Electron transport mechanism through polyimide Langmuir-Blodgett films containing porphyrin

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Thin Solid Films, 243 (1994) 472 475

Electron transport mechanism through polyimide Langmuir-Blodgett films containing porphyrin Mitsumasa Iwamoto, Masami Wada and Tohru Kubota Department of Physical Electronics, Tokyo Institute of Technology, 2-12-I, O-okayama, Meguro-ku, Tokyo 152 (Japan)

Abstract We fabricated junctions with a single PORPI monolayer, where PORPI is a polyimide (PI) Langmuir-Blodgett (LB) film containing a tetraphenylporphyrin (POR) moiety. We then investigated the electron transport properties of the junctions from current-voltage (I-V) and inelastic electron tunnelling spectroscopy measurements. PI LB films without POR were used as tunnelling barriers. Many peaks were seen in junctions with a PORPI monolayer at low voltages, due to the excitation of vibrational modes of the PORPI molecules, whereas no peak was observed in junctions without PORPI monolayers. One large peak was seen at a voltage around 1.9 V, due to the excitation of electron transitions in PORPI molecules, whereas negative electrical resistance was not observed.

1. Introduction Polyimide (PI) Langmuir-Blodgett (LB) films with a monolayer thickness of about 0.4 nm and with a small number of defects find many potential applications in electronic device applications, e.g. as a good electrical insulator [1-3]. In our previous study, we fabricated tunnel junctions with structures of A u / P I / ( P b - B i ) (30 wt.% Bi) [4] and N b / A u / P I / ( P b - B i ) [5], and then observed the tunnelling conduction current predicted by the Bardeen-Cooper-Schrieffer (BCS) theory. It was concluded that PI LB films work as a good tunnelling barrier. We also investigated the electron transport properties of the junctions b y inelastic electron tunnelling spectroscopy (IETS) [6]. We then concluded that the inelastic tunnelling process accompanied by the energy loss due to the excitation of vibrational modes of PI molecules does not make a significant contribution. Recently, there has been growing interest in the negative electrical resistance due to a molecular resonance tunnelling effect in the field of molecular electronic device applications [7, 8]. For example, it is expected that the transmission probability for quantum mechanical tunnelling through double-barrier systems exhibits pronounced maxima at the energies at which the potential well between the two barriers possesses energy states [9]. Beyond that, it is essential to clarify the electron transport mechanism through organic ultrathin films in the fields of physics, chemistry and electronics. In the present study, we fabricated junctions with a PORPI monolayer [10]. Here PORPI is a PI LB film containing a tetraphenylporphyrin moiety, and it possesses energy states originating in electron energy levels

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of a molecular orbit in the porphyrin. We then investigated the electron transport properties of the junctions from the current-voltage ( I - V ) and IETS measurements. In the IETS measurements, it is essential to fabricate tunnelling junctions incorporating a good tunnelling barrier. In the present study, we used PI LB films as a tunnelling barrier.

2. Experimental details 2.1. Sample description

The chemical structure of the PI used here is shown in Fig. 1. Au/PI/Pb, Au/PORPI/PI/Pb and Au/PI/ PORPI/PI/Pb junctions were fabricated on an n-type silicon substrate in a manner similar to that described in previous papers [5, 6]. The electrode configuration of the junctions was also the same as that of Au/PI/Pb junctions used in the previous study [6]. The area of each junction was about 0.03 mm 2. The monolayer thickness of PI was about 0.4 nm and that of PORPI was about 0.5 nm. The number of deposited PI layers was between 0 and 33, and there was one deposited PORPI layer. 2.2. Electrical measurements 2.2.1. I - V measurements

Each junction was placed in a liquid helium bath and was connected in a standard four-terminal arrangement. A current I was applied to the junction from a ramp voltage generator with a cycle time between 10 ms and 100 s. The I - V characteristics were recorded at a temperature between room temperature and liquid helium temperature (4.2 K) by using a microcomputer.

