International Journal of Engineering Trends and Technology (IJETT) – Volume X Issue Y- Month 2015
The influence of surface state density at the interfaces of n-Ge
Schottky contacts modified with conducting polymer
Dr. A. Ashok Kumar#
#
Assistant Professor, Department of Physics
Y.S.R. Engineering College of Yogi Vemana University, Proddatur, Andhra Pradesh, India.
1
drashok.yvuce@gmail.com
Abstract—The influence of organic interlayer poly (3,4ethylene dioxythiophene) doped with poly (styrene sulphonate
(PEDOT:PSS) on the Schottky barrier parameters of Pt/n-Ge
Schottky contacts has been investigated. The Schottky barrier
height and ideality factor of the Pt/PEDOT:PSS/n-Ge
Schottky contact is found to be 0.69 eV and 1.58. The series
resistance of the Schottky contacts is evaluated using Cheung
and Cheung functions. It is more important to know how the
barrier height varies with the applied voltage in the forward
bias condition. Because of the potential drop across the
interfacial layer, the zero bias barrier height is different from
that expected in an ideal diode. Similarly the potential across
the interfacial layer varies with bias because of the electrical
field present in the semiconductor. The change in the interface
state charge as a result of the applied voltage thus modifies
the barrier height. By taking into account the bias dependence
of the effective barrier height, the energy distribution of
interface state density determined from the forward bias I–V
characteristics decreases exponentially with bias from
2.72×1012 eV−1 m−2 to 0.15×1012 eV−1 m−2. The space
charge limited current (SCLC) and trap charge limited
current (TCLC) are observed the dominant transport
mechanisms at large forward-bias voltages.
Keywords — Schottky contact, n-Ge, PEDOT:PSS, Series
Resistance, Photovoltaic cells.
I. INTRODUCTION
This Germanium semiconductor as an alternative channel
material for high-speed complementary metal–oxidesemiconductor (CMOS) devices due to its higher mobility of
both the electrons and holes compared to Si [1,2]. However,
Fermi-level pinning (FLP) at the charge neutrality level close
to the valence band edge of Ge results in high electron barrier
of metal contact to n-type Ge, serving as a major barrier for
future Ge-based devices.
Until now, considerable efforts have been made to
minimize the effects of FLP on metal/Ge interfaces by the
insertion of an insulator layers such as oxides and nitrides in
between the metal and Ge. Similarly, the introduction of an
organic interlayer to the metal/Ge interface is another
approach to modulate the Schottky barrier height. Poly(3,4ethylene dioxythiophene): poly (styrene
sulfonate)
(PEDOT:PSS) conducting polymer has superior properties
such as high conductivity, structural stability, optical
ISSN: 2231-5381http://www.ijettjournal.org
transparency, and processability makes it suitable for various
applications in electronic devices [3]. This paper discusses
about the fabrication of Pt/n-type Ge Schottky rectifier with a
PEDOT:PSS interlayer and demonstrate its electrical
properties.
II. EXPERIMENTAL DETAILS
N-type Ge (100) (Sb-doped) wafers with a doping
concentration of 2x1018 cm-3 were used as a starting material.
After removing native oxide using a diluted HF solution, the
PEDOT:PSS films were spin-coated at 2000 rpm for 60 s,
followed by baking at 150 C for 30 min. Finally, 30-nm-thick
Pt electrodes were formed on the top of the PEDOT:PSS thin
films by means of Pt sputtering through a metal mask with
diameter of 500 μm. The I–V characteristics of the
Pt/PEDOT:PSS/n-type Ge Schottky contacts were measured at
room temperature using a precision semiconductor parameter
analyzer (Agilent 4155C).
III. Results and Discussion
The Current-Voltage (I-V) characteristics of the
Pt/PEDOT:PSS/n-type Ge Schottky rectifier measured at
room temperature is shown in Fig. 1. The device exhibited
good rectification, implying the thermionic emission theory
can be used to assess the electrical parameters of the device.
The values of ideality factor and barrier height were found to
be 1.58 and 0.69 eV, respectively. The relative deviation of
ideality factor from unity could be attributed in part to the
effects of the voltage drop across the PEDOT:PSS interlayer
and to secondary mechanisms such as interface dipoles or
specific interface structure as well as fabrication-induced
defects at the interface [4].
