Electronic devices incorporating stable phthalocyanine Langmuir-Blodgett films

Thin Solid Films, 132 (1985) 113-123 ELECTRONICS

AND OPTICS

113

ELECTRONIC DEVICES INCORPORATING PHTHALOCYANINE LANGMUIR-BLODGETT G. G. ROBERTS?,

M. C. PETTY,

S. BAKER$,

STABLE FILMS

M. T. FOWLER$

*

AND N. J. THOMAS

Department of Applied Physics and Electronics. University of Durham. Durham (Gt. Britain) (Received June 19, 1985; accepted

July 4, 1985)

This paper describes the critical role of the solvent in the preparation of monolayers of phthalocyanine. Pressure-area isotherms demonstrate that realistic monomolecular areas can be achieved for a number of compounds provided that the central hydrogen atoms in the ring system are replaced by appropriate metal ions. Particularly good results have been obtained with an asymmetrically substituted copper phthalocyanine. Transmission electron diffraction data for copper phthalocyanine tris(CH,NHC,H,-iso) show a preferred orientation over a sample region of 3 mm. The excellent stability of the Langmuir-Blodgett films has enabled us to incorporate them in a number of electronic devices. These include a gas-sensitive structure based on silicon, a bistable switch using GaAs and an electroluminescent metal-insulator-semiconductor ZnSeS diode.

1.

INTRODUCTION

The excellent stability characteristics of phthalocyanine (PC) compounds have led to their widespread use as a dye and colourant in the chemical industry’. They are resistant to bleaching under high intensity illumination and the majority can be heated to at least 400 “C before sublimation, The phthalocyanines have also played a historic role in the area of crystal and molecular structure determination*. They exist in several morphological forms, some of which have been exploited as photoconductors in copying machines 3. Phthalocyanines have received little attention in the Langmuir-Blodgett (LB) field on account of their almost total insolubility in organic solvents. Other less stable dyes have been used successfully in fundamental research investigations but normally only after substitution with long *Paper presented at the Second International Conference on Langmuir-Blodgett Films, Schenectady, NY, U.S.A., July l-4, 1985. t Present address: Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3P3, Ct. Britain. $ Present address: Sony Corporation Research Center, 174 Fujitsuka-cho, Hodogaya-ku, Yokohama 240, Japan. 5 Present address: NEC Corporation, l-l Miyazaki 4-Chome, Miyamae-ku, Kawasaki, Kanagawa 213, Japan. 0040-6090/85/$3.30

0 Elsevier Sequoia/Printed

in The Netherlands

114

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hydrocarbon chains4 or dilution with a fatty acid to form a mixed monolayer”. For stability reasons, such materials will have limited practical applicability and therefore it is important to produce more robust LB film dye assemblies. The preparation of multilayer films of phthalocyanine and its derivatives was first reported by us in the Proceedings of the First International Conference on Langmuir-Blodgett Films’; a related patent’ describes a range of phthalocyanine compounds which can be successfully deposited using the Langmuir trough. Since then we have also demonstrated the potential of monolayers of an asymmetrically substituted copper phthalocyanine in the field of gas detection’. We now discuss an extension of our previous work. Firstly, we demonstrate the critical role of the solvent and the subphase in the preparation process and report the optimum conditions required to form multilayer films with improved characteristics. Secondly, we describe a range of electronic devices whose performance is enhanced by the incorporation of a few monolayers of phthalocyanine. These include a gassensitive structure based on silicon, a bistable switch using GaAs and an electroluminescent metal-insulator-semiconductor (MIS) ZnSeS diode. 2. PREPARATION

OF PHTHALOCYANINE

LB FILMS

The Langmuir troughs used to deposit the phthalocyanine films are of constant-perimeter barrier design’ and were situated on antivibration tables in a microelectronics clean room. The pressure-area isotherms and the deposition profiles were carefully monitored in order to assess the optimum preparation conditions. A range of phthalocyanines has been studied but the majority of the work has involved the compounds listed in Table I. In all cases it has been possible to discover a suitable solvent that yields a cross-sectional area consistent with the molecule sitting edge-on in the liquid. The data provided in Table I represent only a small fraction of the systematic experiments that have been carried out to help to avoid aggregation and to overcome the difficulties in deposition due to the high surface viscosity of the Langmuir film. Similar results for a related series of compounds have been reported by Snow and Jarvis”. A few relevant comments relating to each of the cases mentioned in Table I are given below. TABLE LANGMUIK

