Un doped and doped with Al ZnS thin Films have been fabricated by vacuum evaporation technique under the vacuum of 10-5 Torr on glass substrate at room temperature and with different ratio of Al concentration of thickness (0.8µm). The optical properties were revealed by UV-Visible transmittance spectra and the band gap energy was determined. Transmission spectra indicate a high transmission coefficient (¨95%). The results showed that films have direct optical transition, and the values of energy gap were found to decrease with doping concentrations. Also the optical constants such as absorption coefficient, refractive index, extinction coefficient and dielectric constant have been calculated. The effect of doping concentration on the electrical properties has been studied

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CHARACTERIZATION OF ZNS AND ZNS: AL THIN FILMS
Eman M. Nasir
Asst. Professor, Physical Dept, College of Science/ University of Baghdad, Jadirya, Baghdad, Iraq
ABSTRACT
Un doped and doped with Al ZnS thin Films have been fabricated by vacuum evaporation
technique under the vacuum of 10-5 Torr on glass substrate at room temperature and with different
ratio of Al concentration of thickness (0.8µm). The optical properties were revealed by UV-Visible
transmittance spectra and the band gap energy was determined. Transmission spectra indicate a high
transmission coefficient (¨95%). The results showed that films have direct optical transition, and the
values of energy gap were found to decrease with doping concentrations. Also the optical constants
such as absorption coefficient, refractive index, extinction coefficient and dielectric constant have
been calculated. The effect of doping concentration on the electrical properties has been studied
Keywords: ZnS, Thin Films, Doping, Optical Properties, Structure Properties
I.
INTRODUCTION
Zinc sulfide (ZnS) is a wide gap semiconductor. With the solution of chemical bath
containing the metal consequently, it is a potentially important material to be and chalcogen ions.
The film formation on substrate takes used as an antireflection coating for hetero junction solar place
when ionic product exceeds solubility product. The cells. It is an important device material for the
detection, precipitate formation in the bulk of solution is an inherent emission and modulation of
visible and near ultra violet problem in CBD technique. This normally results light. With a large
band gap, Zinc sulfide (ZnS) becomes unnecessary precipitation and loss of materials during film a
highly efficient luminescent material, when doped deposition on glass substrate. In order to avoid
such with manganese, copper or other ions. Owing to wide problem, proper optimization of chemical
constituents of band gap value of 3.7 eV, it can be used for fabrication bath deposition is necessary.
The purpose of this work of optoelectronic devices such as blue light-emitting was to study the
effects of the growth conditions, such as diodes, electroluminescent devices, electro optic deposition
time on optical and structural properties of the modulator, optical coating, n-window layers for thin
film chemical bath ZnS thin films [1]. Recent investigations have evoked considerable interest in
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2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME
ZnS thin films due to their vast potential for use in thin film devices such as photo luminescent and
electroluminescent devices and more recently as n-type windowlayer heterojunction solar cells [2].
The optical properties of the prepared film depend strongly on the manufacturing technique. Two of
the most important optical properties; refractive index and the extinction coefficient are generally
called optical constants. The amount of light that transmitted through thin film material depends on
the amount of the reflection and absorption that takes place along the light path [3]. This class of new
materials has not only provided many unique opportunities but also exhibited novel optical and
transport properties, which are potentially useful for technological applications [4]. The study of the
optical properties of doping material such as ZnS:Cu nanocrystals is important for the development
of nanocrystal phosphors with efficient luminescence and the understanding of the impurity related
optical transitions in semiconductor nanocrystals. A variety of methods have been deployed in order
to synthesize and control the size of nanoparticles [5]. Many researchers investigated the structural,
electrical and optical properties of ZnS films deposited at different temperatures evaporation for
photovoltaic applications [6]-[12].
The ZnS higher band gap energy (3.68 eV) [6] compared to that one of the CdS (2.42 eV)
makes the ZnS wavelength range of transparency wider (above 330 nm) than the CdS one (above
520 nm). Many deposition techniques have been used to prepare ZnS thin films, such as sputtering,
pulsed laser deposition, chemical vapor deposition, electron beam deposition, thermal evaporation,
photochemical deposition and
Chemical bath deposition [3]. Among these methods, thermal evaporation is the most
common method in producing thin film because the advantages of thermal evaporation are stability,
reproducibility and high deposition rates. It is well known that the physical properties such as the
structure and the surface are generally different from those of the same materials in bulk form[12][16]. In this work pure zinc sulfide and doped with Al are prepared by vacuum evaporation technique.
The aim of this work is to preparing ZnS and ZnS:Al thin films and studying the effect of
concentration of Al on the optical and electrical properties.
