Revista de Ciencias Tecnológicas (RECIT). Volumen 3 (1): 10-22
Revista de Ciencias Tecnológicas (RECIT). Universidad Autónoma de Baja California ISSN 2594-1925
Volumen 7 (4): e375. Octubre-Diciembre, 2024. https://doi.org/10.37636/recit.v7n4e375
1 ISSN: 2594-1925
Research article
Optical and Methanol Sensing Properties of Al-doped ZnO Thin
Film
Propiedades de detección óptica y de metanol de una película delgada de
ZnO dopada con Al
Sumitra Pandey1* , Samundra Marasini1, Rishi Ram Ghimire2
1Goldengate International College, Tribhuvan University, Kathmandu, Nepal
2Department of Physics, Patan Multiple Campus, Tribhuvan University, Patandhoka, Lalitpur, Nepal
*Corresponding author: Sumitra Pandey, Goldengate International College, Tribhuvan University, Kathmandu, Nepal. E-
mail: sumitrapandey12@gmail.com. ORCID: 0009-0008-9627-7595.
Received: September 19, 2024 Accepted: November 19, 2024 Published: November 20, 2024
Abstract.- The study investigates the optical and electrical properties of undoped and aluminum (Al)-doped zinc oxide (ZnO)
films, focusing on their performance as gas sensors and their potential applications. Optical analysis, conducted using UV-visible
spectrophotometry, reveals that 1% Al-doped ZnO films exhibit the highest transmittance of 91%, indicating superior optical
clarity and suitability for applications like solar cell electrodes. In contrast, 3% Al-doped ZnO films show significantly lower
transmittance due to increased light scattering and photon absorption. The bandgap of ZnO films decreases with higher Al doping
concentrations, from 3.3 eV for undoped ZnO to 3.15 eV for 3% Al-doped ZnO, suggesting enhanced electrical conductivity due
to reduced bandgap. The extinction coefficient data demonstrate that 2% Al-doped ZnO has the highest extinction coefficient,
reflecting improved light absorption and scattering properties. Electrical characterization through I-V curves indicates that 1%
Al-doped ZnO films have higher current (121 µA) compared to undoped (431 µA) and higher doping concentrations, attributed
to enhanced carrier concentration and mobility. Sensitivity tests show that 2.5% Al-doped ZnO films exhibit the highest sensitivity
to methanol vapor, with a significant reduction in resistance compared to 0.5% Al-doped ZnO films. Resistance measurements
with varying methanol volumes reveal a rapid decrease upon gas introduction, stabilizing and then increasing as the gas is
removed. Sensitivity analysis indicates that 100 µL methanol provides the highest sensitivity (97%) at 60°C, while 2% Al-doped
ZnO films show consistent sensitivity at 60 °C and 100 °C, but not at 80 °C.
Keywords: ZnO films; Aluminum doping; Optical transmittance; Bandgap reduction; Electrical conductivity; Gas sensor
sensitivity.
Resumen. - El estudio investiga las propiedades ópticas y eléctricas de películas de óxido de zinc (ZnO) dopadas con aluminio
(Al) y sin dopar, centrándose en su rendimiento como sensores de gas y sus posibles aplicaciones. El análisis óptico, realizado
mediante espectrofotometría UV-visible, revela que las películas de ZnO dopadas con Al al 1 % exhiben la transmitancia más
alta del 91 %, lo que indica una claridad óptica superior y su idoneidad para aplicaciones como electrodos de células solares.
