Materials Transactions, Vol. 55, No. 5 (2014) pp. 758 to 762
© 2014 The Japan Institute of Metals and Materials
Electrical Properties and Carrier Transport Mechanism of Au/n-GaN Schottky
Contact Modified Using a Copper Pthalocyanine (CuPc) Interlayer
V. Janardhanam1, I. Jyothi1, Ji-Hyun Lee1, Jae-Yeon Kim1,
V. Rajagopal Reddy2 and Chel-Jong Choi1,+
1
School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center (SPRC),
Chonbuk National University, Jeonju 561-756, Korea
2
Department of Physics, Sri Venkateswara University, Tirupati-517 502, India
We investigated the electrical characteristics of Au/n-GaN Schottky rectifier incorporating a copper pthalocyanine (CuPc) interlayer using
current­voltage (I­V), capacitance­voltage (C­V) and conductance­voltage (G­V) measurements. The barrier height of the Au/CuPc/n-GaN
Schottky contact was higher than that of the Au/n-GaN Schottky diode, indicating that the CuPc interlayer influenced the space charge region of
the n-GaN layer. The series resistance of Au/CuPc/n-GaN Schottky contact extracted from the C­V and G­V methods was dependent on the
frequency. In addition, the series resistance obtained from C­V and G­V characteristics was comparable to that from Cheung’s method at
sufficiently high frequencies and in strong accumulation regions. The forward log I­log V plot of Au/CuPc/n-GaN Schottky contact exhibited
four distinct regions having different slopes, indicating different conduction mechanisms in each region. In particular, at higher forward bias, the
trap-filled space-charge-limited current was the dominant conduction mechanism of Au/CuPc/n-GaN Schottky contact, implying that the most
of traps were occupied by injected carriers at high injection level. [doi:10.2320/matertrans.M2013449]
(Received December 16, 2013; Accepted February 28, 2014; Published April 11, 2014)
Keywords: GaN, Schottky contact, CuPc, series resistance, space-charge-limited current
1.
Introduction
GaN has established a place for itself in the field of solidstate devices, with applications ranging from light-emitting
diodes1) and visible-blind photodetectors2) to high power
Schottky diodes3) and high electron mobility transistors.4)
Among them, Schottky devices such as ultraviolet photodetectors, solar-blind diodes and high electron mobility
transistors have recently experienced spurts of further
development.5­7) Since the performance of these devices is
often limited by the quality of the Schottky contacts, a full
understanding of the nature of the electrical characteristics is
of vital importance to the control of this phenomenon. It is
well known that the interfacial properties of metal-semiconductor contacts have a dominating influence on the device
performance, reliability and stability.8) It is possible to
modify the Schottky barrier heights of metal-semiconductor
contacts by the deposition of an organic film between the
metal and inorganic semiconductor;8­10) this modulation is
often attributed to the insertion of a dipole layer between the
semiconductor and the organic film despite the clean and
unreacted appearance of the organic­inorganic interface.11,12)
This modification could lead to an efficient barrier height and
interface modification. Some studies have shown that the
electronic parameters of the metal-inorganic diodes were
modified with various organic materials.13­17) The control of
metal/semiconductor diodes with organic materials may thus
be the key to the fabrication of reproducible metal/organic
semiconductor/inorganic semiconductor rectifying devices.
