Chapter 1 Introduction
1.3 Motivation
In this essay, we used the APPCVD system to deposit the ZnO based transparent conductive oxides (TCO), including aluminum doped zinc oxide (AZO), gallium doped zinc oxide (GZO) and indium doped zinc oxide (IZO). The three group-III elements are common used to dope the ZnO that could be occupied a substitutional site for Zn2+ cation of higher valency. The optical and electrical properties on Al3+-, Ga3+- and In3+- ZnO films have been widely published with lots of physical and chemical deposition method. However, little information is known about the opto-electrical properties of above TCO thin films which are fabrication in the APPCVD system. So, we interested on the opto-electrical characteristics of the TCO films, and compared them in some of aspect.
Post annealing treatments are frequently conducted to optimize the electrical and optical properties. The thermal budget for opto-electronic devices required serious considerations. As a result, studies on the thermal stability of TCO films are in demand. In modern years, the thermal stability of the indium tin oxide (ITO) film of their synthesized
optoelectronic devices had been discussed. Zinc oxide film is a promising candidate for replacing the commercial ITO film. So, the thermal stability of ZnO is a worthy work to investigate.
Table 1-1 The basic properties of three in common used TCO films [42]
Material In2O3 SnO2 ZnO
Crystal structure bixbyite rutile wurtz Band gap (eV) 3.5 - 4.0 3.8 - 4.0 3.3 - 3.6
Main dopant Sn4+ F1+, Sb5+ Al3+, Ga3+ , In3+
Mobility (cm2/V-s) 103 18 - 31 28 - 120 Carrier concentration
(cm-3)
1.4×1021 2.7×1020 - 1.2×1021 1.1×1020 - 1.5×1021
Resistivity (Ω-cm) 4.4×10-5 7.5×10-5 – 7.5×10-4 1.9×10-4 – 5.1×10-4
Table 1-2 Application of TCO films [42]
Material Application Property
SnO2:F Low radiation building glasses at the frigid zone
Plasma wavelength～3um
Ag, TiN Heat insulating glasses at the torrid zone
Plasma wavelength≦1um
SnO2:F Outer surface of solar cell Thermal stability, low cost ITO, SnO2:F Electrochromic windows Chemical stability, High
transparency, low cost ITO Electrodes of flat panel
display
Easy to etching, low process temperature, low
resistance ITO, ZnO Window layers of light
emitting diode
High transparency, low resistance
ITO, Ag, Ag-Cu alloy
Defogger glasses Low cost, good endurance, low resistance SnO2 Glasses of the toaster Thermal stability at high
temperature, chemical and mechanical endurance,
low cost
ITO, SnO2 Touch panels Low cost, good endurance Ag, ITO Electromagnetism screening Low resistance Ag/ZnO Concealed safety circuits of
the display cabinets
Good endurance, UV protection
Chapter 2
Literature Review
2.1 Properties of ZnO
Most of the group II-VI binary compound semiconductors crystallize in either cubic zinc blende or hexagonal wurtzite structure where each anion is surrounded by four cations at the corners of a tetrahedron, and vice versa. This tetrahedral coordination is typical of sp3 covalent bonding orbital, but these materials also have a substantial ionic character that tends to increase the band gap beyond the one expected from the covalent bonding. ZnO is an II-VI compound semiconductor which ionicity resides at the borderline between the covalent and ionic semiconductors.
The crystal structures composed by ZnO are wurtzite, zinc blende, and rocksalt. Under ambient conditions, the thermodynamically stable phase is the wurtzite symmetry. The zinc blende ZnO structure can be stabilized only by growth on cubic substrates, and the rocksalt structure may be obtained at relatively high pressures, as in the case of GaN.
The wurtzite structure has a hexagonal unit cell with two lattice parameters a and c in the ratio of c/a = (8/3)1/2 =1.633. The structure is composed of two interpenetrating hexagonal close-packed sub-lattice, each of which consists of one type of atom displaced with respect to each other along the threefold c-axis by the amount of u = 3/8 = 0.375. The internal parameter u is defined as the length of the bond parallel to the c-axis divided by the c lattice parameter. The basal plane lattice parameter is always depicted by a; the axial lattice parameter, perpendicular to the basal plane, is described by c. Every sub-lattice
includes four atoms per unit cell, and every atom of group II atom is surrounded by four atoms of the group VI atom, or vice versa, which are coordinated at the edges of a tetrahedron.
