electrical conductivity studies in single and...
TRANSCRIPT
ELECTRICAL CONDUCTIVITY STUDIES IN SINGLE AND MULTILAYER THIN FILMS OF
CuS, PbS, CdS and CuPc
4.1 Introduction
The basic property of a semicor~ductor is its electrical conductivity, which
depends on the mobility and concentration of the charge carriers. In
semiconductors, the electrical properties are sensitive to the impurity content and
doping.'4 Inorganic semiconductors are characterized by covalent bonding
between ions of the crystal. Electrons c& be excited optically or thermally,
promoting free electrons into the conduction band and leaving holes in the
valence band. Under an applied electric field, the free charge carriers are
transported causing conduction.
Phthalocyanines are organic semiconduct.ors, whose electrical properties have
considerable importance owing to their potential application in electronic devicessb and
sensor The electronic transport in phthalocyanines show ohmic conduction
at low voltages and space charge limited conduction at high ~ o l t a ~ c s . ' ~ " Discrete trap
levels located in the conduction band is dominated either by an exponential trap
dist~ibution'~ or by a uniform trap di~tribution.'~ Qin Zou et.alI5 liave measured the
dielectric properties of sol-gel derived rnultilayer films of Pb and Ba. Mirkarirni et. a1'
have suggested a method for preparing MoISi rnultilayer films. Pontes et. all7 also
measured the dielectric properties and microstructure of SrTiOIBaTiO multilayer films
70
prepared by chemical methods. Neyts et. all8 have investigated the electrical properties
of white SrSIZnS rnultilayer thin films. Shihub and ~ o u l d ' ~ have calculated the
activation energy of cobalt phthalocyanine (CoPc) films as 0.54 eV. Gould and
assa an^' have also measured the activation energy of CoPc films. h this chapter we
present the electrical conductivity studies on chemically prepared thin films of CuS,
PbS, CdS, rnultilayer PbS-CuS, vacuum sublimed CuPc and multilaycr films of
metallic sulphides with copper phthalocynine.
4.2 Theory
The electrical conductivity in semiconductors is caused by thermal excitation
of electrons, impurities, lattice defects and nonstoichiometry. A highly purified
semiconductor exhibits intrinsic conductivity. In the temperature range at which the
intrinsic conductivity is exhibited, the electrical properties of the crystal are not
modified by impurities. In inorganic semiconductors as the temperature is increased
from absolute zero, electrons are thermally excited from the valence band to the
conduction band. The conductivity due to the electrons and holes is,
where n and pt are the carrier concentration and mobility of the electrons and p
and FL~, are the corresponding quantities for the holes. In an intrinsic
semiconductor, the number of electrons is equal to the number of holes. The
expression for carrier concentration is given by
ni = Nc exp Er/ kB T
pi = Nv exp - (EF+ Eg)/ kH T
7 1
where N, and N, are the density of states in the conduction band and valence
band Eg is the forbidden energy gap, ko and T are the Boltzmann's constant and
absolute temperature respectively.
N, and Nv are given by
2 3/2 N, = 2(2xm,* k ~ T / h )
2 312 Nv = 2(2 n mh* kB T 1 h )
where m* and mh* are the effective masses of the e1,ectrons and holes respectively.
Since nj = pi
2 312 ni = pi = 2 ( 2 x k s T / h ) (rn,*rnh* )312 e x p ( - E g / 2 k R T )
= A e x p ( - E g / 2 k B T ) 4.2.4
where A is a constant.
If we assume that the variation of mobility of the electrons and holes in an
electric field with temperature is small, then conductivity, which is proportional
to the number of carriers has a variation of the form
where 00 is a constant. Such an exponential variation of electrical conductivity is
known for semiconductors. Multiple donor levels exist within the forbidden
energy gap and deeper levels can be frozen out as the temperature is increased.
Conductivity in these films is due to both hopping of holes and charge
transport via excited states. In such a case, the conductivity is given by
72
where El is the intrinsic energy gap and E2, E3, the activation energy needed to excite
the caniers from the corresponding trap levels to the conduction band. A, B, C are
constants.
