Real-time Spectroscopic Ellipsometry Research Articles

Dielectric function ofa-Si:H based on local network structures

For hydrogenated amorphous silicon ($a$-Si:H) layers prepared by plasma-enhanced chemical vapor deposition, we have found clear relationships between the dielectric function in the ultraviolet/visible region and SiH${}_{n}$ ($n$ $=$ 1--2) local bonding states by applying real-time spectroscopic ellipsometry and infrared attenuated total reflection spectroscopy. In particular, the amplitude of the $\ensuremath{\varepsilon}$${}_{2}$ spectra obtained from various $a$-Si:H layers is expressed completely by the SiH${}_{2}$ bond density in the $a$-Si:H and reduces strongly with increasing the SiH${}_{2}$ content, indicating that microvoids present in the $a$-Si:H network are surrounded by the SiH${}_{2}$ bonding state. On the other hand, no significant void formation occurs by the generation of the SiH local bonding due to rather dense surrounding structures. Depending on the SiH${}_{n}$ bonding states, the whole $a$-Si:H dielectric function shifts toward higher energies, as the SiH${}_{n}$ hydrogen contents in the $a$-Si:H increase. Based on these findings, we have established a new $a$-Si:H dielectric function model that incorporates the void structure terminated with SiH${}_{2}$ and $\ensuremath{\varepsilon}$${}_{2}$ spectral shift induced by the SiH${}_{n}$ local structures. This model is appropriate for a wide variety of $a$-Si:H layers deposited at different substrate temperatures and plasma conditions and, conversely, allows the characterization of the SiH${}_{n}$ contents from the $a$-Si:H dielectric functions in the ultraviolet/visible region. From results obtained in this study, the local network structures and electronic states of $a$-Si:H are discussed.

Analysis of interband, intraband, and plasmon polariton transitions in silver nanoparticle films via in situ real-time spectroscopic ellipsometry

The dielectric function of Ag nanoparticle films, deduced from an analysis of in situ real-time spectroscopic ellipsometry (RTSE) measurements, is found to evolve with time during deposition in close consistency with the film structure, deduced in the same RTSE analysis. In the nucleation regime, the intraband dielectric function component is absent and plasmon polariton behavior dominates. Only at nuclei contact, does the intraband amplitude appear, increasing above zero. Both intraband and plasmon amplitudes coexist during surface smoothening associated with coalescence. The intraband relaxation time increases rapidly after surface smoothening is complete, also in consistency with the thin film structural evolution.

Developing an epitaxial growth process for ZnO by MOCVD using real-time spectroscopic ellipsometry

Real-time diagnostics are an essential tool in the development and improvement of growth processes for new materials. Here we use real-time spectroscopic polarimetric observations of zinc oxide deposition, and a chemical model derived therefrom, to develop a method of growing dense, two-dimensional zinc oxide epitaxially on sapphire by metalorganic chemical vapor deposition. With the transition between deposition and etching being 13% in the diethylzinc flow rate, it is unlikely that we would have discovered this process without the use of real-time spectroscopic ellipsometry. New photoluminescence data support our conclusion that using this cyclical growth process yields improved material.

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 ~3000Å 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.

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 6.5eV 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.

Adsorption and desorption kinetics of Ga on GaN(0001): Application of Wolkenstein theory

The kinetics of Ga adsorption/desorption on GaN(0001) surfaces is investigated over the temperature range of $680--750\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ using real-time spectroscopic ellipsometry. The adsorption and desorption kinetics are described in the framework of the Wolkenstein theory, which considers not only the equilibrium between Ga adsorbed on the surface and Ga in the gas phase but also the electronic equilibrium at the surface. It is shown that, because of the fixed polarization charge existing at the GaN(0001) surface, Ga adsorption and desorption processes involve neutral and charged Ga states. By considering the GaN surface charge involved in the surface processes, we demonstrate that a second-order kinetics more accurately describes Ga desorption, in comparison with conventional models, and yields an apparent activation energy of $2.85\ifmmode\pm\else\textpm\fi{}0.02\text{ }\text{eV}$ for Ga desorption consistent with experiments.

