100

CHAPTER-4

ELECTRICAL PROPERTIES OF

NANOCRYSTALLINE SnSe AND ZnSe

THIN FILMS

101

4.1 Introduction

Thin films of semiconductor nanocrystals are promising as an essential class of

materials for electronic and optoelectronic devices such as Field-Effect Transistors (FET) [1-

7], Photodetectors [8-11], Light-Emitting Diodes (LED) [12-16] and solar cells [17-18]. The

synthesis and characterization of semiconductor nanoparticles is an exciting field of research

for future applications in optoelectronics [19-20]. Nanostructured materials and in particular

semiconductor nanostructures and thin films may be exploited for their novel electronic and

optical properties. These structures are of great interest since they have potential applications

in future quantum and photoinic devices [21]. Metal chalcogenides have been the subject of

considerable research due to their technological importance in crystalline and polycrystalline

forms. ZnSe and SnSe is an attractive semiconductor material that exhibits strong size

quantization effects due to the high dielectric constant and the small effective mass of

electron and holes, suggesting that its band gap energy can be easily manipulated from the

bulk value to a few electron volts by the changes in the material’s size [22-24]. These

materials have also been used in many fields such as diode laser, memory switching devices,

holographic recording systems, photoluminescent electroluminescent devices, thin film solar

cells, non-linear optical crystal and infrared electronic devices [25-27].

Utilization of solar energy is the direct conversion of light energy to electrical energy

through photovoltaic devices, especially through solar cells. The developments in the

material science afford an extremely attractive approach towards the production of efficient

and cost effective solar cells through 2D nanostructures. However, high conversion

efficiency is achieved only by selecting suitable semiconductors exhibiting unique optical

and photovoltaic properties. Generation of charge carriers by absorbed photons in

semiconductor materials depends mainly on the energy of the absorbed photons and by

tailoring the forbidden band gap energy. The photo conducting properties of semiconductor

nanoparticles are critically important with regard to their use in electro-optic devices [28].

102

Photoconductivity is a valuable tool to probe charge separation, charge trapping and carrier

recombination mechanism in materials. Measuring the photoconductivity in nanoparticles is

a challenging problem. Furthermore, picoseconds (ps) carrier dynamics play an important

role in efficient charge separation and transport properties of nanomaterials. The electrical

properties of nanocrystalline thin films depend on various growth parameters such as film

composition, thickness, substrate temperature and deposition rate [29-30].

The electrical conductivity of nanocrystalline materials that are affected by grain

boundary scattering and change in microstructure is found to be lower than that in the bulk

materials of the same chemical composition because of the increased volume fraction of

atoms placed at the grain boundaries [31-33]. Actually, the electrical conductivity of

nanocrystalline materials is sensitive not only to the grain boundaries but also to other types

of imperfections and stresses introduced by the synthesis process [34]. The observation that

the electrical conductivity at a constant temperature decreases with decrease in grain size is

consistent with the theoretical analysis of scattering of electrons by grain boundaries. The

magnitude of the electrical conductivity in nanocomposites can be changed by altering the

grain size of the electrically conducting component.

In the present study nanocrystalline thin films of SnSe and ZnSe are deposited using

chemical bath deposition method. The effect of deposition bath temperature and pH of the

bath is studied on electrical properties of these films. The films are annealed at 373 K for one

hour in a vacuum of about 10-3

mbar before the measurements. A vacuum of 2 × 10-3

mbar is

maintained throughout the measurements. For photoconductivity measurements the heat

filtered white light of intensity 8450 lux (200 w tungsten lamp) is made to fall on the film

through a transparent glass window of the sample holder. The temperature dependence of

conductivity measurement of the films was controlled by mounting the heater inside the

sample holder and measured by a calibrated copper constantan thermocouple mounted very

103

near to the film. The heating rate was kept quite small (0.5K/min) for these measurements.

Light intensity is measured using a digital luxmeter. Planar geometry of the films has been

used for the electrical measurements.The photocurrent (Iph) is obtained after current (Id)

subtracting the dark from the current measured in the presence of light. The dark- and photo-

current is measured using a digital picoammeter.

4.2 Theory

Photoconductivity is an important property of semiconductors by means of which the

conductivity of the sample changes due to incident radiation. Photoconductivity is a simple

process in solids which involves the generation and recombination of charge carriers and

their transport to the electrodes. This phenomenon of Photoconductivity also includes the

charge carrier statistics, thermal carrier relaxation process, effects of electrodes, and several

mechanisms of recombination.

