Switching and Other High Field Effects in Organic Films BY SIMON G. E. GARRETT, RONALD PETHIG" AND VIMAL SONI Division of Electrical Materials Science, School of Electronic Engineering Science, University College of North Wales, Bangor, Caernarvonshire Received 2nd May, 1974 The electronic conduction properties and existence of possible reproducible bistable switching in pure naphthalene and anthracene films with mobile electrode material has been investigated. Al- though bistable switching was observed, previously reported conclusions regarding the reproducibility and mechanics of switching in pure organic films have not been confirmed. It is considered that any observed switching effects occur as a result of classical electrical breakdown and not by Ovshinsky type mechanisms. Both Schottky emission and space-charge-limited conduction effects were ob- served, and an observed conductivity frequency dependence of the form G(w) cc wn was interpreted in terms of hopping charge transport. Reproducible bistable switching has been reported for tetracene and perylene thin films when a mobile electrode material such as gallium-indium alloy is used. It was concluded that pure organic films undergo switching via electrode diffusion leading to filament formation of a metallic nature, although the exact mechanism for the production of such filaments was not clear. Such switching with " memory " manifests itself as an initial high resistance OFF state, which, when a threshold voltage is exceeded, rapidly changes to a low resistance ON state that persists when the applied voltage is removed. Switching back from the ON to OFF state is achieved by an appropriate current pulse. Since such phenomena could point to a possible practical application for organic semiconductors, a study of the switching and other high field conduction effects in naphthalene and anthracene films has been made. EXPERIMENTAL Zone refined scintillator grade naphthalene or anthracene was deposited on to various metal electrode substrates at a deposition rate of about 1 ,um min-l using conventional evaporation techniques in a vacuum of 1.3 x Pa. Vacuum evaporated gold or aluminium was used as the counter electrode, or alternatively a gallium-indium eutectic alloy, as used by Kevorkian et aZ.,l was employed. Mercury was also used as a mobile counter electrode. The organic materials were evaporated from ceramic crucibles, whereas clean tungsten filaments were used for the metals. The substrates were held at near room temperature and a stereoscan scanning electron microscope (Cambridge Instrument Co. Ltd., type 961 13 Mk2) was used to inspect the film structures. Pa, in normal atmosphere, or in dry argon at atmospheric pressure, using a stable voltage supply and a Keithley 602 electrometer. A series resistor was used to limit current flow in the transition to the ON state. A.c. measurements over the frequency range 10 Hz to 100 kHz were made using a General Radio Bridge, model 1621. D.c. electrical characteristics were obtained either in vacuum of 1.3 x RESULTS AND DISCUSSION The electron microscope inspection of the organic films showed their basic structures to consist of closely packed irregular shaped crystallites of average dimen- sions I to 3 pm in length. Inokuchi has provided X-ray diffraction evidence to 1732 Published on 01 January 1974. Downloaded by New York University on 02/10/2013 19:39:06. View Article Online / Journal Homepage / Table of Contents for this issue
Transcript
Switching and Other High Field Effects in Organic Films
BY SIMON G. E. GARRETT, RONALD PETHIG" AND VIMAL SONI
Division of Electrical Materials Science, School of Electronic Engineering Science, University College of North Wales, Bangor, Caernarvonshire
Received 2nd May, 1974
The electronic conduction properties and existence of possible reproducible bistable switching in pure naphthalene and anthracene films with mobile electrode material has been investigated. Al- though bistable switching was observed, previously reported conclusions regarding the reproducibility and mechanics of switching in pure organic films have not been confirmed. It is considered that any observed switching effects occur as a result of classical electrical breakdown and not by Ovshinsky type mechanisms. Both Schottky emission and space-charge-limited conduction effects were ob- served, and an observed conductivity frequency dependence of the form G ( w ) cc wn was interpreted in terms of hopping charge transport.
Reproducible bistable switching has been reported for tetracene and perylene thin films when a mobile electrode material such as gallium-indium alloy is used. It was concluded that pure organic films undergo switching via electrode diffusion leading to filament formation of a metallic nature, although the exact mechanism for the production of such filaments was not clear. Such switching with " memory " manifests itself as an initial high resistance OFF state, which, when a threshold voltage is exceeded, rapidly changes to a low resistance ON state that persists when the applied voltage is removed. Switching back from the ON to OFF state is achieved by an appropriate current pulse.
Since such phenomena could point to a possible practical application for organic semiconductors, a study of the switching and other high field conduction effects in naphthalene and anthracene films has been made.
