2246
J . Phys. Chem. 1991, 95, 2246-2250
methylcyclohexane were used for solvents without further purification. Absorption and Fluorescence. Absorption and fluorescence spectra were measured with Hitachi U-3400 and MPF-2A spectrometers, respectively. Fluorescence quantum yield was measured with a Hitachi F-410 spectrometer, with 9,IO-diphenylanthracene and rhodamine 6G being used for the standards. Time-Resolved Optical Absorption (TROA) System. TROA measurement was performed on degassed solutions by using second (532 nm), third (355 nm), or fourth (266 nm) harmonics of a Quanta-Ray DCR-1 Nd:YAG laser as a nanosecond excitation light source. The details of the apparatus were described elsewhere.25 Fluorescence Time Profiles. Fluorescence time profiles shorter than IO ns were measured with a single photon counting system,26 where a Spectra Physics 375B-374 synchronously pumped cavity-dumped dye laser was used as a picosecond excitation light source. This dye laser was excited by the second harmonic of a Spectra Physics 3460 Nd:YAG laser and operated with a pulse width (fwhm) of 6 ps (estimated with an Inrad 5-14 autocorrelator) and with a repetition rate of up to 800 kHz. The excitation wavelength was covered by the lasing region of the second harmonic of rhodamine 6G. The second harmonic was generated with a 0-BBO crystal. Fluorescence was detected by a Hamamatsu Photonics R2809U-01 multichannel plate photomultiplier, and its time profile was measured with an EG&G ORTEC single photon counting system connected to a NIKON G250 25-cm monochromator. Signals from the photomultiplier were amplified with an HP8447F 1.3-GHz preamplifier and shaped with an ORTEC 583 constant fraction differential discriminator. The output pulse was properly line-delayed by an ORTEC 425A delay and used as a stop pulse for an ORTEC 457 biased time-to-pulse height con(25) Sakaguchi, y.; Hayashi, H.; Nagakura, s. J . Phys. Chem. 1982,86, 3177. ( 2 6 ) Yamazaki, I.; Tamai, N.; Kume, H.; Tsuchiya, H.; Oba, K. Rev. Sci. Instrum. 1985, 56, 1187.
Raman Spectral Studies of Ai-Ai0,-MPc-M‘ Dependence
verter. The dye laser pulse was monitored with a Hamamatsu Photonics S 1722-02 high-speed PIN photodiode, discriminated with an ORTEC 436 100-MHz discriminator, and used as a start pulse. An ORTEC 7800 multichannel analyzer was coupled with an NEC PC9801VX microcomputer system for saving the data. Through the modifications of the detecting systems, the apparent pulse width of the exciting laser was found to be about 60 ps. The rise and decay shapes of fluorescence were analyzed by a convolutionsimulation method using the observed time profile of the scattered laser light as a reference. Fluctuations of the laser power result in the timing zitter. In this work, the laser system was stabilized by mechanical fittings and overstepped operations as to ensure the long-time accumulations with minimum time walk. In the case of very weak fluorescence, the monochromator was replaced with a set of filters. Fluorescence time profiles longer than IO ns were measured with the TROA system. Time-Resolved ESR (TRESR) System. TRESR measurement was carried out by a JEOL-FE3X ESR spectrometer without field modulation. A Lumonix H E 440-VB excimer laser (XeCI, 308 nm) was used as a nanosecond excitation light source. The methylcyclohexane solution of each ketone was bubbled with pure nitrogen, followed by flow through an ESR sample tube. The details of the apparatus were described else~here.~’ Acknowledgment. We express our sincere thanks to Professor Noboru Hirota of Kyoto University and Professor Mamoru Ohashi of the University of Electro-Communication for their kindness in putting their TRESR and fluorescencequantum yield measuring systems, respectively, at our disposal. We also thank Professor Iwao Yamazaki of Hokkaido University for his kind suggestions concerning the construction of our picosecond fluorescence measuring system. (27) Terazawa, M.; Yamauchi, S.; Hirota, N. Chem. Phys. Leu. 1983.98, 145.
Tunnel Diodes: Bias and Top Metal
J. J. Hoagland, Jess Dowdy, and K. W. Hipps* Department of Chemistry, Washington State University, Pullman, Washington 991 64-4630 (Received: August 31, 1990)
Raman spectra of AI-AI0,-MPc-M’ tunnel diodes, where M = Cu or H2,M’ = Ag or Pb, and Pc = phthalocyanine, are reported. No evidence is found for chemical modification of either CuPc or HzPc in any step of the device fabrication process. By observing the Raman spectra of functioning tunnel diodes, it is demonstrated that no electrochemical modification of CuPc occurs under bias conditions of f l . 3 V. While ultrathin MPc layer devices prior to M’deposition show strong Raman scattering only for p-polarized radiation, the completed devices (M‘ = Pb or Ag) scatter both s- and p-polarized radiation.
