MSN 307.
01
EXPERIMENTAL TECHNIQUES OF NANOSCIENCE
ZnO Thin Film Synthesis Method, Efficient Parameters and Optimum
Condition
Submitted By Mustafa Berkay Demir
Course Instructor: Zahra Navazi
Date of Submission: 24.12.2023
Yeditepe University Istanbul
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TABLE OF CONTENT
ABSTRACT
ENTRANCE
THE TRANSPARENT CONDUCTIVE OXIDES(TCO)
ZINC OXIDE THIN FILM
• PROPERTIES OF ZINC OXIDE THIN FILMS
• ZnO v ITO: ADVANTAGES OF ZnO
ZnO THIN FILM PRODUCTION METHODS
• CHEMICAL VAPOR DEPOSITION (CVD)
• ATOMIC LAYER DEPOSITION
• SOL-GEL METHOD
• SPUTTERING
EFFICIENT PARAMETERS FOR ZnO THIN FILMS
CONCLUSION
REFERENCE
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ABSTRACT
The deposition, properties, and applications of Zinc Oxide (ZnO) thin films were investigated. ZnO, a
wide bandgap semiconductor, has attracted considerable attention due to its remarkable properties,
including high transparency, excellent electrical conductivity, and versatile applications in
optoelectronics, sensors, and solar This article discusses different deposition techniques used to
produce ZnO thin films, from physical vapor deposition to chemical methods such as sol-gel and
spray pyrolysis. The properties of these thin films are analyzed in detail, highlighting their importance
in the development of next-generation electronic devices. "Various applications of ZnO thin films in
ultraviolet photodetectors, gas sensors, and transparent conductive electrodes are discussed,
highlighting their promising role. Overall, this summary provides a comprehensive overview of the
synthesis, properties, and potential applications of ZnO thin films, highlighting their importance in
modern materials science and technology.
ENTRANCE
The dependence of daily life on the products of the semiconductor industry has led to the tremendous
growth of this industry. Progress requires the development of increasingly smaller devices with greater
speed, flexibility, better performance and lower cost. This demand has led to the development of new
technologies and materials to meet the requirements of the growing semiconductor industry.
Nanotechnology, where products contain very small particles and exhibit special properties, is one of
the newest and most active areas of research. In this context, thin film technology plays an important
role, allowing the deposition of very thin layers (from a few nanometers to the angstrom level) of
semiconductor material on a supporting substrate. The resulting material exhibits new mechanical,
chemical, optical and electrical properties that shrink down to the nanometer scale as a result of
surface and quantum confinement effects. A thin film is defined as a very thin layer of material (10 nm
to 1-2 µm) deposited on a supporting material (substrate) through the controlled condensation of
vapors, ions, or molecules by a physical or chemical process. This technology is called thin film
technology. Thin films are deposited on a wide variety of substrates. Thin films can be divided into
several categories based on material: for example, metallic, dielectric, organic or semiconductor films.
The material can be in monocrystalline, polycrystalline or amorphous forms. The properties of thin
films are completely different from their bulk forms. While bulk materials have fixed properties, the
properties of thin films and devices depend on the quality of the surface rather than the bulk.
Additionally, the properties of thin films can be modulated by various techniques such as doping,
thickness variation or surface treatments. Multilayer thin films can exhibit completely unknown
properties. Thin film technology also enables efficient use of raw materials.
The progressive development of thin film technology has resulted in its widespread use in optics,
electronics, aircraft, defense, space science and other industries. The categories in which thin film
technology finds application are mechanical, chemical, thermal, electrical, magnetic, electronic,
chemical, optical and optoelectronic. The main applications of thin film technology primarily include
optical coatings and semiconductor thin film devices.
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A thin film of material can be deposited from gas, vapor, liquid or solid phase. Various thin film
deposition methods are classified and summarized in Figure 1.
Figure 1.
