Infrared Spectroscopic Ellipsometry Research Articles

Methods for Determining the Infrared Complex Refractive Indices n(λ) and k(λ) from Organic Solid Powders: Comparison of a Sucrose Single Crystal and a Pressed Pellet

There exists a need to measure optical constants n and k for many materials, especially organic solids in the infrared spectral region, but at present, there are only two established methods for making such measurements: single-angle (SA) reflectance and infrared spectroscopic ellipsometry (IRSE). The former derives the n/k vectors via measurement of the change in the amplitude of light reflected from the sample and then uses the Kramers–Kronig transform, while the latter measures the change in polarization of the reflected light from the sample to derive n/k. Here we make a direct comparison between the n/k vectors derived from SA measurements of a sucrose single crystal and the n/k values obtained after the crystal had been crushed, ground to powder, and pressed into a pellet form. The pellet was measured not only by SA but also via the IRSE method, and we found that pressing specular-quality pellets was critical, especially for the SA data. The n/k values of this typical organic molecule all agree to within 0.04 root-mean-square error, confirming that a pressed pellet can be used instead of a crystal to derive the intrinsic n/k vectors. This result is important for the many organic materials that, unlike sugar, do not have centimeter-sized crystals readily available.

Infrared and Raman spectroscopic analysis of functionalized graphene

AbstractIn this mini‐review, we discuss our work in the analysis of material properties of electrochemically functionalized graphene using infrared (IR) and Raman techniques as thin film sensitive vibrational spectroscopies. Multiscale characterization is demonstrated using a combination of IR spectroscopic ellipsometry (IRSE), confocal Raman spectroscopy (RS) and photothermal atomic force microscopy coupled IR analysis (AFM‐IR). IRSE is used for spots with dimensions of a few mm, RS for spots with a diameter of a few micrometer and AFM‐IR at the sub‐100 nm scale. In the first part of the article, functionalized large‐area graphene sheets electrochemically modified by ultrathin oligomer films of maleimidophenyl or 4‐aminophenyl acetic acid are studied. Characteristic molecular vibrations of the functional layers are analyzed by the IR methods, while the graphene‐related phonons are characterized by RS. This dual approach allowed for the determination of the attached functional groups as well as the identification of the nature of chemical coupling of the functional moieties to graphene. The thickness of the deposited layers extracted from IRSE data correlates very well with AFM measurements. In the second part of the article, functionalized graphene sheets are characterized by correlative vibrational optical spectroscopy directly at a device level.

Optical, microstructural and vibrational properties of sol–gel ITO films

The aim of this paper is to prepare multi-layered ITO thin films by a low cost and environmental-friendly method for different applications (optoelectronics, sensors, etc.). ITO films with 15 layers were obtained by successive depositions using the sol-gel & dip-coating method on three different substrates: glass, SiO2/glass and SiO2/Si. Comparative structural, morphological and optical characterization were performed by X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), Cross Section Transmission Electron Microscopy (XTEM) coupled with Selected Area Electron Diffraction (SAED), Infrared Spectroscopic Ellipsometry (IRSE) and Raman spectroscopy analyses. The optical constants (refractive index n and extinction coefficient k) were determined in a large spectral range (300-27500 cm−1) by spectroscopic ellipsometry (SE). The thicknesses determined by SE were confirmed by HRTEM (High Resolution TEM) measurements which also presents in detail the textural properties of the ITO films at nanometric level. A comparison between IRSE and Raman analysis in the infrared active region was presented.

In situ real-time and ex situ spectroscopic analysis of Al2O3 films prepared by plasma enhanced atomic layer deposition

