Journal of Applied Sciences Research, 4(3): 311-318, 2008
© 2008, INSInet Publication
The Electrical Properties of Pbse Thin Films Incorporated with Sn
1
A. Sawaby, 1Z.S. ElMandouh, 2S.A. Nasser and 2M.E.Fathi
1
2
N.R.C., Phys. Division, Cairo, Egypt.
Phys. Dept., Beni-Sauf Faculty of science, Cairo University.
Abstract: Semiconductor thin films of composition Pb 1 -x Sn x Se with x ranging from 0.0 to 0.4 as
deposited on glass substrate are prepared using thermal evaporation technique. X-Ray diffraction
measurements of bulk and thin films of Pb 1 -x Sn x Se were studied. The electrical resistivity of undoped
PbSe and incorporated PbSe with Sn was calculated using conventional I-V curve. T he increase of
electrical resistivity of Pb 1 -x Sn x Se thin films with increasing x at room temperature was attributed to the
carrier composition effect of P- type PbSe and donor impurity of Sn. The increase of thermal activation
energy can be attributed to the increase in barrier height at crystallites and intercrystalline barrier. I-V
Characteristic measurements of films indicate that the space charge limited current may contribute in the
electrical conduction of PbSe and Pb 1 -x Sn x Se thin films. The concentration of charge carrier (n 0 ), the
mobility of the electrons (m 0 ), the effective mobility (m 0 ), Fermi Level (E f), trap level from conduction
band edge (E t ), and the samples at various concentrations were calculated. The type of charge carrier of
films was also determined using thermoelectric probe.
Key words: Thin Films, Electrical Properties, Thermoelectric power, Pb 1 -x Sn x Se
experimental results for n-and p-Pbse led to propose
the existence of a band of quasilocal states the
distribution of holes between this quasilocal band two
valence bands with hole densities 6.75 x10 1 9 and
1.32x10 2 0 gave good results.[1 2 ]
Ali and Khan [1 3 ] studied the thermoelectric power
of Pbse and shows a positive value which signifies a
p-type sample.
INTRODUCTION
PbSe crystallize into f.c.c Nacl-type with structure
(
). It undergoes structured phase transitions to
orthorhombic Tll-type phase (
) [1 ,2 ] .
The resistivity was measured at temperatures
ranging from room temperature to 165 o C by Rodica
and candea [3 ,4 ]. The short-term isothermal stability at
high temperature was tested ATT=300 o C no significant
change was detected but at T>300 o C more or less
pronounced increase in resistivity was recorded.
Numerical calculation [5 ,6 ] and of Hemstreet[7 ,8 ] for
the energy spectrum of intrinsic defects in lead
chalcogenides predict appearance of a group of local
levels associated with the chalcogen vacancies and
located near the bottom vacancies and located near the
bottom of the conduction band of PbTe or PbS.
Investigations of the transport properties and MossBauer effect measurements of the charge state of Sn in
alloys of lead and tin cholcogenides [9 ,1 0 ] have
domenstrated that introducing of acceptors into these
materials results in ionization of tin impurity involving
capture of the holes by Sn 2 + centers and formation of
Sn 4 + centers. In case of Pb 1 -x Sn x Se a band of twoelectron impurity states is observed and the position of
the new band is governed by external conditions and
composition of alloy [1 1 ]. A quantitative analysis of
Experimental Technique: High purity (5N) of Lead,
Selenium, and, Tin were used for preparing Pb 1 -x Sn x
Se. Pb, Sn, and, Se were mixed according to their
atomic weight held at in evacuated silica tube which is
heated to 873K and this temperature for 24 hours. Thin
film was prepared by thermal evaporation. Philips XRay Diffractometer was employed for crystal structure.
Resistance measurements were done by Keithley
Electrometer. Thermoelectric power was measured by
special design circuit.
RESULTS AND DISCUSSION
X-Ray Diffraction: The crystal structure of PbSe
incorporated with Sn was investigated using X-Ray
CuKa radiation with Ni filter. The X-Ray Diffraction
(XRD) pattern of prepared bulk of Pb 1 -x Sn x Se with x
ranging from 0.0 to 0.40 are shown in fig (1) while
XRD pattern of deposited thin films of same
compositions is shown in fig (2).
Corresponding Author: A. Sawaby, N.R.C., Phys. Division, Cairo, Egypt.
311
J. Appl. Sci. Res., 4(3): 311-318, 2008
Fig. 1: XRD pattern of the Pb 1 -x Sn x Se in the bulk form at different values of the additive ratio X : (a) x= 0.00;
(b) x = 0.05; (c) x= 0.15; (d) x=0.25; (e) x = 0.40.
