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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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