IR Spectroscopic Ellipsometry Research Articles

Nano-Bio Interface Engineering Using Cyclovoltammetry and Design of Nanostructured Lanthanide Oxysulfide Thin Films

Nanoparticles (NPs) in which magnetic and optical properties coexist have been intensively explored for diverse biomedical applications including diagnostic, in vivo imaging and therapy. Gadolinium oxysulfide (Gd2O2S) NPs are promising candidates for biotechnologies as they have been used as MRI contrast agent or X-ray absorbing materials. When doped with cerium, they gain an additional functionality as visible light absorber due to the decrease in the material bandgap from 4.7 eV non-doped to 2.5 eV with 5% of Ce. Further biomedical applications may be envisioned thanks to the combined antioxidant properties of cerium and the magnetic properties of gadolinium.Understanding the molecular-level interactions between organisms and nanoparticles to guide and enable the design of novel materials that are less toxic remains a challenge. In order to understand these interactions at the nanoscale, we studied the change in the physico-chemical properties of (Gd,Ce) oxysulfide NPs when they are immersed in a biological medium. In biological medium, pristine nanomaterials transform and acquire a corona of biomolecules, such as proteins, which can modify their identity. This is known as the nano-bio interface. A better understanding of how nanoparticles interaction with a biological system affect their properties is critical to the future development of multifunctional NPs.Our recent work provides us with a privileged access to composition-controlled nanoplates with a narrow size distribution for a large range of Ln. In particular, we mastered the preparation of mixed-lanthanide nanoplates with tunable band gap and magnetic properties while providing an in-depth understanding of their structural and surface features. The challenge addressed here is to prepare and characterize ligand-free nanostructured thin films made from the NPs suspension. Compared to studies realized in solution, thin films allow to reach the surface of the nanoparticles without any interaction from the ligands. Moreover, it allows spectroscopic and electrochemical methods to be used for the monitoring of the NPs intrinsic properties.The design and development of nanostructured thin films started with the colloidal synthesis of (Gd,Ce) oxysulfide NPs which provides a phase-pure powder. The nanoparticles were then dispersed into an optimized mix of solvents in order to get a stable colloidal suspension. Dip coating is an easy method to deposit matter on a substrate while having a fine control of the thickness and homogeneity. This process was optimized to obtain homogenous films with a withdrawal speed of 2 mm/s under low relative humidity. Thin films of optical quality with a thickness from 40 nm to 150 nm were obtained using single-layer and multilayer deposition on various substrates including silicon, FTO and kapton (Figure A). In situ UV and IR Spectroscopic Ellipsometry were used to monitor the variations of thickness and optical constants, together with the chemical species present at the surface, upon heating the films in different conditions. This study allowed to finely tune the thermal treatment in order to get rid of the stabilizing ligands while preserving the integrity of the nanoplates, as confirmed by Transmission Electron Microscopy (Figure B shows the scraped film). As a result, we have access to the bare surface of the NPs, which was hardly possible in solution. The change in the band gap of the material was monitored through UV-Variable Angle Spectroscopic Ellipsometry. This non-invasive technique is the gold-standard for the characterization of optical quality thin films and gives refractive index of the material (Figure C shows the Tauc plot with the linear fit giving the band gap of the material).The film can be immersed into a biological medium where proteins are expected to bind to the surface of the NP and significantly change their reactivity and mobility. Cyclovoltammetry helped us to probe the access to the NPs and monitor the position of the conduction band. We used redox probes of different standard potentials as electrolytes (Figure D). Electron exchange with the semiconductor electrode is supposed to occur through the conduction band as the valence band is too far in energy. For a redox couple with a standard potential negative to the conduction band edge, when the applied potential equals E0 redox, the semiconductor is accumulating electrons at its surface in a so-called space charge layer. Under these conditions, whether a positive or negative bias is applied (compared to E0 redox), the SC is maintained in an accumulation mode. This was very helpful to check whereas the electronical properties of the material changed upon exposition to biological medium. This could lead to a better understanding and prediction of the chemical interactions at the interface with biomolecular systems. Figure 1

Optical-Quality Thin Films with Tunable Thickness from Stable Colloidal Suspensions of Lanthanide Oxysulfide Nanoplates.