In situ real-time spectroscopic ellipsometry measurement for the investigation of molecular orientation in organic amorphous multilayer structures

To investigate molecular orientation in organic amorphous films, in situ real-time spectroscopic ellipsometry measurements were performed during vacuum deposition. Three materials with different molecular shapes were adopted to confirm the generality of the molecular orientation. In all three cases, more than 200 000 values for the ellipsometric parameters measured during deposition were well simulated simultaneously over the entire spectral range and measurement period using a simple model where the films possessed homogeneous optical anisotropy. This demonstrated the homogeneity of the molecular orientation in the direction of film thickness. The molecular orientation can be controlled by the substrate temperature even in multilayer structures. It is also demonstrated that a “multilayer structure” can be fabricated using only one material, where each layer has different optical and electrical properties.

Real-time spectroellipsometric characterization of nucleation, islanding, and coalescence behavior of boron films grown by soft x-ray excited chemical vapor deposition

Real-time spectroscopic ellipsometry was applied to the in situ monitoring of soft x-ray excited chemical vapor deposition of boron films on a Si(001) substrate. While B2H6 molecules cannot reactively stick to a Si clean surface below 400 °C, the decomposition of the precursor molecule through electronic excitation enables boron atoms to secure themselves on the surface. After the incubation period had ended, boron islands began to grow at isolated nucleation centers, exhibiting the Volmer–Weber-type growth. The activation energy for creating the nucleation centers was 2.9 kcal/mol, which presumably corresponded to the clustering of deposited boron atoms. The following growth stage was scaled with an activation energy of 10.3 kcal/mol. The thermally activated process corresponded to the networking of surface-terminating hydrides while releasing H2. As further growth continued, boron islands coalesced, which eventually became two-dimensional boron film. The amorphous boron films could be transformed into a crystallite phase through postannealing at 1000 °C and they were characterized in terms of their optical properties.

Influence of hydrogen dilution on surface roughness development of a‐Si:H thin films grown by remote plasma deposition

AbstractHydrogenated amorphous silicon (a‐Si:H) films are grown at different hydrogen dilutions. For high dilutions we observe a discrepancy in the surface roughness analysis between real‐time spectroscopic ellipsometry (RTSE) and atomic force microscopy (AFM) measurements. With RTSE a much higher roughness layer thickness is measured than with AFM. Additionally we observe what appears to be a strong roughening phase in the first 30‐50 nm of film growth for all conditions. A mechanism that involves the formations of a hydrogenrich overlayer and etching of higher hydrides in this overlayer is suggested.This mechanism explains the difference in RTSE roughness layer thickness evolution. At higher hydrogen dilutions, we obtain a thicker overlayer dominated by lower hydrides. RTSE interprets this overlayer as additional surface roughness and consequently overestimates the surface roughness for higher dilutions. The initial strong roughening phase is related to the built‐up of this over‐layer. In this phase, we obtain much denser films compared to the bulk film. The presence of columnar growth at higher dilutions is attributed to reduced surface mobility under these conditions (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Real-time optical modelling and investigation of inorganic nano-layer growth onto flexible polymeric substrates

A major factor for the achievement of the desirable performance, efficiency and lifetime of flexible organic electronic devices is the optimization of the encapsulation layers that protect the device active layers by atmospheric gas molecule permeation. The active layers consisted of small molecule and/or polymer organic semiconductors as well as the organic conductors need to be encapsulated into a transparent medium that will provide the necessary protection and maintain their charge generation and transport characteristics. The encapsulation layers are generally consisted of inorganic thin films (silicon oxide—SiO x and aluminium oxide—AlO x ) deposited onto the polymeric substrates, such as PolyEthylene Terephthalate (PET). In this work, in situ and real-time Spectroscopic Ellipsometry in the ultraviolet spectral region has been implemented in order to investigate the growth of inorganic SiO x and AlO x nano-layers onto PET flexible polymeric substrates as well as the PET/inorganic interface effects during the early stages of growth. The analysis of the pseudodielectric function < ɛ( ω)> that was measured in real-time in very short time scales (in the order of hundreds of ms) has provided detailed information on the time evolution of the thickness and deposition rate of the inorganic nano-layers during their growth process as well as on their optical and electronic properties. This work proposes a methodology for using real-time optical monitoring technique with the aim to tailor and control the functionality of these materials for application in flexible electronic devices.

