Ellipsometric Spectroscopy Research Articles

Fabrication of WO3 photoanode on crystalline Si solar cell for water splitting

An integrated device of solar cell and photocatalysis (p-Si fronted solar cell/ CoSi2/WO3) has been used as photoanode for water splitting. The WO3 had been synthesized by co-electrodeposition of CoW on the p-Si fronted solar cell and followed by the annealing and acid treatment. The XRD and SEM show the amorphous structure for both as-depostion and after treatment samples. After the treatment, XPS and the ellipsometer spectroscopy shows that the CoSi2 and WO3 was formed with the band gap of 1.32 eV and 2.67 eV, respectively. Without additional power, we had demonstrated that the p-Si fronted solar cell/CoSi2/WO3 can achieve ~ 0.046 mA/cm2 photocurrent.

SURFACE PLASMON RESONANCE IN As2Se3 PLANAR WAVEGUIDES FOR THE IR SPECTRAL REGION

The surface plasmon resonance (SPR) four layers configuration that contains prism of BK7-gold metal film-As2Se3 waveguide-air was realized. As2Se3 thin films with low roughness were obtained by thermal evaporation using cvasi-closed evaporator. The surface roughness’s was less than 2.5 nm. The SPR curves shape calculated by MATLAB solver were identical to these one calculated by matrix method. The As2Se3 films were characterized by ellipsometric spectroscopy and the obtained optical constants were used for calculations. Experimentally obtained SPR curves at the 1064 nm wavelength shown sharp resonance and are in accordance with calculus.

Research on the influence of the filtrating electrode on the structure and optical properties of TiO2 thin films prepared by the energy filtering magnetron sputtering technique

Abstract TiO2 thin films were deposited by the energy filtrating magnetron sputtering (EFMS) technique and the traditional direct current magnetron sputtering (DMS) technique. The influence of the filtering electrode mesh number on the structure and optical properties of TiO2 thin films was investigated. The structure, surface morphology and optical properties were characterized by XRD, SEM and ellipsometric spectroscopy, respectively. Results show that the TiO2 thin films deposited by the DMS and EFMS techniques at the same deposition parameters are composed of the anatase phase exclusively. TiO2 thin films deposited at lower deposition rate by the EFMS technique have lower crystallinity, smaller particle size and smoother surface. With increasing the mesh number, the refractive index, extinction coefficient and optical band gap are larger.

Effect of nanosecond pulsed laser parameters on the color making of 304 stainless steel

Nanosdecond pulsed laser (532 nm) was used to irradiate stainless steel, resulting in various colors permeating the sample surface. The surface morphology, element composition, phase composition, film thickness and surface reflectivity were analyzed by field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), ellipsometer and diffuse reflection spectroscopy (DSR). The effect of laser scanning speed and hatch distance on the resulting surface coloring of stainless steel was investigated. The results showed that a thin oxide film was formed on the sample surface irradiated by the laser. The sample surface presented different colors due to thermal absorption effect caused by the different laser scanning speed and the hatch distance. Furthermore, the effect of laser parameters on the stability of coloration was studied by using the chromatic aberration evaluation formula CIEL*a*b*.

Air-stable alucone thin films deposited by molecular layer deposition using a 4-mercaptophenol organic reactant

Molecular layer deposition (MLD) is a thin film technique to make a pure organic coating or hybrid organic–inorganic film, sequentially dosing organic–organic or organic–inorganic precursors, respectively. In this study, hybrid organic–inorganic alucone films were fabricated via MLD using 4-mercaptophenol and trimethylaluminum as organic and metal precursors, respectively, over the deposition temperature range of 100–200 °C. The fabricated film was very stable without degradation when exposed to the atmosphere, and the characteristic change was confirmed through annealing under vacuum at 300–750 °C. After annealing, the thickness of the alucone films decreased and the bonding of the carbon ring changed, as revealed by the spectroscopic ellipsometer, Fourier-transform infrared, Raman, x-ray diffraction, and x-ray photoelectron spectroscopy results. The annealed alucone films showed thermal polymerization, and their carbon ring structures transformed into graphitic carbon flakes. The alucone film annealed at 750 °C showed an electrical resistivity of 0.55 Ω cm. Annealed MLD alucone films, which are hybrid materials, are potential candidates for applications in electronic, capacitor, and thermoelectric devices.

