Optical anisotropy of Langmuir-Blodgett films of long-chain acetylenic acids

The optical anisotropy of Langmuir-Blodgett films, along with its variation in the course of polymerization under UV radiation, is studied by ellipsometry at a fixed wavelength of 632.8 nm for several angles of incidence of light. Films of the Cd and Pb salts of acetylenic acids with internal and terminal triple bonds—HC≡ C(CH₂)₂₁COOH (I) and CH₃(CH₂)₁₈C≡CCOOH (II)—are studied. The refractive indices and thicknesses of the films are determined on the basis of a model of a biaxial anisotropic film. It is found that the films of the Pb salt of acid II are biaxial, while the films of the Cd salt of acid II and of the Cd and Pb salts of acid I have nearly uniaxial anisotropy. The calculated thicknesses of monolayers coincide with the thicknesses determined from small-angle x-ray scattering. The anisotropy of the films is shown to vary under UV irradiation. After polymerization of the films, the anisotropy in the plane of the layer virtually vanishes, while that in the direction perpendicular to the interface is retained.     

Generalized ellipsometry in unusual configurations

Most ellipsometry experiments are performed by shining polarized light onto a sample at a large angle of incidence, and the results are interpreted in terms of thin film thicknesses and isotropic optical functions of the film or substrate. However, it is possible to alter the geometrical arrangement, either by observing the sample in transmission or at normal-incidence reflection. In both cases, the experiment is fundamentally the same, but the interpretation of the results is considerably different. Both configurations can be used in conjunction with microscope optics, allowing for images to be made of the sample. The results of three examples of these different configurations using the two-modulator generalized ellipsometer (2-MGE) are reported: (1) spectroscopic birefringence measurements of ZnO, (2) electric field-induced birefringence (Pockels effect) in GaAs, and (3) normal-incidence reflection anisotropy of highly oriented pyrolytic graphite (HOPG).     

Magnetic properties of oblique-evaporated Co-Ni thin films

Co_{1 - x}Ni_x (x = 0, 0.2 and 0.3) thin films of thickness about 1500 Å were electron-beam evaporated onto silicon and polymide substrates at various oblique angles . In-plane coercivities and squareness ratios both along and transverse to the incidence plane were examined. Also, the angular variations of coercivity of films prepared at = 0 ° to 85 ° were investigated. The magnetic anisotropy changes from an in-plane anisotropy with the easy axis perpendicular to the incidence plane to an out-of-plane anisotropy parallel to the incidence plane, the transition occuring at about 60 °. Also discussed is the effect of the substrate temperature on the magnetic properties and columnar microstructure of the oblique-evaporated films. At room temperature, there is a small drop in coercivity at = 60 ° before a sharp rise in coercivities to 1400 Oe as the oblique angle increases.     

Some considerations on the determination of optical properties of adsorbed films

A discussion is presented of the criteria involved in the choice of parameters where it is desired to extend either basically ellipsometric or reflectometric measurements to three parameter measurements for refractive index determination of very thin adsorbed films. It is shown that even three parameters often fail to give unique values of film optical properties, and that the requirements for accuracy are frequently extreme for any reasonable error limits in the determination. It is therefore advantageous to extend the measurements to a further angle of incidence, unless the equivalent is provided by measurements at several wavelengths. Although varying the angle of incidence in ellipsometry is of very little value, it is some help in three parameter measurements provided a proper choice of angles is made. As an example the anodic oxidation of Pt and adsorption of benzene on Pt is discussed. Cases where no solution is found for isotropic film models are presented. As these films can well be expected to be uniaxially anisotropic, a search was extended into this case, although by no means exhaustive. Solutions were found for all the examples, but in the benzene case alone, the solutions are plausible.     

Generalized Null-Ellipsometry in the Polarizer–Sample–Analyzer Scheme

Optics and Spectroscopy - The null ellipsometry technique of generalized ellipsometry based on using a compensator-free polarizer–sample–analyzer scheme for the case of incidence of an...     

