Journal paper

1. Materials Science and Engineering B 187 (2014) 83–88
Contents lists available at ScienceDirect
Materials Science and Engineering B
journal homepage: www.elsevier.com/locate/mseb
Band gap engineering of indium zinc oxide by nitrogen incorporation
J.J. Ortegaa,b,∗
, M.A. Aguilar-Frutisc
, G. Alarcónc
, C. Falconyd
,
V.H. Méndez-Garcíab,e
, J.J. Araizaa
a
Unidad Académica de Física, Universidad Autónoma de Zacatecas, Calzada Solidaridad esq. Paseo la Bufa, Fracc. Progreso, C.P. 98060 Zacatecas, Mexico
b
Doctorado Institucional de Ingeniería y Ciencia de Materiales, Universidad Autónoma de San Luis Potosí, Av. Salvador Nava, Zona Universitaria, C.P. 78270
San Luis Potosí, Mexico
c
Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada del Instituto Politécnico Nacional, Unidad Legaría, Calz. Legaría No. 694, Col.
Irrigación, C.P. 11500 México D.F., Mexico
d
Departamento de Física, Centro de Investigación y Estudios Avanzados del Instituto Politécnico Nacional campus Zacatenco, Av. Instituto Politécnico
Nacional 2508, Col. San Pedro Zacatenco, C.P. 07360 México D.F., Mexico
e
Laboratorio Nacional-CIACyT, Universidad Autónoma de San Luis Potosí, Sierra Leona 550, Lomas 2a
Secc, C.P. 78210 San Luis Potosí, Mexico
a r t i c l e i n f o
Article history:
Received 23 December 2013
Received in revised form 8 May 2014
Accepted 12 May 2014
Available online 24 May 2014
Keywords:
Indium zinc oxynitride
Nitrogen incorporation
Band gap engineering
Band gap narrowing
a b s t r a c t
The effects of nitrogen incorporation in indium zinc oxide films, as grown by RF reactive magnetron
sputtering, on the structural, electrical and optical properties were studied. It was determined that the
variation of the N2/Ar ratio, in the reactive gas flux, was directly proportional to the nitrogen percentage
measured in the sample, and the incorporated nitrogen, which substituted oxygen in the films induces
changes in the band gap of the films. This phenomenon was observed by measurement of absorption and
transmission spectroscopy in conjunction with spectral ellipsometry. To fit the ellipsometry spectra, the
classical and Adachi dispersion models were used. The obtained optical parameters presented notable
changes related to the increment of the nitrogen in the film. The band gap narrowed from 3.5 to 2.5 eV as
the N2/Ar ratio was increased. The lowest resistivity obtained for these films was 3.8 × 10−4
cm with a
carrier concentration of 5.1 × 1020
cm−3
.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
The In2O3-ZnO (IZO) system has demonstrated excellent opti-
cal and electrical properties, such as high optical quality, high
mobility, surface uniformity and chemical and thermal stability in
various environments. For these reasons, IZO has been widely used
in applications, such as transparent contacts for solar cells, light
emitting diodes and several other optoelectronic devices [1–6]. For
the application of IZO in optoelectronic devices, one of the rele-
vant properties is the optical band gap, which closely depends on
the change of the growth conditions, the doping impurities and,
consequently, the carrier concentration. In recent years, the best
control in the thin film deposition techniques allows for the design
of band structures with nearly arbitrary and continuous band-gap
variations showing that band-gap engineering is a powerful tech-
nique for the design of new semiconductor materials and devices.
∗ Corresponding author at: Unidad Académica de Física, Universidad Autónoma
de Zacatecas, Calzada Solidaridad esq. Paseo de la Bufa, Fracc. Progreso, C.P. 98060
Zacatecas, Mexico. Tel.: +52 492 924 1314; fax: +52 492 924 1314.
E-mail addresses: jjosila@hotmail.com, jjosila@fisica.uaz.edu.mx (J.J. Ortega).