E X P E R I M E N T A L Zone refined scintillator grade naphthalene or anthracene was deposited on to various
metal electrode substrates at a deposition rate of about 1 ,um min-l using conventional evaporation techniques in a vacuum of 1.3 x Pa. Vacuum evaporated gold or aluminium was used as the counter electrode, or alternatively a gallium-indium eutectic alloy, as used by Kevorkian et aZ.,l was employed. Mercury was also used as a mobile counter electrode. The organic materials were evaporated from ceramic crucibles, whereas clean tungsten filaments were used for the metals. The substrates were held at near room temperature and a stereoscan scanning electron microscope (Cambridge Instrument Co. Ltd., type 961 13 Mk2) was used to inspect the film structures.
Pa, in normal atmosphere, or in dry argon at atmospheric pressure, using a stable voltage supply and a Keithley 602 electrometer. A series resistor was used to limit current flow in the transition to the ON state. A.c. measurements over the frequency range 10 Hz to 100 kHz were made using a General Radio Bridge, model 1621.
D.c. electrical characteristics were obtained either in vacuum of 1.3 x
RESULTS A N D DISCUSSION The electron microscope inspection of the organic films showed their basic
structures to consist of closely packed irregular shaped crystallites of average dimen- sions I to 3 pm in length. Inokuchi has provided X-ray diffraction evidence to
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show that crystallites of vacuum deposited aromatic hydrocarbons are in alignment with their ab crystal planes parallel with the substrate surface. We have therefore assumed that the conduction processes described here are predominantly associated with the c’ crystal direction of monoclinic naphthalene and anthracene.
S. G . E. GARRETT, R. PETHIG AND V. SON1
- 9 -
-10-
- 1 1
-12
- 4 c
-
-
- - c : - 1 - 6 I - a
v 15kV STRESS
10 kV STRESS
I /- I
-13L I I I I I I , I I I 1
0 2 4 6 8 10 12 14 16 18 v* /v+
FIG. 1.-The switching characteristic (Schottky plot) for a naphthalene film of thickness 47 pm, with a negatively biased A1 electrode and a mobile Ga-In counter electrode, to show the effect of high
voltage prestressing treatments. A, 2nd switching cycle.
The electrical characteristics of the films investigated did not appear to be appre- ciably effected by changes in the ambient atmosphere. All of the results discussed here were obtained in an ambient atmosphere of dry argon gas at atmospheric pressure.
For films of thickness less than about 10 pm, any switching effects were found to be erratic and irreproducible. For thicker films two distinct switching characteristics were observed, as outlined in fig. 1, 2 and 3. Fig. 1 shows the typical electrical characteristic of the majority of films to exhibit switching effects and was obtained for a naphthalene film of thickness 47 pm with a negatively biased aluminium electrode and a Ga-In eutectic alloy counter electrode. A 100 ki2 series resistor was used in the OFF state to limit excessive current flow when the films switched to the ON state, and the characteristic was obtained by slowly increasing the voltage across the film. Fig. 1 takes the form of a Schottky plot of log(current) against (voltage)*. According to the Schottky law, the current density against voltage relationship is given by
where A is the emission coefficient, 4 is the zero-field barrier at the electrode/sample interface and
p = (e3[4zdco~,)4
with d the sample thickness and cocr the high-frequency sample permitti~ity.~ A linear plot of log I against Y4, as shown by fig. 1, strongly suggests that the stable conduction mechanism involves field-assisted thermionic emission at an electrode barrier. The bulk analogue of the Schottky effect is the Poole-Frenkel effect of
- 4
- 5
- I 3L I I I 1 I I I I I 1
0.6 0.9 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
log,,( V/V) FIG. 2.-The switching characteristic for an anthracene film of thickness 53 pm, with a negatively biased A1 electrode and a mobile Ga-In counter electrode, which exhibited space-charge effects in the
OFF state. 0, 4th switching cycle ; A, 5th switching cycle ; @, after 5th cycle.
field-assisted thermal ionization of charge from trapping or donor sites. Evidence for this latter process is obtained if the plot of conductivity (log I / V ) against V* is linear. Of the total number of organic films prepared (> 100) using different evapor- ation rates and electrode materials, not one film has exhibited a good linear Poole- Frenkel plot, whereas good Schottky plots were invariably obtained apart from those films which exhibited space charge effects to be described later.