Morphology, UV–visible and ellipsometric studies of sodium lithium orthovanadate

Sodium lithium orthovanadates were prepared by the classic ceramic method technique. Structure, phase purity, composition, and morphology were characterized by X-ray powder diffraction, infrared spectroscopy, energy dispersive spectroscopy, scanning-electron microscopy and atomic force microscopy. The optical properties of NaxLi1−xCdVO4 (x = 0.5, 1) compounds prepared were recorded at room temperature using ultra-visible and ellipsometric spectroscopies in the spectral range (200–800) nm. The average reflectance of the pellets showed high reflectivity (over 80%) in the visible spectrum. The UV–Vis absorption bands are assigned to the charge transfer from the oxygen ligands to the central vanadium atom inside the [VO4]3− groups in the orthovanadate compounds. The optical band gaps Eg of both samples are evaluated using Kubelka–Munk function. The analysis of the data revealed the existence of optical allowed direct transition for NaxLi1−xCdVO4 (x = 0.5, 1). Spectroscopic ellipsometry measurements have been performed on NaxLi1−xCdVO4 (x = 0.5, 1) specimens and their optical properties were analyzed using the Cauchy model.

Band gap renormalization in n-type GeSn alloys made by ion implantation and flash lamp annealing

The last missing piece of the puzzle for the full functionalization of group IV optoelectronic devices is a direct bandgap semiconductor made by CMOS compatible technology. Here, we report on the fabrication of GeSn alloys with Sn concentrations up to 4.5% using ion implantation followed by millisecond-range explosive solid phase epitaxy. The n-type single crystalline GeSn alloys are realized by co-implantation of Sn and P into Ge. Both the activation of P and the formation of GeSn are performed during a single-step flash lamp annealing for 3 ms. The bandgap engineering in GeSn as a function of the doping level and Sn concentration is theoretically predicted by density functional theory and experimentally verified using ellipsometric spectroscopy. We demonstrate that both the diffusion and the segregation of Sn and P atoms in Ge are fully suppressed by millisecond-range nonequilibrium thermal processing.

The role of physisorption in the cryogenic etching process of silicon

The growth mechanism of the passivation layer in the cryogenic process used for silicon deep etching is explored experimentally in an inductively coupled plasma reactor. In particular, the role of SiF4 etching by-products on the SiOxFy layer deposition is investigated. The deposition of a SiOxFy layer using SiF4 and O2 gases is studied by in situ ellipsometric spectroscopy in different experimental configurations to devise the deposition mechanism: SiF4/O2 plasma mixture, alternation of SiF4 plasma and O2 plasma steps and alternation of SiF4 flow without plasma and O2 plasma steps. The refractive index and the thickness of the deposited layer are measured for different substrate temperatures, from −125 °C to 20 °C. Although some of the passivation layer is removed during the wafer warm up, a residual amount remains at the surface. The deposited SiOxFy layer forms more efficiently at low temperature with an optimal temperature of −100 °C in our experimental conditions. The passivation layer was etched by a SF6 plasma without bias versus the deposition temperature, to evaluate its resistance to plasma etching steps. The passivation layer was analyzed by ex situ EDX and XPS. We investigated the role of SiF4 low temperature physisorption in the formation of the passivation layer on the sidewalls of the features that are being etched, which are not submitted to ion bombardment. It is shown that physisorption of SiFx species play an important role because their residence time at the surface is longer, thus increasing the probability of reaction with oxygen.

Modification of the Electrochemical Behavior of InP Electrodes Induced By Ionic Bombardment: The Case of the Response of the Ce4+/Ce3+ Couple