With advances in nanotechnology and thin film deposition techniques, there has been great interest in
the development of photovoltaic devices, batteries, sensors, information storage, lighting and wide-
area electronics in recent years. Various materials such as silicon, GaN, gallium arsenide, and oxide-
based semiconductors (including ZnO) continue to be of great interest for both fundamental and
applied research. However, research interest in ZnO is greatly increasing due to its excellent optical,
electrical, magnetic, piezoelectric, catalytic and gas sensing properties, which make it particularly
attractive for nanoelectronics, optoelectronics, nanophotonics and piezoelectric devices. Different
nanostructures of ZnO, including nanorods, nanowires, nanotubes, and nanoribbons, can be deposited
on a variety of substrates using conventional thin film deposition methods such as radiofrequency (rf)
sputtering, thermal evaporation, and sol–gel. With the availability of large single-crystal ZnO,
epitaxial films with very few defects can be obtained, thus very high-performance electronic and
optoelectronic devices can be fabricated. The processing temperature of ZnO nanostructures is very
low. Therefore, inexpensive substrates such as glass and plastic can also be used to fabricate ZnO-
based devices. Moreover, the electrical and optical properties of ZnO can be easily tuned by post-
deposition processes such as annealing, surface treatments, and doping with materials such as
aluminum, gallium, indium, tin, and copper. It is an n-type transparent material with a direct band gap
of 3.37 eV, with good electrical conductivity.This article presents several important properties that
make ZnO suitable for electronic and optoelectronic applications. Additionally, research by various
research groups on the applications of ZnO thin films and devices such as LED, biosensors, UV
sensors, photodetectors and TFT is presented.
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THE TRANSPARENT CONDUCTIVE OXIDES(TCO)
As the name suggests, transparent conductive films are mainly produced from optically transparent
and electrically conductive materials called Transparent Conductive Oxide (TCO). TCO films are an
important class of materials due to their applications in optoelectronics and solar cells. A film is a
layered deposit of material whose thickness is equal to a certain wavelength of electromagnetic
radiation.
When used as a window for light to pass into the active material underneath, the TCO film acts as an
ohmic contact to transport the carrier out of the photovoltaic. It also acts as a transparent carrier.
Surface mounting devices used between laminated glass and light translucent composites. Transparent
materials have band gaps with energies corresponding to wavelengths shorter than the visible range,
from 380 nm to 750 nm. Thus, photons with energies below the band gap are not collected by these
Figure 2. Thin film interference caused by ITO coating on the Airbus cockpit
window used for de-icing. ITO films placed on windshields are used to de-ice
aircraft windshields. Heat is generated by applying voltage to the film.
materials and thus visible light passes. But applications such as photovoltaics may require an even
wider bandgap to avoid unwanted absorption of the solar spectrum.
Just as many advances in technology rely on the application of TCO materials, TCOs have been the
target of research in recent years. The extensive studies witnessed are due to its distinctive properties
such as high optical transparency in the visible range, significant electrical conductivity and high
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infrared reflectivity. These properties are what make TCO an important component of modern
optoelectronics. TCOs have high transmittance in the visible wavelength range, which characterizes
them as materials with relatively low light absorption. They also have high electrical conductivity
close to metals and are highly flexible intermediate states that have both of these properties.
Figure 3. İndium Tin Oxide thin film.
Production techniques underlie the performance and economy of thin film components. In order to
maintain the good performance of TCO thin films, deposition processes are often used for fabrication.
Indium doped tin oxide, also called Indium Tin Oxide (ITO), is the most commonly used TCO. This is
a result of its outstanding properties, including strong physical and chemical interaction with adsorbed
species, low operating temperature and strong thermal stability in air (up to 500 ̊ C). These properties
make ITO good candidates for many applications such as optoelectronic devices, infrared reflectors,
anti-reflective coatings, thin-film resistors, flat panel display, solar cells, and organic light-emitting
diode.
The scarcity and high cost of indium in the earth's crust has led to changes such as aluminum doped
zinc oxide (AZO), fluorine doped tin oxide (FTO), undoped zinc oxide, barium stannate and the most
popular zinc oxide. It required the discovery and need for various alternatives.
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ZINC OXIDE THIN FILM
Zinc Oxide (ZnO) thin films are structured layers of ZnO with nanoscale thickness, known for their
diverse applications in various fields. These films are typically produced using techniques such as
Figure 4. AFM images of the ZnO thin lms.
physical vapor deposition (PVD), chemical vapor deposition (CVD), and sol-gel methods. The unique
properties of ZnO, including wide bandgap, high transparency, and excellent semiconductor
properties, make these thin films highly attractive for optoelectronic devices, sensors, and energy-
related applications.
Structural and morphological properties of ZnO thin films play a very important role in determining
their performance. These films generally exhibit a number of properties such as high crystallinity,
tunable conductivity and good optical transparency; this makes them suitable for applications in
transparent electrodes, photovoltaic devices, UV photodetectors, gas sensors and more.
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PROPERTIES OF ZINC OXIDE THIN FILMS
Zinc Oxide (ZnO) thin films exhibit a wide variety of properties that contribute to their usefulness in a
multitude of applications:
• Wide Band Gap: ZnO thin films have a wide band gap (about 3.3 eV), providing transparency to
visible light while responding to UV radiation. This property makes them suitable for
optoelectronic devices and transparent conductive electrodes.
