Optical properties of gold island films—a spectroscopic ellipsometry study

Metal island films of noble metals are obtained by deposition on glass substrates during the first stage of evaporation process when supported metal nanoparticles are formed. These films show unique optical properties, owing to the localized surface plasmon resonance of free electrons in metal nanoparticles. In the present work we study the optical properties of gold metal island films deposited on glass substrates with different mass thicknesses at different substrate temperatures. The optical characterization is performed by spectroscopic ellipsometry at different angles of incidence and transmittance measurements at normal incidence in the same point of the sample. Fitting of the ellipsometric data allows determining the effective optical constants and thickness of the island film. A multiple oscillator approach was used to successfully represent the dispersion of the effective optical constants of the films.     

Characterization of pulsed laser deposited hydrogenated amorphous silicon films by spectroscopic ellipsometry

The wide absorption band of hydrogenated amorphous silicon (a-Si:H) is being realized as a key component of solar cells on glass. In this study, a-Si:H films were prepared by reactive pulsed laser deposition onto silicon and glass substrates. Ellipsometry showed that the optical properties of the films are effectively independent on the choice of substrate. According to the optical properties, the character of the films changes from amorphous silicon to dielectric as the hydrogen background pressure increases from 0 to 25 Pa. This observation was attributed to oxygen incorporation indicated by Rutherford Backscattering Spectrometry. Furthermore, a refractive index gradient in depth was revealed, which was attributed to the oxygen concentration gradient.     

Modulation of the structural and optical properties of sputtering-derived HfO2 films by deposition power

High-k gate dielectric HfO₂ thin films have been deposited on Si and quartz substrate by radio frequency magnetron sputtering. The structural and optical properties of HfO₂ thin films related to deposition power are investigated by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), ultraviolet–visible spectroscopy (UV–Vis), and spectroscopic ellipsometry (SE). Results confirmed by XRD have shown that the as-deposited HfO₂ thin films are not amorphous state but in monoclinic phase, regardless of deposition power. Analysis from FTIR indicates that an interfacial layer has been formed between the Si substrate and the HfO₂ thin film during deposition. AFM measurements illustrate that the root mean square (RMS) of the as-deposited HfO₂ thin films’ surface demonstrates an apparent reduction with the increase of deposition. Combined with UV–Vis and SE measurements, it can be noted reduction in band gap with an increase in power has been observed. Additionally, increase in refractive index (n) has been confirmed by SE.     

Virtual separation approach to study porous ultra-thin films by combined spectroscopic ellipsometry and quartz crystal microbalance methods

Spectroscopic ellipsometry (SE) and quartz crystal microbalance with dissipation (QCM-D) techniques have been extensively used as independent surface characterization tools to monitor in-situ thin film formation. They provide different information for ultra-thin films because QCM-D is sensitive to the solvent content while SE is not. For using these two techniques in tandem, we present a virtual separation approach to enable the determination of both ultra-thin film thickness and porosity. Assumptions for the intrinsic molecular polarizability (index of refraction n_o) and density (ρ_o) of the organic adsorbent must be made, and the consequences for these parameters' values are discussed.     

Optical characterisation of SiOxCyHz thin films non-uniform in thickness using spectroscopic ellipsometry, spectroscopic reflectometry and spectroscopic imaging reflectometry

The combined optical method enabling us to perform the complete optical characterisation of weakly absorbing non-uniform thin films is described. This method is based on the combination of standard variable angle spectroscopic ellipsometry, standard spectroscopic reflectometry at near normal incidence and spectroscopic imaging reflectometry applied at normal incidence. The spectral dependences of the optical constants are determined using the non-imaging methods by using the dispersion model based on parametrisation of the density of electronic states. The local thickness distribution is then determined by imaging reflectometry. The method is illustrated by means of the complete optical characterisation of SiO_xC_yH_z thin films.     

Studying early time HWCVD growth of a-Si:H by real time spectroscopic ellipsometry

We have applied real time spectroscopic ellipsometry and secondary ion mass spectrometry to study the growth of amorphous silicon by hot-wire chemical vapor deposition. Differences in temperature and hydrogen content affect the optical properties of the film. These effects provide valuable insight into the growth process. We have compared a-Si:H films grown at two different temperatures to better understand these effects. Our studies reveal the presence of a distinct 100–200-thick layer at the top of the growing film. The properties of this layer are primarily determined by the ambient conditions in the growth chamber and appear relatively independent of substrate temperature. In contrast, the properties of the bulk of the film are strongly influenced by substrate temperature. These results imply that differences in film properties associated with substrate temperature are the result of subsurface reconstruction and diffusion processes.     

Optical properties and switching of a Rose Bengal derivative: A spectroscopic ellipsometry study

Optical properties in terms of the complex-valued dielectric function were determined for spin-coated films of a Rose Bengal derivative using variable angle of incidence spectroscopic ellipsometry in the visible and infrared wavelength regions. In addition, the thickness and roughness of the films were determined and related to the solution concentration of Rose Bengal. Switching between two different oxidation states of the Rose Bengal derivative was investigated. The two states were chemically induced by exposure to vapors of hydrochloric acid and ammonia, respectively. A substantial and reversible change of the optical properties of the films was observed.     

