现代材料分析方法(英文)

Characterization techniques:(A) XPS (X-ray photoelectron spectroscopy):Hydrothermally deposited epitaxial thin films are characterized by XPS to retrieve useful information like composition, chemical structure and local arrangement of atoms that make up few layers of surface of film and also the interfacial layer between the film and substrate.X-ray photoelectron spectroscopy (XPS) was developed in the mid –1960s by Kai Siegnahm and his research group at the University of Uppsala, Sweden.Surface analysis by XPS involves irradiating a solid in vacuum with monoenergetic soft x-rays and analyzing the emitted electrons by energy. The spectrum is obtained as a plot of the number of detected electrons per energy interval versus their kinetic energy. The principle on which the XPS technique is based can explained with the help of figure 1 as shown below. [27]Figure 1. An energy level diagram showing the physical basis of XPS technique.The energy carried by an incoming X-ray photon is absorbed by the target atom, raising it into excited state from which it relaxes by the emission of a photoelectron. Mg Kα(1253.6eV) or Al Kα (1486.6 eV) x-rays are generally used as a source of monoenergetic soft x-rays. These photons have limited penetrating power in a solid on the order of 1-10 micrometers. They interact with atoms in the surface region, causing electrons to be emitted by the photoelectric effect. The emitted electrons have measured kinetic energies given by:KE=hγ-BE -φsWhere hγ is the energy of the photon, BE is the binding energy of the atomic orbital from which the electron originates and φs is the spectrometer work function. The binding energy may be regarded as the energy difference between the initial and final states after the photoelectron has left the atom. Because there are a variety of possible final states of the ions from each type of atom, there is corresponding variety of kinetic energies of the emitted electrons. Photoelectrons are emitted from all energy levels of the target atom and hence the electron energy spectrum is characteristic of the emitting atom type and may be thought as its XPS fingerprint. Each element has unique spectrum .The spectrum from a mixture of elements is approximately the sum of peaks of the individual constituents. Because the mean free path of electrons in the solids is very small, the detected electrons originate from only the top few atomic layers making XPS a unique surface sensitive technique for chemical analysis. Quantitative data can be obtained from peak heights or peak areas and identification of chemical states often can be made from exact measurement of peak positions and separations as well from certain spectral features.The line lengths indicate the relative probabilities of the various ionization processes. The p,d and f levels split upon ionization leading to vacancies in the p1/2,p3/2,d3/2,d5/2,f5/2 and f7/2.The spin orbit splitting ratio is 1:2 for p levels ,2:3 for d levels and 3:4 for f levels .Because each element has a unique set of binding energies, XPS can be used to identify and determine the concentration of the elements in the surface. Variations in the elemental binding energies (the chemical shifts) arise from the differences in the chemical potential and polarizibilty of compounds. These chemical shifts can be analyzed to identify the chemical state of the materials being analyzed.The electrons leaving sample are detected by an electron spectrometer according to their kinetic energy. The analyzer is usually operated as an energy window, referred to as pass energy. To maintain a constant energy resolution, the pass energy is fixed. Incoming electrons are adjusted the pass energy before entering the energy analyzer. Scanning for different energies is accomplished by applying a variable electrostatic field before the analyzer. This retardation voltage may be varied from zero upto and beyond the photon energy. Electrons are detected as discrete events, and the number of electrons for the given detection time. And energy is stored and displayed.In general, the interpretation of the XPS spectrum is most readily accomplished first by identifying the lines that almost always present (specifically those of C and O), then by identifying major lines and associated weaker lines.(B) Auger electron spectroscopy:Auger electron spectroscopy is a very useful technique in elemental characterization of thin films. In the current project this technique has been utilized not only for elemental compositional analysis but also for understanding nucleation and growth mechanism. Auger electron effect is named after the French physicist Pierre Auger who described the process involved in 1925.Auger is process is bit more complicated than the XPS process.The Auger process occurs in three stages. First one being atomic ionization. Second being electron emission (Auger emission) and third being analysis of emitted auger electrons .The source of radiation used is electrons that strike in the range of 2 to 10 kev. The interatomic process resulting in the production of an Auger electron is shown in figure 2 below.Figure 2 showing the interatomic process resulting in production of the Auger electrons. One electron falls a higher level to fill an initial core hole in the k-shell and the energy liberated in this process is given to second electron ,fraction of this