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Photon Absorption Improvement in Reststrahlen Band of Mn1.56Co0.96−x Ni0.48Fe x O4 Series Films

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Photon Absorption Improvement in Reststrahlen Band of Mn1.56Co0.96−x Ni0.48Fe x O4 Series Films

Abstract

Mn1.56Co0.96−xNi0.48FexO4 series films have been fabricated on SiO2/Si(100) substrates by chemical solution deposition and characterized by scanning electron microscopy, and their structural and mid-infrared (IR) properties investigated. The results indicate slight improvement in the microstructure and density of the films with increasing Fe content. The results of Raman spectroscopy showed variation in the local distortion and cation distribution at octahedral sites with elevated Fe content. The IR optical properties of the films were investigated at room temperature in the wavelength range from 1.5 μm to 25 μm. A strong absorption peak corresponding to Reststrahlen band located at 19.5 μm was observed and its absorption intensity found to improve with increasing Fe content in the films. The maximum absorption coefficient was calculated to be about 18,000 cm−1. The results bear technological significance for the design and fabrication of devices for IR detection applications.
Photon Absorption Improvement in Reststrahlen Band
of Mn
1.56
Co
0.96x
Ni
0.48
Fe
x
O
4
Series Films
XIAOBO ZHANG,
1
QIN SHI,
2,3,4
WEI REN ,
2,5
QING ZHOU,
1,6
HEWEI LU,
1
SHUAI BAO,
1
LEI WANG,
2
LIANG BIAN,
2
JINBAO XU,
2
and AIMIN CHANG
2
1.—International Joint Research Center of China for Optoelectronic and Energy Materials, School
of Physics and Astronomy, Yunnan University, Kunming 650091, China. 2.—Key Laboratory of
Functional Materials and Devices for Special Environments of CAS, Xinjiang Key Laboratory
of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics &
Chemistry, CAS, Urumqi 830011, China. 3.—Department of Physics, Shihezi University, Shihezi
832003, China. 4.—University of Chinese Academy of Sciences, Beijing 100049, China. 5.—e-mail:
renw@ms.xjb.ac.cn. 6.—e-mail: zhouqing@ynu.edu.cn
Mn
1.56
Co
0.96x
Ni
0.48
Fe
x
O
4
series films have been fabricated on SiO
2
/Si(100)
substrates by chemical solution deposition and characterized by scanning
electron microscopy, and their structural and mid-infrared (IR) properties
investigated. The results indicate slight improvement in the microstructure
and density of the films with increasing Fe content. The results of Raman
spectroscopy showed variation in the local distortion and cation distribution at
octahedral sites with elevated Fe content. The IR optical properties of the
films were investigated at room temperature in the wavelength range from
1.5 lmto25lm. A strong absorption peak corresponding to Reststrahlen
band located at 19.5 lm was observed and its absorption intensity found to
improve with increasing Fe content in the films. The maximum absorption
coefficient was calculated to be about 18,000 cm
1
. The results bear techno-
logical significance for the design and fabrication of devices for IR detection
applications.
Key words: Thin film, spinel, optical absorption, reststrahlen band, Raman
INTRODUCTION
Over recent decades, transition-metal oxides have
been extensively investigated due to their wide
range of fascinating physical properties and various
physical phenomena, such as spin glass effects,
superconductivity, superparamagnetism, and ferro-
magnetism.
1,2
Among these materials, ternary
spinel oxides of type Mn-Co-Ni-O with general
formula AB
2
O
4
are potential candidates for use in
uncooled infrared (IR) detection
2
due to their excep-
tional negative temperature coefficient, robust ther-
mal stability, moderate resistivity, and rapid
response time.
3
In particular, Mn
1.56
Co
0.96
Ni
0.48
O
4
(MCN) is considered an important composition
because its resistivity is close to the minimum
reported among Mn-Co-Ni-O ternary oxides.
4
More-
over, MCN spinel is a promising material for use in
thermal detection with a wide spectral range. The
thermal, electrical, and optical properties of MCN
film have been extensively studied, and its promis-
ing application in uncooled bolometers systemati-
cally investigated.
