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Materials Letters 139 (2015) 63–65
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Materials Letters
journal homepage: www.elsevier.com/locate/matlet
Nanometric structures of highly oriented zinc blende ZnO thin ﬁlms
L. Martínez-Pérez a, N. Muñoz-Aguirre b,n, S. Muñoz-Aguirre c, O. Zelaya-Angel d
a
Unidad Profesional Interdisciplinaria en Ingeniería y Tecnologías Avanzadas del Instituto Politécnico Nacional, Av. IPN No. 2580,
Col. Barrio La Laguna Ticomán, C.P. 07340 México D.F., México
b
Instituto Politécnico Nacional, Sección de Estudios de Posgrado e Investigación, Escuela Superior de Ingeniería Mecánica y Eléctrica Unidad Azcapotzalco.
Av. Granjas No 682, Colonia Santa Catarina, Del. Azcapotzalco, C.P. 02250 México D.F., México
c
Benemérita Universidad Autónoma de Puebla, Facultad de Ciencias Físico Matemáticas, Av. San Claudio y 18 Sur,
Col. San Manuel (CU), C.P. 72570 Ciudad de Puebla, Puebla, México
d
Departamento de Física del Centro de Investigación y de Estudios Avanzados del IPN, C.P. 07351 México D.F., México
art ic l e i nf o
a b s t r a c t
Article history:
Received 1 September 2014
Accepted 10 October 2014
Available online 18 October 2014
Zinc oxide thin ﬁlms in cubic zinc blende (ZB) crystalline phase on glass substrates by means of the spray
pyrolysis technique were deposited. X-ray diffraction spectra revealed that the ZB–ZnO ﬁlms grow highly
oriented along the (004) crystalline direction with no epitaxial inﬂuence. Optical absorbance measurements indicated that the forbidden energy band gap is 3.18 7 0.02 eV in accordance with reports on the
experimental value of the band gap of ZB–ZnO structures. Atomic Force Microscopy images exhibit
nanometric structures of the surface with the approximated aspect of circular nanodiscs. The thickness
of the thin ﬁlms is 3507 20 nm, which suggests a good stability of the ﬁlms.
& 2014 Elsevier B.V. All rights reserved.
Keywords:
Thin ﬁlms
Deposition
Phase transformation
Cristal structure
Microstructure
Atomic Force Microscopy
1. Introduction
The existence of a speciﬁc material which can be synthesized in a
particular phase from a determined set of possible phases has
widened its spectrum of technological applications [1]. One of the
more versatile solid materials is zinc oxide (ZnO), within the II–VI
semiconductors family, owing to the availability of a large variety of
techniques to prepare it, the large number of structures and
nanostructures in which can be condensed, as well as the large
and varied number of industrial applications [2]. Among the different
phases in which this oxide can solidify in crystalline form, not
considering the amorphous one, are hexagonal wurtzite (WZ) [1],
cubic zinc blende (ZB) [2], cubic rock salt (RS) [3], hexagonal graphite
like [4], tetrapods [5], among others. Wurtzite is the crystalline stable
phase in bulk at normal temperature and pressure. Other phases are
stable in special conditions; for instance, ZB is the stable phase of
nanoparticles condensed from smoke for less than 20 nm in size [6].
As far as, it is well known that ZnO thin ﬁlms in ZB phase have been
obtained only in epitaxial growth on cubic substrates [7] and only
containing a small number of monolayers. In general, ZnO thin
ﬁlms have many important applications principally as sensing and
n
Corresponding author.
E-mail addresses: [email protected] (L. Martínez-Pérez),
[email protected] (N. Muñoz-Aguirre), [email protected] (S. Muñoz-Aguirre),
ozelaya@ﬁs.cinvestav.mx (O. Zelaya-Angel).
http://dx.doi.org/10.1016/j.matlet.2014.10.054
0167-577X/& 2014 Elsevier B.V. All rights reserved.
optoelectronic material [8]; other interesting applications are related
to its use as transparent conductor [9] and photocatalyst. In this way,
important motivation exists for researching their structural, electrical
and optical properties. Martínez Pérez et al. [9] reported the synthesis by means of the spray pyrolysis technique of good quality
WZ–ZnO thin ﬁlms. When chemical methods are used for depositing
ﬁlms, such as the spray pyrolysis, it is well known that the structural
properties of the ZnO thin ﬁlms depend on the molar concentration
and the ambient conditions of the materials source solution [9].