The conductivity 'o' of a film of resistance 'R', length 'I' breadth 'b' and
thickness 't' is given by
4.3 Experiment
CuS, PbS, CdS and multilayer PbS-CuS films are prepared by chemical
deposition technique2 ' as described in chapter 3. The copper phthalocyanine
(CuPc) powder used in this study is obtained from Aldrich chemical company
Inc: USA. Thin films of CuPc are deposited at room temperature onto pre-cleaned
glass substrates with pre-evaporated high purity silver electrodes, at a base
pressure of 10-5 Torr using a Hind Hivac Vacuum coating unit. The evaporation
is carried out by resistive heating of the CuPc powder from a molybdenum boat
and the rate of sublimation is kept constant. The optimum rate of evaporation is
adjusted to be 10-1 5 nm per minute.
For multilayer films, the sulphide films are used as substrates. CuPc is
evaporated onto these sulphide films with pre-evaporated high purity silver
electrodes, at a base pressure of Torr by resistive heating from a
molybdenum boat as per the procedure described in section 3.6 of chapter 3. The
optimum rate of evaporation is adjusted to be 10- 15 nm per minute.
73
For electrical conductivity measurements ohmic electrode contacts are
made on these films. Normally contacts are either ohmic or nonohmic. Ohmic
contact introduces negligible impedances to the flow of current.22 Films are
mounted over the sample holder of the conductivity cell shown in figure 2.10.1 of
chapter 2. Silver and Aluminium are used as contact electrodes. The distance
between the electrodes is 2.5 mm. Copper strands of diameter 0.8 mm are fixed
onto the films by means of the colloidal suspension of silver in a medium of aqua
or alkadag. The temperature of the film i s varied using a heater and measured by
a copper-constantan thermocouple. Resistance of the film is measured at regular
intervals of 5K using a programmable Keithley electrometer (model 110.617) by
the two probe technique shown in figure 2.1 1. l (a) of chapter 2. The electrical
conductivity is obtained using equation 4.2.7. Electrical conductivity
measurements are separately carried out in vacuum of -10" Ton for as deposited
and annealed CuS, PbS, CdS, multilayer PbS-CuS, CuS-CuPc, PbS-CuPc and
CdS-CuPc thin films. Thickness of the films has been measured by Tolanskys
mu1 tiple beam interference technique as described in section 2.8 of chapter 2.
4.4 Results and Discussion
4.4.1 CuPc films
The resistance of CuPc film has been measured in the temperature range
300-500K using a programmable Keithley electrometer. A biasing voltage of 5V
is selected and applied onto the sample. All the measurements are carried out in a
dynarnical vacuum of lom3 Tom. The electrical conductivity o is calculated using
equation 4.2.7 knowing the length, breadth and thickness of the film. Riehl and
I3aessleS3 confirm more than one activation energy in aromatic compounds. In
thc extrinsic conduction region the charge carriers move by hopping along with
the ions or electrons.24 Aoyagi et. a1 25 have reported an activation energy of 1.98
eV for CuPc single crystals. ~ a m a n n ~ ~ have reported an activation energy of 1.96
for CuPc thin films. Hassan and ~ o u l d * ' obtained much lower values of 0.3 eV
and 0.1 eV for CuPc at substrate temperature 423 K.
A gaph is plotted with Ln o along the y-axis and 1000/T along the x-axis.
Figure 4.4.1 gives the Ln 0 versus 1000R plot for CuPc films of thickness 2180A.
There are three lineaf regions for the gaph in figure 4.4.1. From the slope of the linear
portions, the values of the activation energy are calculated. The activation energy is
determined within an accuracy of _+ 0.0 1 eV in all measurements.
The activation cnergy for CuPc is collected in Table 4.4.1 for as deposited
and annealed samples. It is seen that the activation energy decreases with
annealing temperature. El arises from the intrinsic charge carriers and E2 and E3
from the extrinsic conduction due to impurity scattering.