5
However, the optical properties,
in particular the optical absorption in Reststrahlen
band, of MCN film have not yet been completely
clarified.
611
Dannenberg’s group reported rest-
strahlen and Raman absorption from 469 cm
1
to
623 cm
1
from Raman scattering measurements
and Fourier-transform infrared (FTIR) spec-
troscopy,
7
as also confirmed by Kong et al.
6
Huang’s
group also studied the optical properties, in partic-
ular the ultraviolet–visible–near IR and IR optical
constants, of MCN films using spectroscopic
(Received January 4, 2017; accepted April 26, 2017;
published online May 10, 2017)
Journal of ELECTRONIC MATERIALS, Vol. 46, No. 8, 2017
DOI: 10.1007/s11664-017-5556-z
Ó2017 The Minerals, Metals & Materials Society
5349
ellipsometry (SE).
1,11
Furthermore, iron (Fe) ion
exhibits some special features such as the change-
able valency of Fe
2+
and Fe
3+
and flexible occupation
of sublattices at A- and B-sites of the spinel
structure.
9
Therefore, introduction of Fe ion into
MCN spinel may result in interesting changes in
the electrical and optical properties of the material;
For example, Nikolic
´et al. reported the far-IR
optical properties of (Mn,Ni,Co,Fe)
3
O
4
ceramic sam-
ples using FTIR spectrometry.
8,10
In this study,
Mn
1.56
Co
0.96x
Ni
0.48
Fe
x
O
4
(MCNF, x= 0, 0.1, 0.15)
series spinel thin films were synthesized on SiO
2
/
Si(100) substrates by chemical solution deposition.
The films were characterized and their structural
and mid-IR optical properties systematically
explored. Furthermore, the effects of the Fe concen-
tration on the crystallinity, density, local distortion
and cation distribution at octahedral sites, and
intensity of absorption peak of Reststrahlen band
were systematically investigated.
EXPERIMENTAL PROCEDURES
MCNF spinel films were synthesized by chemical
solution deposition. The raw solution was prepared
by mixing analytical-grade Mn(CH
3
COO)
2
Æ4H
2
O
(99% purity), Co(CH
3
COO)
2
Æ4H
2
O (99% purity),
Ni(CH
3
COO)
2
Æ4H
2
O (99% purity), and FeCl
3
(99%
purity) in molar ratio of Mn:Co:Ni:-
Fe = 1.56:(0.96 x):0.48:x(x= 0, 0.1, 0.15; corre-
sponding samples labeled S1, S2, and S3), followed
by addition of the mixture into glacial acid solution.
The contents were then heated at 50°C and stirred
for 3 h until complete dissolution of the mixture.
The concentration of the final solution was adjusted
to 0.15 M. The precursor solution was then filtered
through a 0.45-lm syringe to remove residual
particulates from the solution. The solution was
spin-coated on SiO
2
/Si(100) substrates at 500 rpm
for 5 s and 4000 rpm for 20 s, followed by drying in
a furnace at 350°C for 5 min. The spin-coating and
drying procedures were repeated for ten times, then
the samples were annealed in air at 750°C for 1 h,
which resulted in formation of MCNF spinel oxide
films.
The crystalline phase of the thin-film materials
was identified by x-ray diffraction (XRD, Bruker
D8). The film morphology was characterized by
scanning electron microscopy (SEM, Supra55VP,
Zeiss). To verify the distribution of various elements
in the films, energy-dispersive x-ray (EDX) spectra
and elemental maps were obtained using an EDX
spectroscope (LEO 1430VP). Raman spectra were
measured using a Raman spectrometer (NRS-1000)
with Ar
+
laser (20 mW, 532 nm) as excitation source
in backscattering configuration. The optical proper-
ties were assessed by SE (SENTECH SE850) in the
IR spectral range at angle of incidence of 70°and
room temperature.
