Chapter 2 Literature Review
2.3 Atmospheric-pressure plasma system
A corona is a process by which a current develops between two high-potential electrodes in air, by ionizing that fluid to create a plasma around one electrode, and by using the ions generated in plasma processes as the charge carriers to the other electrode.
Corona discharge usually involves two asymmetric electrodes, one highly curved such as the tip of a needle or a narrow wire, and another one of low curvature such as a plate or the ground. The high curvature assures a high potential gradient around one electrode, for the generation of the plasma.
Coronas may be positive, or negative. This is calculated by the polarity of the voltage on the high curvature electrode. If the curved electrode is positive associated to the flat electrode, it will have a positive corona, and vice visa. The physics of positive and negative coronas are obviously different. This asymmetry structure is a result of the great difference in mass between electrons and positively charged ions, and so only the electron having the ability to undergo a significant degree of ionizing inelastic collision at common temperatures and pressures.
2.3.2 Dielectric barrier discharge (DBD)
Dielectric barrier discharges involve a specific class of high voltage, ac, gaseous discharges that typically operate in the near atmospheric pressure range. Their defining feature is the presence of dielectric layers that make it impossible for charges generated in the gas to reach the conducting electrode surfaces. With each half cycle of the driving
oscillation, the voltage applied across the gas exceeds that required for breakdown, and the formation of narrow discharge filaments initiates the conduction of electrons toward the more positive electrode. As charge accumulates on the dielectric layer at the end of each filament, the voltage drop across the filament is reduced until it falls below the discharge sustaining level, therefore the discharge is quenched. The low charge mobility on the dielectric not only contributes to this self arresting of filaments but also limits the lateral region over which the gap voltage is diminished, thereby allowing parallel filaments to form in close proximity to one another. Thus, the entire gas filled space between parallel electrodes can become, on average, uniformly covered by transient discharge filaments, each roughly 0.1mm in diameter and lasting only about 10ns.
The DBD’s unique combination of non-equilibrium and quasi-continuous behavior has provided the basis for a broad range of applications and fundamental studies. Its use in industrial ozone reactors has generated interest in optimizing conditions for specific chemical reactions. To this end, experimental DBD studies have explored different gas mixtures, electrical characteristics, and geometries. Related work has focused on maximizing the ultraviolet radiation from excimer molecules produced in DBD’s. Several researchers have modeled single filament dynamics in order to account for the many reactions involving electrons, ions, neutral atoms, and photons. These efforts have been moderately successful in explaining and predicting the chemical and radiative properties of various DBD systems. On another research effort, it has been seen that the transverse spatial distribution of discharge filaments in
2D, parallel plate DBD’s can take the form of stable, large-scale patterns reminiscent of those associated with magnetic domains. These patterns have been modeled with some success using methods that apply generally to pattern formation in nonlinear dynamical systems. Thus, the dynamical interactions between filaments, as well as the chemical and electronic interactions within filaments have proven interesting.
2.3.3 Atmospheric pressure plasma jet
Atmospheric pressure plasma jet is a non-thermal glow discharge plasma where operating at atmospheric pressure. The non-thermal plasma generates highly reactive ions, electrons and free radicals. The reactive species are directed onto a surface where the desired chemistry occurs.
The electrons are quite hot, however the overall gas temperature remains quite cold, typically 50-300oC.
2.3.4 Arc Plasma
A plasma arc operates on principles similar to an arc-welding machine, where an electrical arc is struck between two electrodes. The high energy arc creates high temperatures ranging from 3000oC to 7000
oC. The plasma is highly ionized gas which is enclosed in a chamber.
Waste material is fed into the chamber and the intense heat of the plasma breaks down organic molecules into their elemental atoms. In a carefully controlled process, these atoms recombine into harmless gases such as carbon dioxide. Solids such as glass and metals are melted to form materials, similar to hardened lava, in which toxic metals are encapsulated. With plasma arc technology there is no burning or
incineration and no formation of ash. There are two main types of plasma arc processes: plasma arc melter and plasma torch.
Plasma arc melters have very high destruction efficiency. They are very robust; they can treat any waste with minimal or no pretreatment;
and they produce a stable waste form. The arc melter uses carbon electrodes to strike an arc in a bath of molten slag. The consumable carbon electrodes are continuously inserted into the chamber, eliminating the need to shut down for electrode replacement or maintenance. The high temperatures produced by the arc convert the organic waste into light organics and primary elements.
Combustible gas is cleaned in the off-gas system and oxidized to CO2 and H2O in ceramic bed oxidizers. The potential for air pollution is low due to the use of electrical heating in the absence of free oxygen. The inorganic portion of the waste is retained in a stable, leach-resistant slag.
