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Langmuir 2007, 23, 4856-4861
Deposition of Tetracene on GaSe Passivated Si(111) B. Jaeckel,* T. Lim, A. Klein, and W. Jaegermann Surface Science DiVision, Institute of Materials Science, Darmstadt UniVersity of Technology, Petersenstrasse 23, 64287 Darmstadt, Germany
B. A. Parkinson Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523 ReceiVed May 13, 2006. In Final Form: February 9, 2007 The growth of tetracene on GaSe half-sheet passivated Si(111) is investigated under ultrahigh vacuum (UHV) using low-energy electron diffraction (LEED) and photoelectron spectroscopy (PS). A highly ordered thin-film growth was observed in the initial stages of the deposition process. All proposed structures form a coincidence lattice with the underlying substrate, due to the influence of the molecule-substrate interactions and are built up by either flat lying tetracene molecules at low coverage or tilted molecules at higher coverages. Photoelectron spectroscopy (XPS/UPS) shows that the deposited tetracene molecules induce band bending in the silicon substrate. No band bending was observed in the tetracene film, and an interface dipole potential of 0.45 eV was measured between the GaSe passivated Si(111) surface and the tetracene film.
I. Introduction In recent years, electronic devices constructed with organic materials are becoming increasingly important. The properties of organic layers allow the preparation of inexpensive, leightweight, and flexible thin films for the realization of, e.g., flexible displays. Organic field effect transistors (OFETs) are also an essential component of an organic integrated circuit. To optimize device performance, it is crucial to control contact resistance, charge injection, and charge transport1 at the organic/substrate interface. Therefore, control of the interface morphology is a key issue since the mobility of charge carriers is strongly influenced by the crystalline quality of the organic film and the crystal orientation.1,2 Meyer zu Heringdorf et al.3 investigated the growth of pentacene on Si(100) and concluded that epitaxial growth of organic thin films closely mimics the growth of inorganic thin films. A theoretical treatment of this idea, given by Veerlaak,4 presents a detailed evaluation of the growth of pentacene and tetracene on inert substrates. Depending on growth rate and growth temperature, a 2-dimensional or 3-dimensional growth is predicted. The most frequently studied linear aromantic molecules for growth on metal single-crystal are pentacene and tetracene. Detailed studies of initial organic layer growth were performed for pentacene on Au(111)5-8 and Cu(110)9-11 and for tetracene * To whom correspondence should be addressed. Present Address: Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523. E-mail:
[email protected]. Tel: 970-491-3886. Fax: 970491-1801. (1) Gundlach, D. J.; Lin, Y. Y.; Jackson, T. N.; Nelson, S. F.; Schlom, D. G. IEEE Electron DeVice Lett. 1997, 18, 87. (2) Lin, Y. Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE Electron DeVice Lett. 1997, 18, 606. (3) Meyer zu Heringdorf, F.-J; Reuter, M. C.; Tromp, R. M. Nature 2001, 412, 517. (4) Verlaak, S.; Steudel, S.; Heremans, P.; Janssen, D.; Deleuze, M. S. Phys. ReV. B 2003, 68, 195409. (5) Kang, J. H.; Zhu, X.-Y. Appl. Phys. Lett. 2003, 82, 3248. (6) Schroeder, P. G.; France, C. B.; Park, J. B.; Parkinson, B. A. J. Appl. Phys. 2002, 91, 3010. (7) Schroeder, P. G.; France, C. B.; Park, J. B.; Parkinson, B. A. J. Phys. Chem. B 2003, 107, 2253.