RESULTS AND DISCUSSION
The XRD patterns of the MCNF series films are
shown in Fig. 1, where the peaks were labeled by
referring to the standard Joint Committee on
Powder Diffraction Standards (JCPDS) card for
NiMn
2
O
4
(No. 01.1110). The MCNF films exhibited
polycrystalline structure with preferred (311) ori-
entation. The XRD patterns clearly indicate that
substitution of Co with Fe did not lead to a change
in the original spinel structure. This can be
attributed to the fact that the ionic radius of Fe
3+
(0.0645 nm) is close to that of Co
3+
(0.061 nm) and
Fe
3+
replaces Co
3+
at octahedral site.
12
The peaks of
the Fe-containing films showed slight blue-shift
compared with those of pure MCN film, indicating
increase of the lattice constants. This lattice expan-
sion can be explained as follows: for MCN spinel, a
typical cation distribution might be
[Co
0.34
2+
Mn
0.66
2+
]
T
[Ni
0.48
2+
Co
0.62
3+
Mn
0.42
3+
Mn
0.48
4+
]
O
O
4
2
.
12
As
more Fe
3+
substitutes Co
3+
at octahedral site, as
well as more Fe
2+
(0.078 nm) replacing Co
2+
(0.0745 nm) at the tetrahedral site, the calculated
lattice constants increase from 8.308 A
˚for S1, to
8.315 A
˚for S2, to 8.331 A
˚for S3. Moreover, notably,
the peak widths also broadened from S1 to S3,
indicating shrinkage of the MCNF grains. The
average crystallite sizes calculated by using the
Scherrer equation for the (311) peak tended to
decrease from 32.970 nm for S1, to 28.665 nm for
S2, to 25.086 nm for S3, indicating degradation of
film crystallinity.
Figure 2shows the surface morphology of the
MCNF series films, revealing the existence of
particle chains and pores for S1.
13
However, this
particle chain/pore structure tended to disappear on
addition of Fe. The average particle size decreased
from S1 to S3. The film density and surface mor-
phology correspondingly improved, which could
help minimize the effects of ambient humidity or
Fig. 1. XRD patterns of MCNF series films.
Zhang, Shi, Ren, Zhou, Lu, Bao, Wang, Bian, Xu, and Chang5350
absorption of oxygen from the surrounding air onto
the film. The insets in Fig. 2show cross-sectional
SEM images of the thin films, with corresponding
film thickness values of around 400 nm, 395 nm,
and 390 nm for S1, S2, and S3, respectively. The
difference in the film thickness values is small and
will not affect the optical properties of the films as
measured by SE.
EDX spectroscopy was performed to quantify the
composition of the samples. The atomic concentra-
tions of Mn, Co, Ni, and Fe in the series of films
were obtained and are presented in Table I. Based
on these values, the Mn:Co:Ni(:Fe) ratio was calcu-
lated to be 50:33:17 for S1, 50:32:16(:2) for S2, and
50:29:17(:4) for S3.
Figure 3clearly demonstrates that the surface
morphology of film S3 was the least defective.
Elemental distribution maps of Mn, Co, Ni, and Fe
for film S3 are shown in Fig. 3. These maps confirm
that the distribution of these elements was roughly
uniform, except for some holes randomly distributed
on the surface of film S3.
Figure 4shows the Raman spectra for all the
MCNF films at room temperature. According to
previous studies,
1416
the strongest peak at
645 cm
1
is assigned to A
1g
vibration mode, which
originates from the symmetric B–O stretching
vibration and reflects the local lattice bonding of
Mn
3+
–O
2
. The peak centered at 490 cm
1
is
assigned to the F
2g
vibration modes, which reflects
the symmetric bending vibration of Mn
4+
–O bond at
octahedral site. The peak at 820 cm
1
is ascribed to
Si–O bond, which comes from the substrate. Clearly,
the intensity of A
1g
peak improves with Fe addition
and its position shifts slightly toward higher
wavenumber from S2 to S3. When Fe
3+
takes the
place of Co
3+
, an increase in the length of the B–O
bond brings about a reduction of the bond strength
and the corresponding red-shift of the Raman
frequencies.
