by enhancing the tortuosity of the zeolite, randomly blocking pore openings at the surface, and increasing the effective diffusion path length. The final material combines the higher catalyst utilization with enhanced selectivity leading to rates comparable to the parent zeolite, but at significantly higher selectivity.

Critical Crystal Growth of Graphene on Dielectric Substrates at Low Temperature for Electronic Devices

Critical Crystal Growth of Graphene on Dielectric Substrates at Low Temperature for Electronic Devices

Spectrally selective absorber coating from transition metal complex for efficient photothermal conversion

Copper–manganese oxide (CuMnO x ) thin films are proposed as efficient and thermally stable selective solar absorbers. The coatings were deposited on aluminum, stainless steel, and glass substrates by dip-coating method from the alcoholic solution of the Cu and Mn nitrate. An organic filmogen was introduced in order to get better adherence with the substrate hence to get uniform films even for larger substrates. The coated films were dried and subsequently heat-treated at 500 °C. X-ray diffraction spectra of the annealed film showed the formation of pure Cu–Mn oxide spinel structure (Cu1.5Mn1.5O4) in the film. FTIR spectra show complete removal of the organic species after thermal treatment at 500 °C. The solar absorptance and thermal emittance were calculated from the hemispherical reflectance spectra in the UV/Vis/NIR and IR range, respectively. The maximum visible absorptance with minimizing the infrared thermal emittance was optimized by controlling the thickness of the films, choosing substrates, and introducing a SiO2 overlayer.

Effects of He and Ar ion kinetic energies in protection of organosilicate glass from O2 plasma damage

In-situ x-ray photoelectron spectroscopy (XPS) and ex-situ Fourier transform infrared studies of He plasma and Ar+ ion bombardment pretreatments of organosilicate glass demonstrate that such pretreatments inhibit subsequent O2 plasma-induced carbon loss by forming a SiO2-like damaged overlayer, and that the degree of protection correlates directly with increased ion kinetic energies, but not with the thickness of the SiO2 overlayer. This thickness is observed by XPS to be roughly constant and <1 nm regardless of ion energies involved. The data indicate that ion kinetic energies are an important parameter in protective noble gas plasma pretreatments to inhibit O2 plasma-induced carbon loss.

Enhancing shape selectivity without loss of activity – novel mesostructured ZSM5 catalysts for methylation of toluene to p-xylene

An increase in p-xylene selectivity was observed without losing the catalytic activity over novel mesoporous nano-sized ZSM5 crystals covered with an external SiO2 overlayer created by deposition of tetraethyl orthosilicate.

Electroluminescence of Hybrid Self-Organised Fibres Incorporating CdTe Quantum Dots

We demonstrated electroluminescence from hybrid 1D glass fibres incorporating CdTe quantum dots with a thin SiO2 overlayer which contains CdS-like clusters. The self-organised fibres, prepared by refluxing precursor nanowires, exhibited red electroluminescence on Au interdigitated array electrodes at room temperature. Although fluctuation with time was observed in the electroluminescence, relatively low threshold electric field (2.6 × 106 V m–1) suggests that the CdTe quantum dots-based hybrid fibres are expected to be applied to low voltage driven electroluminescent devices.

Reduction of Ordering Temperature of Self-Assembled FePt Nanoparticles by Addition of Au and Ag

The [FePt]94Au6 and [FePt]90Ag10 nanoparticle arrays were synthesized on Si substrates by a reverse micellar method, combined with plasma treatment and in-situ deposition of a SiO2 overlayer, and the post annealing step was performed to drive the face-centered cubic to tetragonal phase transition. These FePt nanoparticles exhibit a quasi-hexagonal order with tailored inter-particle spacing and particle size. The effects of the Ag and Au on the structural and magnetic properties of FePt were investigated. The results indicate that both Au and Ag additives can remarkably enhance the coercivity and reduce the ordering temperature, however, the optimum composition is different for them. The optimum composition is determined to be [FePt]94Au6 and [FePt]90Ag10, respectively, for which the ordering temperature of FePt nanoparticles is reduced by -100 degrees C. After 600 degrees C annealing, the [FePt]94Au6 and [FePt]90Ag10 nanoparticles are totally ferromagnetic with apparent larger coercivities of -7.0 kOe, which is about 3.8 kOe larger than that of the pure FePt nanoparticles. The mechanism of the chemical ordering acceleration may be attributed to the defects and strains caused by the Au/Ag additives.