- Email: [email protected]
The effects of annealing on the AC electrical properties of cobalt phthalocyanine thin films
PHYSICA Physica B 222 (1996) 136-142
ELSEVIER
The effects of annealing on the AC electrical properties of cobalt phthalocyanine thin films S.I. Shihub, R.D. Gould*, S. Gravano Thin Films Laboratory, Department of Physics, Keele University, Keele, Staffs. ST5 5BG, UK
Received 17 November 1994; final revised 18 October 1995
Abstract
A comparative study of the AC electrical properties of cobalt phthalocyanine (CoPc) thin films before and after annealing at 493 K (220°C) has been performed. The films were prepared by evaporation using purified material and measurements were made in the frequency range 102-2 x 104 Hz and for temperatures of 150-500 K. The AC component of conductivity showed an cos dependence, where co is the angular frequency and s is an index less than unity. Features characteristic of free band conductivity at higher temperatures and hopping through a band of localized states at lower temperatures were observed. Hopping appeared to be via multiple hopping through clusters, rather than by a two-site process. After annealing there was an increase in the value of the index s at low temperatures and behaviour consistent with desorption of oxygen impurities and/or a partial phase transformation from the ct- to the 13-form of CoPc. Capacitance and loss tangent normally showed a decrease with increasing frequency and an increase with increasing temperature, with a decrease in absolute values and an improvement in the quality of data after annealing. These results are also consistent with existing equivalent circuit models, with the reduced values after annealing resulting from oxygen desorption and phase transformation behaviour. It is clear that measurements taken after annealing are necessary in order to reveal the intrinsic properties of evaporated phthalocyanine thin films.
I. Introduction
The phthalocyanines are a class of organic materials which possess semiconducting properties and are chemically and thermally stable. They have been found to undergo significant conductivity changes on the adsorption of strong electron-acceptor gases [1]. They are therefore perhaps the most widely investigated organic compounds in the area of gas sensing devices [2]. Most of them are p-type due to adsorbed oxygen which acts as an acceptor level in the band gap [3]. They also show spacecharge-limited conductivity with an exponential trap distribution at relatively high bias voltages * Corresponding author
[4, 5]. Some early s'udies have shown that both the dark conductivity and the photoconductivity of certain phthalocyanines are significantly affected by oxygen [6]. The oxygen may diffuse into the bulk material, for example as observed with nickel phthalocyanine (NiPc) crystals [7], or m a y be bound to the material surface, as found in metal-free phthalocyanine (H2Pc) [8]. The electrical conductivity, capacitance and loss factor in these organic compounds are reduced as oxygen is desorbed from the structure. The oxygen impurities, which are unavoidably introduced in the preparation of organic semiconductors [9], appear to play the dual role of both acceptors and trap levels [10]. However, these impurities, as well as voids and defects which give rise to
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD1 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 8 9 2 - 6
S.I. Shihub et al. / Physica B 222 (1996) 136 142
extrinsic conductivity [11], can be removed by heat treatment. Generally the surface regions of thin films contain a large number of dangling bonds and the bulk crystalline electronic bonds are distorted and shifted by local strains and electric fields [12]. Suri and Chopra [133 reported that the exact nature and concentration of the structural defects depend on the film thickness and the deposition and annealing conditions, resulting in differences in various charge carrier transport properties. A number of phthalocyanine polymorphs have been identified but the most important ones are the a- and 9modifications, with other polymorphs probably being intermediate between these two [14]. Following our earlier work on the effects of annealing on the trap distribution of cobalt phthalocyanine (CoPc) thin films using DC measurements [15], in the present work we report the detailed effects of annealing on the AC properties.
2. Experimental details Conventional thermal evaporation was used to deposit twice-purified 13-CoPc single crystals, from a tantalum boat, onto precleaned Corning 7059 glass substrates. Ohmic contacts [16] were produced by evaporating gold wire from molybdenum boats to produce sandwich structures. The deposition was carried out at room temperature resulting in the growth of 0t-CoPc thin films [17, 18] using a deposition rate of 0.5 nm s- 1. Details of the initial purity of the evaporated material have been reported elsewhere [18]. The evaporation pressure was maintained below 2 x 10-3 Pa. The film thickness was monitored using a conventional quartz crystal system during deposition and accurately measured with a Planer Surfometer SF 200 stylus instrument after deposition. Films were of approximate thickness 2 ~tm, with active area 1.2x 10-5 m z. Electrical contacts to the thin films were made with high conductivity silver paste and copper wires. The electrical measurements were performed inside a separate vacuum system at a pressure of less than 10- 3 Pa in the temperature range 150-500 K. The total conductivity a(co) was measured using a Hewlett-Packard 4276A LCZ
137
meter over the frequency range 102-2 x 104 Hz and the low-field DC conductivity component ~rDcwas determined in the ohmic region at a voltage of 1 V using a Keithley 485 picoammeter. The measurements were carried out prior to annealing and also after annealing at 220°C for several hours inside the vacuum system. Other details are as given previously [19].
3. Results and discussion 3.1. AC conductivity Fig. l(a) represents the variation of total conductivity with inverse temperature in the temperature range 170-500 K and for frequencies of 0.1, 1 and 20kHz. The conductivity is temperature independent and frequency dependent for temperatures up to about 200K. Above 200 K the conductivity starts to increase slowly with temperature, the frequency dependence becoming weak until at even higher temperatures the three curves coincide. Similar behaviour is shown in Fig. l(b) in which the conductivity was measured after the same sample had been annealed at about 220°C for more than 2 h before performing a similar set of measurements. Below room temperature there is a reduction in conductivity after annealing of up to approximately two orders of magnitude, but this reduction is less at higher temperatures. Fig. l(b)
10 ~
1
i
l
i
1
i
i
i
b 10 .5
10 ~ 20 kHz.