As in all solids, the atoms in a semiconductor at nonzero temperature are in unending motion, oscillating about their equilibrium states.
Thermal expansion, specific heat, and pyroelectricity are among the standard material properties that define the linear relationships between mechanical, electrical, and thermal variables. These thermal properties and thermal conductivity depend on the ambient temperature, and the final temperature limit to study these effects is the melting temperature, which is about 1975K for ZnO.
As a direct and large band gap material, ZnO is attracting much attention for a variety of electronic and optoelectronic applications.
Owing to the large energy band gap of ZnO, so it has lots of advantages such as high-temperature and high-power operation, lower noise generation, higher breakdown voltages, and ability to sustain large electric fields. The electron transport in semiconductors can be considered for low and high electric fields. At adequate low electric fields, the energy gated by the electrons from the applied electric field is small compared to the thermal energy of electrons and therefore the energy distribution of electrons is unaffected by such a low electric field.
Because of the scattering rates calculating the electron mobility depend on the electron distribution function, electron mobility remains independent of the applied electric field, and ohmic law is obeyed. When the electric field is increased to a point where the energy gated by electrons from the external field is no longer negligible compared to the
thermal energy of the electron, the electron distribution function changes momentously from its equilibrium value. These electrons become hot electrons characterized by an electron temperature larger than the lattice temperature.
2.2 Deposition methods of TCO films
The methods of deposition TCO films have distinct classification. In this paper, we categorize them in physical and chemical type that representing the source of the TCO films is solid and liquid. Following the exposition will discuss them in detail.
2.2.1 Magnetron sputtering
The sputtering method is working in the glow discharging region, which has higher energy and density of electrons. We put the substrate at anode and set the target at cathode in the Ar ambient, and then the cations which accelerated by the electrical field bombard the target. At this time, the TCO films atoms is leaved out of the target and going to the substrate to form the TCO films. The reason of the cations which is driving to target is the potential of plasma always higher than chamber, target and substrate. Moreover, target connect with cathode will increase the potential difference between plasma and target. If we set a magnet under the target, there were an external magnetic filed to increase the plasma density, so the cations which bombard the target will increase simultaneously. When the low conductivity materials or insulators were to be the deposition substrate, it is difficult to discharge with the DC power. So, we have to use the RF power which the frequency needs to
reach the grade of megahertz (usually 13.56MHz) to be the power supply.
Using the self bias phenomenon in the RF discharge, it can make the target potential will always in negative values to ensure the bombardment will almost continuously which is same capability as the DC discharge.
So far, the sputtering technique is the most common in deposition the TCO films, including electrodes of flat display and energy efficiency windows. The generally properties of sputtering are described following:
(1) Widely scope of the process films such as metal, alloy and insulator.
(2) The films thickness can control by the apply power and process times.
(3) The stable, uniform and large area films can be obtained.
(4) Because of higher bombardment energy, so it can deposit the excellent adhesion and crystallization films.
(5) Long target lifetime, so it can operate at continuous and automatic long time process.
There is another magnetron sputtering method which getting high density plasma by RF-DC couple manner. In general RF magnetron sputtering, the self bias of the target can change with the RF frequency and power, and it controls the ion energy of bombarding to the target.
When the RF frequency increased, the self bias of the target will decreased, the RF power increased, the self bias of the target will also increased. However, in the RF-DC couple magnetron sputtering system, the RF power is mainly to generate high density plasma, and the DC power is to adjust the electric potential of the target, by doing that is easy to control the deposition conditions.
2.2.2 Vacuum evaporation
Vacuum evaporation is a method which evaporating the metal or metal oxide source in the 10-3~10-2Pa pressure, and then deposited on the heating substrate. The ways of evaporating the source material involved resistively heated, electron beam heated and ratio frequency (RF) heated.
Among them, the resistively heated is common used under 15000C, its source is putted in the boat or basket and using the resistance to heat.
Inversely, the regularly used above 15000C is the electron beam heated method which is putted the source materials in the crucible and then heated them by electron beam direct illuminating, electron bean focusing illuminating, electron beam crooked focusing illuminating and electron beam crooked de-focusing illuminating.