In recent years, there has been growing interest in the
fabrication and characterization of Schottky barrier diodes
using organic semiconductor materials due to their wide
range of potential electronics technology applications. Thus
far, many attempts have been made to modify and
+
Corresponding author, E-mail: cjchoi@chonbuk.ac.kr
continuously control the barrier height at metal/inorganic
semiconducting interfaces using an organic semiconducting
layer, thus providing a new approach for the construction of
high-performance electronic devices that take advantage of
both organic and inorganic semiconductors. Among the
various organic semiconductors, pthalocyanine-based materials have attracted considerable attention because of their
chemical and thermal stability and their tendency to form
highly ordered layers, resulting in increased device efficiency.18) Of the pthalocyanines, copper pthalocyanine
(CuPc) has been widely used because of their potential
applications to gas sensors, organic light-emitting diodes,
organic photovoltaic cells and electronic devices including
field effect transistors (FETs).19­23) As the bandgap of CuPc is
about 2.3 eV and the bandgap of n-GaN is 3.4 eV, the vacuum
levels of n-GaN and CuPc generally do not align. This
difference in vacuum levels is attributed to the formation of
interface dipole. The increase or decrease in effective barrier
height for the organic modified Schottky diode can be
correlated with the energy level alignment of the lowest
unoccupied molecular orbital (LUMO) with respect to the
conduction band minimum (CBM) of the inorganic semiconductor at the organic/inorganic semiconductor interface.12) Inspite of these foremost features of an organic
interlayer, the effort in the implementation of the organic
interlayer to GaN-based SBDs is rather scarce. In this work,
we fabricated and investigated the electrical properties of
Au/n-GaN Schottky rectifier modified by CuPc interlayer
using current­voltage (I­V), capacitance­voltage (C­V) and
conductance­voltage (G­V) measurements. It will be shown
that the introduction of a CuPc organic interlayer is effective
in modification of Schottky barrier of the Au/n-GaN
Schottky diode. It will be further shown that carrier
conduction of the Au/CuPc/n-GaN Schottky contact is
dominated by trap-filled space-charge-limited current at
higher forward bias region.
Electrical Properties and Carrier Transport Mechanism of Au/n-GaN Schottky Contact
2.
Experimental Details
Si-doped n-type GaN wafers (2 µm in thickness) grown on a
C-plane Al2O3 sapphire substrate with a metal organic chemical
vapor deposition (MOCVD) technique were used as a starting
material. The typical carrier concentration in the n-GaN layer
as measured by Hall measurements was 4.07 © 1017 cm¹3 and
matched with the value provided by the manufacturer. The nGaN wafer was first ultrasonically degreased in warm trichloroethylene followed by acetone and methanol for 5 min each.
The wafer was then dipped into boiling aqua-regia in a
HNO3:HCl ratio of 1 : 3 for 10 min to remove the native oxide
and later rinsed in deionized (DI) water. The 25 nm thick Ti and
100 nm thick Al films were then deposited on a portion of the
sample defined by photolithography as ohmic contacts by
electron beam evaporation, followed by annealing at 650°C in
ambient N2 for 3 min. A 15-nm-thick CuPc was deposited by
thermal evaporation of powdered CuPc. During the evaporation
process, the chamber pressure was maintained at 1.3 ©
10¹4 Pa. Finally, circular Schottky electrodes of diameter
200 µm were formed by the evaporation of Au (30 nm) on
top of the CuPc film using electron beam evaporation. The
electrical probing of the Au/n-GaN Schottky contacts with the
CuPc interlayer was performed at room temperature using a
precision semiconductor parameter analyzer (Agilent 4155C)
and a precision LCR meter (Agilent 4284A).
3.
759
Results and Discussion
Figure 1 shows the I­V characteristics of Au/n-GaN
Schottky diodes with and without CuPc interlayer. It can be
seen that both devices exhibited good rectification behavior.
The reverse current saturated with the applied bias following
the thermionic emission model. According to thermionic
emission theory, for a Schottky diode with a series resistance
(Rs) and an interfacial layer, the current across a Schottky
diode with (V > 3kT/q) can be expressed as24)
qðV IRS Þ
qðV IRS Þ
I ¼ I0 exp
1 exp
ð1Þ
nkT
kT
where I0 is the saturation current, obtained from the intercept
of the plot of ln(I) versus V at V = 0, given by
qb0
2
I0 ¼ AA T exp ð2Þ
kT
where q is the electron charge, V is the applied voltage, A is
the area of the diode, k is the Boltzmann constant, T is the
absolute temperature, Rs is the series resistance, A** is the
effective Richardson constant, )b0 is the barrier height and n
is the ideality factor. Once the saturation current I0 has been
determined, the barrier height ()b0) can be evaluated using
the expression
kT
AA T 2
ln
b0 ¼
ð3Þ
q
I0
The ideality factor n can be determined from the slope of the
linear region of the forward bias ln(I)­V using
q
dV
n¼
ð4Þ
kT dðln IÞ
Fig. 1 I­V curves of the Au/n-GaN Schottky contacts with and without
CuPc interlayer.
