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PHYSICAL REVIEW B VOLUME 54, NUMBER 19 15 NOVEMBER 1996-I Chemistry and electronic properties of metal-organic semiconductor interfaces: Al, Ti, In, Sn, Ag, and Au on PTCDA Y. Hirose and A. Kahn Department of Electrical Engineering and Advanced Technology Center for Photonics and Optoelectronic Materials, Princeton University, Princeton, New Jersey 08544 V. Aristov Institute of Solid State Physics, Russian Academy of Science, Chernogolovka, Moscow District, Russia 142432 and Department of Physics, Northern Illinois University, DeKalb, Illinois 60115-2854 P. Soukiassian Department of Physics, Northern Illinois University, DeKalb, Illinois 60115-2854, and Commissariat à l’Energie Atomique, Département de Science des Matériaux, DRECAM-SRSIM, Bâtiment 462, Centre d’Etudes de Saclay, 91191 Gif-sur-Yvette Cedex, France V. Bulovic and S. R. Forrest Department of Electrical Engineering and Advanced Technology Center for Photonics and Optoelectronic Materials, Princeton University, Princeton, New Jersey 08544 ~Received 11 April 1996; revised manuscript received 25 July 1996! The chemistry and electronic properties of interfaces formed between thin films of the archetype molecular organic semiconductor 3, 4, 9, 10 perylenetetracarboxylic dianhydride ~PTCDA! and reactive and nonreactive metals are investigated via synchrotron radiation photoemission spectroscopy. In, Al, Ti, and Sn react at room temperature with the anhydride group of the PTCDA molecule, producing heavily oxidized interface metal species and thick interfacial layers with a high density of states in the PTCDA band gap. The penetration of the reactive metal species in the PTCDA film is found to be inversely related to their first ionization energy. The noble metals Ag and Au form abrupt, unreacted interfaces. The chemical and structural results correlate well with the electrical properties of the interfaces that show Ohmic behavior with the reactive metal contacts and blocking characteristics with the noble metals. The Ohmic behavior of the reactive metal contacts is ascribed to carrier hopping and/or tunneling through the reaction-induced interface states. @S0163-1829~96!02044-9# I. INTRODUCTION Organic molecular thin films ~OMTF’s! have attracted considerable attention over the past decade in view of their potential applications in electronic1,2 and optoelectronic devices.3–5 Extensive investigations of bulk properties such as electroluminescence,6 photoconductivity,7 and carrier transport8,9 have produced a fairly advanced understanding of these materials and have led to rapid developments in electroluminescent devices. Yet, in spite of the importance of contacts and heterojunctions in most practical organicorganic or organic-inorganic structures, only a few investigations have focused on the chemical, structural, and electronic properties of interfaces. Chemical bonding and adhesion of metals on polymers have been studied since the early 1980s,10,11 but comparatively little information has been obtained on the chemical nature and electronic properties of metal-OMTF contacts. OMTF-based electroluminescent diodes currently employ a low work function metal as the electron injector and indium tin oxide as the hole injector,6 assuming a classical Schottky model of metalsemiconductor contact where the barrier height depends predominantly on the metal work function.12 In fact, metal contacts and injection mechanisms into molecular crystals are poorly understood. The ‘‘softness’’ of van der Waals– 0163-1829/96/54~19!/13748~11!/$10.00 54 bonded organic molecular solids is likely to lead to considerable chemical and structural deviation from ideal interfaces when the contact is formed. Large densities of interface states induced by metallization chemistry and/or interdiffusion are to be expected, but have so far been largely neglected in the design of practical metal-organic semiconductor interfaces. It is therefore extremely important that systematic investigations of metal-OMTF interfaces be conducted to establish correlations between interface structure and chemistry, injection mechanisms, contact degradation, and the role of the contact region on device performance. In this paper, we report an extensive synchrotron radiation photoemission study of the formation of metal overlayers on an archetypal organic molecular semiconductor, i.e., 3,4,9,10 perylenetetracarboxylic dianhydride ~PTCDA!. We focus on the metal-OMTF chemistry and on the electronic structure of the interface, including the alignment of the metal Fermi level (E F ) with the electronic levels of the organic material. Carrier injection mechanisms at interfaces