In modern laser technologies, there is a need for coatings that would be compatible with flexible substrates while retaining the advantages of inorganic compounds in terms of robustness. As a first step in this direction, we developed here thin films of lanthanide oxysulfide, of optical quality, prepared by low-temperature dip coating. As a model compound in the family of oxysulfides, (Gd,Ce)2O2S anisotropic nanoplates were used. The films were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and in situ UV and IR spectroscopic ellipsometry, showing that the band gap of the materials was preserved through the deposition process. The thickness of the films was tuned in a broad range, from a few nanometers to 150 nm, using different concentrations of the colloidal suspensions as well as single-layer and multilayer deposition. Lastly, thermal treatment of the thin films was optimized to remove the stabilizing organic ligands of the nanoparticles while preserving their integrity, as confirmed by SEM and XRD.

Non-destructive determination of functionalized polyelectrolyte placement in layer-by-layer films by IR ellipsometry.

Layer-by-layer (LbL) assembly facilitates controlled coatings on a variety of surfaces with the ability to manipulate the composition through the thickness by selection of the complementary pairs. However, the characterization of these composition profiles tends to be destructive and requires significant compositional differences that can limit their utility. Here, we demonstrate the ability to non-destructively quantify the depth dependence of the allyl content associated with the selective incorporation of poly(sodium acrylate-co-allylacrylamide) (84 : 16 mol : mol) (allyl-PAA) in LbL films based on the assembly of poly(diallyldimethylammonium chloride) (PDAC)/poly(acrylic acid) (PAA) and PDAC/allyl-PAA. Although the atomic composition of the film is not dramatically influenced by the change between PAA and allyl-PAA, the absorption in the IR near 1645 cm-1 by the allyl group provides sufficient optical contrast to distinguish the LbL components with spectroscopic ellipsometry. The use of IR spectroscopic ellipsometry can determine the thickness of layers that contain allyl-PAA and also gradients that develop due to re-arrangements during the LbL process. With multiple films fabricated simultaneously, the location of the gradient between the 1st and 2nd series of multilayers (e.g., first PDAC/PAA bilayers and then PDAC/allyl-PAA bilayers) can be readily assessed. The results from a variety of different multilayer architectures indicate that the gradient is located within the thickness expected for the 1st deposited bilayer stack (PDAC/PAA or PDAC/allyl-PAA). These results are indicative of a dynamic dissolution-deposition process (in- and out- diffusion) during the fabrication of these LbL films. These results provide additional evidence into the mechanisms for exponential growth in LbL assemblies. The ability to quantify a gradient with the low contrast system examined indicates that spectroscopic IR ellipsometry should be able to non-destructively determine compositional gradients for most polymer films where such gradients exist.

Surface Functionalization toward Biosensing via Free-Standing Si-OH Bonds on Nonoxidized Silicon Surfaces.

Usually hydroxyl groups present on top of oxidized silicon served as binding centers for a silanization reaction toward surface functionalization. In this study, we developed a novel surface functionalization strategy where hydroxyl functionalization on nonoxidized silicon surfaces are obtained. These surfaces were stable for several weeks even in ambient air at room temperature. This high stability indicates a strong spatial isolation of the hydroxyl groups because they keenly tend to undergo a condensation reaction, forming silicon oxide. To prove the applicability of the obtained hydroxylated silicon surface, we further modified the hydroxyl groups with a commonly used silane molecule, (3-aminopropyl)triethoxysilane (APTES). The functional amino groups of the APTES layer bonded to the surface were subsequently altered by N-maleoyl-β-alanin to generate a surface highly specific for the immobilization of thiol-containing biomolecules (like thiolated single-stranded DNA or cysteine-tagged proteins). All modification steps have been investigated by IR spectroscopic ellipsometry measurements.