13
The radius of Fe
3+
(0.0645 nm) is
similar to that of Mn
3+
(0.0645 nm), compared with
that of Co
3+
(0.061 nm), thus inclusion of more Fe
3+
at octahedral site results in reduction of the John
Teller lattice distortion, as confirmed by the
increased peak intensity and narrower peak width
of A
1g
mode. The intensity and position of F
2g
peak
remained stable at elevated Fe content, indicating
insignificant effect of Fe
3+
addition on the content of
Mn
4+
.
SE was used to obtain the optical properties of the
MCNF series films. SE measures the amplitude u
and phase D, which are related to the optical
constants by the equation q¼tanðuÞeiD, where qis
the complex reflectivity ratio. From the ellipsomet-
ric measurements, the pseudodielectric function
(e¼e1þie2) can be obtained by using the following
equation
11
:
e¼sin2/þsin2/tan2/ð1qÞ2
ð1þqÞ2;ð1Þ
where /is the angle of incidence.
Fig. 2. SEM images and cross-sectional SEM images (inset) of MC
0.96x
NF
x
series films with (a) x= 0, (b) x= 0.1, and (c) x= 0.15.
Photon Absorption Improvement in Reststrahlen Band of Mn
1.56
Co
0.96x
Ni
0.48
Fe
x
O
4
Series
Films
5351
To attain precise optical constant curves for the
thin films from ellipsometry, the data processing is
critical. An appropriate model helps significantly in
fitting the data. In this work, a Drude–Lorentz
model of the MCNF film was selected because it can
precisely describe the contribution of both free and
bound electrons responsible for the optical proper-
ties. A three-layer (air/MCNF/SiO
2
) structure was
used to describe the samples, and the mid-IR optical
properties of the MCNF films are shown in Fig. 6.
The Drude–Lorentz oscillator formula below
approximately describes the dispersion relation of
the complex dielectric function (e):
e¼e1þie2¼e1x2
p
t2þixstþX
n
k¼1
X2
p
X2
otðtþiXsÞ
"#
;
ð2Þ
where e
1
,e
1
, and e
2
represent the high-frequency
dielectric constant, and the real and imaginary
parts of the dielectric constant, respectively.
t¼x=2pc=1
=kreflects the influence of the internal
electric field on the Lorentz oscillator.
xp¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Ne2=e0m
pand xs¼e=umshow the relation-
ship among the concentration Nof the free carriers,
the mobility lof the free carriers, and the effective
mass m
*
of the free carriers. In the last part, X
p
,X
o
,
and X
s
represent the amplitude, center frequency,
and damping factor of the Lorentz oscillator, respec-
tively. The dielectric constant can then be trans-
formed to optical properties (refractive index nand
extinction coefficient k) by using the following
equations:
n¼1
ffiffiffi
2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffiffiffiffiffiffiffiffiffiffiffiffiffiffi
e2
1þe2
2
qþe1
r;ð3:1Þ
Table I. EDX spectroscopy results for MCNF series thin films
Element
S1 S2 S3
at.% Error (%) at.% Error (%) at.% Error (%)
Mn 50.45 1.65 49.74 1.27 49.95 1.19
Co 32.98 1.34 31.58 1.05 28.83 0.99
Ni 16.57 0.85 16.27 0.61 16.67 0.69
Fe 2.41 0.22 4.55 0.25
Fig. 3. EDX spectroscopy surface elemental maps for film S3.
Zhang, Shi, Ren, Zhou, Lu, Bao, Wang, Bian, Xu, and Chang5352
k¼1
ffiffiffi
2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffiffiffiffiffiffiffiffiffiffiffiffiffiffi
e2
1þe2
2
qe1
r:ð3:2Þ
The mean square error (MSE) is a measure of the
fitting result, described as follows:
17
MSE ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
2LMX
N
i¼1
umod
iuexp
i
rexp
u;i
!
2
þDmod
iDexp
i
rexp
D;i
!
2
2
43
5
v
u
u
u
t;
ð4Þ
where Land Mare the fitting points and measured
parameters; u
mod
,D
mod
and u
exp
,D
exp
are the
modeled and measured ellipsometric angles, respec-
tively; and r
u
and r
D
are the standard deviations of
the measured ellipsometric angles. Figure 5shows
the measured points and fit lines overlapping each
other for all the films in the series. Furthermore, the
detailed fitting parameters of the Drude–Lorentz
oscillator formula are listed in Table II.