between metals and organic molecular solids are more complex than at metal-inorganic semiconductor contacts where concepts of free delocalized carriers, rigid band structure, and metal semiconductor barrier height are well established. Measurements of contact properties can be obscured by space-charge limitation effects in the OMTF. Furthermore, the injection of 13 748 © 1996 The American Physical Society 54 CHEMISTRY AND ELECTRONIC PROPERTIES OF . . . FIG. 1. Chemical structure of the PTCDA molecule showing the perylene core and the anhydride end groups. carriers in correlated systems such as molecular solids can shift molecular levels, and so can the removal of electrons from a molecule during the photoemission process. The evaluation of energy-level alignment by electrical or spectroscopic means must therefore be undertaken with a thorough understanding of the magnitude of these effects. This has been previously achieved on C60 , for example, with a combination of direct and inverse photoemission spectroscopies.13 The results of such a study of PTCDA are briefly mentioned here and will be reported in detail elsewhere.14 Because PTCDA is known as a hole-transport medium, the alignment of E F with filled molecular levels at the OMTF surface is a priori thought to be a determining factor in the electronic properties of the interface. This issue is investigated here for a series of six metals on PTCDA with different work functions and chemical reactivities, i.e., Al, In, Ti, Sn, Ag, and Au. We find that the energy-level alignment is largely independent of the metal, but that the electrical properties of the interface are essentially dominated by chemistry. The first four metals are highly reactive with PTCDA, resulting in the formation of a mixed interfacial layer with oxidation of the deposited species. Diffusion into the PTCDA film varies significantly from metal to metal and is believed to play a role in the interface electrical behavior. In contrast, Ag and Au are chemically inert and form abrupt interfaces with the OMTF. Finally, and most important, we observe a good correlation between electrical properties of the contacts and the interface chemistry. All the metals investigated lead to nearly identical interface positions of E F with respect to the molecular levels and, as such, should produce similar barriers. We propose that the Ohmic nature of the In, Sn, and Ti contacts15,16 is due to carrier hopping and/or tunneling through reaction-induced electronic states in the PTCDA band gap, whereas the blocking nature of contacts formed with Au and Ag is consistent with the abrupt and unreacted character of these interfaces. 13 749 Prior to introduction in UHV, PTCDA was purified by a three-cycle thermal gradient sublimation process.21,22 The growth of the PTCDA films, the deposition of the metal films, and the photoemission spectroscopy ~PES! measurements were all carried out in situ in an UHV chamber ~base pressure51310210 Torr! connected to the Grasshopper Mark II beam line of the Synchrotron Radiation Center of the University of Wisconsin. PTCDA films were deposited from a Knudsen cell heated at 250 °C–300 °C on roomtemperature ~RT! highly oriented pyrolitic graphite ~grade ZYA HOPG, Advanced Ceramics, Inc.!. The deposition rate, monitored by a quartz crystal microbalance, was typically 10–15 Å/min. It had been previously established that PTCDA grown under these conditions forms large, highly oriented single crystal domains several thousand angstroms in dimension.23–26 The thickness of the films, ranging between 400 and 800 Å, was chosen to ensure complete elimination of substrate signals in PES while minimizing charging effects. Ti was sublimated from a resistively heated Ti wire. All other metals were evaporated from beads or filaments placed on W wires. The metal films were deposited on RT PTCDA at rates ranging from 1 to 50 Å/min. The PES measurements were done with photon energies of 82 eV for the valence band ~VB!, In 4d, and Sn 4d core levels, 120 eV for Al 2p, 148 eV for Au 4 f , 150 eV for Ti 3 p, 340 eV for C 1s, and 450 eV for Ag 3d, providing in each case optimum surface sensitivity, cross section, and resolution. The oxygen 1s core level ~binding energy ;540 eV! was not recorded due to low photon output of the line at high energy. The photoelectron energy was determined with a double pass cylindrical mirror analyzer. With a typical slit width of 25 mm on the beam line, the total instrumental resolutions were ;0.2 eV for the VB, In, and Sn core levels, ;0.3 eV for the Al, Ti, and Au core levels, and ;1.0 eV for the C 1s and Ag 3d core levels. The E F reference was measured with a Au electrode in electrical