electrical conduction mechanisms and dielectric constants...
TRANSCRIPT
Electrical conduction mechanisms and dielectric constantsof nanostructured methyl violet 2B thin films
H. M. Zeyada • M. M. Makhlouf
Received: 24 August 2014 / Accepted: 23 February 2015 / Published online: 4 March 2015
� Springer-Verlag Berlin Heidelberg 2015
Abstract The uniform thin films of methyl violet 2B,
MV2B, with thicknesses ranged from 96 to 300 nm, have
been successfully prepared by spin coating technique.
X-ray diffraction showed that the powder and pristine thin
film of MV2B have amorphous structure. The amorphous
pristine films become polymorphous nanocrystallites after
annealing at 433 K. The electrical properties of MV2B thin
films have been studied. There are a number of operational
environments where the performance of MV2B thin films
is likely to be affected significantly on their electrical
properties and dielectric constants such as the differences
of film thicknesses, temperatures and frequencies. It was
found that the DC conductivity of MV2B films increases
with increasing temperature. The extrinsic conduction
mechanism is operating in temperature range of
288–360 K with activation energy of 0.16 eV, and the
conduction in extrinsic region is explained via applying
Mott model for variable range hopping. The intrinsic
conduction mechanism is operating in temperatures
[360 K with activation energy of 0.91 eV. The conduction
in intrinsic region is explained by applying band to band
transitions theory. The AC electrical conductivity and di-
electric relaxation of MV2B thin films in the temperature
range 365–473 K and in frequency range 0.1–100 kHz has
been also studied. It has been shown that theoretical curves
generated from correlated barrier hopping, CBH, model
gives the best fitting with experimental results. Analysis of
these results proved that conduction occurs by phonon-
assisted hopping between localized states and it is per-
formed by bipolaron hopping mechanism. The temperature
and frequency dependence of both the real and imaginary
parts of dielectric constant have been investigated.
1 Introduction
Organic semiconductors have been extensively studied in
the last two decades due to their unique electronic, optical
and photoelectric properties compared with conventional
inorganic semiconductors [1–4]. One of the main advan-
tages of organic semiconductors is their easy and low cost
in fabrication; therefore, they posses applications in elec-
tronics and optoelectronic devices. Organic semiconduc-
tors can open up avenues for a variety of industrial
applications with large quantities by simple techniques [5,
6]. However, most of fundamental electrical properties for
organic semiconductors have not yet sufficiently been
clarified since most traditional measuring techniques de-
veloped for inorganic semiconductors are not necessarily
applicable to them. In order to improve the performance of
the organic devices, it is necessary to obtain a deep insight
into electronic processes in the organic semiconductors and
junction properties of organic semiconductors/metal
interfaces.
Methyl violets are organic compounds belonging to the
triphenylmethane family that is mainly used as aromatic
dyes [7]. Methyl violets are mixtures of tetramethyl, pen-
tamethyl and hexamethyl pararosanilines, and by blending
H. M. Zeyada � M. M. Makhlouf
Department of Physics, Faculty of Science, University of
Damietta, New Damietta 34517, Egypt
M. M. Makhlouf (&)
Department of Physics, Faculty of Applied Medical Sciences at
Turabah, Taif University, Taif 21995, Saudi Arabia
e-mail: [email protected]; [email protected]
M. M. Makhlouf
Department of Physics, Demiatta Cancer Institute,
Damietta, Egypt
123
Appl. Phys. A (2015) 119:1109–1118
DOI 10.1007/s00339-015-9076-5
the different versions depending on the amount of attached
methyl groups to the amine functional group results in
different types of methyl violet dyes, where tetramethyl
(four methyls) is known as methyl violet 2B, pentamethyl
(five methyls) is known as methyl violet 6B and hexam-
ethyl (six methyls) is known as methyl violet 10B [8, 9].
Methyl violet 2B (MV2B) which is used in the present
work has molecular formula C24H28N3Cl and its molecular
structure is shown in Fig. 1. The skeleton of MV2B has an
extended p-conjugated system due to the delocalization of
electrons in the benzene ring, methyl and amine functional
groups and that leading to a wide range of high optical
absorption in visible spectrum. The spectral characteristics
and nonlinear optical properties of MV2B dissolved in
ethanol and in a dye-doped polymer film have been studied
[10]. The linear optical constants, optical dispersion pa-
rameters and dielectric constant of MV2B thin films de-
posited by the spin coating technique also were measured
[11]. All of these studies showed that MV2B has remark-
able optical absorption in the visible region of spectrum
and low optical band gap recommending it as optical
limiting material in optoelectronic devices. Due to the
richness in its properties, it has assumed a peculiar role in
different fields of disciplines such as applications in a dye
sensitized solar cell, DSSC, [12], solar cells for energy
conversion [13] display devices [14], photo-resistors [15],
nonlinear optical devices [10] and gas sensors [16].