on Ag(111),12 Ag(110),13 and Cu(110).14 More than one stable two-dimensional (2D) structure was often observed depending on the coverage and deposition parameters. Only a few thin film investigations of linear aromatic molecules were performed on semiconducting or insulating substrates. The deposition of tetracene on a bare Si(100) surface results in a chemical reaction between the tetracene molecule and the surface silicon atoms.15,16 A similar behavior was also found for phenylacetylene17 and pentacene18 on Si(100). To avoid chemical reactions, the reactivity of the surface dangling bonds should be reduced by use of a passivation or buffer layer. To prevent chemical reactions the deposition of pentacene was studied on Si(111) with a silver x3 × x3R30° buffer layer where highly ordered pentacene structures forming a coincidence-lattice with underlying Si(111) lattice were found.19 In this study, we used silicon, the most common material in the microelectronics industry, as base material making interfaces between layers deposited on silicon of interest. As described above, chemical reactions can occur on bare silicon and strongly influence the properties of the organic molecule. To avoid any possible reaction between the molecules and the Si surface we used a Si(111) surface passivated by a GaSe half-sheet, schematically shown in Figure 1, panels a and b. This surface (8) France, C. B.; Schroeder, P. G.; Forsythe, J. C.; Parkinson, B. A. Langmuir 2003, 19, 1274. (9) Lukas, S.; Witte, G.; Wo¨ll, Ch. Phys. ReV. Lett. 2002, 88, 028301. (10) Lukas, S.; So¨hnchen, S.; Witte, G.; Wo¨ll, Chr. ChemPhysChem 2004, 5, 266. (11) So¨hnchen, S.; Lukas, S.; Witte, G. J. Chem. Phys. 2004, 121, 525. (12) Langner, A.; Hauschild, A.; Fahrenholz, S.; Sokolowski, M. Surf. Sci. 2005, 574, 153. (13) Lu, B.; Zhang, H. J.; Huang, H.; Mao, H. Y.; Chen, Q.; Li, H. Y.; He, P.; Bao, S. N. Appl. Surf. Sci. 2005, 245, 208. (14) Chen, Q.; McDowall, A. J.; Richardson, N. V. Langmuir 2003, 19, 10164. (15) Rada, T.; Chen, Q.; Richardson, N. V. J. Phys.: Condensed Matter 2003, 15, S2749. (16) Rada, T.; Chen, Q.; Richardson, N.V. Phys. Status Solidi B 2004, 241, 2353. (17) Kim, K. Y.; Song, B. K.; Jeong, S. M.; Kang, H. J. Phys. Chem. B 2003, 107, 11987. (18) Weidkamp, K. P.; Hacker, Chr. A.; Schwartz, M. P.; Cao, X.; Tromp, R. M.; Hamers, R. J. J. Phys. Chem. B 2003, 107, 11142. (19) Guaino, P.; Cafolla, A. A.; Carty, D.; Sheerin, G.; Hughes, G. Surf. Sci. 2003, 540, 107.
10.1021/la061361g CCC: $37.00 © 2007 American Chemical Society Published on Web 03/24/2007
Deposition of Tetracene
Figure 1. Schematic drawing of the GaSe passivated Si(111) surface (a) with both possible domains (Si-like and GaSe-like) and the tetracene molecule with the van der Waals radius.15 The grayscale polygon represents the planar shape of the tetracene-molecule in a little bit more detail.
is known to be van der Waals-like and provides an atomically flat substrate for epitaxial studies with both inorganic and organic materials. The properties of the GaSe passivated surface are described in detail in many publications.20-26 This surface was used to explore the thin film growth of tetracene (see Figure 1c) in the submonolayer to multilayer regime and to demonstrate that highly ordered film growth is possible at room temperature, including interesting interfacial electronic properties. II. Experimental Setup The experiments were performed using a multitechnique surface analysis system with a base pressure better than 5 × 10-10 mbar located at the Surface Science Division at the Darmstadt University of Technology. For this study, a commercial scanning tunneling microscope (RT-Omicron) and an OCI-LEED optic attached to a VG-ESCALAB MK II photoelectron spectrometer were used. The substrate was an n-doped (1015cm-3:P, miscut ≈ 1°) silicon wafer. Before the GaSe half-sheet passivation layer was deposited, a clean Si(111) 7 × 7 surface was first obtained by standard direct current and flash heating procedures. The GaSe half-sheet passivation layer was prepared at a substrate temperature of 550 °C with a GaSe-flux of approximately 2-3 Å/min for about 25 min. After preparation, a sharp 1 × 1 LEED pattern and atomically resolved STM images were obtained which are typical for the GaSe halfsheet passivated surface.20,27 Tetracene