Therefore, for the same molar concentration in the spray pyrolysis
technique, if the ambient conditions of the source solution are
different then the structural properties of the deposited thin ﬁlms
should be different. In this letter we show an example which proves
such hypothesis and as a consequence a phase change from WZ to ZB
was observed at ambient conditions in the growth on glass substrates of undoped ZnO thin ﬁlms. The importance of obtaining
stable ZB phase of ZnO thin ﬁlms for its potential applications has
been remarked by Ashraﬁ and Jagadish [1].
2. Materials and methods
Using the procedure detailed in a previous work [9], two source
materials solutions of 0.3032 M concentrations of zinc acetylacetonate
(SigmaAldrich) dissolved in N,N-dimethylformamide (N,N-DMF) (Mallinckrodt) were prepared. One of the source materials solution was
64
L. Martínez-Pérez et al. / Materials Letters 139 (2015) 63–65
prepared at the same conditions as reported previously [9] (labeled
by solution A) and the second solution (labeled by B) was furthermore reposed at ambient conditions during 72 h. Samples were
prepared from both type of solutions. Corning glasses were used as
substrates, which were carefully cleaned using a well-known
cleaning procedure [10]. The growths were performed at substrate
temperatures of 400, 450, 500 and 550 1C for both A and B
solutions. X-ray diffraction (XRD) patterns were measured using a
commercial Diffractometer (D-5000, Siemens). EDS measurements
were carried out employing a Field Emission Scanning Electron
Microscopy (JEOL). An Atomic Force Microscope (JSPM 5200, JEOL)
was used to observe the surface characteristics of the ﬁlms. A UV–vis
Unicam 8700 model equipment over the 190–1100 nm of wavelength range with an accuracy of Δλ ¼ 7 0.3 nm to register optical
absorption data used to estimate de optical band gap was used.
A proﬁlometer (Dektak3) allowed measure the thickness of the
thin ﬁlms.
3. Results and discussions
Fig. 1. X-ray diffraction patterns of the thin ﬁlms.
EDS measurements showed an atomic concentration of
49.8 71.5% of oxygen and 50.2 71.5% of zinc for samples prepared
using the A solution and 45.37% (O) and 54.63% (Zn) for samples
prepared from the B solution. An absence of oxygen in the order of
5% in the crystalline lattice of B can be observed. Fig. 1 shows the
X-ray diffraction reﬂections of a representative sample (500 1C)
for both A and B types of the thin ﬁlms. The diffractograms of
A samples are associated with the hexagonal WZ crystalline phase
of ZnO with preferred orientation in the (002) direction. The XRD
patterns of B samples have been identiﬁed with the ZB phase of
ZnO [11,12], which possess the (004) reﬂection at 2θ ¼ 44.61. In our
case the pattern is observed highly oriented along this (004)
direction. The unusual ZB phase for ZnO has been reported mainly
in epitaxial growths, for instance C. Tusche et al. [13] reported
epitaxial ZnO ultrathin ﬁlms where the ZB phase was observed as
Fig. 2. Atomic Force Microscopy images obtained by means of the AC mode (tapping) [14–16]. (a) Image of the surface of the WZ–ZnO ﬁlms. (b) The surface of the ZB–ZnO
thin ﬁlms. (c) High resolution AFM images of the WZ–ZnO ﬁlms, the scale is 346 346 nm. (d) High resolution AFM images of the ZB–ZnO ﬁlms, the scale is 254 254 nm.