I FILM ISOTHERM

MIIPC

Tetra-trrr-butyl

ZnPc

Tetra-tert-butyl CuPc Tetra-rert-butyl MnPc Asymmetric CuPc

(‘HAKA(‘TFKISfI~‘S

IOK

A KANGL: 0,

Tetrahydrofuran Chloroform~wylene

76 92

Xylene Xylene4imethylformamide Chloroform

x7 86 57

l’HTHALO(‘YANIihUL

~‘01,,‘01:N,X3

Very rigid Langmuir film Floating film rapidly photooxidize5 Reasonably fluid Langmuir tilm Good film transfer characteristics using 2”,, alcohol in water subphase and pH z 7.0

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2.1. Unsubstitutedmanganesephthalocyanine In a previous publication’ we showed that stable unsubstituted metal-free films could be prepared using Li,Pc and acetone as a solvent; the rigidity of the films could be reduced in the presence of mesitylene but even so the isotherms demonstrated that single monolayer films were not obtained. The calculated areas per molecule were invariably smaller than those expected for a molecule packed edge on (typically about 40 A’) or packed face down (typically about 160 A’) on the subphase. Subsequent measurements using tetrahydrofuran (THF) as the solvent have also confirmed this difficulty. However, using MnPc dissolved in THF, a good isotherm is obtained giving a cross-sectional area of 76 A’, a value consistent with the molecules being stacked vertically edge-on in the film. Clearly the inclusion of a metal in the central coordinating region of the molecule reduces the level of aggregation in the film. Unfortunately the floating monolayer was still quite rigid and, although effective transfer could be achieved during the first dipping cycle, poor pick-up was obtained with subsequent layers. 2.2. Zinc tetra-tert-butylphthalocyanine Several types of tetra-tert-butyl substituted phthalocyanine molecules have been investigated, including the metal-free, zinc, manganese and copper compounds; the main solvents used were chloroform and xylene or appropriate mixtures. If interlocking of the tert-butyl groups is expected, then a minimum crosssectional area of the metal-free molecule would be approximately 60 A’. The best value obtained experimentally is only half this. However, improved characteristics are obtained using metal derivatives. For example, using tetra-tert-butyl ZnPc, the calculated areas per molecule using chloroform and xylene spreading solutions are 92 A’ and 91 A’ respectively. In the former case hysteresis was present in the pressure-area curve indicative of rigid-layer formation and therefore a solvent mixture containing xylene is to be preferred”. Unfortunately, the tetra-tert-butyl ZnPc solution is prone to photo-oxidation and therefore the more stable manganese or copper equivalent compounds were studied in more detail. 2.3. Copper tetra-tert-butylphthalocyanine Figure 1 shows the isotherm for tetra-tert-butyl CuPc spread from xylene; the molecular structure is inset. No isotherm is shown using chloroform because considerable aggregation was observed with this solvent. The area per molecule obtained by extrapolating the curve is approximately 87 AZ, suggesting that the molecule is tilted but standing on its edge on the subphase. It should be borne in mind that this alone does not provide conclusive evidence of the orientation of the molecules. 2.4. Manganese tetra-tert-butylphthalocyanine Chloroform is a protic solvent, i.e. it tends to donate protons and, for example, when mixed with water, tends to render it slightly acidic. It has already been mentioned that a lower surface viscosity is obtained using the neutral solvent xylene. We therefore considered it of some interest to use an aprotic solvent such as dimethylformamide (DMF) which dissolves MnPc. The Langmuir film did indeed

116

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exhibit less rigidity but the isotherm yielded a lower than expected value for the area per molecule. This was attributed to a loss of material to the subphase as DMF is slightly miscible with water. Various combinations of DMF and xylene were then tested and a sharp maximum in the cross-sectional area was obtained using a 78P:, xylene-to-DMF mixture by volume. The area of 86 A’ was close to that observed for the other substituted metal phthalocyanines. These films transferred very effectively onto a range of substrates (transfer ratio, about 0.95) using pH 5.5 and a control pressure of 25 mN m ‘. 2.5. Copper phthalocyanine tris( CH2NHC,H,-iso) There are clearly many problems to be overcome in producing good quality stable phthalocyanine molecular assemblies. All the molecules discussed to date have been symmetric, thus violating one of the genera1 principles thought to be necessary to obtain LB films. The only asymmetric phthalocyanine molecule available to us was the one shown in Fig. 2(a). CuPc tris(CH,NHC,H,-iso) (asyCuPc) produces a curved isotherm with no structure. To test the rigidity of the monolayer, the film was held under constant pressure and material was removed from the dipping area using a suction pipe. The movement of the barriers controlling the dipping area was then carefully monitored to see whether they responded instantly to the removal of the film. All the phthalocyanines mentioned previously, even with an optimized solvent, had shown a rather sluggish response. However, using asy-CuPc a fast stearic-acid-like response was obtained using chloroform. No further advantage could be gained in this case using alternative solvents. The average dimensions of the asymmetric molecule are 4 A x 18 A x 18 .& Therefore, it would appear from the calculated area per molecule of 57 A’ that the molecules are tilted edge-on in the water with the short chains pointing upwards and overlapping

