SiO2 Thin Films Research Articles

Characterization of beam splitters in the calibration of a six-channel Stokes polarimeter

Polarization distortion in a beam splitter is a phenomenon where the polarization state of output light deviates from the theoretical expectation, which is inevitable and will result in significant errors in the optical systems. A theoretical analysis method based on Mueller matrix ellipsometry is proposed for characterizing the beam splitters and the application in the calibration of a six-channel Stokes polarimeter (SP) is shown. In this study, polarization distortions in the beam splitters including depolarization, linear birefringence, circular birefringence, linear dichroism, and circular dichroism have been considered. With the proposed method, the beam splitters are characterized by the polarization distortions and the effective optical parameters extracted. In our experiment, the Mueller matrices of two different commonly used beam splitters measured by a commercial Mueller matrix ellipsometer (MME) are consistently fitted by the proposed method and the residual errors have shown improvement compared to the conventional methods. A practical application of the proposed method is exhibited by calibrating a SP system containing two non-polarization beam splitters and three polarization beam splitters. With the proposed method introduced, the general error of the measured Stokes vector can be reduced from 3% to 1%, and the errors of the thickness measurement of standard SiO2 thin film samples are within 1 nm compared with the results reported by a commercial MME.

Controlling disorder in two-dimensional networks

Two-dimensional networks are constructed by reference to a distribution of ring sizes and a parameter (α) which controls the preferred nearest-neighbour spatial correlations, and allows network topologies to be varied in a systematic manner. Our method efficiently utilizes the dual lattice and allows the range of physically-realisable configurations to be established and compared to networks observed for a wide range of real and model systems. Three different ring distributions are considered; a system containing five-, six- and seven-membered rings only (a proxy for amorphous graphene), the configuration proposed by Zachariasen in 1932, and a configuration observed experimentally for thin (near-2D) films of SiO2. The system energies are investigated as a function of the network topologies and the range of physically-realisable structures established and compared to known experimental results. The limits on the parameter α are discussed and compared to previous results. The evolution of the network structure as a function of topology is discussed in terms of the ring–ring pair distribution functions.

Optical and mechanical properties of Sol-gel prepared Titania (TiO2) –Silica (SiO2) mixed thin films ‘as prepared at 300 K’ without any post heat treatment

The mixed metal oxide (MMO) thin films like titania-silica (TiO2–SiO2) prepared by sol-gel technology have shown a variety of interesting optical, mechanical and thermo-mechanical properties; these thin films are being used in a variety of applications. In the present investigation, the TiO2–SiO2 (TS) thin films (on glass substrates) are prepared at 300 K from non-aqueous individual sols of TiO2 and SiO2 of 0.1 molar (TS-1) and 0.3 molar (TS-3) concentrations; no post heat treatment has been conducted on these films. The thicknesses of the two layer (2 l) and four layer (4 L) thin films prepared by dip coating are in the range 27 nm to 170 nm. The optical and mechanical properties of these ‘as-deposited’ thin films have been evaluated. The ‘as-deposited’ TS thin films show an optical transparency (in the visible region) of ∼90% and optical band gaps are in the range 4.44 eV to 4.50 eV. The optical density has been calculated from the refractive index measurements. The refractive index of TS-1 thin films of thickness 27 nm is practically constant at 1.61 over the wavelength range 400–750 nm. Micro-cracks have been observed (Scanning electron micrographs) in TS-1-4L and TS-3-4L thin films. These micro cracks are more significant in TS-3-4L samples than those of TS-1-4L samples. These cracks are not seen in all other samples (TS-1-2L and TS-3-2L). These mixed titania-silica thin films show higher adherence and higher hardness compared to those of TiO2 thin films. The Raman spectroscopy measurements show the existence of multiple phases of Ti–O–Si. It is known that in the multi-phase structures, the phase boundaries hinder crack propagation and enhances the resistance to plastic deformation enhancing the mechanical hardness of these TS thin films.