Thin films of dyes have been extensively prepared
adopting many techniques including spin coating [11],
thermal evaporation [17], magnetron sputtering [18], sol–
gel [19], chemical vapor deposition [20] and spray py-
rolysis [21]. Application of any technique for thin film
formation depends on molecular size of the dye; spin
coating, sol–gel and spray pyrolysis techniques are applied
for large size molecules other techniques are applied for
small size molecules. With these techniques, thin films of
different thicknesses can be successfully prepared, surface
morphologies and crystallite sizes, electrical and optical
properties depend on applied processing parameters. The
parameters controlling the properties of thin films are as
follows: structure [22, 23], composition [24, 25], film
thickness [26, 27], faults probability [28] and the presence
of impurities [29].
Annealing and substrate temperatures are considered as
processing variables that are used to influence structural
parameters such as: volume fraction of crystallized and
second phase, grain size and its shape, and inter-particle
spacing. Irradiation by ionizing particles such as electrons
or ions and electromagnetic waves such as X and c-rays,laser and UV irradiation may introduce structural defects or
induce phase transformation in the materials depending on
energy of the incident radiation.
In a previous work [11], we investigated the effect of
annealing temperatures on optical constants of MV2B thin
films manufactured by the spin coating technique. The
energy band gap of 1.82 eV, absorption characteristic and
dispersion parameters of MV2B films recommended it for
applications in semiconductors devices applications. In the
present paper, we report on the studies of electrical trans-
port mechanisms, thermoelectric power measurements and
dielectric constants of MV2B thin films prepared by the
spin coating technique and also the influence of different
processing parameters such as film thicknesses, frequencies
and temperatures on the electrical parameters and dielectric
constants. These studies are capable of providing consid-
erable information in order to improve the performance of
organic devices, and it may also be beneficial for the fab-
rication of photoelectric organic devices.
2 Experimental details
2.1 Materials
A green powder of MV2B (IUPAC name: N-(4-(bis(4-
(dimethylamino) phenyl) methylene) cyclohexa-2,5-dien-
1-ylidene) methane aminium chloride) was purchased from
Sigma-Aldrich Co. (USA) and was used in as-received
condition without any further purification. Spectroscopic-
grade ethanol was chosen as a solvent because it possesses
a good solubility for MV2B.
2.2 Thin films preparation
The as-received MV2B dye in a powder form with dif-
ferent weights of 40, 50, 75, 100 and 175 mg individually
was dissolved in 5 mL absolute ethanol and filtered.
Cleaned ordinary glass slides were used as substrates for
depositing films for XRD analysis. The spin coating
NCH3H3C
NH2N
CH3
H3CCl
Fig. 1 Molecular structure of methyl violet 2B (MV2B)
1110 H. M. Zeyada, M. M. Makhlouf
123
technique had been adopted for depositing uniform thin
films onto flat glass substrates. The speed of spin coating
machine was adjusted to 2800 rpm. A drop from each cast
was dropped onto the rotating substrate to form films of
different thickness. The films were dried for 48 h in dark
and at room temperature which results in a film of uniform
thickness. The thickness of deposited films was determined
accurately using an interferometer technique [30]. The
obtained thicknesses were 96, 125, 145, 152 and 300 nm.
The Ohmic contacts at the two ends of MV2B thin films
that used in electrical measurements were provided under
vacuum by thermal evaporation of pure gold using
molybdenum boat.
2.3 Experimental techniques
The structural analysis of MV2B in powder form, pristine
and annealed thin films were analyzed by XRD system
(modelX’ Pert Pro, PhilipsCo.) equippedwithCu target. The
filteredCuKa radiation (k = 1.5408 A)was used. TheX-ray
tube voltage and current were 40 kV and 30 mA, respec-
tively. The transport properties includemeasurements of DC
and AC electrical resistivity, q, of MV2B thin films with
different thicknesses. These measurements were carried out
by adopting the two-point probe technique. The dark elec-
trical resistivity, q, can be calculated from the relation:
q ¼ Rwd
Lð1Þ
where L, w, d and R are the film length, width, thickness
and electrical resistance between the two Au electrodes,
respectively. The resistance of the MV2B films as a func-
tion of temperature was measured in the temperature range
303–408 K using high impedance Keithley 617 pro-
grammable electrometer.