In situ real-time ellipsometry (irtE) with a very high time resolution of 24 ms was applied to monitor the inductively coupled plasma enhanced atomic layer deposition (ALD) process of Al2O3 thin films to precisely resolve each step of the ALD process and its complete cycle. The influence of plasma power, plasma pulse duration, and deposition temperature on the film growth characteristics was investigated. Ex situ ellipsometry [UV-VIS-NIR-SE (ultraviolet-visible-nearinfrared-spectroscopic ellipsometry) and IR-SE (infrared spectroscopic ellipsometry)] and x-ray photoelectron spectroscopy revealed the bulk properties (thickness, refractive index, chemical composition, and carbon incorporation) of the films, which together with the in situ results are compared to those of the films prepared by thermal ALD (T-ALD). The ICPEALD (inductively coupled plasma enhanced ALD) films were deposited at substrate temperatures between 80 and 250 °C and the role of plasma power (50–300 W) and its pulse duration (1–20 s) was investigated at 250 °C. The reference T-ALD layers were prepared at 200 °C. The ICPEALD process of Al2O3 shows an increased growth rate, and the produced films exhibit higher carbon contaminations than the T-ALD Al2O3 films. Plasma pulse times of up to 15 s further increase the content of carbon and CH species; at the same time, the refractive index decreases. The optical properties of ICPEALD deposited Al2O3 films are comparable with those of the T-ALD films for low plasma power and short plasma pulse durations. For the ICPEALD films, UV absorption is found and it is dependent on the deposition parameters. irtE resolves process effects that correlate with the bulk properties of Al2O3, such as impurities and oxygen deficiencies.

In Situ Infrared Spectroscopic Monitoring and Characterization of the Growth of Polydopamine (PDA) Films

Modern infrared (IR) spectroscopic methods as in situ IR spectroscopic ellipsometry (in situ IRSE) and the photothermal atomic force microscopy (AFM)‐IR technique are introduced as novel analysis methods for characterization of polydopamine (PDA) films. The visible (VIS) and IR ellipsometric studies serve for determination of thicknesses, IR optical constants and discussion of specific vibrational bands. The in situ IR ellipsometric studies (IRSE) of the thin film growth of polydopamine (PDA), a versatile material for bioactive surfaces, in tris(hydroxymethyl)aminomethane (Tris) buffer solution are used to monitor the time‐dependent growth process of a PDA film. Complementary methods as X‐ray photoelectron spectroscopy (XPS) and the nanoscale photothermal AFM‐IR technique give access to study chemical homogeneity of the probed spots. All of the results are in qualitative agreement and show up new analysis possibilities, even further work is required to resolve the open questions of the chemical structure of PDA films. As proof of principle reaction for biosensor applications the binding of thiol‐terminated molecules to the PDA film via Michael‐addition was investigated by in situ IRSE.

Characterization of self-assembled monolayers for Cu[sbnd]Cu bonding technology

CuCu bonding process is intensively investigated nowadays because of the possibility of obtaining fine-pitch interconnects in 3D stacks for efficient electronic and sensing applications. These interconnects possess high shear strength, as well as excellent electrical and thermal conductivity compared to commonly used solder joints. The bottleneck of CuCu bonding technology is the rapid oxidation of Cu and adsorption of a contamination layer upon exposure to air. This effect can be strongly reduced by using self-assembled monolayers (SAMs) as temporary protective coatings on Cu substrates.The results of X-ray photoelectron spectroscopy (XPS) yielded a drastic decrease in oxidation rate of both sputtered and electroplated Cu surfaces during storage of passivated samples at −40 °C for 7 days. The results of infrared spectroscopic ellipsometry (IRSE) showed that after 10 days of air exposure at room temperature the long-chain 1-hexadecanethiol (C16) SAMs appear to still remain on the Cu surface and to have much slower oxidation rate in comparison to the short-chain 1-hexanethiol (C6) SAMs. Therefore, we can make a conclusion that the application of long-chain alkanethiols or storage of passivated samples at low temperature can provide a long-term oxidation protection for Cu surfaces stored in air.

Electronic Properties of Ag Reconstructions on Si(111): Coulomb Blockade Behavior at Room Temperature

The electronic properties and optical response of Si(111)–Ag reconstructions, with distinct interface structures, are studied with scanning tunneling microscopy (STM), reflectance anisotropy spectroscopy (RAS), and infrared spectroscopic ellipsometry (IRSE). The results are compared to previous studies on the Si(111)‐(3 × 1)–Ag surface. Depending on the preparation conditions, STM shows hybrid surfaces comprising (√3 × √3) areas with semiconducting (3 × 1)–Ag regions or metallic Ag islands. RAS of the (√3 × √3)–Ag reconstruction mostly shows a flat response, confirming the isotropic nature of the surface while Ag islands on this surface produce a steep increase in the infrared as well as a strong optical anisotropy at 3.5 eV. Scanning tunneling spectroscopy (STS) of the islands shows a high density of states with almost no structure. STS of Ag nanodots on the (3 × 1)–Ag surface shows distinct Coulomb resonances within the Si bandgap while the Si(111)‐(3 × 1)–Ag surface reveals an expected semiconducting behavior with a band gap and no sign of resonances. Ag islands on the (3 × 1)–Ag surface also show weak Coulombic oscillations indicating that local transport depends on the electronic properties of the interface.