In plasma torch systems, an arc is struck between a copper electrode and either a bath of molten slag or another electrode of opposite polarity.
As with plasma arc systems, plasma torch systems have very high destruction efficiency; they are very robust; and they can treat any waste or medium with minimal or no pre-treatment. The inorganic portion of the waste is retained in a stable, leach-resistant slag. The air pollution control system is larger than for the plasma arc system, due to the need to stabilize torch gas.
Table 2-1 Comparison of ALD and CVD
ALD CVD
Highly reactive precursors Less reactive precursors Precursors react separately on the
substrate
Precursors react at the same time on the substrate
Precursors must not decompose at process temperature
Precursors can decompose at process temperature Uniformity ensured by the
saturation mechanism
Uniformity requires uniform flux of reactant and temperature Thickness control by counting the
number of reaction cycles
Thickness control by precise process control and monitoring Surplus precursor dosing acceptable Precursor dosing important
Table 2-2 Thin film deposition methods compared Method ALD MBE CVD Sputter Evaporati Industrial applicability good fair good good good poor
Figure 2-1 ZnO crystal structures: (a) cubic rocksalt (b) cubic zinc blende (c) hexagonal wurzite. Shaded gray and black spheres denote Zn and O atoms, respectively [64-66].
Figure 2-2 Schematic illustration of magnetron sputtering [67].
Figure 2-3 Schematic illustration of ARE which is used the hot electron emitter [68].
Figure 2-4 Schematic illustration of ARE which is used the RF discharge [69].
Figure 2-5 Schematic illustration of a Pulsed laser deposition system [70].
Figure 2-6 Description of the deposition process with raising the substrate temperature [71].
Figure 2-7 Schematic illustration of equipment for spray pyrolysis deposition [71].
Figure 2-8 Steps of the dip coating process: dipping of the substrate into the coating solution, wet layer formation by withdrawing the substrate and gelation of the layer by solvent evaporation [72].
Figure 2-9 Schematic illustration of principle for ALD [73].
Figure 2-10 Sequence of events during CVD: (a) diffusion of reactants through boundary layer, (b) absorption of reactants on substrate surface, (c) chemical reaction takes place, (d) desorption of absorbed species, and (e) diffusion out of by-products through boundary layer [74].
Figure 2-11 Development of boundary layer in a horizontal reactor [75].
Figure 2-12 Surface reaction and mass transit limited growth in CVD [74].
Figure 2-13 Schematic the principle of corona discharge [77].
Figure 2-14 Schematic illustration of the DBD configurations [78].
Figure 2-15 Schematic illustration of the AP plasma jet structures [79].
Figure 2-16 Schematic illustration of the arc plasma [80].
Table 2-3 Density of charge species in the plasma discharge [81].
Source Plasma density (cm-3)
Low pressure discharge 108-1013
Arc and plasma torch 1016-1019
Corona discharge 109-1013
Dielectric barrier discharge 1012-1015
Capacitive discharge 1011-1012
Chapter 3 Experiments
3.1 Experimental procedures
3.1.1 Transparent conductive oxide thin films investigation procedures
Figure 3-1 Schematic illustration of TCO thin films investigation.
3.1.2 Thermal stability investigation of TCO thin films procedure
Glass(2)
Standard glass clean
Depositing IZO, GZO and ZnO thin films by APPCVD
Ultrasonic clean 3min
Thermal treatment
Measurement and analysis
Result and discussion Optical properties
Hall measurement SEM analysis AFM analysis XRD analysis
Annealing ambient Annealing temperature
Figure 3-2 Schematic illustration of thermal stability investigation of TCO thin films.
3.1.3 Standard glass clean procedure
Figure 3-3 Schematic illustration of standard glass clean procedure.
Note: (1) The type of glass is FL (normal float) which is not heat-resistant glass.
(2) The type of glass is AN100 which is heat-resistant glass for the following high temperature treatment.