Temperature-dependent exciton resonance energies and their correlation with IR-active optical phonon modes in β-Ga2O3 single crystals

Temperature-dependent exciton resonance energies Eexciton in β-Ga2O3 single crystals are studied by using polarized reflectance measurement. The Eexciton values exhibit large energy changes in the range of 179–268 meV from 5 to 300 K. The IR-active Au and Bu optical phonon modes are selectively observed in the IR spectroscopic ellipsometry spectra by reflecting the polarization selection rules. The longitudinal optical (LO) phonon energies can be divided into three ranges: ℏωLO = 35–48, 70–73, and 88–99 meV. The broadening parameters, which are obtained from the reflectance measurements, correspond to the lower two ranges of ℏωLO at low temperature and 75 meV above 150 K. The large Eexciton changes with temperature in β-Ga2O3 are found to be originated from the exciton-LO-phonon interaction.

Doping of Direct Gap Ge1 -y Sn y Alloys to Attain Electroluminescence and Enhanced Photoluminescence

Alloying of elemental Ge with Sn permits tuning of the band gap, as well as a transition towards direct gap behavior. Epitaxial deposition of direct gap Ge 1-y Sn y alloys (y≥0.09) requires ultra-low temperatures of ~300°C.1–3 In order to fully realize the electroluminescence and photoluminescence (PL) potential of this class of materials, strategies for p- and n-type doping that are compatible with this temperature range are needed. In our previous work, combination of both traditional and novel dopant sources were utilized for doping of materials ranging from pure Ge to Ge 1-y Sn y alloy at the direct to indirect crossover composition of y=0.09.4,5 Presented here are the extensions of these methods to include alloys beyond the crossover composition. The n-type doped alloys were grown on virtual Ge substrates in a hot wall UHV-CVD reactor using trigermane (Ge3H8) and deuterostannane (SnD4) sources to provide the respective elements. Trisilylphosphine (P(SiH3)3) was used as a P source for n-type doping alloys in the composition range y=0.09-0.13, which required growth temperatures of 300°C-275°C. Choice of this doping agent was due to its greater stability (which facilitates long term storage) and higher vapor pressure (which allows straightforward use in mixture preparation) when compared to trigermylphosphine (P(GeH3)3), another agent capable of delivering donor P atoms at low temperatures. The use of these precursor combinations has enabled the deposition of direct gap Ge 1-y Sn y alloy films with 300-500 nm thicknesseswith dopant concentrations 5x1018-3x1019 cm-3, as determined by IR spectroscopic ellipsometry. RBS channeling spectra and (004) rocking curves show these materials to have excellent crystallinity. Furthermore, analysis using SIMS shows that the incorporated donor atoms are fully activated, and the amount of Si incorporated into the materials from using P(SiH3)3 as the doping precursor is at impurity levels which does not affect the band gap properties. These observations are in agreement with previous experiments in which P(SiH3)3 was used as a doping agent, albeit at higher temperatures.4,5 Using identical protocols but removing P(SiH3)3 from the precursor mixtures, it was possible to obtain intrinsic Ge 1-y Sn y alloys with concentrations up to y=0.14. By choosing n-type doped virtual Ge substrates for these depositions, it was possible to investigate electroluminescence, after growth of appropriate p-type layers to form n-i-p diodes, as described below. The p-type doped alloys were grown on either intrinsic or n-type doped Ge­1-y Sn y alloys with y=0.09-0.14 in order to fabricate n-p or n-i-p device stacks. The depositions were carried out in a hot wall UHV-CVD reactor with a temperature profile that enables the pre-activation of the precursor molecules prior to reaching the growth surface. The precursors used for the depositions were digermane (Ge2H6), SnD4 and diborane (B2H6). The films obtained were 100-300 nm thick, with active dopant concentrations in the 2-3×1019 cm-3 range, and compositions of y=0.08-0.12. As shown in Fig.1, n-type doping of the Ge 1-y Sn y alloys enhances PL in comparison to intrinsic counterparts. Furthermore, prototype LEDs produced with the aid of above doping strategies exhibit strong electroluminescence,2 paving the way for obtaining emission for a broad spectral range, from pure Ge at 1550 nm up to 2700 nm and beyond. References (1) Gallagher, J. D.; Senaratne, C. L.; Kouvetakis, J.; Menéndez, J. Appl. Phys. Lett. 2014, 105, 142102. (2) Gallagher, J. D.; Senaratne, C. L.; Sims, P.; Aoki, T.; Menéndez, J.; Kouvetakis, J. Appl. Phys. Lett. 2015, 106, 091103. (3) Senaratne, C. L.; Gallagher, J. D.; Jiang, L.; Aoki, T.; Menéndez, J.; Kouvetakis, J. J. Appl. Phys. 2014, 116 (13), 133509. (4) Senaratne, C. L.; Gallagher, J. D.; Aoki, T.; Kouvetakis, J.; Menéndez, J. Chem. Mater. 2014, 26 (20), 6033–6041. (5) Xu, C.; Gallagher, J. D.; Sims, P.; Smith, D. J.; Menéndez, J.; Kouvetakis, J. Semicond. Sci. Technol. 2015, 30 (4), 045007. Fig. 1 – PL spectra obtained from a liquid nitrogen cooled InGaAs detector for n-type doped (solid line) and intrinsic (dashed line) Ge 1-y Sn y samples with y=0.09. Both samples show strong luminescence as expected for direct gap materials. However the doped sample, which has an active carrier concentration of 1×1019 cm-3, shows clear enhancement of PL compared to the intrinsic sample. The inset shows the same spectra obtained from a thermoelectrically cooled PbS detector with extended spectral range beyond the cutoff point of the InGaAs detector, albeit at expense of poorer signal-to-noise ratio. Figure 1