The refractive index nand extinction index kwere
determined from uand D. Figure 6a shows that n
generally dropped with increasing wavelength from
1.5 lmto16lm for all the films, dramatically
increased from 16 lmto21lm due to anomalous
dispersion, then remained in the range of 3.4 to 3.7
at wavelength of 21 lmto25lm. Notably, the
decreasing tendency of nfor wavelength of 1.5 lmto
16 lm is similar to that reported in literature,
1
Fig. 4. Raman spectra of MCNF series thin films.
Fig. 5. Experimental and fit SE spectra (/and D)ofMC
0.96x
NF
x
series films with (a) x= 0, (b) x= 0.1, and (c) x= 0.15.
Photon Absorption Improvement in Reststrahlen Band of Mn
1.56
Co
0.96x
Ni
0.48
Fe
x
O
4
Series
Films
5353
although that measurement stopped at 7 lm. More-
over, the nvalues reported in literature are higher
than those of S1, which may be due to the different
film deposition method. On addition of Fe to the
films, the nvalues of S2 and S3 became comparable
to that reported in literature.
1
The improvement in
nvalue with increasing Fe content for S2 and S3
can be ascribed to the significant reduction of pores
in these films, which correspondingly reduces the
effective dielectric constant.
9
Another reason may
be the lower grain size of the Fe-containing films.
13
The minimal value at 16 lm is due to the existence
of a powerful dielectric loss function.
18
Figure 6b shows the kcurves of the MCNF films
in the wavelength range from 1.5 lmto25lm. A
significant absorption region is centered at 19.5 lm
and covers a broad region of 16 lmto22lm, which
can be attributed to reststrahlen absorption.
18
Kong
et al. reported four IR-active frequencies of MCN
(one of them, 555 cm
1
, corresponds to the
wavelength of 18.1 lm).
6
Nikolic
´et al. also reported
the occurrence of two vibration modes (at around
620 cm
1
and 470 cm
1
, corresponding to wave-
lengths of 16.1 lm and 21.3 lm) for MCN spinel
oxide.
8
The intensity of the highest absorption
apparently increased from S1 to S3, probably due
to the following reasons: first, the absorption peak
corresponding to Fe
3+
–O bonds is also found
between 470 cm
1
and 620 cm
1
.
19
The content of
Fe in the films generally increased, while that of Mn
remained constant. Obviously, the optical absorp-
tion between 470 cm
1
and 620 cm
1
was enhanced
with addition of more Fe to the film. Second, the
variation of the cation distribution coupled with the
reduced lattice distortion caused by the replacement
of Co
3+
with Fe
3+
, as evidenced by the Raman
results, may result in improved reststrahlen absorp-
tion. Lutz et al. pointed out that all the allowed IR
modes represent typical lattice vibrations resulting
from the contribution of all atoms and forces of the
Table II. Fitting parameters of Drude–Lorentz oscillator formula
S1 S2 S3
Thickness (nm) 200 157.26 191.62
e10.96 0.99 0.84
e0
10.001 0.016 0.071
xp(1/cm) 1004.2 892.98 1017.54
xs(1/cm) 351.53 0.35 36.59
Xo(1) (1/cm) 12,523.33 13,599.79 12,919.55
Xp(1) (1/cm) 4591.65 3695.59 4600.17
Xs(1) (1/cm) 4500.07 420.02 8.09
Xo(2) (1/cm) 452.24 5904.88 1725.74
Xp(2) (1/cm) 816.94 271.93 1715.38
Xs(2) (1/cm) 57.23 156,544.30 2056.74
Xo(3) (1/cm) 1130.47 1599.63 3282.88
Xp(3) (1/cm) 1276.36 1506.57 2070.23
Xs(3) (1/cm) 1004.25 2676.64 1929.69
MSE 1.37 1.86 1.73
Fig. 6. (a) Refractive index nand (b) extinction coefficient kcurves of MCNF series films in mid-infrared band.
