aluminium interface

Synthetic Metals, 38 (1990) 69-76

69

STUDIES OF THE POLYPARAPHENYLENE/ALUMINIUM INTERFACE T. P. NGUYEN, H. ETrAIK and S. LEFRANT Institut de Physique et Chimie des Matdriau.% UMR CNRS No. 110, Universitd de Nantes, 44072 Nantes Cddex 03 (France) G. LEISING Institut fftr FestkSrperphysik, Technische Universitdt Graz, Petersgasse 15, .48010 Graz

(Austria) F. STELZER Iustitut fi~r Chemische Technologie und Organische Stoffe, Technische UniversitC2t Graz, Petersgasse 15, A8010 Graz (Austria) (Received September 28, 1989; in revised form February 15, 1990; accepted February 27, 1990)

Abstract The dielectric properties of thin film A1/polyparaphenylene/A1 structures have been investigated in the frequency range from 10 -2 to 5)< 10 5 Hz. Experimental data suggest the existence of interfacial layers at the polymer/ metal contacts. An equivalent circuit analysis, taking into account the dielectric response of these elements, is presented. To clarify the nature of the contact, X-ray photoelectron spectroscopy (XPS) has been performed on polyparaphenylene (PPP) films covered with A1 layers. Displacements of the C(ls), O(ls) and Al(2p) lines lead to the conclusion that a chemical reaction between carbon, oxygen and aluminium occurs at the interface to form a met a l - o x y g e n - p o l y m e r complex which is found to be responsible for the electrical a.c. behaviour of AI/PPP/A1 structures.

Introduction

Although the electrical properties of conducting polymers have been extensively studied, little is known on how the contact between the metallic electrode and the polymer influences the transport mechanisms. In semiconductor devices it is known that rectifying or ohmic contacts can be obtained by choosing metals with appropriate work functions with regard to that of the semiconducting material. However, in most cases, it has been demonstrated that the interface Fermi level and the barrier height do not depend greatly on the nature of the metal constituting the electrode. On the

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70

other hand, chemical reactions may occur between the semiconductor and the metal but they generally do not affect significantly the barrier height. Polyparaphenylene (abbreviated hereafter PPP) synthesized by chemical reduction to form thin yellow films has been studied in pure and ion-implanted doped forms by several authors [1, 2]. Their fibrillar morphology, however, does not allow two-port devices, which are helpful for an electrical characterization because of the low conductivity of the polymer, to be obtained. A new preparation method has recently been developed [3, 4] producing films with a much more compact structure as compared to those prepared by the electrochemical synthesis. The present paper deals with some electrical properties of PPP films prepared via the new technique and sandwiched between two aluminium electrodes. From the dielectric response of these structures, we try to point out the role of the interfacial layers and we relate their origin to chemical reactions between the polymer and the metal as observed by means of Xray photoelectron spectroscopy (XPS). Experimental

Thin films of PPP have been synthesized via a precursor polymer route following the method by Ballard et al. [3]. This precursor route provides some important advantages: -- A polymer containing very low catalyst residues can be obtained by dissolving and precipitating the precursor polymer several times. - The PPP films have a compact, non-fibrillar morphology. - - The processability of the precursor polymer allows the production of PPP films of any desired shape and thickness possessing good optical quality. The preparation of PPP via precursor polymer is outlined in Fig. 1. The monomer 5,6-dihydroxy-2-cyclohexen-l,4-ylene diester (CeH4(OCOR)2--) is polymerized to poly(5,6-dihydroxy-2-cyclohexen-l,4-ylene diester) by various methods, which will be described in detail in a subsequent publication. The rest groups used in the current study were R = m e t h y l , biphenyl and 2naphthyl. Thin films were cast from solutions (solvents: acetone, chlorobenzene, methylenechloride or dichloro-l,2-ethane) of the precursor polymer on an appropriate substrate. The films were slowly dried under a pure argon gas flow. The transformation of the precursor polymer film to PPP was carried Precursor-Polymer

PPP

ROCO OCOR R= 2-Naphthyl,Methyl, Biphenyl Fig. 1. PPP synthesis from polymer precursor.

