Numerical ellipsometry: Advanced analysis of thin absorbing films in the n-k plane

A major challenge for those utilizing ellipsometry is numerical processing of the measured data. The transcendental, multivalued equations arising from the physics of simple reflection are problematic for the least-squares numerical methods in common use. Previously we have applied Complex Analysis in the n-k plane and this has led to a growing array of numerical research methods which achieve computational accuracy at a chosen limit (~ 10^− 14) rather than achieving only a mean square value of a large number of measurements, more than 14 orders of magnitude less accurate in this case. The work presented here is the mathematics for film stacks of an arbitrary number of layers required to apply numerical methods for use with absorbing films deposited on transparent and on absorbing substrates. The approach finds intersections of projections of three “twisted curves” resulting from a single light incidence angle and light wavelength measurement for three film thicknesses at a time. The approach employed here determines the thickness and optical properties of chromium films (between 10 and 25 nm nominal thickness) deposited onto silicon substrates. A simple single film model matched the measurements to a first order only. Improved agreement is obtained by the addition of interface layers above and below the main chromium film.     

Spectroscopic ellipsometry analyses of thin films in different environments: An innovative “reverse side” approach allowing multi angle measurements

An innovative ellipsometer sample holder has been designed and tested in order to measure thin films optical properties under different environments and so infer the porosity through effective medium approximation models. Compared to commercial cells that require a fixed angle of incidence or a cell with a cylindrical geometry, we present a simple cell in which the sample is mounted in “reverse side”, allowing multiple angle analyses without the need for cell windows. Standard ellipsometry measurements are compared to the “reverse side” approach in order to confirm the feasibility of this new procedure, obtaining the same refractive index dispersion curves in both cases. Then different samples have been tested in “reverse side” under different environments to measure porosity. The multiangle approach has been found useful to improve the fitting of the experimental data by reducing both the fitting error and the correlation between parameters.     

Ellipsometry study of poly(o-methoxyaniline) thin films

Spin-coated poly(o-methoxyaniline) (POMA) thin films on various substrates were investigated using spectroscopic ellipsometry (SE) in the 1.5–4.5 eV photon energy range. Spin-coating process parameters are reported (spin speed and concentration). Substrates with higher surface energy were found to increase polymer film thickness and decrease roughness. An optical model was developed using SE data along with complementary data from atomic force microscopy and UV–vis spectroscopy to obtain optical properties—refractive index n and extinction coefficient k for POMA. The model includes Lorentz oscillators for the POMA film and a Bruggeman effective medium approximation for roughness. In-plane film optical anisotropy was not observed, but a small out of plane anisotropy was detected for the polymer.     

The dielectric functions and optical band gaps of thin films of amorphous and cubic crystalline Mg~ 2NiH~ 4

Magnesium nickel hydride films have earlier been suggested for several optoelectronic applications, but the optical properties and band gap have not been firmly established. In this work, the dielectric functions and the optical band gaps of thin films of Mg₂NiH₄ have been determined experimentally from optical modeling using spectroscopic ellipsometry and spectrophotometry in the photon energy range between 0.7 and 4.2 eV. Samples were prepared by reactive sputtering, resulting in a single-layer geometry that could easily be studied by ellipsometry. Crystalline samples were prepared by annealing the as-deposited amorphous films ex-situ. The resulting films remained in the high temperature cubic Mg₂NiH₄ structure even after cooling to room temperature. Tauc analysis of the dielectric functions shows that Mg₂NiH₄ films exhibit a band gap of 1.6 eV for the amorphous structure and 2.1 eV for the cubic crystalline structure.     

When Pt meets BaTiO3: Interaction between metal nanoparticles and oxide thin films

With the help of a focused ion beam, we can control the location of BaTiO₃ (BTO) grains during pulsed laser deposition by the use of metal nanoparticles seeds as templates. The metal nanoparticles act as nucleation sites on which to grow the BTO grains. Combined with recent work where control of BTO grain orientation was successfully demonstrated [Thin Solid Films, 518, 5806 (2010)], we can widely use the method to deposit various oxide nanoparticles in particular locations with controlled size and orientation. Such an approach could be useful for nanotechnology and device fabrication.     