Por el contrario, las películas de ZnO dopadas con Al al 3 % muestran una transmitancia significativamente menor debido al
aumento de la dispersión de la luz y la absorción de fotones. La brecha de banda de las películas de ZnO disminuye con mayores
concentraciones de dopaje de Al, de 3,3 eV para ZnO sin dopar a 3,15 eV para ZnO dopado con Al al 3 %, lo que sugiere una
conductividad eléctrica mejorada debido a la brecha de banda reducida. Los datos del coeficiente de extinción demuestran que
el ZnO dopado con Al al 2 % tiene el coeficiente de extinción más alto, lo que refleja propiedades mejoradas de absorción y
dispersión de la luz. La caracterización eléctrica a través de curvas I-V indica que las películas de ZnO dopadas con Al al 1 %
tienen una corriente más alta (121 µA) en comparación con las no dopadas (431 µA) y mayores concentraciones de dopaje,
atribuidas a una mayor concentración y movilidad de portadores. Las pruebas de sensibilidad muestran que las películas de
ZnO dopadas con Al al 2,5 % exhiben la mayor sensibilidad al vapor de metanol, con una reducción significativa en la resistencia
en comparación con las películas de ZnO dopadas con Al al 0,5 %. Las mediciones de resistencia con volúmenes de metanol
variables revelan una rápida disminución al introducir el gas, estabilizándose y luego aumentando a medida que se elimina el
gas. El análisis de sensibilidad indica que 100 µL de metanol proporciona la mayor sensibilidad (97 %) a 60 °C, mientras que
las películas de ZnO dopadas con Al al 2 % muestran una sensibilidad constante a 60 °C y 100 °C, pero no a 80 °C.
Palabras clave: Películas de ZnO; Dopaje de aluminio; Transmitancia óptica; Reducción de la banda prohibida; Conductividad
eléctrica; Sensibilidad del sensor de gas.
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1. Introduction
Zinc oxide (ZnO) is a widely used
semiconducting material known for its ease of
synthesis, cost-effectiveness, non-toxicity,
transparency, and high electron mobility of 2000
cm²/(Vs) at 80 K. It typically crystallizes in the
wurtzite phase and has a direct bandgap of 3.37
eV, with a high exciton binding energy of 60
meV, allowing for efficient excitonic emission at
room temperature [1]. ZnO's various
nanostructures, including films, nanowires,
nanorods, and nanoparticles, are suitable for
applications in sensors, detectors, and thin-film
transistors, with nanostructured thin films
particularly valuable for studying electrical,
thermal, and optical properties [2]. The material's
carrier transport behavior is influenced by light
and other sensing materials, making it attractive
for solar cells, luminescent devices, electrical
and acoustic devices, and chemical sensors [3].
Al-doped ZnO thin films exhibit excellent optical
and electrical properties, such as high electron
mobility, uniformity, and transparency to visible
light, positioning them as promising materials for
next-generation flat panel displays [4]. These
films also possess a broad sensing spectrum
(200-300 nm), making them suitable for UV light
applications, including solar UV radiation
monitoring and ultra-high temperature flame
detection, as well as potential use in transparent
conducting oxide (TCO) electrodes and light-
emitting diodes (LEDs) [5]. The material's
optical absorption is linked to electron transitions
from the valence band to the conduction band and
defect levels, enhancing conductivity through the
desorption of surface oxygen by photogenerated
holes.
Additionally, Zhu et al. [6] developed a novel gas
sensor with high response and selectivity using
molecularly imprinted powders (MIPs). The
sensor demonstrated excellent gas-sensing
properties to methanol vapor, particularly with a
methanol-to-methyl acrylic acid molar ratio of
1:4. At an optimal operating temperature of
130°C, the sensor showed a response of 41 to 1
ppm methanol, with response and recovery times
of 40 seconds and 50 seconds, respectively.
Mingzhi Jiao's research found that ZnO
nanowires, synthesized at 90°C with low
precursor concentration, show better nitrogen
dioxide selectivity compared to other gases, with
higher stability at 600°C [7]. Alaa's study on ZnO
and Al-doped ZnO thin films revealed that
increased Al doping reduced lattice parameters
and bandgap energy [8]. Anandh discovered that
Al doping alters ZnO thin films' structural and
optical properties, increasing the bandgap up to
3% doping before it decreases [9] (Anandh et al.,
2018). Aydın et al. [10] noted that Al doping in
ZnO thin films enhances their suitability for
ammonia gas detection. Kathwate's work
demonstrated that Al doping decreases the
bandgap of ZnO films and improves ammonia
gas sensing [11]. Dubey found that higher Al
doping in ZnO thin films enhances humidity
sensor sensitivity [12]. Khojier showed that Al-
doped ZnO thin films optimize formaldehyde
sensitivity at 2 % Al [13]. Finally, Gulec reported
that 20% Al-doped ZnO films exhibit superior
photocatalytic performance post-annealing,
despite their unique p-type characteristics [14].