10 -r
4 kHz.
10 4
. ~
"~00 Hz
kHz
10 ~ 00 Hz 10-1o
J 3
I I 4 5 1/1 ( x l 0 s K t )
I 6
3
4 5 1/T ( x l 0 ~ K 4)
6
7
Fig. 1. Conductivity as a function of inverse temperature at differentfrequenciesfor (a) a freshlyprepared film and (b) the same film after annealing.
138
S.I. Shihub et al. / Physica B 222 (1996) 136-142
10 ~
i •
10 "s
:
=
i
~L 483 ~<
i
-
;,'~73K
~
i ~
483 I~
Z~-473
__._~,..__.Ji23
I(
I
I
I
a
0.8
I(
373 I~
10 ~s
1.0
0.6 __..._4---'4~373 I<
0.4
10 7 t~ •
10 ~
73 K
0.0 1.0
10 ~ a
104o
10 z
I 103
I 104 f (Hz)
0.2
10 s 102
103
104
10 s
f (Hz)
Fig 2. Conductivityas a function of frequencyat differenttemperatures for (a) a freshlyprepared film and (b) the same sample after annealing.
0.8 0.6
,
'
300 T (K)
400
b
i
0.4 0.2
also shows that the extent of the temperature independence for the conductivity is greater at higher frequencies than at lower frequencies and extends to higher temperatures than in the sample before annealing. The activation energies, calculated from the slopes of the curves in Figs. l(a) and (b), were found to be typically between 0.015 and 0.042 eV at low temperatures. At higher temperatures frequency-independent activation energies of 0.51~).57 eV and 0.42q3.54 eV, for the fresh and the annealed samples, respectively, were obtained. These values are in good agreement with apparent values obtained using DC measurements in the space-charge-limited current region of the same sample under study. The low activation energies suggest that hopping is the dominant conduction mechanism at low temperatures, while at higher temperatures the conduction mechanism is of the free-band type. AC activation energies reported previously for fresh samples have shown similar values [19], while DC measurements have shown an increase in the higher temperature value from 0.45 to 0.69 eV after annealing at a maximum temperature of 200°C [15]. The data in the previous figures are replotted as a function of temperature in Figs. 2(a) and (b), for fresh and annealed samples, respectively, where it is apparent that at low temperatures the conductivity is frequency dependent and at higher temperatures is independent of frequency. As discussed previously this type of behaviour can be described by
0.0 100
200
500
Fig. 3. Dependenceof the index s on temperaturefor (a) a freshly prepared film and (b) the same filmafter annealing. The values were calculated from the data of Fig. 2 after subtraction of the DC component of conductivity. the expression [20] a(~o) = GDC + Aco s,
(1)
where aDC is the DC component of conductivity, ~o is the angular frequency, A is a constant and the index s is the exponent of frequency, s may therefore be derived from the slope of a log(a(co) - a D c ) versus log(frequency) curve. Optimal fitting of these quantities was achieved using a least-squares second-order regression, allowing values of s to be obtained by differentiation. In the present work values of the index s over the frequency range 101-2x 104Hz, and derived from the data of Figs. 2(a) and (b) after subtraction of aDO are plotted against temperature in Fig. 3. The data points represent the mean values of s derived, while the extent of the bars indicate the range of s values obtained across the frequency range. It is clear that s is temperature dependent, decreasing with increasing temperature. Fig. 3 also shows that at low temperatures the mean value of the index s approximately doubled, from 0.4 to 0.8, after annealing. As the temperature increases, the index s tends to a common low value of approximately 0.01, for
139
S.L Shihub et al. / Physica B 222 (1996) 136-142
both freshly prepared and annealed samples. Similar results have also been reported for fresh films of copper phthalocyanine (CuPc) [21], zinc phthalocyanine (ZnPc) [22] and CoPe [19]. Results of this type have traditionally been interpreted in terms of hopping between pairs of closely spaced charged centres, or correlated barrier hopping [23, 24]. Various different AC conductivity models have been reviewed by Elliott [25], and of the models reviewed only correlated barrier hopping is consistent with a decrease in the value of s with increasing temperature and also an increase in s with increasing frequency over the ranges covered in the present work. If the present results are analysed in terms of this model [23] we may use the following expression given for the AC conductivity component:
/z2N2,g~ 8e2 ~6o9s aAC-
24
s = 1 - (2/ln 2 ~)'
fl = 1 - ( 6 k T / W m ) ,
(4)
(2)
where N is the density of states, e the sample permittivity, Zo the effective relaxation time ( ~ 10-~3 s), e the electronic charge and s = 1 -
applicability of the above model. B6ttger and Bryksin [27] have pointed out that an increase in conductivity with increasing frequency is likely to be a consequence of d i s o r d e r rather than a definitive indication of hopping, since with increasing frequency charge carriers can move through clusters of decreasing size, so that well-conducting regions of finite size become more and more effective. If, however, conductivity is via hopping, a two-site hopping model is appropriate only at high frequencies where the current is governed by transitions of electrons between pairs of sites, whereas at lower frequencies the current arises from transitions within highly conducting clusters of sites, and is known as multiple hopping. In this hopping regime the exponent s is given by [27]
(3)
where k is Boltzmann's constant, T is the absolute temperature and Wm is the optical band gap. The density of localized states N was calculated using Eqs. (2) and (3) at T = 293K, a frequency of 2 x 104 Hz and relative permittivity 4.8, as derived from capacitance measurements. The optical band gap Wm was taken to be 1.58 eV, as reported for CuPc by Chadderton [26], giving N = 2.95 x 1022 and 6.56x 1021 m -3 for fresh and annealed samples, respectively. This reduction is consistent with the desorption of oxygen impurities as suggested previously [19] or the occurrence of a structural change within the films. The temperature at which annealing takes place could cause a partial phase transformation from a- to [3-CoPc. In previous work Shihub and Gould [18] showed that a partial phase change in purified CoPc thin films occurred after annealing above 200°C and the films were completely converted to the [3-form above 300°C. Notwithstanding the above analysis, it should be stressed that there are severe doubts concerning the
where (~ is a dimensionless frequency (~>> 1). Thus s varies monotonically with increasing frequency and approaches unity as generally observed [19, 21, 22]. Furthermore, in the frequency range appropriate to the multiple-hopping regime the exponent s decreases with increasing temperature [27], as is also observed in the present work for CoPc. It therefore appears that there is some evidence for a hopping process, and that although a two-site model of the type proposed by Elliott appears to explain the results, it is more plausible that a cluster approximation hopping model is appropriate in view of the relatively low frequency ranges explored. The increase in s at low temperatures after annealing is likely to be related to details of the new cluster population.
