Optical properties of bismuth niobate thin films studied by spectroscopic ellipsometry

We performed optical analysis of bismuth niobate thin films using spectroscopic ellipsometry (SE). The films were grown on Pt/Ti/SiO₂/Si substrates with pulsed laser deposition. Six films were prepared using various deposition temperatures and thermal-annealing times. The room-temperature SE spectra of these films were measured by a rotating-analyzer ellipsometer from 1.12 to 6.52 eV at incidence angles of 50, 55, 60, 65, and 70°. The resulting refractive indices and extinction coefficients show significant changes with deposition temperature and thermal annealing.     

Cubic CdS thin films studied by spectroscopic ellipsometry

CdS polycrystalline films were grown onto glass substrates by chemical bath deposition (CBD) and characterized by spectroellipsometry, X-ray diffraction and transmission electron microscopy. The X-ray diffraction patterns of the samples showed the presence of a CdS cubic phase (β-CdS) and of Cd2SiO4 as interfacial material. Using electron diffraction it was possible to index the films as cubic CdS. From effective dielectric function measurements and from reported optical data for the dielectric function of cubic CdS crystals, grown by vapour phase epitaxy, it was possible to fit the experimental data to an effective medium approximation, and to deduce the film thickness, the void fraction and the field screening. This revised version was published online in August 2006 with corrections to the Cover Date.     

Optical constants of vacuum-evaporated Cd0.96Zn0.04Te thin films measured by spectroscopic ellipsometry

The pseudodielectric-function spectra, (E)=₁(E)+i₂(E), of polycrystalline Cd_0.96Zn_0.04Te thin films in the 1.3–5.5 eV photon energy range at room temperature were obtained using spectroscopic ellipsometry. The measured dielectric-function spectra reveal that distinct structures at energies of the E₁, E₁+₁, and E₂ critical points are due to interband transitions. The Cd_0.96Zn_0.04Te thin films investigated were deposited by vacuum evaporation under a pressure better than 1.3×10^–3 Pa onto well-cleaned glass substrates kept at 300 K. The films exhibited zinc blende structure with predominant (1 1 1) orientation. The root mean square (r.m.s.) roughness of the vacuum-evaporated Cd_0.96Zn_0.04Te thin films evaluated by ex situ atomic force microscopy is 3.7 nm. Dielectric-related optical constants, such as the refractive index (n), extinction coefficient (k), absorption coefficient () and normal incidence of reflectivity (R) determined from the spectroscopic ellipsometry data are presented and analyzed. The optical constants of the films were also determined using optical transmittance measurements and the results are discussed.     

Optical and electronic properties of highly stable and textured hydrogenated ZnO:Al thin films

We have experimentally investigated the effects of hydrogen-annealing on the structural, electrical, and optical properties of Al-doped ZnO (ZnO:Al) thin films prepared by RF magnetron sputtering at room temperature. From the X-ray diffraction observations, the orientation of ZnO:Al films was found to be a c-axis in the hexagonal structure. We found that intentionally incorporated hydrogen plays an important role in n-type conduction as a donor, improving free carrier concentration and electrical stability. We simultaneously obtained improved optical transmission and enhanced absorption edge of the ZnO:Al film due to hydrogen-annealing. Our experimental data suggest the hydrogen-annealing process as an important role in the enhancement of electrical and optical properties, which is promising as a back reflector material for thin-film solar cells.     

Optical properties of GaAs0.9-xNxSb0.1 alloy films studied by spectroscopic ellipsometry

Spectroscopic ellipsometry from 0.73 to 4.75 eV was used to study the optical properties of epitaxial GaAs_0.9-xN_xSb_0.1 layers with x = 0.00, 0.65, 1.06, 1.45 and 1.90%. The ellipsometric experimental spectra were fitted using a multilayer model employing the model dielectric function to describe the GaAs_0.9-xN_xSb_0.1 optical response. We have identified the Γ-point E₀, E₊, and E^# transitions of GaAs_0.9-xN_xSb_0.1 and have determined the effect of nitrogen on the respective transition energies. We have demonstrated that a lower N content can provide an equal E₊-E₀ energy splitting for GaAs_0.9-xN_xSb_0.1 with respect to GaAs_1-xN_x.     