The motivation for this research stems from the
increasing demand for efficient, cost-effective,
and environmentally friendly gas sensors,
particularly for detecting hazardous gases like
methanol vapor. Methanol is widely used in
various industries but poses significant health
and environmental risks due to its toxicity,
making its detection critical. Al-doped ZnO thin
films are promising materials for gas sensing
applications due to their superior optical and
electrical properties, including high
transparency, tunable bandgap, and enhanced
carrier mobility. Previous studies have
demonstrated that Al doping improves ZnO’s
sensitivity and selectivity for various gases, such
as ammonia and formaldehyde, by altering its
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bandgap and structural properties. However,
limited research exists on the optimization of Al-
doped ZnO thin films specifically for methanol
vapor detection. The purpose of this research is
to investigate the optical properties of Al doped
ZnO thin film and methanol sensing properties of
Al-doped ZnO thin films with varying Al
concentrations, focusing on their performance as
methanol vapor sensors. This study's significance
lies in its potential to contribute to the
development of highly sensitive, low-cost, and
reliable methanol sensors for industrial safety,
environmental monitoring, and other
applications requiring accurate detection of toxic
gases.
2. Materials and methods
2.1. Synthesis of aluminum doped zinc oxide
films
Aluminum-doped zinc oxide (Al-doped ZnO)
films are highly sought after due to their
multifunctional properties, including
piezoelectric, electrical, optical, and thermal
characteristics. These films are particularly
useful in applications such as gas sensors,
ultrasonic oscillators, and transparent electrodes
in solar cells. The effectiveness of Al-doped ZnO
films largely depends on their microstructure and
surface nanochemistry. Aluminum doping results
in a reduced material density and smaller grain
size, enhancing the electrical conductivity,
magnetic performance, and optical transparency
of ZnO. Extrinsic doping with elements like
aluminum, indium, gallium, copper, or cadmium
is a common method to improve these properties,
as it can induce defects in the ZnO lattice and
widen the bandgap. Al-doped ZnO is especially
valuable in the fabrication of optoelectronic
devices, heterojunctions, superlattices, and
detectors. The sol-gel method, known for its low
cost and ease of composition control, is
frequently used to synthesize these films,
offering benefits such as precise size control, low
processing temperatures, and the production of
cost-effective semiconducting materials.
2.2. Substrates Cleaning
Before depositing the films, the glass slides were
cleaned with diluted HCl, boiled in acetone, and
then dried completely using a dryer and cotton.
Once fully dry, the slides were prepared for
coating.
2.3 . Preparation of ZnO thin films
To prepare the undoped ZnO precursor solution,
2.74 grams of zinc acetate (ZnC4H6O4) were
dissolved in 25 ml of propanol. This mixture was
then stirred at 60°C for 30 minutes using a
magnetic stirrer, resulting in a curdy mixture.
Diethylamine (C4H11N) was then added drop by
drop while continuing to stir until a clear, water-
like solution was formed. The prepared solution
was stored in a cool, dark place for 24 hours
before use. The spin coating technique was
employed to deposit thin films, as thin as 10 nm,
onto flat substrates. This method involves
placing a liquid on a rotating substrate, with the
material deposited at the center either manually
or robotically. The uniformity of the film
depends on the balance between centrifugal and
viscous forces, which are influenced by the spin
speed, solution viscosity, and spinning time.
Typically used for transparent oxide thin films,
this method ensures the production of uniform
and repeatable films. To achieve consistent
deposition, the spin speed and time (usually 30-
45 seconds) were kept constant. For the
preparation of Al-doped ZnO, 3.0178 grams of
aluminum tetrachloride (AlCl3) were mixed with
25 ml of the previously prepared ZnO solution to
achieve a 0.5M concentration. Five different Al-
doped ZnO samples with varying concentrations
(0.5%, 1%, 1.5%, 2%, and 2.5%) were prepared
using the spin coating technique. During this
process, the slides were placed in the spinner, and
the solution of zinc acetate and propanol was
added dropwise (2 drops per cycle). After each
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coating, the slides were heated to 300°C. This
process was repeated for 10 to 15 coats which is equivalent to 0.001 mm to 0.009 mm thickness
the film's resistance fell within the range.
Figure 1. Flowchart for preparation of ZnO film.