3.2.
Capacitance
and loss tangent
Fig. 4(a) shows capacitance measurements as a function of temperature for a fresh sample. There are two frequency-dependent maxima in these curves which show higher values of capacitance for lower frequencies. Upon annealing (see Fig. 4(b)) the capacitance is temperature independent up to 300 K; however, one of the maxima disappears and a slight reduction in the capacitance is observed at
S.L Shihub et al. / Physica B 222 (1996) 136-142
140 0.6
I
i
1 ji °°.;
1o"
]
I
I
i
i
i
i
i
' b
0.5 1
10 a
.•00
1 kHz
:j:::; kHz
0.4 (,3
0.3
0 kHz
H
102
1 kF 10~
/
/
0
kH
kl
100 10"1
0.2 10.2 0.1 100
I I 300 400 T (K)
I
200
I 500
a I
600100
I
200
I
I
300 400 T (K)
500
600
Fig. 4. Capacitance as a function of temperature at different frequencies for (a) a freshly prepared film and (b) the same film after annealing. 0.6
!
I
10.3 100
'
'
1
200
b
300 400 T (K)
500
473 K ~
[
~
0.4 (b 0.3
I
200
I
I
300 400 T (K)
500 600
i
I
I
b
_473 K
-222
10' 10o 731 10 ~
0.2
600 100
r
102
0.5
f
1
Fig. 6. Loss tangent as a function of temperature at different frequencies for (a) a freshly prepared film and (b) the same film after annealing. 103
I
I
I 0 "2
293 K 0.1
1~ 3
I 104 f (Hz)
10510=
I
I
10~
104
10 s
f (Hz)
I0"~I0~
I
102
I
103 f (Hz)
I
104
10~101
I
102
I
103
I
104
10 s
f (Hz)
Fig. 5. Capacitance as a function of frequency at different temperatures for (a) a freshly prepared film and (b) the same film after annealing.
Fig. 7. Loss tangent as a function of frequency at different temperaturesfor (a) a freshlypreparedfilm and (b) the same film after annealing.
low temperatures. The reduction in the capacitance, and hence in the effective relative permittivity, at higher temperatures has been interpreted by Nalwa and Vasudevan [28] in terms of the difficulty of domains in retaining the charge as the conductivity increases with temperature. Figs. 5(a) and (b) show the sample capacitance, for fresh and annealed samples, respectively, as functions of frequency for five different temperatures. These figures show that the capacitance generally increases with temperature and decreases with frequency as expected, with a possible decrease at high temperature and low frequency. However, the curves in Fig. 5(a) become more ordered after annealing as shown in Fig. 5(b).
The dependence of loss tangent (tan6) on temperature is shown in Fig. 6(a) for a fresh sample. The results show that the loss tangent is reduced by the annealing process (Fig. 6(b)) and the curves also appear "smoother" than those for the fresh sampies. Furthermore, tan 6 was found to decrease with frequency and to increase with temperature as shown in Fig. 7(a), with the results suggesting the occurrence of minima at approximately 10 kHz. After annealing there was no evidence of minima, but there was still a reduction in tan 6 with increasing frequencies as seen in Fig. 7(b). The above results for capacitance and loss tangent as functions of temperature and frequency may be qualitatively explained by the equivalent circuit model
S.I. Shihub et al. / Physica B 222 (1996) 136-142
of Goswami and Goswami [29]. In this model the capacitance should decrease with increasing frequency, as should tan6 at lower frequencies, with tan 6 tending to increase with frequency at higher frequencies after passing through a loss minimum a t (.Omin ~ 1/C'(rR) 1/2, where C' is a frequency-independent capacitive element, r is a constant low value resistance due to the contacts and leads, and R is a temperature-dependent resistive element representing current leakage. The effects of annealing on the values of capacitance and loss tangent are consistent with an increase in the value of R in the model of Goswami and Goswami. This is essentially the same phenomenon as the reduction in a after annealing as discussed in Section 3.1. The increase in tan6 with temperature is believed to be due to the decrease of the resistivity of the CoPc thin films due to thermal activation of carriers when the temperature is increased.