Optical characterization of thin chalcogenide films by multiple-angle-of-incidence ellipsometry

Optical properties of thin chalcogenide films from the systems As-S(Se) and As-S-Se were investigated as a function of the film composition, film thickness and conditions of illumination by light using multiple-angle-of-incidence ellipsometry. Thin films were deposited by thermal evaporation and exposed to white light (halogen lamp) and to monochromatic light from Ar⁺ — (λ = 488, 514 nm) and He-Ne- (λ = 632.8 nm) lasers. The ellipsometric measurements were carried out at three different angles of light incidence in the interval 45-55°, at λ = 632.8 nm. An isotropic absorbing layer model was applied for calculation of the optical constants (refractive index, n and extinction coefficient, k) and film thickness, d. The homogeneity of the films was checked and verified by applying single-angle calculations at different angles. It was shown that the refractive index, n of As-S-Se films is independent of film thickness in the range of 50 to 1000 nm and its values varied from 2.45 to 3.05 for thin layers with composition As₂S₃ and As₂Se₃, respectively. The effect of increasing in the refractive index was observed after exposure to light which is related to the process of photodarkening in arsenic containing layers. The viability of the method for determining the optical constants of very thin chalcogenide films with a high accuracy was confirmed.     

Optical study of Mn-doped Bi4Ti3O,12 thin films by spectroscopic ellipsometry

Mn-doped Bi4Ti3O12(B4T3) thin films grown at 400 degrees C on a Pt/Ti/SiO2/Si substrate through pulsed laser deposition (PLD) were analyzed via spectroscopic ellipsometry (SE). The PLD targets were produced through the conventional solid-state sintering method, and the film samples were annealed at 600 degrees C. The SE spectra of B4T3 films were measured using a rotating analyzer type ellipsometer within the 1.12 to 6.52 eV energy range, with the various incidence angles. The optical properties of the B4T3 films with increasing Mn-mol concentration were extracted using a multilayer model for the whole structure and the Tauc-Lorentz (TL) dispersion relation for the B4T3 film layer. The analysis results clearly showed that the significant changes in optical properties of B4T3 films are caused by thermal annealing procedure and the Mn-mol concentrations. X-ray diffraction (XRD) measurement was also performed to confirm the results of SE analysis.     

Optical properties of CdTe thin films studied by photothermal deflection spectroscopy

Photothermal deflection spectroscopy (PDS) was used to analyze CdTe thin films which are to be used to construct solar cells. It was found that CdTe thin films grown on a conducting SnO₂ layer deposited on glass show deviations from a stoichiometric composition due to an excess of tellurium. This non-stoichiometry modifies the absorption spectrum obtained from the PDS measurements.     

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.     

Electrical and optical properties of ZnO:Al thin films grown by magnetron sputtering

The properties of transparent conductive ZnO:Al thin films grown by R.F. magnetron sputtering method are investigated. The working pressure (argon gas) is changed from 2.5 to 40.0 mTorr to study its influence on the characteristics of ZnO:Al thin films. The ZnO:Al thin films have better texture due to the increase in the surface mobility, which resulted from the increase in the mean free path of sputtering gas under lower working pressure. The microstructure of ZnO:Al films is found to be affected obviously by changing the working pressure. It is shown that the grain size of ZnO:Al thin films decreases with the increase of working pressure. The X-ray diffraction patterns indicate that the poor crystallized structure of ZnO:Al films is obtained at higher working pressure. Except 40 mTorr, the highly (002)-oriented ZnO:Al thin films can be found at the measured range of working pressure. Moreover, the growth rate of the films decreases from 1.5 to 0.5 nm/min as the working pressure increases from 2.5 to 40.0 mTorr. The results of optical transmittance measurement of ZnO:Al thin films reveal a high transmittance (>80%) in visible region and exhibit a sharp absorption edge at wavelength about 350 nm.     