It can be noted that the barrier height of Pt/PEDOT:PSS/ntype Ge Schottky contact is higher than that of Pt/n-type Ge
Schottky contacts [5, 6]. This suggests that the PEDOT:PSS
interlayer modifies the effective barrier height by influencing
the space charge region of the Pt/n-type Ge Schottky junction
[7]. Namely, the PEDOT:PSS interlayer produces a
substantial shift in the work function of the metal and in the
electron affinity of the semiconductor, and in turn confers
excess barrier height.
Usually, the forward bias I–V characteristics are linear in
the semi-logarithmic scale at low voltages. Considerable
deviation from the linearity at higher voltages may be due to
the effects of parameters such as the series resistance R s, the
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International Journal of Engineering Trends and Technology (IJETT) – Volume X Issue Y- Month 2015
interlayer is likely to retain forward current conduction in
Pt/PEDOT:PSS/n-type Ge Schottky contact
At high forward bias voltages, the trapped carriers provide
reliable information about the conduction paths and the
density of states which can be evaluated from the log-log
scales of I-V characteristics. The forward-bias log(I) versus
log(V) plot of the PEDOT:PSS/n-Ge Schottky contact is
Fig 1. Forward and reverse bias current-voltage characteristics of
Pt/PEDOT:PSS/n-type Ge Schottky contacts measured at
room temperature.
interfacial layer and interface states [8]. The parameter Rs
is only effective in the downward-curvature region (non-linear
region) of the forward I–V characteristics at sufficiently high
applied voltage. At high currents, there is always a deviation
of the ideality that has been clearly shown to depend on the
interface state density and bulk series resistance, as one would
expect. The lower the interface state density and the series
resistance, the greater is the range over which ln I(V) does in
fact yield a straight line. Furthermore, ideality factor (n) and
the series resistance (Rs) were evaluated using a method
developed by Cheung and Cheung.
The series resistances extracted from the plot of dV/d(lnI)
vs. I were almost identical to those from the plot of H(I) vs.
I (shown in Fig.2), implying their consistency and validity. It
should be noted that the series resistance of Pt/PEDOT:PSS/ntype Ge Schottky contact is much higher than that of Pt/n-type
Ge Schottky contact [5]. This implies that the series resistance
is a current-limiting factor for Pt/PEDOT:PSS/n-type Ge
Schottky contact. In other words, the voltage drop across
series resistance caused by the presence of an organic
Fig 2. dV/d(lnI) vs. I and H(I) vs. I plots obtained from the forward
I–V characteristics of the Pt/PEDOT:PSS/n-type Ge Schottky
contacts.
ISSN: 2231-5381http://www.ijettjournal.org
Fig 3. Forward bias log (I) versus log(V) characteristics of
Pt/PEDOT:PSS/n-Ge contacts
shown in Figure 3. Figure shows the three distint regions in
the forward-bias current-voltage characteristics. It is observed
an ohmic region at low voltages up to a transition voltage of
about 0.2V with a slope of about 1.28. At increased voltages
current also increases which satisfies the relation IαVm where
m varies with the injection level and trap charges distribution.
The least square fit of region II shows a slope of about 5.61
which represents an exponential distribution of traps in the
band gap of the organic material and the current conduction is
purely due to Space charge limited current. The slope in the
region III is found to be 3.35. The current conduction in
region III could be associated with trap-filled limit. At higher
voltages, the decreased slope is in region III than compared to
region II may be due to the high injection level whose
dependence is the same as in the trap charge limited current
[9].
In n-type semiconductors, the energy of the interface states
ESS with respect to the bottom of the conduction band at the
surface of the semiconductor (EC-ESS) is plotted against the
interface state density (NSS) in fig. 4. As seen from figure, it is
observed that the exponential growth of the interface state
density from midgap towards to the bottom of the conduction
band in the range from (EC-0.352) to (EC-0.677) eV. The
density distribution of the interface states of the
Pt/PEDOT:PSS/n-Ge Schottky diode changes from 2.72×1012
eV−1 m−2 to 0.15×1012 eV−1 m−2.