4. Summary and concluding remarks In common with other metal phthalocyanines the AC component of conductivity of the CoPc films in the present study follows an cos dependence over the investigated temperature and frequency ranges. The results also indicate hopping and free-band conduction processes taking place at low and high temperatures, respectively, similarly to those reported for CuPc [21, 30], magnesium phthalocyanine (MgPc) [30] and previously for CoPc films [19]. It was considered that for the range of frequencies investigated a multiple hopping cluster model was more appropriate in explaining the results than a simple two-site model. The reduction in the conductivity, capacitance and loss tangent after annealing may be explained in terms of a decrease in the concentration of oxygen impurities and of weak paths in the annealed samples in accordance with the reduction in the density of defects [31]. The existence of oxygen and the dislocation of molecules in the freshly prepared s-form thin films would influence the electrical properties of metal phthalocyanines [9]. It has been reported that the absorption of gases forms a relatively thin layer on freshly prepared films. Moreover gas-filled cavities may be formed if the
141
absorption has taken place during the preparation of thin films and consequently the internal electric field distribution may alter, producing erroneous values of the conductivity [3]. Buchholz [32] reported that the rate of oxygen uptake in CoPc was about 1015 molecules m -2 s- 1 and Twarowski [33] found that a uniformly doped layer of about 50 nm depth was present after exposure of ZnPc films to air for two days. In the present work it has been shown that the electrical characteristics are modified after annealing, which may reduce the concentration of the electrically active imperfections caused by voids and dangling bonds [11]. Hence the CoPc thin films only exhibit their intrinsic behaviour after desorption of air and reduction of strains by annealing; subsequently the curves become more well-behaved than before annealing. Therefore, in order to obtain reproducible and consistent data to gain a full understanding of the intrinsic properties of phthalocyanine compounds, suitable annealing must be performed before measurements are made.
Acknowledgements SIS wishes to acknowledge the Libyan Secretariat of Higher Education for the provision of a research studentship.
References [1] T.A.Jones and B. Bott, Sensors and Acuators 9 (1986)27. [2] P.T. Moseley,J.O.W. Norris and D.E. Williams, Techniques and Mechanisms in Gas Sensing (Adam Hilger, New York, 1991). [3] F. Gutman and L.E. Lyons, Organic Semiconductors (Wiley,New York, 1967). [4] G.H. Heilmeierand G. Warfield,J. Chem Phys. 38 (1963) 163. [5] R.D. Gould, Thin Solid Films 125 (1986) 63. [6] S.E.Harrison and K.H. Ludewig,J. Chem. Phys. 45 (1966) 343. [7] T.G. Abdel-Malik, Ind. J. Phys. 57 A (1983)231. [8] A. Wilson and R.A.Collins,Phys. Stat. Sol. A98 (1986)633. [9] M. Martin, J. Andr6and J. Simon, J. Appl. Phys. 54 (1983) 2792. [10] A. Ahmad and R.A. Collins, Thin Solid Films 217 (1992) 75.
142
S.L Shihub et al. / Physica B 222 (1996) 136-142
1-11] A. Lewis, Phys. Rev. Lett. 29 (1972) 1555. [12] M.H. Brodsky, R.S. Title, K.Weiser and G.D. Pettit, Phys. Rev. B 1 (1970) 2632. [13] R. Suri and K.L. Chopra, Thin Solid Films 36 (1976) 47. [14] M.K. Debe and K.K. Kam, Thin Solid Films 186 (1990) 289. [15] S. Gravano, A.K. Hassan and R.D. Gould, Int. J. Electron, 70 (1991) 477. [16] A. Sussman, J. Appl. Phys. 38 (1967) 2738. [17] H. Yoshida, Y. Tokura and T. Koda, J. Chem. Phys. 109 (1986) 375. [18] S.I. Shihub and R.D. Gould, Phys. Stat. Sol. A 139 (1993) 129. [19] S.I. Shihub and R.D. Gould, Thin Solid Films 254 (1995) 187. [20] N.F. Mott and E.A. Davis, Electronic Processes in NonCrystalline Materials (Clarendon Press, Oxford, 1971). [21] R.D. Gould and A.K. Hassan, Thin Solid Films 223 (1993) 334.
[22] A.M. Saleh, R.D. Gould and A.K. Hassan, Phys. Stat. Sol. A 139 (1993) 379. [23] S.R. Elliott, Phil. Mag. 36 (1977) 1291. [24] S.R. Elliott, Phil. Mag. B 37 (1978) 553. [25] S.R. Elliott, Adv. Phys. 36 (1987) 135. [26] L.T. Chadderton, J. Phys. Chem. Solids 24 (1963) 750. [27] H. B6ttger and V.V. Bryksin, Hopping Conduction in Solids (VCH verlagsgesellschaft, Weinheim, Germany, 1985). [28] H.S. Nalwa and P. Vasudevan, J. Mater. Sci. Lett. 2 (1983) 22. [29] A. Goswami and A.P. Goswami, Thin Solid Films 16 (1973) 175. [30] Yu.A. Vidadi, L.D. Rosenshtein and E.A. Chistyakov, Sov. Phys. Solid State 11 (1969) 173. 1,31] A.S.Md.S. Rahman, M.H. Islam and C.A. Hogarth, Int. J. Electron. 62 (1987) 685. [32] J.C. Buchholz, Appl. Surf. Sci. 1 (1978) 547. [33] A. Twarowski, J. Chem. Phys. 77 (1982) 4698.