was deposited onto this surface at a rate of 20-25 Å/min with a source temperature of ≈145 °C and with the substrate held at room temperature. The surface was then extensively studied with low-energy electron diffraction (LEED) and photoelectron spectroscopy (PS). We also performed scanning tunneling microscopy (STM) measurements for various deposition steps. During no deposition/coverage stage were we able to collect atomic/molecular (20) Rudolph, R.; Pettenkofer, Chr.; Bostwick, A. A.; Adams, Ohuchi, J. A., F.; Olmstead, M. A.; Jaeckel, B.; Klein, A.; Jaegermann, W. New J. Phys. 2005, 7, 108. (21) Fritsche, R.; Wisotzki, E.; Islam, A. B. M. O.; Thissen, A.; Klein, A.; Jaegermann, W.; Rudolph, R.;Tonti, D.; Pettenkofer, C. Appl. Phys. Lett. 2002, 80, 1388. (22) Fritsche, R.; Wisotzki, E.; Thissen, A.; Islam, A. B. M. O.; Klein, A.; Jaegermann, W.; Rudolf, R.; Tonti, D.; Pettenkofer, C. Surf. Sci. 2002, 515, 296. (23) Rudolph, R.; Pettenkofer, C.; Klein, A.; Jaegermann, W. Appl. Surf. Sci. 2000, 167, 122. (24) Meng, S.; Schroeder, B. R.; Olmstead, M. A. Phys. ReV. B 2000, 61, 7215. (25) Koe¨bel, A.; Zengh, Y.; Pe´troff, J. F.; Boulliard, J. C.; Capelle, B.; Eddrief, M. Phys. ReV. B 1997, 56, 12296. (26) Jedrecy, N.; Pinchaux, R.; Eddrief, M. Phys. ReV. B 1997, 56, 9583. (27) Jaeckel, B. Ph.D. Thesis, Darmstadt University of Technology: Darmstadt, Germany, 2005.
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Figure 2. LEED patterns obtained during the stepwise deposition of tetracene onto GaSe half-sheet passivated Si(111). (a) Bare Si(111)/GaSe surface, (b) after 5 s, (c) after 10 s, and (d) after 30 s of tetracene deposition with a rate of 20 to 25 Å/min. resolved STM images. The inability to obtain high quality STM images of the investigated system might be due to very weak substrate/ tetracene interactions, resulting from the van der Waals like GaSe passivation layer, leading to movement of tetracene molecules below the STM tip while scanning over the single molecules. Low temperature, to reduce surface diffusion, STM measurements, which are not possible in the used UHV aparatus, are probably required to obtain STM images and so to verify the proposed 2D structures determined from the LEED pattern. After each step, a new GaSe half-sheet was prepared to reduce the influence of accumulated radiation damage induced by repeated X-ray-PS/Ultraviolet-PS and LEED measurements. Herein, we focus on the results obtained by the LEED and UPS measurements.
III. Experimental Results A. Low-Energy Electron Diffraction. LEED patterns were measured after deposition of various coverages of tetracene onto the GaSe half-sheet terminated Si(111) surface (see Figure 2). LEED imaging was performed for electron beam energies from 25 to 53 eV where 53 eV is the best energy to collect an LEED pattern from the substrate and 37 eV is best for the tetracene overlayer. Energies deviating from 37 eV result in a decrease in spot intensity and usually sharpness that was not possible to fully correct by adjusting the focus. Up to coverages of one to two monolayers of tetracene, the substrate reflections (larger spots in Figure 2) in the LEED pattern coexist with spots due to an ordered tetracene film on the surface. With increasing coverage, the substrate reflections are getting weaker and weaker (Figure 2, panels a-c), but due to charging, the tetracene reflections are also getting more and more blurry (Figure 2d). The distance between the GaSe half-sheet (larger ones) spots corresponds to 3.84 Å, equal to the 2D-lattice constant of the Si(111) surface that we used as the reference distance for determinations of all tetracene 2D lattice parameters. The principal symmetry of the GaSe half-sheet is 3-fold. Because of the existence of two different domain types of the GaSe half-sheet (Si-like and GaSe-like,28 see Figure 1, panels a and b), the LEED pattern is usually 6-fold (see Figure 2a), and as a result, six different domains rotated by 60° are expected in the LEED pattern for the overlayer. We attempted to obtain the LEED pattern from films thicker than displayed in Figure 2, but this was not possible due to strong surface charging. Only a weak pattern was briefly (28) Ohta, T.; Klust, A.; Adams, J. A.; Yu, Q. M.; Olmstead, M. A.; Ohuchi, F. S. Phys. ReV. B 2004, 69, 125322.