© 1994-- Elsevier Sequoia. All rights reserved

473

M. lwamoto et al. / Electron transport mechanism through polyimide films

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Fig. 2. 1- V characteristics of an Au/PI/Pb junction with a 27-layer PI LB film. Curves 1 and 2 represent the I - V characteristic at a temperature of 4.2 K and 8.0 K respectively.

Fig. 1. P! used in the present study. (a) PI; (b) PORPI.

2.2.2. l E T S measurements The d 2V/dl 2- V characteristics of each junction were measured by the application of a d.c. biasing current coupled with a small modulation current with a frequency of 5.0 kHz. The d.c. biasing voltage and the small modulation current were generated from a step voltage generator (type 2533, Yew Co. Ltd.) and a function generator (model 278, Wavetek Inc.) respectively. Since the modulation current produces a secondharmonic voltage V2s, which is proportional to d 2 V/dI 2, across the junction, the voltage Vzj was measured by means of a lock-in amplifier (type 5610B, N F Electronic Instrument Inc.). The biasing voltage was measured with a voltmeter. In order to obtain better accuracy and more stability in the IETS measurements, a 10.0 kHz notched filter and a bridge circuit were used in a manner similar to that reported by Adler et al. [ 11]. We calibrated the sensitivity of the circuit used for the IETS measurements on the basis of the report by Hansma [12]. It was found that coverages down to on the order of 1/30 of a benzoic acid monolayer adsorbed on aluminium oxide layers can be detected in our measuring system.

gap A/e of 1.2 meV of a top Pb electrode is clearly seen, possibly due to the occurrence of elastic electron tunnelling based on the BCS theory [5, 6]. Curve 2 represents the I - V characteristic at a temperature of 8.0 K, above the critical temperature of Pb (7.2 K). A linear relationship is seen between I and V, and the superconducting energy gap has disappeared. These results suggest that the 27-layer PI LB film in the junction is a good tunnelling barrier. We looked at the superconducting energy gap of Pb as a test for the junction quality. 3.2. l E T S measurements 3.2.1. At low voltages ~ess than 0.25 V) Figure 3 shows the inelastic electron tunnelling spectra at low voltages at a temperature of 4.2 K. Figure 3(a)

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3. Results and discussion

3. I. I - V measurements We fabricated 77 junctions with an Au/PI/Pb structure. 12 of these junctions showed an I - V characteristic typical of tunnel junctions. Figure 2 shows an example of the I - V characteristic of an Au/PI/Pb tunnel junction with a 27-layer PI LB film. Curve 1 represents the I - V characteristic at a temperature of 4.2 K. The I - V characteristic deviates from a linear relation at a voltage below 3 mV, and the superconducting energy

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M. lwamoto et al. / Electron transport mechanism through polyimide films

gives the inelastic electron tunnelling spectra of an Au/PI/Pb tunnel junction with a 27-layer PI LB film. No peak corresponding to the excitation of vibrational modes of PI molecules, e.g. a C-O stretch peak at 220meV and an N - C stretch peak at 171 meV, was seen similarly to what was reported in our previous paper [6]. This result suggests that the inelastic tunnelling process does not make a significant contribution when electrons tunnel through PI LB films. Figure 3(b) gives the inelastic electron tunnelling spectra of an Au/PORPI/PI/Pb junction with an 18-layer PI LB film. Many peaks were observed in the inelastic electron tunnelling spectra, possibly due to the coupling of tunnelling electrons with the vibrational modes of PORPI molecules. The energy gap of superconducting Pb was also seen in a manner similar to that shown in Fig. 2 for the junction. The vibrational modes of the PORPI molecules were assigned on the basis of the work of Thomas and Martell on porphyrin [13] and are indicated in the figure. Similar experimental results were observed for 25% of the Au/PORPI/PI/Pb junctions we fabricated. It should be noted here that it is essential to prepare excellent tunnel barriers for the IETS measurements. A1203 layers formed on aluminium electrodes are generally used for the measurements [14]. Unfortunately, A1203 layers are unstable. The thickness of A1203 layers increases during the deposition of monolayers onto aluminium electrodes by means of the LB technique, possibly owing to the oxidization of the electrodes. As a result, electron tunnelling through junctions with A1203 layers is prohibited. Therefore it is very difficult to observe inelastic electron tunnelling spectra of single PORPI monolayers deposited onto evaporated AI electrodes with the LB technique. In contrast, PI LB films are chemically stable. We can therefore observe inelastic electron tunnelling spectra of PORPI monolayers as shown in Fig. 3(b). Based on this information, we may conclude that PI LB films can be used as a tunnelling barrier in IETS measurements.