2.4. Fabrication of methanol gas sensor and
gas sensing setup
A cubical aluminum box with each side
measuring 10 mm was constructed, featuring
three air-tight holes sealed with rubber corks.
These holes served different purposes: one for
the methanol gas inlet, another for the outlet, and
the third for the wire connections. The ZnO
sample was placed on the upper side of the box,
and a uniform heat source was applied
consistently. The experiment involved varying
the internal temperature of the box across three
levels. Initially, the temperature was set to 60°C,
and varying volumes of methanol (100µl, 200µl,
300µl, and 400µl) were introduced using a
micropipette. The procedure was repeated at
80°C and 100°C, with the same methanol
volumes being tested at each temperature. The
experimental setup for gas sensing is depicted in
Figure 2.
Figure 2. Experimental setup for gas sensing.
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The heat inside the box facilitated the
vaporization of the methanol. A rheostat was
used to adjust the resistance, and the voltage
supply was managed to maintain the desired
temperature. Together, these components
allowed precise control over the temperature
within the box. The setup featured three holes on
the ceiling of the box, all sealed with rubber
corks. One cork served as the inlet for the gas,
introduced via a syringe, while another cork was
left open as the outlet. The third hole
accommodated the connections for the
multimeter, rheostat, and voltage supply.
Additionally, a heater was positioned at the
bottom of the box, with the ZnO film placed on
the ceiling.
2.5. Mechanism of Conductivity
The mechanism of conductivity in Al-doped ZnO
thin films is primarily influenced by the
incorporation of aluminum ions into the ZnO
crystal lattice, which alters its electrical
properties. ZnO is an intrinsic n-type
semiconductor, primarily due to oxygen
vacancies and zinc interstitials that provide free
electrons. When aluminum is doped into ZnO,
the Al³⁺ ions, having a smaller ionic radius and
higher valency than Zn²⁺ ions, replace some Zn²⁺
in the lattice. This substitution creates extra free
electrons because Al³⁺ donates one more electron
than Zn²⁺, thereby enhancing the electron carrier
concentration, which leads to an increase in
electrical conductivity. Al-doped ZnO
composites exhibit improved charge storage
behavior due to their enhanced carrier mobility
and increased electron density. The higher
concentration of free electrons in Al-doped ZnO
enables greater polarization under an external
electric field, which enhances the dielectric
properties and thus the material's ability to store
charge. This is especially relevant in dynamic
electrical applications, such as capacitors and
sensors, where the charge storage capacity plays
a crucial role.
2.6. Characterization of ZnO films
UV-Visible spectroscopy is used to analyze
materials by measuring their absorbance,
reflectance, or transmittance of light in the UV to
visible range, where electronic transitions occur
in molecules. According to the Beer-Lambert
law, absorbance is directly proportional to both
the concentration of the absorbing species and
the path length. This method is useful for
determining concentration and calculating the
optical band gap of thin films [15].
(1)
A represents the measured absorbance, I0 is the
intensity of the incident light, I is the transmitted
intensity, L denotes the path length through the
sample, and c is the concentration of the
absorbing species. Additionally, the band gap of
a thin film can be calculated using UV-Visible
spectroscopy:
(2)
The optical band gap (Egap) of a semiconductor
can be determined using UV-Visible
spectroscopy. Here, hν represents the incident
photon energy, B is a constant, and m is 0.5 for
direct and 1 for indirect band gaps. The
absorption coefficient (α(ν)) is calculated using
Beer-Lambert's law: α(ν)= (2.303Abs(λ))/d,
where Abs(λ) is the absorbance and d is the film
thickness. When light interacts with a
semiconductor, it can be absorbed, transmitted,
or reflected. The energy required for electron
transitions between the valence and conduction
bands varies with different materials. In direct
optical absorption, the conduction and valence
band edges align at the same wave vector (k),
allowing electrons to transition directly from the
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valence band to the conduction band without a
change in momentum when hν>Eg [16-17].
3. Result and discussion
3.1. Optical properties
Transmittance, which measures the fraction of
incident light passing through a substance, was
analyzed for ZnO films using a UV-visible
spectrophotometer. This analysis compared the
optical transmittance of undoped ZnO films with
those doped with varying concentrations of
aluminum. The thickness of the films was kept
constant (0.001 mm) to accurately determine the
absorption coefficient and evaluate the optical
properties. The transmittance of 1% Al-doped
ZnO films was found to be 91%, indicating a high
level of optical clarity on figure 3a. This high
transmittance suggests that the 1% Al doping
enhances the crystalline quality of the films,
making them suitable for applications such as
solar cell electrodes. The transmittance spectra
for these films displayed interference fringes,
which are indicative of high surface quality.
