Title


Science and Technology Indonesia
e-ISSN:2580-4391 p-ISSN:2580-4405
Vol. 7, No. 3, July 2022

Research Paper

Study of the Electrical, Optical and Morphological Properties in Submicron and
Microstructured ZnO Thin Films Obtained by Spin Coating and Chemical Bath Deposition
C. López-Esmerio1, C. Ruiz-Rojas2, J. Angulo-Rocha3, E. Lizárraga-Medina4, F. Ramos-Brito5*, E. Camarillo-García6, R.
Martinez-Martinez7, M. Aguilar-Frutis8, M. García-Hipólito9
1Master student at Posgrado en Ciencias Físicas, Instituto de Física, Universidad Nacional Autónoma de México, Ciudad de México, 01000, México2PhD student at Posgrado en Tecnología Avanzada at CICATA-Legaria, Instituto Politécnico Nacional, Ciudad de México, 11500, México3Hospital de la Mujer, Secretaría de Salud Sinaloa, Culiacán, 80020, México4Master student at Posgrado en Tecnología Avanzada at CICATA-Legaria, Instituto Politécnico Nacional, Ciudad de México, 11500, México5Laboratorio de Síntesis de Materiales, Facultad de Ciencias Físico Matemáticas, Universidad Autónoma de Sinaloa, Ciudad Universitaria, Culiacán, 80000, México6Instituto de Física, Universidad Nacional Autónoma de México, Ciudad de México, 01000, México7Universidad Tecnológica de la Mixteca, Oaxaca, 69000, México8CICATA-Legaria, Instituto Politécnico Nacional, Ciudad de México, 11500, México9Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Ciudad de México, 04150, México
*Corresponding author: framosbrito@uas.edu.mx, ramosbritof@gmail.com

AbstractIn the present work the synthesis of ZnO semiconductor thin films was performed successively using spin coating and chemicalbath deposition techniques. The deposition was made by varying the concentration of zinc acetate and hexamethylenetetramine(HMTA: ZnAc) in the precursor solution. This process led to two preferred growth directions (002) and (101), both with very similartexture coefficients, too; a noticeable change in morphology of structured surface, variation in unit cell parameters and crystallinegrain size. All the films turned out homogeneously submicro and microstructured and with a wurtzite-type hexagonal crystallinestructure. Using pre-loaded Mathematica 11.3 software functions and an algorithm developed in it, the micrographies were analyzedto calculate the percentage of substrate-covered area which was always greater than 80%. Likewise, it also found that resistivitydecreases at a higher percentage of covered area and that the variation in the shape of the photo luminescent emission spectrumcan be considered as a qualitative indication of the concentration of charge carriers.
KeywordsZnO Semiconductor, Nanostructured Films, Texturized Films, n-Type Semiconductor

Received: 19 February 2022, Accepted: 31 May 2022
https://doi.org/10.26554/sti.2022.7.3.291-302

1. INTRODUCTION

Over the past decade, semiconductor research has grown con-
siderably. This is due to the potential applications they may
have, as well as the fact that nowadays we have better infras-
tructure at hand, such as better labs and supercomputers. The
eld of application of semiconductor materials is very wide and
varied, they are present in products as common as sunscreen
but also in the most sophisticated electronic devices, without
exception of those that are now considered essential by most
human beings to carry out their daily lives, such as: smart-
phones, televisions, tablets, and computers, which are now the
trend for the semiconductor industry (Yeap, 2013). Likewise,
they also made a dierence in the lucratively growing area of
lighting devices, this, since the introduction of LED in the 60’s
and its constant improvement since then (Gayral, 2017; Weis-

buch, 2018). There are other areas where the introduction of
semiconductor materials has played an important role for their
development, such as: food, health, energy, and environment
(Weisbuch, 2018; Nanda et al., 2017; Karthikeyan et al., 2020;
Zhu and Zhou, 2019); a clear example for this is the use of
semiconductors in the production of solar cells that can convert
solar energy into electricity, or the treatment of sewage waters
through photocatalysis using semiconductor materials.

The formerly-mentioned great success of semiconductors
is due to the large amount of knowledge generated on them
as a result of the investigations carried out on their dierent
properties: morphological, optical, electrical, chemical, etc.,
only in this nearly-concluding decade, it has been reported,
according to the Central Library’s online database from Univer-
sidad Nacional Autónoma de México (bc.unam.mx), 977 591

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López-Esmerio et. al. Science and Technology Indonesia, 7 (2022) 291-302

scientic publications that mention the word semiconductor
and are estimated to be around 2,076,660 for the next decade.
The notable development reported by the dierent areas when
theyincorporate semiconductors in its processes has led to hard
work in the semiconductor research from dierent areas: both
experimental, numerical calculations with supercomputers and
theory, all in the same direction, getting better semiconductors
with certain properties that help to solve dierent needs.

Zinc oxide is one of the semiconductors that have recently
caught the attention of multiple research groups, in the rst
decadeof thiscentury,accordingtothedatabaseof thebc.unam.
mx, 413 scientic publications were reported for “zinc oxide
semiconductor” and in this near-to-end decade, it has been
reported in the order of 1420, estimating the order of 5500
publications for the next decade.

ZnO is considered an attractive and potentially promising
semiconductor for a wide variety of applications, this is because
it has an energy gap of 3.37 eV, exciton link energy 60 meV to
300◦K (Özgür et al., 2005a), in addition to being transparent
in the visible region of the wavelength spectrum.

