November 2009 Phys. Chem. News 50 (2009) 98-103
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CONDUCTIVITY OF THIN POLYCRYSTALLINE OF FeSe2 SYNTHESIZED BY VARIOUS SELENIZATION PROCESSES T. Abachi1*, J. C. Bernède2, A. Khelil3, N. Hamdadou3 1
2
Ecole Normale Supérieure E.N.S 16000 Kouba Algérie L.A.M.P, Faculté des Sciences et Technologies, Université de Nantes, 2 rue de la Houssinière, BP92208, 44322, France. 3 L.P.C.M.M.E, Faculté des Sciences, Université d’Oran Es-sénia, 31100 Oran Algérie * Corresponding author. E-mail:
[email protected] /
[email protected] Received: 23 February 2008; revised version accepted: 21 October 2008
Abstract The thin layers of FeSe2 were synthesized by selenization of a thin layer of iron under selenium pressure. Two methods of selenization were used and led to two types of layers: For the first type (IFeSe2), after selenization during half an hour at 723 K, the layers were annealed under selenium pressure during two hours at773 K. For the second type (II-FeSe2), the thin layers were directly annealed under selenium pressure during two hours at 773 K. The two types of layers crystallized in the the orthorhombic structure. The size of the grains of the second type (500nm) is larger than that of the first type (100nm). The measurement of the variations of electric conductivity versus of the reciprocal temperature shows that the two film families have different behaviors. For the first type one observes a continuous decrease of conductivity with the reciprocal of the temperature. Conductivity is limited by the grain boundaries, the model of Werner well explains the behavior of this type of structure in high and low temperatures. For the second type of structures have observes two distinct domains of conductivity; for the domain of the high temperatures as conductivity is close to that of the monocrystals and than the grains are “large” one can propose here that conductivity is limited by the crystals. In this case the energy activation measured imposes that one supposes the presence of states traps not very deep due to defects in the forbidden band. As for conductivity at the low temperatures, it is the tunnel effect which controls it. Keywords: FeSe2 thin films; Selenization; Conductivity; Grain boundary.
apparatus. The substrate temperature was controlled by a chromel-alumel thermocouple attached with silver paste to the sample surface. The substrate was heated using infra red lamps. The deposition rate and the deposited thickness were measured with the help of a quartz monitor. The substrates were at room temperature during iron deposition. A film of iron of thickness 50-100 nm was deposited by joule effect, using a tungsten crucible, at a rate 0.15 nms-1 in a vacuum of 10-4 Pa. In the present study FeSe2 films are issued from two different synthesis processes: - First process [5]: the iron films were heated at 723 K and selenized for half an hour, the Se deposition rate being 2.5 nms-1. Then the films are introduced with some Se powder into a Pyrex tube, which is then annealed 2 h at 773 K. These films were called I-FeSe2. - Second process [6] the iron films are directly introduced with Se powder into a Pyrex tube, then this tube is sealed under vacuum and annealed 2 h at 773 K. The partial Se pressure, estimated from the size of the tube and the selenium mass introduced in it, is around 5 102 Pa. The films which belong to this family are called II-FeSe2. For electrical measurements gold electrodes, in the desired configuration, have been evaporated onto FeSe2 films. Gold was chosen because it
1. Introduction The conductivity of polycrystalline semi conducting thin films depends sensitively on the grain boundaries; that is to say on the potential barrier and space charge regions that are built up around them. The variation of conductivity in a large domain of temperature is observed, these variations are analyzed with models of thermal carrier emission across grain boundary barrier [14]. The grain boundary trapping theory assumes the presence of a large amount of trapping states at the grain boundary able to capture free carrier. This creates the grain boundary barrier, which opposes the passage of carriers from a grain to the neighboring ones [1-4]. During this work, after checking of the crystalline structure of the synthesized layers, we introduced different electric models of conductivity according to the type of layers concerned. 2. Preparation of the layers and characterization The substrates are polished soda lime glass chemically cleaned. Substrates were cleaned using soap and deionized water. After scrubbing with soap, the substrates were rinsed in running deionized water. Then they were dried with N2 gas and immediately introduced into the vacuum 98
T. Abachi et al, Phys. Chem. News 50 (2009) 98-103
the conductivity decreases continuously with the reciprocal of the temperature (1/T). On the other
guarantees a good ohmic contact. An electrometer (Keithley 617) was used to measure the dc resistivity between 30-300 K using a cryostat. The Hall coefficients at room temperature were determined using a four point probe system called Van der Pauw technique. Data acquisition and analysis (calculation of density and mobility of the charge carrier) were carried out using a computer with a standard program. The majority carrier type has been checked by the hot probe technique. The surface topography and cross section of the films were observed with a field effect scanning electron microscope JEOL F 6400a. Electronic microprobe analysis (EMPA) have been preformed using a JEOL 5800 2V scanning electron microscope equipped with a PGT X-ray, microanalysis system, equipped with a germanium detector used for the measurement of X-ray energy. The structure of the films has been checked using an analytical X-ray system-type DIFFRACT at V3.1 Siemens which uses a graphics program EVA. The wavelength was 0.15406 nm.