Microstructure characterization of microcrystalline silicon thin films deposited by very high frequency plasma-enhanced chemical vapor deposition by spectroscopic ellipsometry

We applied ex situ spectroscopic ellipsometry (SE) on silicon thin films across the a-Si:H/μc-Si:H transition deposited using different hydrogen dilutions at a high pressure by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). The optical models were based on effective medium approximation (EMA) and effective to estimate the thickness of the amorphous incubation layer and the volume fractions of amorphous, microcrystalline phase and void in μc-Si:H thin films. We obtained an acceptable data fit and the SE results were consistent with that from Raman spectroscopy and atomic force microscopy (AFM). We found a thick incubation layer in μc-Si:H thin films deposited at a high rate of ~ 5 Å/s and this microstructure strongly affected their conductivity.     

Systematic combination of X-ray reflectometry and spectroscopic ellipsometry: A powerful technique for reliable in-fab metrology

The demand for accurate and highly reliable in-fab characterization of thin layers included in advanced CMOS and MEMS stacks has placed stringent requests on in-line optical metrology methodology and protocols. This work investigates the capability of systematic combination of X-ray reflectometry (XRR) and spectroscopic ellipsometry (SE) to decrease the correlation concerns that sometimes affect the reliability of SE analysis.     

Investigation of Mo/Si and W/Si interfaces by phase modulated spectroscopic ellipsometry and cross-sectional transmission electron microscopy

Mo and W thin films and Mo/Si/Mo and W/Si/W tri-layers have been deposited by r.f. magnetron sputtering on c-Si substrates as a precursor to the fabrication of Mo/Si and W/Si multilayer X-ray mirrors. The Phase Modulated Spectroscopic Ellipsometry (SE) technique has been used for characterizing the single layer films to derive information regarding the thickness and volume fraction of voids present in the surface layers. The Mo/Si/Mo and W/Si/W tri-layer structures have been characterized by the Cross-sectional Transmission Electron Microscopy (XTEM) technique. The inter-diffusion at the interfaces of the tri-layer structures observed by the XTEM technique has been correlated to the thickness of the surface layers of the metal films obtained from the SE measurement.     

In-situ monitoring of alkanethiol self-assembled monolayer chemisorption with combined spectroscopic ellipsometry and quartz crystal microbalance techniques

Self-assembled monolayers (SAMs) formed via chemisorption are important for a variety of surface enhancement and biological applications. We demonstrate that combinatorial spectroscopic ellipsometry (SE) and quartz crystal microbalance with dissipation (QCM-D) provides dynamic, in-situ characterization of the chemisorption process. In agreement with other studies, we find there are two steps for 1-decanethiol, an example alkanethiol SAM, chemisorption onto gold, which are a brief, fast phase followed by one that is long but slower. By using both the optical (SE) and mechanical (QCM-D) techniques, we show that the SAM porosity decreases during the second phase as the coupled ethanol solvent in the disorganized layer is replaced by more alkanethiol.     

Micelle-assisted bilayer formation of cetyltrimethylammonium bromide thin films studied with combinatorial spectroscopic ellipsometry and quartz crystal microbalance techniques

We report on a combinatorial approach to study the formation of ultra-thin organic films using in-situ spectroscopic ellipsometry and quartz crystal microbalance methods. In contrast to the quartz crystal microbalance, which is sensitive to the total mass attached to the surface, including coupled and entrapped solvent, spectroscopic ellipsometry only measures the amount of adsorbent on the surface. By using these two techniques in tandem, we define and determine the solvent fraction of cetyltrimethylammonium bromide thin films adsorbed onto a gold-coated quartz crystal. Cetyltrimethylammonium bromide thin films grown from aqueous solutions above the critical micelle concentration reveal critical phases in thickness and porosity evolution. We relate these effects to the mechanisms of formation and removal and the structure of cetyltrimethylammonium bromide films, which we determine to have systemic defects due to the presence of micelles.     

Free-charge carrier profile of iso- and aniso-type Si homojunctions determined by terahertz and mid-infrared ellipsometry

We present an optical, non-destructive, non-contact method of determining the silicon homojunction epilayer free-charge carrier concentration profile and thickness by means of combined terahertz (0.2-1 THz) and mid-infrared (10-50 THz) spectroscopic ellipsometry investigation. A dual homojunction iso- and aniso-type silicon sample is investigated. Application of analytical models for iso-type and aniso-type homojunctions results in an excellent match between calculated and experimental data. Best-match model calculated parameters are found to be consistent with electrical spreading resistance epilayer thickness and resistivity values.     

