MATERIALSSYNTHESIS AND CHARACTERIZATIONS TECHNIQUES
3.12 Characterization Techniques
For the analysis of the Ba1-xCaxTiO3 thin films, different characterization techniques such as X-ray diffraction, FESEM, EDX, AFM, four-point probe methods are used.
3.12.1 X-ray diffraction
X-ray diffraction is one of the basic tools employed for the determination of structural phase and order of crystallinity of any single crystal, bulk solid matter, and thin films. The structural parameters of the Ba1-xCaxTiO3 thin films was studied using the XRD technique. XRD data was taken using an X-ray diffractometer (3040XPert PRO, Philips) using CuKฮฑ radiation (ฮป = 1.5406 ร ).
Fig. 3.5 Reflection of X-rays from two planes of atoms in a solid.
The basic principle of determining the crystal structure using X-ray is based on the diffraction phenomenon of electromagnetic waves. So, when the matter is irradiated with a beam of X-ray photons, the interaction of the photon with the bounded electron mostly results in a coherent scattering of these photons which can be detected using electromagnetic photon detectors. The scattered or emitted X-ray beams are of equal wavelength or very close to that of the incident beam.
The basic formulation for the determination of crystal structure using the angular distribution of diffraction peaks known as Braggโs law,
2๐โ๐๐๐ ๐๐๐ = ๐๐ (3.1) where 'n' denotes the order of diffraction, โฮปโ is the wavelength of X-ray used, 'dhkl' is the distance between the lattice plane and 'ฮธ' is the angle at which diffraction peak is detected.
3.12.2 Field emission scanning electron microscopy
The surface morphology of the Ba1-xCaxTiO3 thin films was observed by a FESEM using JEOL JSM-7600F at an accelerating voltage of 5 kV. In FESEM, an electron beam is scanned across a sampleโs surface. The electrons are produced by a thermal emission source. The energy of the incident electrons can be as low as 100 eV or as high as 30 keV depending on the evaluation objectives. This electron beam generates several different types of signals include secondary electrons, back-scattered electrons. The electrons are focused into a small beam by a series of electromagnetic lenses in the FESEM column.
Fig. 3.6 A field emission scanning electron microscope set up.
The beam passes through pairs of scanning coils or pairs of deflector plates in electron column, typically in the final lens, which deflects the beam in the x and y axes so that it scans in a raster fashion over a rectangular area of the sample surface. The energy exchange between the electron beam and the sample, results in the reflection of
high-energy electrons by inelastic scattering and the emission of electromagnetic radiation, each of which can be detected by specialized detectors.
Electronic amplifiers of various types are used to amplify the signals, which are displayed as variations in brightness on a cathode ray tube (CRT). The raster scanning of the CRT display is synchronized with the position of the beam on the specimen in the microscope, and the resulting image is therefore, a distribution map of the intensity of the signal being emitted from the scanned area of the specimen. The image may be captured by photography from a high-resolution cathode-ray tube.
3.12.3 Energy dispersive X-ray spectroscopy
The EDX spectrum is a plot of the intensity of X-ray vs energy of the emitted X-ray. An EDX spectrum normally displays peaks corresponding to the energy levels for which the most X-rays had been received. Each of these peaks is unique to an atom and therefore corresponds to a single element. The elements in BTO and the incorporation of the Ca element in BTO films were studied by using EDX spectroscopy.
Fig. 3.7 Schematic diagram of X-ray excitations in EDX analysis.
The incident beam may excite an electron in an inner shell, ejecting it from the shell while creating an electron hole where the electron was stayed.
An electron from an outer, higher energy shell then fills the hole, and the difference in energy between the higher energy shell and the lower energy shell may be released in the form of an X-ray.
3.12.4 Atomic force microscopy
AFM is a very high-resolution type of scanning probe-microscopy with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. The surface morphology of Ba1-xCaxTiO3 thin films was analyzed using AFM. Two and three-dimensional view of the samples and various data were taken using an AFM.
Fig. 3.8 Block diagram of atomic force microscope.
The AFM is one of the foremost tools for imaging, measuring, and manipulating matter at the nanoscale. The information is gathered by "feeling" the surface with a mechanical probe. Piezoelectric elements facilitating tiny, accurate and precise movements on (electronic) command enable a very precise scanning. In some variations, electric potentials can also be scanned using conducting cantilevers. In more advanced versions, currents can be passed through the tip to probe the electrical conductivity of the underlying surface.
3.12.5 Thickness measurement of thin film
One of the most significant film parameters is the thickness measurements. The thickness can be measured by the phase difference of the interferograms generated on the upper and lower surfaces of the thin film. The Fizeau Fringe interferometric method was used to determine the thickness of Ba1-xCaxTiO3 thin films. The technique was used to measure the thickness with the help of optical interference. Since the film thicknesses are generally of the order of a wavelength of light, various types of optical interference phenomena have been found to be most useful for the measurement of film thicknesses.
Fig. 3.9 Interferometer arrangement for producing refection Fizeau fringes of equal thickness.
When two reflecting surfaces are brought into proximity, interference fringes are produced, the measurement of which makes possible a direct determination of film thickness and surface topography with high accuracy. The film whose thickness is to be measured is required to form a step on a glass substrate and over it, another plane glass plate is placed. With the help of a parallel monochromatic beam of light, a fringe system is produced and is viewed with low power microscope.
3.12.6 Optical characterization
The optical transmission spectra were obtained using by a SHIMADZU UV-1601 spectrophotometer in the wavelength range of 190-1000 nm at room temperature. Optical properties characterize the response of materials to incident electromagnetic radiation. Measurements were made by placing the sample in the incident beam and another empty glass substrate in the reference beam of the instrument.
Fig. 3.10 Schematic diagram of Ultraviolet-visible spectrophotometer.
The transmission spectra were recorded both for undoped and Ba1-xCaxTiO3 thin films at Ts of 350 oC thin films of concentration 2-8 at.% Ca.
3.12.7 Electrical characterization
The electrical measurements were performed for Ba1-xCaxTiO3 thin films by using the four-point probe method in the range of 300 ~ 475 K. It consists of four probes arranged linearly in a straight line at an equal distance from each other. The probe array is placed in the center of the material.
A constant current is passed through the two probes and the potential drop across the middle two probes is measured. Thus, an oven is provided with a heater to heat the sample so that the behavior of the sample is studied with the increase in temperature.
Fig. 3.11Experimental set up for electrical measurement.