Whereas some of the films of the order of 1 pm thickness were able to switch, thicker films did not switch even when subjected to very large electric stress, instead the films suffered irreversible damage caused by large scale electrical breakdown. For the thicker films, switching could be made to occur for quite low values of applied field by prestressing for short intervals with large applied voltages. Fig. 1 shows how the electrical characteristic varied after the film had been stressed for 5 s at 10 kV (voltage rising slowly from zero) and then at 15 kV, after which treatment the film could be reversibly switched between the OFF and ON states. Switching from the OFF to the ON state was achieved by slowly increasing the applied voltage from zero
and the re-establishment of the OFF state was achieved by discharging a 100 ,uF capacitor charged at 12 V across the film. The electrical characteristics were very nearly identical on reversal of the electrode polarities and good linear Schottky plots were obtained for both the OFF and ON states, the observed deviations from linearity at low fields being attributed to bulk conduction effects. The switching event was preceded by a fluctuating current flow and because of such current instabilities only an estimate of the order 0.1 to Ips for the switching time could be made.
-2
- 3 -
- 4 -
- 5-
- 6 -
3 -'- % W
tz - 8 -
- 9 -
0 -
-10-
-I1
- 12'-
- I 3-
-
-
1 I i I I I I I I 0 2 4 6 8 10 12 14 16
V+/W
FIG. 3.-The log I against V3 (Schottky plot) relationship for the anthracene film of fig. 2.
Two switching cycles are shown in fig. 1 and it is seen that successive OFF or ON states did not have identical electrical characteristics. On average, the maximum number of times a film could be switched was of the order 10 to 15 times before permanent damage to the film occurred. Scanning electron microscope inspection of various film surfaces showed that localized damage, in the form of surface cracking, occurred after the high voltage stress treatment and that after the first transition to the ON state a Ga-In alloy-filled crater was formed with typical dimensions that ranged between 75 and 175 pm in diameter and with depths that could exceed 80 % of the total film thickness. Similar sized, carbonized walled craters were observed for films with evaporated gold or aluminium counter electrodes for which basically similar, but less reproducible, switching characteristics were sometimes observed. The existence of metallic bridges envisaged by Kevorkian et aZ.l was not confirmed. The bottom of the craters appeared to be formed of recrystallized film material with very small crystallite dimensions. It was considered that the ON state, rather than
representing some new electronic or crystalline state, was simply associated with a localized reduction in the film thickness as a result of electrical failure. If a mobile electrode material is used, the electrical effect of this is enhanced.
TABLE TH THE SLOPES OF THE LINEAR SCHOTTKY PLOTS OF FIG. 1 ARE USED TO DETERMINE THE PERMITTIVITY VALUES cr, USING EQN (1) AND (2). THE PHYSICALLY UNREALISABLE VALUES FOR
Er ARE INTERPRETED AS A CHANGE IN THE EFFECTIVE FILM THICKNESS effective film thicknessfpm
film state slope/V-+ Er (Er = 1.3) ( ~ r = 2.52)
no stress 8.3 x 1.30 47 24.3 10 kV stress 1.54 x lo-' 0.38 13.7 7.1 15 kV stress 2.28 x lo-' 0.17 6.2 3.2 ON 3.45 x lo-' 7.5x 2.7 1.4
Supporting evidence that the switching mechanism was associated with changes in the effective film thickness is given in table 1, where from the slopes of the Schottky plots of fig. 1 and using eqn (1) and (2), values for the relative permittivity E, are derived and the derived changes in E, related to changing values for the effective film thickness. The 4 x lo8 Hz literature value for E, of 2.52 for naphthalene has been quoted in table 1, rather than the 1 kHz cc# value of 4.2 given by Spielberg et aZ.,6 since the high frequency value for 1, is the pertinent one for Schottky emission t h e ~ r y . ~
The second distinct switching characteristic, which was observed less often than the typical Schottky behaviour of fig. 1, is shown in fig. 2 and 3 for an anthracene film, thickness 53 ,urn, with a negatively biased aluminium electrode and a Ga-In alloy counter electrode. A 10 kR series resistor was used to limit current flow in the OFF state, and the film had previously been stressed for 5 s at 0.5 kV and thereafter subjected to three switching cycles. Fig. 2 shows the next two switching cycles after which the film became permanently damaged. A 12V charged 100pF capacitor was used for switching back to the OFF state. The I against V characteristic shown in fig. 2 for the OFF state of the switching cycles appears to take the form of that for space-charge-limited current (SCL) flow in an insulator having charge carrier trapping levels, as described by Lampert and Sirnmon~.~ Although the various conduction regions are not too clearly defined, the following conduction processes can be identi- fied : an ohmic region at low voltages followed by a voltage square-law SCL region, with the current then rising rapidly to another square-law SCL region, after which switching to the ON state occurs. The rapid current rise between the two SCL regions could correspond to the situation where sufficient charge has been injected into the film to fill the traps, and the current rises rapidly by an amount 8-1 to a new SCL conduction region. If this latter region corresponds to trap-free conduction then from Lampert,' 8 corresponds to the ratio of free-to-trapped charge and is given by
with N, the density of states in the conduction band, Nt the trap density and Et the trap depth below the conduction band.
