Material analyses

Material analyses are used to understand the correlation between the thin film quality and the electrical characteristics, such as X-ray diffraction, X-ray photoelectron spectrometer, atomic force microscopy, auger electron spectrometer, scanning electron microscopy, and transmission electron microscopy, which are going to be briefly introduced in the following content. The material analyses and the experimental procedures used in the thesis are summarized and shown in Fig. 2-5.

2.6.1 X-ray diffraction

The structure and crystallinity of the films were investigated by x-ray diffraction (XRD), using Siemens D-5000 diffractometer, operated with monochromatic CuKa radiation source (λ=0.1542 nm), step scanning mode with steps of 0.05° with a scan speed of 4s/step. The schematic illustrateion of the XRD is shown in Fig. 2-4. Based on the XRD theory, the average grain size of the each orientation can be calculated by using the Scherrer ‘s formula

Where D is the average grain size of the film, λ is the wavelength of X-rays source (λ=0.1542 nm), θ is the Bragg‘s angle of the XRD peak, and B is the full-width at half-maximum of the peak.

2.6.2 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to determine the HfOx, Gd2O3, and LAO thin film stoichiometry and binding states. The inner electrons are excited and ionized when the light illuminates the sample surface. The ionized electrons near the surface of the matter have the ability to escape into vacuum, resulting in photoelectron. The schematic illustration of the XPS is shown in Fig. 2-5. This phenomenon is named photoelectric effect. Different components and chemical states of the matters have its specified kinetic energy, which can be used to distinguish the composition elements and the chemical binding states of the surface atoms. XPS data were collected using an Al Kα monochromatic x-ray source, 2 mm filament, 350 W power, >1 mm spot size, with electron charge neutralization and 45 ° takeoff angle.

XPS depth profiling of the samples were performed using Ar⁺ sputtering at 3 KV and 3 nA over a 4×4 mm area. The resulting data was processed using the empirically established XPS sensitivity factors provided by the instrument manufacturer to produce atomic concentration.

2.6.3 Auger electron spectrometer

Auger electron spectroscopy (AES) is a popular technique for determining the composition of the top few layers of a surface. When electron beam of energy 2-20 keV is incident upon an atom or solid surface, these electrons cause core electrons from atoms to be ejected resulting in a photoelectron and an atom with a core hole.

The atom then relaxes via electrons with a lower binding energy dropping into the core hole. The energy thus released can be converted to emit an electron. This electron is called an Auger electron, and the relaxation process was discovered by Pierre Auger.

The schematic illustration is shown in Fig. 2-7.

2.6.4 Atomic force microscopy

The surface topography and surface roughness of the resistive switching thin films are examined by the atomic force microscopy (AFM). First, the AFM tip is manually close to the sample surface, and then the scanner makes a final adjustment in tip-sample distance based on setpoint determined by the user. The tip in contact with the sample surface through any adsorbed gas layer, is then scanned across the sample under the action of a piezoelectric actuator, by moving the tip relative to the other. The average roughness and the root mean square are automatically recorded and calculated by the software. The schematic diagram of the AFM instrument is shown in Fig. 2-8.

2.6.5 Scanning electron microscopy

Scanning electron microscopy (SEM) is a useful method for high-resolution imaging of the surface morphology. The SEM generates a beam of incident electrons in an electron column above the sample chamber. The electrons are produced by a thermal emission source, such as a heated tungsten filament, or by a field emission

cathode. The incident electrons cause electrons to be emitted from the sample due to elastic and inelastic scattering events within the sample‘s surface and near-surface material. The emitted electrons with high-energy by an elastic collision of an incident electron are referred to as backscattered electrons, while the emitted electrons with lower energy are resulting from inelastic scattering, which are named secondary electrons. Two electron detector types are typically used to detect the signal for SEM imaging. The schematic illustration is shown in Fig. 2-9.

2.6.6 Transmission electron microscopy

The thin film morphology including size, shape and arrangement of the particles, the thickness, the interface layer, and the crystallographic information, including the arrangement of atoms at the specific sites and detection of atomic -scale defects in a few nanometers order, can be investigated and examined by transmission electron microscopy (TEM). The TEM samples require very thin samples, typically about 100 nanometers, so the samples have to be prepared before the TEM observation. Focus ion beam (FIB) is a scientific instrument to the sample preparation for TEM investigation. Gallium (Ga+) is chosen because it is easy to build a Ga liquid metal ion source to cause the destructive sputtering onto the sample surface at the micro - and nano-scale.

After the samples preparation by FIB, then the TEM observation can be performed. The electron are accelerated from the electron fun and then focused by the condenser lenses on the sample surface. The sample is so thin and transparent enough for the incident electrons to pass through the specimen. By collecting the transmitted scattered electrons, it forms a diffraction pattern in the back focus plane and a magnified bright image in the main screen. The schematic illustration of the TEM instrument is shown in Fig. 2-9.