The thermoelectric power, S, of MV2B thin films was
measured by depositing masked rectangular thin films of it
onto clean optical flat ordinary glass substrates. The sample
had a dimension &3 9 0.5 cm2. The contacts were made
by evaporating thick pure Cu electrodes on the ends of
MV2B film. A temperature gradient was produced by
heating one end of the film. The resulting potential dif-
ference, DV, was measured by sensitive digital voltmeter
(Keithley Model 182). The temperature difference, DT, wasmeasured by NiCr-NiAl thermocouple. The thermoelectric
power (Seebeck coefficient) was measured by using the
differential technique based on the following equation [31]:
S ¼ DVDT
ð2Þ
The temperatures T1 and T2 of the two ends were increased
by using two different high power resistances R1 and R2 as
a heat source and a heat sink across the thin film under test.
Samples of sandwich structure Au/MV2B/Au are used
for SCLC measurements (cross-plane measurements). The
gold electrodes are considered as Ohmic contact. The
thickness of MV2B layer is 300 nm and that of gold
electrodes is about 100 nm. The active area of the device is
4 9 10-6 m2. The current was measured by high impe-
dance Keithley 617 programmable electrometer. The in-
dependent stabilized DC power supply of the Keithley 617
electrometer was used as a power supply. All measure-
ments were taken in dark at different temperatures in am-
bient atmosphere.
The AC measurements aimed to investigate the AC
conductivity, rAC, of MV2B films as a function of fre-
quency in the range 1–700 kHz, as well as, in the tem-
perature range 300–393 K. The thickness of MV2B layer is
300 nm, and all measurements are taken under dark inside
a tubular furnace. The AC measurements were taken using
Stanford LCR meter model SR720. This LCR meter can
measure the capacitance, C, the resistance, R, the dissipa-
tion factor, tand, (displayed as D) and the quality factor, Q,
in both of parallel and series modes. The bridge SR720 also
measures the impedance, Z, of the device under test by
measuring the voltage across the device and the current
through it. The ratio of voltage to current is equal to the
complex impedance.
3 Results and discussion
3.1 X-ray diffraction analysis
Figure 3 shows the XRD pattern of MV2B in powder form,
pristine film and annealed film at 433 K with a soaking
time of 1 h. The films are of thickness 300 nm. It is ob-
served that the powder form and pristine films of MV2B
have a halo in the 2h range of 13�–37�, indicating their
amorphous structure. Annealing of MV2B film at 433 K
with soaking time of 1 h results in partial transformation of
amorphous MV2B into nanocrystallites structure; this is
confirmed by presence of a major peak at 2h of 31.67� as inFig. 2c. Figure 2 shows that the amorphous structure of the
pristine thin film (Fig. 2b) crystallizes into a single
nanocrystallites phase (Fig. 2c). The results of FTIR
spectroscopy indicate no change in molecular bonds of
MV2B upon deposition or thermal annealing [11]; this
indicates that polymorphous nanocrystallization occurred,
where atoms in the disordered state jump to crystal front
and those in clusters change their orientation to match the
growing crystal and deposit onto the crystal front. The
formation of nanocrystallites structure by annealing indi-
cates a decrease in structural disorder in annealed film in
comparison with that in pristine film.
Electrical conduction mechanisms and dielectric constants 1111
123
3.2 DC electrical measurements
The DC electrical conductivity, rDC, for planar samples of
MV2B has been calculated by using the following equation
[32]:
rDC ¼ r0 exp�Et
kBT
� �ð3Þ
where r0 is the pre-exponential factors, Et is the thermal
activation energy for this process, T is the temperature
expressed in Kelvin and kB is the Boltzmann constant. The
plot of log (rDC) versus reciprocal temperature (1000/T) in
the temperature range 288–470 K for planar samples with
different film thickness in the range 96–152 nm is shown in
Fig. 3, for a fixed temperature log rDC increases as film
thickness increases. A linear relationship between log rDCand 1000/T is observed; the linear relationship consists of
two segments depending on temperature; region, I, lies in
the temperature range from 288 to 360 K, where the ex-
trinsic conduction prevails, and region, II, is in the tem-
perature range from 360 to 470 K where the intrinsic
conduction prevails. The extrinsic and intrinsic activation
energy, DEex and DEin, respectively, are calculated from
the slope of straight lines for each region, and ro is ob-
tained from the intercept of the straight line with ordinate
axis. The values of activation energies and the pre-expo-
nential factor are listed in Table 1. The calculated average
activation energy in extrinsic region is 0.16 eV and in in-
trinsic region is 0.91 eV. The obtained activation energy in
intrinsic region is half the value of onset energy gap ob-
tained from optical measurements of MV2B [11]. For each
conduction region, the conduction is through a thermally
activated process and can be explained by considering the
contribution from extrinsic and intrinsic mechanisms [33].