I’LF C’rRONIC‘

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1.N FILMS

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with the neighbouring molecules. A point of interest with this material is that the amine groups can be protonated. It is surprising, therefore, that variations in pH do not drastically alter the shape of the isotherm. However, the acidity of the subphase does influence the dipping characteristics. The inclusion of I”,, alcohol and raising the pH to above 7.0 produces very good transfer characteristics using a dipping speed of 0.5 cm min ‘. Capacitance and ellipsometry measurements (Fig. 3) indicate that the thickness of a molecular film deposited onto a substrate is approximately 22 A. The good mechanical stability of these layers was confirmed by measuring the absorbance of the film before and after placing Sellotape on the substrate and attempting to peel offthe asymmetric CuPc layer.

118 3.

G. G.

CHARACTERIZATION

OF ASYMMETRIC

COPPER

PHTHALOCYANINE

ROBERTS

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FILMS

In view of the superior dipping qualities of the asy-CuPc molecule, most of our experimental studies using semiconductor substrates have been carried out with this material, although for many of the applications described the tetra-terr-butyl compounds would also have been suitable. The films were examined using transmission electron diffraction (TED), reflection high energy electron diffraction (RHEED) and scanning electron microscopy. All the structural investigations confirmed the superior quality of the asyCuPc LB films. The RHEED investigations indicate that the layers are polycrystalline with the absence of any arcs of intensity, confirming that the grains do not have any preferred orientation. However, using the TED technique, which monitors smaller sample areas, arcs of intensity are observed in the diffraction pattern of the asy-CuPc. An example is shown in Fig. 2 for a sample floated off a specially prepared substrate onto an electron microscope copper gridi2. The d spacings given by the diffraction arcs are 0.329 and 0.12 1 nm, with the larger value corresponding to the interatomic spacing of the single-crystal form. The preferred orientation changed very little as the sample was moved across the beam and persisted in some cases for specimen movements of 3 mm. All the other phthalocyanine compounds examined showed grain sizes of less than 0.1 urn, as reported by Fryer et ~11.l3. For all the phthalocyanine films studied, the absorbance increased correctly in proportion to the number of deposited layers. The spectra showed two peaks, one corresponding to the solution spectrum and the other to the formation of dimers in the LB film. Conductivity measurements were carried out using NESA glass substrates; in the low field region the asy-CuPc films had a conductivity of approximately lo- * fl- ’ m- ‘, sufficiently low for capacitance measurements to be made. Figure 3 shows data for asy-CuPc deposited onto an epitaxial layer of ZnSeS; this substrate was etched for 30 s in a lyO solution of bromine in methanol, followed by a 1Omin rinse in carbon disulphide, prior to LB film deposition. The linear variation of the reciprocal capacitance with number of layers N confirms the reproducibility of the dielectric thickness from one monolayer to the next. If it is assumed that the semiconductor space charge capacitance is independent of N and the single-crystal value of 3.0 is taken for the dielectric constant 14, then a monolayer thickness of 2.25 nm is obtained. This is in agreement with preliminary ellipsometry measurements for this material. 4.

ELECTRONIC

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We have thus shown that it is possible to construct extremely robust monolayer assemblies using suitable phthalocyanine compounds. Their orientation both on the subphase and on the substrate depends on the nature of the metal ions which replace the central hydrogen atom in the ring system. Clearly, the structural quality of the LB layers is as yet relatively imperfect compared with those for stearic or o-tricosenoic acid’“. However, for potential applications, such as those to be discussed here, it is necessary that the films have satisfactory thermal and mechanical stabilities,

