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Electrical behaviour of metal-free phthalocyanine thin layers
Thin Solid Films, 36 (1976) 89-92 © Elsevier Sequoia S.A., Lausanne--Printed in Switzerland
89
ELECTRICAL BEHAVIOUR OF METAL-FREE PHTHALOCYANINE THIN LAYERS* M. FiJST6SS-WI~GNER Central Research Institute for Physics, Budapest (Hungary)
(Received August 25, 1975)
Bistable memory switching and low frequency voltage-controlled negative resistance in metal/metal-free phthalocyanine/metal sandwiches have been reported previously I. Reproducible bistable switching has been reported for organic thin films 3-6. It was concluded that pure organic films underwent switching either via electrode diffusion leading to filament formation of a metallic nature 4, by phase changes induced by ohmic heating along filamentary paths 6 or as a result of classical electrical breakdown s . Metal-free phthalocyanine is known to exist in several polymorphic forms 7 and the crystal modification strongly influences the electrical properties. The purpose of the present report is to investigate the current-voltage characteristics of metal/metal-free phthalocyanine/metal sandwich structures with different electrode metals, varying the thicknesses and crystal modifications of the organic layer. EXPERIMENTAL
Measurements were performed with metal-free phthalocyanine purified by sublimation four times. The thin films of phthalocyanine and the metal/organic material/metal sandwiches were prepared by vacuum evaporation (p ~ 10 -5 Torr) onto KBr and glass substrates, respectively. The thickness was controlled during evaporation by the standard quartz-crystal method and was measured on the completed junctions by use of the Talystep. Gold and aluminium were used as electrodes, and the active areas of the sandwiches were about 1 mm 2. Infrared spectroscopy has been effectively employed to distinguish between the crystalline modifications of phthalocyanine 7 ; the infrared absorption spectra of thin layers on KBr substrates were recorded at room temperature with a Unicam SP 200G infrared spectrometer. We found that vacuum-evaporated layers with thicknesses of 2000-50 000 A were in the a form. According to measurements with a differential scanning calorimeter 8 and infrared spectrometer, the thermal conversion a-+~3 took 15 min at 270 °C. Electrical investigation of the samples was carried out in a vacuum chamber evacuated to about 3 x 10-6-1 x 10 -7 Torr as well as in the ambient atmosphere. Current v e r s u s voltage characteristics were measured with the circuit described in ref. 1. RESULTS AND DISCUSSION
The current ())-voltage (V) characteristics of Au/phthalocyanine (a form)/Au cells were investigated; the thicknesses of the phthalocyanine layers varied from 1 to 5/~m. * Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975; Paper 9-24.
90
M. FUSTOSS-WI~GNER
-5
J t AI0
~0-6 --
3 a
×
10-7
10-8
100
10
V IV)
i
i
4
5
J
1 W
6
Fig. 1. Current-voltage characteristics of metal/phthalocyanine/metal sandwiches at room temperature in a vacuum of 3 x 10-6 Torr: (a) Au/a-phthalocyanine/Au; (b) Au/~/-phthalocyanine/Au; (c) A1/~phthalocyanine/A1. The thickness of the phthalocyanine film is (a), (b) about 2 #m, (c) about 6200 A. Fig. 2. Current-voltage characteristics of Al/a-phthalocyanine/A1 sandwiches at room temperature in a vacuum of 3 x 10 .6 Tort; the thickness of the phthalocyanine film is (a) about 5000 A, (b) about 6200 A. All our junctions exhibited J - V curves which became straight lines when log J/V was plotted against the logarithm of the voltage V. The voltage was changed from 0.25 V to 40 V (Fig. 1(a)). J - V curves of this form lead to the equation J = K V exp (a V) which is expected for space charge limited currents in the case of a linear trap distribution 9 (K and a are constants). Since the evaluation of space charge limited currents is always referred to a single type of carrier, the counterelectrode must not inject carriers of opposite sign. This condition was satisfied because the injecting electrode was of the same material as the counterelectrode. As soon as the voltage reached a value corresponding to a field strength of about 4 x 104 V cm -1 , the exponential dependence of current on voltage changed-log J was in direct ratio to the square root of V. J-V curves of this form are expected both for Schottky emission from the electrodes and for the field-enhanced thermal excitation into the conduction band of electrons trapped by immobile positive charges (Poole-Frenkel emission). When the thin films of a metalfree phthalocyanine were thermally converted into the/3 form, a space charge limited current was measured even at a field strength of 10 s V cm -1. The results are shown in Fig. 1 (curve b). The current varies as Vt+l , where l + 1 = 8. According to Lampert 9 the space charge limited current for an exponential distribution of traps with energy should follow a J ~ Vl÷ 1 relation.