contact with the sample. Finally, current versus voltage measurements previously performed on contacts between PTCDA and In, Al, Ti, Sn, Ag, and Au ~Refs. 15 and 16! were repeated in this work for the Ti-PTCDA system. The structure, consisting of a broad area, 1000-Å-thick Ti contact deposited on ~100! Si, an 1800-Å PTCDA layer and top 0.85-mm-diam, 500-Å-thick Ti dots deposited through a shadow mask, was entirely grown, contacted, and tested in vacuum ~1029 Torr!. II. EXPERIMENTAL CONSIDERATIONS III. RESULTS PTCDA molecules consist of a perylene core with four carboxyl groups at the 3, 4, 9, and 10 positions ~Fig. 1!, which, as will be seen below, play a dominant role in the interface reaction with metals. PTCDA has a 2.2-eV optical band gap.17 The material can be deposited in ultrahigh vacuum ~UHV! to form highly oriented thin films on a variety of substrates.18–21 The molecular structure of these films is believed to be very close to the monoclinic structure of bulk PTCDA with the ~102! crystallographic plane parallel to the substrate surface. The intermolecular plane distance in the ~102! direction is 3.21 Å. A. As-grown PTCDA The lowest spectra in Figs. 2~a!–2~c! show the C 1s core level, the extended VB, and the top of the VB measured on as-grown PTCDA, respectively. Core levels of molecular systems typically exhibit weak features and a broad asymmetric background associated with on-site ~intramolecule! and off-site ~plasmons, etc.! excitations caused by outgoing photoelectrons. PTCDA follows this behavior, as seen from the background discontinuity across the main C 1s component @Fig. 2~a!#. The peak on the high-binding-energy ~BE! 13 750 Y. HIROSE et al. 54 FIG. 2. ~a! C 1s core level, ~b! extended valence band, ~c! top of the valence band, and ~d! In 4d core level as a function of In deposition on PTCDA. side of the main component, however, is due to a different carbon chemical environment. The low-BE component A corresponds to carbon of the perylene core, while the high-BE component B corresponds to carbon of the anhydride end groups.27 The 3.9-eV shift between C 1s (A) and C 1s (B) is due to a transfer of valence charges from carbon to the more electronegative oxygen in the polarized CvO bond of the anhydride group ~Fig. 1!. This shift is nearly equal to that found in poly~ethyl metacrylate!, which has a chemical structure similar to that of PTCDA.28 As will be 54 CHEMISTRY AND ELECTRONIC PROPERTIES OF . . . shown below, the intensity of C 1s (B) is a sensitive indicator of the chemical activity involving oxygen in the anhydride group. The attachment of the electronegative anhydride groups to the perylene core causes the increase in the ionization potential of PTCDA @6.2 eV ~Refs. 29 and 30!# with respect to that of perylene @5.1 eV ~Ref. 29!#. Figure 2~b! shows the extended PTCDA VB with the characteristic p1 , p2 , and p3 peaks 0.6, 3.0, and 4.0 eV below the top of the occupied band, respectively, in good agreement with the results of previous ultraviolet PES studies.31,32 According to recent calculations based on Zerner’s intermediate neglect of differential overlap ~ZINDO! scheme,33 the highest occupied molecular orbital ~HOMO! p1 is primarily associated with p electrons of the perylene core of the molecule. Oxygen-related levels are 1–3 eV below the centroid of the HOMO. This is in contrast with a previous report that attributed the HOMO to the p bonding of the anhydride group.34 The determination of filled and empty states of a molecular solid via electron spectroscopies can be affected by charge correlation effects. In a narrow-band molecular solid, electron and hole wave functions are localized and particles are seen as belonging to systems with a small number of particles, i.e., single molecules. The removal of an electron by PES or the addition of an electron by inverse PES ~IPES! significantly affects the molecular levels of the remaining system, and this change is reflected in the measured energy of the photoemitted electron ~PES! or emitted photon ~IPES!. On C60 , charge correlation gives rise to an approximately 1-eV difference between the band gaps measured by the onset of optical absorption, upon which no removal or addition of an electron occurs, and by the combination of PES ~which gives the position of the HOMO! and IPES @which gives the position of the lowest unoccupied molecular orbital ~LUMO!