The barrier height and ideality factor were calculated to be
1.02 eV and 1.33 for the Au/CuPc/n-GaN structure and
0.89 eV and 1.14 for the Au/n-GaN Schottky diode. The
barrier height value calculated for the Au/n-GaN Schottky
diode matched well with values reported previously in the
literature.25­27) However, the barrier height calculated for the
Au/CuPc/n-GaN diode was higher than the conventional
Au/n-GaN diode. This indicates that the CuPc organic film
formed as interlayer in Au/n-GaN Schottky diode has
modified the barrier height in a significant rate by influencing
the space-charge region of the inorganic substrate. This
organic layer is believed to produce a substantial shift in the
work function of the metal and in the electron affinity of the
semiconductor, and, in turn, affects the electrical properties
of the structures. The CuPc organic layer forms a physical
barrier between the Au metal and the n-GaN substrate,
thereby preventing the metal to directly contact the semiconductor’s surface. According to Roberts and Evans, the
change in the barrier height is attributed to the change in the
inorganic substrate band bending due to interaction between
the metal layer and the organic modified substrate.28,29)
Typical ideality factors determined by the effect of image
force lowering are close to 1.01 or 1.02.30,31) The higher
ideality factor values in the Au/CuPc/n-GaN structure could
be attributed to secondary mechanisms such as interfacial
dipoles caused by the organic interlayer or a specific interface
structure or even fabrication-induced interfacial defects.
According to Tung et al.,32) larger values of ideality factor
may be associated with the presence of a wide distribution of
low-Schottky barrier patches caused by a laterally inhomogeneous barrier. Also, the image-force effect, recombinationgeneration and tunneling are all possible mechanisms that
could lead to an ideality factor greater than unity.24)
It is well known that the downward concave curvature of
forward bias I­V plots at sufficiently large voltages is caused
by the effect of series resistance (RS) and the presence of
interface states. The series resistance value can be calculated
using a method developed by Cheung and Cheung.33)
Cheung’s function (H(I)) can be expressed as
dV
nkT
¼
þ IRS
dðln IÞ
q
ð5Þ
760
V. Janardhanam et al.
Fig. 2 Plots of dV/d(ln I) vs. I and H(I) vs. I for Au/CuPc/n-GaN Schottky
contact obtained from the experimental forward I­V curves shown in
Fig. 1.
kT
q
and H(I) is given as follows:
HðIÞ ¼ V ln
I
AA T 2
HðIÞ ¼ nb0 þ IRS
ð6Þ
ð7Þ
Figure 2 shows the plot of dV/d(ln I) versus I and H(I) versus
I for the Au/CuPc/n-GaN structure. The plot of dV/d(ln I) is
linear and gives RS as the slope and nkT/q as the y-axis
intercept from eq. (5). The series resistance and ideality
factor were calculated as 1.9 k³ and 2, respectively. It was
observed that there was a large difference between the value
of ideality factor obtained from the forward bias ln(I)­V plot
and that obtained from the dV/d(ln I)­I curve. This may be
due to a combination of the voltage drop across the CuPc
organic interlayer caused by the series resistance. According
to eq. (7), the plot of H(I) versus I must be linear, as shown in
Fig. 2. The slope of this plot provides a second method to
calculate the series resistance. By using the value of ideality
factor from eq. (5), the barrier height can be obtained from
the intercept of H(I) versus I; the series resistance and barrier
height obtained from this plot were 2.4 k³ and 0.82 eV,
respectively. Differences in the barrier height values obtained
from the I­V and Cheung’s method may be attributed to the
extraction from different regions of the forward-bias current­
voltage plot.