hand, in the case of the type
domains of variation are visible; this cannot be explain by the classical theory of the semiconductors. The conductivity of the semiconductors in polycrystalline thin films depends mainly on the grain boundaries. This theory was developed by Seto and others [1]. These authors highlight the effects of trapping of the carriers at the grain boundaries in the electric conductivity of the polycrystalline thin films. They suppose the creation of a depopulated zone of carriers due to the capture of mobile electric charges by the traps present at the grain boundaries (highly disordered medium). It results from this the formation of barrier of potential at the accesses of the grains which reduce considerably the passage of mobile charges of a grain to other what implies an increase in the electrical resistance. The conductivity of the polycrystalline thin layers undergoes strong variations according to the temperature. If we analyze this variation from a thermionic model of emission of carriers to the top of the barrier of potential present at the grain boundaries similar to Seto type [1-4], one expects to obtain a line while tracing
3. Experimental Results and discussion The diagrams of diffraction of x-rays presented in figure -1 show the same peaks of diffraction for the two types of films. The layers of FeSe2 crystallized in the orthorhombic structure. Figure -2 shows the visualization of the two films, these last show that the size D of the grains of II − FeSe2 type is larger than that of
(
one (see table -1). The technique of the hot point shows that all the layers are of the type p, the same type of majority carriers is obtained by measurements of Hall effect. The electric values deduced from measurements of Vander Pauw at the ambient temperature are reported in table -1. σ (Ω.cm)1
I-FeSe2 II-FeSe2
0.294 3.84
p (cm-3) 18
2.3 10 4 1018
µ (cm2
D(nm)
V-1s-1) 0.8 6
100 500
(
)
is not the case and we note that the curves of Arrhenius are strongly curved because the energy of activation increases with the temperature. The inhomogeneousness of the grain boundaries was not considered by the models of Seto type. This problem was considered by Werner [7]. This new model attributes the increase of the energy of activation of the curve of Arrhenius to the fluctuations heights of barriers of potential at the grain boundaries. It has the great merit to make it possible to obtain a good agreement between the experimental results and the theoretical curves while utilizing one model on a great domain of temperature where before, it was necessary to use several of them. The model of the fluctuations of potential supposes a continuous distribution with the interface between metal and the semiconductor.
Table 1: Electrical parameters for I-FeSe2 and II-FeSe2 deduced from the Van der Pauw measurements
Figure -3 shows the variations of conductivity between 30 K and 300 K for the two types of films, these figures show that the variation of conductivity does not follow the simple law of Arrhenius. In the case of the type
)
ln σ T n = f 10 3 / T . However, very often it
I − FeSe2
Sample
II − FeSe2 , two
I − FeSe2 99
T. Abachi et al, Phys. Chem. News 50 (2009) 98-103
1000 1000
(210)
(310)
400 200 20
30
40
50
(120)
400 400
200 200
0010 0
60
2 θ (deg.)
(220)
600 600
(211)
(220)
600
0 10
(110)
(200)
800
(120)
20
30
20
30
40
50
40
50
2θ(deg.) (deg.) 2θ
60
60
Figure 1: X-ray diffractogramms in the case of : a) I-FeSe2 b) II-FeSe2.