The photoconductivity process due to the absorption of photons in the material

occurs by several mechanisms that compete with generation of the charge carriers. The

contribution of photoconductivity with respect to the energy range is due to the possible

phenomena of band-to-band transitions, impurity levels to band edge transitions, ionization

of donors, and deep-level (located in the valence band) to conduction band transitions. When

incident photons have energy greater than the band gap, it will create electrons and holes in

the conduction and valence bands respectively, and provides the main contribution to the

photoconductivity. If in a doped semiconductor, we measure Photocoductivity and the

photon energy is slightly less than the band gap, then the impurity atom can absorb the

photons and creates a free electron in the conduction band. In this case photoresponse

originates from the low-energy side of the band gap and photoconduction is possible due to

excitation near the band edge. It is also observed that photoconductivity is also possible

when the energy of the incident photon is much less than that of the band gap. When the

energy of the incident photons is close to the ionization energy of the impurity atoms, they

104

are ionized, creating extra electrons in the conduction band, and hence there occurs an

increase in photoconductivity. Whatever the main contribution to the photoconductivity is at

a specific energy, Photoconductivity is the result of the absorption of photons, either by an

intrinsic process or by impurities with or without phonons, leading to the creation of free

charge carriers in the conduction band and/or the valence band.

Photoconductivity measurement method is a valuable diagnostic tool for the material

quality. Here, the photoconductivity is measured by two different ways: (a) Steady state

photoconductivity measurement (b) Transient photoconductivity measurement.

In steady state photoconductivity measurement the light is made to fall on the sample

until the steady state is reached and in transient photoconductivity measurements, the rise

and decay of photocurrent is measured after shining light on the thin film.

4.2.1 Steady state photoconductivity

When a quantum of radiation is absorbed, charge carriers are generated, which can

follow two different paths. In the first case, the charge carriers are trapped and recombine at

the states near Fermi level. In this case, photocurrent (Iph) is proportional to generation rate

and mono-molecular behaviour takes place. In the second case, Iph is proportional to drift

mobility ( ) and the carriers form a quasi-equilibrium between the states near the mobility

edge. If the recombination takes place from the lowest states and is not temperature

activated, then the temperature dependence of Iph is similar to that of . So, the

photocurrent becomes proportional to square root of the generation rate.

If Δn is the excess density of carriers due to relaxation and e is the charge for the

carriers, then the photocurrent is given by [35]

ph (4.1)

where E is the applied field and is drift mobility for the carriers. τ is the life time of one

of these carriers in the states between which the quasi-equilibrium is maintained

105

and Δn is given by

(4.2)

where G is the number of carrier pairs generated per unit time per unit volume and

τ =

(4.3)

where b = pa3, a is the spatial extent of the state and p is the chance per unit time that an

electron in a band edge localized state, overlapping the recombination center, recombine.

Hence

Δn =

(4.4)

and after the radiation is cut off, Δn decays as

Δ

= -

Δ

= -b Δn Δ (4.5)

According to Simmons and Taylor [36], the variation of photocurrent with

temperature can be divided into three regimes. In regime I, the photocurrent is less than the

dark current and increases with decreasing temperature. This increase may be due to the fact

that at high temperatures, concentration of thermal carriers exceeds photo current carriers.

The rate of recombination in this regime is determined by the dark carrier concentration as

Iph < Idark (hence Δ < ) and in this regime Δ can be

Δ =

(4.6)

Or

Δ exp (- ) (4.7)

Hence

Iph =

G (4.8)

106

In regime II, the photocurrent is greater than the dark current and it decreases with

decreasing temperature. The recombination in this regime is by electrons and holes, both of

which are generated by radiation. Here (1/τ) is proportional to the number of photo excited

carriers. As Δ , therefore

Δ =

(4.9)

and

Δ

=

Δ = bt + Constant (4.10)

which is a characteristic of bi-molecular decay. In regime III, value of photocurrent is

proportional to G and photocurrent falls rapidly, approaching a temperature independent

value.

4.2.2 Transient photoconductivity

The transient photoconductivity employs the method of measuring the rise and decay

of Iph with respect to time after illuminating the sample with light radiations. On starting the

illumination, the traps start to fill up and density of the photon generated carriers increases.

The increase in the number of filled traps and mobile carriers continue until generation rate

approaches recombination rate and equilibrium is reached in carrier production and a steady

state is observed in the conductivity. Here, since the traps are not activated by holes, they

have no effect while radiation continues. The steady state will be maintained as long as the

illumination continues. Once the illumination is turned off, the decay in photoconductivity

will follow. Transient photoconductivity is a very useful method to determine the energy

distribution of various species of gap states which influences the carrier mobilities and life

times in materials, assuming the to be controlled by multi-trapping processes [37-38]. This

method provides very valuable information about the material quality for various

photoconductive applications.

107

The presence of traps (or gap states) plays a significant role in the recombination

mechanism. When the material is exposed to light, a certain proportion of generated free

carriers are captured by these traps. These filled traps will be emptied after the exciting light

is switched off at a rate depending upon their cross-section and ionization energy.

If it is assumed that the traps, which are contributing to photoconductivity, are of

same kind and are located very close to the Fermi level, then the decay time constant ( )

will have a single value and the equation of decay can be written as [39]

(4.11)

(4.12)

where is the density of carriers at t = 0. However, if different kinds of traps with

different energetic depths in the band gap are present, then the decay time constant ( will

have different values. The situation can still be represented by the relation similar to Eq.