In this way, there are many reports related to the production of
shifts in the band gap of IZO [6–9]. Several of these works modified
the band gap by changing the ratio between indium and zinc metal
or including new metallic species within of the IZO matrix, which
often results in a more complicated, costly and less reproducible
process, and the shifts of the band gap are relatively small. How-
ever, before this work, nobody has studied or reported the band
gap engineering of the IZO system by substituting oxygen with
nitrogen.
In the present work, a method to change the optical band gap of
IZO films grown by RF reactive magnetron sputtering is reported.
The synthesis and the optical, structural and electrical character-
ization of the IZON thin films obtained by RF reactive magnetron
sputtering are presented. The nitrogen incorporation in the films
is also studied as a function of the nitrogen concentration in the
sputtering atmosphere. The refractive index and extinction coef-
ficient were determined as a function of the photon energy using
spectroscopic ellipsometry (SE). The optical constants derived from
the experimental techniques are presented, and a significant study
of the optical band gap was realized from the dependence of the
absorption coefficient on the photon energy. Optical band gap shifts
as large as 1 eV are observed for the IZON films.
http://dx.doi.org/10.1016/j.mseb.2014.05.005
0921-5107/© 2014 Elsevier B.V. All rights reserved.

2. 84 J.J. Ortega et al. / Materials Science and Engineering B 187 (2014) 83–88
0 1 2 3
0
10
20
30
40
50
60
70
Gas flux ratio N
2
/Ar
%(Atomiccontent)
N
O
Zn
In
N+O
Fig. 1. Atomic percentages for the different species present in the IZON films as a
function of the gas flux ratio N2/Ar in the reactive atmosphere.
2. Experimental details
Indium zinc oxynitride thin films were deposited on Si(1 0 0)
substrates and corning glass substrates by RF reactive magnetron
sputtering under different nitrogen concentrations in the reac-
tive atmosphere. IZO (In2O3/ZnO, 90/10 wt.%, with a purity of
99.99%) was used as the sputtering target. For this purpose, the
deposition chamber was evacuated to a base pressure lower than
1.5 × 10−6 Torr. Room temperature sputtering depositions were
performed with Ar (99.995%) and N2 (99.99%) as the reactive
sputtering gases at a total pressure of 6 × 10−3 Torr. The gas flux
ratio N2 (sccm)/Ar (sccm) was set at: 0/5, 5/5, 10/5 and 15/5. The
deposited films were characterized in a Jeol Scanning Electron
Microscope, model JSM-6390LV. An X-ray energy dispersive spec-
trometer (INCA X-sight Oxford Inst. Model 7558) was attached to
the microscope. The crystalline structure of the IZON thin films was
studied in a Siemens D-5000 diffractometer using the Cu K␣ line
( = 0.1541 nm). The X-ray diffraction patterns were obtained in a
grazing angle mode at 1.5◦. The electrical resistivity, mobility, and
carrier concentration were measured in an Ecopia HMS-3000 Hall
Effect Measurement System, using the Van der Pauw configuration.
The optical transmittance was measured in a Perkin Elmer Lambda
35 UV–vis spectrophotometer in the wavelength range from 300
to 1000 nm. Finally, spectroscopic ellipsometry (SE) measurements
were acquired in a Yobin Ivon Ellipsometer, model UVISEL, in an
energy range from 1.5 to 4.5 eV. The latter measurements were
employed to find the refractive index and extinction coefficient of
the films by comparison with the theoretical computer-calculated
spectra of the films using the software provided with the ellipsome-
ter (Psi-Delta 2.0).