Ellipsometric study of refractive index anisotropy in porous silicon

Porous Si layers of different thicknesses were prepared by anodising p⁺-type Si substrates with a resistivity of 0.01 Ω cm. The porosity of the samples ranged from 23% to 62%. The refractive index values for the ordinary and extraordinary rays were determined by multiple angle of incidence ellipsometry, from which an optical anisotropy parameter varying from 13% to 20% was obtained. The porous Si layers were modelled as uniaxially anisotropic films on an isotropic substrate, with an optical axis perpendicular to the sample surface. The morphological anisotropy which is typical for the p⁺-type porous Si with a predominating cylindrical geometry is responsible for these optical properties. All the porous Si layers studied were found to be optically negative.     

Effects of methyl red acidity and UV illumination on absorption coefficient of MR/PVA thin films

The absorption coefficient spectra of poly(vinyl alcohol), PVA, mixed with methyl red (MR) thin films on glass substrates, prepared by the spin coating method has been investigated using scanning electron microscopy (SEM) and spectroscopic ellipsometry. SEM imaging indicates that the surface of the MR/PVA film is smooth, uniform, and no crack could be observed. Spectroscopic ellipsometry measurements of PVA and MR/PMMA thin films were carried out at three angles of incidence, over the wavelength range 400-800 nm. Optical models were used to obtain the absorption coefficients for the prepared samples. These models include Cauchy formula for the glass substrate and PVA film, Lorentz model with three oscillators for MR layer, and a Bruggeman effective medium approximation for MR/PVA films. Absorption coefficients were found to be in the range 5×10³−5×10⁴ cm⁻¹ with a maximum being at about 475 nm. Changing the absorption coefficient as a function of increasing the pH of MR causes a shift of the absorption band toward higher wavelengths. Our results show that the absorption coefficient of the film decreases upon increasing the UV illumination time.     

High-frequency excitations of stripe-domain lattices in magnetic garnet films

Magnetic garnet films of composition (Y,Bi)₃(Fe,Al)₅O₁₂ have been grown by liquid phase epitaxy on [111] and [110] oriented substrates of gadolinium gallium garnet. The domain wall resonance and the two branches of the domain resonance of periodic stripe domains are measured as function of the bias induction applied in the film plane parallel to the stripes. Resonance frequencies up to 7.5 GHz are observed. An improved version of the hybridization model is developed to describe these resonances. It turns out that hybridization of the domain resonance branches is determined by the cubic anisotropy for [111] oriented films, while for [110] oriented films coupling of the domain resonances is mainly caused by the orthorhombic anisotropy. The theoretical model is in excellent agreement with experiments, no fitting parameters are used. It is also used to derive the phase relation between the precessing magnetizations of neighbouring domains.     

Determination of optical constants of a biaxially anisotropic film by standard multiangle monochromatic ellipsometry

Possibilities of standard multiangle monochromatic ellipsometry in the determination of parameters of a uniform biaxially anisotropic layer are studied for the case of an arbitrary orientation of the two principal axes in the plane of incidence of the light beam and perpendicularity of the third axis to the plane of incidence. Using numerical simulation, it has been found that the measurement accuracy that is necessary in the determination of all the three principal components of the dielectric permittivity tensor ε and tilt angle of the axes using only angular dependences of ellipsometric parameters must be no worse than 0.0001°, which is far beyond the accuracy limits provided by present-day ellipsometers. If the tilt angle is known, standard multiangle monochromatic ellipsometry provides the determination of thickness and all three principal components of the dielectric permittivity tensor. This method allows one to determine the layer thickness and tensor component for the axis perpendicular to the plane of incidence, as well as the average value of components for the axes lying in the plane of incidence without involving the data about the tilt angle of the axes. This is demonstrated by an example of experimental data for biaxially anisotropic SiO _x films obtained by oblique deposition of silicon monoxide SiO evaporated in vacuum.     

Spectral responses of gyrotropic chiral sculptured thin films to obliquely incident plane waves

Gyrotropic chiral sculptured thin films (STFs) exhibit optical activity due to their structural chirality, local anisotropy, and magneto-optic gyrotropy. We adapted two algorithms for nongyrotopic chiral STFs to investigate the circular-polarization-sensitivity of gyrotropic chiral STFs to incident plane waves. The impacts of gyrotropy and oblique angles of incidence on the reflectances and the transmittances were examined, and several conclusions drawn. In particular, we found that the incorporation of gyrotropy results in a blueshift of the Bragg regime.     