Among the possible applications for ZnO there are: ultra-
violet (UV) lasers (ZKTang et al., 1998), light emitting diodes
(LED) (Park et al., 2010), PN junction devices (Bian et al.,
2004), transparent thin lm transistors (Carcia et al., 2006),
gas sensors (Chou et al., 2006), and dental cements considering
that its biosecurity has been proven (Nguyen et al., 2019). Fur-
thermore, 10 years ago it was revealed that ZnO nanoparticles
have the potential to selectively kill tumor cells, performing
studies with dierent tumor entities (Hanley et al., 2008; Van-
debriel and De Jong, 2012; Pandurangan et al., 2016; Wahab
et al., 2016; Chandrasekaran and Pandurangan, 2016; Gupta
et al., 2015; Kc et al., 2016; Hassan et al., 2017), although
the exact mechanism of Cytotoxicity is still subject to debate
(Vandebriel and De Jong, 2012).

Various synthesis techniques have been used to grow ZnO
lms (Lu et al., 2007; Kim et al., 2000; Lu et al., 2000; Zhang
et al., 2010; Shinde et al., 2010; Muiva et al., 2011) but it is
important to highlight those that oer simplicity, high growth
rate, homogeneity and production at a low cost. Among these,
some of the most common are Spin Coating and Chemical
Bath Deposition. There are works that report the growth of
microandnanoZnObarsonbuersdepositedbySpinCoating
using Chemical Bath Deposition (Chen and Ting, 2016; Wu
et al., 2010; Rana et al., 2017; Suhaimi and Yuwono, 2019;
Chang et al., 2017), that is, using a process that involves these
two techniques. The virtue of employing this process is that
it allows growing micro/nano ZnO bars aligned and oriented
(Chang et al., 2017). The buer layer deposition provides
nucleation centers that reduce interface energy between the
substrate and the reagents and allows the vertically aligned
bars to form at the substrate surface due to heterogeneous
nucleation. The morphology, the degree of alignment, the
density, crystalline quality and optical properties of micro and
nano bars are directly related to the parameters of the buer
(Chang et al., 2017). Likewise, the study on the eect that this

buer has on the properties of the micro and nano bars has
concluded that the diameter of these increases linearly with the
crystalline grain size of the buer (Chang et al., 2017), that the
length of these is proportional to the time in the chemical bath
as t1.5 and that the area density decreases with the roughness
of the buer layer (Chen and Ting, 2016).

There are very signicant advances in potential applica-
tions for ZnO, specically in those that imply that it has an
optimal semiconductor property, two examples of this are: 1)
as a transparent conductive oxide, there are works where the
properties of commercial ITO, and ZnO thin lms doped with
indium (I) gallium (Ga) and aluminum (Al) are compared,
having very similar resistivities, of the order of 10−4 Ω.cm,
with ZnO presenting higher mobility (Gonçalves et al., 2011)
and 2) in LED’s (Rahman, 2019), ZnO is presented as the
naturally most viable semiconductor to replace GaN in diode
manufacturing for short wavelength light emitters due to the
ZnO energy gap and its exciton link energy that it is more
than twice that of GaN; however, much remains to be done.
The research reported in this direction shows that a way to
deal with the vicissitudes that have been presented, is through
systematic studies on the synthesis of ZnO as a function of the
dierent synthesis parameters, this through synthesis processes
that allow: reproduce the ZnO obtained, such as Spin Coating
and Chemical Bath Deposition, as well as induce changes in
its electrical properties based on its other properties such as:
morphology, crystal structure, texture and optical properties,
among others. This in order to study the former as a function
of the latter and fully understand the dierent processes that
give it its semiconductor property and the correlation between
them. Surely this will allow, in the future, to develop a syn-
thesis process that is attractive and scalable at the industrial
level to manufacture ZnO in line with specic semiconductor
characteristics that allow its incorporation in the market and
the partial or total displacement of the material to replace.

This work presents an investigation that deals with the syn-
thesis and systematic characterization of ZnO micro / nano
structured semiconductor lms by deposition by Chemical
Bath Deposition on glass substrates previously coated with a
layer ZnO buer by Spin Coating. For this work, a systematic
synthesis of microstructured ZnO was achieved through a pro-
cess employing spin coating for the deposition of a buer layer
over glass substrates, followed by Chemical Bath Deposition
to grow a microstructured thin lm of ZnO over the buer.
The objective for this investigation work is to obtain lms with
varying morphology and texture, in addition to seeing how
those variations can aect optical and electrical properties for
ZnO. Among the results obtained there is the fact that: the
synthesis process used favors two preferred texture directions,
(002) and (101) depending on the synthesis conditions and
without noticeable changes between the coecients of texture
both for address (002) and address (101); the concentration of
charge carriers was found to be higher for the textured lms in
the address (101) than for those in address (002); the higher
the percentage of substrate-covered area, the lower the resistiv-

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López-Esmerio et. al. Science and Technology Indonesia, 7 (2022) 291-302

ity, regardless of the morphology and texture; the dierence
between photoluminescent emissions associated with donor
defects (Zni0 y Zni) and VZn acceptors can be considered as a
parameter indicative of the charge carrier concentration; the
greater the dierence, the greatest the concentration of n-type
charge carriers. These results encourage the use of high resolu-
tionphotoluminescencespectroscopytoquantitativelymeasure
the dierence between type “n” and “p” charge carriers in ZnO.
This could place photoluminescence spectroscopy as an optical
method for characterizations that until this day are done only
via electrical techniques.