Introduction Many of the present and proposed microelectronic devices function because of a buried interface. Often, this is a M’-I-M or M’-S-M interface where the insulator or semiconductor passes electrical current to or from the metal layers that enclose it. The observation of the chemical and electrochemical processes that occur in these buried interfaces is quite difficult since the metal layers shield the interfaces of interest from photon and electron probes. One general exception to this rule is inelastic electron tunneling (IET, or tunneling) spectros~opy.’-~In IETS, one
extracts both electronic and vibrational spectra by measuring (in a special way) the voltage-dependent currents flowing through M-I-M or M-I-S-M devices. Thus, tunneling spectroscopy is the technique of choice for studying certain types of imbedded interfaces. Further, given the current interest in spectroscopic (1) Hansma, P. K. In Vibrational Specrroscopy o j Adsorbed Molecules; Yates, J. T., Madey, T. E., Eds.; Plenum Press: New York, 1987. (2) Tunneling Spectroscopy; Hansma, P. K . , Ed.; Plenum Press: New York, 1982. (3) Hipps, K. W.J . Phys. Chem. 1989, 93, 5958.
0022-365419 112095-2246%02.50/0 0 1991 American Chemical Society
Raman Spectra of AI-A10,-MPc-M'
Tunnel Diodes
methods carried out in a scanning tunneling microscope? it is critical that we have a firm grasp upon the basic tunneling processes. Until recently, we were quite sure that we had such an understanding of these processes. However, the tunneling spectrum of copper phthalocyanine (CuPc) in AI-A10,-(Pb or T1) junctions is not consistent with our previous e~perience.~ When these devices are biased such that the aluminum layer is positive, a spectrum similar to that of CuPc results. If these devices are oppositely biased, a very different spectrum appears. Further, the spectra depend on the top metal when aluminum is biased negatively. Either a reversible electrochemical process is occurring or some very unusual aspect of the tunneling theory is being displayed. If CuPc is electrochemically modified within the device, all is well with the tunneling theory. If CuPc is stable within the device throughout the -1 to +1 V bias region, then our theory of tunneling is failing, In order to decide between these alternatives, it is critically important to ascertain the state of the CuPc layer within the device. To do this, we must turn to a different spectroscopic method. The only nondestructive alternative, Raman spectroscopy, has been applied to only a few complete device structure^.^-^ And, in most of these cases, the sample configuration never matched that of a typical biased d e ~ i c e . ~Ching - ~ et aL5 utilized the E, gap resonance enhancement in n- and p-InAs to study the effects of gate voltage on a complete (working) MOS junction. They found a narrow bias dependent band intermediate between the transverse optical (TO) phonons and the unscreened longitudinal optical (LO) phonons. Kirtley et aL6 utilized several different enhancement methods to observe Raman scattering from AlAI0,-A-Ag and A1-AI0,-A-Sn tunnel diodes, where A is an organic adsorbate. They published a single spectrum of a biased device [an Al-AlO,-(4-pyridinecarboxylic acid)-Ag structure biased at 0.5 VI; but, the device was not constructed in a manner appropriate for tunneling spectroscopy. A very rough 120-nm CaF, foundation layer was used to produce an irregular AlAI0,-A-Ag interface. This substrate-induced roughening of the Ag top layer provided the primary Raman intensity enhancement mechanism. Nemanich and co-workers' utilized interference enhanced Raman spectroscopy to study the metalsilicon interface in model Schottky barrier devices. The electrically insulating interference layer required by this technique precluded the application of a voltage across the M-Si interface. The intrinsic resonance enhancement of the pentacyanopropenide ion (PCP) was used to provide Raman spectra of AI-AIO, structures having submonolayer coverage of PCP.* These structures were then coated with Pb, and tunneling spectra were obtained. A comparison of Raman and IETS spectra was presented. The devices studied by Raman spectroscopy could not be biased. Within the narrow confines of MPc-containing structures, we know of no Raman studies of complete electronic devices. There are, however, several groups considering the Raman spectra obtained from Ag-MPc contacts formed by depositing silver onto relatively thick (> I4 nm) MPc films9 In these studies a com(4) (a) Demuth, J. E.; Hamers, R. J.; Tromp, R. M. Springer Ser. Surf. Sci. 1988,14,236. (b) Hamers, R. J. Annu. Rev. Phys. Chem. 1989,40,531. IC) Kaiser. W. C.: Bell. L. D. Phvs. Reu. Lett. 1988. 60. 1406. (5) Ching, L. Y.; Burstein, E.,Buchner, S.; Wieder, H. H. J . Phys. SOC. Jpn. 1980, 49, 95 I . (6) (a) Tsang, J . C.; Kirtley, J. R.; Theis, T. N.; Jha, S.S. Phys. Reu. B 1982, 25. 