Analysis of controlled mixed-phase (amorphous+microcrystalline) silicon thin films by real time spectroscopic ellipsometry

Engineered thin films consisting of periodic arrays of silicon microcrystallites in a hydrogenated amorphous silicon host matrix have been prepared by plasma-enhanced chemical vapor deposition where the hydrogen dilution of silane is modulated in multiple cycles. These types of films have been guided by a phase evolution diagram, depicting the deposition conditions and film thickness at which the material exhibits amorphous, microcrystalline, or mixed-phase (amorphous+microcrystalline) characteristics, developed for intrinsic Si:H prepared with varying H2 dilution on unhydrogenated a-Si:H. Real time spectroscopic ellipsometry (RTSE) has been used in situ to noninvasively determine the phase evolution of the resulting hydrogenated mixed-phase (amorphous+microcrystalline) silicon thin films and corroborated with dark-field transmission electron microscopy. Such tailored microstructures are of growing interest as components of thin film photovoltaic devices, and RTSE is shown to be a key technique for structure verification.

Electronic and structural properties of molybdenum thin films as determined by real-time spectroscopic ellipsometry

Real-time spectroscopic ellipsometry (RTSE) is shown to be an effective contactless probe of radio frequency magnetron sputtered molybdenum thin films used as the back electrode in chalcopyrite [Cu(In,Ga)Se2] solar cells. A series of Mo thin films was sputtered onto soda-lime glass substrates at Ar pressures ranging from 4 to 20 mTorr. RTSE measurements reveal how Ar pressure affects the nucleation and growth mechanisms that influence the films’ ultimate grain structure and properties. Determinations of the free electron relaxation times at optical frequencies reveal that higher pressures lead to a smaller average grain size and increased void volume fraction.

Observation of SiC Oxidation in Ultra-Thin Oxide Regime by &lt;i&gt;In Situ&lt;/i&gt; Spectroscopic Ellipsometry

Thermal oxidation process of silicon carbide in ultra-thin oxide regime has been studied by performing in-situ and real time spectroscopic ellipsometry. We found the thermal oxidation at 700°C forms no or extremely thin interface layers between SiC and oxide layers. In contrast, the oxidation at 850°C forms an interface layer of around 1 nm in thickness, having similar thickness and optical constants of the interface layers formed by the oxidation at higher temperature than 1000°C. To make clear the conditions no interface layer is formed, i.e., whether low temperature growth or thin oxide thickness is crucial, we have performed the oxidation at 850°C in the reduced oxygen pressure. Based on the results of these experiments, we discussed the origin of the formation of interface layers as well as the oxidation mechanism of SiC.

Roughening during XeF2 etching of Si(100) through interface layers: H:Si(100) and a-Si∕Si(100)

Real-time spectroscopic ellipsometry has been applied in situ in an Ar+∕XeF2 beam-etching experiment to study the roughening of Si(100) etched by XeF2 at room temperature. The role of initial surface conditions has been examined. For the etching of hydrogen-terminated (H:)Si(100), the roughness evolution as a function of XeF2 dose can be characterized by an initially fast roughening phase followed by a slower, final roughening phase. Similar behavior is observed when etching through an amorphous silicon (a-Si) layer on top of crystalline Si(100) bulk as obtained by sputter cleaning of Si(100) substrates. These observations can be explained as follows. Both H termination and a-Si lead to patch formation on the surface where etching is impeded and hence, high aspect-ratio etch pits develop. The quantitative differences in roughening can then be attributed to the duration and timing of the influence of the H-terminated and a-Si patches on the etch process until H-bonded Si surface atoms or a-Si are totally removed from the surface. Surface area increase due to the roughening can therefore be held responsible for observed trends and differences in etch rates, reaction layer thickness, and composition as a function of etch time.

Real-Time Spectroscopic Ellipsometry of Sputtered CdTe Thin Films: Effect of Ar Pressure on Structural Evolution and Photovoltaic Performance

AbstractIn this study, 1 μm thick polycrystalline CdTe films were deposited by magnetron sputtering using a variable argon pressure, 2.5 ≤ pAr ≤ 50 mTorr, and a fixed substrate temperature, Ts = 230°C. Real time spectroscopic ellipsometry (RTSE) was performed during deposition in order to analyze the nucleation and coalescence, as well as the evolution of the surface roughness thickness ds with bulk layer thickness db and the depth profile in the void volume fraction fv. A linear correlation was found between the final ds value measured by RTSE at the end of deposition and the root-mean-square (rms) surface roughness measured by atomic force microscopy (AFM) ex situ after deposition. A monotonic decrease in RTSE-determined roughness thickness is observed with decreasing Ar pressure from 18 to 2.5 mTorr. The lowest pressure also leads to the greatest bulk layer structural uniformity; in this case, fv increases to 0.04 with increasing CdTe thickness to 1 μm. The photovoltaic performance of CdTe films prepared with the lowest pressure of pAr = 2.5 mTorr is compared with that of previously optimized CdTe solar cells with pAr = 10 mTorr.

