Download carrier transport properties of aluminum oxide/polypyrrole

CARRIER TRANSPORT PROPERTIES OF ALUMINUM
OXIDE/POLYPYRROLE FILMS DOPED WITH
NAPHTHALENE-1,5-DISULFONIC ACID
M. Campos
Universidade Paulista (UNIP), Grupo de Pesquisa “Ciência dos Materiais”
14025-270 Ribeirão Preto-SP [email protected]
Filmes de Al/Al2O3/PPy-NSA/Au foram investigados com a utilização de medidas de corrente elétrica em função da
voltagem externa aplicada (I-V) e, de capacitância em função da voltagem (C-V), em temperaturas que variaram no
intervalo de 90-350 K. As medidas de (C-V) foram realizadas em freqüências de 1kHz a 20 MHz. Para analise das
características elétricas dos filmes, foram considerados efeitos de resistência elétrica em série, camada interfacial e
estados interfaciais. Foi observado que (I-V) segue um modelo de emissão termiônica dos portadores sobre uma barreira
de potencial, supondo uma distribuição gaussiana para a altura da barreira. As medidas de (C-V) exibiram um pico,
cujo máximo é deslocado para baixas voltagens e, apresenta também uma diminuição de intensidade, com o aumento da
freqüência de medida.
Palavras-chave: corrente, capacitância, barreira de Schottky, polímero condutor.
Carrier transport properties of aluminum oxide/polypyrrole films doped with naphthalene-1,5-disulfonic acid
The electrical structure of the Al/Al2O3/PPy-NSA/Au has been investigated by means of current-voltage (I-V) and
capacitance-voltage (C-V) measurements, in a temperature range of 90-350 K. The forward C-V measurements have
been carried out in the range of frequency of 1 kHz to 20 MHz. The effects of series resistance, interfacial layer and
interface states on I-V and C-V characteristics are investigated. At high current densities in the forward direction, the
series resistance effect has been observed for voltages greater than 0.7 V. The analysis of I-V characteristics based on
the thermionic emission mechanism has been explained by the assumption of a Gaussian distribution of barrier heights,
due to barrier height inhomogeneities that prevail at the interface. It has been observed that the forward C-V plot
exhibits a peak, whose position shifts towards lower voltages and that decreases with increasing frequency. The nonlinearity of 1/C2 versus V plot at high frequency was explained with the assumption that only some of the interface
states follow the applied ac signal.
Keywords: conducting polymer, capacitance, Schottky barrier, current.
1. Introduction
The presence of a thin insulator layer between metal and semiconductor in the MIS structure gives
these devices the properties of a capacitor, which stores the electric charge due to the presence of
oxide layers. In a MIS structure, if the thickness δ of the interfacial insulator layer is larger than 50
Ǻ, the interface states are in equilibrium with the semiconductor. If the insulator layer thickness is
less than 50 Ǻ, direct tunneling becomes possible in MIS structure [1]. In the case that δ is smaller
than 10 Ǻ, the interface states are in equilibrium with the metal [2]. Hence, current transport may be
dominated by tunneling and non-rectifying devices are expected [3].
The characterization of interface states and series resistance in MIS structure has become a subject
of very intensive research for more than four decades [4]. A number of workers have suggested
several ways of characterization of these behavior [5]. It is well known that interface states cannot
follow ac signal at higher frequencies [6]. Thus, low frequency capacitance of Schottky contacts is
applied to interpret interface states [7]. Also, those interface states can affect the C-V characteristics
of MIS structures causing a bending of the C-2 versus V as well increasing the ideality factor. An
anomalous peak in forward C–V characteristics, attributed to interface states and series resistance,
has been reported earlier [4].
The objective of this study is to obtain some properties of Al/Al2O3/PPy-NSA/Au structure and
investigate the effects of the interface states and series resistance by measuring current-voltage (IV) characteristics in a temperature range of 90-350 K. Measurements also have been made for C-V
characteristics under forward and reverse bias at frequencies changing from 1 kHz to 20 MHz.
Temperature dependences of the ideality factors and barrier height have been analyzed in the light
of the inhomogeneity model.