71 out in a high v a c uum furnace at t em pe r at ures ranging from 170 to 400 °C for several hours. The purity of the PPP films concerning residues of the p r e c u r s o r p o ly m er (incomplete transformation) and the eliminated molecule -R C O2 H was verified by infrared (IR) s p e c t r o s c o p y of thin films on KBr substrates treated simultaneously with the samples used for this investigation. A1/PPP/A1 structures were obtained by casting the polymer f i l m on glass substrates containing pre-deposited metal electrode. After heat treatment: of these two layers, the film and the b o t t o m electrode were annealed as described above. The subsequent metallization step was carried out at ambient temperature to obtain the top electrode. All evaporation p r o c e d u r e s were performed under high va c uum conditions ( ~ 10 -6 Torr). The active area of the samples was 2 X 2 m m 2 and the thickness of the metal electrodes was about 100 nm to insure low contact resistance conditions. The samples were m ount e d in a cryostat working in the t em perat ure range from 77 to 420 K. The conductance and the capacitance were m easured by a Hewlett-Packard I m pe da nc e ve c t or HP 418 for the frequency range 1 0 - 5 x 105 Hz and by the ellipse m e t hod [5] for the low frequency range down to 0.01 Hz. The XPS experiments were per f or m ed in a Leybold LH ESCA analyser (Universit@ de Nantes-CNRS). Thin films of PPP with and without the A1 layer were deposited on inox or c o p p e r substrates. The core levels of carbon, oxygen and aluminium were r e c o r d e d by using Mg Ka radiation ( h v = 1253.6 eV). The pressure of the cham be r was kept in the 10 -~ T orr range during the experiment. Binding energy data were r e f e r e n c e d to the Au(4fT/2) line (84 eV) of a gold plate fixed on the sample holder. The data were p r o c e s s e d using a c o m p u t e r allowing background and satellites subtraction, smoothing and integration.

Results and discussion A.c. m e a s u r e m e n t s Figure 2 shows the capacitance C and the conductance G of an AI/PPP/ A1 structure as a function of the frequency f at ambient temperature. It can be seen that t h e C(/) characteristic has u p p e r and lower limits when the frequency tends to low and high values respectively. The dielectric constant, e, at high frequency determined from geometrical param et ers of the structure and the lower limit of the C(f) characteristic is ~ 3. This value agrees well with that r e p o r t e d by Stubb et al. [6]. The conduct ance curve G(f) shows only one limit in the high frequency range; the d.c. conduct ance limit ( ~ 5 x 10 -11 £t-1 estimated from d.c. measurements) could not be observed in the investigated frequency range. Preliminary work on films with chromium electrodes shows that the limit of G(f) at high frequency is not reached with the highest frequency used. For t h e structures investigated, this reflects a behaviour typical for devices with a contact resistance in series with a bulk sample [7]. To describe the dielectric response of the metal/insulator/metal

72

Log G (t"l-I )

Log C (F) -8

-8

-2 -4

-9

-6

I

-9

-8 RI

-10 0

R3

R2

I

I

I

I

I

I

2

3

4

5

Log f (Hz)

-10

-10

I

I

I

I

2

3

4

5

6

,

Fig. 2. Plots of the capacitance C and the conductance G vs. frequency at T = 294 K. The thickness of the sample is 260 nm. The full line curves are c o m p u t e d from the equivalent circuit shown in the inset (see text for details). Fig. 3. Plots of capacitance vs. frequency for different temperatures: (a) 294 K; Co) 187 K; (c) 129 K. The thickness of the sample is 260 nm. The full line curves are c o m p u t e d from the equivalent circuit (see text for details). TABLE 1 Values of the c o m p o n e n t s used to compute the response curves in Fig. 2 Component

R (k~)

C (nF)