Real time spectroscopic ellipsometry of Ag/ZnO and Al/ZnO interfaces for back-reflectors in thin film Si:H photovoltaics

Real time spectroscopic ellipsometry (RTSE) has been applied to analyze the optical characteristics of Ag/ZnO and Al/ZnO interfaces used in back-reflector (BR) structures for thin film silicon photovoltaics. The structures explored here are relevant to the substrate/BR/Si:H(n-i-p) solar cell configuration and consist of opaque Ag or Al films having controllable thicknesses of microscopic surface roughness, followed by a ZnO layer up to ~ 3000 Å thick. The thicknesses of the final surface roughness layers on both Ag and Al have been varied by adjusting magnetron sputtering conditions in order to study the effects of metal film roughness on interface formation and interface optical properties. The primary interface loss mechanisms in reflection are found to be dissipation via absorption through localized plasmon modes for Ag/ZnO and through intraband and interband transitions intrinsic to metallic Al for Al/ZnO.     

Properties of composite a-C:H/metal layers deposited by combined RF PECVD/magnetron sputtering techniques

A sequential deposition method for synthesis of composite materials is presented. It consists of the periodical exposing the substrate to separated plasma assisted chemical vapor deposition and magnetron sputtering deposition sources. The plasma sources are operated independently. Using this deposition method, a-C:H/W composite layers were deposited on silicon substrates. The material morphology, structure and composition were investigated using specific techniques (AFM, SEM, EDX, SIMS and XRD). The material composition, as example the W concentration in the range of 10-50 at.%, can be tuned in a-C:H/W composites by proper setting the exposure time of the substrate to each of the plasma sources. The process limit that makes the difference between deposition of composites and multilayers is discussed.     

Optical characterisation of radio frequency plasma polymerised Lavandula angustifolia essential oil thin films

Optically transparent RF plasma polymerised thin films were fabricated from Lavandula angustifolia essential oil under varying RF power levels and their optical properties investigated. The refractive index, extinction coefficient, absorption and optical band gap of the thin films in addition to their thickness and roughness were investigated using the spectroscopic ellipsometry and UV-Vis spectroscopy in the wavelength range 200-1000 nm (6.199-1.239 eV). For films fabricated under the RF power from 10 W to 75 W, the refractive index values vary from 1.530 to 1.543 at 500 nm. Even though the refractive index is unaffected by the RF power, the optical band gap tends to decrease with increasing RF power, with 2.75 at 10 W and 2.34 at 75 W.     

Characterization of carbon thin films prepared by the thermal decomposition of spin coated polyacrylonitrile layers containing metal acetates

Polyacrylonitrile (PAN) layers were cast from dimethyl-formamide solutions onto quartz substrates by spin coating and subsequently annealed at up to 1000 °C in N₂ atmosphere. Carbonization was catalyzed by nickel or cobalt added to the solution as acetate salts. The synthesized films were approx. 970 nm thick and were characterized by Raman and infrared spectroscopy as well as thermogravimetric and electrical conductance measurements. We discuss the effects of carbonization temperature and metal concentration on the morphology, composition and electrical properties of the formed carbon layer. Increasing the amount of catalyst and the pyrolysis temperature was beneficial for the process and resulted in carbonaceous films with a higher degree of structural order as evidenced by the decreasing Raman I_D/I_G ratio and the increasing electrical conductivity of the films. Cobalt is a better catalyst for PAN carbonization than nickel as far as the structure of the product film is concerned.     

Comparative study of erbium disilicide thin films grown in situ under ultrahigh vacuum or ex situ with a capping layer

Erbium disilicide (ErSi_2-x) thin films grown by two different techniques are compared using a variety of characterization techniques, both electrical and physical. The first technique involves Er deposition and annealing under ultrahigh vacuum and the second one focuses on Ti/Er/Si(100) stacks evaporated under high vacuum and heated ex situ by rapid thermal annealing. Crystalline phase identification by X-ray diffraction reveals the formation of ErSi_2-x for all the studied samples. Cross-sectional transmission electron microscopy shows that the Ti cap transforms into Ti-Si compounds. The efficient stripping of the capping layer is also demonstrated. Atomic force microscopy evidences the formation of inverted pyramidal defects in both cases, with some improvement for the Ti-capped samples. X-ray photoelectron spectroscopy depth profiles show that the ErSi_2-x films and the ErSi_2-x/Si interfaces are oxygen-free. The extracted Schottky barrier height of ErSi_2-x/n-Si contacts lies around 0.3 eV notwithstanding the annealing temperature or the growth technique. It thus demonstrates a route to form ErSi_2-x thin films that advantageously compares with reference ultrahigh vacuum samples with less stringent fabrication conditions.     