The interface layer, interface states and fixed charges
between
the
polymeric
organic
compound/
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International Journal of Engineering Trends and Technology (IJETT) – Volume X Issue Y- Month 2015
semiconductor structures play an important role in the
determination of the characteristic parameters of the devices
which generally deviates the forward and reverse bias
characteristics of the Pt/PEDOT:PSS/n-Ge from its ideal
characteristics.
3.0
[3]
[4]
[5]
Pt/PEDOT/n-Ge
-2
NSS ( 10 eV m )
2.5
[6]
12
-1
2.0
1.5
[7]
1.0
Y. Chen, K.S. Kang, K.J. Han, K.H. Yoo, J. Kim, ―Enhanced optical
and electrical properties of PEDOT: PSS films by the addition of
MWCNT-sorbitol‖ Synth. Met. Vol. 159, pp. 1701–1704, sept.2009.
W. Monch, ―Barrier heights of real Schottky contacts explained by
metal-induced gap states and lateral inhomogeneities‖ J. Vac. Sci.
Technol. B, Vol. 17, pp.1867-1876, May 1999.
A. Ashok Kumar, V. Rajagopal Reddy, V. Janardhanam, Min-Woo Seo,
Hyobong Hong, Kyu-Sang Shin, Chel-Jong Choi, ―Electrical
Properties of Pt/n-Ge Schottky Contact Modified using Copper
Phthalocyanine (CuPc) Interlayer‖ J. Electrochem. Soc. Vol. 159, pp.
H33–H37, 2012.
A. Ashok Kumar, V. Rajagopal Reddy, V. Janardhanam, Hyun-Deok
Yang, Hyung-Joong Yun, Chel-Jong Choi, ―Electrical properties of
Pt/n-type Ge Schottky contact with PEDOT:PSS interlayer‖, J. Alloys
Comp. Vol. 549, pp. 18-21, 2013.
M. Cakar, N. Yildirim, S. Karatas, C. Temirci, A. Turut, “Currentvoltage and capacitance-voltage characteristics of Sn/rhodamine-101∕n-
0.5
0.0
0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75
Ec-ESS (eV)
Fig 4. Energy distribution of the interface state density of
Pt/PEDOT:PSS/n-Ge Schottky contacts.
Briefly, polymeric organic/inorganic semiconductor diodes
may be useful to increase the quality of devices fabricated by
using the semiconductor in establishing processes for
minimizing surface states, surface damage and contamination
and actively tunable barrier heights.
[8]
[9]
Si and Sn/rhodamine-101∕p-Si Schottky barrier diodes” J. Appl. Phys.
Vol. 100, pp. 074505-74510, Oct. 2006.
A.R.V. Roberts, D.A. Evans, ―Modification of GaAs Schottky diodes
by thin organic interlayers’ Appl. Phys. Lett. Vol. 86, pp. 072105072107, Feb. 2005.
T. Kılıcoglu, M.E. Aydın, G. Topal, M.A. Ebeoglu, H. Saygılı, ―The
effect of a novel organic compound chiral macrocyclic tetraamide-I
interfacial layer on the calculation of electrical characteristics of an
Al/tetraamide-I/p-Si contact‖, Synthetic Metals vol. 157, pp. 540–545,
July 2007.
III. CONCLUSIONS
The fabrication and analysis of distribution of surface states
of Pt/PEDOT:PSS/n-Ge Schottky contacts has been
investigated. The Schottky barrier height and ideality factor of
the Pt/PEDOT:PSS/n-Ge Schottky contact is found to be 0.69
eV and 1.58. It is observed that the potential across the
interfacial layer varies with bias because of the electrical field
present in the semiconductor. The change in the interface state
charge as a result of the applied voltage thus modifies the
barrier height. The energy distribution of interface state
density determined from the forward bias I–V characteristics
decreases exponentially with bias from 2.72×1012 eV −1 m−2 to
0.15×1012 eV−1 m−2. The dominant transport mechanisms at
large forward-bias voltages may be attributed to space charge
limited current (SCLC) and trap charge limited current (TCLC)
mechanisms.
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ISSN: 2231-5381http://www.ijettjournal.org
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