0
a
Ln (σ)
-4
-8
-12 10
20
30
40
-1
1000/T (K)
b
2.4
b Ln (σ)
Intensity (a.u.)
1000
a
800 800
(111)
1200
Intensity (a.u.) Intensity (a.u.)
b
(110)
1400
1.6 0.8 0.0 0
1 µm
10
20
30 -1
1000/T (K ) Figure 3: Plots of Ln ( σ ) vs. inverse temperature in the case of a) I-FeSe2 layer , b) II-FeSe2 layer
Figure 2: Microphotographs in the case of : a) I-FeSe2 b) II-FeSe2
100
T. Abachi et al, Phys. Chem. News 50 (2009) 98-103
⎛ q r A* ⎞ qαφ qφ(T = 0) 1 q2σφ2 1 ⎛ σ gb ⎞ ⎟⎟ − − ⋅ + 2 ⋅ 2 ln⎜⎜ ⎟⎟ = ln⎜⎜ k k T 2k T ⎝T ⎠ ⎝ k ⎠ The derivative of this equation with 1/T gives the equation of the energy of activation. Thus, the
The spatial distribution of the tension ΦB to the interface metal/ semiconductor of the Schottky. contacts is stimulated by a Gaussian distribution:
( )
P φB =
1
σφ
(
⎡ φ −φ exp ⎢− B 2 B 2σ φ 2π ⎢⎣
) ⎤⎥ 2
⎥⎦
with being the mean barrier height and the standard deviation. In this model the density of the thermionic current through the grain boundaries is given by the expression:
of the temperature according to a parabolic law such as: y = A X + B X + C , starting from A and of B one respectively determines the standard 2
deviation
and the height of the barrier
2
As we saw in the case of the Schottky contact
φ (T )
The values of
varies linearly with the temperature as:
and
φ
are obtained from (A) ,
and (B) respectively, such as
φ (T ) = φ (T = 0) + α φ T
φ = 20.33meV
The energy activation is given by: ⎡ qσ φ2 ⎤ ⎛ σ gb ⎞ d ⎜ ⎟ ( ) φ ln 0 E act (T ) = − k q T = = − ⎥ ⎢ kT ⎦⎥ dT −1 ⎜⎝ T ⎟⎠ ⎣⎢
(table-2).
For the second type of layer II − FeSe2 , the value of conductivity at the ambient temperature is , which is close to that of the monocrystals of FeSe2 measured by HARADA [8]. For as much one notes two distinct domains of conductivity; this soft brings to propose a conductivity dominated by the grains in the field of the high temperatures and a conductivity controlled by tunnel effect with the grain boundaries in the field of the low [9] figure 6 . temperatures
-9 Ln (σ/T)
φ.
In the case of I − FeSe2 figure 4 well corresponds to the model suggested by WERNER [7]; this curve is followed by the equation:
⎡ β q q φ B (T = 0) qσ φ2 ⎤ J 0 = A T exp ⎢− + − ⎥ 4kT ⎥⎦ kT ⎢⎣ kT0 *
⎛ σ gb ⎞ ⎟⎟ must vary with the reverse ln ⎜⎜ T ⎠ ⎝
variation of
-12
-15 0
12
24
36
48
2.4
-1 1000/T (K )
With
σ gb
being
the
grain
Ln (σ)
Figure 4: Plot of Ln (σ/T) vs. inverse temperature in the case of -I-FeSe2 layer. ■: experimental results. ---- : Werner theory[7].