Zhang, Shi, Ren, Zhou, Lu, Bao, Wang, Bian, Xu, and Chang5354
spinel structure, though some of them can be more
affected by the metal ions at octahedral or tetrahe-
dral site.
19
However, further studies are definitely
needed for comprehensive understanding of the
distribution of cations due to Fe doping. Finally,
the improved film quality proved by SEM images
can promote the absorption intensity. Addition of Fe
to the MCN films generated smaller grains, facili-
tated densification of the thin film, and improved
the film morphology, thus increasing oscillators at
the absolute volume. Participation of more oscilla-
tors in lattice vibrations leads to occurrence of more
prominent optical absorption. Moreover, the absorp-
tion coefficient can be calculated by using the
formula a=4pk/k, where kis the wavelength. Based
on this formula, the strongest absorption coefficient
is 17,974 cm
1
, calculated for S3 at 19.5 lm.
CONCLUSIONS
MCNF series films were synthesized on SiO
2
/
Si(100) substrate by chemical solution deposition.
Pure MCN film was polycrystalline structured with
poriferous surface; however, increasing Fe content
coupled with decrease of Co in the films helped
smoothen the film morphology and improved the
reststrahlen absorption, although the crystallinity
degraded. The strong absorption peak located at
19.5 lm is attributed to characteristic lattice vibra-
tions. The heightening of the peak with Fe doping
was due to the increasing number of oscillators or
the disorder of the lattice structure. The maximum
value of awas approximately 18,000 cm
1
. Addition
of Fe to MCN film modified the microstructure, film
densification, lattice distortion, and cationic distri-
bution, and helped to improve the reststrahlen
absorption.
ACKNOWLEDGEMENTS
This work was supported by the Thousand Youth
Talents Plan (Y42H831301), National Natural Sci-
ence Foundation of China (61504168), West Light
Foundation of the Chinese Academy of Sciences
(XBBS-2014-04), Science and Technology Program
of Hebei Province (D2016403064, 16044601Z), He-
bei Outstanding Young Scholars and Hebei Science
and Technology Support Program (15211121), and
Foundation of Director of Xinjiang Technical Insti-
tute of Physics & Chemistry, CAS (2015RC010).
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Photon Absorption Improvement in Reststrahlen Band of Mn
1.56
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Ni
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Fe
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O
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Series
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... Manganese-based transition metal oxide (MTMO) ceramics, including Mn-Co-Ni-O, Mn-Co-Cu-Ni-O, Mn-Cu-Zn-Ni-O and Mn-Fe-Ni-O [1][2][3][4][5][6], have been applied as thermistor materials in industry for years. Their abundancy and appealing and tunable physical properties, including small-polaron-hopping transport, variation of cationic distribution at the tetrahedral (A) site and octahedral (B) site and infrared absorption, have contributed to attract intensive research [7][8][9][10]. To meet the requirements from industry and military applications, namely for achieving fast and precise temperature measurements, during the last decades researchers tried to prepare low-dimensional thermo-sensitive materials, combining thin/thick films, ultrathin chip and micro-beads [6,8,11,12]. ...
Article
We investigate the synthesis of Mn2Zn0.25Ni0.75O4 (MZN) thin films by radio frequency (RF) sputtering method at different sputtering powers. The variation of the film morphology, the crystalline structure, the cationic distributions and the optical properties are reported and explained by the variation of the film growth rate. Scanning electron microscopy results indicate that a too fast growth rate of the film results in dense surface defects or the degradation of the film crystallinity. X-ray photoelectron spectroscopy analysis suggests that oxygen defects are almost inevitable in such thin films deposited by RF sputtering. At a moderate (120 W) sputtering power the Mn²⁺ content increases 10–20% and the Mn⁴⁺ content is accordingly reduced. The detailed cationic configuration is proposed, where Zn²⁺ ions occupy both the tetrahedral site and the octahedral site. Raman and spectroscopic results allows asserting the relation between Mn valence states and the sputtering power.