Optical and Physical Characterization of Cu2-xSe Thin Films for Real Time Spectroscopic Ellipsometry on Cu(In,Ga)Se2-based Photovoltaic Devices

AbstractIn situ, real time spectroscopic ellipsometry (RTSE) has been used to study the growth processes and optical properties of Cu2-xSe - an important binary compound in the fabrication of high efficiency copper indium gallium diselenide (CIGS) photovoltaic devices. It was found that the high surface roughness of the Cu2-xSe layers necessitated a “graded” optical model in order to extract meaningful dielectric functions at both 550 °C and room temperature. The optical model was verified at room temperature against SEM micrographs and reflectance measurements carried out ex situ. The growth temperature dielectric functions presented in this study are expected to allow for a greater level of control and understanding of the so-called 2- and 3-stage processes for CIGS fabrication in which a Cu2-xSe phase, present at the CIGS grain boundaries, acts as a fluxing agent for the growth of photovoltaic quality CIGS. Real time optical feedback via RTSE combined with the growth temperature dielectric functions presented here could play an important role in improving material fabrication on both the laboratory and industrial scales.

UV impact on the optical properties of thin films of positive tone chemically amplified resist

Chemically amplified photo resist are blend materials whose main compounds are a polymer matrix and an added photo acid generator. Lithography processes that allow imaging patterns in a CAR resist films consist in exposing at first the film to UV light and then to post-bake the film in order to initiate a chemical reaction that induces a local solubility switch of the resist. Sensitivity of the resist is measured by monitoring the UV dose needed to induce the proper solubility switch. For positive tone resist, the efficiency of the photolysis of the PAG to an acid and the deprotection kinetic during the post-exposure bake directly govern the resist sensitivity. This article deals with measurements of the PAG photolysis efficiency by real time spectroscopic ellipsometry. C Dill parameter of 193 nm photoresists is then measured for various film thicknesses and PAG loading.

Growth, optical and nanostructural properties of magnetron sputtered ZnO thin films deposited on polymeric substrates

AbstractZnO thin films were deposited onto flexible Poly‐Ethylene Terephthalate and Poly‐Ethylene Naphthalate substrates in an ultra‐high vacuum chamber by Pulsed DC Magnetron Sputtering. The process took place at room temperature and in argon atmosphere with fixed pressure, by applying two different power values on the target, in order to investigate their effect on the growth, optical, nanostructural and topographical properties of the films. In‐situ and real‐time Spectroscopic Ellipsometry in the 1.5–6.5 eV energy region has been employed to monitor the ZnO deposition, which revealed information about the growth mechanisms, interface phenomena and optical properties of the films. Transmission Electron Microscopy was used in order to investigate the structural characteristics of the films and to examine the film/polymer interface properties. The X‐ray Diffraction measurements revealed the changes in films structure caused by the change of the target power, nevertheless all films were preferentially grown along the (002) axis. Finally, with Atomic Force Microscopy the surface roughness of the films was defined correlated also with the target power applied. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Transient growth and thinning of the barrier oxide layer on iron measured by real-time spectroscopic ellipsometry

Abstract The transient growth and thinning of the passive film on iron was investigated in ethylenediaminetetraacetic acid, disodium salt (EDTA) containing borate buffer solutions of pH 8.4 using real-time spectroscopic ellipsometry under potentiostatic control. EDTA effectively suppressed the formation of the outer layer of the passive film, thereby rendering the barrier layer amenable to direct examination. It was shown that the barrier layer growth was completed in about 10 s upon potential stepping in the anodic direction. On the other hand, the thinning of the barrier oxide layer upon potential stepping in the cathodic direction occurred at a rate that was two orders in magnitude lower than the growth rate. The steady-state barrier layer thickness varied linearly with applied potential, whereas the steady-state current density did not depend on the formation potential. These spectroscopic ellipsometric measurements are qualitatively explained by the point defect model (PDM).

Cu underpotential deposition on Au controlled by in situ Spectroscopic Ellipsometry

AbstractWe have studied Cu electrodeposition on well defined Au films using real time Spectroscopic Ellipsometry (SE). SE allows to discriminate the under‐potential (UP) and over‐potential (OP) regimes. In the UP regime, tiny yet reproducible variations of Ψ and Δ parameters indicate the formation of two phases with slightly different optical behavior. The phase at the largest coverage is assigned to a Cu monolayer. The SE response in the OP regime shows a marked dependence on the potential scan rate. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)