Table 3-1 Composition and characteristics of FL glasses FL glasses
SiO2 [wt%] 70~73
Al2O3 [wt%] 1.7~1.9
RO (CaO + MgO) [wt%] 11.5~13.0
R2O (Na2O + K2O) [wt%] 13.0~14.0 Composition
Fe2O3 [wt%] 0.05~0.15
Softening Point [oC] 720~730
Annealing Point [oC] ≒550
Strain Point [oC] ≒510
Specific Gravity ≒2.5
Coefficient of linear expansion [×10-6/oC (K)] 8.5~9.0
Mohs’ hardness ≒6.5
Young’s Modulus [MPa] ≒7.16×104
Poisson’s Ratio ≒0.23
Refractive index (at 589.3nm) ≒1.52
Reflectivity [%] ≒4
Water resistance [mg] ≦0.5
Transmittance at 5mm [%] ≧88
Table 3-2 Characteristics of AN100 glasses AN100
Density (g/cm3) 2.51
Thermal shrinkage (ppm) 8
Strain point (oC) About 670 Young’s modulus (kg/mm2) 7900
3.2 Experimental equipments and parameters
The AZO, GZO and IZO films are deposited by the APPCVD systems as shown in Figure 3-4. First, we prepare the glasses which the area is 5mm×5mm, then purge it with standard clean process and segment them into the area of 2.5mm×2.5mm. Second, the films precursors are prepared by 1M Zn(NO3)2 and 0.1M A(NO3)2 (“A” represents the Al, Ga and In) which are mixed by atomic percentage. Next, we put them in a bottle with an ultrasonic which its frequency is 2.45MHz and used as the precursors to deposit thin films. Third, we apply nitrogen as the carrier gas and main gas to deliver the precursors into the inner nozzle and convey the reactants to the substrate surface with spiral airstream which is decomposed by arc plasma.
The APPCVD systems have lots of parameters including dopant concentration, substrate temperature, gap distance, scan times, power, carrier gas flow rate, main gas flow rate, different carrier gas kinds and Zn(NO3)2 concentration. All the parameters are shown in Table 3-2.
Among above parameters, we choose the dopant concentration and substrate temperature to be the main topic of our research. Another
parameter will unalter and maintain the optimum condition which is investigated by the anteriorly researchers. The variation of parameters is also shown in Table 3-2 which is described in the parenthesis.
Figure 3-4 Schematic illustration of TCO thin films prepared with APPCVD.
Figure 3-5 APPCVD system of ITRI.
Table 3-3 Parameters of TCO thin films
Parameter Value
Doping concentration (at%) 0, 2, 4, 6, 8, 10, 12 and 20 Substrate temperature (0C) 100, 200 and 300
Gap distance (mm) 5
Scan times 10
Power (Watt) ~625
Carrier gas flow rate (sccm) 30
Main gas flow rate (SLM) 35
Carrier gas nitrogen
Zn(NO3)2 concentration (M) 0.2
Nozzle speed (mm/s) 20
Ultrasonic frequency (MHz) 2.45
In order to investigate the thermal stability of zinc oxide based thin films such as GZO, IZO and ZnO, we put above-mentioned samples in the backend atmosphere anneal furnace at different ambient gases and annealing temperatures to observe the variation and stability of them.
According to the previously experiments, we can conclude the AZO thin film which is deposited by APPCVD systems is very unstable, so the APPCVD systems are unsuitable to deposit AZO thin films. Therefore, we only investigate the GZO and IZO in the thermal stability experiments.
The GZO and IZO thin films are prepared by the above experiment procedure which used the optimum parameter, but the power supply
voltage is changed from 250 volt to 280 volt which is attributed to the reason of the power transformer decayed. And the zinc oxide is also deposited by APPCVD systems with the same conditions to GZO and IZO thin films, all the experiment and variation parameters are shown in Table 3-3.
Table 3-4 Parameters of thermal stability experiments
Parameter GZO IZO ZnO
Doping concentration (at%) 8 8 0
Substrate temperature (0C) 100
Gap distance (mm) 5
Scan times 10
Power (Watt) ~825
Carrier gas flow rate (sccm) 30
Main gas flow rate (SLM) 35
Carrier gas Nitrogen
Zn(NO3)2 concentration (M) 0.2
Nozzle speed (mm/s) 20
Ultrasonic frequency (MHz) 2.45
Annealing gases Nitrogen and Oxygen
Annealing temperatures (0C) 200, 300, 400 and 500
3.3 Characterization analysis equipments 3.3.1 Scanning Electron microscope (SEM)
SEM stands for scanning electron microscope. The SEM is a microscope that uses electrons instead of light to form an image. Since their development in the 1950, scanning electron microscopes have developed new areas of study in the medical and physical science communities. SEM uses a focused beam of high energy electrons to generate a variety of signals at the surface of solid samples. The signals that derive from electron and sample interactions reveal information about the sample including external morphology, chemical composition, and crystalline structure and orientation of materials making up the sample. The SEM has allowed researchers to inspect a much larger variety of samples.
Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron and sample interactions when the incident electrons are decelerated in the solid sample. These signals include secondary electrons, backscattered electrons, diffracted backscattered electrons, photons and heat. Secondary electrons and backscattered electrons are commonly used for imaging samples. Secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples. X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete orbital of atoms in the sample.