Article
Herein, NiMn2O4 (MNO) spinel oxide thermistor films were synthesized on a SiO2/Si substrate via annealing the electron beam evaporated Mn–Ni–Mn metal trilayers in air at different temperatures. The X-ray diffraction (XRD) results indicate that polycrystalline spinel-structured MNO thermistor films were formed. The surface particle size of the series MNO films quickly reduced from ~300 to ~120 nm with a temperature increase from 650 to 750 °C, and then, slowly reduced to 80 nm or even smaller with a temperature increase from 750 to 950 °C. Specifically, 750 °C anneal formed the spinel MNO film with largest B value of 5067 and Ea value of 0.4366. The proposed synthesis route for MNO spinel oxide film has been proven to be feasible.
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The optical properties of spinel Mn1.56Co0.96Ni0.48O4 thin films have been investigated by spectroscopic ellipsometry in the temperature range from 20 to 260 degrees C. By fitting the measured ellipsometric parameter data with a three-layer model by Tauc-Lorentz oscillator dispersion formula, the refractive index and extinction coefficient of the thin films are determined in the spectral range of 280-850 nm. The refractive index decreases in the short-wavelength region but increases in the long-wavelength region with increasing temperature. The extinction coefficient increases with increasing temperature in all visible region.
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Mn1.4Co1.0Ni0.6O4 (MCN) thin films are prepared by RF sputtering deposition method on amorphous Al2O3 substrate. Microstructure and X-ray photoelectron spectroscopy analyses suggest improvements in crystallinity and stoichiometry for MCN films with post-annealed process. Infrared (IR) optical constants of the MCN films are obtained by IR spectroscopic ellipsometer (SE) in the range of 1500 cm-1 to 3200 cm-1 (2.8-6.7 μm). The derived effective charge supports the increase of the oxidation after annealing. The dielectric function of the films is also extracted by SE in the range of 300-1000 nm adopting a double Lorentz model together with a Tauc-Lorentz model. The mechanism in electronic transition process is discussed based on the variation observed in the optical absorption spectra of the as-grown and post-annealed samples. The optical absorption peaks located at 1.7 eV, 2.4-2.6 eV, and 3.5-4 eV are attributed to the charge-transfer transitions of 2p electrons of oxygen ions and 3d electrons of Mn and Co ions. Our results are very important to understand the optoelectronic mechanism and exploit applications of metal oxides.
Article
Full-text available
The influence of the Fe on the microstructure, electrical and dielectric properties of Ni0.6Cu0.4FeyMn2−yO4 (0.1 ≤ y ≤ 0.5) negative temperature coefficient (NTC) thermistors prepared by well known simple chemical co-precipitation method were studied. The replacement of manganese by iron plays an important role in changing the lattice parameter, X-ray density, sintered density, porosity, DC resistivity at different temperatures and dielectric properties at different frequencies. The X-ray and sintered density increased linearly and porosity decreased with iron. The room temperature resistivity of nickel copper manganite NTC ceramic decreased from 1 MΩ cm to 68 KΩ cm and dielectric constant increased from ∼9 × 107 to 1.5 × 109 at 20 Hz as iron content increased.
Article
(Mn1.56Co0.96Ni0.48O4)1 − x(LaMnO3)x serial thin films have been synthesized on Si substrates through the chemical solution deposition method. The film crystal structures varied from a pure spinel phase to a mixed perovskite/spinel phase according to the increase in La concentration. Similarly, film density and surface roughness were improved with an increase in La concentration. According to the optical constant spectra of the composite thin films, two strong absorption structures appeared in the visible spectral range. Resistance–temperature curves indicated that all of the prepared thin films have negative temperature coefficient effects. In particular, the x = 0.5 film offered the lowest resistance and highest B25/50 constant at 4022 K, compared with the remaining films.
Article
Mn1.56Co0.96Ni0.48O4 (MCN) thin films with cubic spinel phase were prepared by chemical solution deposition. The variable temperature electrical properties showed that the temperature coefficient of resistance reached -4% K-1 at 295 K. The immersed and non-immersed infrared (IR) detectors based on MCN films were fabricated and characterised. The experiment results indicated that the performance of immersed detector was better than the non-immersed one. It exhibited a responsivity of 4.4×103 V W-1, detectivity of approximately 5×108 cm Hz1/2 W-1, and thermal time constant of 18 ms at 295 K and 10 Hz. These results provide a possible avenue to develop a high-performance uncooled IR bolometer based on the thermistor materials.