#.13 The Hubbard U, which is the energy needed to remove an electron from one molecule and place it on another, is ;1.6 eV and is considerably larger than the width of the molecular bands which is estimated to be ;0.5 eV. C60 is said to be a highly correlated system. On the other hand, the effect is negligible in valence-band spectroscopy of conventional wideband semiconductors where the removal ~addition! of an electron from ~to! a large pool of delocalized particles has negligible impact on the energy of the system. In order to evaluate the magnitude of such correlation effects in PTCDA, we have performed PES-IPES measurements and found a 3.660.2 eV energy difference between the energy centroids of the HOMO and LUMO.14 Given that the Frenkel exciton energy measured in optical absorption is ;2.6 eV,17 the HOMO-LUMO gap leads to a value of the Hubbard U of ;1 eV which is slightly larger than the estimated band width of 0.7 eV. The results indicate therefore that correlation effects in PTCDA, although smaller than in C60, are not negligible. The strong overlap of p-electron wave functions in the direction perpendicular to the plane of the PTCDA molecules is a key factor in making this molecular system not as strongly correlated as C60. E F is found 1.860.1 eV above the top of the occupied band at the as-grown PTCDA surface, corresponding to ;0.4 eV below the bottom edge of the LUMO-derived states. In a nominally undoped material with low free carrier concentration like PTCDA, the bulk and surface positions of 13 751 FIG. 3. Schematic energy diagram of PTCDA showing the 2.2-eV HOMO-LUMO gap, the ionization energy ~IE!, and the alignment of the metal Fermi level based on a simple alignment of the metal and PTCDA vacuum levels. E F are entirely determined by small densities of defects or impurities and as such are not necessarily significant. At the same time, the surface position of E F acquires considerable importance upon metal deposition as we begin the discussion of energy-level alignment and interface electrical barriers. B. Reactive metal contacts: In, Al, Ti, and Sn The chemical reaction between In and PTCDA is apparent upon deposition of the smallest amount of metal ~nominally 1 Å!. One observes a rapid attenuation of C 1s (B) @Fig. 2~a!#, indicative of an interaction that reduces the carbonyl carbon in the upper molecular layers of the OMTF. The C 1s (B)/C 1s (A) ratio plotted versus metal coverage in Fig. 4 also shows that the carbon atoms mostly affected by the reaction are in the molecular end groups. The reduction of the carbonyl carbon requires a charge transfer from the In atom, which becomes positively charged, as indicated by the BE shift of the In 4d core level with respect to metallic In @Fig. 2~d!#. At the same time, peaks R 1 and R 2 appear in the PTCDA gap @Figs. 2~b! and 2~c!#. A third VB feature, R 3 @Fig. 2~b!#, is also consistent with a reaction between In and the anhydride group since, according to the ZINDO calculation,33 the states due to the end groups are located at energies near R 3 . These observations are consistent with a strong chemical interaction between In and the anhydride group, resulting in either In-O-C-R ~In attached to the molecule! or In-oxide complexes detached from the molecule. The perylene core of FIG. 4. Plots of the intensity ratio of the B and A components of the C 1s core level C1s (B)/C1s (A) as a function of deposition of various metals on PTCDA. 13 752 54 Y. HIROSE et al. FIG. 5. ~In 4d!/~C 1s! integral intensity ratio as a function of In deposition. the PTCDA molecule retains its original structure, as seen from the C 1s (A) component, which remains unchanged except for a gradual energy shift of ;0.7 eV toward lower BE @Fig. 2~a!#. Given that no other comparable molecular level shifts are observed @e.g., Fig. 2~b! around 6–8 eV below the top of the valence band#, the 0.7-eV shift can be attributed to either a redistribution of charge in the molecule due to the reduction of the carbonyl carbon or additional screening of the photogenerated core hole upon increasing In coverage. In either case, the rigid shift of molecular levels near the surface, referred to as ‘‘band bending’’ in conventional inorganic semiconductors, is small. The In-PTCDA reaction produces multiple chemical states that give rise to a broad-shaped core level on which the In spin-orbit splitting cannot be directly resolved. The 1.7-eV shift of the In core level with respect to metallic In @Fig. 2~d!# is considerably larger than that found in oxides such as In2O3 ~1.1 eV!.34 This anomalously large shift is attributed in part to the reduction in screening of the photogenerated In core hole in the PTCDA matrix as compared to that in metals or semimetals such as In2O3 . The spectra of Fig. 2~a!–2~d! also demonstrate that In does not accumulate at the PTCDA surface until the nominal coverage reaches several hundred angstroms. First, these surface-sensitive spectra remain essentially unchanged beyond the initial 1-Å coverage. The integrated intensity ratio In 4d/C 1s remains constant beyond 10 Å ~Fig. 5!. Second, the absence of density of states at E F @Fig. 2~c!