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Table 1. Measured Distances for the LEED Pattern Obtained at 37 eV (Figure 2c)a
a
OA
OB
BC ) DE
CD
BD ) CE
BE
3.84
5.3 ( 0.2
75.0 ( 3
27.0 ( 1
18.0 ( 1
13.5 ( 0.5
OG ) OF
FG
OH ) OK
OI ) OJ
BG ) DG
CF ) EF
4.4 ( 0.2
54.0 ( 2
3.6 ( 0.2
3.4 ( 0.2
18.0 ( 1
18.0 ( 1
HK
IJ
HJ ) IK
BI′ ) EJ
21.0 ( 1
42.0 ( 2
23.5 ( 1
18.0 ( 1
All distances are given in Å and the labels are shown in the scheme in Figure 3b. Table 2. Measured Angles from the LEED Pattern Obtained at 37 eV (Figure 2 c) between Important Directions
∠BOA
∠ COA
∠ GOA
∠OBCdOEC ∠ ODCdOCE
∠ OCD
∠ OBE
∠ BDGdDGBdGBA
∠CEFdEFCdFCE
20°
23°
25°
82°
83°
78°
60°
60°
observable after the sample was moved below the electron gun. Accumulation of radiation damage can be ruled out because UPS spectra collected before and after the LEED measurements were unchanged, and for each deposition step, a new surface/ interface was prepared, reducing any accumulated damage. Substrate and over-layer reflections are labeled in Figure 3, and a summary of the measured spot distances is listed in real space dimensions in Table 1 and all important angles between the two directions in Table 2. The increased error margins for the larger distances result from the difficulty in localizing the exact maximum and the short length in the reciprocal space images. The center of the additional reflections B-G are rotated by 30° with respect to the substrate reflections, and the four reflections around the substrate spots are most likely higher order spots from the 2D-structure of tetracene. The origin of the obtained reflections B-G can be interpreted in two ways as follows. The likelihood of each interpretation will then be discussed. The first interpretation leads to three different structures, where the first is created by the vectors OB and BE, the second by OC and CD, and the third by OG and GF (see the scheme in Figure 3 for labels). Due to the substrate symmetry, each structure is present in 6 domains, rotated by 60°, on the surface. The parameters of these three structures are summarized in Table 3. The measured lengths of the first two structures lead to the conclusion that the tetracene molecules are lying flat on the surface and both structures have a similar molecular density (≈1.4 × 1014 molecules per cm2) with the main difference being the length of the unit cell (see models 1a and 2b in Figure 4). The second molecular row in model 2 is rotated to allow two molecules in the unit cell to establish the larger unit cell vector b2. The third structure shows a very large unit cell containing four molecules which are tilted along their long axis (see Figure 4c).
∠OBI′dOEJ 98°
Table 3. Listing of the Determined Primitive Lattice Constants for the Structures 1-3 for the First Interpretationsa primitive lattice structure
b1 [Å]
b2 [Å]
R [°]
1 2 3
5.3 ( 0.2 5.3 ( 0.2 4.4 ( 0.2
13.5 ( 0.5 27.0 ( 1.0 54.2 ( 2.0
102 ( 3 98 ( 3 95 ( 3
a The error margins result from the relative distances between the spots and the accuracy of locating the spot maximum.
Figure 4. Models of the different tetracene-structures resulting from the first interpretation. (a) Structure 1, (b) structure 2, and (c) structure 3. The corresponding model-parameters are listed in Table 3.