These fringes result from reflections at the film
surface with minimal absorption and scattering in
the bulk, further confirming the excellent quality
of the 1% Al-doped ZnO films [18].
Figure 3. (a) Transmittance with 1% Al doped for TM(Tmax) and Tm (Tmin), and (b) transmittance of undoped, 1%, 2%, 3% Al
doped ZnO.
In contrast, the 3% Al-doped ZnO films
exhibited a much lower transmittance of 11%,
which increased to 60% with varying methanol
volumes. This significant reduction in
transmittance is attributed to the increased
doping concentration, which introduces higher
charge density and consequently enhances
photon absorption. The presence of defect sites
from higher doping levels leads to increased light
scattering, which degrades the optical
performance of the films as shown in figure 3b.
This scattering effect is evident from the lower
transmittance values observed for the 3% Al-
doped ZnO films [18]. The transmittance of 2%
Al-doped ZnO films was recorded at 74%. This
value is intermediate between the 1% and 3%
doping levels, reflecting a balance between
improved crystalline quality and the introduction
of defect sites. The decrease in transmittance
compared to the 1% doping level indicates that
while some benefits of doping are retained,
further increasing the concentration introduces
more defects, leading to increased light scattering
and reduced transmittance [19]. Undoped ZnO
films exhibited a transmittance of 62%, which is
lower than that of 1% Al-doped ZnO but higher
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than that of 3% Al-doped ZnO shown in figure 3.
This suggests that while undoped ZnO films have
relatively lower optical clarity compared to the
1% Al-doped films, they perform better than the
higher doping concentrations in terms of light
transmission.
Figure 4a illustrates the variation in the direct
bandgap of ZnO films with increasing Al doping
concentration. The bandgap of undoped ZnO is
measured at 3.3 eV. For 1% Al-doped ZnO, the
bandgap decreases to 3.25 eV. Further increasing
the Al concentration to 2% and 3% results in
bandgaps of 3.2 eV and 3.15 eV, respectively.
This decrease in bandgap with higher doping
concentrations can be attributed to the
interactions between the dopant and the host
material. The reduction in bandgap with
increased Al doping is explained by the
incorporation of Al3+ ions, which introduce
additional electronic states within the bandgap.
Specifically, the 3d levels of the Al3+ ions
interact with the sp-electrons of the ZnO lattice,
resulting in strong sp-d exchange interactions.
This interaction modifies the electronic structure
of ZnO, leading to a narrowing of the bandgap
[20]. As shown in Figure 4b, the bandgap
reduction is linearly related to the Al
concentration, indicating a consistent impact of
doping on the electronic properties of the ZnO
films.
Figure 4: (a) Comparison bandgaps of undoped, 1%, 2%, and 3% Al-doped ZnO and (b) Band gap vs doping concentration
A narrower bandgap generally enhances
electrical conductivity, as it reduces the energy
required for electron excitation from the valence
band to the conduction band. Therefore, the
observed decrease in bandgap with increased Al
doping suggests that the electrical conductivity
of the ZnO films is likely to improve with higher
doping concentrations. The improved
conductivity is beneficial for applications
requiring efficient charge transport, such as in
transparent electrodes and optoelectronic
devices.
Figure 5 presents the extinction coefficient for
ZnO films with varying Al doping
concentrations. Notably, the 2% Al-doped ZnO
sample exhibits a higher extinction coefficient
compared to the 1% Al-doped ZnO sample. The
extinction coefficient is a measure of how much
light is absorbed and scattered by a material,
influencing its dielectric loss. A higher extinction
coefficient indicates greater light absorption and
scattering. The extinction coefficient trends
observed in the samples correlate with their
doping levels. For the 2% Al-doped ZnO, the
extinction coefficient initially decreases and then
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increases with wavelength. This behavior
suggests that the optical properties of the film are
significantly influenced by the doping
concentration. Conversely, undoped, 1%, and 3%
Al-doped ZnO films show a similar extinction
coefficient pattern, indicating a more stable
relationship between wavelength and extinction
coefficient within these samples.