Quality diagnostics of nanoscale AlAs/GaAs resonant tunnelling heterostructures based on IR-spectroscopic ellipsometry

In this work, the procedure of assessing the quality of AlAs/GaAs resonant tunnelling heterostructures with relation to their resistance to diffusion destruction was developed. The diffusion blur of the AlAs/GaAs heterostructure layers was detected by means of infrared-spectroscopic ellipsometry. The diffusion coefficients of Al and Si in GaAs were also calculated.

Control and stability of self-assembled monolayers under biosensing conditions

The ability to stabilize and control the attachment of cells on the surfaces of a variety of inorganic materials is important for the development of biomedical devices and sensors. An important intermediate step is the functionalization of semiconducting surfaces with a self-assembled monolayer (SAM) with an appropriate surface termination to interact with proteins. The stability of such SAMs is critical to withstanding subsequent processing and measurement conditions (e.g. long exposure to a buffer solution) to avoid artifacts resulting from such deterioration during electrical measurements. This work highlights the importance of surface cleaning and SAM chain length by comparing two commonly used short alkyl chains, aminopropyltriethoxysilane (APTES) or 3-(trimethoxysilyl)propyl aldehyde (C4-ald) molecules, with their long-chain counterparts, amino-undecilenyltriethoxysilane (AUTES) and 11-(triethoxysilyl)undecanal (C11-ald). Using IR spectroscopy, spectroscopic ellipsometry, and electrical measurements, a cleaning method is developed, based on a short room temperature (RT) SC-1 treatment, to remove photoresist without degrading device performance as is the case with currently used oxygen plasma methods. The spectroscopic and electrical measurements also show that short-chain SAMs, typically used for pH- or bio-sensing, do not have the stability suitable for biosensor environments. In contrast, long-chain SAMs display much higher stability and can be reproducibly grafted. These findings are the basis for a reliable preparation and robust operation of biosensors.