In the low temperature region; the conduction is due to
intermolecular conduction process and is done if the in-
termolecular orbital overlap in the organic compound and it
is caused by transfer of charge carriers between molecules.
At high temperature region, intrinsic conduction is due to
electron transport from HOMO to LUMO orbital. The
electrical conduction in the extrinsic region can be ana-
lyzed by applying Mott model for variable range hopping
in localized states near Fermi level [34]. In this model, the
conductivity as a function of temperature for three-di-
mensional hopping is given by:
r ¼ roffiffiffiv
pexp � 1
v1=4
� �; ro ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið9=8pÞ18:1
p e2
jTtpha
ð4Þ
and
v ¼ NF
No
; No �18:1a3
jTð5Þ
where NF is the density of states at Fermi level, vph is the
frequency of phonons assisted with the hopping process
and a is the inverse of Bohr radius. Figure 4 illustrates the
relation between Ln rT0.5 and T-0.25 in temperature region,
I. Straight lines are obtained indicating that the operating
0
50
100
150
200
250
0.0
10 20 30 40 50 60
(c)
(b)
Powder Pristine film Annealed film at 423K
(2
(a)
Inte
nsity
(cou
nts/
Sec.
)
(
Fig. 2 XRD patterns for MV2B: (a) powder, (b) pristine thin film
and (c) annealed thin film at 433 K for 1 h
2.4 2.6 2.8 3.0 3.2 3.4
e-17
e-16
e-15
e-14
e-13
e-12
e-11
e-10
e-9 d,nm 96 125 145 152Average value
E1 = 0.16 eVE2 = 0.91eV
log
DC,
-1 . cm
-1)
1000/T ,K-1
(I)
(II)
Fig. 3 DC conductivity of Au/MV2B/Au in planar configuration as a
function of reciprocal temperature for different film thickness
Table 1 Activation energies of DC conductivity for planar samples
with different film thickness
Film thickness (nm) Activation energy (eV)
DEex DEin
96 0.187 0.894
125 0.186 0.901
145 0.204 0.909
152 0.213 0.916
1112 H. M. Zeyada, M. M. Makhlouf
123
conduction mechanism is a variable range hopping in lo-
calized states near Fermi level. The values of parameters
such as NF, Ro(hopping distance) and Df (average hoppingenergy between two hopping states) have been evaluated
using the following formulae [34]:
Df ¼ 3
4pR3oNF
ð6Þ
Ro ¼9
8pakBTNF
ð7Þ
together with slopes obtained from Fig. 4 and taking into
account that phonon energy is 100 meV, Bohr radius is
1 nm and the measurements are taken at 300 K. The ob-
tained results are presented in Table 2. Results shown in
Table 2 indicate that NF increases and Ro and Df decrease
with increasing film thickness (Table 3).
3.3 Thermoelectric power measurements
Hot probe test showed that MV2B films exhibit p-type
semiconductor conductivity. Results of determination of
Seebeck coefficient, S, as a function of inverse temperature
for pristine MV2B films of four different thicknesses are
depicted in Fig. 5. S decreases as film thickness and tem-
perature are increased; a linear relationship between S and
1000/T for all thickness values in the investigated tem-
perature range suggests that Seebeck coefficient obeys the
relation [35]:
S ¼ � kB
e
� �ES
kBT� ckB þ 1
� �ð8Þ
where Es is the thermal activation energy and c is the
temperature coefficient of thermal expansion. Es is
determined from the slope of straight lines, and it takes a
value of 0.67 eV.
The polaron activation energy, Ep, is deduced according
to [35] as:
Ep ¼ Et � ES ð9Þ
The calculated polaron activation energy is 0.24 eV.