One-step growth of thin film SnS with large grains using MOCVD

Thin film tin sulphide (SnS) films were produced with grain sizes greater than 1 μm using a one-step metal organic chemical vapour deposition process. Tin–doped indium oxide (ITO) was used as the substrate, having a similar work function to molybdenum typically used as the back contact, but with potential use of its transparency for bifacial illumination. Tetraethyltin and ditertiarybutylsulphide were used as precursors with process temperatures 430–470 °C to promote film growth with large grains. The film stoichiometry was controlled by varying the precursor partial pressure ratios and characterised with energy dispersive X-ray spectroscopy to optimise the SnS composition. X-ray diffraction and Raman spectroscopy were used to determine the phases that were present in the film and revealed that small amounts of ottemannite Sn₂S₃ was present when SnS was deposited on to the ITO using optimised growth parameters. Interaction at the SnS/ITO interface to form Sn₂S₃ was deduced to have resulted for all growth conditions.     

Analytical model for the optical functions of indium gallium nitride with application to thin film solar photovoltaic cells

This paper presents the preliminary results of optical characterization using spectroscopic ellipsometry of wurtzite indium gallium nitride (In_xGa_1−xN) thin films with medium indium content (0.38 < x < 0.68) that were deposited on silicon dioxide using plasma-enhanced evaporation. A Kramers-Kronig consistent parametric analytical model using Gaussian oscillators to describe the absorption spectra has been developed to extract the real and imaginary components of the dielectric function (?₁, ?₂) of In_xGa_1−xN films. Scanning electron microscope (SEM) images are presented to examine film microstructure and verify film thicknesses determined from ellipsometry modeling. This fitting procedure, model, and parameters can be employed in the future to extract physical parameters from ellipsometric data from other In_xGa_1−xN films.     

Direct patterning of double-layered metal thin films by a pulsed Nd:YAG laser beam

Metal thin-film patterning is of technological significance because modern electronic devices commonly require an electrode or metallization pattern. There are many cases where this pattern consists of two different metallic layers in order to improve the mechanical and electrical contact. We here show that double-layered metal thin films evaporated on glass can be directly patterned by a spatially-modulated pulsed Nd-YAG laser beam incident from the backside of the substrate. This method utilizes a pulsed laser-induced thermo-elastic force exerting on the film which plays a role to detach it from the substrate. Since the film is polycrystalline with nano-sized grains, a spatially-modulated thermo-elastic force may enable selective removal of the material by shearing along the weakly-bonded grain boundary regions. Many different combinations of Al, Ag, and Au layers have been investigated and their pattern fidelity and morphology are discussed, along with the simulation results for double-layered nanocystalline films.     

A review of Fe3O4 thin films: Synthesis,modification and applications

Magnetite (Fe₃O₄) has been used for thousands of years as one of the important magnetic materials. The rapid developments of thin film technology in the past few decades attract the attention of material scientists on the fabrication of magnetite thin films. In this article, we present an overview of recent progress on Fe₃O₄ thin films. The widely used preparation methods are surveyed, and the effect of substrates is discussed. Specifically the modified Fe₃O₄ thin films exhibit excellent electrical and magnetic properties compared with the pure films. It is noteworthy that modified Fe₃O₄ thin films can be put into two categories: (1) doped films, where foreign metal ions substitute iron ions at A or B sites; and (2) hybrid films, where magnetite phases are mixed with other materials. Notably, Fe₃O₄ thin films show great potentials in many applications such as sensors and batteries. It is expected that the investigations of Fe₃O₄ thin films will give us some breakthroughs in materials science and technology.     

Layer-by-layer assembly of polyelectrolyte/TiO2 thin films with reflection-enhancing function

Colloidal TiO₂ was prepared by hydrolyzing tetra-n-butyl titanate. Composite multilayer films of poly(sodium 4-styrenesulfonate) (PSS) and colloidal TiO₂ particles were layer-by-layer assembled onto optic fibers and microscope glass slides. As the PSS/TiO₂ film was deposited onto the end face of a glass fiber, the reflected optic intensity periodically oscillated as the bilayer number of the film increased. After a 24-bilayer film was coated onto the both sides of a glass slide, the transmittance at 850 nm decreased more than 20%, which means that the film could serve the function as a reflection-enhancing coating. X-ray diffraction analysis and data of TEM electron diffraction analysis show that the colloidal TiO₂ particles are mainly brookite nanocrystals and that the PSS/TiO₂ films are polycrystalline films. Scratching experiments indicate that the composite films are of relatively high hardness.     