The voltage V, at which the transition from ohmic to SCL conduction occurs is given by
and the voltage V, at which the SCL trap-filled limit transition occurs is given approxi- mately by
with e the electronic charge and n the volume-generated free charge concentration.
The existence of SCL conduction processes implies that the electrical contacts were ohmic. The electrical characteristics were identical irrespective of the electrode polarities. The Schottky plot of fig. 3 shows that Schottky emission controlled the ON state conduction as well as that for the permanently damaged film after the fifth switching cycle.
TABLE 2.--C0NDUCnON PARAMETERS DERIVED FROM EQN (3)-(5) film 8 t ~ l r n - ~ Ntlm-3 EtleV
anthracene 3-7x 0.9-3.5 x 1014 3-7 x 1019 0.75-0.80 naphthalene 5-2 X 1-5x 1013 0.6-2x lo2' 0.76-0.82
Table 2 gives the spread of values obtained for the various conduction parameters given in eqn (3), (4) and (5) derived from some of the naphthalene and anthracene films that exhibited the characteristics of fig. 2. A value * of 4 x lo2' m-3 was taken for N, the density of states in the conduction band. For the anthracene films a value for E, of 3.74 was used, corresponding to the c' crystal direction, and for naphthalene films the value 2.52 was assumed.
The energy band gaps for crystalline naphthalene and anthracene are of the order 4eV, which precludes intrinsic free charge carrier concentrations as high as those given in table 2. The bulk generation of free charge carriers in these films must therefore be an extrinsic process involving impurities or defect states. Also, with trap depths as large as those given in table 1, it is unlikely that the second, higher voltage, SCL region represents the transition to a trap-free condition, but rather that it represents a trap-limited conduction process involving more shallow traps than those at around 0.78 eV. In this way, the effective values for 0 given in table 2 could be an overestimate, resulting in the values derived for the carrier concentration n also being an overestimate and the trap depths Et being slightly underestimated.
The switching process from the OFF to ON state can be envisaged to involve classical electrical breakdown at an electrical weak-spot in the film, followed by the sudden release of the trapped charge through this weak-spot, leading to local melting and a marked decrease in the effective film thickness. The formation of craters of the
7r
1 i I I I I
- 4 -3 - 2 - I energy/J
FIG. 4.-Variation of film resistance with the energy (CY2/2) successively discharged by a 100 pF capacitor across the film when initially in the ON state. 0, naphthalene, 179 p m thick, Al/In-Ga
same dimensions as those previously described was also observed for filins with this second type of switching characteristic for which space-charge effects were observed.
In fig. 4 the variation of film resistance is plotted against the energy (+CV2) successively discharged by a 100 pF capacitor across a naphthalene and anthracene film, each being initially in the ON state. Essentially the same plots were obtained for different capacitor values, but as each switching event did not have reproducible electrical characteristics, a comparison of the results for different capacitor values was not too meaningful. The transition from the ON to OFF state is seen to be a gradual process, especially evident for the anthracene film. Another interesting feature was the existence of a maximum and minimum in the resistance values before the OFF state was finally attained, a phenomenon possibly related to the physical destruction of the electrical weak-spot. The energy values of fig. 4 are enormous when considering the effective volume of the organic films through which they are dissipated. For the naphthalene film, for example, if we assume that the ON state represents conduction in a weak-spot at the base of a typical metal filled crater, of dimensions one-tenth the crater diameter and one-tenth the base thickness, then the energy dissipated is of the order lo5 J kg-' or 780 kJ mol-l, an energy in excess of that required to break either a C-C bond (346 kJ mol-') or C-H bond (413 kJ mol-l). The reason for the existence of carbonised crater walls in films that had undergone a switching transition would therefore appear to be evident.?
J
log o( frequency / Hz) FIG. 5.-Variation of film conductance with frequency and the effect of a 100 V d.c. bias. 0, naphthalene, 30 pm thick ; 0, anthracene, 25 pm thick ; A, anthracene, 30 pm thick. Open symbols correspond to no external bias. All electrodes were evaporated aluminium and the a.c.
amplitude was 5 V.
t Note added irz proofi If the ON state corresponds to conduction through sample material at the base of the observed craters, then for typical crater dimensions and ON state resistances, the effective resistivity of the crater base material is of the order 1-10 ohm m. Such a low resistivity value is compatible with the existence of pyrolysis products (e.g. graphite) created in regions of excessively high current density just prior to the OFF-ON switching event.