2. Experimental
The samples were chemically obtained. For that, pyrrole monomer (Sigma-Aldrich) was purified by
distillation under reduced pressure prior to use. Ammonium persulfate (APS) (Sigma-Aldrich) was
used as oxidant. The dopant used was naphthalene-1,5-disulfonic acid (NSA) (Sigma-Aldrich), used
without further purification. The doped PPy was chemically synthesized by in situ doped oxidative
coupling polymerization. The solution of 0.68 g (3.0 mmol) ammonium persulfate in 10.0 mL
deionized water was slowly added to the mixture of 1.2 mmol of dopant and 1.0 mL (14.5 mmol)
pyrrole in 20.0 mL deionized water. The reaction was maintained at 0°C for 2 hours. The
polypyrrole was precipitated by pouring the reaction mixture into a large excess amount of
deionized water. The PPy powder was washed several times with deionized water and methanol
before drying in a vacuum at room temperature. Films were prepared dissolving PPy powder into
m-cresol. The thickness of the prepared films was typically 10-25 µm, and the total active area of
about 0.85 cm2. Aluminum oxide (Al2O3), with estimated thickness to be about 23 Ǻ obtained from
high frequency measurement of the oxide capacitance in the strong accumulation region at high
frequency (1 MHz), was deposited on one surface of PPy by atomic layer chemical vapor deposition
[8]. Gold circular electrode of about 0.20 cm2 were deposited on one side, with vacuum of 10-5
Torr, while Al circular electrodes of the same area were vacuum deposited on the aluminum oxide
surface using suitable mask, in order to allow the I-V measurements. Therefore, we have in one side
of the sample a rectifying contact (Al) and an ohmic contact in the other side (Au), similar to
conventional inorganic semiconductor Schottky barrier diodes.
Current-voltage measurements were made with the use of a Keithley 6517A electrometer, and a
Keithley 230 programmable voltage source. The capacitance-voltage characteristic (C-V) was
measured using a Solartron 1260 impedance/gain-phase analyzer coupled with a Solartron 1296
Anais do 10o Congresso Brasileiro de Polímeros – Foz do Iguaçu, PR – Outubro/2009
dielectric interface in the frequency range from 1 kHz to 20 MHz. All measurements were carried
out with the help of a microcomputer trough an IEEE-488 ac/dc card in dark. Measurements were
made between 90-350 K using a homemade temperature controlled cryostat.
3. Results and Discussion
3.1 Current-voltage characteristics
The (I-V) characteristics of the Al/Al2O3/PPy-NSA/Au structure, is shown in Fig. 1, for temperature
of 330 K. As we can see from Fig. 1 the electrical current in forward bias is quickly dominated by
series resistance given rise to the curvature observed at high electrical current. In general, the
forward bias I-V characteristics is linear on a semi-logarithmic scale at low forward bias voltage,
but deviates considerably from linearity due to the effect of the series resistance (RS), the interfacial
layer, and interface states when the applied voltage is sufficiently large. The parameter resistance is
only effective in the downward curvature region of the I-V characteristics, but the other two
parameters play significant role in both the linear and non-linear regions of these measurements. It
was observed a weak voltage dependence of the reverse bias current and the exponential increase of
the forward bias current. This behavior is a characteristic property of rectifying interfaces. Since we
know that we have an interfacial oxide layer (Al2O3) between Al and the conducting polymer, we
are in the MIS condition.
The transport of charge over the potential barrier, from semiconductor into the metal, is generally
described by the thermionic emission theory without diffusion and tunneling, and it can be
expressed for the forward bias case as [9]
I = I0 exp [qV/(nkT)]
(1)
where q is the electronic charge, T the temperature, V the applied voltage, and k the Boltzmann’s
constant. I0 is the saturation current, and can be expressed as
I0 = A A* T2 exp [− qφB/(kT)]
(2)
where φB is the effective barrier height at zero bias, A* and A are, respectively, the effective
Richardson constant and effective diode area.
The forward bias current-voltage characteristics due to thermionic emission of a Schottky diode
with the series resistance (RS) and V > 3kT/q can be expressed as [10]
I = I0 exp [q(V − VS)/(nkT)]
(3)
The term VS = IRS represents the voltage drop across series resistance of diode, and n is the ideality
factor (n=1 in the ideal case).