• High Transparency:These films exhibit high transparency in the visible range of the
electromagnetic spectrum; This makes them valuable for transparent electronics, displays and solar
cells.
• Semiconductor Nature:ZnO is a semiconductor with both n-type and p-type doping properties. This
property provides control over its conductivity, which is very important in electronic and
optoelectronic applications.
• Tunable Properties:The properties of ZnO thin films, including electrical conductivity, optical
behavior, and morphology, can be tuned by controlling deposition parameters and post-processing
methods, offering versatility in device design and functionality.
• Good Electron Mobility:ZnO thin films show high electron mobility, making them suitable for
high-speed electronic devices such as transistors and sensors.
• Chemical Stability:ZnO exhibits good chemical stability, making it resistant to oxidation and
degradation in various environments and increasing its durability in different applications.
• Piezoelectric and Photocatalytic Properties: ZnO thin films have piezoelectric properties that find
use in sensors and actuators. Additionally, their photocatalytic structure makes them useful in
environmental applications for the degradation of organic pollutants under UV light.
• Surface Sensitivity:The surface properties of ZnO thin films make them susceptible to gas
adsorption, making them suitable for gas sensing applications.
These properties collectively make ZnO thin films versatile and preferred for a wide range of
applications in electronics, optoelectronics, energy harvesting, sensing, and environmental
remediation. Researchers continue to explore and improve these features to unlock new possibilities
and improve performance in various technological fields.
Figure 5. ZnO thin lms deposited on (a) glass, (b) silicon and (c) kapton substrates.
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ZnO v ITO: ADVANTAGES OF ZnO
Zinc Oxide (ZnO) and Indium Tin Oxide (ITO) both serve as transparent conductive materials but
differ in several aspects, each offering distinct advantages:
I. Abundance and Cost: ZnO is more abundant and affordable compared to Indium, which is a
relatively rare and expensive material. This makes ZnO a more economically viable option for
large-scale production.
II. Mechanical Flexibility: ZnO thin films can be deposited on flexible substrates, allowing their use
in flexible and bendable electronic devices. ITO, on the other hand, is more brittle and less
suitable for flexible applications.
III. Improved Transparency: ZnO exhibits better transparency in the visible spectrum, especially in
the blue and UV regions, compared to ITO. This property is valuable for optoelectronic devices
such as displays and photovoltaics.
IV. Compatibility with Low Temperatures: ZnO thin films can be deposited at lower temperatures
compared to ITO, thus reducing thermal stress on the substrates and expanding the range of
materials that can be used as substrates.
V. Environmentally Friendly: ZnO is considered more environmentally friendly than ITO due to its
abundance in nature and reduced reliance on rare and potentially toxic materials such as indium.
VI. Tunable Properties:Properties of ZnO, such as electrical conductivity, band gap, and morphology,
can be easily tuned by adjusting deposition conditions, offering greater flexibility in optimizing
performance for specific applications.
Figure 6. Schematic representation of ZnO applications.
However, it is important to note that ITO has historically been widely used in the industry and still has
some advantages, including slightly higher conductivity and established manufacturing processes.
Both materials have their unique advantages and limitations, and the choice between ZnO and ITO
often depends on the specific requirements of the application, cost considerations, and desired
performance characteristics.
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ZnO THIN FILM PRODUCTION METHODS
Zinc Oxide (ZnO) thin films can be produced using a variety of deposition methods, each offering
distinct advantages in terms of control over film properties and scalability.
Chemical Vapor Deposition (CVD)
In CVD, precursors containing volatile Zn and
O are prepared. Common precursors for zinc
include diethylzinc (DEZn) and oxygen
sources such as oxygen gas or water vapor. The
substrate, usually glass or silicon wafer, is
meticulously cleaned to obtain a pristine
surface. This step is crucial for the subsequent
growth of uniform and high-quality thin films.
The precursors are placed in a chamber
containing the substrate. When the precursors
come into contact with the heated substrate,
they undergo chemical reactions. For example,
DEZn can react with oxygen sources to form
ZnO and gaseous byproducts. ZnO thin film
deposition occurs when reaction byproducts
accumulate on the substrate, forming a thin
ZnO layer.
Precise control of various parameters is
required for optimum film properties, including
temperature, pressure, precursor flow rates, and
substrate orientation. These factors affect film
thickness, crystallinity, morphology and other
properties.
Annealing or additional treatments may be Figure 7. Schematic representation of CVD
performed following deposition to improve
film quality, remove defects, or change properties.
Advantages of CVD for ZnO Thin Films:
• Uniform Deposition: CVD allows the growth of uniform and large-area ZnO thin films.