ELSEVIER
The effects of annealing on the AC electrical properties of cobalt phthalocyanine thin films S.I. Shihub, R.D. Gould*, S. Gravano Thin Films Laboratory, Department of Physics, Keele University, Keele, Staffs. ST5 5BG, UK
Received 17 November 1994; final revised 18 October 1995
Abstract
A comparative study of the AC electrical properties of cobalt phthalocyanine (CoPc) thin films before and after annealing at 493 K (220°C) has been performed. The films were prepared by evaporation using purified material and measurements were made in the frequency range 102-2 x 104 Hz and for temperatures of 150-500 K. The AC component of conductivity showed an cos dependence, where co is the angular frequency and s is an index less than unity. Features characteristic of free band conductivity at higher temperatures and hopping through a band of localized states at lower temperatures were observed. Hopping appeared to be via multiple hopping through clusters, rather than by a two-site process. After annealing there was an increase in the value of the index s at low temperatures and behaviour consistent with desorption of oxygen impurities and/or a partial phase transformation from the ct- to the 13-form of CoPc. Capacitance and loss tangent normally showed a decrease with increasing frequency and an increase with increasing temperature, with a decrease in absolute values and an improvement in the quality of data after annealing. These results are also consistent with existing equivalent circuit models, with the reduced values after annealing resulting from oxygen desorption and phase transformation behaviour. It is clear that measurements taken after annealing are necessary in order to reveal the intrinsic properties of evaporated phthalocyanine thin films.
I. Introduction
The phthalocyanines are a class of organic materials which possess semiconducting properties and are chemically and thermally stable. They have been found to undergo significant conductivity changes on the adsorption of strong electron-acceptor gases [1]. They are therefore perhaps the most widely investigated organic compounds in the area of gas sensing devices [2]. Most of them are p-type due to adsorbed oxygen which acts as an acceptor level in the band gap [3]. They also show spacecharge-limited conductivity with an exponential trap distribution at relatively high bias voltages * Corresponding author
[4, 5]. Some early s'udies have shown that both the dark conductivity and the photoconductivity of certain phthalocyanines are significantly affected by oxygen [6]. The oxygen may diffuse into the bulk material, for example as observed with nickel phthalocyanine (NiPc) crystals [7], or m a y be bound to the material surface, as found in metal-free phthalocyanine (H2Pc) [8]. The electrical conductivity, capacitance and loss factor in these organic compounds are reduced as oxygen is desorbed from the structure. The oxygen impurities, which are unavoidably introduced in the preparation of organic semiconductors [9], appear to play the dual role of both acceptors and trap levels [10]. However, these impurities, as well as voids and defects which give rise to
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD1 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 8 9 2 - 6
S.I. Shihub et al. / Physica B 222 (1996) 136 142
extrinsic conductivity [11], can be removed by heat treatment. Generally the surface regions of thin films contain a large number of dangling bonds and the bulk crystalline electronic bonds are distorted and shifted by local strains and electric fields [12]. Suri and Chopra [133 reported that the exact nature and concentration of the structural defects depend on the film thickness and the deposition and annealing conditions, resulting in differences in various charge carrier transport properties. A number of phthalocyanine polymorphs have been identified but the most important ones are the a- and 9modifications, with other polymorphs probably being intermediate between these two [14]. Following our earlier work on the effects of annealing on the trap distribution of cobalt phthalocyanine (CoPc) thin films using DC measurements [15], in the present work we report the detailed effects of annealing on the AC properties.
2. Experimental details Conventional thermal evaporation was used to deposit twice-purified 13-CoPc single crystals, from a tantalum boat, onto precleaned Corning 7059 glass substrates. Ohmic contacts [16] were produced by evaporating gold wire from molybdenum boats to produce sandwich structures. The deposition was carried out at room temperature resulting in the growth of 0t-CoPc thin films [17, 18] using a deposition rate of 0.5 nm s- 1. Details of the initial purity of the evaporated material have been reported elsewhere [18]. The evaporation pressure was maintained below 2 x 10-3 Pa. The film thickness was monitored using a conventional quartz crystal system during deposition and accurately measured with a Planer Surfometer SF 200 stylus instrument after deposition. Films were of approximate thickness 2 ~tm, with active area 1.2x 10-5 m z. Electrical contacts to the thin films were made with high conductivity silver paste and copper wires. The electrical measurements were performed inside a separate vacuum system at a pressure of less than 10- 3 Pa in the temperature range 150-500 K. The total conductivity a(co) was measured using a Hewlett-Packard 4276A LCZ
137
meter over the frequency range 102-2 x 104 Hz and the low-field DC conductivity component ~rDcwas determined in the ohmic region at a voltage of 1 V using a Keithley 485 picoammeter. The measurements were carried out prior to annealing and also after annealing at 220°C for several hours inside the vacuum system. Other details are as given previously [19].
3. Results and discussion 3.1. AC conductivity Fig. l(a) represents the variation of total conductivity with inverse temperature in the temperature range 170-500 K and for frequencies of 0.1, 1 and 20kHz. The conductivity is temperature independent and frequency dependent for temperatures up to about 200K. Above 200 K the conductivity starts to increase slowly with temperature, the frequency dependence becoming weak until at even higher temperatures the three curves coincide. Similar behaviour is shown in Fig. l(b) in which the conductivity was measured after the same sample had been annealed at about 220°C for more than 2 h before performing a similar set of measurements. Below room temperature there is a reduction in conductivity after annealing of up to approximately two orders of magnitude, but this reduction is less at higher temperatures. Fig. l(b)
10 ~
1
i
l
i
1
i
i
i
b 10 .5
10 ~ 20 kHz.