5070. (b) Tsang, J. C.; Avouris, P.; Kirtley, J. R. J . Chem. Phys. 1983, 79,493. (c) Tsang, J. C.; Avouris, P.; Kirtley, J. R. Chem. Phys. Lett. 1983, 94, 172. (7) (a) Tsai, C. C.; Thompson, M. J.; Nemanich, R. J. J. Phys. (Les Ulis, Fr.) 1981.42, C4-1077. (b) Nemanich, R. J.; Thompson, M. J.; Jackson, W. B.; Tsai, C. C.; Stafford, B. L. J . Vac. Sei. Techno/., B 1983, 519. (c) Thompson, M. J.; Johnson, N . M.; Nemanich, R. J.; Tsai, C. C. Appl. Phys. Lett. 1981, 39, 274. (8) Hipps, K. W.; Keder. J. W. J . Phys. Chem. 1983,87, 3186. (9) (a) Ferris, N . S.;Littman, J. E. SPIE 1989, 1055, 117. (b) Saad, E.; Lippitsch. M. E.; Leitner, A.; Aussenegg, F. R. Springer Ser. Chem. Phys. 1984. 39, 486. (c) Aroca. R.; Loutfy, R. 0.J . Raman Spectrosc. 1982, 262. \
,
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2247 LASER BEAM
V Figure 1. Schematic of Raman spectroscopy sampling configuration.
bination of intrinsic and silver-promoted resonance enhancements provides observable Raman spectra. These authors report that the MPc layer is not chemically affected by the Ag overlayer. However, because of the thickness of the MPc films used, it is unlikely that a localized interfacial reaction could be observed. It is also worth noting that Ferrisgasuggests that the deposited Ag layer may actually penetrate the organic layer. In this report, we take advantage of the intrinsically intense Raman scattering of MPc complexes coupled with optically thin but conducting electrodes in order to observe Raman spectra from buried MPc-metal interfaces in biased devices. The importance of these observations goes well beyond the confines of tunneling spectroscopy. The metal-MPc interface is a critical component in a large number of current and proposed microelectronic dev i c e ~ . ' ~The ' ~ insights gained here and/or the methodology used may provide a better understanding of the role of the organicmetal interface in molecular electronic devices. Experimental Section
Materials. CuPc was purchased from commercial sources and then multiply sublimed to obtain very high purity material. High purity H,Pc was used as purchased from Fluka. CuPc and H,Pc were deposited by use of a baffled W or Mo box source (ME-1 from R.D. Mathis Co.). Both the aluminum metal (99.999% purity) and the silver metal (99.99+%) were deposited from high purity tungsten wire coils. Lead (99.999%) was deposited from a Mo or W dimpled boat. Sample Preparation. Approximately 80 nm of A1 was first deposited over a 1-cm X 2-cm area of a clean Corning microscope slide. The slides had been previously cleaned by heating in an aqueous solution of nitric acid and hydrogen peroxide. Next, the fresh aluminum surface was exposed to an oxygen plasma (0.10 Torr, ac discharge) for 5 min. Then, room air was admitted into the vacuum system for a period of 1 h. At this point, the system was reevacuated to a pressure of 20 MR. To prevent this, we enclosed the sample compartment of the Raman spectrometer in a plastic and quartz enclosure and continuously purged with N2 gas. While purging for a period of 12 h, resistance increases of less than 35% were observed. A very weak (about 50 counts/s) sharp band near 1557 cm-’ appears in all spectra taken with aluminum mirrors in our spinner geometry. This band is an atmospheric band and has been removed from the displayed data by spectral subtraction. Other Measurements. The optical absorbance measurements were performed as follows. A measured thickness of metal was deposited onto a clean microscope slide. This slide, and a clean reference slide, were placed in the sample compartment of a Perkin-Elmer 330 UV-vis spectrophotometer. Absorbance was measured at 647.1 and 514.5 nm. Silver films had absorbances ranging from 0.03 to 0.5 while the absorbance of the Pb films ranged between 0.9 and 1.5. Crude resistance measurements were performed in the following way. A two-point measurement was taken by placing the test leads of a digital ohmmeter about 1.5 cm apart on a 1 .O-cm-wide strip. Results and Discussion The Lead Film. While silver has been extensively used to provide light-transmitting conductive films, lead is not commonly used for this purpose. Therefore, our measurements of the resistance and absorbance of Pb films of various thickness is reported are Figure 2. The solid line is a least-squares fit to the absorbance vs thickness data. The broken curve is a guide to the eye in viewing the thickness dependence of the Pb resistance. Note the very sharp increase in this resistance as the film thickness decreases toward 50 nm. This thickness dependence is consistent with relatively late formation of a continuous film. For comparison, an 18-nm(14) Strommen, D. P.; Nakamoto, K. Laboratory Raman Spectroscopy; Wiley: New York, 1984.