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.

CMOS Compatible in-Situ N-Type Doping of Ge Using New Generation Doping Agents P(MH3)3 and As(MH3)3 (M=Si, Ge)

New in-situ methods have been developed to dope Ge-on-Si films with P and As using reactions of novel compounds P(MH3)3 and As(MH3)3 (M=Si, Ge) and UHV-CVD and molecular source epitaxy methods. In these experiments Ge3H8, Ge4H10 hydrides were used to deposit intrinsic Ge layers on Si(100) to produce i-Ge/Si(100) platforms upon which the n-Ge layers were grown at ultra-low temperatures of 300-330°C. The resultant n-Ge/i-Ge/Si(100) samples exhibited device quality crystallinity and defect-free microstructures as evidenced by XTEM. Infrared spectroscopic ellipsometry (IRSE) and Hall revealed carrier concentrations, mobilities and resistivities that are on par with values observed in bulk Ge. SIMS gave flat dopant profiles and abrupt transitions at the n/i interfaces. The highest carrier concentration was achieved in As doped samples at 8.44×1019 cm-3 and the lowest resistivity was observed in P doped samples at 4.2×10-4 Ω∙cm. Comparison between IRSE and SIMS data revealed near full activation (>80%) of the absolute dopant concentrations in the range of 1×1020 cm-3. The methods reported here have the potential for applications in group-IV semiconductor technologies that require high doping levels, low resistivities and shallow junction depths.

Temperature dependent spectroscopic ellipsometry of Ag2Se and Ag2S with phase transitions from ionic to superionic conductivity state

AbstractTemperature dependent infrared (IR) and optical spectroscopic ellipsometry methods are used to investigate peculiarities of the transitions from the ionic conductivity to the superionic conductivity states in Ag2S and Ag2Se compounds. The structural phase transitions (SPTs), known as β→α transition in these semiconductors, are investigated also with spectroscopic ellipsometry methods. The electronic band structure of Ag2S at the SPT changes sharply from the monoclinic to the bcc phase, which occurs at T = 453 K in the super ionic conductivity state, where all the interband transitions in optic region, corresponding to the monoclinic structure of β‐phase, disappear. The α‐phase of Ag2S is characterised with new lines in the dielectric functions ε1 and ε2, which arise at 5‐6.5 eV. The interband transitions do not change in the SPT from the orthorhombic to the bcc in Ag2Se, which occurs at T > 406 K. In IR ellipsometry for the degenerated Ag2Se, the free carriers' plasma energy is displaced abruptly from ħω=0.075 eV at T=300 K to ħω=0.125 eV at T=338 K. The same effect is seen in the IR ellipsometry for Ag2S, where negative ε1 appears at T=434‐440 K. These facts indicate a drastic increase of the free carriers concentrations in both of the semiconductors at temperatures before the β→α SPT. In Ag2S the second abrupt shift of the free carriers plasma energy from 0.035eV to 0.1 eV occurs at T=453 K, corresponding to β→ α SPT. The second shift is due to the electronic band structure change of Ag2S. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ infrared ellipsometric monitoring of the growth process of polyaniline thin films and doping with poly(4-styrenesulfonate)

In this work, different undoped and doped polyaniline (PANI) films were synthesized by electrochemical pulse deposition. The growth process was in situ investigated using a combined infrared spectroscopic ellipsometry (IRSE) and optical spectroscopy set-up in the visible spectral range [Vis-ellipsometry and reflectance anisotropy spectroscopy (RAS)]. A growth acceleration effect and thicker PANI films in the presence of poly(4-styrenesulfonate) (PSS) are concluded from the measurements in the infrared and visible spectral range. IR-microscopic spectra revealed a strong inhomogeneity of the obtained PANI film doped with PSS. Moreover, the interpretation of the in situ IR ellipsometry spectra not only delivered qualitative (e.g., chemical structure and composition) information, but also quantitative information (e.g., film thickness) in the growth process of the PANI films by applying an optical layer model.