energy is retained by auger electron as kinetic energy.X-ray nomenclature is used for the energy levels involved and the auger electron is described as originating from for example ,an ABC auger transition where A is the level of the original core hole,B is the level from which core hole was filled and C is the level from which auger electron was emitted. In above figure 2 shown above the auger transition is described as L3M1M2, 3.The calculation of energies of the lines in the Auger electron spectrum is complicated by the fact that emission occurs from an atom in an excited state and consequently the energies of the levels involved are difficult to define precisely.Each element in a sample being studied gives rise to characteristic spectrum of peaks at various kinetic energies. Area generally scanned is 1 mm2.To understand the variation in the concentration with the distance from the surface depth profiling can also be carried out. For depth profiling the surface has to be etched away by using argon beam.The principle advantage that AES hold over XPS is that the source of excitation in case of AES is electrons which allows it to take a spectra from micro-regions as small as 100 nm diameters or less instead of averaging over the whole of the surface of the sample as is done generally in XPS.(C) Atomic force Microscope:Atomic Force Microscope (AFM ) is being used to solve processing and materials problems in a wide range of technologies affecting the electronics, telecommunications, biological, chemical, automotive, aerospace, and energy industries. The materials being investigating include thin and thick film coatings, ceramics, composites, glasses, synthetic and biological membranes, metals, polymers, and semiconductors.In the current work AFM was used to understand the nucleation and growth mechanism of the epitaxial thin films and to understand the surface morphology of totally grown films in terms of surface coverage and surface roughness.In the fall of 1985 Gerd Binnig and Christoph Gerber used the cantilever to examine insulating surfaces. A small hook at the end of the cantilever was pressed against the surface while the sample was scanned beneath the tip. The force between tip and sample was measured by tracking the deflection of the cantilever. This was done by monitoring the tunneling current to a second tip positioned above the cantilever. They were able to delineate lateral features as small as 300 Å. This is the way force microscope was developed. Albrecht, a fresh graduate student, who fabricated the first silicon microcantilever and measured the atomic structure of boron nitride. The tip-cantilever assembly typically is microfabricated from Si or Si3N4. The force between the tip and the sample surface is very small, usually less than 10-9 N.According to the interaction of the tip and the sample surface, the AFM is classified as repulsive or Contact mode and attractive or Noncontact mode. In contact mode the topography is measured by sliding the probe tip across the sample surface. In noncontact mode, topography is measured by sensing Van de Waals forces between the surface and probe tip. Held above the surface. The tapping mode which has now become more popular measures topography by tapping the surface with an oscillating probe tip which eliminates shear forces which can damage soft samples and reduce image resolution. 1. Laser2. Mirror3. Photo detector4. Amplifier5. Register6. Sample7. Probe8. CantileverFigure 3 showing a schematic diagram of the principle of AFM.Compared with Optical Interferometric Microscope (optical profiles), the AFM provides unambiguous measurement of step heights, independent of reflectivity differences between materials. Compared with Scanning Electron Microscope, AFM provides extraordinary topographic contrast direct height measurements and unobscured views of surface features (no coating is necessary). One of the advantages of the technique being that it can be applied to insulating samples as well. Compared with Transmission Electron Microscopes, three dimensional AFM images are obtained without expensive sample preparation and yield far more complete information than the two dimensional profiles available from cross-sectioned samples.(D) Fourier Transform Infrared Spectroscopy:Infrared spectroscopy is widely used chemical analysis tool which in addition to providing information on chemical structures also can give quantitative information such as concentration of molecules in a sample.The development in FTIR started with use of Michelson interferometer an optical device invented in 1880 by Albert Abraham Michelson. After many years of difficultiesin working out with time consuming calculations required for conversion intereferogram into spectrum, the first FTIR was manufactured by the Digilab in Cambridge Massachusetts in 1960s .These FTIR machines stared using computers for calculating fourier transforms faster.The set up consists of a source, a sample and a detector and it is possible to send all the source energy through an interferometer and onto the sample. In every scan, all source radiation gets to the sample. The interferometer is a fundamentally different piece of equipment than a monochromater. The light passes through a beamsplitter, which sends the light in two directions at right angles. One beam goes to a stationary mirror then back to the