Dielectric properties and thickness metrology of strain engineered GaN/AlN/Si (111) thin films grown by MOCVD

Maturity of silicon nanoelectronics and the high quality of 300mm epi-Si wafers make these substrates an ideal choice for the growth of high quality III-Nitride devices. The results of our substrate engineering technique, which involves implantation of nitrogen into Si through an AlN thin film, have shown to simultaneously and significantly reduce the dislocation density and macro-cracks in epitaxially grown 2μm GaN films. In this study, high quality strain engineered GaN films were grown by metalorganic chemical vapor deposition (MOCVD) and spectroscopic ellipsometry was used to characterize the dielectric properties, thickness, and stress of the complex structure. The uniaxial, anisotropic dielectric functions of wurtzite GaN and AlN were determined for the processes used in this study, and using this information, the thickness of each layer was determined in the completed film stack. IR spectroscopic ellipsometry (IRSE) was used as the non-destructive characterization technique to identify the IR sensitive phonon modes in AlN. The stress evolution in the films was investigated as a function of the phonon frequency shift and the broadening of the phonon modes. The results obtained by IRSE were further complemented by high resolution X-ray diffraction (HRXRD) and Raman scattering measurements.

Multimode phonon structure of Be‐containing II ‐ VI mixed crystals determined by IR spectroscopic ellipsometry

AbstractSpectroscopic ellipsometry in the infrared spectral range 250‐5000 cm‐1 is used for analysis of the dielectric response of Zn1‐x‐yBexMgySe and Zn1‐x‐yBexMnySe crystals grown by a high‐pressure Bridgman method. Ellipsometric spectra display features in the spectral range 390‐500 cm‐1 associated with BeSe‐type phonon modes. In the optical spectra of Zn1‐x‐yBexMgySe crystals both BeSe‐type and MgSe‐type lattice absorption bands are detected. The MgSe‐like modes are located at approximately 300 cm‐1. The complex dielectric functions can be reproduced using a model with two or three and one or two classical damped oscillators corresponding to the BeSe‐like and the MgSe‐like transverse‐optical phonon modes, respectively. The frequencies of longitudinal‐optical phonons have been derived from the dielectric loss functions. A red‐shift of the BeSe‐like phonons frequencies with a mean rate 0.42 cm‐1 (0.50 cm‐1) per mole percent of Mg (Mn) incorporated to the alloy has been found for examined concentration range x, y ≤ 0.25. A noticeable damping the intensities of BeSe‐type modes with increasing fraction of Mg and Mn dopant is observed in comparison to the strengths of BeSe‐type modes in Zn1‐xBexSe crystals. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Methodology for Quantitative Measurements of Multilayer Polymer Thin Films with IR Spectroscopic Ellipsometry

A new methodology to quantify the sensitivity and resolution of infrared spectroscopic ellipsometry (IRSE) measurements was developed for probing the depth profile of composition in thin polymer films. A multilayer system comprised of sub-100 nm films of poly(4-hydroxystyrene) on top of poly(methyladamantyl methacrylate) was used as a model test structure for our IRSE measurements of bilayer film thickness and composition profile. The IRSE results are further validated by comparing to high-resolution neutron reflectivity results also from these samples. We demonstrated that the substrate type, incident angle, and specific spectral band regions strongly influence the sensitivity of IRSE; a judicious choice of measurement variables and infrared spectral regions can substantially improve the measurement in terms of sensitivity. Most strikingly, an incidence at the Brewster angle of the substrate does not lend to the maximum sensitivity. Discussions of the origin of these findings and metrics are provided.