(4.12), where single valued is replaced by a multi-valued . As is multi-valued

function, its value changes with time during decay and thus, will result in a non-exponential

decay curve. By calculating the slope at any time t, one can calculate the value of at time t

using the relation [39]

(4.13)

Eq. (4.13) is just a mathematical manipulation of Eq. (4.11). Fuhs and Stuke [40] have

defined this decay time constant as differential life time. To analyse the decay rate at various

intensities and temperatures, the concept of differential life time ( ) has been used.

According to them

(4.14)

108

The values of at different times can be calculated using Eq. (4.18) from the slopes (at

different times) of Iph vs. time plots.

4.3 Electrical properties of nanocrystalline SnSe thin films

The SnSe is a narrow band gap, binary IV–VI semiconductor, suitable for various

optoelectronic applications like memory switching devices, photovoltaic, light emitting

devices (LED), and holographic recording systems [41-43]. Large availability of constituents

of SnSe compound in nature has been raised attention for the cost effective solution in

photovoltaic applications. Recently, Mathews [43] reported for the first time a CdS/SnSe

solid state heterojunction solar cell. SnSe has a orthorhombic crystallographic structure with

lattice parameters: a = 11.50Å, b = 4.15 Å, and c = 4.44 Å, which may be viewed as a

distorted NaCl rock-salt structure in order to obtain layers made up of double planes [44].

The structure of the SnSe thin films strongly influences the optical and electrical properties,

which is dependent on the preparation technique. Various deposition techniques are reported

for the preparation of SnSe thin films viz. chemical bath deposition [45], atomic layer

deposition [46], thermal evaporation [47-48], hot wall epitaxy [49], flash evaporation [50].

In the present study, chemical bath deposition method is used to synthesize

nanocrystalline SnSe thin films and the electrical properties of these films are studied. Dark

and photo-conductivity measurements are done on these films with the variation of

temperature. Also, transient photoconductivity behaviour is studied using rise and decay

analysis. The present section describes the effect of deposition temperature and pH on

electrical properties of SnSe films.

4.3.1 Effect of deposition temperature

a) DC conductivity measurements

The Dark conductivity ( ) measurements are carried out over the films after

mounting them in the metallic sample holder (Figure 2.8). The DC conductivity

109

measurements yield valuable information about conduction mechanism in semiconductors.

SnSe normally shows an activated temperature dependent dark conductivity according to the

Arrhenius relation:

where is the activation energy for dc conduction, the pre -exponential factor and K is

the Boltzmann's constant. The values of dark conductivity at room temperature comes out to

be (1.22 ± 0.02) × 10-6

Ω-1

cm-1

, (5.39 ± 0.02) ×10-6

Ω-1

cm-1

(1.01 ± 0.02) × 10-5

Ω-1

cm-1

for

SnSe thin films deposited at bath temperatures of 318 K, 333 K and 353 K, respectively. In

the given conductivity values, the least count error of instruments is included.

2.6 2.8 3.0 3.2 3.4 3.6

-17

-16

-15

-14

-13

-12

-11

-10

ln

do

hm

-1c

m-1

)

1000/T(K-1)

[a]

[b]

[c]

SnSe

Figure 4.1: Plot of ln σd vs. 1000/T of SnSe films deposited at temperatures [a] 318 K [b]

333 K and [c] 353 K

Figure 4.1 shows the temperaturee dependence of dark conductivity for the thin films

of SnSe deposited at different bath temperatures in the temperature range 318 K to 353 K.

The plots of ln σd vs. 1000/T are straight lines in the measured temperature range. This

110

implies that the conduction in SnSe thin films is an activated process having single activation

energy. The activation energies for dc conduction have been calculated from the slopes of ln

σd vs 1000/T curves. The values of σd and Δɛd are listed in Table 4.1. The value of σd

increases as the temperature of deposition of SnSe thin fllms increases. This increase in

conductivity is due to the increase in particle size of SnSe nanocrystals with temperature.

2.6 2.8 3.0 3.2 3.4 3.6

-14

-13

-12

-11

-10

-9

ln

pho

hm

-1c

m-1

)

1000/T(K-1)

[a]

[b]

[c]

SnSe

Figure 4.2: Plot of ln σph vs. 1000/T of SnSe films deposited at temperatures [a] 318 K

[b] 333 K and [c] 353 K

The effect of size on electrical conductivity of nanostructures is a resultant of the

following mechanisms: surface scattering, coulomb charging, quantized conduction and

tunnelling widening and discrete band gap and change of microstructure. So, increased

conductivity in case of SnSe thin films may be due to the decrease in grain boundary

scattering, structural defects and dislocations and improvement of the nano particle size. The

activation energy (Δɛd) in SnSe thin films are 0.94, 0.58 and 1.03 at temperature 318 K,

323 K and 333 K respectively which shows that the dark conduction in SnSe thin films is an

activated process having single activation energy.

