This document summarizes a study on the effect of doping on electrical properties of ZnS thin films. Pure ZnS and Pb-doped ZnS thin films were prepared by thermal evaporation. Hall effect measurements showed that conductivity changed from n-type to p-type with doping. I-V characteristics exhibited diode-like behavior. Doping increased carrier concentration and conductivity. The ideality factor was calculated from I-V curves. Overall, doping improved film properties and ZnS:Pb could potentially be used in solar cell devices.

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EFFECT OF DOPING ON D.C. CONDUCTIVITY FOR ZnS:Pb THIN FILMS
Farah T. M. Noori1, Salma M. Shaban1, Zaidoon T. M. Noori2 and Raad M. S. Al-Haddad1
1
Department of physics, College of science, University of Baghdad
2
Dijla University College, Baghdad, Iraq
ABSTRACT
Pure ZnS and ZnS:Pb thin films were prepared by vacuum thermal evaporation onto glass
substrates at room temperature and 200 nm film thickness to study effect of doping on electrical
measurements. Hall measurements indicated that the type of conductivity for pure films is n-type but
transforms to p-type after doping. The behavior of I-V characteristics can be considered as ideal
diode. The ideality factor for the I-V characteristics at forward bias voltage was calculated. The
concentration of carriers increases with doping of Pb atoms.
Keywords: ZnS, Junction, Thin Films.
1. INTRODUCTION
Thin films of lead and Zinc sulfate show promise in many physical applications, such as
optoelectronics, infrared photo detectors, optical switches, and solar cells J. Twidell et al. (2006)[1],
A. Shah et al. (1999)[2] studied the doping effect of Cu & Al on thin films structure of PbS and ZnS
which have generated significant interest because they offer the advantage of tunable optical and
opto-electronic properties of PbS, ZnS. The band gap, electrical conductivity, and structural
properties doping ZnS were studied by A. Goetzberger et al. (2002)[3] and R. Strandberg et al.
(2010)[4] . From such results, doping of PbS by Cd has been found to influence film growth and
result in reductions in crystallite size. In the present work, the effect of doping on Hall measurements
and D.C. conductivity were studied for ZnS:Pb thin films which prepared by thermal evaporation.
The Hall Effect is important in the analysis of semiconductor material (in which the charge carriers
can be either positively or negatively charged). It also has many important practical applications in
detecting and measuring magnetic fields.
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2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 2, February (2014), pp. 01-05, © IAEME
2. EXPERIMENTAL PART
Pure and ZnS:Pb thin films were prepared by thermal evaporation using an Edward E306 A
coating unit onto glass substrates which were cleaned by distilled water , pure alcohol , and then by
ultrasonic vessel in order to deposit thin films at room temperature of 200 nm film thickness for
electrical measurements. A molybdenum boat was used as the source holder and the pressure inside
the chamber was better than 2*10-5 mbar. Pure ZnS and ZnS:Pb thin films were evaporated thermally
with a deposit rate of 0.8 nm/sec to get 200 nm thickness. Aluminum electrodes were evaporated
from a molybdenum boat by the previous coating unit for electrical measurements. The electrical
resistance and then D.C. conductivity were calculated from I-V measurements. Also Hall
measurements were obtained using two sensitive digital electrometer type Keithley 616 to measure
the voltage and current as shown in Fig. 1.
Fig. 1 the circuit diagram for Hall measurement
3. RESULTS AND DISCUSSION
In general for D.C. measurements the electrical conductivity σ was calculated from
equation [5]:
σ = (1/ρ) = (R*A)/L ………………..(1)
where ρ , R , A , and L are the resistively , the resistance , cross section area , and the distance
between electrodes respectively . The resistivity can found from the relation between current and the
applied voltage. Figure 2 shows the current voltage characteristics of the electrical conductivity for
ZnS
and
ZnS:Pb
films
.
As
seen
from
fig
ure 2, the relation is exponential with the increase of positive bias voltage and it shows saturated
current with the increase of reverse bias voltage. So the I-V characteristics for ZnS:Pb films can be
considered as ideal diode. Also with doping of Pb atoms in pure ZnS, the increase of saturated
current with the increase of reverse bias voltage and this can be attributed to increase the electrical
conductivity with doping then these films can be used as intermediate layer in solar cell devices[6].
From figure 2, the initial part of the forward branch of the plot (V<0.3) represents the recombination
current while tunneling current dominates in the rising part of the forward branch of the plot (V>0.3 )
volt. Figure 3 shows the logarithm of the initial part of the forward branch, the ideality factor for the
diode can be calculated as listed in Table 1 .The values of the ideality factor are in agreement with
M. Green whose thin films were prepared by (Chemical Bath Deposition) method.
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3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 2, February (2014), pp. 01-05, © IAEME
Fig. 2 I- V characteristics of pure ZnS and ZnS:Pb Films
Fig. 3 the logarithm of the initial part of the forward branch of pure ZnS and ZnS:Pb Films
Table 1 Values of ideality factor for ZnS :Pb Films
ZnS Films
β
Pure
9.1
Doped
9.3
Figs. 4 and 5 represent the relation between Hall voltage VH and the current I for pure ZnS and
ZnS:Pb films. Hall coefficient RH, carrier concentration n, and mobility µ can be calculated from the
slope of the figure and by using equations:
RH = (VH/I)*(t / B)
(3)
n = (-1 / RH*q)
(4)
µ = (σ RH)
(5)
where t , B represent the thickness and magnetic field respectively .
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4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 2, February (2014), pp. 01-05, © IAEME
Fig.5 Hall voltage as a function of current for pure ZnS
Fig.4 Hall voltage as a function of current for ZnS:Pb films
Values of Hall parameters were indicated in Table 2. From the Table we noticed that the type
of conductivity transforms from n-type to p-type and the carrier concentration increases after
doping[7,8,9].
Table 2 Values of Hall measurements
Type of
Type of
Hall
Carrier
Conductivity
Thin
Conductivity
effect
Concentration
(1/ῼ Cm)
Films
RH
( cm-3 )
Carrier
Mobility
cm2/v. s.
Pure
ZnS
n - type
1.69E+7
3.68 *1011
1.38E-6
24.8
ZnS:Pb
p- type
1.01E+7
6.12 *1011
1.46E-5
142
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5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 2, February (2014), pp. 01-05, © IAEME
4. CONCLUSION
The behavior of I-V characteristics for Pure ZnS and ZnS:Pb can be considered as good ideal
diode. The doped films ZnS:Pb have higher concentration and p-type conductivity. From these
results, ZnS:Pb films can be used as intermediate layer in solar cell devices.
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