Optical Tamm state aided room-temperature amplified spontaneous emission from carbon quantum dots embedded one-dimensional photonic crystals

Optical Tamm state (OTS) aided room-temperature amplified spontaneous emission (ASE) from carbon quantum dots (CQDs) embedded all-dielectric one-dimensional photonic crystals (1DPhCs) is presented. 1DPhCs, constituting twelve pairs of alternating quarter wave thick SiO2 and TiO2 thin films are fabricated by sol-gel synthesis route. The 1DPhCs are covered with ~50 nm thick silver thin film to obtain the Tamm structures. CQDs are prepared using organic precursors (onion pulp) and incorporated in the TiO2 matrix of the final four pairs of the 1DPhCs. OTSs are observed in the reflection spectra at a detection angle of ~15°, at ~648 nm and ~618 nm, for the samples with and without CQDs respectively. Comparisons of enhancement of photoluminescence from samples with and without CQDs are presented. ASE at ~648 nm corresponding to the OTS of the CQDs incorporated Tamm structure, and suppression of emission within the photonic stop-band is demonstrated.

A systematic study of multifunctional xTiO2/(100 − x)SiO2 thin films prepared by sol–gel process

It is known that TiO2/SiO2 nanocomposite thin films have several properties improved when compared with the TiO2 or SiO2 pure thin films. In this work, a systematic study of xTiO2/(100 − x)SiO2 films (x = 0, 20, 40, 60, 80, and 100%) with different numbers of layers (1–5), i.e., different thicknesses, prepared by sol–gel process using dip-coating deposition was made. The films were thermally treated and characterized by UV–Vis spectroscopy and ellipsometry to evaluate the optical properties and thicknesses. Transmittance of the films were discussed by means of the interference fringes and refractive index modulation as a TiO2/SiO2 ratio function. The values of refractive index, extinction coefficient, and band gap obtained from ellipsometric analyses were related to the changes in UV–Vis spectra. The construction of the xTiO2/(100 − x)SiO2 structural models for a precise ellipsometric analyses were based on a comprehension of theoretical models. The glass substrate and SiO2 film were modeled by Cauchy; TiO2 film by Tauc–Lorentz and TiO2/SiO2 by Bruggeman theories. A graded refractive index function was necessary to fit the thicker films with low mean square error (MSE). Moreover, the refractive index values permitted the calculation of total porosity of the films, using Lorentz–Lorentz equation. Atomic force microcopy images showed clearly the effect of SiO2 addition on TiO2, reducing the particle size and root mean square roughness (from 1.4 to 0.3 nm). The lowest roughness of TiO2/SiO2 films was correlated with their enhanced hardness. All TiO2/SiO2 films showed a persistent super hydrophilicity that makes them suitable for applications in environmental and energy fields.

Growth of nanocolumnar thin films on patterned substrates at oblique angles

The influence of one dimensional substrate patterns on the nanocolumnar growth of thin films deposited by magnetron sputtering at oblique angles is theoretically and experimentally studied. A well‐established growth model has been used to study the interplay between the substrate topography and the thin film morphology. A critical thickness has been defined, below which the columnar growth is modulated by the substrate topography, while for thicknesses above, the impact of substrate features is progressively lost in two stages; first columns grown on taller features take over neighboring ones, and later the film morphology evolves independently of substrate features. These results have been experimentally tested by analyzing the nanocolumnar growth of SiO2 thin films on ion‐induced patterned substrates.

The Characteristics of Silica Nano-powder and Thin Films Prepared from Rice Husk Ash

In this study we have conducted laboratory research on the characteristics of silicanano-powder andthin films prepared from rice husk ash by extractionmethod. Rice husk ash was obtained fromAcehBesar district, Aceh province, Indonesia. The extraction process was carried out using 1.5% KOH and 10% HCl solution. The deposition of SiO2 thin films was carried out by sol-gel method. SiO2 thin films were grown on a glass substrate by a dip coating technique. The precursor solution was prepared by dissolving 1 gram of silica nano-powder into 20 ml of ethanol and stirring for 60 minutes at 50 0C using a hot plate stirrer. The SiO2 thin films were then annealed at various temperature of 450 - 600 0C with a holding time of 60 minutes. XRD pattern showed that silica powder from rice husk ash has a nano-crystal structure. We found that there are other peaks besides SiO2. This indicated that silica nano-powder obtained by extraction method was still contains impurity elements. The characteristics of SiO2 thin films strongly depend on annealing temperature. The optical bandgap of SiO2 thin films varies from 3.63 - 3.68 eV.