• Precise Control: Parameters can be precisely adjusted, providing control over film properties.
• Scalability: CVD is suitable for industrial scale production.
However, challenges such as precursor stability, equipment complexity, and optimization of
deposition conditions for specific applications are issues that need to be considered in CVD
processes for ZnO thin film production.
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ATOMIC LAYER DEPOSITION (ALD)
Zinc Oxide (ZnO) thin film fabrication using Atomic Layer Deposition (ALD) is a precise and
controlled process that involves depositing ZnO layer by layer onto a substrate. ALD is a technique
that allows excellent thickness control, uniformity and consistent coating, making it advantageous
for thin film production.
ALD is based on self-limiting surface
reactions. The substrate is exposed to a
precursor gas (such as diethylzinc) in a reaction
chamber and a single layer of Zn is formed on
the substrate surface. Any excess precursor is
purged or removed from the chamber using an
inert gas, ensuring that only a single layer of
precursor molecules adheres to the
substrate.The substrate is exposed to an
oxygen-containing gas (such as water vapor or
ozone) that reacts with the deposited Zn layer
to form ZnO. This reaction completes the
cycle. Any remaining reactants or byproducts
are cleared from the chamber, preparing it for
the next cycle.The cycle of precursor addition,
cleaning, oxidation and cleaning is repeated
many times to form the desired thickness of the
ZnO film; Each cycle typically adds a layer of
atomic material.
Figure 8. Schematic presentation of ALD.
ALD offers several advantages for ZnO thin
film production:
• Precise Control: ALD provides precise control over film thickness at the atomic level, ensuring
uniformity and repeatability.
• Compatible Coating: It provides excellent conformal coating even on complex and irregular
surfaces, allowing the formation of uniform films on 3D structures.
• Low Temperature Deposition: ALD can be performed at relatively low temperatures, preserving
the integrity of sensitive substrates and allowing compatibility with a wide range of materials.
• High Quality Films: The process produces high quality films with low defect density, making
them suitable for a variety of applications in electronics, optics and sensors.
While ALD offers exceptional control and precision in ZnO thin film fabrication, optimizing
deposition parameters and understanding the specific properties required for a particular application
is crucial to achieving the desired film properties and performance.
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SOL-GEL METHOD
The sol-gel method to produce Zinc Oxide (ZnO) thin films involves a solution-based process in
which precursor materials are converted into a sol (a colloidal suspension of nanoparticles) and then
applied to a substrate to form a thin film.
The process begins with the preparation of a sol containing the ZnO precursor. This usually
involves dissolving a zinc salt, such as zinc acetate or zinc nitrate, in a suitable solvent (usually
alcohol or water) to form a clear solution. Once the precursor solution is prepared, hydrolysis
occurs by adding a hydrolyzing agent (such as water or ammonia). This causes the precursor
molecules to react with water to form metal hydroxides. Through further reactions and controlled
conditions (such as temperature and pH adjustments), the metal hydroxides undergo condensation
reactions and bond together to form a three-dimensional network called a gel. This gel contains
nanoscale ZnO particles dispersed in a liquid medium.
The gel is then applied onto a substrate using a variety of techniques, such as spin coating, dip
coating, or spray deposition. The choice of deposition method affects the thickness and
homogeneity of the resulting ZnO film. Following coating, the coated substrate is subjected to a
drying process to remove excess solvent, leaving a thin film of ZnO precursor. The film is then
annealed at high temperatures (typically around 300-500°C) to promote crystallization and convert
the precursor into a stable ZnO thin film.
Figure 9. Schematic representation of Sol-Gel.
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The sol–gel method offers advantages such as relatively low processing temperatures, the ability to
produce thin films on a variety of substrates (including flexible ones), and the potential to control
film properties by tuning synthesis parameters. However, achieving precise control over film
thickness, uniformity, and crystallinity can be challenging and require optimization of process
parameters.
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SPUTTERING
Sputtering is a widely used method to produce Zinc Oxide (ZnO) thin films.
Sputtering involves ejecting material from a target (in this case, a ZnO target) by bombarding it
with energetic ions. These ejected atoms or molecules then settle on a substrate, forming a thin film.
The process takes place inside a vacuum chamber to create a low-pressure environment. The ZnO
target, typically made of ZnO ceramic or sintered ZnO, is placed in the chamber. The substrate
material on which the thin film will be placed is positioned facing the target. In a vacuum
environment, high-energy ions (typically from an inert gas such as argon) are generated and
accelerated towards the ZnO target. Energetic ions collide with the ZnO target, removing ZnO
atoms/molecules from the target surface. The dislodged ZnO atoms/molecules move in a straight
line and accumulate on the substrate surface, forming a thin film with controlled thickness.