Physical and sensing properties of ZnO:F:Al thin films deposited by sol-gel

Fluorine and aluminum-doped zinc oxide thin films, ZnO:F:Al, were prepared on soda-lime glass substrates by the sol-gel method and repeated dip-coating. The effect of the solution ageing and film thickness on the physical characteristics of the films was studied. Two ageing times, namely, two and seven days, and three different thicknesses, in the order of 220, 330, and 520 nm, were the main variables used in this work. As-deposited ZnO:F:Al films showed a high electrical resistivity, however after a vacuum thermal treatment, it was registered a significant decrease. Structural, optical, and morphological characterizations were carried out in vacuum-annealed films. The X-ray diffraction (XRD) patterns revealed that both as-deposited and vacuum-annealed ZnO:F:Al thin films were polycrystalline with a hexagonal wurtzite-type structure with a well-defined (002) diffraction peak, irrespective of the ageing time of the starting solution. The (002) peak shows a proportional increase with the thickness magnitude. An average crystallite size of about 20 nm was estimated using the well-known Scherrer's formula. From the surface morphological study it was observed that the grain size is almost independent of the ageing time of the starting solution, and the film thickness. Films presented an average optical transmittance in the visible range (400-700 nm) in the order of 90%, as well as a band gap of 3.3 eV. The gas-sensing properties of ZnO:F:Al thin films in an atmosphere containing different concentrations of carbon monoxide, and at different operation temperatures were probed. The highest sensitivity registered was of the order of 93%.     

铌酸锶钡薄膜制备及椭偏光谱研究

用溶胶-凝胶法成功地制备出了退火温度分别为500、600、700、800、900℃的铌酸锶钡(SBN)薄膜;对制备出SBN薄膜分别进行了椭偏光谱测量研究,得到了不同退火SBN薄膜椭偏光谱参数曲线;并对测得的椭偏光谱进行了数值反演计算,得到了不同退火温度的SBN薄膜的光学常数谱.结果发现SBN薄膜的折射率和消光系数都随着退火温度的增高而增大.     

ZnO:Al thin films deposited by RF-magnetron sputtering with tunable and uniform properties

Nanostructured, high quality and large area Al-doped ZnO (ZnO:Al) thin films were obtained by radiofrequency (RF) magnetron sputtering. The sample rotation during deposition has resulted in excellent spatial distribution of thickness and electro-optical properties compared to that obtained under static conditions. ZnO:Al thin films are employed in a large number of devices, including thin film solar cells, where the uniformity of the properties is a key factor for a possible up-scaling of the research results to industrially relevant substrate sizes. A chemical post etching treatment was employed achieving tunable surface nanotextures to generate light scattering at the desired wavelength for improved cell efficiency. Since the film resistivity is only slightly increased by the etching, this post-deposition step allows separating the optimization of electro-optical properties from light scattering behavior. The thin films were characterized by FE-SEM, XRD, UV-VIS spectroscopy, four probe and van der Paw techniques.     

Optical and photoluminescent properties of sol-gel Al-doped ZnO thin films

Al-doped zinc oxide (AZO) thin films have been prepared via a sol-gel process. Optical and photoluminescent properties of the AZO films have been investigated. The UV absorption edge was blue shifted with increasing Al doping concentration. Efficient green–yellow emission was obtained after annealing at 850 °C. For the 850 °C-annealed samples, the green peak was red shifted from 518 to 565 nm as the Al doping concentration increased from 0 to 2.0 at.%. In addition, violet emission in the range of 400–450 nm was observed in the 850 °C-annealed AZO films. The possible origins responsible for these emission bands have been discussed.     

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.     