Enhanced Self-Cleaning Performance of Ag-F-Codoped TiO2/SiO2 Thin Films

Highly self-cleaning thin films of TiO2-SiO2 co-doped with Ag and F are prepared by the sol-gel method. The asprepared thin films consist of bottom SiO2 and top TiO2 layers which are modified by doping with F, Ag and F-Ag elements. XRD analysis confirms that the prepared thin film is a crystalline anatase phase. UV-vis spectra show that the light absorption of Ag-F-TiO2/SiO2 thin films is tuned in the visible region. The self-cleaning properties of the prepared films are evaluated by a water contact angle measurement under UV light irradiation. The photocatalytic performances of the thin films are studied using methylene blue dye under both UV and visible light irradiation. The Ag-F-TiO2/SiO2 thin films exhibit higher photocatalytic activity under both UV and visible light compared with other samples of pure TiO2, Ag-doped TiO2, and F-doped TiO2 films.

Pulse Energy Effect on the Optical Properties of Pulse Laser Deposited SiO2 Thin Films

Pulse Energy Effect on the Optical Properties of Pulse Laser Deposited SiO2 Thin Films

Magnetic domain structure and domain wall analysis of ferromagnetic MnAs nanodisks selectively-grown on Si (111) substrates for spintronic applications

We report on the experimental and analytic results on magnetic domain and domain wall structures of MnAs nanodisks on AlGaAs nanopillar buffers selectively grown on Si (111) substrates partially covered with dielectric SiO2 thin film mask patterns using selective-area metal-organic vapor phase epitaxy. The results on the size dependence of the magnetic domain structure in MnAs nanodisks investigated by magnetic force microscopy show that a single domain is predominant in the MnAs nanodisks with an area of approximately 3 × 104 nm2 or less. It is also indicated that in the nanodisks with an area of approximately 6 × 104 nm2 or more, multiple domains, in particular, two magnetic domain structures with a 180° domain wall, are predominant. In addition, in the case of nanodisks with multiple domains, not only Néel walls but also Bloch walls are possibly formed, according to the detailed analyses of the magnetic force microscope images obtained. These results suggest that the magnetic domains and domain walls can be tuned by the control of the MnAs nanodisk size making them interesting nanostructures for spintronic applications.

Dielectric and energy storage properties of PbO–SrO–Nb2O5–Na2O–Si thin films by annealing

Dielectric and energy storage properties of PbO–SrO–Na2O–Nb2O5–SiO2 (PSNNS) thin films with annealing temperature from 700 to 850 °C were investigated by measuring their capacitance-electric filed curve and hysteresis loops. The results show that the highest dielectric constant and energy density are 81.2 and 17.0 J·cm−3, respectively, which is obtained in the sample with annealing temperature of 800 °C. Annealed from 700 to 800 °C, the dielectric constant and energy storage performance of PSNNS films are continuously improved. However, with annealing temperature up to 850 °C, their dielectric constant decreases, which might be related with the removal of interfacial defects as a function of annealing temperature. Defect is one of the causes of space charge phenomenon, resulting in the increase in dielectric constant. Moreover, the microstructure analysis by X-ray diffraction (XRD) and transmission electron microscope (TEM) indicates that the change of crystallization phase and interfacial polarization takes responsibility to the results.

Monitoring of Film Qualities of Amorphous SiO2 films/Si(100) Substrates by Measuring Bulk and Surface Acoustic Waves in Terms of Brillouin Spectroscopy

Acoustic properties of amorphous SiO2 thin films deposited on the Si(100) substrate were investigated by using Brillouin spectroscopy. The Brillouin spectrum was obtained by tilting the sample to investigate both the longitudinal acoustic (LA) mode and the surface acoustic wave (SAW) modes. Sound velocities of both modes, the two elastic moduli (longitudinal and shear moduli), and the Poisson ratio of the film could be obtained by combining the Brillouin data and the measured refractive index. The longitudinal sound velocity and the longitudinal elastic modulus increased with increasing Ar ratio in the Ar:O2 gas mixture, which was attributed to the better stoichiometry of the film grown at higher Ar ratio.

Enhanced Sensitivity of a Love Wave-Based Methane Gas Sensor Incorporating a Cryptophane-A Thin Film.