3.4 AC electrical measurements
The AC electrical measurements were taken on thin film
sample in the sandwich structure with active area
0.15 9 1.8 cm2 and thickness of 300 nm. Temperature and
0.232 0.233 0.234 0.235 0.236 0.237 0.238 0.239 0.240-14.4
-14.2
-14.0
-13.8
-13.6
-13.4
-13.2
-13.0
-12.8
-12.6
-12.4
-12.2
-12.0 d, nm 96 nm 125 nm 145 nm 152 nm
T-1/4 , k-1/4
ln (
DCT1/
2 , -1cm
-1k1/
2 )
Fig. 4 Dependence of ln rDC T1/2 on T-1/4 for MV2B thin films with
different thickness in the extrinsic conduction region
Table 2 Dependence of Mott parameters on film thickness for the
pristine films of MV2B
d (nm) NF (cm-3 eV-1) Ro (cm) Df (meV)
96 5.37 9 1018 3.19 9 10-6 1.26
125 7.52 9 1018 2.93 9 10-6 1.16
145 1.02 9 1019 2.71 9 10-6 1.08
152 1.39 9 1019 2.51 9 10-6 1.00
Table 3 Dependence of electrical, thermal and polaron activation
energies on film thickness of MV2B
d (nm) DEt (eV) DES (eV) EP (eV)
96 0.87 0.63 0.24
125 0.88 0.67 0.21
145 0.90 0.68 0.22
152 0.91 0.71 0.20
2.45 2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85
e-16
e-15
e-14
e-13
e-12
e-11
average ( )S(
V/K
)
1000/T , K-1
DC
, -1
cm-1
100
150
200
250
300
350
400
450 d [ = nm ]
96 125 145 155 average (S)
Fig. 5 Dependence of electrical conductivity and thermoelectric
power coefficient on temperature for MV2B films of different
thickness
Electrical conduction mechanisms and dielectric constants 1113
123
frequency dependence of the electrical properties have
been studied.
AC electrical conductivity measurements of semicon-
ductors have been extensively used to understand the elec-
tronic transport mechanisms and are a powerful tool for
obtaining information about the defect states in amorphous
semiconductors [36]. Various models such as quantum
mechanical tunneling (QMT) model [37] and correlated
barrier hopping (CBH) model [36] have been proposed to
explain the AC conduction mechanisms. The total conduc-
tivity rtotal of many semiconductors [36] over a wide range
of frequencies, x, and temperature, T, can be written as:
rtotal ¼ rAC þ rDC ð10Þ
where rAC is the AC conductivity and rDC is the DC
conductivity. This equation is valid only when the AC and
DC conductions arise from completely separate mechan-
isms, but when x ? 0, the AC conductivity represents the
DC conductivity.
Figure 6 shows the AC conductivity as a function of the
reciprocal temperature at nine fixed frequencies for MV2B
sandwich film with a thickness of 300 nm. A linear rela-
tionship is obtained between ln rAC and 1000/T, it consists
of two segments intersecting at a transition temperature, Tt,
depending on temperature and frequency. It is evident that
the conductivity increases with increasing both frequency
and temperature, such a trend indicates that MV2B films
behave as a semiconductor and the temperature depen-
dence of AC conductivity is also represented by Eq. 3. The
values of activation energies and transition temperatures
were calculated for different frequencies and are listed in
Table 4. The activation energies decrease and transition
temperature increases with increasing frequency; that is
because increasing the applied frequency enhances the
electronic jumps between the localized states [38]. The
small values of activation energy confirm that hopping
conduction is the dominant current transport mechanism.
The frequency dependence of rAC (x) at various tem-
peratures is shown in Fig. 7. It is evident from these curves
that AC conductivity has a frequency dependence given by
Eqs. 11 and 12. The frequency dependence of the real part
of the total AC conductivity, rAC, [39] can be given by:
rACðxÞ ¼ eoxe2ðxÞ ð11Þ
rAC ¼ A0xs ð12Þ
where A0is a constant depending on temperature, x is the
angular frequency (x = 2v where v is the frequency), e0 isthe permittivity of free space, e2 is the imaginary part of the
dielectric constant and s is an exponent, generally less than
or equal to unity. This exponent, s, has been used fre-
quently to characterize the AC electrical conduction in
different semiconductor materials. It is clear from Fig. 7
that the variation in the logarithmic AC conductivity is
almost linear with the variation in logarithmic frequency
and rAC (x) increases with increasing frequency and
temperature. The frequency exponent s was obtained from
the slope of ln rA versus ln x and is plotted as a function of
temperature in Fig. 8. The exponent s has temperature
dependence and generally it decreases as the temperature
increases. The observed frequency dependence reveals that
the responsible mechanism for AC conduction could be
due to hopping [36]. The AC conductivity, having fre-
quency dependence (rAC � xs) with s\ 1, has been ob-
served in organic semiconductors thin films [40]. We
invoke the correlated barrier hopping CBH model to ex-
plain the observed behavior in Fig. 8. In the CBH model,
the temperature dependent exponent sis predicted where s
decreases with increasing temperature and it approaches
unity as temperature tends to zero K, in marked contrast to
the quantum mechanical tunneling QMT [36] or simple
hopping over a barrier HOB mechanism [41].