Anais do 10o Congresso Brasileiro de Polímeros – Foz do Iguaçu, PR – Outubro/2009
Fig. 2 shows the experimental series resistance values, as obtained from I-V characteristics as a
function of temperature. The effective barrier height (φB) can be obtained from Richardson plot of
the saturation current. Eq. (2) can be rewritten as
ln (I0/T2) = ln (A A*) − qφB/(kT)
(4)
The ideality factor n is determined from the slope of the straight-line region of the semi log
forward-bias I-V characteristics through the relation
n = (q/kT) dV/d[ln (I)]
(5)
300
10-3
250
Series Resistance (Ω)
Current (A)
10-4
10-5
10-6
10-7
200
150
100
10-8
-1,5
-1,0
-0,5
0,0
0,5
1,0
1,5
2,0
50
100
V (Volt)
Figure 1-Plot of (I-V) characteristics for temperature of
330 K.
150
200
250
300
Temperature (K)
Figure 2-Electrical series resistance as a function of temperature
According to Eq. (4), if the barrier height φB is independent of temperature, we should obtain a
linear dependence on the ln (I0/T2) versus T−1 curve. However, the corresponding results presents a
non-zero curvature, thus indicating that φB is temperature dependent.
Because was observed a increase of zero-bias barrier height and decrease of the ideality factor with
increase in temperature, to explain the experimental data one has to look for other possibility, for
example that we have barrier inhomogeneities [11]. As far as the barrier height distribution is
concerned, different functions, including Gaussian distribution, have been proposed [12,13]. Let us
assume that we have Gaussian distribution of the barriers heights with a mean value φm and a
standard deviation σ [12], in the form
P(φ) = (1/σ) (2π)−1/2 exp [− (φ −φm)2/(2 σ2)]
(6)
The total current under the forward bias V is given by
I(V)= ∫
∞
−∞
{I0 exp[qV/(nkT)] (1/σ)(2π)−1/2 exp[−(φ−φm)2/(2σ2)]} dφ
(7)
After the integration we have:
I(V)=A A* T2 {1−exp [−(qV/(kT)]} exp [(qV)/(nakT)] exp {−(q/(kT)][φm −σ2/(2kT)]}
and
Anais do 10o Congresso Brasileiro de Polímeros – Foz do Iguaçu, PR – Outubro/2009
(8)
I0 = A A* T2 exp [− qφa/(kT)]
(9)
where φa and na are, respectively, the apparent barrier height and apparent ideality factor, and given
by
φa = φm0 − qσ02/(2kT)
(10)
[(1/na) − 1] = −ξ+ qψ/(2kT)
(11)
where φm0 stands for the mean barrier height at zero bias, and σ0 is the standard deviation, also at
zero bias. Here ξ and ψ are the corresponding voltage coefficients which may depend on T and
quantify the voltage deformation [14]. Assuming linear dependence for the Gaussian parameters we
have:
φ = φm0 + ξV
(12)
σ = σ0 +ψV
(13)
As was considered in Eq. (13), the temperature dependence of σ is very small and can be neglected
[15].
Thus, fitting Eq. (8) to the experimental results for I-V, one gets φa and na which are temperature
dependent as was seen in Eqs. (10) and (11). So the plot of φa versus T−1 should be a straight line,
what allow us to obtain the values of φm0 = 0.84 eV and σ0 = 0.086, as can be seen in Fig. 3. The
values of ξ = − 0.0632 and ψ = − 0.00315 V were obtained from the linear dependence of [(1/na) −
1] versus T−1, represented in Fig. 4.
The Richardson plot can be modified by combining Eqs. (9) e (10):
ln(I0/T2) − q2σ02/(2k2T2) = ln(AA*) − qΦm0/kT
(14)
A plot of Eq. (anterior) as a function of T−1 result in a straight line, providing the values φm0=0.83
0,9
-0,3
0,8
-0,4
0,7
-0,5
0,6
[(1/na ) - 1]
Apparent barrier height (eV)
eV and A*= 138 A cm−2 K−2.
0,5
0,4
0,3
-0,6
-0,7
-0,8
0,2
-0,9
0,1
2
4
6
8
10
103/T (K-1)
Figure 3- Apparent barrier height as a function of T−1.
12
2
4
6
8
10
12
103/T (K-1)
Figure 4-Apparent ideality factor as a function of T−1.