3.2.2. At higher voltages 3.2.2.1. Au/PORPI/PI/Pb junction. Figure 4 shows the inelastic electron tunnelling spectra observed for Au/PORPI/PI/Pb and Au/PI/Pb junctions at higher voltages at a temperature of 4.2 K. For the Au/PORPI/ PI/Pb junction with a 16-layer PI LB film, one large peak is clearly seen at a voltage of 1.95 V, as shown in Fig. 4(b). In contrast, no peak was seen at voltages around 1.9 V for the Au/PI/Pb junctions, as shown in Fig. 4(a). It is interesting here to compare the inelastic electron tunnelling spectra with the absorption spectrum of a PORPI LB film deposited on a glass slide. Figure 5 shows the absorption spectrum of a 20-layer PORPI LB film. One main absorption peak around 430 nm (Soret

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band) and several absorption peaks in the visible region (Q band) are due to electronic transitions originating in porphyrin incorporated in PORPI molecules. In the figure, the photon energy is also indicated. It was found that the inelastic electron tunnelling peak of the Au/ PORPI/PI/Pb junction appears at an energy corresponding to the position of absorption peaks in the visible region (Q band). These results suggest that the inelastic tunnelling process accompanied by the energy loss due to the excitation of electronic transitions occurs in junctions with a PORPI LB film. It should be noted here that most of our Au/PI/Pb and Au/PORPI/PI/Au junctions were electrically destroyed when a biasing voltage more than 2.0 V was applied to the junctions. For the junctions used in the lETS measurement, the Au/PI/Pb and Au/PORPI/PI/ Au junctions broke at voltages of 2.5 V and 2.2 V respectively.

M. lwamoto et al. / Electron transport mechanism through polyimide films i

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4. Conclusions

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Fig. 6. Inelastic electron tunnelling spectra at high voltages at room temperature: (a) Cr/PI/Hg junction; (b) Cr/PI/PORPUPI/Hg junction. A two-layer PI LB film was used.

3.2.2.2. Cr/PI/PORPI/PI/Hg junction. As described in Section 1, it is expected that the negative electrical resistance due to a molecular resonance tunnelling effect is created by the construction of double-barrier systems. We fabricated 12 Cr/PI/PORPI/PI/Hgjunctions with two two-layer PI LB films. Here, Hg electrodes were used in order to avoid the destruction of the junctions by the application of top electrodes. The electrode configuration of the junctions was similar to that of the Au/PI/Pb junctions. Two junctions were not electrically shorted; we therefore investigated their electron transport properties. Unfortunately, the negative electrical resistance was not seen in the I - V curve of the junctions, whereas one large peak appeared at a voltage of 1.9 V in the inelastic electron tunnelling spectra, as shown in Fig. 6(b), possibly due to the excitation of an electronic transition in the POPRI LB film. In Fig. 6(a), the inelastic electron tunnelling spectrum of a Cr/PI/Hg junction is also given. No peak was seen at voltages around 1.9 V, as in the inelastic electron tunnelling spectrum of the Au/PI/Pb junction. At the present stage, it is a hard task to construct junctions incorporating LB films. Further investigation is required in order to improve the junction quality.

We fabricated junctions with a single PORPI monolayer, and then investigated the electron transport properties of the junctions from I - V and IETS measurements. It was found that PI LB films without porphyrin can be used as a tunnelling barrier for IETS measurements. Using a PI LB film as a tunnelling barrier, we observed the inelastic electron tunnelling spectra of the junctions with a PORPI LB film. Many peaks were seen at low voltages, due to the excitation of vibrational modes of PORPI molecules, and one large peak was seen at a voltage around 1.9 V, due to the excitation of electron transitions in PORPI molecules.

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