The AC conductivity in the CBH model [36, 41] is given
by:
rACðxÞ ¼ ðp3=24Þn0N2e1eoxR6x ð13Þ
where N is the concentration of sites in pairs, n0 is the
number of electrons that hop over the barrier, e1 is the realpart of the dielectric constant and Rx is the hopping dis-
tance which is given by:
Rx ¼ n0e2=pe1eo½WM þ kBT lnðxsoÞ� ð14Þ
kB is Boltzmann constant, e is the electron charge, WM is
the maximum barrier height and the frequency exponent s
for this model is given by:
2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4
e-29
e-27
e-25
e-23
e-21
e-19
e-17
e-15
Tt
DC 1kHz 5KHz 10KHz 30KHz 50KHz 100KHz 300KHz 500KHz700KHz
1000/T , K-1
AC
,DC ,
-1cm
-1
(I)(II)
Tt
Tt
Fig. 6 Temperature dependence of the AC conductivity at different
frequencies for MV2B thin films
1114 H. M. Zeyada, M. M. Makhlouf
123
s ¼ 1� 6kBT=WM ð15Þ
The temperature dependence on s shown in Fig. 8 is con-
sistent with Eq. 15 indicating the dominance of CBH
model as electrical transport mechanism for MV2B thin
films. According to the CBH model [36], the theoretical
AC electrical conductivity can be calculated by using
Eq. 13 over all the range of the temperatures and
frequencies used in this study. Fitting is made at constant
frequency of 1 kHz, and the same values are used for the
other frequencies. Figure 8 shows that there is agreement
between experimental and theoretical results of electrical
conductivity. This agreement is satisfied only when the
number of polaron, the values of density of states N and the
relaxation time so are 2, 5.51 9 1028 m-3 eV-1 and
1.3 9 10-14 s, respectively. The solid line is the best fit
predicted by CBH model as shown in Fig. 8. As it is clear,
the fitting is reasonably good, indicating that the conduc-
tion is through bipolaron hopping mechanism.
3.5 Dielectric properties
Studies of dielectric properties are important to understand
the nature and the origin of dielectric constants, which in
turn may be useful in the determination of structure and
defects in solids. The dielectric behavior of thin film de-
vices depends not only on their material properties, but also
on the substrate used for fabrication and the type of the
metal electrodes.
Figure 9 shows the capacitance–temperature, C–T,
characteristics of MV2B sandwich thin film with thickness
of 900 nm at various constant frequencies ranging from 1
to 700 kHz. It can be seen that as the temperature in-
creases, the capacitance increases nonlinearly. It is also
observed that the rate of increase in the capacitance in the
low temperature region,\353 K, is lower than that for the
high temperature region, [353 K. Inspection of Fig. 9
shows that for constant temperature the capacity of MV2B
films decreases as the applied frequency is increased. Such
a behavior of the capacitance of MV2B thin film is the
ordinary one for the semiconductor materials [33, 41].