In contrast with the magnetron sputtering, the kinetic energy of the deposited film atoms is lower than the sputtering method even at the highest temperature region, so it needs to more substrate temperature. But this property may not a disadvantage surely. For example, when deposition the window layer of LED which is deposited on the finished epitaxial layer, if we use the sputtering method to deposit this film, the high energy particles may be destroyed the epitaxial layer, so the vacuum evaporation method is more convenient in this case.
In order to improve the low reaction ability of the vacuum evaporation method, researchers invent a method which is generating high density plasma between the source and substrate to activate the source material and oxygen called activated reactive evaporation (ARE).
The generation of high density plasma nearby the substrate is putted a RF coil or a tungsten filament by the side of substrate and applying current to
activate it to release hot electrons, letting them to move to the anode side by biasing voltage. Besides, we can enhance the reaction by installing a magnetron coil to hamper the plasma.
The familiar properties of activated reactive evaporation are expounded below:
(1) Extremely high deposition rates, variety of coating compositions.
(2) Precise control of stoichometry.
(3) Better adhesion.
(4) Denser microstructure than direct evaporation.
(5) High substrate temperature.
(6) Addition of an extra electrode, slightly complicated compared to evaporation.
(7) Substrate must generally be rotated for uniform coating.
2.2.3 Pulsed laser deposition
Pulsed laser deposition is a technology where a high energy density focusing pulsed laser beam is struck a target of the material that is to be deposited. The target is vaporized which deposits it as a thin film on substrate. This method can be applied on many materials, so it can deposit lots of thin films. But the growth rate of PLD is extremely slow;
therefore, it is not a mass production technology. This process can occur in ultra high vacuum or in the presence of a ambient gas, such as oxygen which is commonly used when depositing oxides to fully oxygenate the deposited films.
The PLD basic machinery is simple relative to many other deposition techniques, the physical phenomena between laser and target
interaction and the film growth are quite complex. When the laser pulse is absorbed by the target, the absorption energy is converted to electronic excitation and thermal energy resulting in evaporation and plasma formation. The ejected varieties full of the surrounding vacuum including atoms, molecules, electrons, ions, clusters, particulates and molten globules, before depositing on the typically hot substrate.
There are a number of advantages of PLD over other thin film deposition methods, these include:
(1) The largest advantage is that it is versatile. This method can be applied on many thin films including metal, oxides, semiconductors and even polymer. It is unlike MBE and CVD, where different source of precursors are required for each element of the desired compound.
(2) It can be maintained the target composition in the deposited thin films. Because of the very short duration and high energy of the laser pulse, target material immediately toward the substrate, every component of the phase has an analogous deposition rate, so the thin films composition is almost unchanged.
(3) The energy associated with the high ionic content in laser ablation plumes and high particle velocities appear to aid crystal growth and lower the substrate temperature required for epitaxy.
(4) PLD is clean, low cost and capable of producing simply by switching several different targets.
There are also a heaps of advantages of PLD, these include:
(1) PLD brings difficulty to controlling thickness uniformity across the sample, but this problem can be overcome, to some extent, by scanning the laser beam on a larger size target.
(2) The plume of ablated material is highly forward directed, which causes poor conformal step coverage. It also makes thickness monitoring difficult.
(3) There is an intrinsic splashing associated with laser ablation itself, which produces droplets or big particles of the target material on the substrate surface. From an industrial perspective, this is particularly serious as it will result in device failure
2.2.4 Spray pyrolysis
Deposition TCO thin films by the pyrolysis method has been used for a long time. The deposition material can use solid or liquid source, according to our previous statement of the definition of the deposition method, it may be categorized to the physical type, but it is similar to the CVD method, so we still categorizing it in chemical manner.
Spray pyrolysis is the most in common uses in the pyrolysis manners.