Anais do 10o Congresso Brasileiro de Polímeros – Foz do Iguaçu, PR – Outubro/2009
3.2 Capacitance-voltage characteristics
When an ac signal is applied to the diode, there is a frequency dependence of the C-V curve, which
does not exist in the ideal case. When the dc voltage corresponds to a reverse bias, the differential
capacitance represents the response of the depletion layer to the ac signal. In simple Schottky
barrier theory, the capacitance of the barrier varies with the applied reverse voltage V in such a way
that a straight line is obtained if 1/C2 is plotted against V. With the use of the simple depletion
layer theory, this relation is given by
C−2 = 2 (Vd + V)/(εs ε0 q A2 NA)
(15)
where Vd is the diffusion potential at zero bias, which can be determined from the C−2versus V
results, εs is the dielectric constant of the semiconductor, NA the carrier concentration and A is the
effective area of the diode. However, since we have interfacial layer, extra terms have to be
included [16]
C−2 = [2(Vd + V)/εsε0q A2 NA ] [1 − 4 (εsd/εmw)2 + ...........]
(16)
where d and εm are, respectively, the thickness and the dielectric constant of the organic layer, and
w is the width of the interface layer. For non-ideal structures, the barrier height can be calculated by
[17]
φB = βVd + Vp
(17)
where β = n−1 , and Vp is the potential difference between the Fermi level and the top of the valence
band in the neutral regime, given by
Vp = kT ln (Nv/NA)
(18)
where Nv is density of states in the valence band [18]. From the plot of C−2 versus V characteristics
of Al/Al2O3/PPy-NSA/Au Schottky diode at 250 kHz and room temperature, the Vd value was
found to be 0.82 V. The barrier height φB = 0.79 eV was calculated from C-V curve, using the
obtained Vd and Vp (0.29 V) values. The φB value obtained from C-V measurements is higher than
the φB value obtained from I-V measurements. The discrepancy between these values may occur
due to existence of barrier inhomogeneities [19], such as non-uniformity of the interfacial layer
thickness and distribution of the interfacial charges [20]. As the potential drops across the
interfacial layer, this potential changes with applied voltage and therefore the interface state charge
changes, resulting in an increase in barrier height.
Fig. 5 shows the experimental C-V curves of the Al/Al2O3/PPy-NSA/Au structure measured in the
frequencies of 300 kHz and 2 MHZ at room temperature. As can be seen in Fig. 5 the capacitance
peak values decrease with increasing frequency. Furthermore, the peak position shifts towards a
lower voltage values with increasing frequency, and also with decreasing of interface state density.
Anais do 10o Congresso Brasileiro de Polímeros – Foz do Iguaçu, PR – Outubro/2009
It is well known that the capacitance of Schottky barrier diodes is extremely sensitive to the
interface properties, as the interface states respond differently to low and high frequencies
450
400
Capacitance (pF)
350
300
250
200
150
100
-1,0
-0,5
0,0
0,5
1,0
Voltage (V)
Figure 5-Plot of capacitance as a function of voltage measured in frequencies of 300 kHz (O) and 2 MHz (O) at room temperature.
4. Conclusions
Some electrical properties of the Al/Al2O3/PPy-NSA/Au structure were investigated using
I-V and C-V characteristics in the temperature range of 90-350 K, in the absence of light. It
was found that the value of the ideality factor n as calculated from forward bias I-V
measurement was greater than unity. This behavior can be ascribed to the interfacial
layer, interface states and series resistance. The applied bias voltage decreases across
the oxide layer causing the forward current also to decrease, thus producing a deviation
from the ideal I-V characteristics. The values of ideality factor and barrier height have been
obtained at room temperature (25°C) from I-V characteristics as 1.65 and 0.68 eV
respectively, and were found to be strongly temperature dependent. It is shown that the
series resistance values decreased as the temperature is increased. Such behavior has
been attributed to distribution of interface states, interface layer and barrier height
inhomogeneities.
The forward-bias C-V plot exhibits a peak due to substrate series resistance. It has been
found that the peak value of the capacitance and its position depend on the interface
states. At high frequency, where the influence of interface states decreases, the peak
value of the capacitance decreased and the peak position shifted towards lower voltages.
The non-linearity of the C−2 versus V plot at high frequency can be explained with the
assumption that only some of the interface states follow the applied ac signal.
Anais do 10o Congresso Brasileiro de Polímeros – Foz do Iguaçu, PR – Outubro/2009
Acknowledgments
This work was supported by Vice-Reitoria de Pós-Graduação e Pesquisa da Universidade Paulista
(UNIP), São Paulo, Brazil.
The author wish to thank Dr R.W. Xingwu for the supply of the samples.
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Anais do 10o Congresso Brasileiro de Polímeros – Foz do Iguaçu, PR – Outubro/2009