Figure 5. Extinction coefficient of undoped, 1% Al, 2% Al, 3% Al-doped ZnO vs wavelength.
The general trend for all samples shows that the
extinction coefficient decreases with increasing
wavelength. This decrease is attributed to the
scattering of light, which reduces absorbance. As
the wavelength of light increases, the absorption
decreases, leading to a lower extinction
coefficient. This trend is consistent across the
undoped, 1%, and 3% Al-doped ZnO films. The
increase in the extinction coefficient for higher
Al doping levels, specifically for the 2% Al-
doped ZnO, is due to reduced light scattering and
increased absorbance. With more Al doping, the
ZnO film's microstructure becomes more
conducive to absorbing light, leading to higher
absorbance and, consequently, a higher
extinction coefficient. This improvement in
absorbance is linked to the enhanced electronic
interactions introduced by the Al dopants, which
modify the optical properties of the ZnO film.
Figures 6a and 6b illustrate the real and
imaginary parts of the dielectric functions for
ZnO films with varying Al doping
concentrations. The dielectric functions provide
crucial insights into the optical properties of the
films, with the real part related to
the refractive index (n) and the imaginary part
( ) associated with the extinction
coefficient (k). The real part of the dielectric
function is generally higher than the
imaginary part () across all ZnO samples. This
suggests that the films exhibit good transparency,
as a higher is indicative of a higher refractive
index, which typically corresponds to reduced
light absorption. Among the samples, the 1% Al-
doped ZnO films show a higher compared to
the 2% Al-doped ZnO films, indicating superior
transparency and optical quality. The real part's
graph, however, exhibits increased noise at
higher doping concentrations, complicating
accurate analysis and interpretation. The
imaginary part of the reflects the material’s
absorption characteristics. For the ZnO films,
is generally lower than , signifying relatively
low absorption. Specifically, the 1% Al-doped
ZnO films have a higher compared to the 2%
Al-doped ZnO films, supporting the notion that
1% Al-doped films exhibit better transparency
and reduced absorption. The data for shows
consistent behavior across different Al
concentrations, which aids in understanding the
films' absorption properties. Analysis and
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Implications: The analysis of dielectric functions
indicates that the 1% Al-doped ZnO films have
better transparency and lower absorption
compared to films with higher Al doping
concentrations. This suggests that a lower doping
concentration, such as 1%, is advantageous for
applications requiring high optical clarity, such
as transparent electrodes in solar cells. The
observed noise in the real part of the dielectric
function for higher doping levels reflects
challenges in measurement accuracy, likely due
to increased light scattering and other optical
effects. Sensitivity and Application: The
dielectric property findings are crucial for
optimizing ZnO films in gas sensing
applications. The lower absorption and higher
transparency of 1% Al-doped ZnO films could
offer a balance between optical performance and
sensor sensitivity. Conversely, higher doping
levels might alter the sensor response due to
increased absorption. Thus, tailoring the doping
concentration is essential for achieving the
desired balance in various applications.
Figure 6. (a) Real part of dielectric of 1% and 2% Al-doped ZnO vs wavelength, and (b) Imaginary part dielectric of vs
wavelength
3.2. Electrical properties
The gas sensing mechanism in Al-doped ZnO
thin films is primarily based on surface sensing,
where the interaction between the target gas
(methanol vapor) and the film's surface leads to
changes in electrical properties. When exposed to
air, oxygen molecules adsorb onto the ZnO
surface, trapping free electrons from the
conduction band and forming negatively charged
oxygen species (O₂⁻, O⁻). This creates a depletion
layer near the surface, reducing the film's
conductivity.
When methanol vapor is introduced, it reacts
with these oxygen species, releasing the trapped
electrons back into the conduction band, thereby
reducing the depletion layer and increasing
conductivity. The extent of this change in
conductivity depends on the concentration of
methanol vapor and the efficiency of electron
transfer. Al-doping enhances this surface
interaction by increasing the electron density,
which boosts the sensor's sensitivity, allowing it
to detect lower concentrations of methanol with
faster response and recovery times. This surface
reaction mechanism is central to the gas sensing
properties of Al-doped ZnO thin films.