ELECTRICAL BEHAVIOUR OF METAL-FREE PHTHALOCYANINE
91
J
VT
VM
V
Fig. 3. Solid line: d.c. or low frequency current-voltage characteristics of an A1/phthalocyanine (1000 A)/A1 sandwich; the broken lines are the a.c. characteristics of the same device.
The J- V characteristics of A1/phthalocyanine (a form)/A1 are shown in Fig. 2. In this case the thicknesses of the samples varied from 3000 to 8000 A. The current may be carried by field-emitted electrons in the interval of voltage investigated. At a thickness of about 4000 )k bistable switching appeared in the phthalocyanine. When the threshold voltage (~40 V) was exceeded, switching from a high impedance (> 108 ~2) to a low impedance on state was observed. After the switching the resistance of the sample of 1 mm 2 active area was reduced to l0 s ~. A current pulse of a few milliamp seconds was required for switching the film back to the off state. In Fig. l(c) the J - V characteristics of the same sandwiches are shown after a -~/~ thermal conversion of the phthalocyanine layers. According to Fig. 1(c) the presence of the ~ polymorph has caused an exponential distribution of traps in the thin organic films. Bistable memory switching was observed only in phthalocyanine layers thinner than 3000 A. The reproducibility was rather poor. It should also be noted that in free air we could not observe bistable switching of c~-phthalocyanine layered between two A1 electrodes. In the case of Au electrodes an a-phthalocyanine film of thickness 1500-5000 A could be switched from the high resistance state of about 108 ~2 to the on state with a resistance of 10-100 f2. In the on state the J - V characteristics were linear and reproducibility of switching was excellent. In Fig. 3 are shown the d.c. or low frequency current-voltage characteristics of A1/phthalo cyanine (500-2500 A)/A1 sandwiches at room temperature and 3 x 10 -6 Torr pressure. In the low voltage range the current characteristics show a voltage-controlled negative resistance after forming, as reported previously by us 1. The on state with low resistance remained stable in the subsequent cycle up to the threshold voltage VT. At voltage VM the sample turned into the off state which-compared with the previous one-had high resistance, and on increasing the voltage between the two electrodes the current increased again. As the frequency of the applied voltage was raised, the hysteresis of the currentvoltage characteristic increased. At 7 = 103 cycles s-1 it gave a steady state conductance (broken lines) which depended on the voltage amplitude. This behaviour is qualitatively similar to J - V characteristics observed in A1/SiO/Au 2 and in A 1 / A 1 2 0 3 / A u lO cells. When the a-phthalocyanine was converted into the/3 form, the character of the J - V curve did not change, but the value of VT increased from 2.6-3 V to 4-5 V.
92
M. FOSTOSS-WI~GNER
CONCLUSIONS We conclude that three types of current-voltage characteristic may be observed in the case of metal/metal-free phthalocyanine/metal sandwiches. For thin layers (5002500,8,) voltage-controlled negative resistance appears at room temperature in the vacuum chamber. Bistable memory switching may be predominant in layer thicknesses of 1500-5000 A in free air as well as in a vacuum of 3 x 10 -6 Tort. As soon as the thickness of the metal-free phthalocyanine exceeds a value of 5000 A, the current will be space charge limited or controlled by field emission, and at high field strength electrical breakdown occurs instead of switching. The electrode materials and the crystal modifications of phthalocyanine only modify a few parameters of the J - V characteristics. REFERENCES 1 M. Ffist~Sss-W~gnerand K. Ritvay-Emandity, KFKI- 75-17, 1975. 2 G. Dearnaley, D. V. Morgan and A. M. Stoneham, J. Non.Cryst. Solids, 4 (1970) 593. 3 J. Kevorkian, M. M. Labes, D. C. Larson and D. C. Wu, Discuss. Faraday Soc., 51 (1971) 139. 4 D.C. Larson, N T I S A D - 7 5 5 267, 1972. 5 G. E. Garret, R. Pethig and V. Soni, J. Chem. Soc. Faraday Trans. 2, 70 (1974) 1732. 6 W. P. BaUard and R. W. Christy, J. Non-Crvst. Solids, 17 (1975) 81. 7 J. H. Sharp and M. Lardon, J. Phys. Chem., 72 (1968) 3230. 8 M. FiJst6ss-W~gner,in preparation. 9 M. A. Lampert and P. Mark, Current Injection in Solids, Academic Press, New York, 1970, p. 27. 10 L. T6th, personal communication, 1974.
