electrical conductivity studies in single and...

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


-.-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


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


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


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



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


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



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



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


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



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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