Article
Mn1.56Co0.96Ni0.48O4 (MCN) free-standing ultrathin chips are successfully fabricated by using screen printing. The structure, electrical and IR absorption properties have been investigated as a function of the sintering temperature. The X-ray diffraction, X-ray photoelectron spectroscopy and field emission scanning electron microscope analyses show remarkable improvements in crystallinity, stoichiometry and relative density for MCN chips. From the electrical experimental results, it is found that the resistivity of the chip samples sharply decreases from 2660 to 21.9 Omega cm. Such feature is attributed to the decrease in the grain boundary resistance and the increase in Mn3+/Mn4+ ratio with increasing sintering temperature. Furthermore, Fourier transform infrared spectra show that the absorption bands nu 1 (around 1160 cm(-1)) at 1100 degrees C almost disappears while the others still remain. It is ascribed to the increase in the density of samples. The intensities of symmetry bands nu 2 (850 cm(-1)) are also found to be consistent with an increase in ratio of Mn3+ to Mn4+ ions with increasing sintering temperature.
Article
Polycrystalline films of Mn1.56Co0.96Ni0.48O4 (MCN) were evaluated for uncooled bolometric applications grown by chemical solution deposition on amorphous Al2O3 substrate. The microstructural characterizations showed that the films were of excellent crystallization and compact surface morphology. Electrical results showed that the temperature coefficient of resistance reached −3.8%/K at 295 K. Low excess noise (normalized Hooge parameter αH/n of 7.6 × 10−28 m3) was achieved owing to the good epitaxial quality of the prepared films. Infrared bolometers were fabricated to evaluate the performance on infrared detection. It exhibited a noise equivalent temperature as low as 2.1 × 10−7 K/Hz1/2, responsivity of 330 V/W, detectivity of 0.6 × 108 cm Hz1/2/W, and noise equivalent power of 3.7 × 10−10 W/Hz1/2 at 30 Hz. The feasibility of the MCN films was demonstrated to be used for uncooled bolometric applications by thermal imaging. One can expect to get a responsivity of about 1 × 103 V/W and detectivity higher than 6 × 108 cm Hz1/2/W at 30 Hz for thermally isolated MCN film bolometer. The results of MCN showed its great potentiality for future room-temperature detection.
Article
NiMn2−xMgxO4 (0 ≤ x ≤ 0.4) ceramics have been studied by powder x-ray diffraction (XRD), infrared (IR) spectroscopy, and thermogravimetric analysis. NiMn2−xMgxO4 ceramics are all single-phase with spinel structure. XRD and IR spectroscopy results indicate that Mg2+ ions occupy A- and B-site of spinel lattice, which inhibits the formation of cation vacancies. Moreover, Mg2+ substitution enhances the tolerance of the oxidation in air. As a result, Mg substitution leads to a significant increase in ρ25, temperature coefficient of resistivity B25/85, and activation energy, which improves the aging property of NiMn2−xMgxO4 negative temperature coefficient thermistors.
Article
Single phase complex spinel (Mn, Ni, Co, Fe)3O4 samples were sintered at 1050, 1200 and 1300 °C for 30 min and at 1200 °C for 120 min. Morphological changes of the obtained samples with the sintering temperature and time were analyzed by X-ray diffraction and scanning electron microscope (SEM). Room temperature far infrared reflectivity spectra for all samples were measured in the frequency range between 50 and 1200 cm−1. The obtained spectra for all samples showed the presence of the same oscillators, but their intensities increased with the sintering temperature and time in correlation with the increase in sample density and microstructure changes during sintering. The measured spectra were numerically analyzed using the Kramers–Krönig method and the four-parameter model of coupled oscillators. Optical modes were calculated for six observed ionic oscillators belonging to the spinel structure of (Mn, Ni, Co, Fe)3O4 of which four were strong and two were weak.