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J. Appl. Sci. Res., 4(3): 311-318, 2008
Fig. 2: XRD pattern of the as deposited Pb 1 -x Sn x Se thin films at different values of the additive ratio X: (a)
x= 0.00, (b) X = 0.05, (c) x=0.15, (d) x=0.25; (e) x= 0.40.
Comparing the calculated d-values with that
reported indicate that PbSe was prepared in a single
polycrystalline phase. W hile XRD of Pb 1 -x Sn x Se was
identified and the variation of d-spacing as function of
the additive ratio x is clarified in figs (1,2).
film during heating and cooling cycles as recorded in
fig (3). It was observed that during heating an
anomalous behaviour was observed where resistivity
decreases up to 322K o indicating semiconductor
behaviour; above this temperature the resistivity starts
to increase as in metals and then decreases again at
temperature 355K. this behaviour was detected before
by Ali, et al[1 3 ]. The variation of electrical resistivity
Electrical Properties: The electrical resistivity as
function of temperature was measured for PbSe thin
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J. Appl. Sci. Res., 4(3): 311-318, 2008
Fig. 3: Electrical restistivity (p) vs. Temperature (T) for the undoped PbSe thin films.
Ln ñ against 1000/T for Pb 1 -x Sn x Se thin films
with x ranging from 0.0 to 0.4 are presented in fig (4).
The resistivity was calculated from the relation:
Table 1:
C
0.0
0.05
0.15
0.25
0.40
X
ñ= ñ 0 exp (DE / KT)
W here:
C
ñ 0 resistivity constant.
C
DE activation energy.
C
K Boltzmann constant.
this films such as scattering of crystallites,
intercrystallites barrier and thermal Oxygen desorptionadsorption process which affects the values of
activation energy and energy gap. [4 ,1 7 ] Gurieva et al [1 8 ]
suggested the existence of a second valence band for
Pb 1-x Sn x Se with x < 0.35 which may play a role in
interpreting the electrical behaviour of these thin films.
It is clear from fig (4) that the temperature
dependence of Lnp had a maximum typical of a phase
transition from cubic phase to orthorhombic T II type
phase
(
) occurring
T herm al activation energy for as deposited Pb 1 -x S n x S e
thin film s.
Activation Energy (ev)
------------------------------------------------------------------Above Anom aly
After Anom aly
0.1438
0.0502
0.1947
0.0679
0.2019
0.0732
0.2034
0.1113
0.1766
0.05934
at temperature T 0 . This
structural phase transition was observed for all
compositions in the range 0.0 < x < 0.4. T his transition
temperature increases with increasing tin content [1 4 ,1 5 ].
Such phase transitions are usually associated with a
soft
vibration mode of the transverse optical
p h o nons [ 1 6 ] which is a co nsequence of the
anharmonicity of such vibrations. The slope of this
anomalous peak was calculated before and after
transition temperature and the values of activation
energies were listed in table (1).
The activation energy increases as tin content
increases till x=0.25, it decreases, which may be
attributed to the change in the barrier height due to the
change in size of the grain in a polycrystalline film.
Normally some factors affect the electrical resitivity of
C ha racteristics: I-V C hara cte ristic s at room
temperature showed Ohmic behaviour as shown in
fig(5). I-V Characteristics for PbSe and Pb 1 -x Sn x Se
(x = 0.4) at temperature 390 and 395K respectively
above the transition temperature are shown in fig (6) as
log-log scale. Such figure is divided into three regions
depending on the applied voltage. First region of I-V
was Ohmic, while second region was a square law
relation I 4V 2 and the third region I4V n where 2< n <
7 at the trap filled limit. However; the presence of
three regions may lead to suggestion that the
conduction mechanism may be due to space change
limit.[1 9 ,2 0 ] the current in the square law region was
described by:
C
314
J = 9/8 è g ì 0 V tr /d 3
J. Appl. Sci. Res., 4(3): 311-318, 2008
Fig. 4: The variation of the Pb 1 -x Sn x Se thin films resistively with temperature for different values of tin content.
Fig. 5: The variation of the I-V characteristic curve as a function of the tin content at room temperature.
C
W here q is the ratio between the free
carrier n 0 in conduction band to total density
(n 0 +n t) where n is the density of the trapped
electrons
W here E electric field (v/d) we can determine total
concentration of traps which defined by:
C
C
è = n 0 / n 0 +nt
the free carrier density in conduction band n 0 could be
determined using relation.
J = q n0 ì 0 E
N t = 2g V T FL / qd 2
W here V T E L is taken as voltage at which the
current rises sharply with time (third region) [1 9 ,2 1 ,2 2 ].
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J. Appl. Sci. Res., 4(3): 311-318, 2008
Fig. 6: I-V characteristics on a log-log- scale for typical sample for pure PbSe at T=398 o K and as deposited
Pb 0 .6 Sn 0 .4 at T=423 o K thin films.