2. EXPERIMENTAL DETAILS

2.1 Synthesis
Thin lms of ZnO were synthesized on glass substrates, the
techniques used in the synthesis were spin coatingand chemical
bath deposition, by means of the rst one the lm that served
as a buer layer to later grow the second one on it. For buer
lms the precursor solution was prepared with zinc acetate
dehydrate ((CH3COO)2Zn.2H2O) from the brand J.T. Baker
USA and methanol with a purity of 99.8% as solvent, with a
concentration of 0.06 M (200 mL of methanol and 2.63 g of
zinc acetate). The spin coating process was done with a KW-4A
spin-coater model from Chemat Technology, the equipment
was congured with two turning stages, the rst at 800 RPM for
9 s and the second at 2500 RPM for 30 s. For each buer lm,
was used a droplet of 100 `L, then, after spinning the glass was
placed on an IKA brand grill HS-7 model for 10 minutes at
80◦C.

The synthesis of ZnO thin lms was achieved by Chemical
Bath Deposition, using the same zinc precursor as above men-
tioned and adding deionized water with a resistivity of 18 MΩ
as solvent, in addition, an organic compound that acts as a com-
plexingagentwasadded, hexamethylenetetramine ((CH2)6N4).
For the precursor solution, the zinc acetate solution was dis-
solved in deionized water at 0.024 M, this solution was divided
into 7 beakers to which hexamethylenetetramine (HMTA) was
added. To achieve homogeneity in the solution this was placed
for5 minutes in an ultrasonic bath in a from the brand Branson,
model 1800. The samples were then placed in the chemical
bath at a temperature of 78◦C for 12 hours. The parameter
that was varied was the relative concentration between zinc
acetate and HMTA (HMTA: ZnAc) in the precursor solution,
the ratios were: 0.0625: 1, 0.125: 1, 0.1875: 1, 0.3125: 1,
0.375: 1, 0.54: 1, and 0.625: 1, which from here on will be
named C1, C2, C3, C4, C5, C6, and C7, respectively.

2.2 Characterization
The diraction patterns of all ZnO lms, Figure 1, were ob-
tained with a diractometer from the brand BRUKE model
D8 ADVANCE. The crystallite size (D) strain (Y), stress (𝜎)
and energy density (u) values were calculated by using Scherrer
formula and the modied Williamson-Hall method (W-H)
(Kalita and Kalita, 2015), the results are shown in Table 1. The
texture coecient was obtained at from the intensities of (100),

(002), and (101) peaks and considering the following equation
(Wang et al., 2015):

Figure 1. a) Results of X-Ray Diraction (XRD) Analysis for
All Samples Showing Their Miller Indexes Corresponding to
Wurtzite Type Crystalline Structure; b) Enlarged XRD Pat-
tern for The Zone of The (100), (002), and (101) Planes, The
Inserts are an Enlargement of The Diraction Peaks Corres-
ponding to The Planes (002) and (101) that Show The Varia-
tion of The Unit-Cell Dimensions

TChkl =
Ihkl
I0hkl

{
1
N

∑︁
N

Ihkl
I0hkl

}−1
(1)

where TC(hkl) is the texture coecient of the planes hkl,
I(hkl) is the measured intensity of the peak, I0(hkl) is the in-
tensity of the planes hkl in a sample that shows no growth or
preferential orientation and N is the number of diractions
considered in the analysis. The micrographs of the samples
were obtained using the Atomic Force and Scanning Electron
Microscopies (AFM and SEM), using the Thermo Microscopes
VeecomodelAutoprobeCPandJEOLmicroscopemodel JSM-
6390LV, respectively. The electrical properties of the ZnO
lms were obtained with a kit from the brand ECOPIA, model
HMS-3000 that measures Hall voltage. Photoluminescence
spectra of the samples was obtained with an Edinburg FLS980
spectrometer. Theexcitationwavelengthofchoicewas325nm,
corresponding to a local maximum for the excitation spectrum
of sample C7, which resulted to appear in all samples. The

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López-Esmerio et. al. Science and Technology Indonesia, 7 (2022) 291-302

Table 1. D, Y, 𝜎 and u Values for All Samples Extracted from Plots in Figure 2 That Correspond to Scherrer and W-H Analysis
of XRD Patterns of The Samples

Sample

Scherrer method W-H method
(100), (002),
(101) peaks

All peaks
UDM USDM UDEDM

D D Y D Y 𝜎 D Y 𝜎 u
(nm) (nm) 10−3 (nm) 10−3 (Mpa) (nm) 10−3 (Mpa) (Kjm−3)