As the excited electrons return to lower energy states, they yield X-rays that are of a fixed wavelength. Thus, characteristic X-rays are produced
for each element in a mineral that is excited by the electron beam. SEM analysis is considered to be non-destructive; that is, x-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyze the same materials repeatedly.
The scanning electron microscope has many advantages over traditional microscopes. We describe it following:
(1) It has a large depth of field, which allows more of a specimen to be in focus at one time.
(2) SEM also has much higher resolution, so closely spaced specimens can be magnified at much higher levels.
(3) Because the SEM uses electromagnets rather than lenses, the researcher has much more control in the degree of magnification.
3.3.2 Atomic Force Microscope (AFM)
Atomic force microscopy is a manner of measuring surface morphology on a scale from angstroms to 100 microns. The technique involves imaging a sample through the use of a probe or tip, with a radius of 20 nm. The tip is held several nanometers above the surface using a feedback mechanism that measures surface tip interactions. Variations in tip height are recorded while the tip is scanned repeatedly across the sample, producing a topographic image of the surface.
In addition to basic AFM, the instrument in the Microscopy Suite is capable of producing images in a number of other modes, including tapping, magnetic force, electrical force and pulsed force. In tapping mode, the tip is oscillated above the sample surface, and data may be
collected from interactions with surface morphology, stiffness and adhesion. This result in an expanded number of image contrast methods compared to basic AFM. Magnetic force mode imaging utilizes a magnetic tip to enable the visualization of magnetic domains on the sample. In electrical force mode imaging a charged tip is used to locate and record variations in surface charge. In pulsed force mode, the sample is oscillated beneath the tip, and a series of pseudo force distance curves are generated. This permits the separation of sample topography, stiffness, and adhesion values, producing three independent images, or three individual sets of data, simultaneously.
3.3.3 X-Ray diffraction (XRD)
X-ray diffraction is a very important method to characterize the structure of crystalline material. The technique can typically be used for the lattice parameters analysis of single crystals, or the phase, texture or even stress analysis of polycrystalline materials. The technique is widely used in research and development applications and its use for production or quality control issues is also growing, benefiting from developments in hardware and software for high throughput capability.
Most of the applications of X-ray diffractometry require a beam with well defined spatial and spectral characteristics. X-ray optics is a critical component for obtaining the required beam specifications at the sample.
Multilayer X-ray optics is now widely used in X-ray diffraction due to their balanced performance in terms of divergence, spectral purity, and flux.
3.3.4 Photoluminescence (PL)
Photoluminescence spectroscopy is a contactless, nondestructive method of probing the electronic structure of materials. Light is directed onto a specimen, where it is absorbed and imparts excess energy into the material in a process called photo-excitation. One manner this excess energy can be dissipated by the sample is through the emission of light, or luminescence. In the case of photo-excitation, this luminescence is called photoluminescence. The intensity and spectral content of this photoluminescence is a direct measure of various important material properties.
Photo-excitation causes electrons within the material to move into permissible excited states. When these electrons return to their equilibrium states, the excess energy is released and may include the emission of light or may not. The energy of the emitted light relates to the difference in energy levels between the two electron states involved in the transition between the excited state and the equilibrium state. The quantity of the emitted light is related to the relative contribution of the radiative process.
3.3.5 Four point probe
A four point probe is a common apparatus for measuring the resistivity of semiconductor samples. By passing a current through two outer probes and measuring the voltage through the inner probes allows the measurement of the substrate resistivity.
The sheet resistivity of the top emitter layer is very easy to measure experimentally using a four point probe. A current is passed through the outer probes and induces a voltage in the inner voltage probes.
Using the voltage and current readings from the probe:
simply the voltage reading in mV.
3.3.6 Hall measurement
If an electric current flows through a conductor in a magnetic field, the magnetic field exerts a transverse force on the moving charge carriers which tends to push them to one side of the conductor. This is most evident in a thin flat conductor as illustrated. A buildup of charge at the sides of the conductors will balance this magnetic influence, producing a measurable voltage between the two sides of the conductor. The presence of this measurable transverse voltage is called the Hall Effect after E. H.
Hall who discovered it in 1879.
Note that the direction of the current I in the diagram is that of conventional current, so that the motion of electrons is in the opposite direction. That further confuses all the right-hand rule manipulations you have to go through to get the direction of the forces.
The Hall voltage is given by:
ned
V
H= IB
(Eq. 3-3)Where, n=density of mobile charges; e=electron charge
The Hall Effect can be used to measure magnetic fields with a Hall probe.
The Hall Effect can be used to measure magnetic fields with a Hall probe.