Optical study of ion-deposited diamond-like carbon films using spectroscopic ellipsometry

Spectroscopic ellipsometry was used for the characterization of ion-deposited diamond-like carbon (DLC) films, including the determination of film thickness and optical properties of DLC. The measured spectra in the wavelength range from 300 to 850 nm were analyzed with an appropriate fitting model, which was constructed according to the nominal sample structure in which the optical properties of DLC were described by a Cauchy dispersion model. Reasonably good agreement was found between the measured and calculated spectra for all samples studied, indicating that the models used were appropriate and that the calculated results were reliable. The results of our analysis suggest that, under the same deposition conditions (i.e., same substrate temperature and same chamber pressure), the optical properties of ion-deposited DLC film did not change much even if the film was prepared with quite different gas flow ratios.     

Unambiguous determination of thickness and dielectric function of thin films by spectroscopic ellipsometry

We present a spectroscopic method of determining both thickness and dielectric functions of thin films on previously characterized substrates. The method requires that spectroscopic ellipsometric data be taken over an energy range where the substrate has a sharp optical structure, e.g. a critical point. These data are used to determine a “pseudodielectric” function for the film, which necessarily depends on the assumed film thickness. We develop a general first-order theory which shows that the correct thickness and dielectric function of the film are those for which the substrate feature vanishes in the calculated pseudodielectric function. The theory is also used to investigate sensitivity and applicability. We show that sensitivity can be enhanced by differentiating the pseudodielectric function with respect to energy. The method is demonstrated with several numerical and experimental examples, which illustrate that monolayer thickness accuracy can be achieved.     

Optical properties of SiO2/nanocrystalline Si multilayers studied using spectroscopic ellipsometry

Using variable-angle spectroscopic ellipsometry, we measure the pseudo-dielectric functions of as-deposited and annealed SiO₂/SiO_x multilayers. The SiO₂(2 nm)/SiO_x(2 nm) multilayers are prepared under various deposition temperatures by ion beam sputtering. Annealing at temperatures above 1100 °C leads to the formation of Si nanocrystals (nc-Si) in the SiO_x layer of multilayers. Transmission electron microscopy images clearly demonstrate the existence of nc-Si. We assume a Tauc-Lorentz lineshape for the dielectric function of nc-Si, and use an effective medium approximation for SiO₂/nc-Si multilayers as a mixture of nc-Si and SiO₂. We successfully estimate the dielectric function of nc-Si and its volume fraction. We find that the volume fraction of nc-Si decreases after annealing, with increasing x in as-deposited SiO_x layer. This result is compared to expected nc-Si volume fraction, which is estimated from the stoichiometry of SiO_x.     

Optical properties of soda-lime float glass from spectroscopic ellipsometry

Optical properties of soda-lime glass manufactured by the float process were investigated using spectroscopic ellipsometry and intensity transmission measurements. Thickness and optical properties of surface layers on the air and tin sides were determined with ellipsometry. The tin side surface layer shows a graded refractive index with a non-linear profile. Intensity transmission data were used to quantify absorption in the bulk glass. Transmission-mode generalized ellipsometry characterized residual birefringence in the bulk glass. Birefringence effects on ellipsometric delta data were corrected using a simple empirical offset with 1/wavelength dependence. A general optical model for soda-lime glass is presented which can be used for subsequent analysis of coated soda-lime glass and other transparent substrates.     

溅射功率对ZnO:Al薄膜光电性能的影响

采用射频磁控溅射工艺,以Al掺杂ZnO(ZAO)陶瓷靶为靶材在石英玻璃基片上制备出具有优良光电性能的ZAO透明导电薄膜,研究了溅射功率对薄膜光电性能的影响。在不同溅射功率条件下制备的ZAO薄膜具有很好的c轴择优取向。较大功率溅射有利于薄膜晶粒尺寸的增大、电阻率降低。ZAO薄膜在可见光区的透过率平均值高达90%以上,受溅射功率影响不大。在340nm-420nm波长附近ZAO薄膜透过率急剧下降,呈现明显的紫外吸收边;高的溅射功率提高了ZAO薄膜的光学带隙宽度。