An electron spectrometer for LEED fine structure measurements

LEED fine structure analysis requires intensity versus energy data for very low energy electrons (typically 5–25 eV) at various angles of incidence and azimuth. An electron spectrometer capable of fulfilling these requirements was built and used to obtain data for Cu(111). This spectrometer differs from others used for these type of measurements in that it used a retarding field analyzer to collect the elastically scattered beams instead of a deflecting analyzer. This considerably simplifies the apparatus required and the experimental procedure as the collector can be used at a fixed angle to collect data from a number of different angles of incidence. The experimental design is discussed and sample results, showing that three peaks were resolved, are presented.     

Mǒsbauer study of the field induced uniaxial anisotropy in electro-deposited FeCo alloy films

Thin ferromagnetic films with in-plane magnetic anisotropy are promising materials for obtaining high microwave permeability.The paper reports a Mo¨ssbauer study of the field induced in-plane uniaxial anisotropy in electro-deposited FeCo alloy films.The FeCo alloy films were prepared by the electro-deposition method with and without an external magnetic field applied parallel to the film plane during deposition.Vibrating sample magnetometry and Mo¨ssbauer spectroscopy measurements at room temperature indicate that the film deposited in external field shows an in-plane uniaxial anisotropy with an easy direction coinciding with the external field direction and a hard direction perpendicular to the field direction,whereas the film deposited without external field does not show any in-plane anisotropy.Mo¨ssbauer spectra taken in three geometric arrangements show that the magnetic moments are almost constrained in the film plane for the film deposited with applied magnetic field.Also,the magnetic moments tend to align in the direction of the applied external magnetic field during deposition,indicating that the observed anisotropy should be attributed to directional ordering of atomic pairs.     

Ultrafast optical study of magnons in the ferromagnetic semiconductor GaMnAs

Coherent oscillations of the magnetization were observed in magneto-optical Kerr measurements in thin films of the ferromagnetic semiconductor GaMnAs. For magnetic fields oriented in the film plane, two precession modes were observed. Their frequencies increase with the field when it is along the [100] axis, whereas they behave non-monotonically when the field is oriented along the in-plane hard axis [110]. Spectra are also presented for fields applied normal to the film plane. From the measured field-dependence of the magnon frequency, the spin stiffness and magnetic anisotropy constants were obtained.     

Effect of thermal annealing on the magnetic properties of thin films of Co-Pd alloys

The results of studies of the effect of thermal annealing on the magnetic properties of thin films of Co-Pd alloys are described, along with a method for obtaining easy-axis perpendicular anisotropy in these materials. The method consists of depositing layers of the alloy on a substrate whose thermal expansion coefficient is considerably smaller than that of the film, and then annealing it in a vacuum. This method is used to prepare samples with rectangular hysteresis loops for magnetization perpendicular to the plane of the film, coercive forces of 750 Oe, and Kerr rotation angles of 0.21°. Also presented are the results of experiments on thermomagnetic recording on the samples. Zh. Tekh. Fiz. 68, 66–70 (April 1998)     

Anisotropy of light propagation in thin opal films

Thin opal films are prepared by crystallization in a moving meniscus, and their optical transmission spectra are recorded in polarized light and studied. It is shown that the anisotropy of light propagation in the films is unambiguously related to the photonic band structure of opal and depends on the angle of incidence, the orientation of the incidence plane with respect to the opal lattice, and the wavelength and polarization of the incident light. Azimuthal diagrams of transmitted polarized light are constructed in the range of photonic band gaps of three orders for oblique incidence of a light beam. The anisotropy is found to vary with the light wave-length independently in perpendicular polarizations. A model of the band structure of opal wherein opal is represented as an fcc lattice of close-packed spheres adequately describes the optical transmission of opal films only in the range of the first-order photonic band gap.     

A rotating-compensator fourier ellipsometer

A new digital Fourier ellipsometer is described which employs a rotating compensator and a fixed analyzer to measure the polarization of light reflected from the sample. This technique combines the advantages of null ellipsometry and previously reported rotating- analyzer Fourier ellipsometry (RAE) in that it is fast, precise, has data processing flexibility and operates at high detected levels, but removes the handedness ambiguity present in RAE. The instrument may be called a Fixed-Analyzer, Rotating-Compensator Ellipsometer, but in the interest of brevity the authors have chosen the acronym RCE.     