The dielectric loss, in which a part of the energy of an
electric filed is dissipated as heat in the dielectric, is com-
prised of two parts: the first part, which arises due to elec-
trodes resistance, is frequency independent. This can be
minimized using electrodes of highly conducting metal
(Ohmic electrodes). The second part is a property of the
material itself, which is frequency dependent. The dielectric
permittivity, e, is a complex function expressed as
e ¼ e1 þ ie2. The dissipation factor, tan d = e2e1where e2 is a
measure of the energy required for molecular motion and e1is related to the capacitive nature of the material and is a
measure of the stored energy in the material by polarization
Table 4 DC and AC conductivity parameters for MV2B sandwiched film with thickness of 300 nm
kHz 0 (DC) 1 5 10 30 50 100 300 500 700
DE1 0.16 0.09 0.077 0.072 0.056 0.043 0.032 0.018 0.015 0.002
DE2, (eV) 0.92 0.875 0.871 0.863 0.851 0.84 0.8 0.743 0.65 0.62
Tt, (K) 346 353 357 359 365.8 365.8 365.8 365.8 365.8 365.8
8 9 10 11 12 13 14 15 16
-23
-22
-21
-20
-19
-18
-17
-16
-15
-14
TA,K 303 313 323 333 343 353 363 373 383
ln(
AC ,
-1cm
-1)
ln( , rad/s )
Fig. 7 Frequency dependence of the AC conductivity at various
temperatures
310 320 330 340 350 360 370 380 3900.70
0.75
0.80
0.85
0.90
0.95
T , K
Freq
uenc
y ex
pone
nt ,
S
Expermintal Theoretical fit
Fig. 8 Temperature dependence of the frequency exponent, s
Electrical conduction mechanisms and dielectric constants 1115
123
[42]. Figure 10 shows the loss tangent–temperature char-
acteristics of MV2B thin film of thickness 300 nm at var-
ious constant frequencies ranges from 1 to 700 kHz. It can
be seen that the loss tangent, tand, increases with increasingthe temperature and the rate of its increase with tan d is
approximately constant and have a small value at low
temperature region\353 K. For temperatures[353 K, tandincreases nonlinearly with temperature and for a constant
temperature it decreases with increasing frequency [33].
Temperature and frequency dependence of dielectric
constant, e1, are studied for film sample of thickness
300 nm in the temperature range 300–393 K and frequency
range of 1–700 kHz, such a dependence is shown in
Fig. 11. It is observed that e1 increases with increasing
temperature at different constant frequencies. The variation
of e1 with the temperature is related to the charge carriers
which at low temperatures some of them can orient
themselves to the direction of the applied field; therefore,
they possess a weak contribution to the polarization and the
dielectric constant e1. As the temperature increases, a lot of
the bound charge carriers get enough thermal excitation
energy to be able to obey the change in the external field
more easily. This in turn enhances their contribution to the
polarization leading to an increase of the dielectric constant
e1 of the sample [43]. Inspection of Fig. 11 shows that at a
fixed temperature, e1 decreases with increasing the fre-
quency, and this is a result of the dipoles which will no
longer be able to rotate sufficiently rapidly with increasing
frequency, so that, their oscillation will lag behind that of
the field [43]. As the frequency is further increased, the
dipole will be completely unable to follow the field and the
orientation stopped, so e1 decreases at a high frequency
approaching a constant value due to the interfacial
polarization.
Temperature dependence of the dielectric loss e2 for
MV2B sandwich thin film with thickness of 300 nm at
different but constant frequencies in the range 1–700 kHz
is shown in Fig. 12. It is observed that dielectric loss e2increases as the temperature increases for the considered
frequencies; at low temperatures up to 353 K, this increase
is linear and the variation of e2 with the frequency is very
small. For temperatures[353 K, e2 increases nonlinearly.
This behavior can be clarified by plotting ln e2 versus ln xfor various temperatures as shown in Fig. 13. It is observed
that a series of straight lines with different slopes are ob-
tained, e2 decreases as the frequency increases and it in-
creases with the temperature increase, such a behavior has
been realized in many other organic [44–46] and inorganic
semiconductors [47]. The frequency dependence of the
320 340 360 380 4001.6x10-12
2.0x10-12
2.4x10-12
2.8x10-12
3.2x10-12
3.6x10-12
4.0x10-12
4.4x10-12
1kHz 5kHz 10kHz 30kHz 50kHz 100kHz 300kHz 500kHz 700kHz
C ,
F
T , K
Fig. 9 Capacitance dependence on temperature at different
frequencies
320 340 360 380
0.1
0.2
0.3
0.4
1kHz 5kHz 10kHz 30kHz 50kHz 100kHz 300kHz 500kHz 700kHz
tan
T , K
Fig. 10 Loss tangent dependence on temperature at different fre-
quencies for MV2B thin film of thickness 900 nm
320 340 360 380
5
6
7
8
9
10
11
12
13 1kHz 5kHz 10kHz 30kHz 50kHz 100kHz 300kHz 500kHz 700kHz
1
T , K
Fig. 11 Temperature dependence of dielectric constant, e1, at differ-ent frequencies for MV2B thin film
1116 H. M. Zeyada, M. M. Makhlouf
123
dielectric loss, e2, is given by Giuntini model [48] and can
be written by the following expression:
e2 ¼ Yxm ð16Þ
where Y is a constant and m is an exponent given by:
m ¼ �4kBT=WM ð17Þ
To estimate a value for the parameter m, a plot of ln e2versus ln x in the temperature range 303–393 K is illus-
trated in Fig. 13, straight lines are obtained each of them
represent a definite temperature in the temperature range
303–393 K. The power m is calculated from the slopes of
the curves, and it was found that the values of m are
negative and linear with an increase in temperature as
shown in Fig. 14; this is in agreement with the prediction
of the theory [48]. The parameter m confirms the observed
variations as a function of the measuring temperature. In
fact, this result is satisfying if we consider the empirical
law rAC / xs [49]. It is obvious that if s is temperature
dependent, m should consequently depend on temperature.