1.2
boundaries
conductivity. This equation shows that the energy activation Eact of the curve of Arrhenius of
σ gb T
1.8
3
4
5
6
-1
1000/T (K )
varies linearly
Figure 5: The variation of Ln (σ) vs. inverse temperature in the case of -II-FeSe2 layer. ■:experimental results.___: monocrystal conductivity[8].
with 1 /T. If one goes up with the curves of conductivity temperature σ/T function of the temperature: 101
T. Abachi et al, Phys. Chem. News 50 (2009) 98-103
0.7
Indeed as the grains are “large” one figure-5 can propose that here the conductivity, which moreover is higher than for type I, is limited by the crystals and not by the grain boundaries, at least for a temperature higher or equal to the ambient temperature. By applying the law of
σ = σ 0e
Ea kT
-1
σ (Ω. cm)
conductivity
−
0.6
to this sample, one
notes that the energy of activation is of:
E a = 41, ,32meV ≺
Eg 2
0.4
= 480meV , in this
50
temperature range. In this case the activation energy measured forces that one supposes the presence of not very major states traps due to defects, this soft pleasing to propose a conductivity dominated by the grains in the field of the high temperatures. When with there conductivity in low temperatures its variations are weak (figure-6) and a good agreement between the experimental results and the theory is obtained by using the model of the tunnel effect describes by GARCIA - CUENCA and al [9], in this model the simple expression, deduced from the theory of the tunnel effect, of the variation conductivity according to the temperature is: [9]. The figure-6 shows a good agreement between the curve and the theoretical expression, the deduced values of are: . When with the size of the grains of the two types of layers, the pressure partial of Se during the preparation of the layers is smaller for the first type than that for the second one; this justifies the low speed of growth for the first type what induces the formation of small grains. For the second case, the pressure of selenium is important what justifies a fast growth of the size of the grains.
Sample
I-FeSe2
75
100
125
T (K) Figure 6: The variation of electrical conductivity
σ
versus T of - II-FeSe2 layer. ■: experimental results. ___: Garcia-Cuenca model [9].
4. Conclusion The thin layers of were synthesized by selenisation of the thin films of Iron, the conductivity of these films was discussed using the theory of conduction of the grain boundaries. The layers of bi selenium of iron ( FeSe2 ) crystallized in the orthorhombic structure, they are a semiconductors of the P type with a gap about 1eV [10]-The growth of the films by selenization of the films of Iron depends on the temperature of the substrate and the selenium pressure in the reaction room [5]. We defined two types of films according to the method of synthesis used. For the first type of layer I − FeSe2 , thin films synthesized in two times, considering the low size of the grains, the low mobility of the carriers and their high resistivity; conductivity is governed by the grain boundaries. It seems that the model of WERNER [7] explains well the continues growth of energy of activation with the temperature; this model introduces the fluctuations of the potential to the grain boundaries to explain the variation of conductivity according to the temperature. For the second type of layer II − FeSe2 , thin films synthesized only in one stage, the value of conductivity to ambient is about 3.5(Ω.cm)-1 , which is close to that of the monocrystals of FeSe2 measured by HARADA. For as much one note two distinct fields of conductivity; this soft brings us to propose a conductivity dominated by
σΦ(meV)
20.33
0.5
6.317
Table - 2: Electrical parameters for I-FeSe2 deduced from Werner model.
102
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Ben Nasrallah T., J. Phys. III., 4 (1994) 677. [5] N. Hamdadou, J. C. Bernède, A. Khelil, J. Crystal Growth, 241 (2002) 313. [6] N. Hamdadou, A. Khelil, M. Morsli, J.C. Bernède, Vacuum, 77 (2005) 151. [7] J.H. Werner, Solid State Phenom., 37/38 (1994) 213. [8] T. Harada, Journal of the Physical Society of Japan, 67 (1998) 1352. [9] Garcia-Cuenca., J. L. Morenza, J. Esteve, J. Apl. Phys., 56 (1984) 1738. [10] L.D. Dudkin, V.I. Vaidanich, Sov. Phys. Solid State, 6 (1962) 1384.
the grains in the domain of the high temperatures and a conductivity controlled by tunnel effect with the grain boundaries in the domain of low temperatures [9]. References [1] J. W. Seto, J. Apl. Phys., 46 (1975) 5247. [2] G. Baccarini, B. Ricco, G. Spadini, J. Apl. Phys., 49 (1979) 5565. [3] C. H. Seager, G. E. Pike, J. Appl. Phys., 50 (1979) 3414. [4] J. C. Bernède, J. Pouzet, R. Le Ny,
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