10 -r
4 kHz.
10 4
. ~
"~00 Hz
kHz
10 ~ 00 Hz 10-1o
J 3
I I 4 5 1/1 ( x l 0 s K t )
I 6
3
4 5 1/T ( x l 0 ~ K 4)
6
7
Fig. 1. Conductivity as a function of inverse temperature at differentfrequenciesfor (a) a freshlyprepared film and (b) the same film after annealing.
138
S.I. Shihub et al. / Physica B 222 (1996) 136-142
10 ~
i •
10 "s
:
=
i
~L 483 ~<
i
-
;,'~73K
~
i ~
483 I~
Z~-473
__._~,..__.Ji23
I(
I
I
I
a
0.8
I(
373 I~
10 ~s
1.0
0.6 __..._4---'4~373 I<
0.4
10 7 t~ •
10 ~
73 K
0.0 1.0
10 ~ a
104o
10 z
I 103
I 104 f (Hz)
0.2
10 s 102
103
104
10 s
f (Hz)
Fig 2. Conductivityas a function of frequencyat differenttemperatures for (a) a freshlyprepared film and (b) the same sample after annealing.
0.8 0.6
,
'
300 T (K)
400
b
i
0.4 0.2
also shows that the extent of the temperature independence for the conductivity is greater at higher frequencies than at lower frequencies and extends to higher temperatures than in the sample before annealing. The activation energies, calculated from the slopes of the curves in Figs. l(a) and (b), were found to be typically between 0.015 and 0.042 eV at low temperatures. At higher temperatures frequency-independent activation energies of 0.51~).57 eV and 0.42q3.54 eV, for the fresh and the annealed samples, respectively, were obtained. These values are in good agreement with apparent values obtained using DC measurements in the space-charge-limited current region of the same sample under study. The low activation energies suggest that hopping is the dominant conduction mechanism at low temperatures, while at higher temperatures the conduction mechanism is of the free-band type. AC activation energies reported previously for fresh samples have shown similar values [19], while DC measurements have shown an increase in the higher temperature value from 0.45 to 0.69 eV after annealing at a maximum temperature of 200°C [15]. The data in the previous figures are replotted as a function of temperature in Figs. 2(a) and (b), for fresh and annealed samples, respectively, where it is apparent that at low temperatures the conductivity is frequency dependent and at higher temperatures is independent of frequency. As discussed previously this type of behaviour can be described by
0.0 100
200
500
Fig. 3. Dependenceof the index s on temperaturefor (a) a freshly prepared film and (b) the same filmafter annealing. The values were calculated from the data of Fig. 2 after subtraction of the DC component of conductivity. the expression [20] a(~o) = GDC + Aco s,
(1)
where aDC is the DC component of conductivity, ~o is the angular frequency, A is a constant and the index s is the exponent of frequency, s may therefore be derived from the slope of a log(a(co) - a D c ) versus log(frequency) curve. Optimal fitting of these quantities was achieved using a least-squares second-order regression, allowing values of s to be obtained by differentiation. In the present work values of the index s over the frequency range 101-2x 104Hz, and derived from the data of Figs. 2(a) and (b) after subtraction of aDO are plotted against temperature in Fig. 3. The data points represent the mean values of s derived, while the extent of the bars indicate the range of s values obtained across the frequency range. It is clear that s is temperature dependent, decreasing with increasing temperature. Fig. 3 also shows that at low temperatures the mean value of the index s approximately doubled, from 0.4 to 0.8, after annealing. As the temperature increases, the index s tends to a common low value of approximately 0.01, for
139
S.L Shihub et al. / Physica B 222 (1996) 136-142
both freshly prepared and annealed samples. Similar results have also been reported for fresh films of copper phthalocyanine (CuPc) [21], zinc phthalocyanine (ZnPc) [22] and CoPe [19]. Results of this type have traditionally been interpreted in terms of hopping between pairs of closely spaced charged centres, or correlated barrier hopping [23, 24]. Various different AC conductivity models have been reviewed by Elliott [25], and of the models reviewed only correlated barrier hopping is consistent with a decrease in the value of s with increasing temperature and also an increase in s with increasing frequency over the ranges covered in the present work. If the present results are analysed in terms of this model [23] we may use the following expression given for the AC conductivity component:
/z2N2,g~ 8e2 ~6o9s aAC-
24
s = 1 - (2/ln 2 ~)'
fl = 1 - ( 6 k T / W m ) ,
(4)
(2)
where N is the density of states, e the sample permittivity, Zo the effective relaxation time ( ~ 10-~3 s), e the electronic charge and s = 1 -
applicability of the above model. B6ttger and Bryksin [27] have pointed out that an increase in conductivity with increasing frequency is likely to be a consequence of d i s o r d e r rather than a definitive indication of hopping, since with increasing frequency charge carriers can move through clusters of decreasing size, so that well-conducting regions of finite size become more and more effective. If, however, conductivity is via hopping, a two-site hopping model is appropriate only at high frequencies where the current is governed by transitions of electrons between pairs of sites, whereas at lower frequencies the current arises from transitions within highly conducting clusters of sites, and is known as multiple hopping. In this hopping regime the exponent s is given by [27]
(3)
where k is Boltzmann's constant, T is the absolute temperature and Wm is the optical band gap. The density of localized states N was calculated using Eqs. (2) and (3) at T = 293K, a frequency of 2 x 104 Hz and relative permittivity 4.8, as derived from capacitance measurements. The optical band gap Wm was taken to be 1.58 eV, as reported for CuPc by Chadderton [26], giving N = 2.95 x 1022 and 6.56x 1021 m -3 for fresh and annealed samples, respectively. This reduction is consistent with the desorption of oxygen impurities as suggested previously [19] or the occurrence of a structural change within the films. The temperature at which annealing takes place could cause a partial phase transformation from a- to [3-CoPc. In previous work Shihub and Gould [18] showed that a partial phase change in purified CoPc thin films occurred after annealing above 200°C and the films were completely converted to the [3-form above 300°C. Notwithstanding the above analysis, it should be stressed that there are severe doubts concerning the
where (~ is a dimensionless frequency (~>> 1). Thus s varies monotonically with increasing frequency and approaches unity as generally observed [19, 21, 22]. Furthermore, in the frequency range appropriate to the multiple-hopping regime the exponent s decreases with increasing temperature [27], as is also observed in the present work for CoPc. It therefore appears that there is some evidence for a hopping process, and that although a two-site model of the type proposed by Elliott appears to explain the results, it is more plausible that a cluster approximation hopping model is appropriate in view of the relatively low frequency ranges explored. The increase in s at low temperatures after annealing is likely to be related to details of the new cluster population.