beamsplitter. The other goes to a moving mirror. The motion of the mirror makes the total path length variable versus that taken by the stationary-mirror beam. When the two meet up again at the beamsplitter, they recombine, but the difference in path lengths creates constructive and destructive interference: an interferogram:The recombined beam passes through the sample. The sample absorbs all the different wavelengths characteristic of its spectrum, and this subtracts specific wavelengths from the interferogram. The detector reports variation in energy versus time for all wavelengths simultaneously. A laser beam is superimposed to provide a reference for the instrument operation.Energy versus time was an odd way to record a spectrum, until the point it was recognized that there is reciprocal relationship between time and frequency. A Fourier transform allows to convert an intensity-vs.-time spectrum into an intensity-vs.-frequency spectrum.The advantages of FTIR are that all of the source energy gets to the sample, improving the inherent signal-to-noise ratio. Resolution is limited by the design of the interferometer. The longer the path of the moving mirror, the higher the resolution.One minor drawback is that the FT instrument is inherently a single-beam instrument and the result is that IR-active atmospheric components (CO2, H2O) appear in the spectrum. Usually, a "Background" spectrum is run, and then automatically subtracted from every spectrum.(E) Scanning Electron Microscopy:Scanning electron microscopy is one the most versatile characterization techniques that can give detailed information interms of topography, morphology, composition and crystallography. This has made it widely useful in thin film characterization.The scanning electron microscope is similar to its optical counterparts except that it uses focused beam of electrons instead of light to image the specimen to gain information about the structure and composition.A stream electron is accelerated towards positive electrical potential. This stream is confined and focused using metal apertures and magnetic lenses into a thin, focused, monochromatic beam. This beam is focused onto the sample using a magnetic lens. Interactions occur inside the irradiated sample, affecting the electron beam. These interactions and effects are detected and transformed into an image. The electron detector collects the electrons and then image is created. Scanning with SEM is accomplished bytwo pairs of electromagnetic coils located within the objective lens, one pair deflects the beam in x-direction across the sample and the other pair deflects it in the y direction. Scanning is controlled by applying an electric signal to one pair of scan coils such that the electron beam strikes the sample to one side of theFigure 4 Schematic view of a SEM instrument.center axis of the lens system. By varying the electrical signal to this pair of coils as a function of time, the electron beam is moved in a straight line across the sample and then returned to its original position. Thus by rapidly moving the beam the entire sample surface can be irradiated with the electron beam. The output signal consists of backscattered and secondary electrons which generally serve as basis of scanning electron microscope and whereas the x-ray emission serves as the basis of the energy dispersive spectroscopy as shown in figure 4.Figure 5.Schematic presentation of the interaction of the electron with the sample.Energy dispersive spectroscopy is analytical method which is used in determination of elemental composition of the specimen.EDS uses the electrons generated characteristic x-radiation to determine elemental composition. The SEM/EDS combination is a powerful tool in inorganic microanalysis, providing the chemical composition of volumes as small as 3 m3.(F) Transmission Electron microscopy:Transmission electron microscopy was used to analyze the interface between the BaTiO3 on SrTiO3 single crystals.For TEM specimen must be specially prepared to thicknesses which allow electrons to transmit through the sample, much like light is transmitted through materials in conventional optical microscopy. Because the wavelength of electrons is much smaller than that of light, the optimal resolution attainable for TEM images is many orders of magnitude better than that from a light microscope. Thus, TEMs can reveal the finest details of internal structure - in some cases as small as individual atoms. Magnifications of 350,000 times can be routinely obtained for many materials, whilst in special circumstances; atoms can be imaged at magnifications greater than 15 million timesThe energy of the electrons in the TEM determine the relative degree of penetration of electrons in a specific sample, or alternatively, influence the thickness of material from which useful information may be obtained.Cross-sectional specimens for TEM observation of the interface between the film and the substrate were prepared by conventional techniques employing mechanical polishing, dimpling and ion beam milling.TEM column is shown in figure 6 consists of gun chamber on the top to the camera at the bottom everything is placed under vacuum.Figure 6. Main components of TEM system. [28]At the top of the TEM column is the filament assembly, which is connected to thehigh voltage supply by insulated cable. In standard TEM, normal accelerating