Characterisation of Thin Composite Films from Hyperbranched Polyphenylene and Thermolabile Hyperbranched Polycarbonate

AbstractThin composite films made from a thermally stable hyperbranched polyphenylene and a thermolabile hyperbranched polycarbonate were prepared on silicon wafers and characterised. The end groups of the polycarbonate were varied in order to accomplish an optimised interaction between the two components and thus the formation of homogeneous blends. The composite films were annealed at 200 °C in order to decompose the hyperbranched polycarbonate which can act as a porogen. The composition of the films before and after the annealing process was investigated applying IR spectroscopy and spectroscopic ellipsometry. Analysis of the data allowed calculating the volume fractions of the two materials within the films.magnified image

Nitrogen‐Rich Plasma‐Polymer Films for Biomedical Applications

AbstractLow‐pressure plasma co‐polymerisation of binary gas mixtures of ethylene and ammonia was investigated in order to deposit N‐rich plasma polymer coatings for biomedical applications. Deposition kinetics and relevant surface characteristics were investigated as a function of r.f. power, gas pressure, and NH3/C2H4 ratio. Physico‐chemical properties of the coatings were determined by surface‐sensitive techniques: for example, total nitrogen concentrations up to 40 at.‐% were measured by XPS, while those of primary amines were determined by chemical derivatisation followed by XPS analysis. L‐PPE:N films were further characterised by UV‐vis and IR spectroscopic ellipsometry and by CAG.magnified image

Studies of electrochemically grafted thin organic layers on inorganic surfaces with infrared spectroscopic ellipsometry

We present IR spectroscopic ellipsometry (IRSE) measurements of electrochemically grafted organic ultrathin films down to monolayer thickness. The formation of organic layers of 4-methoxybenzene (anisole) on TiO2, Au, and Si(111) surfaces was confirmed from observation of the respective absorption bands in the ellipsometric spectra. We discuss the orientation of molecules in a thin film on an Au substrate and in a thick film grafted on a TiO2 substrate. The appearance of silicon oxide related bands in the IRSE spectra shows that oxidation is a side reaction of the electrochemical grafting on Si substrates. These results demonstrate the potential of IRSE application in studies of interfacial structures and thin film engineering.

Self-assembly of terminally aryl-substituted long-chain alkanethiols on silver

Self-assembled monolayers (SAM) from para-substituted benzyl-16-mercaptohexadecanoates (R = –Cl; –OCH 3; –NO 2; –CN) were prepared using standard deposition procedures on silver substrates. The substituents were attached to the terminal benzyl group of the SAM precursor molecules in order to modify the properties of the outer film surface and to probe their influence on the organizational order of the monolayers. The SAMs were characterized by infrared spectroscopic ellipsometry and contact angle measurements. The carbonyl stretching as well as the methylene symmetric and antisymmetric stretching vibrations were employed to determine the tilt angle of the methylene chains. The water advancing contact angles implied that the wetting of the film's outermost surface is substantially different from that expected for more polar groups like NO 2, CN and OCH 3 and is much closer to that reported for aryl terminated SAMs. The flexible methylene linkage can facilitate the free rotation around C–Ph bond that leads to a specific orientation of the terminal phenyl rings in which they are closely exposed to the interface. An attempt was made to qualitatively characterize the phenyl ring orientations of the different SAMs using the IR spectroscopic ellipsometry data and IR absorption spectra of the same molecules.

Investigation of the optical anisotropy of PET and PEN films by VIS-FUV to IR spectroscopic ellipsometry

In the last years, a significant amount of research is being performed in the field of polymer research for novel applications, such as flexible electronic devices, photovoltaic cells, high performance optics, data storage, etc. Toward this direction, in this work, the optical anisotropy of biaxially stretched poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) films has been extensively investigated. The optical properties of the films have been studied in terms of their optical, electronic and vibrational response, by Fourier transform IR spectroscopic ellipsometry (FTIRSE) (900–3500 cm −1) and Vis-fUV variable angle SE (1.5–6.5 eV) techniques. The films optical anisotropy is the result of the stretching procedure during their fabrication, which results to the structural rearrangement of the macromolecular chains parallel to the stretching direction and to a higher structural symmetry. During the SE spectra analysis, the films have been approximated as uniaxial materials with their optic axis parallel to the sample/ambient interface leading to the accurate determination of the principal components ɛ ||( ω) and ɛ ⊥( ω) of the dielectric function ɛ( ω). The detailed study of the electronic transitions has been performed in the Vis-fUV region, where the characteristic features corresponding to the n → π* electronic transitions of the carbonyl –C O group and the 1A 1g → 1B 1u transition due to the π → π* excitation of the π-electron structures have been identified and analysed. Furthermore, the FTIRSE spectra allowed the accurate identification and assignment of the features of ɛ( ω) to the vibrational modes of the various bonding structures characteristic of the PET and PEN macromolecular chains.