3.2.
Capacitance
and loss tangent
Fig. 4(a) shows capacitance measurements as a function of temperature for a fresh sample. There are two frequency-dependent maxima in these curves which show higher values of capacitance for lower frequencies. Upon annealing (see Fig. 4(b)) the capacitance is temperature independent up to 300 K; however, one of the maxima disappears and a slight reduction in the capacitance is observed at
S.L Shihub et al. / Physica B 222 (1996) 136-142
140 0.6
I
i
1 ji °°.;
1o"
]
I
I
i
i
i
i
i
' b
0.5 1
10 a
.•00
1 kHz
:j:::; kHz
0.4 (,3
0.3
0 kHz
H
102
1 kF 10~
/
/
0
kH
kl
100 10"1
0.2 10.2 0.1 100
I I 300 400 T (K)
I
200
I 500
a I
600100
I
200
I
I
300 400 T (K)
500
600
Fig. 4. Capacitance as a function of temperature at different frequencies for (a) a freshly prepared film and (b) the same film after annealing. 0.6
!
I
10.3 100
'
'
1
200
b
300 400 T (K)
500
473 K ~
[
~
0.4 (b 0.3
I
200
I
I
300 400 T (K)
500 600
i
I
I
b
_473 K
-222
10' 10o 731 10 ~
0.2
600 100
r
102
0.5
f
1
Fig. 6. Loss tangent as a function of temperature at different frequencies for (a) a freshly prepared film and (b) the same film after annealing. 103
I
I
I 0 "2
293 K 0.1
1~ 3
I 104 f (Hz)
10510=
I
I
10~
104
10 s
f (Hz)
I0"~I0~
I
102
I
103 f (Hz)
I
104
10~101
I
102
I
103
I
104
10 s
f (Hz)
Fig. 5. Capacitance as a function of frequency at different temperatures for (a) a freshly prepared film and (b) the same film after annealing.
Fig. 7. Loss tangent as a function of frequency at different temperaturesfor (a) a freshlypreparedfilm and (b) the same film after annealing.
low temperatures. The reduction in the capacitance, and hence in the effective relative permittivity, at higher temperatures has been interpreted by Nalwa and Vasudevan [28] in terms of the difficulty of domains in retaining the charge as the conductivity increases with temperature. Figs. 5(a) and (b) show the sample capacitance, for fresh and annealed samples, respectively, as functions of frequency for five different temperatures. These figures show that the capacitance generally increases with temperature and decreases with frequency as expected, with a possible decrease at high temperature and low frequency. However, the curves in Fig. 5(a) become more ordered after annealing as shown in Fig. 5(b).
The dependence of loss tangent (tan6) on temperature is shown in Fig. 6(a) for a fresh sample. The results show that the loss tangent is reduced by the annealing process (Fig. 6(b)) and the curves also appear "smoother" than those for the fresh sampies. Furthermore, tan 6 was found to decrease with frequency and to increase with temperature as shown in Fig. 7(a), with the results suggesting the occurrence of minima at approximately 10 kHz. After annealing there was no evidence of minima, but there was still a reduction in tan 6 with increasing frequencies as seen in Fig. 7(b). The above results for capacitance and loss tangent as functions of temperature and frequency may be qualitatively explained by the equivalent circuit model
S.I. Shihub et al. / Physica B 222 (1996) 136-142
of Goswami and Goswami [29]. In this model the capacitance should decrease with increasing frequency, as should tan6 at lower frequencies, with tan 6 tending to increase with frequency at higher frequencies after passing through a loss minimum a t (.Omin ~ 1/C'(rR) 1/2, where C' is a frequency-independent capacitive element, r is a constant low value resistance due to the contacts and leads, and R is a temperature-dependent resistive element representing current leakage. The effects of annealing on the values of capacitance and loss tangent are consistent with an increase in the value of R in the model of Goswami and Goswami. This is essentially the same phenomenon as the reduction in a after annealing as discussed in Section 3.1. The increase in tan6 with temperature is believed to be due to the decrease of the resistivity of the CoPc thin films due to thermal activation of carriers when the temperature is increased.