Quantitative analysis of the supramolecular structure of metastable polycrystalline films by infrared spectropolarimetry

A method of quantitative estimation of the structural anisotropy of metastable organic polycrystalline films based on infrared spectropolarimetry is used to investigate the structural features films of the α form of copper phthalocyanine. Due to the separation of the inverse problem of reconstruction of the optical parameters of the medium into a number of stages with optimization over no more than two unknown parameters, the graphical tracking of the optimization process and, as a consequence, the minimization of errors and on-line correction of ambiguity are performed at each stage of the solution. It is demonstrated that the structure of copper phthalocyanine films vacuum-deposited onto a substrate can be changed by varying the substrate temperature, the angle of incidence of a molecular beam during the deposition, and the electrostatic field applied to the substrate.     

硬X射线微束掠入射实验的控制和数据采集系统及其应用

在国内发展了硬X射线微束掠入射实验方法,并将此具有微米级高空间分辨率的方法应用于纳米厚度薄膜的微区分析。该实验方法对分析样品表面或薄膜在微小区域的不均匀组分、结构、厚度、粗糙度和表面元素化学价态等信息具有重要意义。基于X射线全反射原理,以微聚焦实验站的高通量、能量可调的单色微束X射线为基础,通过集成运动控制、光强探测、衍射和荧光探测,设计了掠入射实验方法的控制和数据采集系统。此系统采用分布式控制结构,并基于Experimental Physics and Industrial Control System （EPICS）环境设计SPEC控制软件。通过建立SPEC和EPICS的访问通道,实现SPEC软件对EPICS平台上设备的控制和数据获取。在所设计的控制和数据采集系统中,运动控制系统控制多维样品台电机的运动,实现定位样品位置和调节掠入射角;光强探测系统则监测样品出射光强度,通过样品台运动控制和光强探测的联控,实现样品台的扫描定位控制;通过衍射和荧光探测系统获取不同入射深度下样品的衍射峰强度和荧光计数。此外,为准确控制掠入射角角度,必须确定样品平面与X射线平行的零角度位置,对此给出一种自动定位零角度的方法,编写了该方法的控制算法,设计了相应的控制软件。零角度自动化定位的扫描结果表明,实验系统微区分析的空间分辨率达到2.8 μm,零角度定位精度小于±0.01°。利用该系统在上海光源微聚焦实验站首次实现了具有自动化准确控制零角度的微束掠入射X射线衍射和荧光同步表征的实验方法,实验中被测样品为10 nm Au/Cr/Si薄膜材料,Si基底最上层为10 nm厚的Au薄膜,其间为一层很薄的Cr粘附层。在不同掠入射角下测量样品的衍射信号,获取不同入射深度下样品的衍射峰强度,并实现在同一掠入射角下,同步采集样品的荧光计数信号,从而确定了样品表层的相结构信息以及荧光信号强度与入射角关系,实现了对纳米厚度薄膜在微小区域的相结构和组分分析。此外,通过该技术能够选取荧光计数最大值对应的入射角度,有助于提高后续发展的低浓度样品掠入射X射线吸收近边结构实验方法的信噪比。     

IR Absorption Anisotropy of Films of Copper Phthalocyanine and Composites Based on It in the Region of γ-(CH) Oscillations

By the IR spectrophotometric method, metastable vacuum-deposited films of copper phthalocyanine and composites based on it have been investigated. For copper phthalocyanine films, an intense absorption band in the region of 729 cm^–1, which appears at oblique incidence of linearly polarized radiation with oscillations lying in the incidence plane, has been established. Its origin is associated with the features of the supermolecular architecture of polycrystalline films particular, with the character of packing of copper phthalocyanine microcrystallites.     

Total internal reflection ellipsometry: ultrahigh sensitivity for protein adsorption on metal surfaces

Total internal reflection ellipsometry (TIRE) is used to study adsorption of human serum albumin and fibrinogen on thin gold films. TIRE shows very high sensitivity for protein monolayers adsorbed on metal surfaces when surface plasmon resonance effects are utilized. The measured data, expressed in ellipsometric angles psi and D are of several orders of magnitude larger in comparison with those from similar experiments performed with traditional ellipsometry. TIRE in spectral mode opens a new path for precise studies of organic layers adsorbed on metal surfaces, with a potential for resolving the adsorbed layer microstructure.