II.
EXPERIMENTAL WORK
ZnS and ZnS:AL thin films with different Al concentration of thickness(0.8µm)were
prepared by vacuum evaporation technique under lower pressure (10-6 Torr) on Si and glass substrate
by using Edward (E306A) coating system. The prepared films have been annealed at 400K.
Spectrophotometer, type Shimadzu and UV-Visible recorder Spectrophotometer UV-160, is used to
measure the transmittance and absorbance spectrum in the range (300-1100) nm region for ZnS thin
films. The electrical resistance has been measured as a function of the temperature (T). The
measurements have been done with used sensitive electrometer type of Keithly Digital Electrometer
(616) and vacuum electric oven. The resistivity and conductivity as a function of Al concentration
can be calculated from these measurements.
III.
RESULTS AND DISCUSSION
A.
Optical Properties
The study of optical absorption is important to understand the behavior of semiconductor
nanocrystals. A fundamental property of semiconductors is the band gap-the energy separation
between the filled valence band and the empty conduction band. Optical excitation of electrons
across the band gap is strongly allowed, producing an abrupt increase in absorption at the wavelength
corresponding to the band gap energy. This feature in the optical spectrum is known as the optical
absorption edge [4]. The optical properties of un doped and doped ZnS thin films with Al were
determined from transmittance measurement in the range 300-900 nm. Fig. 1 shows the optical
absorbance of ZnS and ZnS:Al thin films at different Al concentration (pure, 1%, 2%) in the
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wavelength range of 300nm to 900nm. The optical studies show that the absorbance of these films
increases as Al concentration increases as in Table III. All the samples show very high absorption in
UV region and low absorption in the VIS and NIR region. This make the material to be useful in
windscreen coating and driving mirror to prevent the effect of dazzling light into driver’s eyes from
oncoming vehicle and following vehicle [17]. It is observed that increasing of Al concentration shifts
the peak of absorption spectrum to the red shift. The shift in the peak position films may be
attributed to the crystallite of film structure by increasing the grain size which is confirmed by XRD
results and AFM. Also the spectrum of transmittance has been studied; it is obviously that its
behavior is opposite to that of the absorbance spectrum. From Table III the value of transmittance
increases with increase of Al concentration, and it is quite clear that all films have excellent
transparency in the visible range ൒95% transmission. The film is actually an efficient transmitting
and antireflective material. Because of the high optical transmission and anti-reflective properties of
ZnS film, ZnS may play an important role in photovoltaic devices. So they are useful as an
antireflection coating for the optical transmission window. Similar behaviour was observed by
researchers [15]-[18]. Also the values of reflectance are shown in Table I.
2.5
nn doped ZnS
1% Al
2
2% Al
A
1.5
1
0.5
0
200
250
300
350
400
450
500
550
600
650
700
λ(nm)
Fig. 1: The absorption spectrum of pure ZnS and ZnS:Al thin films at Al concentration of 1%, 2%.
TABLE I: SHOWS THE VLUE OF ATR FOR ZNS AND ZNS:AL THIN FILMS
Sample
pure
1%
2%
A
T
R
0.279
0.433
0.765
0.554
0.363
0.116
0.0244
0.2034
0.1187
The absorption coefficient increases with increasing Al concentration and shifted to higher
wavelength for all prepared samples as shown in Table II and Fig. 2.This is due to the decreasing
value of energy gap with increasing Al concentration. The absorption coefficient (α) value varies
between 1.4x104 and 9.97x104cm-1 with increasing of Al concentration from pure to 3% which are
nearly agrees with [17]-[20]]. The energy band gaps were calculated with the help of the optical
absorption spectral. To determine the energy band gap, we plotted (αhυ)2 against hυ. Absorption
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coefficient (α) associated with the strong absorption region of the films was calculated from
absorbance (A) and the film thickness (t) using the relation,
α =2.303A/t (1)
(1)
The theory of inter band absorption shows that at the optical absorption edge, the absorption
coefficient α varies with the photon energy hυ according to:
α (hυ)=A(hυ - Eg) n
(2)
where Eg is the optical band gap, A is a constant and the exponent n = ½, 1, 2, 3, depending
on the types of electronic transition in k-space. To determine the optical band gap of the ZnS and
ZnS:Al thin films, taking n = ½ gives the best fit for the films. The optical band gap, Eg, was
determined by extrapolating the linear part of the optical absorption spectral. The band gap energy
increases as Al concentration increases as in Table II. It could be observed that the optical band gap
was decreasing with increasing Al concentration as shown in Fig. 3 and Table II. Also the decrease
in band gap of the films could be attributed to improvement in the crystals and increases of the grain
sizes of the films with increase Al concentration. This shift in the optical band gap may be attributed
to the band shrinkage effect because of increasing carrier concentration [19] The ZnS is very useful
as a window layer in heterojunction photovoltaic solar cells, because the high band gap energy will
decrease the window absorption losses. This also leads to the improvement of the short circuit
current. Similar results was observed by researchers [17]-[21]
TABLE II: SHOWS THE OPTICAL PARAMETERS FOR ZNS AND ZNS: AL THIN FILMS
B2X109 eV/cm2
α cm-1x104
Eu(eV)
Sample
Eg(eV)
pure
3.8
0.31
1.4
0.465
1%
3.74
0.58
1.68
0.305
2%
3.45
0.81
9.97
0.277
30
nn doped
ZnS
Series2
25
2% Al
20
α cm -1x104
15
10
5
0
200
300
400
500
600
700
λ(nm)
Fig. 2: The absorption coefficient of pure ZnS and ZnS:Al thin films at Al concentration of 1%, 2%.