Table 4.4.1. Variation of activation energy with annealing tempera turc for CuPc thin film of thickness 2180 A
Figure 4.4.1 Plot of Ln (o) Vs 10001T for CuPc thin films of thickness 21 80 A
Samples
1. As deposited
2. Amealed at 523 K
3. Annealed at 573 K
Activation energy ( eV
Et E2 -E3
0.70 0.60 0.18
0.58 0.58 0.17
0.50 0.27 0. 05
4.4.2 CuS films
Electrical studies are done to determine the thermal activation energy and the
effect of annealing on activation energy.28 The studies are carried out in the
temperature range 300-500 K in a vacuum of lo5 Torr to avoid contamination of the
film. The resistance of CuS film has been measured in the temperature range 300-
500K using a programmable Keithley electrometer. A biasing voltage of 5V is selected
and applied onto the sample. All the measurements are carried out in a dynamical
vacuum of 1 o5 Torr. The electrical conductivity o is calculated using equation
4.2.7. A graph is plotted with Ln a along the y-axis and 1000IT along the x-axis.
The electrical conductivity as a function of inverse of temperature of as deposited
and annealed CuS films of thickness 3120 A are given in figure 4.4.2. From the
slope of the graph the activation energy is calculated. The activation energy is
determined within an accuracy of + 0.01 eV. The activation energy of the
samples varies with annealing tern perature. Each curve has three linear regions,
which give E l , Ez, and E3. The activation energies in the intrinsic region (El) and
impurity scattering regions (Ez and E3) are calculated. The activation energy
for CuS is collected in table 4.4.2. From the present study it is seen that the
activation energy of the samples decreases with annealing temperature.
Table 4.4.2. Variation of activation energy with annealing temperature for CuS film of thickness 3120 A
Samples
1 3. Annealed at 573 K 1 0.34 0.25 0.16 I
1. As deposited
2. Annealed at 523 K
-.-CuS-As dep. --0-Ann.523K -A-Ann.573K I,
0.73 0.33 0.17
0.53 0.5 1 0.16
Figure 4.4.2 Plot of Ln (G) Vs 1 OOOm for CuS thin films of thickness 3 120 A
4.4.3 Multilayer CuS-CuPc films
Electrical studies are done to determine the thermal activation energy and the
effect of annealing on activation energy.'"he studies are carried out in the
temperature range 300-500 K in a vacuum of 10" Ton to avoid contamination of the
film. The resistance of CuS-CuPc film has been measured in the ternpcrature range
300-500K using a programmable Keithley electrometer. All the measurements are
carried out in a dynarnical vacuum of lo5 TOIT. The electrical conductivity o is
calculated using equation 4.2.7 knowing the length, breadth and thickness of the
film. A graph is plotted with Ln o along the y-axis and 1000/T along the x-axis. The
electrical conductivity as a function of inverse of temperature of the as deposited and
the annealed CuS-CuPc films of thickness 5340 A are given in figure 4.4.3. From the
slope of the graph the activation energy is calculated. The activation energy is
determined within an accuracy of 2 0.01 eV. The activation energy of the samples
varies with annealing temperature. Each curve has three linear regions, which give
El , E2, and E3. The activation energies in the intrinsic region (El) and impurity
scattering regions (E2 and Ej) are calculated. The activation cncrgy for ZnS-CuPc is
collected in table 4.4.3 for as deposited and annealed samples. The activation
energy of the samples decreases with annealing temperature.
Table 4.4.3. Variation of activation energy with annealing temperature for multilayer CuS-CuPc film of thickness 5340 A
1 -.- CuS-CuPc-As dep. 1
Samples
1. As deposited
2 . Annealed at 523 K
3. Annealed at 573 K
Figure 4.4.3 Plot of Ln(cr) Vs 1000/T for rnultilayer CuS-CuPc thin films of thickness 5340 A
Activation energy ( eV )
El E2 E3
0.34 0.26 0.06
0.26 0.13 0.06
0.24 0.06 0.05
4.4.4. PbS films
Electrical studies are done to determine the thermal activation energy and the
effect of annealing on activation energy. The studies are carried out in the temperature
range 300-500 K in a vacuum of 10" Torr to avoid contamination of the film. I l ~ e
resistance of PbS film has been measured in the temperature range 300-500K using a
programmable Keithley electrometer. A biasing voltage of 5V is selected and
applied onto the sample. All the measurements are carried out in a dynamical
vacuum of 1 0-J Torr. The electrical conductivity cr is calculated using equation 4.2.7
knowing the length, breadth and thickness of the film. A graph is plotted with Ln o
along the y-axis and 1000/T along the x-axis. The electrical conductivity as a function
of inverse of temperature of as deposited and annealed PbS films of thickness 4 150 A
are given in figure 4.4.4. Each curve has three linear regions, which give El, E2, and E3.