111

b) Steady state photoconductivity

Figure 4.2 shows the temperature dependence of photoconductivity for SnSe thin

films deposited at different deposition temperatures.The values of photoconductivity (σph)

from Arrhenius relation are calculated as (9.30 ± 0.02) × 10-6

Ω-1

cm-1

, (1.50 ± 0.02) × 10-5

Ω-1

cm-1

and (2.73 ± 0.02) × 10-5

Ω-1

cm-1

at deposition temperatures of 318 K, 333 K and 353

K. The photo activation energies (Δɛph) have been calculated using the slopes of Figure 4.2

and are given in the Table 4.1. The value of σph increases with the increase in particle size of

SnSe films deposited at different substrate temperatures.

0.0

2.0x10-9

4.0x10-9

6.0x10-9

0.0

4.0x10-9

8.0x10-9

1.2x10-8

0 100 200 3000.0

5.0x10-9

1.0x10-8

1.5x10-8

[a]

SnSe

I Ph

(A)

[b]

Time(sec)

[c]

Figure 4.3: The rise and decay curves of Iph for SnSe thin films at different deposition

temperatures [a] 318 K, [b] 333 K and [c] 353 K

Photosensitivity (σph/σd) is an effective parameter in determining photoconductivity

and defined as ratio of photoconductivity to dark conductivity. It decides the use of the given

material in the photoconductive devices, such as solar cells. The value of (σph/σd) a have been

calculated for SnSe thin films deposited at different temperatures are given in table 4.1.

112

1.5 2.0 2.5 3.0 3.5 4.0

2.4

3.0

3.6

4.2

4.8

5.4

[b] N = 0.675

[c] N = 0.515

[a] N = 0.48

ln

d(s

ec

)

lnt (sec)

SnSe

Figure 4.4: Plot of lnτd vs. lnt for SnSe thin films deposited at deposition temperatures

[a] 318 K, [b] 333 K and [c] 353 K

c) Transient photoconductivity

Figure 4.3 shows the rise and decay curves of Iph for SnSe thin films at different

substrate temperatures. Iph rises to a steady state value and a peak is observed in rise curves

of SnSe films deposited at different deposition temperature. In materials with traps in the

mobility gap, when free carrier density is more than trapped carrier density because the

recombination time of carriers is same as carrier life time [51]. When the free carrier density

is much smaller than the trapped carriers, then the recombination process is dominated by the

rate of trap emptying and is much more than carrier life time, resulting in a slow decay.

During decay, the photocurrent does not reach zero for a long time after the incident light is

switched off. This type of photoconductive decay has also been reported in various other

semiconductors [51-53]. In the present case, the non-exponential decay of photoconductivity

is observed.The values of τd at different times have been calculated using Eq. (4.13), for

113

SnSe thin films deposited at different substrate temperatures from the slopes (at different

times) of decay curves of Figure 4.3.

The decay times observed for SnSe thin films, deposited at different deposition

temperatures, are found to be time dependent.The value of increases with time which

confirms the non-exponential decay of photocurrent. Figure 4.4 shows the plots of ln vs.

lnt for all samples at intensity 8450 lux. The extrapolation of the curves at t = 0, give the

values of the carrier life time [53] and are found to be 1.85, 2.23 and 2.53 seconds for films

deposited at substrate temperatures 318 K, 333 K and 353 K respectively. Clearly, the carrier

life time increases with increasing deposition temperature (size). The straight lines in Figure

4.4, obey a power law of the form t-N

, with N = d(ln /lnt) and the values of N are found to

be 0.480, 0.515 and 0.675 at deposition temperatures 318 K, 333 K and 353 K respectively.

Table 4.1: List of various electrical parameters for SnSe thin films deposited at various

temperatures

Temp

(K)

σd

(Ω-1

cm-1)

Ed

(eV)

σph

( Ω-1

cm-1

)

Eph

(eV)

σph/ σd lnτd(t=0)

(sec)

318 1.22 × 10-6

0.94 9.3 × 10-6

0.63 7.62 1.847

333 5.39 × 10-6

0.58 1.50 × 10-5

0.29 2.78 2.231

353 1.01 × 10-5

1.03 2.73 × 10-5

0.24 2.70 2.530

4.3.2 Effect of pH on SnSe thin films

a) DC conductivity measurements

The Dark conductivity ( ) measurements are carried out over the films after

mounting them in the metallic sample holder. The values of room temperature dark

conductivity comes out to be (2.05 ± 0.02) × 10-6

Ω-1

cm-1

, (6.32 ± 0.02) × 10-6

Ω-1

cm-1

and

(1.81 ± 0.02) × 10-5

Ω-1

cm-1

for SnSe thin films deposited at different pH values 11.0, 11.4

and 11.8 of the bath. Figure 4.5 shows the temperature dependence of dark conductivity for

the thin films of SnSe deposited at different pH of the bath in the range from 11.0 to 11.8.

The plots of ln σd vs. 1000/T are straight lines in the measured temperature range. This

114

implies that the conduction in SnSe thin films is an activated process having single activation

energy. The activation energies for dc conduction have been calculated from the slopes of ln

σd vs 1000/T curves. The values of σd and Δɛd are listed in Table 4.2.