The concept of modified electrode is very important for a lot of topics related to interfacial electrochemistry. It is particularly true when semiconductor (SC) interfacial electrochemistry is considered. Due to the specificities of the SC/liquid interfaces, modified SC electrodes have to be examined from different angles. The external part of the surface can be strongly modified due to the variations of surface /solution equilibrium inducing drastic transformations of the energy diagram at the interface. A direct consequence is a possible alteration of electronic transfers. Moreover the modification of the SC electrode can also concern deeper region through variation of the depletion layer distribution even of buried diffusion layer. Evolution of these parameters could again modify the energy diagram and the related electronic transfer abilities. Finally surface modifications can induced also evolution of the photoelectrochemical properties through evolution of the optical properties of the SC surface. All this different parameters are more or less related to the global response of the interface. It is in this context that our paper is being taken. For this study, our tool of surface modification is based on ion bombardment through mono-atomic or cluster Argon beams. Modulation of the perturbation induced by ion / SC surface interaction can be largely modulated by using mono or cluster Argon beam conditions [1]. In UHV the level and the nature of the perturbation can be easily in-situ evaluated from XPS responses evolutions from the core levels and also from the valence band. At this in-situ approach can be also associated optical responses such as ex-situ Ellipsometric Spectroscopy giving a global evaluation of the SC modification. As supporting SC material we choose InP whose surface, optical and interfacial electrochemical responses are well known in their initial stage before ionic bombardment. Based on well characterized InP (-n or -p) electrodes effect of ionic bombardment can be explored through a lot of electrochemical responses. Among the more attractive modification towards electronic transfer on InP, one can consider the addition in the electrolyte of a redox probe which acts as a perfect tool giving quantitative information for post bombardment. Ce4+/Ce3+ couple is particularly efficient due to its transfer specificities when its oxidant state (Ce4+) is only present [2-6]. Indeed pure holes injection inside the valance band (BV) is observed during the Ce4+ reduction mechanism which is only governed by the Ce4+ diffusion at the surface. As a direct consequence, the evolution of the reduction rate of Ce4+ can be easily measured vs the level of ionic bombardment. Moreover when InP is properly polarized all the holes injected are used in the anodic dissolution process with a very attractive and unusual conduction band (BC) injection phenomenon due to corrosion intermediates [6]. This is again a very sensitive probe to the level of modification of the InP surface after ionic bombardment. Note that this induced anodic process as a very similar potential distribution as the one obtained under illumination for which the holes creation is nevertheless only associated to the photon absorption. Common and difference points are also a subject of discussion for a characterization of the induced modifications. Finally we will discuss how an anodic and/or a chemical etching are able to regenerate the initial chemical state of the surface. The underlying question is how the anodic process linked to Ce4+/Ce3+ is sufficient to perform the surface regeneration even if it is initially modified by the ionic bombardment or if it requires the assistance of high positive polarization of the modified InP electrode.

Morphology and element composition profiles of alumina films obtained by mixed thermal oxidation of AlN epitaxial layers

The mixed thermal oxidation of the high temperature aluminium nitride (HT-AlN) epitaxial layer leads to the formation of the stable aluminium oxide having a developed surface area, potentially suitable for sensor applications. In this work, we studied the process of oxide formation on HT-AlN layers versus oxidation time. We applied scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to perform analysis of the morphology and chemical depth profiles of the oxide/AlN structures obtained at gradually increased time of oxidation. The SEM images revealed porous, semi-crystalline columnar structure of HT-AlN layers and largely developed surface of aluminium oxide films. We recognized two oxidized regions with different features: stoichiometric Al2O3 layer near the top structure surface and partially oxidized, gradually changing Al-oxynitride region between Al2O3 and AlN, without clearly determined boundaries. We estimated the thicknesses of these regions and compared the results with the ellipsometric and energy-dispersive X-ray spectroscopy measurements. We proposed a morphological and chemical model of the oxidized structures, including both the fully oxidized alumina film and oxynitride region adjacent to the AlN region. We found two competitive components of the oxidation process, namely an almost linear Al2O3 growth and gradual decay of the Al-oxynitride layer with oxidation prolongation.

Efficacy of Raman mapping over ellipsometric spectroscopy and XRD for characterization of structurally heterogeneous PLD nc-Si thin films