C1 29.73 112.72 59.74 1.15 59.11 1.06 133.25 60.85 1.11 139.43 77.51
C2 34.25 126.05 63.87 1.10 61.10 0.96 120.50 63.79 1.03 129.44 66.80
C3 27.73 254.31 66.96 1.63 62.15 1.45 182.54 65.82 1.55 193.84 149.80
C4 33.85 93.68 54.35 0.76 54.24 0.67 84.39 55.48 0.72 89.83 32.17
C5 32.20 114.59 56.80 1.02 54.03 0.86 107.39 56.57 0.93 117.40 54.95
C6 28.09 171.70 53.31 1.35 53.24 1.26 157.50 54.65 1.31 163.83 107.28
C7 34.31 117.50 60.00 0.91 57.47 0.77 96.69 59.84 0.84 105.20 44.13

transmittance of the samples was obtained using the UV-VIS-
NIR Cary 5000 spectrophotometer. The percentage of the
covered areaof the substrate was calculated using the procedure
shownin theelectronic supplementarymaterial, which includes
preloaded functions of Mathemática 11.3 and an algorithm
developed in it. The dimensions of the individual elements that
made up the microstructure of the lms were obtained by ana-
lyzing the SEM images using the "imageJ" software (Rasband,
2012).

3. RESULTS AND DISCUSSION

3.1 Crystalline Structure
Figure 1a shows the diractograms of all samples, all of them
present the peaks corresponding to the planes (100), (002),
(101), (102), (110), (103), and (112) corresponding to the
wurtzite type crystalline structure of ZnO. The gure shows
that, except samples C4 and C6 that presented a preferential
growth of the planes (101) and texturization of the surface in
<101> direction, all the others presented a preferential growth
of the planes (002) and texturization of the surface in direction
<001>. The texture coecients in the direction <001> for sam-
ples C1, C2, C3, C5, and C7 were: 2.7947, 2.8696, 2.9338,
2.7768, and 2.7794, respectively; while texture coecients
in direction <101> for samples C4 and C6 were 1.7857 and
1.9303, respectively.

Figure 1b presents the diractograms for all samples, for a
2\ interval of (28.0, 37.8)◦, which corresponds to the region
of diraction peaks associated with crystalline planes (100),
(002) and (101) of ZnO. With the intention of pointing out
the existing dierences between all diraction peaks of a same
crystallographic plane for each sampes, an enlargement of re-
spective zones was made for the diractogram; these enlarge-
ments can be found in the inserts of Figure 1b. Insert 1 shows
a small shift of the (002) peak towards the right in comparison
to the position of that same peak for sample C2, this indicates
a dierence between interplanar distances for the samples with
a prefered growth in the <001> direction, being C2 and C3 the

highest and lowest values, respectively. Insert 2 shows that the
peaks corresponding to planes (101) are all centered arround
the same value, which would mean that the interplanar spacing
did not varyforsamples with apreferential growth in the <101>
direction.

Taking the Bragg angle values of diraction peaks, the inter-
planar distances “d” of that planes were calculated through the
equation known as the Bragg’s law (Cullity and Stock, 2001):

n_ = 2dsin \ (2)

Where n is the order of diraction, _ the wavelength of the
x-rays and \ the diraction peak in question.

Once the interplanardistances were calculated, the unit-cell
parameters were calculated, this by means of the equation that
relates “d”, the Miller indices of the planes (hkl) and the “a”
and “c” unit-cell parameters for a hexagonal structure (Cullity
and Stock, 2001):

1

d2
=
4
3
(h2 + hk + k2)

a2
+
l2

c2
(3)

The volume V of the unit-cell was obtained knowing that V=
0.866a2c (Wang et al., 2015). Table 2 shows the results ob-
tained from V, a, c for all samples. The cell volume reached
vary from 47.4344 to 47.6035 Å3 and the unit cell parameter
"c" from 5.1886 to 5.1999 Å.

The corresponding W-H plots (Figure 2) showed that
broadening of the diraction peaks was essentially isotropic.
This indicates that diracting domains were isotropic and there
was also a microstrain contribution. Table 1 shows D, Y, 𝜎, and
u values for all samples. There are two columns for D value by
Scherrer equation, for the rst column was considering only
(100), (002) and (101) diraction peaks and for the second one
all diraction peaks were considered. There is a remarkable
dierence between values of D obtained in these two dier-
ent ways, which drives to consider the lattice strain factor for

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López-Esmerio et. al. Science and Technology Indonesia, 7 (2022) 291-302

Figure 2. . Estimation of Crystallite Size (D), Lattice Strain (Σ),
Stress (𝜎) and Energy Density (u) by Scherrer and W-H Met-
hods. (a) Scherrer Method to Calculate D of All Samples Con-
sidering All Peaks in Their Corresponding Diractions Pat-
terns. Plot of cos \ vs 1/𝛽. (b) Uniform Deformation Model
of W-H Method. (c) Uniform Stress Deformation Model of
W-H Method. (d) Uniform Deformation Energy Density
Model of W-H

Figure 3. Atomic Force Microscopy Results for All Samples.
Each Sample was Analyzed at Two Dierent Resolutions in
Order to Show Microstructure Details. a, b) Sample C1; c, d)
Sample C2; e, f) Sample C3; g, h) Sample C4; i, j) Sample
C5; k, l) Sample C6; and m, n) Sample C7

Table 2. Results Obtained from a, c and V for All The Samples

Sample
a c V (Å3)
(Å) (Å) = 0.866 a2c

0.0625:1 (C1) 3.2528 5.1905 47.5600
0.1250:1 (C2) 3.2513 5.1999 47.6022
0.1875:1 (C3) 3.2510 5.1886 47.4900
0.3125:1 (C4) 3.2497 5.1999 47.5554
0.3750:1 (C5) 3.2500 5.1943 47.5129
0.5400:1 (C6) 3.2468 5.1958 47.4331
0.6250:1 (C7) 3.2507 5.1999 47.5846