3. Results and discussion
3.1. Thin film composition
X-ray energy dispersive spectroscopy (EDX) analysis confirmed
the presence of In, Zn, O and N in all the deposited films. The
atomic concentrations are shown in Fig. 1. As the ratio of nitro-
gen increased in the reactive gas flux, the amount of nitrogen in
the films increased. At the same time, the percentage of oxygen in
the film decreased, inversely proportional to the amount of nitro-
gen incorporated. In addition, the atomic percentages of indium
and zinc in the films did not change with the N2/Ar gas flux ratio. In
Fig. 1, the sum N + O is also plotted, and its behavior with the varying
N2/Ar gas flux ratio is nearly constant. These results directly suggest
that oxygen ions are partially substituted by nitrogen ions in the
structure of the film. Even when no nitrogen flux was supplied to
20 30 40 50 60 70
(622)In2
O3
(102)InN
(611)In2
O3
(101)InN
(222)In2
O3
0N2
/5Ar
5N2
/5Ar
10N2
/5Ar
15N2
/5Ar
2θ (Degree)
Intensity(a.u.)
Fig. 2. XRD spectra of the IZON thin films as a function of the growth parameters.
the sputtering chamber (0 N2/5 Ar), the background nitrogen inside
was enough to be introduced in the films. This finding is most likely
observed due to the large affinity of nitrogen for indium [10]. In fact,
this feature has led to improvement of the incorporation of nitrogen
in the films. However, the increment of nitrogen in the films could
lead to the appearance of new phases besides the original phases
that were present in the sputtering target used for depositing these
films.
Fig. 2 shows the XRD pattern obtained for the different films.
For the first three conditions of deposition, amorphous films were
generated, and the related spectra showed a broad band in the
diffracted intensity associated with the amorphous phase in the
range of 20–40◦ with the maximum approximately 32.4◦. This band
is related to the IZO matrix, so it is not possible to observe any effect
of nitrogen for these cases. However, the films deposited with a gas
flux ratio of 15 N2/5Ar showed a sharp peak at 2Â = 30.5◦ associated
with the (2 2 2) plane of the In2O3 bixbyite structure. This peak
has been previously reported for the In2O3-ZnO system [11–15]. In
addition, four peaks located at 33.2◦, 43.5◦, 56.1◦ and 61.7◦ corre-
sponded to the (1 0 1) and (1 0 2) planes of the hexagonal structure
of pure InN and the (6 1 1) and (6 2 2) planes of In2O3 bixbyite struc-
ture. The XRD results showed that for the largest N2 flux in the
chamber, the active N species formed by the RF readily competed
for sites and substituted oxygen in the films, as confirmed by EDX,
leading to the formation of additional crystalline phases of InN in
the films.
3.2. Electrical properties
The carrier concentration (n), the mobility ( ) and the resis-
tivity ( ) of the IZON thin films were analyzed as a function of
the N2/Ar gas flux ratio. Fig. 3 shows the evolution of the elec-
trical properties. All of the films presented n-type conductivity,
which is the typical conductivity reported for IZO films [14–17].
The carrier concentration was related to the nitrogen in the films,
i.e., the carrier concentration decrease was inversely proportional
to the amount of nitrogen incorporated in the films. The elec-
trical conduction in intrinsic indium oxide (In2O3) is due to free
electrons originating from two mechanisms. The related mecha-
nisms are oxygen vacancies and the excess of indium atoms in
the films [18]. Nevertheless, a complementary scenario for n-type
electrical conductivity is expected for intrinsic zinc oxide (ZnO).
In addition to interstitial zinc in the films, oxygen vacancies are
historically attributed as free charge carriers via the formation of
shallow donor levels [19]. However, first-principles calculations

3. J.J. Ortega et al. / Materials Science and Engineering B 187 (2014) 83–88 85
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
15
18
21
24
27
30
33
0 1 2 3
0.3
0.6
0.9
1.2
1.5
1.8
Carrierconcentration
(x10
20
cm
-3
)
Mobility
(cm
2
/Vs)
Resistivity
(x10
-3
Ωcm)
Gas flux ratio N2
/Ar
Fig. 3. Dependence of the electrical properties: (a) carrier concentration, (b) elec-
tron mobility and (c) electrical resistivity on the N2/Ar gas flux ratio introduced as
reactive atmosphere.