The tilt of the molecules allows for a much higher packing density (1.7 × 1014 molecules per cm2) than for the flat lying molecules. Each of the three structures forms a coincidence lattice with the substrate with an increasing number of coincidence points per unit area from structure 1 to 3, leading to the assumption that the third structure is the most stable, high coverage structure. The superstructure matrices of the coincidence lattices are
Figure 3. (a) Selected area of the LEED pattern obtained after 10 s (see Figure 2c, here ≈ 3-5 Å) at an electron energy of 37 eV. Panel b shows a schematic representation of the selected area with spot labels used in the text.
[ ] [ ]
7 4 7 #1 M2 ) 4 M#1 1 )
0 8 0 3
Deposition of Tetracene
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Figure 5. Model structures for the second interpretation. Panel a shows the simplest, but unlikely structure containing flat-lying molecules. Panel b shows a model structure containing tilted molecules according to a more bulk-like structure. Table 4. Summary of Parameters for All Determined Structures of Both Interpretations, Including the Number of Molecules Per Unit Cell, the Overall Molecular 2D Packing Density, and the Symmetry of the Primitive Unit Cell
1
“primitive” (b) coincidence lattice (S) “primitive” (b) coincidence lattice (S) “primitive” (b) coincidence lattice (S)
2D Unit Cell Parameters of Interpretation #1 5.37 13.56 101.3 1 26.9 26.6 90 10 5.37 26.7 96 2 26.9 13.8 74 5 4.39 53.4 94.7 4 30.7 15.4 60 7
“primitive” (b) coincidence lattice (S)
2D Unit Cell Parameters of Interpretation #2 5.4 18.1 82.5 1 or 2 46.1 73.0 106 24
3
1
b2/S 2 [Å]
molecules per unit cel
lattice type
2
b1//S1 [Å]
R [°]
structure
and
M#1 3 )
[ ] 8 0 4 4
for structures 1, 2, and 3, respectively. The optimized model parameters of all three structures are summarized in Table 4, including the sizes of the coincidence lattices, the number of molecules per unit cell, the molecule density and the symmetry of the proposed structure. The second interpretation only involves one structure and is spanned by the vectors OB ) b a and BD ) B b leading to unit cell constants of a ≈ 5.3 Å and b ≈ 18.0 Å with an angle between b a and B b of 98° in real space. Due to the substrate symmetry, the structure of the vectors spanned by OE and EC has a mirror symmetry toward OB and BD, respectively, leading to 12 equivalent domains on the Si(111)/GaSe surface. Tetracene has a van der Waals radius of approximately 5.3 × 13.6 Å. The observed value of a ) 5.3 Å fits the short length of the tetracene molecule very well. The second value b ) 18 Å is much larger than the van der Waals length of tetracene but too short for two molecules lying flat on the surface. This raises the question of how the unit cell is built up and if flat lying molecules are present. Figure 5a shows the simplest structure containing flat lying molecules showing an extraordinary large spacing along the long axis. This structure might also be possible by inserting a standingup tetracene molecule between the flat-lying molecules. Lukas et al.9 observed something similar for pentacene on Cu(110) in a STM study but here the relative pentacene coverage was much lower and also the substrate has a different symmetry, providing
density (×1014) [molecules/cm 2 ]
unit cell symmetry
1.391
P2
1.408
P2
1.71
P2
1.05 or 2.1
P2
a symmetry more favorable for rectangular structures and a head to tail alignment of the molecules making this a weak comparison. According the bulk unit cell of tetracene29,30 where different orientations of tetracene molecules exist, we propose a model with tilted molecules as shown in Figure 5b. Due to the fact that two molecules are present in the unit cell, the molecular density of this structure is 2.1 × 1014 molecules per cm2. The 2D-structure for the second interpretation forms also a coincidence lattice with the substrate, indicated by the big circles in Figure 5. The superstructure matrix of this coincidence lattice is
M#2 )
[
11 1 5 16
]
The optimized 2D-structure parameters are included in Table 4 for comparison to the first interpretation. By comparing the distances between BG, GD, and BD as well as between CE, EF, and FC a constant distance of 5.3 Å and an angle of 60° is found implying the conclusion that the spots F-G are not correlated to an other 2D-structure and are second-order spots, just like the spots H to K. Discussion. The first interpretation has the advantage that the over-layer incorporates flat lying molecules which can be expected for low coverages of flat molecules like tetracene. Its major disadvantage is that three structures with different packing densities are necessary to explain the main observed LEED spots (29) Robertson, J. M.; Sinclair, V. C.; Trotter, J. Acta Crystallogr. 1961, 14, 697. (30) Overney, R. M.; Howald, L.; Frommer, J.; Meyer, E.; Guentherodt, H.-J. J. Chem. Phys. 1991, 94, 8441.