D calculation. Table 1 shows the values of D obtained from
UDM, USDM, and UDEDM, which are approximately similar,
indicating that the inclusion of strain in various forms of W-H
method has a very small eect on the average crystallite size of
sub-microstructured ZnO lms. The crystallite size resulted
from W-H method was in a size of around 58.73 nm, which
is in agreement with microstructure results obtained by AFM
for all samples, for example: AFM images in Figures 3b and 3f
show the microstructure of samples C1 and C3, respectively.
However, the dierence between D values obtained from the
Scherrer formula and W-H methods is large, showing that is
essential to estimate D by considering both crystallite size and
lattice strain contributions in the widening of the diraction
peaks. This large dierence was associated with the great con-
tribution of heterogeneous microstrains in the widening of the
diraction peaks (Zhang et al., 2020). These microstrains are
due to dislocations in grain boundaries regions (Zhang et al.,
2020). Zhang et al. (2020) conclude in their work that assum-
ing grain boundary structures in nanocrystalline and coarse
grain materials are very similar, then the dislocation density
in the grain boundary (GB) regions is almost like a material
constant depending mostly on the misorientation and structure
of GBs. They proposed a model that shows, on one hand, that
when the grain size is smaller than about 20 nm and the grain
interior regions become more-or-less free from dislocations
the average dislocation density in the crystal can still be substan-
tially large. At these small grain size values, the volume fraction
of GBs becomes signicant and the dislocation density in GBs
becomes dominant in the entire crystal. On the other hand,
even in coarse grain polycrystals, the GB regions do consist of
substantial dislocation densities. Other authors whose research
is about nanoparticles (Bindu and Thomas, 2014; Solati and
Dorranian, 2017) and nanolms (Yamada et al., 2007), they
have reported only small dierences between results obtained
from Scherrer formula and W-H method attributing this to
the dierence in averaging the particle size distribution (Bindu
and Thomas, 2014; Solati and Dorranian, 2017; Yamada et al.,
2007).

Figure 4 presents the "c" parameter and volume of the unit-
cell plotted as a function of the relative concentration of HMTA
in the precursor solution. It is appreciated that apparently there

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López-Esmerio et. al. Science and Technology Indonesia, 7 (2022) 291-302

is no correlation between these physical characteristics of the
sample and the HMTA concentration.

Figure 4. Parameters and Volume of The Unit-Cell Plotted as
a Function of The Relative Concentration of HMTA in The
Precursor Solution

3.2 Microstructure
Figures 3 and 5 show the AFM and SEM micrographs of all
the samples, respectively. The congruence between the images
obtained for each of the samples by both techniques, AFM,
and SEM, denote the good quality of the deposit in terms
of homogeneity along the entire surface of the substrate. All
samples resulted in microstructured lms. The C1, C2, C3,
C4, and C5 lms were made up entirely of ZnO bars that
were moderately oriented perpendicular to the substrate and
with apparently hexagonal cross section, while lms C6 and
C7 were made up of corn ake-like structures oriented almost
perpendicular to the substrate plane.

At rst glance it can be seen that the samples C1, C2 y
C4 show empty spaces between the bars, that is to say, spaces
where the deposition of ZnO has relatively poor, the Table 3
presents the results obtained for the percentage of substrate-
covered area, in addition to: the diameter average of the cross
section of the bars for samples C1, C2, and C5, the edge of
the hexagonal cross section for sample C4, the thickness of the
sheets in the samples C6 and C7 and the texture coecient.
The results for the diameter show a dierence of up to an order
of magnitude between the thinnest bars of the C2 sample and
the thickest of the C3 sample. All ZnO bars show a nodular
type growth and nanostructured hexagonal cross section with
an average roughness of the order of crystallite grain size, i.e.
for sample C1 the average roughness obtained by AFM was
24.9 nm that is consistent with its grain crystalline size of
26,975 nm calculated by Scherrer formula for the planes (002),
as was shown in Table 1, which seems to indicate that the

Figure 5. Scanning Electron Microscopy Results for All Samp-
les. Each Sample was Analyzed at Two Dierent Resolutions
in Order Two Show Microstructure Details. a, b) Sample C2;
c, d) Sample C3; e, f) Sample C4; g, h) Sample C5; i, j) Sam-
ple C6; and k, l) Sample C7

nanostructures of the hexagonal upper face are single crystals
of ZnO.

Table 3 shows the values of roughness considering an area
of 10×10 `m2. These values turned out to be: 63.1, 33.0,
50.0, 46.4, 47.8, 66.2, and 35.4 nm, for samples C1, C2, C3,
..., C7, respectively. All the samples presented Gauss-type
histograms for the distribution of the measured heights, practi-
cally symmetrical, with no apparent bias, except for sample C1,
which showed little bias pronounced towards heights of lesser
value. This rearms what has been said about the degree of
homogeneity of the coatings.