indicate that oxygen vacancy is actually a deep donor, and it is
more likely that the n-type conductivity might be caused by the
unintentional incorporation of impurities [20,21]. For example, it
has been suggested that hydrogen can incorporate on oxygen sites
(HO), acting as a shallow donor. Consequently, the drop in the car-
rier concentration could have been caused by the displacement of
impurities with nitrogen, not only inside the growth chamber but
also on those oxygen sites where the impurities are incorporated
as shallow donors. However, when nitrogen replaced oxygen, new
phases were generated, and the oxygen vacancies were partially
suppressed or extinct, causing a decrease in the carrier concentra-
tion. The mobility results were low (∼15 cm2/V s) for the IZON films
deposited with flux ratios of 1, 2 and 3. In contrast, the mobility
for the films deposited with a flux ratio of 0 sccm of nitrogen was
almost double the value (∼32 cm2/V s) compared with the previ-
ous values of flux ratio. Although the mobility may decrease due to
different scattering mechanisms, the process that is likely occur-
ring with the mobility in the IZON films is related to the a further
increase in the nitrogen concentration. The ionized impurity scat-
tering is expected to dominate, and as a consequence, the mobility
associated with the free carriers would be decreased [18,22]. The
resistivity of the IZON films showed a trend similar to the relation
for semiconductors, 1/ = ne , where the resistivity is restricted
by the mobility and carrier concentration. As shown in Fig. 3 c,
when n and decreased as the nitrogen in the films increased, the
resistivity increased.
3.3. Optical properties
Fig. 4 shows the thin films structure model used in the SE analy-
sis, in which the thin film was considered to be composed of three
layers: an interfacial layer of SiO2, a bulk layer and a roughness
Fig. 4. Model thin film structure used for the spectral ellipsometry characterization:
surface layer, the IZON bulk layer, and the interface SiO2 layer on the substrate.
layer. A combination of the bulk layer material and voids in differ-
ent contents were used for the roughness layer. For obtaining the
optical parameters, it is necessary to model the experimental data
with an analytical model. The bulk layer was simulated using two
different models, the classical dispersion model and the Adachi dis-
persion model [23]. The classical model is based on the harmonic
oscillator approach and the dielectric function consists of four con-
tributions which are the high energy dielectric constant, the Lorenz
oscillator term, the Drude model and a damped harmonic oscillator
contribution as it is shown the following expression,
ε = ε∞ +
(εS − ε∞)ω2
t
(ω2
t − ω2) + i 0ω
+
ω2
p
−ω2 + i Dω
+
2
j=1
fjω2
0j
(ω2
0j
− ω2) + i jω
where ε∞ is the high energy dielectric constant, εs is the static
dielectric constant, ωt is the single oscillator resonance frequency,
0 is the single oscillator resonance width, ωp is the Plasmon
frequency and D is Drude broadening parameter in angular fre-
quency. On the other hand, the Adachi model is a standard critical
point model derived from the Kramers–Kronig transformation and
it is strongly connected with the electronic energy band structures
of the material. It is widely used for calculation of the real and imag-
inary parts of the dielectric function of semiconductors at energies
below and above the lowest band gaps, in which the model reveals
distinct features at energies of the E0, E0 + 0, E1, E1 + 1, and E2
critical points. The optical parameters obtained using both disper-
sion models are presented in Fig. 5. The refractive index showed
a notable change; it increased as the nitrogen content in the films
increased. The values obtained for the refractive index are higher
than those reported for the IZO [14,24], and they matched the val-
ues reported for zinc nitrides [25]. Similarly, for the films grown
with a large N2 concentration in the atmosphere, the extinction
coefficients presented a notable increment as a result of the incor-
poration of nitrogen into the films.