# and the nonmetallic energy position of the In core level @Fig. 2~d!# show that the overlayer remains nonmetallic even at coverages of the order of 100 Å. Clustering, which could explain the first effect, is therefore ruled out by the second. Assuming that the sticking coefficient of the adatom is ;1 ~comparable to that found for the other metals considered below!, we conclude that In initially diffuses deep into the PTCDA film at room temperature without appreciable surface accumulation. The resulting In/PTCDA interface is a thick intermixed layer with a high density of states in the PTCDA gap region @peaks R 1 and R 2 in Fig. 2~c!#. We have previously proposed27 a diffusion mechanism based on the ionization of the metal adatoms and the subsequent ion-ion repulsion in the PTCDA matrix. This issue will be revisited in Sec. IV when we discuss the dependence of diffusion of the reactive metals on ionization energy and the role of the gap states induced by the reaction. Al/PTCDA, Ti/PTCDA, and Sn/PTCDA interfaces show both similarities and differences with the In/PTCDA interface. Like In, these metals ~a! react with the anhydride groups of the molecule, as evidenced by the rapid disappearance of C 1s (B) in all three cases @Figs. 4, 6~a!, and 7~a!#, and ~b! induce states in the gap above the PTCDA HOMO @Figs. 6~b! and 7~b!#. The energy position and intensity of the gap state peaks vary somewhat from one interface to the next, but the characteristic R 1 -R 2 doublet of the reacted In/ PTCDA interface is present at all three interfaces. Similarly, C 1s (A) shifts gradually toward lower BE in all three cases due to charge transfer to the molecule and additional screening by the metal. The main difference among the four reactive interfaces concerns the degree to which the metal adatoms diffuse into the organic film. Al behaves like In at low coverage and diffuses into the film, as shown by the absence of metallic Al 2p component at thicknesses 20 Å @Fig. 6~c!# and by the absence of a significant density of states at E F below 20–50 Å @Fig. 6~b!#. However, a metallic Al overlayer is achieved at a coverage considerably smaller than that required for In. The persistence of the C 1s (A) peak and of the oxidized Al 2 p peak up to coverages of at least 180 Å also show that Al clusters form on top of the reacted layer. Ti behaves similarly, although diffusion into the organic film and clustering in the thick metal film are somewhat reduced with respect to Al. Finally, a marked reduction in interdiffusion is observed with Sn. The metallic component of the Sn 4d core level appears with the initial 1-Å deposition @Fig. 7~c!# and, although the high BE oxidized component of the core level is present, the metallic part dominates the spectrum above 2 Å. Furthermore, a significant density of states appears at E F at or below 10 Å @Fig. 6~b!#, confirming that the overlayer rapidly becomes metallic for the thinnest layers. Diffusion into PTCDA is therefore considerably suppressed compared to the other reactive metals studied. The persistence of the C 1s core-level signal beyond a nominal coverage of 125 Å @Fig. 7~a!# shows that clustering is also prevalent in the thick Sn layer. C. Unreactive metal contacts: Ag and Au Ag and Au show considerably less chemical reactivity with PTCDA than the four metals presented above. Figures 8~a!–8~c! present the evolution of the C 1s core level, top of the VB, and Ag 3d core level spectra as a function of Ag deposition, respectively. The C 1s (B)/C 1s (A) ratio remains approximately constant throughout the deposition of the first 20 Å @Figs. 4 and 8~a!#, indicating that the anhydride group is not affected by the presence of Ag. The PTCDA HOMO peak also retains its original shape @Fig. 8~b!# and reaction-induced gap state peaks R 1 , R 2 , and R 3 do not appear. In addition, the Ag 3d core level appears from the first deposition at the energy of metallic Ag and exhibits a wellresolved spin-orbit splitting @Fig. 8~c!#. Thus, unlike for reactive metals, the states in the PTCDA gap region that appear at coverages of 5 Å and extend to E F at 20 Å are due to metallic Ag rather than to the products of a chemical reaction. The interface is therefore unreacted and abrupt. The C 1s signal persists for coverages larger than 40 Å due to clustering of the metallic layer. Finally, the 0.5-eV shift of C 1s toward lower BE with the initial coverage is again ascribed either to an increased screening of the core hole by the elec- 54 CHEMISTRY AND ELECTRONIC PROPERTIES OF . . . 