In spray pyrolysis, the precursor solution is pulverized as affine mist via a spray nozzle and a carrier gas at high pressure. The so produced mist condenses on a preheated substrate, and is instantly pyrolysed (spray pyrolysis). The process can be conducted in one or more pulses to obtain uniform films. Spray pyrolysis is suitable for substrate with complex geometry, and can be used for a variety of oxide materials. Although the first impression of spray pyrolysis is simple to do, but it concerns at least
seven parameters, including heater temperature, carrier gas flow rate, gap distance, solution drop size, solution component, solution flow rate and substrate velocity through the heater
When the solution drops leave from nozzle to the substrate, occurs different reaction with increasing the substrate temperature. From Figure 2-6, in process A, the solution drop sprinkled on the substrate, vaporizes, then leaves a dry precipitate in which decomposition occurs; in process B, the solvent evaporates before the solution drop arrives at the surface and the precipitate bombards upon the surface where decomposition occurs;
in process C, the solvent vaporizes as the solution drop accesses the substrate, then the solid melts and vaporizes, its vapor diffuses to the substrate to undergo a heterogeneous reaction there; in process D, at the hugest temperatures, the metallic compound vaporizes before it arrives the substrate and the chemical reaction takes place in the vapor phase.
Apparently, we hope not to happen to the process A and D, because it will cause rough and viscosity thin films. So, select the appropriate substrate temperature and make the uniform and equal size of droplet will help the reaction perfectly.
The advantages of spray pyrolysis are summarized below:
(1) The spray pyrolysis can be easy and cheap.
(2) Substrate with complex geometries can be coated.
(3) Leads to uniform and high quality coatings.
(4) Low crystallization temperatures.
(5) Porosity can be easily tailored.
2.2.5 Dip coating
Dip coating is a conventional way of deposition thin films for research purpose. Uniform films can be applied onto planar substrate. For industrial processes, spin coating is used more often. Dip coating is putted the substrate in the deposition solution first, and then pull up the substrate in a regular speed, after that the successful thin film will obtained by drying and annealing. This deposition thin film way is one of the most common used in sol-gel method.
There are a lot of properties of dip coating manner:
(1) It can be deposited on the irregular surface or double-faced substrate.
(2) Few nanometers of thin films can be acquired.
(3) Simple operation, but usually unstable.
(4) Unfit to high viscosity fluid.
(5) The edge of substrate will gathered deposition solution to cause un-uniform films.
The dip coating process can be separated into five procedures:
(1) The substrate is immersed in the solution of the coating material at a constant speed.
(2) The substrate has remained inside the solution for a while and is started to been pulled up.
(3) The thin layer deposits itself on the substrate while it is pulled up.
The withdrawing is carried out at a constant speed to avoid any vibration. The speed determines the thickness of the coating layer.
(4) Excess fluid will drain from the substrate surface.
(5) The solvent evaporates from the fluid, forming the thin film. For volatile solvents, such as alcohols, evaporation starts already during the deposition and drainage steps.
2.2.6 Atomic layer deposition (ALD)
Atomic layer deposition (ALD) also called atomic layer epitaxy (ALE) or chemical beam deposition where is reacted on the thin film surface. The reaction can be categorized to two chemical reactions, A and B. The product of reaction A is the reactant of reaction B, and vice versa.
Therefore, if only introduce the precursor gases each other, deposition reaction will proceeding alternate continually in ABAB… form. ALD is a method of applying thin films such as compounds and elements to various substrates with atomic scale precision. Similar in chemistry to chemical vapor deposition (CVD), except that the ALD reaction breaks the CVD reaction into two half-reactions, keeping the precursor materials separate during the reaction. ALD film growth is self-limited and based on surface reactions, which makes achieving atomic scale deposition control possible. By keeping the precursors separate throughout the coating process, atomic layer thickness control of film grown can be obtained as fine as atomic/molecular scale per monolayer.
The two reactions of ALD is reacted between gas phase molecule precursor and surface functional group, it can simply express below:
Reaction A: A*+PA →FA +B*+VA
( )
↑ (Eq. 2-1) Reaction B: B*+PB → FB + A*+VB( )
↑ (Eq. 2-2)In above expression, * is the functional group, P the precursor, F the composition of thin films and V the volatile molecules. Introduce PA in the chamber will occur reaction A, formation a new layer FA, functional group B* and volatile molecule VA; VA pump out by the vacuum system, and B* is the demand functional group of reaction B. Reaction A will proceeding continually until the substrate surface cover the FA and B*
absolutely. After reaction A, it wills not reacting continually if there are surplus PA, so ALD film growth is self-limited. Due to the characteristics
absolutely. After reaction A, it wills not reacting continually if there are surplus PA, so ALD film growth is self-limited. Due to the characteristics