Fig. 7: Plot of thermal emf versus temperature for films of different concentrations of tin for the asdeposited
Pb 1 -x Sn x Se thin films.
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J. Appl. Sci. Res., 4(3): 311-318, 2008
Table 2: The variation of the transport param eters for the pure PbSe thin film s and as deposited Pb 1 -x Sn x Se with x=0.40.
Sam ples
ó Ù -1 cm -1 è x10 -2
ì 0 cm 2 V -1 S -1 ì e cm 2 V -1 S -1 n 0 cm -3
N t cm -3
n t cm -3
N c cm -3
E F eV
E t eV
PbSe
(T=390 o K)
2.93x10 2
22.95
1.7x10 2
0.4x10 2
1.05x10 1 9
1.57x10 1 5
3.3x10 1 9
1.7x10 1 8
0.061
0.285
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Pb 1 -x Se x Se
(T=395 o K)
2.07x10 2
20.26
1.472
0.297
8.8x10 2 0
3.9x10 1 6
3.4x10 2 1
1.76x10 1 8
0.214
0.184
Table 3: Com position dependence of g, E 0 , A and Z.
Sam ple
Tem perature
Activation Energy at O k (ev)
Ferm i level E F (e.V) Scattering param eter A
Figure of M erit Z(K-1)
PbSe
3.24
0.093
0.0631
2.47
1.4x10 -4
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Pb 0 .9 5 Sn. 0 5 Se
3.25
0.09
0.063
2.51
0.99x10 -4
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Pb 0 .8 5 Sn. 1 5 Se
1.87
0.0518
0.0611
2.49
2.8x10 -4
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Pb 0 .7 5 Sn. 0 25 Se
3.33
0.0908
0.0641
2.49
1.8x10 -4
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Pb 0 .6 Sn. 0 4 Se
3.7
0.099
0.065
2.42
0.96x10 -4
Fig. 8: Thermoelectric power variation with l000/T for different addition ratio of tin atoms to the PbSe thin films
W hen a trap level exists the electron mobility m 0 is
reduced by a factor 1 /q and effective electron mobility
in a trap level was:
C
Thermoelectric Power: Some transport parameters
such
as
thermal
activation
energy at 0K,
temperature coefficient of activation energy g and
concentration
of
majority
carriers
could be
evaluated from TEP measurements. Fig (7) shows
the variation of thermal emf with temperature for
PbSe and Pb 1 -x Sn x Se thin films during heating. The
re sults in d ic a t e th a t t h e film s are p -typ e
semiconductors. Seebeck coefficient was evaluated
from the obtained data of thermal emf as a function
of temperature.
ìe = ì0 q
The effective density of states in the conduction
band is calculated according to:
C
N c = [2 ð mKT/h 2 ] 3 /2
The transport parameters were calculated and listed
in table 2. it is clear from the values that the
mobility and the number of change carriers are
increased with increasing tin and temperature in the
film. The conduction mechanism of these films may be
attributed to the localized defects.
where: r : is a constant depending on the mechanism
of scattering.
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J. Appl. Sci. Res., 4(3): 311-318, 2008
C
ì p : is the distance from the Fermi level to the
valence band.
9.
The values of temperature coefficient of activation
energy (g) and activation energy at 0 0 K could be
determined by plotting thermoelectric power S against
inverse temperature and extrapolating the linear
portions to the value 1/T=0K -1 as shown in fig (8)
according to the equation [2 3 ]:
10.
11.
12.
13.
W here: e : electronic , K: Boltzmann constant.
A: Transport term depends on the nature of
scattering process and varies from 0-4 [2 4 ].
From the graph; thermoelectric power increases
with temperature in the low temperature region,
increasing temperature TEP tends to saturate. The slope
of the plot will yield E 0 and it changes with
temperature, this suggested that E 0 - g T=E F -E v and the
Fermi level is pinned at high temperature, very close
to band edge E v [2 3 ] . The pinning of the Fermi level
occurs at high temperatures because the calculated
activation energy for conduction does not exceed
0.2 e.V and hence there is abundance of excited
carriers (holes) from the acceptor levels. See table (3).
The table (3) shows the values of the temperatures
coefficient of activation energy (g) E 0 , the position of
Fermi level E F , the value of the scattering parameter
(A); and, the Figure of Merit Z calculated for PbSe
and Pb 0 .9 5 Sn.0 5 Se films for different compositions.
The value of A=2.5 suggests that the predominant
scattering mechanism is lattice scattering. In thin films
the surface and grain boundaries scattering should be
taken into account. The values of Figure of Merit are
in the range expected for substituted Sn in Pb ternary
alloys.[2 5 ,2 6 ]
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
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