From the slope of the gaph the activation energy is calculated within an accuracy of
+ 0.0 1 eV and is found to vary with annealing temperature. The activation energy for -
PbS is collected in table 4.4.4 for as deposited and annealed samples. The activation
energy of the samples decreases with annealing temperature.
Table 4.4.4. Variation of activation energy with annealing temperature for PbS film of thickness 4150 A
Figure 4.4.4 Plot of Ln(o) Vs lOOO/T for PbS thin films of thickness 41 50 A
Samples
1. As deposited
2. Annealed at 523 K
3. Annealed at 573 K
Activation energy ( eV
El E2 E3
0.38 0.32 0.19
0.29 0.26 0.17
0.22 0.19 0.14
4.4.5 Multilayer PbS-CuPc films
Electrical studies are done to determine the thermal activation energy and the
effect of annealing on activation energy. The studies are canied out in the temperature
range 300-500 K in a vacuum of lo5 Torr to avoid contamination of the film. The
resistance of P bSCuPc film has been measured in the temperature range 300-500K
using a programmable Keithley electrometer. All the measurements are canied out in a
dynamical vacuum of 10;' Ton: The electrical conductivity cr is calculated using
equation 4.2.7 knowing the length, breadth and thickness of the film. A graph is plotted
with Ln o along the y-axis and 1 000K along the x-axis. The electrical conductivity as
a function of inverse of temperature of as deposited and annealed PbS-CuPc films of
thickness 6450 A are given in figure 4.4.5. From the slope of the graph the activation
energies in the intrinsic region (El) and impurity scattering regions (Ez and E3) are
calculated. The activation energy is determined within an accuracy of + 0.01 eV. The
activation energy for PbS-CuPc is collected in table 4.4.5 for as deposited and annealed
samples. The activation energy of the samples varies with annealing tempenture.
Table 4.4.5. Variation of activation energy with annealing temperature fur multilayer PbS-CuPc thin film of thickness 6450 A
--a- Ann. 523 K --A- Ann. 573 K
Samples
1. As deposited
2. Annealed at 523 K
3. Annealed at 573 K
Figure 4.4.5 Plot of Ln(o) Vs 10001T for multilayer PbS-CuPc thin film of thickness 6450A
Activation energy ( eV )
El E2 E3
0.68 0.65 0.56
0.67 0.60 0.18
0.62 0.47 0.15
4.4.6 CdS films
The resistance of CdS film has been measured in the temperature range
300-500K using a programmable Keithley electrometer. A biasing voltage of 5V
is selected and applied onto the sample. All the measurements are carried out in a
dynamical vacuum of 10" Torr. The electrical conductivity o is calculated using
equation 4.2.7 knowing the length, breadth and thickness of the film. A graph is
plotted with Ln o along the y-axis and 1000/T along the x-axis. Figure 4.4.6
gives the Ln o versus 1000/T plot for CdS films of thickness 6100A. The
activation energy is determined within an accuracy o f t 0.01 eV. The activation
energy for CdS is collected in table 4.4.6 for as deposited and annealed samples.
There are three linear regions for the graph in figure 4.4.6. From the slope of the
linear portions, the values of the activation energy are calculated. From the
present study, it is seen that the activation energy decreases with annealing
temperature. E l arises from the intrinsic charge carriers and Ez and E3 depend on
the extrinsic conduction due to impurity scattering.