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4.1. Electroluminescent diodes Many efficient luminescent materials are not available in n- and p-type form and hence cannot form useful p-n junctions. In order to exploit these in an electroluminescent device, Schottky barrier structures are required but to enhance the minority carrier injection efficiency a thin insulating layer is incorporated between the metal and the semiconductor, thus forming an MIS diode. The thin insulator must be sufficiently stable to withstand high current densities and yet have reproducible characteristics over a large area. The properties of a model system based on gallium phosphide and LB films have already been reported by US~~-‘~. However, the Group II-VI semiconductors are of more practical interest because of their intrinsically higher internal efficiencies. We have therefore studied electroluminescence from films of ZnSeS deposited using organometallic chemical vapour deposition onto GaAs. Films of the asymmetrically substituted phthalocyanine molecule deposit well onto this substrate (see Fig. 3). D.c. drive densities of up to 5 A cmw2 were used to produce the light emission. A typical spectral response using forward bias is shown in Fig. 4. The peak at 580 nm with a halfwidth of 80 nm is well documented as being due to a complex localized state involving a zinc vacancy and an aluminium impurity. The maximum power efficiency estimated for a typical diode incorporating ten monolayers of asy-CuPc is about 2 x 10e4%. This low value is due to a lattice mismatch at the interface between the ZnSeS and the GaAs. There is some evidence that the problem can be avoided using lower growth temperatures. Then it will be possible to assess the injection efficiency using the LB film. By adjusting the growth conditions it should also be possible to observe the more desirable blue band-to-band electroluminescence. When this stage is reached it will also be worthwhile to optimize the thickness of the insulating film as was done with the GaP devices”.

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600

650

WAVELENGTH

inm)

Fig. 4. Spectral response of an Au/LB film/ZnSeS MIS diode using a forward density of 0.5 A cm-‘. The device contains ten monolayers of asy-CuPc.

bias of 5 V and a current

120

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ROBERTS

rt Cd.

The metal/thin insulator/n-p+ (or equivalent ppn+)-(MISS) structure is of interest because of its bistable switching characteristics. Potential applications of these devices, if a reproducibly thin high quality semi-insulating layer can be found, include memories and shift registers. With silicon it is possible to use oxide layers approximately 3 nm thick, even though they are difficult to grow uniformly. For high mobility Group III-V compounds the difficulties are more severe owing to the lack of a suitable native oxide. However, we have already shown”) that it is possible to use w-tricosenoic acid LB films as the insulating layer on GaAs substrates. Figure 5(b) displays the current-voltage switching characteristics for the device shown in the accompanying diagram incorporating four monolayers of asy-CuPc. Similar data obtained using different doping concentrations in the n-type epilayer help to confirm that the switching is due to a punch-through mechanism. Operation for lo7 cycles at 100 Hz does not degrade the characteristics; a slight increase in the holding voltage with time over long intervals of operation at high current densities is the only noticeable change. However, this effect is reduced using asy-CuPc rather than w-tricosenoic acid. Future work will concentrate on exploring the potential of MISS switching devices as transducers. With an appropriate choice of organic molecule, the device could be made to switch selectively between its high and low impedance states. 4.3. Silicon metal-o.\-icie-.~~nziLont~u~tor do& In a previous publication8 we have shown that the conductivity of a phthalocyanine film is very sensitive to the presence of NO,; the saturation current density in a simple lateral structure on glass scales linearly with gas concentration. The influence of a change in ambient can also be detected using the device shown in Fig. 6. The lower curve shows capacitance-voltage curves for this MOS diode where zero, one, three and five monolayers of asy-CuPc have been deposited onto the silicon oxide. These characteristics are typical of those expected when the applied voltage drives the accumulated semiconductor surface through depletion to inversion. In the accumulation region below - 2 V, it is convenient to measure the capacitance and to assume that it approximates to that for the insulator. The reciprocal capacitance tlersus N plot, such as that shown in Fig. 3, is a good straight line and yields approximately the same value (1.9 nm) for a monolayer thickness. Figure 7 shows the influence of different concentrations of NO, gas on the conductance-voltage characteristic for the five-layer device. The conductance peak centred near 0 V may be attributed to surface states and/or unsaturated dangling bonds at the Si-SiOz interface. This is displaced slightly to higher voltages in the presence of a gas. We interpret this as being due to a change in the effective work function of the top electrode. A more noticeable feature in the conduction data is the introduction of an additional peak whose magnitude is very sensitive to the ambient; it vanishes when the sample chamber is subsequently flushed with dry nitrogen. Its appearance can be attributed to the onset of lateral conduction. It is known that an inversion layer is very sensitive to potential gradients along the oxide surface”. In our case, the change in the conductivity of the asy-CuPc layer allows such a charge distribution to occur. Thus the production of the new lateral

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DEVICES INCORPORATING

PHTHALOCYANINE

Au

lIcCtrode~

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LB FILMS

electrodes

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Fig. 5. (a) The structure of a GaAs MISS device containing four monolayers of asy-CuPc; (b) a typical MISS device characteristic using a series resistance of 1 kR (the diode area is approximately 2 x 10m3 cm’). Fig. 6. (a) The metal-oxide-semiconductor (MOS) diode incorporating a staircase assembly of asy-CuPc monolayers; (b) capacitance vs. voltage curves for the device structure shown above containing zero, one, three and five monolayers of asy-CuPc.

conduction peak in the conductance spectrum directly reflects the additional charge introduced by the NO, at the oxide-phthalocyanine interface. Naturally, such charges would also manifest themselves in the corresponding transistor structure. 5.

