Physical Characterization Techniques

In this subsection physical characterization was explained in detail.

3.5.1. Measurement of Surface Resistance

Surface resistance is commonly used to define the resistance of thin films with a unit of ohms/square. The working principle in the background of the four-point probe device is illustrated in Figure 3.3.

Figure 3.3. Schematic diagram of the four-point probe method.

The two outer probes are connected to the Keithley model constant current source, the other two inner probe record the drop in voltage to get the potential difference. The current source is programmed to the desired current output and the current in a finite plate indicates a logarithmic increasing potential. The electrostatic analysis of the distribution of the electrical potential and area in the film gives the equation Rs = DV / I.Where D is a constant and its value equals to 4.5324 which varies depending on space and configuration of the contacts. Thus, the surface resistivity of the measured sample is calculated as Rs = 4.5324 V/I. Theory of four-point probe was explained in the next paragraph.

21 For a thin film sample, the thickness is smaller than the spacing between the probe (t<<s) case of the current ring is essential.

ܣ ൌ ʹߨݔݐ (3.1)

R=׬ ߩ_௫ଵ^௫ଶ _ଶగ௫௧^ௗ௫ =׬ ߩ_௦^ଶ௦ _ଶగ௫௧^ௗ௫ = ^ఘ

ଶగ௧ (ln x)| = ^ఘ

ଶగ௧ln2 (3.2) Superposition of current at outer two tips cause to R = V/2I, Therefore, the sheet resistivity of a thin film specimen is

ߩ = ^గ௧

and hence, sheet resistivity takes the simple form,

•ൌͶǤͷ͵ሺȀ) (3.5) 3.5.2. Stylus Method Profilometry

Profilometer actuated with the electromechanical system. Electromechanical system which is in control of mechanical systems with the electronic system does not contain software. Aim of the stylus method is to trace the topography of a film-substrate step. The working principle in the background of the device is shown in Figure 3.4. The stylus is acted with a fixed velocity and force through the layer. There is a tip which is made of diamond on the top of the stylus with a radius of approximately 10 μm. The force of stylus is adjustable from 1 to 30 mg. Vertical deviation of the stylus is measured through the instrument of Linear Variable Differential Transformer (LVDT) sensor. As a consequence, the height profile results obtained. Height of the resulting step-contour trace shows directly film thickness. There are several restrictions about the accuracy of stylus-model such as stretching and penetration of film, substrate roughness and vibration of the equipment. Substrate roughness introduces extravagant noise into measurement, which creates uncertainty in the position of the step (Milton Ohric, 1992).

22 In this thesis, all deposited sample thickness were determined stylus method and using Veeco DEKTAK 150 Profilometer.

Figure 3.4. Illustration of the working principle of a profilometry.

3.5.3. Transmission Measurements

Transmittance-wavelength spectra of optical thin films can be divided into three parts. The desired and required transmission value is obtained in the short wavelength absorption edge which is dependent on the electronic structure of the material and between the long-wavelength limit. The characteristic of the transparent region depends to a large extent on the material itself, in particular, the stoichiometry and impurity of the material causing the absorption. The transparency of thin films is usually slightly less than that of the substrate material and it is largely associated with growth conditions. A slight deviation in stoichiometry or actual absorption due to contamination is the cause of the increase in the damping coefficient of film. Other reasons include surface roughness, inner grain boundary, density fluctuation due to the crystal structure, microstructure, fine holes, cracks and similar reasons (Danielzik et al., 2003). The spectrophotometer is fastest method among interference systems to obtain the optical properties of the films in visible and near IR spectral regions, These films generally have a thickness of 100 nm and their spectra give an advanced pattern of interference.

23 3.5.4. X-ray Diffraction (XRD) Analysis

The purpose of XRD measurement is to identify the crystal structure of hybrid IAI thin film. It can determine the internal strain and crystallite size of material and XRD gives information about the quality of the film. Scherrer’s equation is the estimated average of crystallite size and calculated with the following Equation 3.6,

ࡸ ൌ ^࢑O

ࢼࢉ࢕࢙ࣂ (3.6) λ is the wavelength of X-ray source with a unit of nanometers. Constant which is k related to the crystallite shape which taken as 0.89. β is the full width of half maximum the diffraction peak at half maximum height (FWHM) in unit radians and ߠ refers to Bragg’s diffraction angle which is shown in Figure 3.5 (Monshi et al., 2012).

Figure 3.5. Bragg’s diffraction (Source: Anton Paar Wiki).

Bragg-Brentano diffractometer configuration that on the Philips X’Pert Pro X-ray diffractometer was used for XRD measurements. Bragg’s diffraction law illustrated as the following Equation 3.7,

2dsinT=nλ (3.7) where d is the plane spacing, θ is the diffraction angle, n is an integer and λ is the incident wavelength.

3.5.5. Optical Microscope Analysis

It can be constitutively categorized optical microscopy system under six main heading. Based on their kind of utilization area brightfield optical microscopy, darkfield

24 microscopy, oblique illumination, phase contrast light microscopy, differential interference contrast microscopy, confocal laser scanning have emerged.

3.5.6. Scanning Electron Microscopy (SEM) Analysis

Surface morphologies of hybrid ITO/Ag/ITO layer which was deposited with introducing 40 sccm Ar rate at room temperature with various deposited Ag time were analyzed by SEM. How it works will be explained in a simplistic way referring to Skoog and Leary’s book. There is 3 main part of the scanning electron microscope. The first part is the electron gun. After electron gun part secondly focusing part appears.

Finally, the third part is the sample chamber which is contained in the sample. All three part is kept under ultra-high vacuum (UHV). The energy of a motionless electron outside the metal defines as a vacuum level. In an electron gun, electron beam produced and accelerated. Electromagnetic lenses that are contained in the second part makes to electron gun condensed and focused. Lastly, the specimen surface has been scanned by the electron beam and their signals are collected by the detector. Metallic filament, Wehnelt cylinder, and anode are part of the electron gun. Electron gun source is generally as a tungsten, Lanthanum hexaboride (Lab6) and field emission gun (FEG -ZrO/W). In order to create a free electron, these guns heated nearly up to melting points.

Obtained free electron accelerated towards to anode part. Transportation is supplied by an applied voltage up to a maximum of 50 kV. Wehnelt cylinder which has a negatively charged is blocked the electron which has a small velocity. The only fastest electron can pass from the Wehnelt cylinder. Topological information is obtained as a function of x-y direction scanning. When the electron hits the specimen surface, manx-y interactions can occur. Resulting in inelastic scattering, the secondary electron emerges depth between 0.5 nm to 5 nm. Due to the energy transfer, specimen electrons leave the sample if gained enough energy. Interaction volume depends on the acceleration voltage and the atomic number of the specimen.

3.5.7. Energy Dispersive X-Ray Spectroscopy (EDS) Analysis

EDS measurements which are a tool of qualitative and quantitative examination of the specimen were obtained by BSD detector. The energy of the electron specifies the depth of the region where the X-ray comes from the specimen. Interaction kinds

25 between the electron beam and specimen are shown in Figure 3.6. depend on the incident energy of the beam.

Figure 3.6. Interaction types between the beam electron and the sample surface.