2.6 2.8 3.0 3.2 3.4 3.6

-17

-16

-15

-14

-13

-12

-11

-10

-9

ln

do

hm

-1c

m-1

)

1000/T(K-1)

[a]

[b]

[c]

SnSe

Figure 4.5: Plot of lnσd vs. 1000/T of SnSe films deposited at different pH [a] 11.0, [b]

11.4 and [c] 11.8

The value of σd increases as the pH of the bath of SnSe thin fllms increases. This

increase in conductivity is due to the increase in particle size of SnSe nanocrystals with pH

values. The increased conductivity of SnSe thin films may be caused by decrease in grain

boundary scattering, structural defects and dislocations and improvement of the nanoparticle

size.

b) Steady state photoconductivity

Figure 4.6 shows the temperature dependence of photoconductivity for SnSe thin

films deposited at different pH values from 11.0 to 11.8. The values of photoconductivity

(σph) from Arrhenius formula are calculated as (4.87 ± 0.02) × 10-6

Ω-1

cm-1

, (1.92 ± 0.02) ×

10-5

Ω-1

cm-1

and (4.38 ± 0.02) × 10-5

Ω-1

cm-1

at different mention values. The photo

115

activation energies (Δɛph) have been calculated using the slopes of Figure 4.6 and are given

in the Table 4.2. The value of σph increases with the increase in particle size of SnSe.

2.6 2.8 3.0 3.2 3.4 3.6

-16

-15

-14

-13

-12

-11

-10

-9

ln

Pho

hm

-1c

m-1

)

1000/T(K-1)

SnSe

[a]

[b]

[c]

Figure 4.6: Plot of ln σph vs. 1000/T of SnSe films deposited at different pH [a] 11.0,

[b] 11.4 and [c] 11.8

The valuable parameter in photoconductivity measurement is the photosensitivity

(σph/σd) and the value of (σph/σd) a have been calculated for SnSe thin films deposited at

different pH of the solution bath are given in table 4.2.

c) Transient photoconductivity

Figure 4.7 shows the rise and decay curves of Iph for SnSe thin films at different pH

of the bath. Iph rises to a steady state value and a peak is observed in rise curves of SnSe

films deposited at different pH. In materials with traps in the mobility gap, when free carrier

density is more than trapped carrier density then the recombination time of carriers is same

as carrier life time [51]. If the free carrier density is much smaller than the trapped carriers,

then the recombination process is dominated by the rate of trap emptying and is much greater

than carrier life time, resulting in a slow decay. During decay, the photocurrent does not

116

reach zero for a long time after the incident light is switched off. This type of

photoconductive decay has also been reported in various other semiconductors [51-53].

0.0

1.0x10-9

2.0x10-9

0.0

1.0x10-9

2.0x10-9

0 100 200 3000.0

1.0x10-9

2.0x10-9

[a]

SnSe

I Ph

(A)

[b]

Time(sec)

[c]

Figure 4.7: The rise and decay curves of Iph for SnSe thin films at different pH [a] 11.0,

[b] 11.4 and [c] 11.8

In the present case, the non-exponential decay of photoconductivity is observed. The values

of τd at different times have been calculated using Eq. (4.14), for SnSe thin films deposited at

different pH of the solution from the slopes (at different times) of decay curves of Figure 4.7.

The decay times observed for SnSe thin films, deposited at different pH, are found to be time

dependent. The value of increases with time, which confirms the non-exponential decay

of photocurrent. Figure 4.8 shows the plots of ln vs. lnt for all samples at intensity 8450

lux. The extrapolation of the curves at t = 0, give the values of the carrier life time [53] and

are found to be 1.88, 2.58 and 2.71 seconds for films deposited at substrate temperatures 318

K, 333 K and 353 K respectively.

117

1.5 2.0 2.5 3.0 3.5 4.0

2.8

3.2

3.6

4.0

4.4

ln

ds

ec

lnt (sec)

[a] N= 0.565

[c] N = 0.447

[b] N = 0.342

SnSe

Figure 4.8: Plot of lnτd vs. ln t for SnSe thin films deposited at different pH [a] 11.0, [b]

11.4 and [c] 11.8

Clearly, the carrier life time increases with increasing deposition temperature (size). The

straight lines in Figure 4.8, obey a power law of the form t-N

, with N = d(ln /lnt) and the

values of N are found to be 0.565, 0.342 and 0.447 at pH of the solution 11, 11.4 and 11.8

respectively.