The present report demonstrates the efficacy of Raman mapping over X-Ray diffraction (XRD) and spectroscopic ellipsometry (SE) in structural characterization of nanocrystalline Silicon (nc-Si) films having spatial heterogeneity in crystallinity and structure. The Si films were deposited via pulsed-laser deposition (PLD) technique at room temperature and elevated substrate temperature (Ts) of 400 °C and 700 °C, under vacuum (∼10−6 mbar). XRD showed that all films were polycrystalline while SE studies showed that only at 700 °C, the film was nanocrystalline in nature otherwise largely amorphous at lower Ts. This discrepancy in results is well explained by Raman maps which confirmed the presence of micron sized structures composed of nc-Si embedded in otherwise uniform matrix comprised of amorphous Silicon (a-Si) for films fabricated at room temperature and 400 °C whereas at 700 °C the uniform background matrix is dominated by nc-Si. The real and imaginary parts of pseudo dielectric function (ε1,ε2) versus energy spectra obtained for films grown at RT, 400 °C, and 700 °C were modeled and fitted to substrate/film/top SiO2 layered structure. The simulated (ε1,ε2) patterns are in excellent agreement with the recorded spectra. From the SE studies the band gap and thickness of the films were estimated. Band gap energy was found to vary from 1.35 to 1.30 eV approximately with increasing Ts from room temperature to 400 °C, then gradually increases to 1.55 eV at 700 °C. The variation of other optical constants of the films as a function of Ts also reflects the structural transition from amorphous-like to nanocrystalline-like with increasing Ts.

Fabrication and properties of high B value [Mn1.56Co0.96Ni0.48O4]1−x[SrMnO3]x (0 ≤ x ≤ 0.5) spinel–perovskite composite NTC films

The [Mn1.56Co0.96Ni0.48O4]1−x[SrMnO3]x ((MCN)1−x(SM)x, 0 ≤ x ≤ 0.5) ceramic thin composite films were grown on SiO2/Si (100) substrates by the chemical solution deposition (CSD) method. The results of X-ray diffraction showed that the (MCN)1−x(SM)x (0 < x ≤ 0.5) films were a resemblance of the spinel–perovskite composite structure. The field emission scanning electron microscope analyses presented a considerable improvement in relative density with the increasing of the SM contents. In particular, the thermal constant of (MCN)1−x(SM)x ceramic film increased from 3289 to 5390 K with the increasing of the SM contents (x) from 0 to 0.5. This feature was attributed to the increase in the grain boundary resistance and the increase in Mn3+/Mn4+ pairs as the SM contents increased. From the spectroscopic ellipsometer spectroscopy results, it was found that the (MCN)0.5(SM)0.5 film had the biggest refractive index n value of 2.36 at 1.5 eV and a strong absorption peak at 4.0 eV. The sensitivity of MCN spinel ceramic film was enhanced by the compounding of SM. It was a significant disposition in the study of negative temperature coefficient thermistors.

Performance analysis of undoped and Mg-doped ZnO/p-Si heterojunction diodes grown by sol–gel technique

This paper reports the comprehensive analysis on the effect of Mg doping on structural, optical and electrical characteristics of MgxZn1−xO/p-Si heterojunction diodes. In this work, undoped and Mg-doped ZnO thin films were directly synthesized on p-Si (silicon) substrate using sol–gel spin coating method. The structural and optical properties of as-fabricated thin films are analyzed by means of XRD, AFM, SEM, ellipsometer and Photoluminescence spectroscopy. The as-grown films are having polycrystalline nature with preferred growth orientation along (002) direction. Although, no peaks due to Mg doping is exhibited in 5% Mg-doped ZnO thin films, however, due to phase segregation, the additional peaks of MgO phase at (200) and (220) direction has started visible for higher Mg-doped ZnO films. The Grain size of the films is diminished from 42.51 to 28.35 nm for Mg-doping increment from 0 to 15%, where as bandgap of the films is increased from 3.27 to 3.83 eV for same doping variation. Electrical parameters of MgxZn1−xO/p-Si heterojunction diodes are studied after forming large area ohmic contacts of Al/Ti and Al on top of ZnO thin film and bottom of Si layer respectively. The significant electrical parameters of the diodes are calculated from current–voltage characteristics measured by semiconductor parameter analyzer and capacitance–voltage characteristics measured by LCR meter. With very high rectification ratio of 41741 and low reverse saturation current of 1.52 nA, Mg0.1Zn0.9O/Si heterojunction diode has potential to be used in future nanoelectronic devices with wider bandgap.