R,, Cx R2, Ce Ra, Ca

6.6 12 2 × 106

0.4 10 5.3

structure, it is often convenient to use the well-known Maxwell-Wagner representation [8]. From the different preparation conditions of the interfaces, we will represent each interface by an RC combination, i.e. R1C1 and R2C2, and the bulk by another combination R8C8. Table 1 gives the values of the components used to fit both cO') and GOO characteristics (full line curves of Fig. 2). It should be noted that the values of RC could be permuted for the calculation but only the proposed value for R8C8 leads to acceptable resistivity for PPP f i l l (>3)<1012 ~ cm). The resistivity of the interface contact layers estimated from geometrical parameters is much lower than that of PPP ( ~ 5 × 1 0 s ~ cm). Figure 3 shows the evolution of the capacitance as a function of the frequency for different temperatures. As one can see, the COO characteristic shifts to higher frequencies with increasing temperature. Following the above equivalent circuit analysis, this observation suggests that the conduction mechanism is thermally activated [9]. The activation energy can be easily determined from the plot of the constant values of the capacitance versus the inverse of the temperature. This plot gives a straight line and from the slope we derive an activation energy of AE--0.06 eV. The activation energy

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can also be obtained by plotting the frequency corresponding to the maximum of the loss tangent fo versus the inverse of the temperature [10]. On the other hand, the variation of the conductance G(f) as a function of the temperature (Fig. 4) clearly suggests that the contact resistance is thermally activated. Thus, using the calculated activation energy and assuming a variation of these resistances as R1.2 =Ro(1.2) exp( -

zlE/kT)

(1)

we compute the theoretical curves CO') and G00 from the p r o p o s e d equivalent circuit. The full line curves in Figs. 3 and 4 result from the calculation and fit the experimental data fairly well. From this behaviour, we conclude that the temperature dependence of the device is controlled by the interfacial layers. With this hypothesis, we note that the resistance of the polymer film is practically independent of temperature (very little change of R3 is needed for computing the theoretical response curves). In fact, the activation energy observed in doped PPP [6] is much higher ( > 0 . 6 eV) than that observed in our films. This difference can be explained in two ways: (i) the same conduction mechanism occurs in both doped and pristine PPP and with the Al-sandwiched configuration, the conduction through the bulk would be masked by the large effect of the contact resistances; (ii) the conduction mechanism is different in pristine PPP film and hopping of carriers between localized states would explain the low value of the activation energy [11 ]. It should also be noted that the two interfaces are different concerning the computed values (R1, C1 and Re, C2) and this can be explained by the different temperature conditions under which they were obtained. The values of the interfacial capacitances indicate that the ratio of their thickness is about 25 and this would explain the high value of the bulk capacitance (5.3 nF). In fact, using the value of the dielectric constant at high frequency, we found an effective thickness of the PPP film of about 20 nm. Therefore, interfacial layers are predominant and seem to control the transport properties by the same mechanism (with identical activation energy).

I -4 -6 -8

S I

2

I

|

I

3 4 5 6 Log f (Hz) ,~ Fig. 4. Plots of conductance vs. frequency for different temperatures: (a) 294 K; Co) 187 K; (c) 129 K. The thickness of the sample is 260 nm. The full line, dashed and dash-dotted curves are computed from the equivalent circuit (see text for details).

74

XPS study o f the A1/PPP interface The electrical behaviour of AI/PPP/A1 structures suggests that the contact layers at the interfaces are fundamentally different from the polymer film. Thus, we tried to identify the nature of these layers using the XPS technique to study the chemical bondings and compare them to those in PPP films. For this purpose, we analysed PPP films deposited on inox or copper substrates before and after evaporation of an A1 layer. In order to achieve a reliable comparison, the A1 film was deposited under the same conditions as those described above. Erosion of the metallic electrode by Ar + bombardment with controlled flux and accelerating voltage was then performed for a given time. Core level spectra of carbon, aluminium and oxygen were recorded and the process was repeated until the pure PPP film was obtained (by comparing the C(ls) position with that of uncovered PPP films). The spectra of Al(2p), C(ls) and O(ls) lines are shown in Fig. 5. For uncovered films, the C(ls) peak position is located at 285 eV with a full width at half maximum (FWHM) of 1.5 eV (curve e). We also found that oxygen contamination of the films occurred despite the fact that samples had been stored in vacuo before analyses. The O(ls) line is located at 533.5 eV but its intensity is rather weak and would correspond at most to only a few atomic percent. For the covered A1 layer, the C(ls) is located at 284.5 eV throughout the interfacial layer. This layer is defined as the one in which C(ls) and Al(2p) lines appear simultaneously. Its thickness is estimated to be 5 n m in all the samples studied. It should be noted that this value corresponds to the layer underneath the top electrode of the A1/PPP/AI structures; the thickness of the interfacial layer on the bottom side cannot be evaluated by ESCA measurement. The Al(2p) spectra show an evolution in lineshape and in peak position throughout the interface. For the outermost layer, the peak C(ls)