CONCLUSION

We have shown that the solvent plays a crucial role in the deposition of phthalocyanine LB films. Pressure-area isotherms displaying realistic monomolecular areas have been obtained for a variety of compounds all of which contain a central metal ion. Particularly good results have been obtained with an asymmetricElectron dtffractton data for CuPc ally substituted copper phthalocyanine. tris(CH,NHC,H,-iso) show a preferred orientation with a domain size close to 0.3 mm. The stability of this molecule has enabled us to incorporate it in LB film form in a number of semiconductor devices. More research is required to improve

(i.

the structural quality of the phthalocyanine achieved without sacrificing its good thermal

G. RoBERrS

et r/l.

monolayers further but this must be and mechanical properties.

A(‘KNOWI.EIX;MENT‘S

The authors wish to thank Dr. G. J. Russell for his assistance with the electron diffraction studies and Mr. C. Pearson for his help with the LB film deposition. The phthalocyanine materials were kindly supplied by Dr. M. Ahmad and Dr. D. Thompson. The authors also acknowledge receipt of the electroluminescent diodes from the Royal Signals and Radar Establishment (Malvern) and the GaAs substrates from the Science and Engineering Research Council Ill~V Semiconductor Growth Facility at Sheffield University. REFERENG-3 I , 3 4 5 6 7 x 9 10 II

I’. H. Moser and A. L. Thomas. 771~/‘h/hrrb~~i,trrli,r[,\. Vol. 2. CRC‘ Pre\\. 1983 J. M. Robertson, R. P. Linstead and C’. E. Dent. IVoruw i I.o~~tk~rr i. ( 1935)I 15 J. H. Sharp and M. Lardon. J. P/I!.\. (‘lwi?.. 72 (196X)3230 M. Sugi and S. Iizima. Tlzi,~So/idFi/t~~.v. 68 ( 1980) 199. N. Yamamoto. T. Ohnishi. M. Hatakeyama and H. Tsubomura. T/f/u SolrrlF/ln~.\. 68 (19X0) 191 S. Baker. M. C. Petty. G. G. Roberts and M. V. Twig. Thin So/ir/F~/m~. YY (1983) 53, G G Roberts and M. V. Twigg, Br. I’orritr H.I2Y,O/H. 1981. S. Baker. G. G. Roberts and M. C. Petty. Pwt IEE 1. 130 ( 19X3)260. G. G. Roberts. P. S. Vincett and W. A. Barlow. Ph~,c. Twh.. I.? ( 19XI ) 6Y. A. Snow and N. L. Jarvis. J. -Iru. C/~CUI.Sw., 106 ( 1984) 4706. R. A. f-lann. W. A. Barlow. J. H. Steven. B. L. Eyes. M. V. Twig and G. Cr. Robert\. Prop ldlrrr U~wl\.dwp on Molt~cdrrr Elcctrorric~ Lk~riw.\, IY83. in the press.

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S. Baker, Ph.D. Thesis, University of Durham, 1985. J. R. Fryer, R. A. Hann and B. L. Eyres, Nuture (London), 313 (1985) 382. Y. A. Vidadi, E. A. Chistyakov and L. D. Rozenshtein, Soti. Ph_vs. Solid Stare, II (1970) 1946. I. R. Peterson, G. J. Russell and G. G. Roberts, Thin Solid Films, 109 (1983) 371. J. Batey, G. G. Roberts and M. C. Petty, Thin Solid Films, 99 (1983) 283. J. Batey, M. C. Petty and G. G. Roberts, Proc. INFOS83 Conf:. North-Holland, Amsterdam, 1983. p. 141. J. Batey, M. C. Petty and G. G. Roberts, Electron. Left.. 20 (1984) 838. N. J. Thomas, M. C. Petty, G. G. Roberts and H. Y. Hall. Elrcrron. Lrtt., 20 (1984) 383. W. Shockley. H. J. Queisser and W. W. Hooper, Phys. Rev. Lerr.. II (1963) 489.