4. Summary and concluding remarks In common with other metal phthalocyanines the AC component of conductivity of the CoPc films in the present study follows an cos dependence over the investigated temperature and frequency ranges. The results also indicate hopping and free-band conduction processes taking place at low and high temperatures, respectively, similarly to those reported for CuPc [21, 30], magnesium phthalocyanine (MgPc) [30] and previously for CoPc films [19]. It was considered that for the range of frequencies investigated a multiple hopping cluster model was more appropriate in explaining the results than a simple two-site model. The reduction in the conductivity, capacitance and loss tangent after annealing may be explained in terms of a decrease in the concentration of oxygen impurities and of weak paths in the annealed samples in accordance with the reduction in the density of defects [31]. The existence of oxygen and the dislocation of molecules in the freshly prepared s-form thin films would influence the electrical properties of metal phthalocyanines [9]. It has been reported that the absorption of gases forms a relatively thin layer on freshly prepared films. Moreover gas-filled cavities may be formed if the
141
absorption has taken place during the preparation of thin films and consequently the internal electric field distribution may alter, producing erroneous values of the conductivity [3]. Buchholz [32] reported that the rate of oxygen uptake in CoPc was about 1015 molecules m -2 s- 1 and Twarowski [33] found that a uniformly doped layer of about 50 nm depth was present after exposure of ZnPc films to air for two days. In the present work it has been shown that the electrical characteristics are modified after annealing, which may reduce the concentration of the electrically active imperfections caused by voids and dangling bonds [11]. Hence the CoPc thin films only exhibit their intrinsic behaviour after desorption of air and reduction of strains by annealing; subsequently the curves become more well-behaved than before annealing. Therefore, in order to obtain reproducible and consistent data to gain a full understanding of the intrinsic properties of phthalocyanine compounds, suitable annealing must be performed before measurements are made.
Acknowledgements SIS wishes to acknowledge the Libyan Secretariat of Higher Education for the provision of a research studentship.
References [1] T.A.Jones and B. Bott, Sensors and Acuators 9 (1986)27. [2] P.T. Moseley,J.O.W. Norris and D.E. Williams, Techniques and Mechanisms in Gas Sensing (Adam Hilger, New York, 1991). [3] F. Gutman and L.E. Lyons, Organic Semiconductors (Wiley,New York, 1967). [4] G.H. Heilmeierand G. Warfield,J. Chem Phys. 38 (1963) 163. [5] R.D. Gould, Thin Solid Films 125 (1986) 63. [6] S.E.Harrison and K.H. Ludewig,J. Chem. Phys. 45 (1966) 343. [7] T.G. Abdel-Malik, Ind. J. Phys. 57 A (1983)231. [8] A. Wilson and R.A.Collins,Phys. Stat. Sol. A98 (1986)633. [9] M. Martin, J. Andr6and J. Simon, J. Appl. Phys. 54 (1983) 2792. [10] A. Ahmad and R.A. Collins, Thin Solid Films 217 (1992) 75.
142
S.L Shihub et al. / Physica B 222 (1996) 136-142
1-11] A. Lewis, Phys. Rev. Lett. 29 (1972) 1555. [12] M.H. Brodsky, R.S. Title, K.Weiser and G.D. Pettit, Phys. Rev. B 1 (1970) 2632. [13] R. Suri and K.L. Chopra, Thin Solid Films 36 (1976) 47. [14] M.K. Debe and K.K. Kam, Thin Solid Films 186 (1990) 289. [15] S. Gravano, A.K. Hassan and R.D. Gould, Int. J. Electron, 70 (1991) 477. [16] A. Sussman, J. Appl. Phys. 38 (1967) 2738. [17] H. Yoshida, Y. Tokura and T. Koda, J. Chem. Phys. 109 (1986) 375. [18] S.I. Shihub and R.D. Gould, Phys. Stat. Sol. A 139 (1993) 129. [19] S.I. Shihub and R.D. Gould, Thin Solid Films 254 (1995) 187. [20] N.F. Mott and E.A. Davis, Electronic Processes in NonCrystalline Materials (Clarendon Press, Oxford, 1971). [21] R.D. Gould and A.K. Hassan, Thin Solid Films 223 (1993) 334.
[22] A.M. Saleh, R.D. Gould and A.K. Hassan, Phys. Stat. Sol. A 139 (1993) 379. [23] S.R. Elliott, Phil. Mag. 36 (1977) 1291. [24] S.R. Elliott, Phil. Mag. B 37 (1978) 553. [25] S.R. Elliott, Adv. Phys. 36 (1987) 135. [26] L.T. Chadderton, J. Phys. Chem. Solids 24 (1963) 750. [27] H. B6ttger and V.V. Bryksin, Hopping Conduction in Solids (VCH verlagsgesellschaft, Weinheim, Germany, 1985). [28] H.S. Nalwa and P. Vasudevan, J. Mater. Sci. Lett. 2 (1983) 22. [29] A. Goswami and A.P. Goswami, Thin Solid Films 16 (1973) 175. [30] Yu.A. Vidadi, L.D. Rosenshtein and E.A. Chistyakov, Sov. Phys. Solid State 11 (1969) 173. 1,31] A.S.Md.S. Rahman, M.H. Islam and C.A. Hogarth, Int. J. Electron. 62 (1987) 685. [32] J.C. Buchholz, Appl. Surf. Sci. 1 (1978) 547. [33] A. Twarowski, J. Chem. Phys. 77 (1982) 4698.