of sculpting, patterning and fabricating structures at the nano- or micro-scale with the FIB and to analyze them simultaneously with the high resolution, non-destructive, SEM probe. Unlike an electron microscope, FIB is inherently destructive to the spec-imen. When the high-energy gallium ions strike the sample, they will sputter atoms from the surface. Gallium atoms will also be implanted into the top few nanometers of the surface, and the surface, in general, change its crystalline structure. Because of the sputtering capability, the FIB is used as a micro- and nano-machining tool, to modify or materials at the micro- and nanoscale. Also FIB micro machining becomes a broad field of its own, but this is a field still developing. Commonly the smallest beam size for imaging is 2.56 nm. The smallest milled features are somewhat larger (1015 nm) as this is dependent on the total beam size and interactions with the sample being milled.
2.2 Structural analysis
Crystal structure of thin films and multilayers have been analyzed by means of X-Ray Di↵raction (XRD) and High Resolution Transmission Electron Microscopy (HRTEM), in particular in the latter case by means of selected area electron di↵raction (SAD).
When coupling and comparing the information obtained by the two techniques, we obtain an accurate analysis of the samples characteristics on the atomic scale and determinate the epitaxial order.
2.2.1 X-Ray Di↵raction
X-ray di↵raction has been a well-established technique in the field of structural investi-gations for decades, applied not only by physicists. It represents an important tool for chemists and biologists, too, and played a decisive role in the discovery of the structure of the DNA in 1953.
X-rays are electromagnetic radiation with typical photon energies in the range of 100 eV - 100 keV. For di↵raction applications, only short wavelength x-rays (hard x-rays) in the range of a few angstroms to 0.1 angstrom (1 keV - 120 keV) are used. Because the wavelength of x-rays is comparable to the size of atoms, they are ideally suited for probing the structural arrangement of atoms and molecules in a wide range of materi-als. The energetic x-rays can penetrate deep into the materials and provide information about the bulk structure. Common targets used in x-ray tubes include Cu and Mo,
2. EXPERIMENTAL TECHNIQUES
which emit 8 keV and 14 keV x-rays with corresponding wavelengths of 1.54 and 0.8 , respectively. (The energy E of a x-ray photon and its wavelength is related by the equa-tion E = hc/l, where h is Planck’s constant and c the speed of light). In recent years synchrotron facilities have become widely used as preferred sources for x-ray di↵raction measurements. Synchrotron radiation is emitted by electrons or positrons traveling at near light speed in a circular storage ring. When x-ray photons collide with electrons, some photons from the incident beam will be deflected away from the direction where they originally travel. If the wavelength of these scattered x-rays did not change (mean-ing that x-ray photons did not lose any energy), the process is called elastic scatter(mean-ing (Thompson Scattering) in that only momentum has been transferred in the scatter-ing process. These are the x-rays that we measure in di↵raction experiments, as the scattered x-rays carry information about the electron distribution in materials. On the other hand, in the inelastic scattering process (Compton Scattering), x-rays transfer some of their energy to the electrons and the scattered x-rays will have di↵erent wave-length than the incident x-rays. Di↵racted waves from di↵erent atoms can interfere with each other and the resultant intensity distribution is strongly modulated by this interaction. If the atoms are arranged in a periodic lattice, as in crystals, the di↵racted waves will consist of sharp interference maxima peaks with the same symmetry as in the distribution of atoms. Measuring the di↵raction pattern therefore allows us to de-duce the distribution of atoms in a material. The peaks in a x-ray di↵raction pattern are directly related to the atomic distances. Let us consider an incident x-ray beam interacting with the atoms arranged in a periodic manner as shown in 2 dimensions in the following illustrations (see figure 2.4). The atoms, represented as green spheres in the graph, can be viewed as forming di↵erent sets of planes in the crystal. For a given set of lattice planes with an inter-plane distance of d, the condition for a di↵raction (peak) to occur can be simply written as
n = 2d ˙sin(✓) (2.1)
which is known as the Bragg’s law, after W.L. Bragg, who first proposed it. In the equation, is the wavelength of the x-ray, ✓ the scattering angle, and n an integer rep-resenting the order of the di↵raction peak. The Bragg’s Law is one of most important laws used for interpreting x-ray di↵raction data. It is important to point out that the law holds true if the atoms are replaced by molecules or collections of molecules, such
2.2 Structural analysis
Figure 2.4: - Bragg Law representation
as colloids, polymers, proteins and virus particles. There are di↵erent XRD techniques that can be applied to the studies of thin film samples grown on substrates. Nowadays, thin films have important technological applications in microelectronic and optoelec-tronic devices, where high quality epitaxial films are critical for device performance.
Thin film di↵raction methods are used as important process development and control tools, as hard x-rays can penetrate through the epitaxial layers and measure the prop-erties of both the film and the substrate. There are several special considerations for using XRD to characterize thin film samples. First, the film substrates are generally enough thick to avoid transmission and allow to define a specific crystallographic direc-tion along which film grows making easier XRD measurement. Second, high angular resolution is required because the peaks from similar thin film materials are very sharp.
Consequently, carefully measures and data analysis have been done. During this work X-ray di↵raction (XRD) in ✓ 2✓ configuration, using a Brucker AXS D8 and Phillips XPert high resolution X-ray di↵ractometers, have been used to characterized samples.
Basic XRD measurements made on thin film samples include:
• Precise lattice constants measurements derived from ✓ 2✓ scans, which pro-vide information about lattice mismatch between the film and the substrate and therefore is indicative of strain and stress
• Rocking curve measurements made by doing a ✓ scan at a fixed 2✓ angle, the width of which is inversely proportionally to the dislocation density in the film and is therefore used as a gauge of the quality of the film.
• Superlattice measurements in multilayered heteroepitaxial structures, which man-ifest as satellite peaks surrounding the main di↵raction peak from the film. Film
2. EXPERIMENTAL TECHNIQUES
thickness and quality can be deduced from the data.
• Glancing incidence x-ray reflectivity measurements, which can determine the thickness, roughness, and density of the film. This technique does not require crystalline film and works even with amorphous materials.
Also structural and residual stress in materials can be determined from precision lattice constants measurements.
2.2.2 Selected Area Electron Di↵raction (SAED)
The traditional method for orientation determination in transmission electron mi-croscopy (TEM) (see section 2.3.1 on TEM technique) is the use of selected area electron di↵raction (SAED). This tool is used like common XRD techniques, but allows also de-termining the orientation relationship between two di↵erent crystalline structures and the epitaxial relationship of a specific region of the sample at the nanoscale. The SAED method selects the di↵racting are of the specimen by placing a diaphragm in the first image plane below the objective lens, so only the part of the image selected by the aperture contributes to the di↵raction pattern. Di↵raction from single-crystal volumes yields characteristic patterns that are composed of a regular arrangement of individual di↵raction spots, as shown in figure 2.5 , which can be evaluated for orientation deter-mination. The di↵raction spots are formed by coherent elastic scatter of the electrons at the crystal lattice. Because of the very short wavelength of the electron radiation,
Figure 2.5: - SAD pattern of ZnO nanorod grown on Si