13 753 FIG. 6. ~a! C 1s core level, ~b! top of the valence band, and ~c! Al 2p core level as a function of Al deposition on PTCDA. trons of the metal atom or to a redistribution of charges in the molecule. ‘‘Band bending’’ is negligible, as indicated by the constant energy position of the HOMO as a function of metal deposition @Fig. 8~b!#. Au shows essentially the same behavior as Ag, resulting in an unreacted and abrupt interface, as shown by the persistence of C 1s (B) ~Fig. 4! and the development of metallic states at E F . In contrast to Ag, however, Au appears to follow a quasi-two-dimensional growth mode leading to a rapid attenuation of the C 1s intensity with increasing coverage. IV. DISCUSSION A. Chemical reactivity of metals with PTCDA In, Al, Sn, and Ti react with PTCDA, whereas Ag and Au do not. The reaction, which involves predominantly the an- hydride end groups of the organic molecule and leads to the oxidation of the adatoms and the reduction of the carbonyl carbon, is the natural consequence of the high affinity of these metals for oxygen. The heats of formation of the reactive metal oxides ~Table I! are an order of magnitude large than those of the noble metal oxides @DH 298 ~oxide!# and are higher than those of the metals themselves @DH 298 ~vapor!#, justifying the formation of oxides rather than of metal-metal bonds. The rather open bonding configuration of oxygen in the end groups of the molecule also favors the reactions observed here. The heats of formation of noble metal oxides, on the other hand, are smaller than those of the metals, consistent with the absence of observed reaction and with the formation of metallic clusters or layers. The level of oxidation of the reactive metal species and the nature of the oxide complexes detached or attached to the 13 754 Y. HIROSE et al. 54 FIG. 7. ~a! C 1s core level, ~b! top of the valence band, and ~c! Sn 4d core level as a function of Sn deposition on PTCDA. molecule remain difficult to unambiguously ascertain by core-level photoemission spectroscopy because of possible final-state effects. The dilution of oxidized species in a low dielectric constant matrix such as PTCDA, where core-hole screening is poor, can result in a shift of the metal core-level spectra toward high BE beyond the amount corresponding to the real oxidation level. As an example, the shift of the reacted In 4d core level relative to metallic In is 1.7 eV, or approximately 0.6 eV higher than the chemical shift corresponding to In in In2O3 .34 Similarly, the shift of reacted Al in Fig. 6~c! is 2.3 eV with respect to metallic Al, whereas it is 1.0–1.5 eV in Al2O3 .35 Whether the oxidation level is effec- tively higher than in these oxides or the shift is in part an artifact of the measurement is therefore unknown. In either case, indiffusion and a high oxidation level are consistent with the formation of metal-oxide complexes detached from the molecules or with interstitial metal atoms bound to the end groups of surrounding organic molecules, in bridge-type positions. B. Diffusion versus ionization potential We have seen large differences in the level of diffusion of the various reactive metals into PTCDA, as inferred from 54 CHEMISTRY AND ELECTRONIC PROPERTIES OF . . . 13 755 FIG. 8. ~a! C 1s core level, ~b! top of the valence band, and ~c! Ag 3d core level as a function of Ag deposition on PTCDA. different rates of attenuation of substrate photoemission signals with metal deposition and from the different amounts of deposited metal necessary to reach a metallic overlayer. The first of these two criteria is quantified in Fig. 9, where the normalized C 1s integrated intensity is plotted as a function of metal coverage. In stands out at the high limit of diffusion into the OMTF. On the other hand, a rapid initial decrease followed by a slower attenuation supports limited diffusion and surface clustering for Sn, consistent with evidence for a metallic overlayer at very low coverage @Figs. 7~b! and 7~c!#. Al and Ti fall between these two extremes. Below 25 Å, the substrate signals remain strong, which we ascribe both to diffusion and clustering. The latter is most evident at thicknesses greater than 20–25 Å with Al, as indicated by the metallic Al 2 p component @Fig. 6~c!