A Love wave-based sensing chip incorporating a supramolecular cryptophane A (CrypA) thin film was proposed for methane gas sensing in this work. The waveguide effect in the structure of SiO2/36° YX LiTaO3 will confine the acoustic wave energy in SiO2 thin-film, which contributes well to improvement of the mass loading sensitivity. The CrypA synthesized from vanillyl alcohol by a double trimerisation method was dropped onto the wave propagation path of the sensing device, and the adsorption to methane gas molecules by supramolecular interactions in CrypA modulates the acoustic wave propagation, and the corresponding frequency shifts were connected as the sensing signal. A theoretical analysis was performed to extract the coupling of modes for sensing devices simulation. Also, the temperature self-compensation of the Love wave devices was also achieved by using reverse polarity of the temperature coefficient in each media in the waveguide structure. The developed CrypA coated Love wave sensing device was connected into the differential oscillation loop, and the corresponding gas sensitive characterization was investigated. High sensitivity, fast response, and excellent temperature stability were successfully achieved.

Laser Induced Damage Threshold of Ta2O5 and Ta2O5/SiO2 Films at 532 and 1064 nm

Laser induced damage threshold (LIDT) of conventional electron beam deposited Ta2O5 and SiO2 thin films on BK-7 substrate are studied to see the effect of multilayer boundary and related effect for their combination as high and low index material films when number of layers increases as well as improvement on upper two non-quarter wave thickness layers. Reflection spectra of these samples show a relatively small decrease in reflection in non-quarter wave layer design and Grazing Incidence X-ray Diffraction (GIXRD) patterns show that deposited films are amorphous, having high (4.2 eV) band gap, sufficient high refractive index and very low extinction coefficient or low absorption which are factors favourable for high laser induced damage threshold. LIDT measured for single layer at 1064 nm is higher than that of multilayers and vice-versa at 532 nm wavelength. However, improvement of LIDT with non- quarter wave outer layers is more at 532 nm than 1064 nm.

Angle dependent localized surface plasmon resonance from near surface implanted silver nanoparticles in SiO2 thin film

Near surface silver nanoparticles embedded in silicon oxide were obtained by 40 keV silver negative ion implantation without the requirement of an annealing step. Ion beam induced local heating within the film leads to an exodiffusion of the silver ions towards the film surface, resulting in the protrusion of larger nanoparticles. Cross-sectional transmission electron microscopy reveals the presence of poly-disperse nanoparticles (NPs), ranging between 2 nm and 20 nm, at different depths of the SiO2 film. The normal incidence reflectance spectrum shows a double kink feature in the vicinity of 400 nm, indicating a strong localized surface plasmon resonance (LSPR) from the embedded NPs. However, due to overlap of the bilayer interference and LSPR, the related features are difficult to separate. The ambiguity in associating the correct kink with the LSPR related absorption is cleared with the use of transfer matrix simulations in combination with an effective medium approximation. The simulations are further verified with angle dependent reflectance measurements. Additionally, transfer matrix simulation is also used to calculate the electric field intensity profile through the depth of the film, wherein an enhanced electric field intensity is predicted at the surface of the implanted films.

All Solution-Processed Quintuple-Layer Organic Light-Emitting Diodes Containing Organic-Inorganic Hybrid Active Layers Fabricated by Sol-Gel Method

Organic-inorganic hybrid thin films were fabricated using the sol-gel method and applied to multi-layer organic light-emitting diodes (OLEDs) as an active layer. A polymer emissive material poly (9,9-dioctyl-fluorene-co-N-4-butylphenyl-diphenylamine) (TFB) was solved in a sol-gel reaction accelerator perhydropolysilazane (PHPS). The PHPS solution turned into SiO2 by humidity treatment. Thus, the TFB:PHPS solution became SiO2 thin films in which the organic emissive material TFB was dispersed. All the organic layers of OLEDs can be fabricated using solution-process because the organic-inorganic hybrid active layer is not soluble with the adjacent organic layers. Consequently, we successfully fabricated quintuple-layer OLEDs consisting of the following five organic films: hole-injection, hole-transporting, active (organic-inorganic hybrid) electron-transporting, and electron-injection layers. Electroluminescence (EL) was successfully observed.

Gas-Phase Electron-Impact Activation of Atomic Layer Deposition (ALD) Precursors: MeCpPtMe3.