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5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
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Fig 3: The (αhν)2 as a function of photon energy of pure ZnS and ZnS:Al thin films at Al
concentration of 1%, 2%
The coefficient (A) which is Tauc slope in the Tauc equation (2) has been obtained from the
root square of the straight line slope in the Fig.(3). From this figure the value of A is increased with
increasing Al concentration form pure to 3 wt% as shown in Table II. The factor A is inversely
proportional to amorphosity and the width of the band tails [22].This mean that amorphosity
decreases with increasing Al concentration.
The density of localize states in the band can be evaluated from the Urbach energy (Eu) at α <104cm-1
which is referred to absorption tails at energies smaller than the optical energy gap. In the
exponential edge region, Urbach rule is expressed as [23]
α= α o exp(hν/Eu)
(3)
where α o is a constant, Eu is the Urbach energy, which characterizes the slope of the
exponential edge. Equation (3) describes the optical transition between the occupied states in the
valence band tail to the unoccupied states of the conduction band edge. By plotting ln α as a function
of hν, and the reciprocal slope of the linear part give the value of Eu. It is obvious from Table II and
Fig.(4) that Eu decreases from 0.645 to 0.2777eV with increasing Al value from pure to 2%.This
may be attributed to improvement in the structure by doping and decreasing the degree of amorphous
films leading to decrease the localized states and fill all dangling bond
Fig. 4 The Ln(α) as a function of photon energy (Urbach plots) of pure ZnS and ZnS:Al thin films
at Al concentration of 1%, 2%.
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The refractive index is an important parameter for optical materials and applications. Thus, it
is important to determine optical constants of the films Table III shows the variation of refractive
index for prepared of pure ZnS and ZnS:Al thin films at Al concentration of 1%, 2%. . It is obvious
that n decreases with increasing Al concentration. This behavior is due to increase the crystalanity by
adding Al which causes to expand the lattice and grow the grain size and decreases the defect (as in
XRD and AFM) which means increasing the absorption and decreasing the reflection which the
refractive index depend on it and this is an agreement with [17]. The behavior of extinction
coefficient (k) is nearly similar to the corresponding absorption coefficient at different Al
concentration; It is clear from Table V that k increasing with Al concentration. This attributed to the
same reason, which mention previously in absorption coefficient [2]-[6]. The variation of real
dielectric constant (ε1) and imaginary dielectric constant (ε2) with different Al concentration has been
shown in Table III. The behavior of ε1 is similar to refractive index because the smaller value of k2
comparison of n2, while ε1 is mainly depends on the k values, which are related to the variation of
absorption coefficient [17].
TABLE III: SHOWS THE OPTICAL CONSTANTS FOR ZNS AND ZNS: AL THIN FILMS
ε2
Sample
n
k
ε1
pure
2.46
0.047
6.05
0.23
1%
2.03
0.25
4.09
1.04
2%
2.015
0.41
3.89
1.65
B.