In Situ Infrared Ellipsometry for Protein Adsorption Studies on Ultrathin Smart Polymer Brushes in Aqueous Environment

The protein-adsorbing and -repelling properties of various smart nanometer-thin polymer brushes containing poly(N-isopropylacrylamide) and poly(acrylic acid) with high potential for biosensing and biomedical applications are studied by in situ infrared-spectroscopic ellipsometry (IRSE). IRSE is a highly sensitive nondestructive technique that allows protein adsorption on polymer brushes to be investigated in an aqueous environment as external stimuli, such as temperature and pH, are varied. These changes are relevant to conditions for regulation of protein adsorption and desorption for biotechnology, biocatalysis, and bioanalytical applications. Here brushes are used as model surfaces for controlling protein adsorption of human serum albumin and human fibrinogen. The important finding of this work is that IRSE in the in situ experiments in protein solutions can distinguish between contributions of polymer brushes and proteins. The vibrational bands of the polymers provide insights into the hydration state of the brushes, whereas the protein-specific amide bands are related to changes of the protein secondary structure.

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.

Infrared optical properties of ferroelectric 0.5BaZr0.2Ti0.8O3–0.5Ba0.7Ca0.3TiO3 thin films

0.5BaZr0.2Ti0.8O3–0.5Ba0.7Ca0.3TiO3 (BCZT50) thin films have been prepared on platinized silicon substrates by a modified sol–gel technique and proved to be well-crystallized perovskite ferroelectrics at room temperature. Optical properties of these films in the 2.5–12.5μm wavelength range have been investigated by infrared spectroscopic ellipsometry (IRSE) at room temperature. The measured ellipsometry parameters are well fitted by Brendel oscillator dispersion formula. The optical functions and thickness of films have been uniquely extracted. As the wavelength increases, the refractive index decreases while its extinction coefficient increases. The calculated infrared absorption coefficient increases with increasing wavelength in measured wavelength range. It is greater than 1000cm−1 for the wavelength over 11.3μm. And it is very small and less than 100cm−1 for the wavelength under 7.5μm. The relatively large of the thin films reveals that it is a promising material for infrared pyroelectric detectors and arrays.

In-situ characterization of the temperature-sensitive swelling behavior of poly(N-isopropylacrylamide) brushes by infrared and visible ellipsometry

In-situ infrared-spectroscopic characterization of polymer brushes plays an important role for a deeper understanding of the brushes' structural and chemical properties during stimuli-induced switching experiments. In this work, we used infrared-spectroscopic ellipsometry (IR-SE) to investigate the temperature-dependent swelling behavior of poly(N-isopropylacrylamide) [PNIPAAm] brushes below and above their lower critical solution temperature. IR-SE spectra were evaluated using optical-layer calculations in order to determine swollen-brush thickness and water content inside the brush. For a ddry=12.6nm thin PNIPAAm brush, we found swollen- and deswollen-brush thicknesses of respectively (43±8)nm and (20±9)nm, which is in good agreement with ellipsometry results in the visible (VIS) spectral range. We determined water contents of (86±13)% and (68±15)%, and by correlating IR-SE with VIS-ellipsometric measurements, we were even able to obtain values of up to 2% accuracy.

Near-Ideal Complete Coverage of CD3 onto Si(111) Surfaces Using One-Step Electrochemical Grafting: An IR Ellipsometry, Synchrotron XPS, and Photoluminescence Study

A one-step electrochemical grafting process using Grignard reagents has been performed to achieve a complete monolayer methyl-terminated Si(111) surfaces. Anodic treatment (0.5 mA/cm2 for 300 s) has been applied to atomically flat H-terminated Si(111) surfaces in methylmagnesium bromide (CH3MgBr), methylmagnesium iodide (CH3MgI), and methyl-d3-magnesium iodide (CD3MgI) to obtain methylated Si(111) surfaces. Infrared spectroscopic ellipsometry (IRSE) clearly reveals a vibrational shift of the symmetric “umbrella” mode characteristic for methyl groups (CH3 and CD3) with a preferential z-orientation. Additionally, X-ray photoelectron spectroscopy using synchrotron radiation (SXPS) shows a well-defined splitting of the Si 2p core level spectra from the methylated Si(111) surfaces. This splitting is more pronounced for the CD3-terminated Si(111) surfaces. Moreover, the C 1s spectra also confirm the well-defined structure of the CD3-terminated Si(111) surfaces by the presence of C–D vibrational stretching features. Photoluminescence (PL) measurements reveal better surface passivation in the case of CD3-terminated Si(111) surfaces. Finally, the quality of the surfaces depends strongly on the counterions. Grignard reagents containing iodine show the best performance in the formation of complete monolayer methylated Si(111) surfaces.