L. Martínez-Pérez et al. / Materials Letters 139 (2015) 63–65
65
energy. Fig. 3 displays the plot for Tauc's method to calculate the
band gap for the ZB and WZ structures, Eg-ZB ¼3.187 0.02 eV and
Eg-WZ ¼3.29 70.01 eV. The error bars are due to the average
calculation in a set of four samples for both WZ–ZnO and ZB–ZnO
thin ﬁlms. These results were compared with the values of the Eg
calculated using the derivative method [17]. The Eg values resulted
larger than those calculated using Tauc's method. It is important to
mention that the derivative method is only used to approximate the
Eg [17]. However, these results support the fact that the band gap for
WZ–ZnO is larger than for ZB–ZnO thin ﬁlms, which is in agreement
with experimental results reported by other authors [13].
On the other hand, under the same growth conditions of the
ZnO ﬁlms, the thickness of the WZ–ZnO thin ﬁlms was 150 nm and
the ZB–ZnO ﬁlms was 350 nm. This fact could mean that the high
preferred orientation of ZB–ZnO ﬁlms favors a faster growth rate.
The lack of oxygen in the ZnO thin ﬁlms could inﬂuence the
growth in ZB phase.
4. Conclusions
Fig. 3. The Tauc method employed to calculate the band gap energy of the WZ–ZnO
and ZB–ZnO thin ﬁlms. The inset illustrates the estimation of the gap using the
derivative of the optical absorption with respect to the photon energy [17] d(OA)/d (hν).
In summary, the growth of nanostructured ZnO thin ﬁlms in cubic
zinc blende crystalline phase was successfully achieved. The ZnO thin
ﬁlms synthesis was made on glass substrates (no epitaxial inﬂuence)
by means of the spray pyrolysis growth technique at substrate
temperatures in the range of 400–550 1C. The Atomic Force Microscopy images reveal that the ZB phase grows with the aspect of
hexagonal and quadrangular nanochains. The ﬁlms were relatively
thick (350 nm) and highly oriented in the (004) direction. The
bandgap of this elusive phase of ZnO resulted in 3.1870.02 eV,
which is lower than the gap of the wurtzite phase that is
3.2970.01 eV, both of the values are the average of the four samples
synthetized at different temperatures. The preparation of ZB–ZnO
nanostructured thin ﬁlms can open new possible applications of this
material in electronic and chemical industries.
Acknowledgments
a modiﬁed hexagonal phase. They reported that ultrathin ﬁlms
were characterized by a signiﬁcant amount of under-coordinated
atoms, and their underlying bulk. Consequently, atomic structures
and depolarization mechanisms distinctly different from those of
the bulk can be expected. The ZB–ZnO thin ﬁlms are also closely
related to tetrapods and RS structures as has been reported by
other authors [1,13]. It has been reported that the RS structure is
only possible to be synthesized at high pressure [3].
Atomic Force Microscopy images allow observing truncated
discs and craters for WZ–ZnO ﬁlms and well-ordered like discs for
ZB–ZnO thin ﬁlms as shown in Fig. 2a and b. When a high
resolution AFM image was obtained, the discs and craters for
WZ–ZnO thin ﬁlms are perfectly observed and approximately
circular discs for ZB–ZnO ﬁlms as shown in Fig. 2c and d. It is
very important to mention that ZB–ZnO ﬁlms did not show craters.
The average size of the craters was about of 50 nm as can be
observed in Fig. 2c. The average size of the circular discs was about
90 nm and the size of the pentagonal discs was around 60 nm by
edge. From Fig. 2c and d it can also be observed that the discs and
the craters are constituted by hexagonal nano-chains of
pyramidal-like clusters of atoms for WZ–ZnO thin ﬁlms, and
hexagonal and quadrangular nano-chains for ZB–ZnO thin ﬁlms.
The size of these chains is of the order of 8 nm for WZ–ZnO ﬁlms
and 4 nm for ZB–ZnO thin ﬁlms (see Fig. 2).
From optical absorbance measurements the (αhν)2 data was
obtained and, following the method reported by Tauc, were
plotted as a function of hν to estimate the energy band gap (Eg).
Here α is the optical absorption coefﬁcient and hν the photon
Work supported by Instituto Politécnico Nacional from México
with Project number SIP-20144145. The authors also would like to
acknowledge the technical assistance of Ing. Ana Berta Soto and QFB.
Marcela Guerrero from Physics Department of CINVESTAV-IPN.
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