The use of gas-phase electron-impact activation of metalorganic complexes to facilitate atomic layer depositions (ALD) was tested for the case of (methylcyclopentadienyl)Pt(IV) trimethyl (MeCpPtMe3) on silicon oxide films. Uptake enhancements of more than 1 order of magnitude were calculated from X-ray photoelectron spectroscopy (XPS) data. On the basis of the measured C:Pt ratios, the surface species were estimated to mainly consist of MeCpPt moieties, likely because of the prevalent formation of [MeCpPtMe x- nH]+ ions after gas-phase ionization (as determined by mass spectrometry). Counterintuitively, more extensive adsorption was observed on thick SiO2 films than on the native thin SiO2 film that forms on Si(100) wafers, despite the former having virtually no surface OH groups. The adsorption of MeCpPt fragments onsilicon oxide surfaces was determined by density functional theory (DFT) calculations to be highly exothermic and to favor attachment to Si-O-Si bridge sites.

The SiO2 and SiNx Multilayer Thin Film Deposition for Encapsulation of OLED Using a NSi-01 Single Precursor By Peald

OLEDs (Organic Light-Emitting Diodes) are already being used in many mobile devices and TVs, and the next generation of OLED displays will be flexible and foldable. These foldable and flexible OLED displays tend to reduce the radius of curvature under 2.5R and require excellent encapsulation characteristics at thin thicknesses. To satisfy these requirements, multilayer barrier films are considered to be indispensable for the OLEDs. Unlike a single thin film such as SiO2 and SiNx, which is currently applied, the multi-layer itself can produce an antireflection film effect due to a difference in refractive index of each thin film in laminated thin films, thereby reducing diffuse reflection. Also It is effective for blocking ultraviolet rays and loss stress.[1], [2]. Recently, The ALD (Atomic Layer Deposition) method is used for developing thin film encapsulation technology, due to advantages of self limiting surface reaction and controlling the fraction of reaction. In this paper, SiO2, SiNx and multi-layer thin films were deposited on PEN (polyethylene naphthalate) films by using the NSi-01 single precursor by PEALD (Plasma Enhanced Atomic layer Deposition) technique at low temperature (90℃), and thin film thickness and refractive index were measured using a M2000D Spectroscopic Ellipsometer from Woollam. Also, the WVTR (Water Vapor Transmission Rate) characteristics of the deposited films were confirmed using a MOCON Aquatran 2. The refractive index of the SiO2 and SiNx film was 1.47 and 1.85 respectively, which confirmed the possibility of antireflection effect through the multi-layer thin film. The WVTR characteristics were measured according to the thickness of SiO2 and SiNx film over 100hrs. The both deposited SiO2 and SiNx thin film below 150Å showed poor or breakdown encapsulation property (Fig.1). In order to overcome this, the SiO2 film and SiNx film were stacked in order. The deposited multi-layer film showed excellent WVTR characteristics with a breakdown prevention of encapsulation property at thin thickness. Also, 2 to 3% of the carbon in the deposited SiNx film is expected to lower film stress and maintain the flexibility of the entire film (Fig. 2). [3] From this research, we confirmed the possibility of deposit multi-layer thin film consist of silicon oxide film and nitride film in one chamber using a one precursor by PEALD. Especially, multilayer type thin film deposition suggested the possibility of preventing reflection and water vapor simultaneously. References Hemant Kumar Raut,*a V. Anand Ganesh,a A. Sreekumaran Nairb and Seeram Ramakrishna*acd “Anti-reflective coatings: A critical, in-depth review” Energy Environ. Sci.,3784, 2011S. Chhajed, M. F. Schubert, J. K. Kim, and E. F.Schubert, “Nanostructured Multilayer Graded-indexAntireflection Coating for Si Solar Cells with Broadbandand Omnidirectional Characteristics”, Applied Physics Letters, vol. 93, p. 251108, 2008KwanDo Kim, SeokHee Jang, JongMin Kim* and SangMok Chang, “Characterization of Thin Film Passivation for OLED by PECVD” , Korean Chem. 575, 2012 Figure 1

Significant Structural Distortion in the Surface Region of 4H-SiC Induced By Thermal Oxidation and Recovered By Ar Annealing