T O( 1s)

Al(2p)

d c

a

c

m

b c

a

I

540

I

I

I

I

[//I

530 290

I

I

I

I

[//I

280

I

I

I

80 BINDING

I

I

70 ENERGY

(¢V)

Fig. 5. Photoelectron spectra of PPP films: (1), without AI layer (curve e); (2), in the interracial PPP/A1 layer (curves a, b, c, d) with successive removal of the A1 layers.

75 position corresponds to Al(2p) of oxidized aluminium AI~O~ (75.6 eV, curve a). When approaching the PPP film, it shifts to an intermediate value (74.2 eV, curve d) between those of A1 clean metal (72.8 eV) and A1203 (75 eV, curve b). On the other hand, we observed a growth of oxygen species in the interface. The O(ls) line is shifted from O(ls) in AI~O~ (532.6 eV) to values higher than that from PPP contamination (final position at 531.8 eV, curve d). The peak position of the O(ls) line of curve b (532 eV) is identified as that of A1203 [ 12]. Comparison of the binding energy for the C(ls), O(ls) and Al(2p) lines for A1/PPP with known literature [13] and measured values of A1 and A1203 indicates that A1 is in an oxidized state and that both carbon and oxygen have reacted with the A1 layer. The origin of the oxygen presence is not clear; it might be due to contamination of the A1 electrode on handling and transferring the sample in the spectrometer. Traces of unconverted precursor polymer sequences would also contribute to the observed oxygen signal. The shift of the C(ls) line towards lower binding energy is compatible with findings in metal carbides [14]. Combined with the Al(2p) and O(ls) line shifts, we conclude that there is formation of an A1 oxide-carbide complex on the PPP surface covered by an A1 layer. The stoichiometry of this complex is not known but optical characterization such as IR or Raman spectroscopy should be useful to obtain further information about its structure. Our observations are also in good agreement with recent conclusions on the nature of the species formed at the metal/polymer interface. In fact, it has been shown that alumim'um is very reactive when brought into contact with polymers such as polyethylene [12] or polyamide [15, 16]. In these cases, it was suggested that A1 breaks the C--O bonds by forming A1-O-C complex and charge transfer occurs from A1 to C via O. The gradual change of both Al(2p) and O(ls) peak positions observed here does not support this hypothesis. It seems that A1 has diffused into the PPP films and combined with both residual oxygen and carbon, giving rise to the observed interfacial layer.

Conclusions In conclusion, the present work has been performed on PPP films synthesized by a new method. The a.c. properties of A1/PPP/AI structures were studied and some effects of the metal/polymer contacts on the dielectric response have been evidenced. An equivalent circuit analysis has been proposed to estimate the different contributions of bulk and contact parts. The XPS technique has been used to identify the nature of the contact layer. It was found that a reaction between carbon, oxygen and aluminium occurred at the interface to form A1 oxide-carbide complex over 5 nm of the film thickness. The interfacial layer is related to the contact effect on a.c. behaviour of the structures studied.

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Acknowledgement T h e a u t h o r (G. L.) t h a n k s t h e J u b i l a e u m s f o n d N a t i o n a l b a n k ( C o n t r a c t No. 3 1 0 0 ) .

der C)sterreichischen

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