Figures 3(a) and 3(b) represent the C­V and G­V
characteristics of an Au/CuPc/n-GaN structure at different
frequencies, respectively. The insets show the capacitance­
frequency (C­f ) and conductance­frequency (G­f ) characteristics at various bias voltages. It was observed that both the
capacitance and conductance were strongly dependent on
frequency. For instance, capacitance and conductance
increased and decreased with increasing frequency, respectively. This could be attributed to many different types of
states typically existing at interface between metal and
semiconductor.34) Capacitance is high at low frequencies
because all of the interface states are affected by the applied
signal and are able to give up and accept charges in response
to this signal. The interface state capacitance acts in parallel
with the depletion capacitance and results in a higher total
capacitance value at low frequencies. Conversely, at high
frequencies, the capacitance is low because the interface state
charges do not contribute to the diode capacitance. Namely,
Fig. 3 (a) C­V and (b) G­V curves at different frequencies obtained from
the Au/CuPc/n-GaN Schottky contact. Corresponding C­f and G­f
curves at various bias voltages are shown in insets of Figs. 3(a) and 3(b),
respectively.
the interface states could be a main cause of the discrepancy
between total capacitances measured at high and low
frequencies. Furthermore, frequency dependent G­V behavior could be associated with the enhancement of the jumping
of interface charges caused by increasing frequency. Figure 4
shows the room temperature 1/C2­V plot of the Au/n-GaN
Schottky diodes with and without CuPc interlayer measured
at a frequency of 1 MHz. The C­V relationship for the
Schottky diode is given by10)
1
2
kT
V
ð8Þ
¼
Vbi C2
¾s qND A2
q
The x-intercept (Vo) of the 1/C2­V plot is related to the builtin potential (Vbi) expressed by Vbi = Vo + kT/q. The barrier
height is given by
b0ðCV Þ ¼ V0 þ Vn þ
kT
q
ð9Þ
where Vn = (kT/q) ln (NC/Nd). The density of states in the
conduction band edge, determined from NC = 2(2³m*kT/
h2)3/2, where m* = 0.22mo, h is Planck’s constant and value
of NC is 2.53 © 1018 cm¹3 for GaN at room temperature. The
barrier heights are obtained as 1.50 and 1.20 eV for Au/nGaN Schottky diodes with and without CuPc interlayer,
respectively. The barrier heights of Au/n-GaN Schottky
Electrical Properties and Carrier Transport Mechanism of Au/n-GaN Schottky Contact
Fig. 4 1/C2­V plots of Au/n-GaN Schottky diodes with and without CuPc
interlayer measured at frequency of 1 MHz.
diodes with and without CuPc interlayer obtained from the
C­V method were higher than those obtained from the I­V
method; they are not always the same because of the different
natures of the I­V and C­V measurement techniques
themselves. It is well known that the C­V method yields
the average barrier height for the entire diode. The observed
barrier height dependence on the measurement technique is
likely due to the existence of inhomogeneities in the Schottky
barrier. When the Schottky interface is relatively rough, there
are the spatial fluctuations in the barrier height. The
capacitance is insensitive to potential fluctuations, whereas
the current across the interface depends exponentially on the
barrier height and the current flows preferentially through the
barrier minima, which can lead to underestimation of the
barrier height extracted using the I­V method.
The series resistance was calculated using C­V and
G­V characteristics in accumulation region at various
frequencies (Fig. 3), as proposed by Nicollian and Brews.35)
The series resistance RS can be determined by the following
relation:
RS ¼
G
G2 þ ð½CÞ2
ð10Þ
where ½ is the angular frequency. Based on eq. (10), the
series resistance profiles of the Au/CuPc/n-GaN structure
were extracted as a function of applied bias at various
frequencies, as shown in Fig. 5. The profiles clearly showed
that the series resistance was strongly dependent upon
frequency, which could be attributed to the particular
distribution of interface states. This suggests that special
attention be paid to the effects of the series resistance in the
application of the admittance-based measurement methods
(C­V and G­V characteristics). It should be noted that the
series resistance gave a peak depending on frequency and
decreased with increasing frequency. This behavior is
considered indicative that the trap state charges have
sufficient energy to escape from the traps located at Schottky
interface as increasing frequency. According to this method,
real series resistance values can be obtained at sufficiently
high frequencies and in strong accumulation regions.
Furthermore, the series resistance obtained from this method
at high frequency and in the accumulation region matched
with that obtained from the I­V method.