Polyacrylic acid polymer brushes as substrates for the incorporation of anthraquinone derivatives. Unprecedented application of decorated polymer brushes on organocatalysis

Abstract The synthesis of amino-terminated anthraquinone derivatives and their incorporation onto polymer brushes for the fabrication of silicon-based nanometric functional coatings are described for the first time. The general process involves the covalent grafting of anthraquinone 1 onto two different polymer-brushes by amidation reactions. They are composed by amino- and carboxy-terminated poly(acrylic acid) chains (PAA-NH2- and PAA-COOH, respectively) tethered by one end to an underlying silicon oxide (SiO2) substrate in a polymer brush configuration. A third substrate is fabricated by UV induced hydrosilylation reaction using undecenoic acid as adsorbate on hydrogen-terminated Si(111) surfaces. One- and two-dimensional nuclear magnetic resonance (NMR), FT-IR, MS and X-ray diffraction (XRD) were used to characterize anthraquinone 1. Ellipsometric and X-ray photoelectron spectroscopy (XPS) measurements demonstrated the presence of the polymer brushes on the silicon wafers, and atomic force microscopy (AFM) was used to study its surface morphology. The covalent linkage between anthraquinone and polymer brushes was proven by XPS and confocal fluorescence microscopy. The resulting surfaces were assayed in the heterogenous organocatalytic transformation of (1H)-indole into 3-benzyl indole with moderate yields but with high recyclability.

Influence of Ag volume fraction on microstructure and optical constants of Ag-WO3 thin films

The WO3 film and Ag-WO3 nanocomposite films with Ag volume fractions (3.0, 6.0, and 12.0 vol. %) were deposited on Si substrates by radio-frequency cosputtering. X-ray diffractometer analyses show that the microstructure of WO3 films transforms from the tetragonal phase to monoclinic phase when the metal Ag was embedded into the WO3 matrix. Scanning electron microscope images demonstrate that the density of particles on Ag-WO3 films surface increases as the Ag volume fraction increases. X-ray photoelectron spectroscopic measurements display that Ag is in the form of Ag atom clusters in the film. Optical constants confirmed by ellipsometric spectroscopy indicate that the value of refractive index decreases from 2.1 to 1.1 at 300 nm with the increase in Ag content. The extinction coefficient increases with the increase in Ag content.

Symmetry driven control of optical properties in WO3 films

In this work, we demonstrate that the optical bandgap of WO3 films can be continuously controlled through uniaxial strain induced by low-energy helium implantation. The insertion of He into epitaxially grown and coherently strained WO3 films can be used to induce single axis out-of-plane lattice expansion of up to 2%. Ellipsometric spectroscopy reveals that the optical bandgap is reduced by about 0.18 eV per percent expansion of the out-of-plane unit cell length. Density functional theory calculations show that this response is a direct result of changes in orbital degeneracy driven by changes in the octahedral rotations and tilts.

Functionalization of gold and graphene electrodes by p-maleimido-phenyl towards thiol-sensing systems investigated by EQCM and IR ellipsometric spectroscopy

Electrografting of gold and graphene surfaces by functional p‐(N‐maleimido)phenyl groups was performed by reduction of p‐(N‐maleimido)phenyldiazonium tetrafluoroborate. The reduction was carried out using cyclic voltammetry coupled with micro-gravimetric measurements by means of electrochemical quartz crystal microbalance (EQCM). The overall deposited mass on gold was higher than on graphene. However, the Faradaic efficiency was lower on Au (14%) compared to graphene (22%) after the first potential scan. Subsequently, the maleimide functional groups have been tested for immobilization of terminal thiols using (4-nitrobenzyl)mercaptan for the functionalized graphene surface and a cysteine-modified peptide for the functionalized gold surface. The functionalization by p‐(N‐maleimido)phenyl groups and the following thiol coupling of the particular surface was proven by infrared spectroscopic ellipsometry (IRSE). In addition, the interaction of the tetrabutylammonium and tetrafluoroborate ions present in the electrolyte with the Au and graphene electrodes was investigated by EQCM and revealed less electrostatic interaction of graphene with these ions in solution compared to the metal (Au) surface.