The optical transmittance in the wavelength range from 300
to 1000 nm for the IZON films is presented in Fig. 6a. As the
amount of nitrogen increased, the transmittance in the visible
region decreased. The average transmittance in the visible part of
the spectra (400–700 nm) for the thin films grown in the highest
N2 concentration decreased from 80% to 50%. The absorption coef-
ficient is shown in Fig. 6b. The absorption increased monotonically
as the concentration of N2 increased in the reactive atmosphere as
a result of the incorporation of nitrogen in the films. For this reason,
the films were more absorbent. The same features were observed
on the extinction coefficient because both the absorption and the
extinction coefficient are related by the equation ˛ = 4 k/ , where

4. 86 J.J. Ortega et al. / Materials Science and Engineering B 187 (2014) 83–88
1.8
1.9
2.0
2.1
2.2
2.3
2.4
0.0
0.1
0.2
0.3
0.4
0.5
0.6
1.5 2.0 2.5 3.0 3.5 4.0 4.5
1.8
1.9
2.0
2.1
2.2
2.3
2.4
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Adachi model
0 N2
/5 Ar
5 N2
/5 Ar
10 N2
/5 Ar
15 N2
/5 Ar
Adachi model
0 N2
/5 Ar
5 N2
/5 Ar
10 N2
/5 Ar
15 N2
/5 Ar
RefractiveIndexRefractiveIndex
Energy (eV) Energy (eV)
Classic model
0 N2
/5 Ar
5 N2
/5 Ar
10 N2
/5 Ar
15 N2
/5 Ar
ExtinctioncoefficientExtinctioncoefficient
Classic model
0 N2
/5 Ar
5 N2
/5 Ar
10 N2
/5 Ar
15 N2
/5 Ar
Fig. 5. Refractive index and extinction coefficient as a function of the N2/Ar gas flux ratio in the reactive atmosphere.
300 400 500 600 700 800 900 1000
0
20
40
60
80
100
1.5 2.0 2.5 3.0 3.5
0.0
0.5
1.0
1.5
2.0
2.5
Transmittance(%)
Wavelength (nm)
0N2
/5Ar
5N2
/5Ar
10N2
/5Ar
15N2
/5Ar
a)
0N2
/5Ar
5N2
/5Ar
10N2
/5Ar
15N2
/5Ar
Absorption
Energy (eV)
b)
Fig. 6. (a) Transmission spectra and (b) absorption coefficient for IZON thin films as
a function of the wavelength for different reactive N2/Ar gas flux ratios.
˛ is the absorption, k denotes the extinction coefficient and is the
wavelength associated to the photon energy.
In Fig. 7, the absorption spectra measured directly and indi-
rectly are plotted, and Fig. 7a–c show the absorption obtained
from the SE data from the classical and Adachi models and exper-
imentally measured by the UV–vis technique. In all the graphs,
the absorption increased monotonically with the incorporation of
nitrogen in the films, as previously mentioned. Simultaneously,
the optical absorption coefficient shifted systematically toward a
lower energy, indicating a continuous decrease of the optical gap
as an effect of the increment of nitrogen in the film. The optical
band gap Eg was determined according to Tauc’s equation [26],
˛hv = A(hv − Eg)
n
, where A is a constant, hv is the photon energy
and the exponent is n = 1/2 for allowed direct, n = 2 for allowed
indirect, n = 3/2 for forbidden direct and n = 3 for forbidden indirect
transitions [24,26,27]. Thus, the Eg can be determined by extrapo-
lating the linear portion of the curves to zero absorption. In the case
of IZON films, direct allowed transitions were selected (n = 1/2), and
this is related with previous observations for the precursor oxides,
such as IZO [4,7], In2O3 [28,29] and ZnO [30,31].