761
Fig. 5 Plot of series resistances calculated using C­V and G­V characteristics (Fig. 3) as a function of bias voltages at different frequencies
obtained from the Au/CuPc/n-GaN Schottky contact.
Fig. 6 Forward log I­log V characteristics of the Au/CuPc/n-GaN
Schottky contact.
Charge transport mechanism through the Au/CuPc/n-GaN
structure was obtained from analysis of the I­V curves. The
variation of log I with log V for the Au/CuPc/n-GaN structure
is shown in Fig. 6. The log I­log V plot showed that there
existed four distinct regions that changed the power-law
exponent of the form of I £ Vm, indicating different
conduction mechanisms in each region. The exponent m
values were calculated from the slope of linear fit to the plot of
log I versus log V. At region-I, the slope was close to unity
showing a linear dependence as I ³ V. This implies that
conduction mechanism at low voltage follows Ohm’s law
controlled by thermally activated carriers predominant over
the injected charge carriers.36) In this region, the current can be
described by the thermionic emission of carrier over the
Schottky barrier of Au/CuPc/n-GaN structure.37,38) On the
other hand, the current increased exponentially with a value of
m = 4.1 and 12.6 in region-II and region-III, respectively.
This power law relation, with m > 2, is characteristic of
space-charge-limited current associated with the filling of
traps distributed exponentially in energy.39,40) Generally, due
to the low mobility of charge carriers in organic thin films, the
injected carriers are localized by the trap states, resulting in
forming a space charge with the competition against applied
bias that limits carrier conduction.40) The traps may originate
762
V. Janardhanam et al.
from structural defects caused by nonuniformity and subatomic structure of the organic CuPc layer. Additionally, slope
in this region can be used as a criterion for comparison of the
stretching of the exponential distribution. Low slope indicates
gradual (extended) distribution, while higher slope implies
abrupt distribution.41) The region-IV showed the power-law
dependency illustrated by a decrease in the slope to a value of
m = 2.1, indicating the carrier transport in Au/CuPc/n-GaN
Schottky contact approached the trap-filled limit at high
injection level whose dependency is the same as in the trapfree space-charge-limited current.18,38,42,43) Namely, in higher
voltage region, the most of traps were occupied by injected
carriers, and then the accumulation of space charge near the
electrode resulted in the creation of a field impeding further
injection. Thus, Au/CuPc/n-GaN Schottky contact would
behave as if there are no traps in it, and the current would vary
as the square of the voltage.
4.
Conclusions
The electrical properties of the Au/n-GaN Schottky
structure modified using CuPc interlayer have been investigated using I­V, C­V and G­V characteristics. The barrier
height of the Au/CuPc/n-GaN structure was higher than that
of Au/n-GaN structure, which could be attributed to the
modification of the effective barrier height by a CuPc
interlayer causing a substantial shift in the work function of
the metal and electron affinity of the semiconductor. Due to
the different response of traps prevailing in Schottky interface
to frequency, the Au/CuPc/n-GaN Schottky contact exhibited a strong frequency dependence of series resistance. In
particular, the series resistance obtained from C­V and G­V
characteristics measured at sufficiently high frequencies and
in strong accumulation regions showed a relatively good
agreement with that from Cheung’s method. The forward
log I­log V characteristics revealed that the ohmic conduction
was the dominant carrier conduction mechanism of Au/
CuPc/n-GaN Schottky contact at low voltage, controlled by
thermally activated carriers. On the other hand, the main
feature of the forward log I­log V plot was indicative of the
domination of space-charge-limited current transport mechanism at higher voltage in Au/CuPc/n-GaN Schottky contact
depending on the traps affected by injected carriers.
Acknowledgements
This work was supported by the Priority Research Center
Program (2011-0031400) and the Converging Research
Center Program (2012K001428) through the National
Research Foundation of Korea (NRF) funded by the Ministry
of Education, Republic of Korea. It was also supported by the
R&D Program (Grant No. 10045216) for Industrial Core
Technology funded by the Ministry of Trade, Industry and
Energy (MOTIE), Republic of Korea.
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