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The I-V curve illustrates the relationship between
the applied voltage and the resulting current in
ZnO films. As depicted in Figure 7a, there is a
clear linear relationship where an increase in
voltage corresponds to an increase in current.
This behavior is typical of conductive materials,
where the current is directly proportional to the
applied voltage, following Ohm's Law. However,
the magnitude of the current varies depending on
the doping concentration. The undoped ZnO film
exhibits a high current of 431 µA at elevated
voltages. This high current is attributed to the
specific dimensions of the sample, which can
affect the current measurement. Undoped ZnO
generally has lower resistivity due to the absence
of doping-induced defects that would otherwise
impede charge carrier movement. In contrast, the
2.5% Al-doped ZnO film shows a significantly
lower current of 17 µA. This reduction is likely
due to the increased defect density and potential
scattering of charge carriers at higher doping
concentrations, which impedes carrier mobility.
The current increases from 69 µA at 0.5% Al
doping to 121 µA at 1% Al doping. This increase
is due to the enhanced carrier concentration
resulting from Al doping. Aluminum introduces
extra electrons into the ZnO lattice, which
increases electrical conductivity by providing
additional charge carriers. Carrier Concentration:
The rise in current with moderate Al doping
(0.5% to 1%) indicates improved carrier
concentration and enhanced electrical
conductivity. As Al concentration increases,
more electrons are introduced into the ZnO
lattice, enhancing carrier mobility and
conductivity. At higher doping levels (e.g., 2.5%
Al), the observed decrease in current can be
attributed to increased defect density. Excessive
doping creates a higher number of scattering
centers and traps, which can hinder carrier
mobility and reduce the overall current. This
phenomenon reflects a common trade-off where
initial increases in carrier concentration improve
conductivity, but excessive doping can lead to
diminished performance due to defect-related
scattering and carrier recombination [22].
Figure 7. (a) Current vs Voltage and (b) Comparison of resistance of undoped, 0.5%Al, 1%Al, 1.5%Al, 2%Al, and 2.5%Al
doped ZnO vs time at 100l of serum at 60 oC
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Figure 7b shows the resistance of ZnO films with
varying Al concentrations over time. The
resistance initially decreases when gas is
introduced, stabilizes for approximately 130
seconds, and then gradually returns to its original
value within 100 seconds. Upon gas introduction,
resistance drops, indicating a quick reaction to
the gas exposure. This decrease is likely due to
the interaction between the gas and the ZnO film,
which alters the carrier concentration or surface
properties temporarily. After the initial drop, the
resistance stabilizes and eventually increases
back to the original value. This behavior suggests
that the gas exposure causes a transient change in
carrier dynamics or surface chemistry, which
later returns to equilibrium. The resistance of
ZnO films decreases with higher Al doping
concentrations. This is because high doping
concentrations lead to increased defect density
by trapping or scattering electrons which lead to
higher resistance over time [23].
Figure 8 shows that sensitivity to methanol vapor
increases with Al doping concentration in ZnO
films, with 2.5% Al-doped ZnO exhibiting the
highest sensitivity and 0.5% Al-doped ZnO the
lowest. Higher doping concentrations introduce
more free electrons into the ZnO lattice, which
enhances the film's response to methanol vapor.
The presence of methanol, a reducing agent,
decreases the resistance of the film by reducing
oxygen species at the surface and grain
boundaries, leading to increased sensitivity.
Specifically, the 2.5% Al-doped ZnO film shows
a significant reduction in resistance due to a
higher concentration of free electrons and more
active reaction sites, whereas the 0.5% Al-doped
ZnO film, with fewer free electrons and reaction
sites, demonstrates lower sensitivity. Thus,
optimizing Al doping levels improves the
sensitivity of ZnO films for gas detection
applications. Also, Al-doped ZnO thin films
offer high sensitivity, fast response, and recovery
times, making them ideal for developing
efficient, low-cost sensors for real-time methanol
vapor monitoring. This aligns with the need for
accurate, portable sensors in industrial and
environmental applications, ensuring safe
handling and compliance with regulations.
Figure 8. Comparisons of sensitivity of undoped, 0.5% Al, 1.5% Al, 2% Al, 2.5% Al-doped ZnO vs Time at 100 l and 60 oC.