Table 4.4.6. Variation of activation energy with annealing temperature for CdS thin film of thickness 6100 A
Figure 4.4.6 Plot of Ln (o) Vs 1000/T for CdS thin films of thickness 6100 A
Samples
1. As deposited
2. Annealed at 523 K
3. Annealed at 573 K
Activation enerev ( eV )
El E2 E3
0.64 0.54 0.13
0.4 8 0.36 0.1 2
0.28 0.23 0.1 1
4.4.7 Multilayer CdS-CuPc thin films
The resistance of multilayer CdS-CuPc film has been measured in the
temperature range 300-500K using a programmable Keithley electrometer. A
biasing voltage of 5V is selected and applied onto the sample. All the
measurements are carried out in a dynarnical vacuum of 10" Ton. The electrical
conductivity o is calculated using equation 4.2.7. A graph is plotted with Ln cr
along the y-axis and 10001T along the x-axis. Figure 4.4.7 gives the Ln o versus
lOOO/T plot for CdS-CuPc films of thickness 8250A. There are three linear
regions in all the curves. The activation energy is determined within an accuracy
of + 0.01 eV. The activation energy for multilayer CdS-CuPc is collected in
table 4.4.7 for as deposited and annealed samples. From the slope of the linear
portions, the values of the activation energy are found out. From the present
study, it is seen that the activation energy decreases with annealing. El arises
from the intrinsic charge carriers and E2 and E3 depend on the extrinsic
conduction due to impurity scattering.
Table 4.4.7. Variation of activation energy with annealing temperature for multilayer CdS - CuPc thin film of thickness 8250 A
Samples Activation energy ( eV 1
El E2 E3
1. As deposited
2. Annealed at 523 K
1 3. Annealed at 573 K 1 0.36 0.25 0. 18
Figure 4.4.7 Plot of Ln(c~) Vs 1000/T for multilayer CdS-CuPc films of thickness 8250 A
4.4.8 Multilayer PbS-CUS films
The resistance of multilayer PbS-CuS film has been measured in the
temperature range 300-500K using a programmable Keithley electrometer. A
biasing voltage of 5V is selected and applied onto the sample. All the
measurements are carried out in a dynamical vacuum of 10') Torr. The electrical
conductivity o is calculated using equation 4.2.7 knowing the length, breadth and
thickness of the film. A graph is plotted with Ln o along the y-axis and 1 OOOIT
along the x-axis. Figure 4.4.8 gives the Ln o versus 1000/T plot for PbS-CuS
films of thickness 9350A. The activation energy is determined within an accuracy
of + 0.0 1 eV. The activation energy for P bS-CuS is collected in table 4.4.8 for as
deposited and annealed samples. There are three linear regions in all the curves.
From the slope of the linear portions, the values of the activation energy are
determined. E l arises from the intrinsic charge carriers and E2 and E3 depend on
the extrinsic conduction due to impurity scattering. From the present study, it is
seen that the activation energy decreases with annealing temperature.
Table 4.4.8. Variation of activation energy with annealing temperature for multilayer PbS-CuS thin film of thickness 9350 A
Samples Activation energy ( eV ')
1. As deposited
2. Annealed at 523 K
3. Annealed at 573 K
I -=- PbS-CuS-As dep. I
Figure 4.4.8 Plot of Ln(o) Vs 1000TT i'or multilayer PbS-CuS film of tllickness 9350 A
4.5 Conclusion
Electrical conductivity and thermal activation enerLy of the a5 deposited and
annealed CuPc, CuS, PbS, CdS, multilayer CuS-CuPc, PbS-CuPc, CdS-CuPc and
PbS-CuS thin films have been studied. Electrical conductivity by thermal activation
process is found to involve different conduction mechanisms. For CuPc in the high
temperature range, intrinsic conductivity by holes are found to contribute to the
conduction process whereas in the low temperature range, impurities are found to
play an active role. The change in canier activation energy is indicated by the
change in the slope of the plot.
For all the samples, activation energy is found to decrease with annealing
temperature. The activation energy measurements provide a measure of the
trapping levels. Annealing causes a redistribution of traps and hence a drop in
activation energy.
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