3.3 Electrical Properties
Figure6showstheresultsobtainedforresistivity(Ω·cm), charge
carrier concentration (1/cm3) and mobility (cm2/V·s) of all
samples. It is observed that samples that resulted with higher
resistivity and practically in the same order of 60 Ω·cm were
C1 and C2, while the rest presented lower resistivities in up
to one order of magnitude, as is the case of C3, which turned
out to be the one with the lowest resistivity with 4.02 Ω·cm.
Likewise, samples C1 and C2 also had the fewest charge carrier
concentration with values of 1.68 and 1.19×1017 1/cm3, re-
spectively, while samples C4 and C6 textured in address <101>
resulted in the highest number of charge carriers with values of
6.84 and 6.31×1018 1/cm3, but with the lowest mobility with
values of 0.34 and 0.29 cm2/V·s, respectively. Sample C1,
which, as already mentioned, turned out to be the sample with
greater mobility of 3.08 cm2/V·s, up to an order of magnitude
greater than those of C4 and C6, however; with a very low
number of charge carriers.

Figure 6 shows that the number of charge carriers for sam-
ples C1, C2, and C5 is smaller than that for sample C3, while
for samples C4, C6, and C7 it is bigger. Additionally, carrier
mobility values for samples C1, C2, and C5 are much higher
than for sample C3, but lower for samples C4, C6, and C7.
This is not in agreement to the results presented formicrostruc-

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López-Esmerio et. al. Science and Technology Indonesia, 7 (2022) 291-302

Table 3. Results Obtained for The Percentage of Substrate-Covered Area, The Diameter Average of The Bars, The Thickness of
The Sheets of The Samples, and The Texture Coecient from All Samples

Sample
Substrate-
covered

Diameter
average

Edge of the
hexagonal

Thickness
of the Texture coecient

area (%) (nm) cross section sheets <001> <101>

0.0625:1 (C1) 94.72 338 —– —– 2.7947 0.0789
0.1250:1 (C2) 81.52 126 —– —– 2.8696 0.0745
0.1875:1 (C3) 99.61 1268 —– —– 2.9338 0.0300
0.3125:1 (C4) 98.46 —– 402 —– 0.6374 1.7857
0.3750:1 (C5) 85.33 186 —– —– 2.7768 0.1353
0.5400:1 (C6) 99.85 —– —– 25 0.5820 1.9303
0.6250:1 (C7) 99.81 —– —– 30 2.7794 0.1000

Figure 6. Results Obtained for Resistivity (Ω·cm), Charge
Carrier Concentration (1/cm3), and Mobility (cm2/V·s)
Plotted as a Function of The Relative Concentration of
HMTA in The Precursor Solution

ture, as samples C1, C2, and C5 show spacings between rods,
where material deposition is insucient, which was expected to
aect the mobility of the carriers, and was not the case. On the
other hand, the surface of the ZnO rods in these same samples,
appears to be relatively smooth, which could represent fewer
defects and consequently a lower quantity of charge carriers.
Contrastingly, samples C4, C6, and C7, are practically free of
gaps and are composed by sub-micrometric structures with a
nanostructured surface resulting in higher values for charge
carriers.

3.4 Optical Properties
Figure 7 shows the photoluminescent emission spectra of all
samples when excited them with a _ex= 325 nm, as well as
excitation spectra for sample C7 when considering the emis-
sion _em= 395, 420 and 620 nm. Photoluminescence emission

spectra for all samples (Figure 7b) presents the characteristic
emission spectra for ZnO (Angulo-Rocha et al., 2017; Félix-
Quintero et al., 2017; Ramos-Brito et al., 2011). It shows a
narrow emission band centered around near-UV wavelength,
which is associated to an exciton-related electronic transition.
In addition, it exhibits a broad emission band in the visible
range of the electromagnetic spectrum, typically associated
with defects in the crystalline structure of ZnO. Additionally,
besides the se two particular bands, an emission band can be
found around 410-490 nm, which intensity will vary depend-
ing on the sample. Figure 8 presents a deconvolution of the
emissionspectra forsampleC3, it canbeobservedthat thespec-
trum itself can be seen as one composed of distinct emission
bands centered around 387, 395, 411, 439, 602, and 688 nm.
Referring to the works previous (Angulo-Rocha et al., 2017;
Félix-Quintero et al., 2017; Ramos-Brito et al., 2011), emis-
sion bands were associated with electronic transitions between
dierent states of energy introduced into the ZnO energy band
gap due to its following intrinsic defects: neutral interstitial
zinc (Zni0), zinc vacancies (VZn), interstitial zinc (Zni), oxygen
antisite (OZn), and Interstitial oxygen (Oi). This according to
the energy diagram in Figure 9. In this it is observed that the
emission centered at 387 nm is associated with the electronic
transition Zni0 → VB, emission centered at 395 nm with the
CB → VZn transition, emission centered at 411 nm at Zni0
→ VZn electronic transition, the emission of 439 nm to the
electronic transition Zni → BV. The band centered at 602 nm
at the CB → Ozn and CB → Oi transitions and nally, the
band focused on 688 nm was associated with the electronic
transitions Zni → Ozn and Zni → Oi, which is consistent with
what was reported Özgür et al. (2005b) and recently by Siva
et al. (2019). These latter observed a yellow band (574-587)
nm that they associate with the presence of Oi and a band
between (678-729) nm associated with excess oxygen or oxy-
gen vacancies (V0) sites. Surely at the time of deconvolution
of the emission spectrum (Figure 8) it was also possible have
considered the occurrence of V0, as reported in previous work
for deposited ZnO using Spray Pyrolysis (Angulo-Rocha et al.,
2017) where the presence was considered of V0 due to the rel-

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López-Esmerio et. al. Science and Technology Indonesia, 7 (2022) 291-302

atively intense emission in the region of (460-510) nm, but as
already it had been observed in previous work (Félix-Quintero
et al., 2017; Ramos-Brito et al., 2011) and now in this one,
apparently when the synthesis is by chemical bath deposition
the emission in this region is relatively poor, which is why it
was not considered. Low intensity Luminescent in the region
of (460-510) is associated with the poor presence of V0 be-
cause the synthesis is carried out in a oxygen-rich environment
(Özgür et al., 2005b).