Electrical properties
The physical properties of prepared films can be improved mainly by many parameters such
as the substrate temperature, nature of the dopant, the amount of absorbed oxygen that appears
during the deposition process, and the annealing process in controlled environments [24]. In this
respect, in order to improve and control the electrical properties of ZnS films, they have been doped
with Al. The incorporation of the impurity into the ZnZ lattice can increase the electrical
conductivity. In order to study conductivity mechanisms, it is convenient to plot logarithm of the
conductivity (ln σ) as a function of 1000/T for pure ZnS and ZnS:Al thin films at Al concentration of
1%, 2%.
as shown in Fig. 5. It is seen that resistivity decreases with temperature, indicating
semiconducting nature of films. Also It is clear from these figures that there are two transport
mechanisms, giving rise to two activation energies Ea1 and Ea2. This result is conform with [11] . At
higher temperature range, the conduction mechanism is due to carrier excited into the extended states
beyond the mobility edge and at lower temperature range; the conduction mechanism is due to
carrier excited into localized states at the edge of the band[25]. Table VII shows the relation between
the conductivity and film Al concentration. It is clear that the σR.T increases with increasing of Al
concentration. This increase in conductivity is related to the improvement of crystallinity. The
thermal activation energy was calculated using the relation
ρ = ρο exp(Eο/KT)
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Fig. 5: The Ln σ versus 1000/T for pure ZnS and ZnS:Al thin films at Al concentration of 1%, 2%
TABLE IV: SHOWS THE VARIATION OF D.C CONDUCTIVITY PARAMETRES
Thickness (nm)
(293 – 453)K
(463 –513)K
σR.T × 10-5 (Ω.cm)-1
Ω
Ea1 (eV)
Ea2 (eV)
Pure
1%
2%
0.080
0.085
0.104
0.257
0.216
0.183
2.85
3.25
3.616
where, ρ is resistivity at temperature T, ρο is a constant, K is Boltzmann constant (8.62 × 10−5
eV/K) and Eο is the activation energy required for conduction. Table IV shows the variation in
activation energy as a function of Al concentration. It is clear that the activation energies Ea2
decrease and Ea1 increases with increasing of Al concentration due to the tailing both of the
conduction and the valence band is diminished thus moving the conduction into regions with higher
density of states and mobility also may be the variation of crystallinty of film with increasing Al
concentration. From the Hall measurement, the type of charge carriers, carrier concentration (nH),
Hall mobility (µH) drift velocity (υd) has been calculated as in Table V. It was noticed from this table
that the films of all samples have a negative Hall coefficient. It can be observed from these
calculations that both the conductivity increases with increasing of Al concentration, and the carrier's
concentration, mobility and drift velocity increases with increasing of Al concentration.
TABLE V: SHOWS THE VARIATION of electrical parameters
Thicknes
RH x104
nH x1015
µ x10-2
σ R.T x10-5
υd x10-2
3
-3
-2
-1
s (nm)
(cm /c)
(cm)
(cm /V.s)
(cm/s)
(Ω.cm)
Ω
Pure
1%
2%
2.85
3.25
3.616
174.25
130.56
71.30
108.9
81.60
44.56
272
0.1636
0.2489
0.5071
1.635
2.489
5.071
Type
n
n
n

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For applications as transparent contacts, the films must have a low resistivity and a low
absorption coefficient in the visible region. A way for evaluating this compromise is by means of the
figure of merit (FOM), defined as:
FOM=1/αρ
(5)
where α and ρ are the visible absorption coefficient and the electrical resistivity, respectively.
As the visible absorption coefficients of all the films were similar, the figure of merit was strongly
dependent on electrical resistivity. As shown in Fig. 6, the variation of FOM shows the same trend
with that of electrical conductivity, due to the variation observed in the electro-optical properties of
films. The FOM is increases with increasing of Al concentration and the highest figure of merit
value was 3.15×10–5 –1, which was obtained for the ZnS:Al (2%) film.
3.2
Figure of merit x10-5
-4
3.1
3
2.9
2.8
2.7
2.6
0
0.5
1
1.5
Al content (%)
2
2.5
Fig. 6: Variation of figure of merit with the dopant concentration.
ACKNOWLEDGEMENT
The authors wish to acknowledge the support rendered by the laboratories of thin films in
physical department, college of science, Baghdad University.
CONCLUSION
Thin films of ZnS and ZnS:Al have been fabricated successfully by vacuum evaporation
technique on glass substrate. The value of optical energy gap decreases with increasing Al
concentration. The amorphosity decreases with increasing Al concentration. The width of tail
decreases with increases Al concentration. The refractive index decreases with increasing Al
concentration. The behavior of extinction coefficient is nearly similar to the corresponding
absorption coefficient. The behavior of ε1 is similar to refractive index, while ε1 is mainly depends on
the k values. The electrical resistivity and therefore activation energy are observed to be Al
concentration dependent.
The Hall effect measurements confirm the n-type nature of ZnS and ZnS:Al thin films. Also
the conductivity, Hall mobility, drift velocity and figure of merit increases with increasing Al
concentration, whereas, the carrier concentration, decreased with increasing of Al concentration.
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