# and the density of states at E F . With increasing coverage, clustering dominates and eventually leads to surface metallization. We observe that In is followed by Al, Ti, and Sn in decreasing order of diffusivity into PTCDA ~Fig. 9!. The PTCDA films were all prepared under identical conditions and contained similar densities of defects and grain boundaries. Differences in interface diffusion or reaction must therefore be attributed to specific properties of the metallic species. The atomic radius ~Table II! and other measures of atomic size that could be relevant to the motion of atoms in a loose matrix like PTCDA do not correlate with the relative amounts of diffusion of the metals. The 4–5 Å-wide channels between molecular stacks are large compared to metal atomic or ionic radii ~Table II! and could, in principle, provide an easy diffusion path for all the metals investigated here. Similarly, the heat of formation of the oxide that determines the affinity of adatoms for oxygen in the bulk of the film does not justify the extensive penetration of In as compared to Al, Ti, or Sn. Given our limited under- 13 756 TABLE I. Heat of formation of metal oxides and vapors ~Ref. 37!. Element Al Ti In Sn Ag Au 54 Y. HIROSE et al. DH 298 ~oxide! ~kJ/mol! 21675.7 ~Al2O3! 2542.7 ~TiO! 21520.9 ~Ti2O3! 22459.1 ~Ti3O5! 2944.0 ~TiO2! 2925.9 ~In2O3! 2280.7 ~SnO! 2577.6 ~SnO2! 231.1 ~AgO! 23.4~Au2O3! DH 298 ~vapor! ~kJ/mol! 330 473 TABLE II. First ionization energy ~IE! and atomic radius of metals ~Ref. 38!. Element IE ~eV! Atomic radius ~Å! In Al Ti Sn Ag Au 5.79 5.99 6.82 7.34 7.58 9.23 2.00 1.82 2.00 1.72 1.75 1.79 240 301 C. Electrical properties of the metal-PTCDA interfaces 285 368 standing of the structural and chemical details of these interfaces, the only parameter that appears to provide a coherent explanation of the diffusion phenomenon is the first ionization energy ~IE! of the metal atom. Ionization leads to Coulomb repulsion between positively charged ions at interstitial sites. In analogy with the well-known intercalation process in layered materials,36 this repulsion could act as the driving factor in the diffusion process. Because of their relatively low first IE’s, In and, to a lesser degree, Al are more easily ionized than other metals in the PTCDA matrix. On the other hand, metal atoms with higher IE such as Sn and the noble species Ag and Au remain neutral at the surface of the film and form clusters. As formulated, this mechanism presents a simplified view of a complex process that results from a detailed balance between the ability of the metal species to move through the film, the mechanism that drives their penetration deep into the film, and the chemical reaction with molecules that should limit the motion of the metal species. It leads, however, to a likely scenario according to which the diffusing species are pushed by Coulomb repulsion through the matrix, passing unavailable molecular sites already occupied by metal species and reaching unreacted PTCDA layers. Diffusion eventually stops when the organic film is saturated or when the front of unreacted molecules is too deep below the surface of the film. We approach the problem of the metal-organic contact in terms of interface energy-level alignment and assume that the relevant energy barrier for a contact with a hole-transport layer such as PTCDA is between E F in the metal and the HOMO in the organic film. In order to extract this barrier from the photoemission data, we correct the position of E F measured on the as-grown PTCDA surface for subsequent rigid shifts, or band bending, induced by the deposition of the metal. The rigid shift, which is generally measured through a combination of core level and valence band shifts, is relatively difficult to measure precisely because of distortions of the organic level line shapes. Thus it is measured either directly from the shift of the HOMO when its shape is preserved upon deposition of the unreactive metals ~Ag and Au! or from deeper, unperturbed parts of the occupied band and from the C 1s core level when the HOMO is distorted by chemical reaction ~with In, Al, Ti, and Sn!. On the as-grown PTCDA surface, the high-energy tail of the HOMO is consistently found at 1.860.1 eV below E F @bottom of Figs. 2~c!, 6~b!, 7~b!, and 8~b!#. Upon metal deposition, the shift of the C 1s core level, beyond the initial 0.5-eV shift due to charge redistribution in the molecule @Figs. 2~a!, 6~a!, 7~a!, and 8~a!#, and the shift of the occupied band @Figs. 2~b! and 8~b!# remain below 200 meV. We conclude therefore that the HOMO in the PTCDA film remains 1.5–1.8 eV below E F , giving a barrier that is larger than half of the band gap and essentially independent of the work function of the deposited metal ~Table III!. When the interface is abrupt and free of gap states, the contact properties should be determined by the 1.5–1.8 eV energy offset and the barrier should lead to a blocking contact. This is indeed the case with the two noble metals investigated here, which have been reported to produce blocking contacts on PTCDA ~Table III!.16 A schematic diagram of TABLE III. Work function of metals and barriers on PTCDA ~Refs. 15 and 16!. FIG. 9. C 1s integral intensity plotted as a function of metal coverage. Metal Work function ~eV! Contact In Sn Ti Al Ag Au 4.12 4.42 4.33 4.25 4.52 5.1 ohmic ohmic ohmic blocking blocking blocking 54 CHEMISTRY AND ELECTRONIC PROPERTIES OF . . . 