Sputtering process parameters such as gas pressure, applied power, target-substrate distance and
deposition time are carefully controlled to optimize the properties of the film.
The properties of the resulting ZnO thin film, including thickness, crystal structure, morphology,
and electrical/optical properties, can be customized by adjusting the process parameters.
In some cases, post-deposition annealing (heating) may be performed to increase the crystallinity of
the film and improve its properties.
Sputtering offers advantages such as precise control over film thickness and composition,
compatibility with a variety of substrates, and scalability for industrial production.
Figure 10. Schematic representation of sputtering method.
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EFFICIENT PARAMETERS FOR ZnO THIN FILMS
The efficiency and quality of Zinc Oxide (ZnO) thin films depend on various deposition parameters
that affect their structural, optical and electrical properties. Some key parameters and their effective
ranges for producing high-quality ZnO thin films are:
1.Surface Temperature:
• Range: Typically between 100°C and
500°C.
• Effect: Controls the crystallinity, grain
size and orientation of the film.
Higher temperatures may increase
crystallinity but may also affect
substrate integrity.
2. Deposition Rate:
• Speed: Controlled by spray power or
deposition time.
• Effect: Affects film thickness,
u n i f o r m i t y a n d m o r p h o l o g y.
Optimizing this parameter ensures
consistent film quality across the
substrate.
3.Spray Power:
• R a n g e : Va r i e s d e p e n d i n g o n
equipment and installation.
• Effect: Affects film density, adhesion
and crystallinity by affecting the
intensity of ion bombardment on the
target. Higher power can lead to
denser films.
Figure 11. AFM images of ZnO grown on sapphire substrate at
various growth temperatures (Tg)
• 4. Gas Pressure:
• Range: Typically between 1 and 20 mTorr.
• Effect: Controls the mean free path of the sprayed particles and affects the film morphology and
density. Higher pressures can lead to better film integrity.
5. Target-Substrate Distance:
• Range: Usually between 5 and 15 cm.
• Effect: Affects film properties such as thickness uniformity and composition by affecting the
angular distribution and energy of sputtered species.
6. Oxygen Partial Pressure (For Reagent Spraying):
• Range: Controlled by adjusting the ratio of oxygen to inert gas (e.g. argon).
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• Effect: ZnO affects the stoichiometry and crystallinity of the film. Oxygen content affects
electrical and optical properties.
7. Annealing After Deposition:
• Temperature: Generally between 300°C and 600°C.
• Duration: Can vary from minutes to hours.
• Effect: ZnO increases the crystallinity of the film, removes defects and improves its electrical
properties.
It is crucial to optimize these parameters according to the specific requirements of the desired ZnO
thin film application. Balancing these factors allows the production of high-quality ZnO thin films
with desired properties such as good transparency, high conductivity, and morphology suitable for
various applications in electronics, optoelectronics, and sensors. Trials and adjustments within these
ranges are often required to achieve the desired film properties.
CONCLUSION
In conclusion, Zinc Oxide (ZnO) thin film synthesis methods offer a versatile and promising route
for the fabrication of materials with different applications in various fields. The efficiency and
quality of these thin films largely depend on numerous deposition parameters and optimizing
conditions for their production.
Various synthesis methods, including sputtering, chemical vapor deposition, and sol–gel techniques,
provide ways to tailor the structural, optical, and electrical properties of ZnO thin films. Among
these methods, sputtering stands out for its versatility, scalability, and precise control over film
properties.
Efficient parameters vital for obtaining high-quality ZnO thin films include substrate temperature,
deposition rate, sputtering power, gas pressure, target-layer distance, oxygen partial pressure (in
reactive sputtering) and post-deposition annealing conditions. Balancing and optimizing these
parameters within their effective range is fundamental in controlling film properties such as
crystallinity, morphology, electrical conductivity and optical transparency.
The search for optimum conditions in ZnO thin film synthesis requires a delicate balance between
various parameters to achieve the desired film properties suitable for specific applications. This
pursuit often requires systematic experimentation and fine-tuning to achieve ideal conditions that
provide superior film quality, uniformity, and functionality.
Overall, the synthesis of ZnO thin films combined with efficient parameter optimization holds
significant promise for emerging technologies in optoelectronics, transparent electrodes, sensors,
and solar cells. Ongoing research and development in this field aims to unlock the full potential of
ZnO thin films and expand their applications in various industries, promoting innovation in
materials science and technology.
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