The obtained experimental results confirm the values of
s obtained during studies on conductivity. Indeed, if we
consider, the value of m = 0.028 at T = 303 K, the ob-
tained value of WM is &3.7 eV for the investigated films.
Such a value of WM is the fundamental energy gap of
MV2B films [11], and it is consistent with the theory of
hopping of charge carriers over a potential barrier as sug-
gested by Elliott [36].
Comparative study of these figures indicates that e1 ande2 increase with the increase of temperature (the rate of
increase varies with the different frequencies), and they
decrease with increasing frequency. This type of behavior
has also been reported in organic films [33, 39].
4 Conclusions
Spin coating technique has been successfully applied to
deposit uniform methyl violet 2B thin films. X-ray
diffraction analysis showed that MV2B in a powder form
and pristine thin films has an amorphous structure. An-
nealing MV2B thin films at 433 K decreased the structure
disorder of these films and partially transformed the
amorphous structure of pristine film into a polymorphous
nanocrystallites structure.
The dark electrical resistivity of MV2B films decreases
nonlinearly with increasing film thickness. The transport
electrical properties of MV2B thin films have been inves-
tigated. Analysis of results of DC conductivity reveals the
presence of two conduction mechanisms: extrinsic
mechanism with average activation energy of 0.16 eV and
320 340 360 380
1
2
3
4
5 1kHz 5kHz 10kHz 30kHz 50kHz 100kHz 300kHz 500kHz 700kHz
T , K
2
Fig. 12 Temperature dependence of dielectric loss, e2, at differentfrequencies
8 9 10 11 12 13 14 15 16 17-1.8-1.6-1.4-1.2-1.0-0.8-0.6-0.4-0.20.00.20.40.60.81.01.21.41.61.8
ln ( , rad/s)
ln 2
TA, K 303 313 323 333 343 353 363 373 383 393
Fig. 13 Frequency dependence of dielectric loss, e2, at different
temperatures
300 320 340 360 380 400-0.18
-0.16
-0.14
-0.12
-0.10
-0.08
-0.06
-0.04
-0.02
m
T , K
Fig. 14 Temperature dependence of parameter m
Electrical conduction mechanisms and dielectric constants 1117
123
intrinsic mechanism with activation energy of 0.91 eV.
The conduction in extrinsic region is explained via apply-
ing Mott model for variable range hopping. The operating
conduction mechanism is a variable range hopping in lo-
calized states near Fermi level. The values of parameters
such density of localized states near Fermi level, hopping
distance and average hopping energy between two hopping
states have been evaluated as 5.4 9 1018 cm-1 eV-1,
3.2 9 10-6 cm and 1.26 meV, respectively, the values of
these parameters are influenced by annealing temperatures.
In intrinsic region, the conduction is by electron transitions
from HOMO to LUMO orbital with activation energy of
0.91 eV. Thermoelectric power measurements showed that
MV2B is p-type semiconductor and the polaron activation
energy is 0.26 eV.
Analysis of AC conductivity data on the basis of corre-
lated barrier hopping (CBH) model predicts that bipolaron
hopping mechanism is the dominant conduction mechan-
ism. The deduced values of density of states and relaxation
time are 5.5 9 1028 m-3 eV-1 and 1.3 9 10-14 s, respec-
tively. The values of real part of dielectric function, e1, anddielectric loss, e2, increase with increasing temperature, and
they decrease with increasing frequency. Analysis of data of
e2 showed that the maximum barrier height is 3.7 eV.
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