Growth of beta barium borate (β-BaB2O4) thin films by injection metal organic chemical vapour deposition

Thin films containing beta barium borate (β-BaB₂O₄ so called β-BBO) were grown on silicon (100) substrates by injection metal organic chemical vapour deposition for different deposition temperatures. The films were characterized by optical microscopy, micro-Raman spectroscopy and X-ray photoelectron Spectroscopy (XPS). The micro-Raman spectra show an intense peak at 637 cm^− 1 that is the fingerprint of β-BBO. Our XPS analysis permits the measurement of the Ba, B and O core levels, which are reported here for the first time for β-BBO thin films. The formation of a new spectral component appearing with lower growth temperatures has been observed as well.     

Band gap and dispersive behavior of Ge alloyed a-SbSe thin films using single transmission spectrum

The transparency of SbSeGe glasses in the IR region makes them attractive candidates for low transmission loss applications. The samples of Sb₁₀Se_90−xGe_x (x = 0, 19, 21, 23, 25, 27) glasses have been prepared by melt quench technique. The thin films of these glasses have been deposited by vacuum evaporation technique. The optical study of thin films has been carried out. The refractive index, oscillator parameters, optical band gap and dielectric parameters have been calculated from optical measurements. The optical study reveals that the variation in the density of localized defect states on Ge addition affects the optical parameters of the system. The variation in concentration of localized defect states has been interpreted in terms of the change in structural network of the system.     

Numerical modeling of SiH4 discharge for Si thin film deposition for thin film transistor and solar cells

Amorphous and microcrystalline silicon thin films are used in solar cells as a multi-junction photovoltaic device. Plasma enhanced chemical vapor deposition is used and high deposition rate of a few nm/s is required while keeping film quality. SiH₄ is used as a precursor diluted with H₂. Electron impact processes give complex interdependent plasma chemical reactions. Many researchers suggest keeping high H/SiH_x ratio is important. Numerical modeling of this process for capacitively coupled plasma and inductively coupled plasma is done to investigate which process parameters are playing key roles in determining it. A full set of 67 volume reactions and reduced set are used. Under most of conditions, CCP shows 100 times higher H/SiH₃ ratio over ICP case due to its spatially localized two electron temperature distribution. Multi hollow cathode type CCP is also modeled as a 2 × 2 hole array. For Ar, the discharge is well localized at the neck of the hole at a few Torr of gas pressure. H₂ and SiH₄ + H₂ needed higher gas pressure and power density to get a multi hole localized density profile. H/SiH₃ was calculated to be about 1/10.     

Can micro-compression testing provide stress-strain data for thin films?: A comparative study using Cu, VN, TiN and W coatings

Micro-compression testing using an instrumented micro- or nanoindenter equipped with a flat punch is a promising approach to measure the stress-strain response of miniaturized materials and to complement hardness and modulus measurements gained by nanoindendation. Focussed ion beam milling is employed to fabricate micron-sized compression pillars from 1 µm thick single crystal Cu(001), TiN(001), and VN(001) films grown on MgO(001), and from a 6.7 µm thick polycrystalline W coating deposited on Si(001). In situ micro-compression tests in a scanning electron microscope reveal reproducible stress-strain curves for W, a considerable statistical scatter in the flow stress for Cu and VN, and failure of TiN pillars by cleavage. A linear work-hardening rate of 2.7 ± 1.2 GPa is determined for the polycrystalline W coating. The results are critically discussed taking into account material defects and the stiffness of the film-substrate system.     

Hydrothermal in-situ growth of Tris(8-hydroxyquinolate) aluminum nanorods thin films

Tris(8-hydroxyquinolate) aluminum(Alq₃) thin films assembled with large-scaled nanorods have been fabricated on Al substrates through hydrothermal in-situ growth method assisted by the surfactant of sodium dodecylbenzenesulfonate. The obtained Alq₃ thin film is composed of uniformly sized (500-800nm × 4-10 μm) nanorods with regular hexagonal cross section, which are assembled to form dense nanorod arrays perpendicularly to the Al substrate. X-ray diffraction revealed that the prepared Alq₃ nanorods were the α-phase. Photoluminescence spectra showed that the Alq₃ nanorods thin film possessed a spectral blue-shift (10 nm) compared with the Alq₃ solution. The hydrothermal growth mechanism of nanorods was studied, which implied that the hydrothermal in-situ growth process on the metal substrate played an important role in the formation of the Alq₃ nanorods thin film. This simple hydrothermal method provides a convenient fabrication approach for nanocrystalline functional organic/metal interface.