Table 4.2: List of various electrical parameters for SnSe thin films deposited at

different pH values of the bath

Temp σd

(Ω-1

cm-1)

Ed

(eV)

σph

( Ω-1

cm-1

)

Eph

(eV)

σph/ σd lnτd(t=0)

(sec)

11.0 2.05 × 10—6

1.02 4.87 × 10-6

0.93 2.37 1.88

11.4 6.32 × 10-6

0.86 1.92 × 10—5

1.08 3.04 2.59

11.8 1.81 × 10-5

1.21 4.38 × 10-5

0.67 2.43 2.72

4.4 Electrical properties of nanocrystalline ZnSe thin films

The ZnSe nanoparticles have wide-ranging applications because it has wide and

direct band gap. It is transparent over a wide range of visible spectrum and has a relatively

118

large non-linear optical coefficient. It is well known as a high refractive index material. ZnSe

possesses unique optical and photovoltaic properties and exhibits great potential

applications, such as blue–green light emitting diodes, photoluminescent and

electroluminescent devices, lasers, thin film solar cell non-linear optical crystal and infrared

optical materials [54-55]. It is also used to increase the open circuit voltage of solar cells

[56]. A great number of devices at present are made by using ZnSe thin films.

The as deposited ZnSe thin films are very less conductive and their room

temperature dark conductivity is of the order of 10-8

Ω-1

cm-1. The very low electrical

conductance indicates that the as-deposited thin films are strongly quantized as described in

chapter 3. The free electrons and holes are actually confined within the ZnSe quautum dots.

The electrical isolation between zinc selenide quantum dots deposited as thin films signifies

a large potential barrier between them. From conductivity measurements, additional

information can be obtained about the band structure of investigated semiconductor in thin

film form mainly regarding the so-called band gap states.

4.4.1 Effect of deposition temperature on ZnSe thin films

In the present work, the electrical properties of ZnSe thin films deposited by

chemical bath deposition method are studied by doing the dark and photo-conductivity

measurements (with temperature) on these films. The transient photoconductivity behaviour

is also studied using rise and decay analysis. The present section describes the effect of

deposition temperature and pH on electrical properties of ZnSe films

Thin films of ZnSe are deposited by chemical bath deposition method at different

temperatures 318 K, 333 K and 353 K. All the parameters except temperature are kept

constant during deposition.

119

2.6 2.8 3.0 3.2 3.4 3.6

-20

-18

-16

-14

ln

d(o

hm

-1c

m-1

)

1000/T(K-1)

[a]

[b]

[c]

ZnSe

Figure 4.9: Plot of lnσd vs. 1000/T of ZnSe films deposited at temperatures [a] 318 K [b]

333 K and [c] 353 K

a) DC conductivity measurements

The values of room temperature dark conductivity comes out to be (4.88 ± 0.02) ×

10-8

Ω-1

cm-1

, (2.14 ± 0.02) × 10-7

Ω-1

cm-1

and (2.66 ± 0.02) × 10-7

Ω-1

cm-1

for ZnSe thin films

deposited at bath temperatures of 318 K, 333 K and 353 K, respectively.

Figure 4.9 shows the temperature dependence of dark conductivity for the thin films

of ZnSe deposited at different bath temperatures in the temperature range 318 K to 353 K.

The plots of ln σd vs. 1000/T are straight lines in the measured temperature range. This

implies that the conduction in ZnSe thin films is an activated process having single

activation energy. The activation energies for dc conduction have been calculated from the

slopes of l nσd vs 1000/T curves. The values of σd and Δɛd are listed in Table 4.3. The value

of σd increases as the temperature of deposition of ZnSe thin fllms increases. This increase in

conductivity is due to the increase in particle size of ZnSe nanocrystals with temperature.

The increase in electrical conductivity with increase in deposition temperature was due to

120

improvement in crystallite size, decrease in (1) density of grain boundary intercrystallite, (2)

grain boundary discontinuities, (3) defects such as pinholes, voids, etc. and (4) improvement

of nanoparticles size and/or recrystallization of ZnSe films. So increase in conductivity of

ZnSe thin films may be due to the decrease in grain boundary scattering, structural defects

and dislocations and improvement of the nanoparticle size. Kishore et.al [57] have been also

prepared the ZnSe crystals using melt cooling technique and measure the electrical

conductivity at different temperature and reported as the temperature is increased the

amplitude of the atoms situated at the boundaries of the barrier increases. As a result of this

enhanced motion the area of contact of the grains increases, which in turn decreases the

barrier height and hence reduces the grain boundary resistance due to production of parallel

resistances at the contact points between grains. As the temperature increases, there is

enhancement in the atomic motion leading to more drop in the value of the potential barrier.

This explains the higher values of electrical conductivity at higher temperatures. Hankare

et.al [58] have also been reported the increase of electrical conductivity with temperature.

2.6 2.8 3.0 3.2 3.4 3.6

-18

-17

-16

-15

-14

ln

Ph

(oh

m-1

cm

-1)

1000/T(K-1)

[b]

[b]

[a]

ZnSe

Figure 4.10: Plot of ln σph vs. 1000/T of ZnSe films deposited at temperatures [a] 318 K

[b] 333 K and [c] 353 K

121

b) Steady state photoconductivity

Figure 4.10 shows the temperature dependence of photoconductivity for ZnSe thin

films deposited at different deposition temperatures. The values of photoconductivity are

calculated to (2.15 ± 0.02) × 10-7

Ω-1

cm-1

, (2.53 ± 0.02) × 10-7

Ω-1

cm-1

and (5.52 ± 0.02) ×

10-7

Ω-1

cm-1

at deposition temperatures 318 K, 333 K and 353 K.