Infrared ellipsometry and near-infrared-to-vacuum-ultraviolet ellipsometry study of free-charge carrier properties in In-polar p-type InN

ABSTRACTWe apply infrared spectroscopic ellipsometry (IRSE) in combination with near-infrared to vacuum-ultraviolet ellipsometry to study the concentration and mobility of holes in a set of Mg-doped In-polar InN samples of different Mg-concentrations. P-type behavior is found in the IRSE spectra for Mg-concentrations between 1x1018 cm-3 and 3x1019 cm-3. The free-charge carrier parameters are determined using a parameterized model that accounts for phonon-plasmon coupling. From the NIR-VUV data information about layer thicknesses, surface roughness, and structural InN layer properties are extracted and related to the IRSE results.

Optical arrangement and proof of concept prototype for mid infrared variable angle spectroscopic ellipsometry

We present an optical setup for variable angle mid infra red spectroscopic ellipsometry. The arrangement can be placed into the sample compartment of a Bruker ifs66v/s vacuum Fourier transform infrared spectrometer. A first prototype of the setup has been tested in the spectral range from 650 cm −1 to 4000 cm −1 and can measure incidence angles between 8° and 87°. We compare the measured data to reference measurements with a commercial variable angle infrared spectroscopic ellipsometer. The comparison gives a proof of concept for the discussed optical arrangement.

Infrared spectroscopic ellipsometry (IRSE) and X‐ray photoelectron spectroscopy (XPS) monitoring the preparation of maleimide‐functionalized surfaces: from Au towards Si (111)

AbstractThe IR ellipsometric technique was used to identify the surface species and to control the preparation of maleimide‐terminated surfaces. Because of higher s/n ratios for metallic substrates, the protocol was initially developed on Au surfaces, was later successfully transferred to technologically more relevant Si (111) substrates. The functionalized surfaces were achieved by electrochemical deposition of diazonium linker films and following chemical adsorption steps. Complementary XPS was also employed to detect the surface species in the process of preparation. The immobilization of different functional molecules was proven by interpreting the specific vibrational bands in IR spectra and additionally confirmed by XPS experiments. The surface homogeneity was investigated by FT‐IR synchrotron mapping ellipsometry. This work shows that the proposed protocol is an effective pathway to achieve the desired functionalized surfaces. Copyright © 2010 John Wiley & Sons, Ltd.

Synchrotron infrared spectroscopic ellipsometry for characterization of biofunctional surfaces

AbstractThe structural and thickness homogeneity of different biofunctional surfaces after coating an Au substrate was studied by infrared spectroscopic ellipsometry (IRSE). Monolayer coverage was achieved by washing the samples in aqueous solution. A selected cysteine‐modified CHL peptide and an anti‐glutathione S‐transfer (GST) antibody produced a characteristic change in the IR ellipsometric spectra after peptide and antibody adsorption, confirming the biosensing ability of this technique. IR ellipsometric mapping at the synchrotron storage ring BESSY II in Berlin allowed laterally resolved characterization of the protein films. The IR ellipsometric tanΨ and Δ maps revealed that the protein films are inhomogeneous in structure and thickness after adsorption of the peptide and antibody. The absolute thickness of the protein films varied considerably by up to 3 to 7 nm. We also studied cytosine films with different thicknesses (d = 58 nm and 125 nm) and a guanine film (d = 84 nm). For the two cytosine films the synchrotron spectra revealed that the thicker film in particular was rather homogeneous in thickness but inhomogeneous in structure. For the guanine film the shape of vibrational bands in the ellipsometric spectra correlated with anisotropic molecular orientations.