Controlling the optical parameters of self-assembled silver films with wetting layers and annealing

We investigated the influence of presence of Ni and Ge wetting layers as well as annealing on the permittivity of Ag films with thicknesses of 20, 35 and 65nm. Most of the research on thin silver films deals with very small (<20nm) or relatively large (≥50nm) thicknesses. We studied the transition region (around 30nm) from charge percolation pathways to fully continuous films and compared the values of optical parameters among silver layers with at least one fixed attribute (thickness, wetting and capping material, post-process annealing). Our study, based on atomic force microscopy, ellipsometric and X-ray photoelectron spectroscopy measurements, shows that utilizing a wetting layer is comparable to increasing the thickness of the silver film. Both operations decrease the roughness-to-thickness ratio, thus decreasing the scattering losses and both narrow the Lorentz-shaped interband transition peak. However, while increasing silver thickness increases absorption on the free carriers, the use of wetting layers influences the self-assembled internal structure of silver films in such a way, that the free carrier absorption decreases. Wetting layers also introduce additional contributions from effects like segregation or diffusion, which evolve in time and due to annealing.

Cost-effective fabrication of perdurable electrodeposited TiO2 nano-layers on stainless steel electrodes applicable to photocatalytic degradation of methylene blue

Titanium oxide nano-layers were fabricated by a direct electrodeposition method, including Ti as anode and stainless steel as cathode in a non-aqueous media. The applied voltage, electrodeposition time and amount of water in the electrolyte were taken as control parameters to obtain highly stable nano-layers. The prepared samples were characterized by scanning electron microscopy, atomic force microscopy, ellipsometric spectroscopy and energy dispersive X-ray analysis. The fabricated samples showed remarkably high stability and acceptable photocatalytic behavior towards the photodegradation of methylene blue (MB). The best result found for degradation of MB is obtained by nano-layer electrodeposited at 15 V for 30 min. These nano-layers could be reused at least 30 times without any perceptible loss of the photocatalytic activity. The attractive features of this simple process are operational simplicity, decrease of precursors consumption, photocatalytic performance of thin layers without any heat treatment (no annealing is required), and, above all, high stability of thin layers in both static and flowing environments, which make it a useful strategy for the degradation of the pollutants such as MB.

Dual-modulated photoreflectance spectra of semi-insulating GaAs

For a semiconductor material, the characterization of its electronic band structure is very important for analyzing its physical properties and applications in semiconductor-based devices. Photoreflectance spectroscopy is a contactless and highly sensitive method of characterizing electronic band structures of semiconductor materials. In the photoreflectance spectroscopy, the modulation of pumping laser can cause a change in material dielectric function particularly around the singularity points of joint density of states. Thus the information about the critical points in electronic band structure can be obtained by measuring these subtle changes. However, in the conventional single-modulated photoreflectance spectroscopy, Rayleigh scattering and inevitable photoluminescence signals originating from the pumping laser strongly disturb the line shape fitting of photoreflectance signal and influence the determination of critical point numbers. Thus, experimental technique of photoreflectance spectroscopy needs further optimizing. In this work, we make some improvements on the basis of traditional measurement technique of photoreflectance spectroscopy. We set an additional optical chopper for the pumping laser which can modulate the amplitude of the photoreflectance signal. We use a dual-channel lock-in amplifier to demodulate both the unmodulated reflectance signals and the subtle changes in modulated reflectance signals at the same time, which avoids the systematic errors derived from multiple measurements compared with the single-modulated photoreflectance measurement. The combination of dual-modulated technique and dual-channel lock-in amplifier can successfully eliminate the disturbances from Rayleigh scattering and photoluminescence, thus improving the signal-to-noise ratio of the system. Under a visible laser (2.33 eV) pumping, we measure the room-temperature dual-modulated photoreflectance spectrum of semi-insulating GaAs in a region from near-infrared to ultraviolet (1.1 ～6.0 eV) and obtain several optical features which correspond to certain critical points in its electronic band structure. Besides the unambiguously resolved energy level transition of E0 and E0+0 around the bandgap, we also obtain several high-energy optical features above the energy of pumping laser which are related to high-energy level transitions of E1, E1+1, E0' and E2 in the electronic band structure of GaAs. This is consistent with the results from ellipsometric spectroscopy and electroreflectance spectroscopy. The results demonstrate that for those high-energy optical features, the mechanism for photoreflectance is that the photon-generated carriers modulate the build-in electric field which affects the overall electronic band structures, rather than the band filling effect around those critical points. This indicates that dual-modulated photoreflectance performs better in the characterization of semiconductors electronic band structure at critical point around and above its bandgap.