Hoagland et al. 2.0
y
1.6
0
9
t 0, a 4
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Figure 2. Absorbance and resistance of a l-cm2 Pb film deposited on
glass. The broken line guides the eye in recognizing the resistance data.
I 200
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E N E R G Y (cm-1) Figure 3. Comparison of Raman spectra obtained from CuPc in KBr, a 20-nm CuPc film on oxidized aluminum, and an AI-AI0,-CuPc( 1 nm)-Ag tunnel diode. The intensity scales have been adjusted for ease of comparison. Spectra were recorded with 514.5-nm laser excitation.
thick silver film had a resistance of IO R under the same deposition and measurement conditions. Top Metal Effects. As the physical environment of the MPc changes from bulk solid to an ultrathin film embedded in an M-I-M device, the peak positions remain essentially the same. As shown in Figure 3, the Raman spectrum of 20 nm of CuPc deposited on oxidized aluminum has exactly the same major peak positions as does the spectrum of CuPc supported in KBr. Further, the Raman spectrum of an AI-AIO,-CuPc( 1 nm)-Ag device at zero bias also has the same peak positions. There are differences in intensity, but these are due to orientational effectsI5 and to the selective enhancement afforded by the Ag layer.I6 It has been shown that the first few layers of CuPc or H2Pc deposited onto the oxidized aluminum surface are present in the a-phase modification.15J7 Very thin films of CuPc preferentially orient with the 4-fold CuPc axis parallel to the metal surface.” Presumably, therefore, 1 nm of CuPc is about one monolayer thick. Thicker films of CuPc and H,Pc tend to adopt a more ordered structure with the 4-fold axis making an angle of about 26’ with respect to the surface Thus, some variation in the (15) Dowdy, J.; Hoagland, J. J.; Hipps, K. W . J. Phys. Chem., in press. ( 1 6 ) Campion, A. In Vibrational Specrroscopy of Adsorbed Molecules;
Yates, J. T.,Madey, T. E.,Eds.;Plenum Press: New York, 1987. (17) (a) Debe, M. K. J . Vac. Sci. Techno/. 1982, 21, 74. (b) Debe, M. K. J. Appl. Phys. 1984.55, 3354.
Raman Spectra of AI-AI0,-MPc-M’
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2249
Tunnel Diodes
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ENERGY (cm-1) Figure 6. Raman spectra obtained from differently doped tunnel junctions. Spectra were taken with 8-cm-l resolution and 647.1-nm excitation. The CuPc and H2Pc layers were 1.3 nm thick.
E N E R G Y (cm-1) Figure 4. Raman spectra of AI-AIO,-CuPc(l nm)-M (M = Ag or Pb) junctions taken with 8-cm-l resolution and 647.1-nm excitation.
100
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Figure 5. Raman spectra of H2Pc in different environments. The upper trace is the polycrystalline material pressed into KBr. The lower trace was obtained from an AI-AI0,-H2Pc( 1 nm)-Pb junction. Both spectra were taken with 8-cm-I resolution and 647.1-nm excitation. TABLE I: Raman Peak Positions and Assignments for AI-AIO,-CuPc (1 nm)-(Ag or Pb) Tunnel Diodes position,“ position,O cm-’ symmetryb intensity cm-‘ symmetryb intensity 485 A2, medium 1003 AZ8 very weak 595 BO weak 1 IO9 BI, weak 682 A,, strong 1 I43 Blg medium 149 B,, strong 1 I95 A2$ broad 171 BZg very weak I307 BI, weak 835 AI8 weak 1341 A,, strong 850 weak shoulder 1453 B2, medium 955 BZp medium 1529 A,, strong
“Values accurate to &3 cm-I. bTaken from refs 20 and 21.