89
ELECTRICAL BEHAVIOUR OF METAL-FREE PHTHALOCYANINE THIN LAYERS* M. FiJST6SS-WI~GNER Central Research Institute for Physics, Budapest (Hungary)
(Received August 25, 1975)
Bistable memory switching and low frequency voltage-controlled negative resistance in metal/metal-free phthalocyanine/metal sandwiches have been reported previously I. Reproducible bistable switching has been reported for organic thin films 3-6. It was concluded that pure organic films underwent switching either via electrode diffusion leading to filament formation of a metallic nature 4, by phase changes induced by ohmic heating along filamentary paths 6 or as a result of classical electrical breakdown s . Metal-free phthalocyanine is known to exist in several polymorphic forms 7 and the crystal modification strongly influences the electrical properties. The purpose of the present report is to investigate the current-voltage characteristics of metal/metal-free phthalocyanine/metal sandwich structures with different electrode metals, varying the thicknesses and crystal modifications of the organic layer. EXPERIMENTAL
Measurements were performed with metal-free phthalocyanine purified by sublimation four times. The thin films of phthalocyanine and the metal/organic material/metal sandwiches were prepared by vacuum evaporation (p ~ 10 -5 Torr) onto KBr and glass substrates, respectively. The thickness was controlled during evaporation by the standard quartz-crystal method and was measured on the completed junctions by use of the Talystep. Gold and aluminium were used as electrodes, and the active areas of the sandwiches were about 1 mm 2. Infrared spectroscopy has been effectively employed to distinguish between the crystalline modifications of phthalocyanine 7 ; the infrared absorption spectra of thin layers on KBr substrates were recorded at room temperature with a Unicam SP 200G infrared spectrometer. We found that vacuum-evaporated layers with thicknesses of 2000-50 000 A were in the a form. According to measurements with a differential scanning calorimeter 8 and infrared spectrometer, the thermal conversion a-+~3 took 15 min at 270 °C. Electrical investigation of the samples was carried out in a vacuum chamber evacuated to about 3 x 10-6-1 x 10 -7 Torr as well as in the ambient atmosphere. Current v e r s u s voltage characteristics were measured with the circuit described in ref. 1. RESULTS AND DISCUSSION
The current ())-voltage (V) characteristics of Au/phthalocyanine (a form)/Au cells were investigated; the thicknesses of the phthalocyanine layers varied from 1 to 5/~m. * Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975; Paper 9-24.
90
M. FUSTOSS-WI~GNER
-5
J t AI0
~0-6 --
3 a
×
10-7
10-8
100
10
V IV)
i
i
4
5
J
1 W
6
Fig. 1. Current-voltage characteristics of metal/phthalocyanine/metal sandwiches at room temperature in a vacuum of 3 x 10-6 Torr: (a) Au/a-phthalocyanine/Au; (b) Au/~/-phthalocyanine/Au; (c) A1/~phthalocyanine/A1. The thickness of the phthalocyanine film is (a), (b) about 2 #m, (c) about 6200 A. Fig. 2. Current-voltage characteristics of Al/a-phthalocyanine/A1 sandwiches at room temperature in a vacuum of 3 x 10 .6 Tort; the thickness of the phthalocyanine film is (a) about 5000 A, (b) about 6200 A. All our junctions exhibited J - V curves which became straight lines when log J/V was plotted against the logarithm of the voltage V. The voltage was changed from 0.25 V to 40 V (Fig. 1(a)). J - V curves of this form lead to the equation J = K V exp (a V) which is expected for space charge limited currents in the case of a linear trap distribution 9 (K and a are constants). Since the evaluation of space charge limited currents is always referred to a single type of carrier, the counterelectrode must not inject carriers of opposite sign. This condition was satisfied because the injecting electrode was of the same material as the counterelectrode. As soon as the voltage reached a value corresponding to a field strength of about 4 x 104 V cm -1 , the exponential dependence of current on voltage changed-log J was in direct ratio to the square root of V. J-V curves of this form are expected both for Schottky emission from the electrodes and for the field-enhanced thermal excitation into the conduction band of electrons trapped by immobile positive charges (Poole-Frenkel emission). When the thin films of a metalfree phthalocyanine were thermally converted into the/3 form, a space charge limited current was measured even at a field strength of 10 s V cm -1. The results are shown in Fig. 1 (curve b). The current varies as Vt+l , where l + 1 = 8. According to Lampert 9 the space charge limited current for an exponential distribution of traps with energy should follow a J ~ Vl÷ 1 relation.