The photo activation energies (Δɛph) are calculated using the slopes of Figure 4.10

and are given in the Table 4.3. The value of σph increases with the increase in particle size of

ZnSe. The useful parameter in photoconductivity measurement is the photosensitivity

(σph/σd), which determines the use of a particular material in photoconductive devices such as

solar cells. The value of σph/σd a have been calculated for ZnSe thin films deposited at

different temperatures are given in table 4.3.

0.0

2.0x10-9

4.0x10-9

0.0

2.0x10-9

4.0x10-9

0 100 200 3000.0

2.0x10-9

4.0x10-9

[a]

ZnSe

I Ph

(A)

[b]

Time(Sec)

[c]

Figure 4.11: The rise and decay curves of Iph for ZnSe thin films at different deposition

temperatures [a] 318 K, [b] 333 K and [c] 353 K

122

c) Transient photoconductivity

Figure 4.11 shows the rise and decay curves of Iph for ZnSe thin films at different

substrate temperatures. Iph rises to a steady state value and a peak is observed in rise curves

of ZnSe films deposited at different substrate temperatures. During decay, the photocurrent

does not reach zero for a long time after the incident light is switched off. A persistent

photocurrent is observed in all the cases. This type of photoconductive decay has also been

reported in various other semiconductors [51-53]. In the present case, the non-exponential

decay of photoconductivity is observed.The values of τd at different times have been

calculated using Eq. (4.14) for ZnSe thin films deposited at different deposition temperatures

from the slopes (at different times) of decay curves of Figure 4.11.

1.5 2.0 2.5 3.0 3.5 4.0

2.0

2.4

2.8

3.2

3.6

4.0

4.4

4.8

ln

d(s

ec

)

lnt (sec)

[a] N = 0.476

[b] N = 0.759

[c] N = 0.861ZnSe

Figure 4.12: Plot of lnτd vs. lnt for ZnSe thin films deposited at deposition temperatures

[a] 318 K, [b] 333 K and [c] 353 K

The decay times observed for ZnSe thin films, deposited at different substrate

temperatures, are found to be time dependent. The value of increases with time, which

confirms the non-exponential decay of photocurrent. Figure 4.12 shows the plots of ln vs.

lnt for all samples at intensity 8450 lux. The extrapolation of the curves at t = 0, give the

123

values of the carrier life time [53] and are found to be 1.85, 2.23 and 2.53 seconds for films

deposited at substrate temperatures 318 K 333 K and 353 K respectively. Clearly the carrier

life time increases with increasing substrate temperature (size). The straight lines in Figure

4.12, obey a power law of the form t-N

and the values of N are found to be 0.476, 0.759 and

0.861 at substrate temperatures 318 K, 333 K and 353 K respectively.

Table 4.3: List of various electrical parameters for ZnSe thin films deposited at various

temperatures

Temp

(K)

σd

(Ω-1

cm-1)

Ed

(eV)

σph

( Ω-1

cm-1

)

Eph

(eV)

σph/ σd lnτd(t=0)

(sec)

318 4.88 × 10-8

0.71 2.15 × 10-7

0.36 4.41 1.225

333 2.14 × 10-7

0.58 2.53 × 10-7

0.86 1.18 1.383

353 2.66 × 10-7

0.82 5.52 × 10-7

0.47 2.07 1.448

4.4.2 Effect of pH on ZnSe thin films

a) DC conductivity measurements

The values of room temperature dark conductivity comes out to be (2.84 ± 0.02) × 10-7

Ω-

1cm

-1, (2.96 ± 0.02) × 10

-7 Ω

-1cm

-1 (1.76 ± 0.02) × 10

-6 Ω

-1cm

-1 for ZnSe thin films deposited

at different pH of the solution 11.0, 12.0 and 13.0.

Figure 4.13 shows the temperaturee dependence of dark conductivity for the thin

films of ZnSe deposited at different pH of the bath in the range from 11.0 to 13.0. The plots

of ln σd vs 1000/T are straight lines in the measured temperature range. This implies that the

conduction in ZnSe thin films is an activated process having single activation energy. The

activation energies (Δɛd) for dc conduction are calculated from the slopes of ln σd vs. 1000/T

curves. The values of σd and Δɛd are listed in Table 4.4. The value of σd increases as the pH

of the bath of ZnSe thin fllms increases. This increase in conductivity is due to the increase

in particle size of ZnSe nanocrystals with pH values. The increase in conductivity of ZnSe

124

thin films may be due to the decrease in grain boundary scattering, structural defects and

dislocations and improvement of the nanoparticle size.