Figure 7. a) Excitation Spectra for Sample C7 when Conside-
ring The 395, 420 and 620 nm Emission Wavelengths as
Well as b) Photoluminescent Emission Spectra of All Samples
when Excited Them with a Wavelength of 325 nm

Figure 8. Deconvolution of The Emission Spectrum of Sample
C3. It Shows Bands of Emission Centered at 387, 395, 411,
439, 602 and 688 nm that were Associated with The Presence
of Defects in The Crystalline Structure of ZnO. The Insert
Shows The Transmittance Spectrum for Sample C3 which is
Representative of All Samples

Due to the position of the donor and acceptor defects in
the “band gap” (see Figure 9) it can be concluded that the
ones that mostly aect the conductivity of ZnO are: Zni0, Zni,
and VZn, since the energy distances of OZn and Oi from VB
are large enough involving high activation energy; considering
this fact and in order to investigate a possible correlation, at
least qualitative, between the quantities of these defects and the
concentration of charge carriers measured by Hall eect, the

Table 4. Relative Percentage Amounts of The Donor Defects
Zni0, Zni, and Acceptors VZn Calculated from The Intensities
of Their Corresponding Photoluminescent Emission Bands

Sample
Amount of
donor defects

"n" (%)

Amount of
acceptor

defects "p" (%)

Dierence "n"-"p"
(%)

C1 0.701092351 2.546962517 -1.845870166
C2 12.32504424 8.688540241 3.636504000
C3 17.58026106 11.78783564 5.792425420
C4 20.05841157 9.975875551 10.08253602
C5 1.340338616 5.404254271 -4.063915654
C6 14.90788871 10.93850050 3.969388210
C7 21.13800263 12.48812969 8.649872935

percentage intensities of the emission bands centered at 387,
395, and 439 nm were calculated, and as a rst approximation
associated with the relative percentage amounts of the donor
defects Zni0, Zni and acceptor VZn. Table 4 presents these
results, it can be seen that samples C4, C7, C3, and C6 are
the highest value, in order from highest to lowest, for the dif-
ference between the number of defects Zni0, Zni donors and
VZn acceptor, consistent with those with the highest number
of C4, C6, C7, and C3 charge carriers, almost in the same
order from highest to lowest, except for the C6 shows that
in principle the dierence between defects should have been
greater than C7 and C3 and less than C4. This seems to indi-
cate that a correlation exists between the relative concentration
of type “n” and “p” charge carriers and the dierence of in-
tensities for emission peaks associated with the bands for Zni0
→ VB, CB → VZn and Zni → BV. These results encourage
the use of high resolution photoluminescence spectroscopy to
deconvolute said emission bands to obtain enough precision to
quantitatively measure the dierence in intensities and there-
fore the dierence between type “n” and “p” charge carriers
in ZnO. This could place photoluminescence spectroscopy as
an optical method for characterizations that until this day are
done only via electrical techniques.

The insert of Figure 8 shows the transmittance spectrum
for sample C3 which is representative of all samples. The
transmittance for all samples was not greater than 70% in the
visible region, being C1 the one with the lowest transmittance
40%. They all started with their value highest at 750 nm, in the
case of C3 it starts at ∼ 70% for 750 nm, and they presented
a drop pronounced due to intrinsic defects in its crystalline
structure. Theenergybandgapofall thesampleswereobtained
using the Tauc method (Viezbicke et al., 2015), resulting in:
3.19, 2.8, 3.18, 3.25, 3.05, 3.26, and 3.19 eV for, C1, C2, ...,
C7, respectively.

3.5 Implications of Morphology, Texture, and Crystallite
Size on Electrical Properties

Figure 10 shows the resistivity as a function of: percentage of
substrate-covered area, average diameter of the bars or thick-

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López-Esmerio et. al. Science and Technology Indonesia, 7 (2022) 291-302

Figure 9. Energy Diagram that Shows Indistinctly The Die-
rent De-Excitation Paths for Bars or Sheets of ZnO

ness of the sheets, average crystalline grain size and texture di-
rection, of samples. Figure 10a shows that for all samples there
is the tendency that the highest the percentage of substrate-
covered area, the smaller the resistivity except for C1. This
exception can be attributed to the fact that it is C1 the one hav-
ing the least percentage of donor defects (see Table 4). Figures
10a and 10b show for samples C1, C2, C3, and C5 that the
greatest the diameter of the bars, the greater the percentage of
substrate-covered area, therefore, the contribution that diame-
ter of the bars has in the resistivity is implicitly contemplated
when considering the contribution that substrate-covered area
has in the resistivity.