voltagesranges from 20,000 to 100,000V.Intermediate-voltage and high voltage TEMs may use accelerating voltages of 200,000 V to 1000000 V.The higher the accelerating voltage, the greater the theoretical resolution. Below the filament tip and above it the anode is a beam volume called crossover. In this area of the filament chamber, the electron beam volume iscondensed to its highest density. There are more electrons per unit area at the cross over than at any other place in the microscope. Crossover is the effective electron source for image formation. In a TEM, the diameter of the electron beam at crossover is approximately 50 μm.The anode or positively charged plate, is below the filament assembly.Electron beam then travels to the condenser –lens system.TEMs has two condensers lenses. Condenser system lens system controls electron illumination on the specimen and on the viewing screen for such functions as viewing, focusing and photography. Condenserlenses are fitted with apertures which are usually small platinum disks or molybdenum strips with holes of various sizes ranging from 100 to 400 μm and it protects specimen from too many stray electrons which can contribute to excessive heat and limit X-ray production farther down the columnObjective lens is the first magnifying lens and the specimen is inserted into the objective lens, which must be designed so that the specimen can be moved in both X and Y directions and have tilting and rotating capabilities. As the electron beam interacts with the specimen, a number of signals useful in the formation of the TEM image occur: absorption, diffraction, elastic scattering and inelastic scattering.(H) X-ray Diffraction (XRD):X-ray diffraction is the most commonly known technique which I used to determination of the phase formed in films and also to assess texture and crystallinity.X-rays were discovered in 1895 by the German physicist Wilhelm Conrad Röntgen - in some languages x-rays are called Röntgen-rays - and x-ray diffraction was discovered in 1912.The X-rays used in diffraction experiments all have a wavelength of 0.5-2.5 Å. The intensity of a beam of x-rays is the rate of transport of energy flow through a unit area perpendicular to the direction of propagation. To produce x-rays, a source of electrons, a high accelerating voltage and a target are needed. To get the voltage, the metal target is grounded and a cathode is at 30-50 kV. To get the electrons a metal filament is resistively heated (the tube is called a filament tube). The filament current is 3-5 amps. The cathode and the filament is one and the same thing and surrounding the target and the filament is an air evacuated envelope.The electrons from the filament are accelerated towards the target. They bombard the target in a rectangular shaped area called the focal spot. From there the x-rays are emitted in all directions. The walls of the tube are impenetrable for the x-rays except where beryllium windows are inserted. Beryllium has a very low absorption coefficient for the x-rays.The amount of x-rays produced depends on the number of electrons emitted and their energy when they reach the target. The number of electrons in turn depends on the filament temperature, and thus the filament current. The current of electrons from the filament to the target is measurable and usually 25-55 mA. This current can be chosen freely as a feedback loop will feed the filament with the current needed. The energy ofthe electrons depend on the accelerating voltage. Thus the total intensity emitted by thex-ray tube depends on both the operating voltage and the tube current.In general, diffraction is possible when the length of the wave is of the same order of magnitude as the distance between the regularly spaced scattering objectsTwo scattered rays are in phase, if their path difference is equal to a whole number n of wavelengths. Scattered rays emerging from a plane surface as a result of a beam incident on that surface, have a path difference equal to a whole number of wavelengths, if n l = 2 d' sinq (The Bragg Law),where d' is the distance between the diffracting planes in the crystal and q is the angle between the incident beam and the surface. n is the order of reflection and n can be any integral number as long as sin q < 1. n is also equal to the number of wavelengths in the path difference of two rays scattered from adjacent planes (e.g. If n = 2 then a ray scattered from one plane will have a path that is two wavelengths shorter than a ray scattered from a deeper lying neighbor plane).The basis for phase analysis is that the crystal of a certain phase will have interatomic distances peculiar to that phase and these different distances will cause a series of reflections as the detector are shifted through 2theta.Two phases can have similar or almost similar structures and hence interatomic distances. This makes identifying phases in an unknown sample very difficult, but knowing what elements are present in the sample will narrow the possibilities down quite a bit. Also crystallite size using XRD .X-ray pole figure measurements are used to characterize the film with respect to any preferred orientation with which growth has taken place. Rocking curve is another application to characterize the film with respect to its quality ofcrystallinity comparing to the single crystals or polycrystalline materials.。