ELECTRICAL BEHAVIOUR OF METAL-FREE PHTHALOCYANINE
91
J
VT
VM
V
Fig. 3. Solid line: d.c. or low frequency current-voltage characteristics of an A1/phthalocyanine (1000 A)/A1 sandwich; the broken lines are the a.c. characteristics of the same device.
The J- V characteristics of A1/phthalocyanine (a form)/A1 are shown in Fig. 2. In this case the thicknesses of the samples varied from 3000 to 8000 A. The current may be carried by field-emitted electrons in the interval of voltage investigated. At a thickness of about 4000 )k bistable switching appeared in the phthalocyanine. When the threshold voltage (~40 V) was exceeded, switching from a high impedance (> 108 ~2) to a low impedance on state was observed. After the switching the resistance of the sample of 1 mm 2 active area was reduced to l0 s ~. A current pulse of a few milliamp seconds was required for switching the film back to the off state. In Fig. l(c) the J - V characteristics of the same sandwiches are shown after a -~/~ thermal conversion of the phthalocyanine layers. According to Fig. 1(c) the presence of the ~ polymorph has caused an exponential distribution of traps in the thin organic films. Bistable memory switching was observed only in phthalocyanine layers thinner than 3000 A. The reproducibility was rather poor. It should also be noted that in free air we could not observe bistable switching of c~-phthalocyanine layered between two A1 electrodes. In the case of Au electrodes an a-phthalocyanine film of thickness 1500-5000 A could be switched from the high resistance state of about 108 ~2 to the on state with a resistance of 10-100 f2. In the on state the J - V characteristics were linear and reproducibility of switching was excellent. In Fig. 3 are shown the d.c. or low frequency current-voltage characteristics of A1/phthalo cyanine (500-2500 A)/A1 sandwiches at room temperature and 3 x 10 -6 Torr pressure. In the low voltage range the current characteristics show a voltage-controlled negative resistance after forming, as reported previously by us 1. The on state with low resistance remained stable in the subsequent cycle up to the threshold voltage VT. At voltage VM the sample turned into the off state which-compared with the previous one-had high resistance, and on increasing the voltage between the two electrodes the current increased again. As the frequency of the applied voltage was raised, the hysteresis of the currentvoltage characteristic increased. At 7 = 103 cycles s-1 it gave a steady state conductance (broken lines) which depended on the voltage amplitude. This behaviour is qualitatively similar to J - V characteristics observed in A1/SiO/Au 2 and in A 1 / A 1 2 0 3 / A u lO cells. When the a-phthalocyanine was converted into the/3 form, the character of the J - V curve did not change, but the value of VT increased from 2.6-3 V to 4-5 V.
92
M. FOSTOSS-WI~GNER
CONCLUSIONS We conclude that three types of current-voltage characteristic may be observed in the case of metal/metal-free phthalocyanine/metal sandwiches. For thin layers (5002500,8,) voltage-controlled negative resistance appears at room temperature in the vacuum chamber. Bistable memory switching may be predominant in layer thicknesses of 1500-5000 A in free air as well as in a vacuum of 3 x 10 -6 Tort. As soon as the thickness of the metal-free phthalocyanine exceeds a value of 5000 A, the current will be space charge limited or controlled by field emission, and at high field strength electrical breakdown occurs instead of switching. The electrode materials and the crystal modifications of phthalocyanine only modify a few parameters of the J - V characteristics. REFERENCES 1 M. Ffist~Sss-W~gnerand K. Ritvay-Emandity, KFKI- 75-17, 1975. 2 G. Dearnaley, D. V. Morgan and A. M. Stoneham, J. Non.Cryst. Solids, 4 (1970) 593. 3 J. Kevorkian, M. M. Labes, D. C. Larson and D. C. Wu, Discuss. Faraday Soc., 51 (1971) 139. 4 D.C. Larson, N T I S A D - 7 5 5 267, 1972. 5 G. E. Garret, R. Pethig and V. Soni, J. Chem. Soc. Faraday Trans. 2, 70 (1974) 1732. 6 W. P. BaUard and R. W. Christy, J. Non-Crvst. Solids, 17 (1975) 81. 7 J. H. Sharp and M. Lardon, J. Phys. Chem., 72 (1968) 3230. 8 M. FiJst6ss-W~gner,in preparation. 9 M. A. Lampert and P. Mark, Current Injection in Solids, Academic Press, New York, 1970, p. 27. 10 L. T6th, personal communication, 1974.