A correlation between resistivity and texture coecient was
not found. Samples C1, C2, C3, C5, and C7, textured in the
<001> direction have too similar texture coecients, with a
relative percentage dierence between them less than 5% (see
Table 4), while the resistivity between them diers from up to
an order of magnitude (see Figure 6).

The mobility of the charge carriers in a thin lm is nec-
essarily inuenced by the crystalline grain size, percentage of
substrate-covered area, texturing direction, and microstructure
of the sample in question. Figure 6 and Table 3 show that
mobility for samples C1, C2, C3, C5, and C7, textured in
the direction <001>, increases as the substrate-covered area
increases even if the crystalline grain size decreases, being then

Figure 10. It Shows The Resistivity of All Samples as a Func-
tion of: a) Percentage of Substrate-Covered Area, b) Average
Diameter of The Bars or Thickness of The Sheets, c) Average
Crystalline Grain Size and d) Texture Direction

the substrate-covered area a preponderant factor in mobility.
This happens even when a decrease in the crystalline grain
size could implies an increase of the crystal dislocations that
should means greater dispersion centers for charge carriers and
therefore less mobility of them.

The results shown in Figure 6, Table 1 and Table 1 show
that when you have a covered percentage of the area larger than
95% the major factor in mobility appears to be the crystalline
grainsize, just as if theamountofcrystaldislocationsdominates
mobility. Thiswasobservedwhenmobilitycomesfrom3.0803
cm2/V·s for sample C1 that has a substrate-covered area of
94.72% and particle size of 29,731 nm to mobility of 0.8334
cm2/V·s for the C3 sample that has a substrate-covered area of
99.61% and grain size of 27.731 nm.

The change in the microstructure of the thin lm could be a
relevant factor in mobility. This was observed when mobilityof
sample C7 that has substrate-covered areaof 99.81% was less in
an up to a 65.2% than mobilityof C2 that has substrate-covered
area of 81.52%.

Indealingwith thesamplesC4andC6textured in thedirec-
tion <101> their values for the crystalline grain size, percentage
of substrate-covered area, texturing direction and microstruc-
ture are too similar, which is consistent with the similarity
between its values for resistivity, charge carrier concentration
and mobility. The small dierence between their values for the
charge carrier concentration is consistent with the dierence
between their values for the relative percentage amounts of
donor and acceptor defects (see Table 4).

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López-Esmerio et. al. Science and Technology Indonesia, 7 (2022) 291-302

4. CONCLUSIONS

The deposit by spin coating followed by chemical bath allowed
the synthesis of ZnO lms. The seven samples resulted in
microstructured lms. The C1, C2, C3, C4, and C5 lms were
made up entirely of ZnO bars that were moderately oriented
perpendicular to the substrate and with apparently hexagonal
cross section, while lms C6 and C7 were made up of corn
ake-like structures oriented almost perpendicular to the sub-
strate plane. All ZnO bars show a nodular type growth and
nanostructured hexagonal cross section with an average rough-
ness of the order of crystallite grain size.

The experiment allowed the synthesis of ZnO lms with
hexagonal crystalline structure (a= 3,250 Å and c= 5,195 Å),
average crystalline grain size of 31.95 nm and a shaped surface,
for the most of the samples, by hexagonal bars with diameters
that varies between 126 and 1268 nm. The increase in the
diameter of the bars induces a higher substrate-covered area.

The synthesis process favors two texture directions, (002)
and (101) without noticeable changes between the texture coef-
cients for both. The <101> textured ZnO lms resulted with
thehighestnumberofchargecarriervaluesofaround6.5×1018
1/cm3, but with the lowest mobility with values of around the
0.32 cm2/V·s. The higher the percentage of substrate-covered
area of ZnO lms, the lower their resistivity, regardless of their
morphology and texture. The <001> texturized sample C3
resulted with the lowest resistivity of 4.02 Ω·cm. The mobility
in ZnO lms resulted mainly inuenced by substrate-covered
area, this when its value is less than 95%, while forhighervalues
the mobility is dominated by the crystalline grain size.

A new optical parameter resulted from the dierence be-
tween photoluminescent emissions associated with donor de-
fects (Zni0 y Zni) and VZn acceptors that could be considered
as an indicator of the charge carrier concentration. The greater
the dierence, the greatest the concentration of n-type charge
carriers. These results encourage the use of high resolution
photoluminescence spectroscopy to quantitatively measure the
dierence between type “n” and “p” charge carriers in ZnO.
This could place photoluminescence spectroscopy as an optical
method for characterizations that until this day are done only
via electrical techniques.

5. ACKNOWLEDGMENT

Authors wish to express their gratitude to the National Coun-
cil of Science and Technology (CONACYT) for the master
scholarships granted to: C. López-Esmerio and C. Ruiz-Rojas.
They would also like to acknowledge the technical support of:
Dr. F. J. S. Rodriguez in synthesis process, Dr. Omar Novelo
in SEM characterization and Dr. Rogelio Fragoso in AFM
analysis.

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	INTRODUCTION
	EXPERIMENTAL DETAILS
	Synthesis
	Characterization

	RESULTS AND DISCUSSION
	Crystalline Structure
	Microstructure
	Electrical Properties
	Optical Properties
	Implications of Morphology, Texture, and Crystallite Size on Electrical Properties 

	CONCLUSIONS
	ACKNOWLEDGMENT