A broadband analysis of the optical properties of silver nanoparticle films by in situ real time spectroscopic ellipsometry

A broadband analysis of the optical properties of silver nanoparticle films over the range from 0.75 to 6.5 eV was performed by applying in situ real-time spectroscopic ellipsometry (RTSE) during the nucleation, coalescence, and bulk thin film growth regimes. The dielectric functions of the particulate films were found to depend strongly on the particle size and film thickness from the nucleation regime throughout coalescence. These dependences were analyzed by separately characterizing the three types of transitions evident in the dielectric function: intraband, particle plasmon polariton, and interband. Throughout the film growth regimes, the thickness evolution of the amplitude, energy, and broadening parameter for each type of transition is discussed in view of the structural characteristics of the films, as corroborated ex situ by atomic force microscopy for films deposited over different time durations.     

Band offset of SnS solar cell structure measured by X-ray photoelectron spectroscopy

The energy band offset at the heterointerface is one of the most important properties of semiconductor heterostructures, particularly in solar photovoltaic devices. Band discontinuities of CdS/SnS and SnS/SnO₂ heterointerfaces were measured by X-ray photoelectron spectroscopy and capacitance-voltage measurements. The valence band offsets were determined to be approximately 1.5 eV for CdS/SnS and 3.5 eV for SnS/SnO₂ interfaces whereas the conduction band discontinuities for these junctions were respectively found to be 0.4 eV and 1.0 eV. Using these values and the energy band gaps of the corresponding layers, the energy band diagram was developed and it was considered to be a TYPE-II heterostructure. The Fermi level was found to be much closer to the valence band maximum for SnS, whereas it appeared in the upper half of the band gap for both CdS and SnO₂.     

Internal photoemission spectroscopy measurement of Alq3/cathode interface by three layered electron only device

We have investigated the Schottky barrier height for electrons at aluminum tris (8-hydroxyquinoline) (Alq₃)/cathode interfaces by internal photoemission (IPE) measurement. A three layered device consisting of ITO/TiO₂/intermediate layer (IL)/Alq₃/cathode structure was used to fabricate the “electron-only device” and to reduce the equivalent thickness of TiO₂ and IL stack. The measured barrier heights were 0.86, 1.05, 1.3 and 1.55 eV for MgAg, Al, Ag and Au electrodes, and it gradually decreased with external voltage and it was explained as Schottky effect. Barrier height showed linear relationship with work-function and electronegativity of cathode materials. The slope parameter of 0.6 with electronegativity was attributed to the interfacial charge transfer through the metal-induced gap states at the cathode/Alq₃ interface.     

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.     

Investigation of adenine, uracil, and ribose phosphate thin films prepared by electrospray in vacuum deposition using photoemission spectroscopy

Previous results showed significant ionization energy differences between thin films of the four ribonucleic acid (RNA) polynucleotides. The experiments reported here aim at the investigation of the origin of these differences. Since the ribose phosphate backbone is common to all RNA nucleotides, the nucleobases are the most likely candidate defining the ionization energy of RNA. Consequently, experiments were performed to investigate the electronic structure and ionization energies of thin films of two nucleobases (adenine and uracil, representative for purines and pyrimidines), and ribose phosphate. These experiments were performed using x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) in conjunction with an electrospray based in vacuum multi-step deposition technique. The presented results clearly demonstrate a significant ionization energy difference between the two nucleobases, qualitatively matching the previous results on the homopolymers of adenosine and uridine (poly rA, poly rU). As expected, the ionization energy of the prepared ribose phosphate thin films was much larger than those of the nucleobases.     

Determination of optical constants of Cd1−xZnxTe thin films by spectroscopic ellipsometry

The optical response of vacuum-evaporated Cd_1−xZn_xTe thin films in the 1.5-5.6 eV photon energy range at room temperature has been studied by spectroscopic ellipsometry. The films of Cd_1−xZn_xTe (x=0.04) were deposited at room temperature onto well-cleaned glass substrates of film thickness 450 nm. The measured dielectric-function spectra reveal distinct structures at energies of the E₁, E₁+Δ₁ and E₂ critical points corresponding to the interband transitions. Dielectric related optical constants such as complex refractive index, the absorption coefficients and the normal incidence reflectivity, are presented. Results are in satisfactory agreement with the calculations over the entire range of the photon energies.     

Hot-wire deposition of a-Si:H thin films on wafer substrates studied by real-time spectroscopic ellipsometry and infrared spectroscopy

Film growth of hydrogenated amorphous silicon (a-Si:H) by hot-wire chemical vapor deposition was studied simultaneously and in real-time by spectroscopic ellipsometry and attenuated total reflection infrared spectroscopy. The a-Si:H films were deposited on native oxide-covered GaAs(100) and Si(100) substrates at temperatures ranging from 70 to 350 °C. A temperature dependent initial growth phase is revealed by the evolution of the surface roughness and the surface and bulk SiH_x absorption peaks. It is discussed that the films show a distinct nucleation behavior by the formation of islands on the surface that subsequently coalesce followed by bulk a-Si:H growth. Insight into a temperature-activated smoothening mechanism and the creation of a hydrogen-rich interface layer is presented.