S . G . E . G A R R E T T , R. PETHIG AND V. SON1 1739
Finally, typical results obtained using the General Radio Bridge are shown in fig. 5 and 6. Over the frequency range covered (10 Hz to 100 kHz) the room temper- ature conductances G(w) of the various films (fig. 5) were found to obey the relation- ship
with values of n in the range 0.5 to 0.7. This behaviour has been previously des- cribed l o for a number of organic materials and is consistent with a hopping charge
G(o) cc con
I 2 3 4 5 log, ,,(frequency/Hz)
FIG. 6.-Variation of conductance with frequency for an anthracene film of thickness 35 pm (A1 electrodes) to show the effect after a 1 kV stress treatment (closed symbols).
transport mechanism as originally analysed by Pollak and Geballe.ll The low field conductivity of most of the films investigated was of the order l2-l m-1 and taking a value for the free charge concentration n of around loi4 m-3 (table 2) then from the conductivity relationship
0 = izep the typical charge carrier mobility p is of the order 6 x lo-'' m2 V-l s-l. Mobility values of this order are consistent with a hopping conduction mechanism. Similar results and conclusions have also been obtained recently for compressed anthracene powder by Sakai and Sadaoka.
From fig. 5, the effect of a d.c. bias of 100 V was to increase the a.c. conductivity, possibly resulting from an increase in injected charge carriers into the film. Fig. 6 shows the difference in the as . conductivity characteristic of a 35 pm thick anthracene film before and after a brief 1 kV stress treatment. The increase in conductance and deviation at higher frequencies towards a frequency square-law dependence for the
stressed film indicates that a physical change has occurred. This physical change may be associated with damage at an electrode/film interface, since an w2 conductivity dependence is known to be due in some cases to contact effects.I3
CONCLUSIONS
Basically, the observation by Kevorkian et aZ.l that pure organic films with mobile electrode material can exbibit bistable switching phenomena has been confirmed. On the other hand, the present authors have not reached the same conclusions regarding the reproducibility of this switching phenomena, or the existence of a filament formation of a metallic nature in the ON state. The switching character- istics of more than 100 films of different thicknesses have been investigated, and for all these films the switching event was considered to involve physical damage to the test film with only a limited number of switching cycles being possible before large- scale irreversible damage resulted. Apart from naphthalene and anthracene, a limited number of films of pure p-chloranil, perylene, and 2,3-dichloro-5,6-dicyano-p- benzoquinone, have also been investigated and the same basic conclusions derived. For the organic films investigated in this laboratory, the switching processes can in no way be described in terms of Ovshinsky switching, but rather in terms of classical electrical failure and breakdown,14 particularly for the so-called ON to OFF transi- tion.
We thank the S.R.C. for an Advanced Studentship (to V. S.) and Professor T. J. Lewis for useful discussions.
J. Kevorkian, M. M. Labes, D. C. Larson and D. C . Wu, Disc. Faruduy Soc., 1971, 51, 139. H. Inokuchi, Disc. Faraday Soc., 1971, 51, 161. J. G. Simmons, J. Phys. D: Appl. Phys., 1971, 4, 613. F. C. Aris and T. J. Lewis, J. Phys. D: Appl. Phys., 1973, 6,1067. Handbook of Chemistry and Physics (Chemical Rubber Co., Ohio, 197111972), vol. 52, E46. D. H. Spielberg, A. I. Korn and A. C. Damask, Phys. Rev. B, 1971, 3, 2012. ’ M. A. Lampert, Rep. Prop . Phys., 1964, 27, 329. * W. Helfrich and P. Mark, 2. Phys., 1963, 171, 527.
R. W. Mum, J. R. Nicholson, H. P. Schwob and D. F. Williams, J. Chem. Phys., 1973,58,3828. lo R. Pethig, Proc. 3rd Int. Symp. Chemistry of the Organic Solid State (Glasgow, Sept. 1972), pp.
95-100. M. Pollak and T. H. Geballe, Phys. Rev., 1961, 122, 1742.
l2 Y. Sakai and Y . Sadoaka, Japan. J. Appl. Phys., 1973, 12,1463. l3 A. K. Jonscher, J. Phys. C: Solid State Phys., 1973, 6, L235. l4 J. J. O’Dwyer, The Theory of Electrical Conduction and Breakdown in Solid Dielectrics (Claren-