relative intensities of bands with film thickness is to be expected. Figure 4 contrasts the spectra obtained from tunnel diodes containing 1.3 nm of CuPc and topped with different metals. Excitation a t 647.1 nm was used because it provides a Raman spectrum with more lines of similar intensity (but weaker) than does 514.54111 excitation. Table I provides a listing of the CuPc bands observed in Pb and Ag topped junctions and their assignments. Within the limits of our calibration accuracy ( f 2 cm-I), there are no shifts in the vibrational bands. Similarly, no shifts are observed between the Raman spectrum obtained from the free acid (H,Pc) pressed into KBr and that resulting from an AlAI0,-H2Pc( l nm)-Pb junction as shown in Figure 5 . Note that the spectrum of microcrystalline HzPc in KBr is especially noisy because of the presence of intense fluorescence. The upper trace in Figure 5 has been adjusted by removing a high-order polynomial background from the raw Raman spectrum.
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E N E R G Y (cm-1) Figure 7. Polarization dependence of the Raman scattering from an AI-AI0,-CuPc( 1 nm)-Ag tunnel diode. The upper spectrum was obtained with p-polarized 647.1-nm radiation while the lower was obtained with s-polarized light.
The central metal ion plays a strong role in the positions of a number of the MPc vibration^.^^,'^ In fact, Saad et al.9bproposed that Raman spectroscopy could be used to detect variations in metal doping of phthalocyanine-based extrinsic semiconductors. Thus, the data presented in Figures 3-5, indicate that top metal deposition has no chemical effect on these ultrathin MPc layers. This is further supported by a comparison of the Raman spectra of diodes made with HzPc and CuPc (Figure 6). If even a small amount of the H2Pc were converted to PbPc, we would observe bands in the 800-1000-cm-’ region. While application of a top metal does not produce chemical modification of the MPc layer, it dramatically effects the polarization of the Raman scattering. Classically, one should observe Raman scattering only for ppolarized excitation of species located near a metal s ~ r f a c e . ’ ~In ~ ’the ~ absence of a top metal, all of our Al-AlO,-MPc( 1 nm) films are strongly ppolarized.Is Upon completion of the diode, however, a significant component of the scattering comes from s-polarized excitation. As seen in Figure 7, the s-polarized spectrum of this particular Ag-covered CuPc (18) Sidorov, A. N.; Kotlyar, I. P.Opt. Spectrosc. 1961, 11, 92. (19) (a) Greenler, R.G.; Sager, T. L.Spectrochim. Acta 1973, A29, 193. (b) Efrima, S.;Matiu, H. J . Chem. Phys. 1979, 70, 1602. (20) Bovill, A. J.; McConnell, A. A.; Nimmo, J. A.; Smith, W.E.J . Phys. Chem. 1986, 90, 569. (21) Bartholomew, C. R.; McConnell, A. A,; Smith, W. E . J. Raman Spectrosc. 1989, 20, 595.
Hoagland et al.
2250 The Journal of Physical Chemistry, Vol. 95, No. 6,1991
> I-
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Figure 8. Comparison of Raman spectra obtained from AI-AIO; CuPc(l nm)-Pb tunnel diodes biased at 0.0 and -1.3 V. The sign is
chosen such that the AI electrode is positive when the bias is greater than zero. Spectra were recorded with 647. I-nm excitation.