13 757 FIG. 11. Current vs voltage characteristics of a Ti/1800 Å PTCDA/Ti structure grown and tested in UHV. The diameter of the Ti top contact is 0.85 mm, corresponding to a device area of 5.731023 cm2. Al oxide layer could have formed, leading to poor interface conductivity. Investigations of Al contacts formed in UHV remain to be done. FIG. 10. Schematic diagram of energy-level alignment at ~a! unreactive and ~b! reactive metal-PTCDA contacts. the corresponding interface band alignment is given in Fig. 10~a!. When the interface is reacted and the density of gap states is large, the contact properties could be drastically different. Far from the interface, the Fermi level of the metal and the PTCDA HOMO remain separated in energy by 1.5– 1.8 eV. In the interface region, however, these two levels are connected by the large density of reaction-induced states R 1 and R 2 distributed throughout the interfacial layer, as shown schematically in Fig. 10~b!. The contact becomes Ohmic, as was indeed previously found for In/PTCDA, Ti/PTCDA, and Sn/PTCDA.15,16 Hole conduction in the interface region occurs via hopping ~at moderate temperature! and/or tunneling between gap states into the PTCDA occupied molecular levels. In order to verify the early current vs voltage measurements, we grew a Ti/1800 Å PTCDA/Ti structure and characterized its electrical properties without removal from the UHV environment. The current-voltage (I-V) characteristics, given in Fig. 11, show a linear dependence clearly indicative of an Ohmic contact with an area of 5.731023 cm2. The resistance given by the slope of the straight line is due to conduction through the PTCDA layer with a resistivity of 0.14 MV cm, consistent with resistivities for PTCDA films exposed to air.17 An exception to the Ohmic nature of the reactive contacts was noted with early measurements on Al/PTCDA, which appeared to be blocking.16 The photoemission measurements @Figs. 6~a!–6~c!# indicate that the contact should be Ohmic based on evidence of reaction and diffusion and the large density of interface gap states. However, the early electrical measurements were performed in air on contacts deposited under vacuums ;1026 Torr, where an insulating interfacial V. CONCLUSIONS This investigation suggests that interface chemistry has a profound impact on the electrical properties of contacts formed between metals and organic molecular semiconductors such as PTCDA. Among the metals investigated, Al, Ti, In, and Sn are found to react extensively with the oxygencontaining anhydride groups of the PTCDA molecule, creating a large density of interface states in the PTCDA energy gap. The reaction is consistent with a high affinity for oxygen. In and Al are found to diffuse deep into the organic film. We note that diffusion correlates with ionization energy whereby ionized species are driven into the OMTF by Coulomb repulsion. The thickness of the interfacial layer is found to be well above 100 Å with In. The two noble metals Ag and Au are chemically inert on PTCDA, consistent with the low heat of formation of their oxides. The resulting interfaces are abrupt and free of gap states. This difference in interface chemistry, morphology, and electronic structure is consistent with the different electrical properties of the contacts formed with these metals. The Ohmic nature of the reactive metal contacts is attributed to carrier conduction through a high density of chemistry-induced interface gap states, whereas the blocking character of the noble-metal contacts is expected from the abruptness of the interface and the 1.5–1.8-eV barrier between the Fermi level of the metal and the PTCDA HOMO. Although specific chemical species and interface stoichiometries are as yet undetermined, our results suggest that the classical Schottky model of contact formation is inapplicable in the case of many metal-organic interfaces. ACKNOWLEDGMENTS This work was supported primarily by the MRSEC program of the National Science Foundation under Grant No. 13 758 Y. HIROSE et al. DMR-9400362. Support by a National Science Foundation grant at Northern Illinois University ~DMR-92-23710! is also gratefully acknowledged. V.A. also thanks the NIU Graduate School for support. Finally, this work was based upon re- 1 J. H. Burroughes, C. A. Jones, and R. H. 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