The variation of the optical band gap as a function of the N2/Ar
flux ratio is shown in Fig. 8. The values for the band gap were
shifted toward lower energies, from 3.5 to 2.5 eV, as the nitrogen
concentration in the film increased. The optical properties of n-
type semiconductors strongly depend on their electronic behavior
[16,32]. In the case of degenerated semiconductors, such as IZON,
the optical band gap is influenced by changes in the carrier concen-
tration, which for instance, blocks the lowest states of the bottom
of the conduction band, resulting in the Burstein–Moss band-filling
shift [33,34]. In addition, other phenomena are present due to the
many body interactions for high carrier densities, i.e., for the car-
rier concentration with values approximately 1019 cm−3 [35,36], a
band gap narrowing phenomenon is presented due to the effect
of electron–electron and electron-impurity scattering. This phe-
nomenon is called band gap renormalization [6,8,9,37]. Band gap
renormalization can affect the band gap simultaneously with the
Burstein–Moss effect [29,31]. Therefore, the explanation for the
optical band gap decreasing is because, in the case of a highly doped

5. J.J. Ortega et al. / Materials Science and Engineering B 187 (2014) 83–88 87
1.5 2.0 2.5 3.0 3.5
0
2
4
6
8
10
1.5 2.0 2.5 3.0 3.5
0
2
4
6
8
10
1.5 2.0 2.5 3.0 3.5
0
5
10
15
20
25
0N2
/5Ar
5N2
/5Ar
10N2
/5Ar
15N2
/5Ar
(αhν)
2
(x10
14
eV/m
2
)
a) Classical model
(αhν)
2
(x10
14
eV/m
2
)
0N2
/5Ar
5N2
/5Ar
10N2
/5Ar
15N2
/5Ar
b) Adachi model
(αhν)
2
(a.u.)
Energy (eV)
0N2
/5Ar
5N2
/5Ar
10N2
/5Ar
15N2
/5Ar
c) Uv-Vis
Fig. 7. Absorption coefficient plotted as a function of the photon energy for different
N2/Ar gas flux ratios in the reactive atmosphere obtained from the SE parameters
(a) using the classical model, (b) using the Adachi model and (c) directly measured
with a UV–vis spectrometer.
0 1 2 3
2.6
2.8
3.0
3.2
3.4
3.6
Eg
(eV)
Gas flux ratio N2
/Ar
Fig. 8. Average band gap Eg obtained from the values of the intercept at ˛ = 0 of
the linear extrapolations of the absorption coefficient graphs for the different IZON
films.
semiconductor, the effect of band gap renormalization is dominant
and determines the total optical band gap.
4. Conclusions
IZON films were deposited on Si(1 0 0) substrates by radio fre-
quency magnetron sputtering using an oxide ceramic In2O3-ZnO
target. The changes in the structural, electrical and optical proper-
ties of the IZON films were analyzed as a function of the nitrogen
in the film. The IZON films maintained the amorphous structure
even after an N2/Ar gas flux ratio of 15/5, where it tended to crys-
tallize in phases of In2O3 and InN. As a result of the increase in the
nitrogen atomic percentage in the film, the electrical properties
showed slight changes: the resistivity in the film increased from
10−4 to 10−2 cm. The carrier concentration decreased moder-
ately, but it remained over 1020 cm−3. The mobility presented the
same features, and it decreased to nearly half its original value. The
transmittance fell from 80% to 50% in the visible region at the same
time that the absorption coefficient shifted to lower energies. The
films tended to be more absorbent in the visible region. Finally, this
behavior resulted in a shift of the optical band gap toward lower
energies (from 3.5 to 2.5 eV), and the same behavior was found
by an SE characterization using the classical and Adachi dispersion
models. This result was observed after increasing the N2/Ar gas flux
ratio from 0 to 3, which can be potentially utilized for the devel-
opment of IZON based band gap engineering heterostructures and
devices.
Acknowledgments
This work was supported by CONACyT-México; Projects #
CB/2010-105723 and CB/2009-129227. The authors are grateful to
Zacarias Rivera and Marcela Guerrero for their technical assistance.
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