Download Chemistry and electronic properties of metal

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
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© 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!
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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
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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-
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