2.6 2.8 3.0 3.2 3.4 3.6 3.8

-19

-18

-17

-16

-15

-14

-13

-12

ln

d(o

hm

-1 c

m-1

)

1000/T(K-1)

[a]

[b]

[c]

ZnSe

Figure 4.13: Plot of lnσd vs. 1000/T of ZnSe films deposited at different pH [a] 11.0, [b]

12.0 and [c] 13.0

2.6 2.8 3.0 3.2 3.4 3.6 3.8

-15

-14

-13

-12

-11

ln

ph

(oh

m-1

cm

-1)

1000/T(K-1)

[a]

[b]

[c]

ZnSe

Figure 4.14: Plot of logσph vs. 1000/T of ZnSe films deposited at different pH [a] 11.0

[b] 12.0 and [c] 13.0

125

b) Steady state photoconductivity

Figure 4.14 shows the temperature dependence of photoconductivity for ZnSe thin

films deposited at different pH values from 11.0 to 13.0. The values of photoconductivity are

calculated to (1.92 ± 0.02) × 10-6

Ω of the bath-1

cm-1

, (1.68 ± 0.02) × 10-5

Ω-1

cm-1

and (8.67 ±

0.02) × 10-5

Ω-1

cm-1

at different pH.

Table 4.4: List of various electrical parameters for ZnSe thin films deposited at

different pH values of the solution

pH σd

(Ω-1

cm-1)

Ed

(eV)

σph

( Ω-1

cm-1

)

Eph

(eV)

σph/ σd lnτd(t=0)

(sec)

11.0 2.84 × 10-7

0.51 1.92 × 10-6

0.36 6.78 1.71

12.0 2.96 × 10-7

0.55 1.68 × 10-6

0.49 5.67 2.05

13.0 1.76 × 10-6

0.75 8.67 × 10-6

0.41 4.92 2.16

The photo activation energies (Δɛph) have been calculated using the slopes of Figure

4.14 and are given in the Table 4.4. The value of σph increases with the increase in particle

size of ZnSe. The activation energy for photoconduction is much less than for the dark

conduction.

The values of photosensitivity (σph/σd) for ZnSe thin films deposited at different pH

of the solution bath are given in table 4.4. The value of photosensitivity is maximum at pH

=11.0.

c) Transient photoconductivity

Figure 4.15 shows the rise and decay curves of Iph for ZnSe thin films at different pH

of the bath which shows Iph rises to a steady state value and a peak is observed. A persistent

photocurrent is observed in all the ZnSe thin films at different pH values. In the present case,

the non-exponential decay of photoconductivity is observed. The values of τd at different

times have been calculated using Eq. (4.14), for ZnSe thin films deposited at different pH of

the solution from the slopes (at different times) of decay curves of Figure 4.15.

126

0.0

2.0x10-9

4.0x10-9

0.00

1.50x10-9

3.00x10-9

0 100 200 3000.0

2.0x10-9

4.0x10-9

[a]

ZnSe

I Ph

(A)

[b]

Time(Sec)

[c]

Figure 4.15: The rise and decay curves of Iph for ZnSe thin films at different pH [a]

11.0, [b] 12.0 and [c] 13.0

1.5 2.0 2.5 3.0 3.5 4.0

2.4

2.8

3.2

3.6

4.0

ln d

(s

ec

)

lnt (sec)

[a] N =0.305

[b] N =0.357

[c] N = 0.362

ZnSe

Figure 4.16: Plot of ln τd vs. lnt for ZnSe thin films deposited at different pH [a] 11.0,

[b] 12.0 and [c] 13.0

.

127

The decay times observed for ZnSe thin films, deposited at different pH, are

found to be time dependent. The value of increases with time, which confirms the non-

exponential decay of photocurrent. Figure 4.16 shows the plots of ln vs. lnt for all samples

at intensity 8450 lux. The extrapolation of the curves at t = 0, give the values of the carrier

life time [53] and are found to be 1.71, 2.05 and 2.16 seconds for films deposited at different

pH 11.0, 12.0 and 13.0. Clearly, the carrier life time increases with increasing pH of the

solution (size). The straight lines in Figure 4.16, obey a power law of the form t-N

and the

values of N are found to be 0.305, 0.357 and 0.362 at pH of the solution 11.0, 12.0 and 13.0

respectively.

4.5 Conclusions

In SnSe and ZnSe thin films the values of the dark conductivity (σd) and steady state

photoconductivity (σph) increases with increase of deposition temperature and pH of the

solution bath. Actually, conductivity is also size dependent and increases with increase of

size of particle. The dark conductivity (σd) is thermally activated processes, having single

activation energy. No maximum in photoconductivity has been observed with temperature in

all samples. The effect of size on conductivity of nanostructures is a resultant of the

following mechanisms: surface scattering, quantized conduction, coulomb charging and

tunneling, widening and discrete band gap and change of microstructure. A peak in the rise

curves is observed in all MSe thin films deposited at higher substrate temperatures and at

higher pH value of the solution. The carrier life time increases with increase in substrate

temperature and pH of the solution (size). The value of decay time constant (τd) increases

with time, which confirms the non-exponential decay of photocurrent.

128

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