Investigation of II –VI alloy lattice dynamics by IR spectroscopic ellipsometry

Optical properties related to lattice vibrations of bulk Zn1-xBexSe, Zn1-x-yBexMnySe and Cd1-xBexSe alloys are reported. Optical phonons of the mixed crystals grown by the Bridgman method have been investigated by infrared spectroscopic ellipsometry in the wave-number range 300-8000 cm-1. The complex dielectric functions can be reproduced using a model with two or three classical damped oscillators corresponding to the BeSe-like transverse-optical phonon-modes and a Drude contribution from the free carriers. The frequencies of longitudinal-optical phonons have been found from the imaginary parts of the dielectric loss functions. The influence of the alloy composition on the zone-centre optical phonon frequencies is discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

On the optical anisotropy of poly(ethylene terephthalate) and poly(ethylene naphthalate) polymeric films by spectroscopic ellipsometry from visible-far ultraviolet to infrared spectral regions

The optical anisotropy of biaxially stretched poly(ethylene terephthalate) and poly(ethylene naphthalate) films has been investigated, by visible-far ultraviolet to IR spectroscopic ellipsometry (SE). The consistency of the SE spectra analysis results between the two energy regions, in a full rotational scan of the angle θ between the plane of incidence and the stretching direction, justified the approximation of the films as uniaxial materials with their optic axis parallel to the surface.

Optical investigations of the effect of temperature and plasma conditions on the growth of sp 3-bonded BN thin films

Boron nitride (BN) films with high cubic (sp 3) content using various conditions were deposited on c-Si substrates by rf magnetron sputtering. Fourier Transform IR Spectroscopic Ellipsometry (FTIRSE) has been employed for the investigation of the effect of the substrate temperature ( T s) and gas partial pressure on the films bonding structure. A two-layer optical model was applied for the analysis of the FTIRSE spectra leading to the determination of the individual layer thickness and optical properties, as a function of the deposition parameters in order to get better insights on the phase evolution and microstructure of the BN films. As a result, the critical experimental conditions required to sustain c-BN growth were determined. It was found that at higher T s, the formation of c-BN is favored and followed by a significant reduction of the corresponding layer thickness as a combined effect of the reduction of the B atoms sticking coefficient, the preferential etching of the h-BN regions and the existence of thermal spikes in the growing BN films. Furthermore, the investigation of the effect of plasma parameters on BN phase evolution and c-BN optical properties showed that the addition of N atoms in the plasma promotes the growth of c-BN. These results have been supported and justified by the study of the mechanical behavior of the BN films by additional depth sensing nanoindentation measurements.

Optical and chemical characterization of expanding thermal plasma deposited silicon dioxide-like films

In this paper we report on the use of expanding thermal plasma technique for the deposition of carbon-free silicon dioxide films by means of argon/hexamethyldisiloxane/oxygen mixtures, at growth rates in the range of 6–12 nm/s, as a function of the substrate temperature. The variation of substrate temperature has been investigated as an alternative route to ion bombardment in promoting the densification of SiO 2-like films. Information concerning film chemical composition was obtained by means of Fourier transform IR absorption and X-ray photoelectron spectroscopies. The films are stoichiometric and a residual hydrogen content of 3.5·10 21 at/cm 3 is detected in the film deposited at 300 °C (SiO 2H 0.17). The film optical characterization by means of UV–VIS and IR spectroscopic ellipsometry has highlighted the beneficial role of substrate temperature in the densification of the SiO 2 film matrix and the decrease of film porosity.