4H-SiC has attractive properties for power devices of high voltage applications, however, the performance of MOSFETs is often severely limited by the existence of electrically active defects at SiO2/SiC interface. The formation of interface states is often attributed to the carbon-related byproducts of thermal oxidation, however as suggested by theoretical study, it may also partly originate from the non-uniformity in electrical structure of 4H-SiC, due to the oxidation-induced local strains at the 4H-SiC surface [1]. In this study we systematically investigated the lattice distortions introduction in 4H-SiC surface region after various oxidation processes and the recovery of that directly by using surface sensitive in-plane x-ray diffractometry (XRD). 4º off-axis 4H-SiC (0001) wafers covered with 5 μm-thick n-type epitaxial layers were employed in this study. The wafers were cleaned by HF solution, then thermal oxidation process in dry oxygen at 1300ºC for various oxidation time were performed. For some of the samples, the thermally grown SiO2 layer was removed by chemical etching in HF solution, followed by annealing in Ar at 1150ºC or 1300ºC for various annealing time. The shallow incident angles less than or slightly more than the critical angle were used for in-plane XRD measurements to limit the x-ray penetration depth which can be widely tuned simply by changing the incident angle.In the in-plane XRD pattern, a peak corresponding to the diffraction of (1-100) planes was detected. The interplanar spacing was characterized for the result obtained with different incident angles. Before thermal oxidation, as expected, no significant peak position shift was observed even by changing the incident angle, since the lattice constant should be uniformly distributed in the epitaxial layer. In contrast, the peak position of oxidized 4H-SiC sample shifted to the lower angle for small incident angle measurement and gradually shifted back by increasing the incident angle [2]. In addition, when we changed the thermal oxide thicknesses by performing dry oxidation for different time, the shift of the peak position became larger for the longer oxidation time, and reaches to as large as +0.4% distortion for >40nm-thick oxide case [2]. The x-ray penetration depth where such a significant shift of the lattice constant was observed, was estimated to be only several hundreds of nanometers from the SiC surface, by a simple approximation of the penetration depth taking account of the critical angle and the x-ray absorption coefficient of 4H-SiC. These results clearly indicate that the dry oxidation process induces a significant lattice distortion locally in the surface region of 4H-SiC wafer. We also found that the lattice distortion of SiC surface remained even after the complete removal of SiO2 thin film by HF etching, though it was partially reduced. Thus the detected local strain in the surface region of SiC is distinguishable from an intrinsic strain due to the thermal expansion coefficient difference and atomic density difference of Si between SiC and SiO2 [3]. A further recovery of the lattice constant in the surface region was observed by Ar annealing. It should be noted that the relaxation occurs gradually by extending the Ar annealing time to a few hours even at 1300ºC, which is equivalent to the oxidation temperature in this study. This suggests a thermally-activated process inside SiC is required for the relaxation of the surface distortion. Actually a higher recovery rate was observed for 1300ºC than that for 1150ºC annealing. From these results we speculate that the observed surface distortion of 4H-SiC is caused by oxidation-induced defects which are accumulated only in the surface region and induce a significant strain locally in the surface region. Then the gradual relaxation by Ar annealing would be corresponding to the processes of decomposition of the defects followed by migration and desorption of the ejected atoms from the defects. Even though the relationship between the observed surface strain and the electrical structure of SiC surface is not clarified yet, such significant strain should have a certain impact on the electrical properties of SiO2/SiC interfaces.

Epitaxial graphene thermistor for cryogenic temperatures

The thermal responsivity of monolayer epitaxial graphene grown on the Si-face surface of semi-insulating SiC substrate is investigated as a function of temperature below 300 K. The measurements showed that adsorption/desorption of atmospheric adsorbates can randomly modify the electrical characteristics of graphene which is indeed undesirable for consistent temperature sensing operations. Therefore, in order to avoid the interaction between graphene layer and adsorbates, the grown graphene layer is encapsulated with a thin SiO2 film deposited by Pulsed Electron Deposition technique. Temperature dependent resistance measurement of encapsulated graphene exhibited a clear thermistor type behavior with negative temperature coefficient resistance character. Both the sensitivity and transient thermal responsivity of the SiO2/graphene/SiC sample were found to be enhanced greatly especially for the temperatures lower than 225 K. The experimentally obtained results suggest that SiO2 encapsulated epitaxial graphene on SiC can be used readily as an energy efficient and stable temperature sensing element in cryogenic applications.