film is about 30% as intense as the p-polarized spectrum. In other experiments with varying excitation, top metal, and thickness, anywhere from 15% to 50% of the scattered radiation was s-polarized. That is, the s- and p-polarized spectra were sometimes of equal intensity. Further, all of the bands observed in p-polarization are seen in s-polarization. Therefore, the mechanism by which the s-component becomes allowed is not mode sensitive. Kirtley6 also reported a strong s-polarized scattering in the case of unroughened silver topped tunnel diodes. A possible explanation for the observed s-polarized Raman spectra can be found in the suggestion of Ferris and Littmanga that vapor-deposited silver tends to penetrate or diffuse into the CuPc layer. This could produce nanometer scale roughness that would destroy the “normal component” selection rule. Lead and thallium topped junctions, however, provide tunneling spectra having intense CuPc bands.3 This, we believe, indicates that Pb and TI do not penetrate CuPc layers of the order of 1 nm thick. Since s-polarized Raman scattering is seen in both Pb and Ag topped junctions, its origin must depend on something other than simple metallic penetration of the CuPc layer. Bias Dependence. The application of an electric field across the AI0,-MPc composite layer could produce one or more of the following outcomes. The most likely is that charge will be transmitted across the barrier via resonant or nonresonant tunneling. In this case, the energy levels of the barrier material are unaffected but the populations may change. Second, one might expect levels to shift slightly due to the Stark effect. The local field at I-V bias is quite large, of the order of 5 X lo6 V/cm. The free CuPc molecule, however, has an inversion center and a nondegenerate ground electronic state. Thus, only a second-order field dependence is expected for the free molecule. Incorporation of CuPc into the device certainly lowers its symmetry, but second-order Stark shifts are still most likely. Electric field induced shifts are therefore expected to be small and symmetrical in positive and negative bias. Finally, an electrochemical process might occur. The one-electron oxidations of CuPc and of H2Pc are well-known.”*’* Both macrocycle and metal localized oxidations can occur. These electrochemical processes would depend on the bias direction and would produce large changes in the vibrational spectrum. When we originally reported the dramatic bias dependence of the tunneling spectrum, we thought that oxidation of CuPc was the most likely cause of those differences. Thus, we expected to see dramatic bias-dependent changes in the Raman spectra of AI-AI0,-CuPc( 1 nm)-Pb devices. Quite the opposite actually occurred. The lower trace of Figure 8 shows the Raman spectrum obtained from an AI-AI0,-CuPc( 1 nm)-Pb junction at zero bias. The Pb layer in this particular diode had a resistance of 175 Q.
712 ENERGY
775
(cm-1)
Figure 9. Comparison of Raman spectra obtained from AI-AI0,CuPc( 1 nm)-Pb tunnel diodes biased at 0.0 and -1.3 V. Spectra were recorded with 647.1-nm excitation at a resolution of 3 cm-’ and one data point taken every 0.3 cm”.
As with other spectra taken from junctions of this type, no band shifts relative to uncovered films were observed. The upper trace in Figure 8 shows a typical spectrum obtained from an AlAlO,-CuPc( l nm)-Pb device biased at -1.33 V (Pb positive). The Pb layer in this case had a resistance of 53 9. Comparison of the traces in Figure 8 demonstrates that no significant changes in spectra occur as a function of bias. In fact, higher resolution scans like those presented in Figure 9 show that static field induced shifts are less than 2 cm-’. Taken together, the Raman spectra presented here provide solid evidence that CuPc is not significantly modified by being deposited onto the AIO, layer, by being coated with Ag or Pb, or by the application of a static voltage of magnitude less than 1.3 V across the device. Even the Stark shift is small, as expected, due to the symmetry and ground electronic state of CuPc. It appears, therefore, that the anomalies observed in the tunneling spectra of CuPc-containing diodes are due to phenomena associated with the tunneling process, such as resonance t ~ n n e l i n g . ~ ~Since **~ this issue is unrelated to bias or top metal induced changes in state of the MPc layer, it will be discussed in a forthcoming publication. Conclusions
Copper phthalocyanine and the free acid are hearty materials that resist chemical changes induced by thin film preparation methods (vacuum deposition). When incorporated into M-I-M devices, these materials are inert to electrochemical modification over a voltage range exceeding f 1.3 V. This is true even for near monolayer thickness CuPc and H2Pc films. While ultrathin MPc layer devices prior to M’deposition show strong Raman scattering only for p-polarized radiation, the completed devices (M’ = Pb or Ag) scatter both s- and p-polarized radiation. Resonance Raman spectroscopy is a powerful tool for studying molecular electronic materials. By adjusting the thickness of metal over layers, it is possible to construct working devices that give good Raman spectra. Because the Raman spectra of many organic semiconductors (phthalocyanines, tetracyanoethylenides, etc.) are highly sensitive to metal and charge state, it may be possible to identify the molecular species present in charge depletion regions, Schottky barriers, and many other interfacial regions present in microelectronic devices. Acknowledgment. We thank the Environmental Protection Agency (Grant R-816329-01-0) and the Department of Chemistry at Washington State University for their gracious support of this work. J.J.H. thanks the Grant in Aid program of the WSU Graduate School for a summer research assistantship. (22) Hipps, K. W.; Hoagland, J. J.; Dowdy, J. Langmuir, in press. (23) Person, B. N . J. Phys. Scr. 1988, 38,282.