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Figure 7. Band-energy diagram of the interface GaSe passivated Si(111) with tetracene. All values are given in eV, and the band gap of tetracene is taken from the Landolt Bo¨rnstein series.
final tetracene film where no charging is present. Because of the presence of band bending in the substrate, the difference in the workfunction is the sum of band bending EBB and the dipole potential δ at the interface. Therefore Figure 6. Set of UPS measurments obtained during the stepwise deposition of tetracene on GaSe passivated Si(111). In the beginning, substrate band bending is visible (indicated with the double arrows). The shift of the secondary electron onset is the sum of substrate band bending EBB and the formation of an interface dipole potential δ. No energy-shifts are detectable in the tetracene HOMO level emissions.
B-G (see Figure 2). From the different packing densities, it can be expected that the low coverage spots should disappear with increasing tetracene coverage. It appears from the sequence of the LEED pattern shown in Figure 2 that this is not the case. This makes the first interpretation unlikely due to the expectation that spot intensity for structure 3 (high-density structure) should increase. From this viewpoint, the second interpretation seems more reasonable. However the remaining question is how the molecules are arranged in the 2D/3D unit cell. The 3D is added here because the second model (see Figure 5b) involves a more complex arrangement of the tetracene molecules in the unit cell, where it is possible that the molecules are tilted toward the surface and against each other. It was not possible to acquire the necessary data to conclusively determine this structure. To do so, either a detailed SPA-LEED or/and a low-temperature STM study would be necessary. Unfortunately, both techniques are not available on the used UHV system. Nevertheless, the one, but more complex, structure of the second interpretation explains more observed features compared to the first interpretation and is therefore favored to explain the obtained LEED pattern. B. Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements were performed after several deposition steps to investigate the electronic structure of the GaSe passivated Si(111) tetracene interface. In Figure 6, a set of UPS measurements is displayed. Initially an energy-shift to higher binding energies is detected for the GaSe half-sheet emissions (Ga and Se orbitals20) which is indicated by the little double arrows in Figure 5. This shift is simultaneously observed in the core-level emissions of the Si 2p, Se 3d, and Ga 2p emissions (not shown here), and so this shift is assigned to band bending EBB ) 0.25 eV in the silicon substrate. No energetic shifts are present in the growing tetracene overlayer as marked with the vertical lines for the different HOMO level emissions of tetracene in Figure 6. The evaluation of the secondary electron onset results in a workfunction of 5.35 eV for the substrate and of 4.65 eV for the
δ ) ∆Φ - EBB ) 5.35 eV - 4.65 eV - 0.25 eV ) 0.45 eV Using all of the experimentally determined values, the knowledge of the built-in potential between the silicon substrate and the GaSe half-sheet layer (+0.51 eV)20,27 and the literature values for the band gaps of tetracene and silicon, the band energy diagram can be drawn as shown in Figure 7. The given value of 0.7 eV for the width of the HOMO level is obtained from the width of the emission in the UP spectra in Figure 6. The LUMO level is assumed to be symmetric to the HOMO level. The discontinuities HOMO ∆LUMO CBM and ∆VBM at the conduction-band minimum and the valence-band maximum, respectively, indicate that there are energetic barriers for the electron and hole transport across the interface.
IV. Conclusions LEED patterns obtained from tetracene layers on GaSe passivated Si(111) have two distinct interpretations. The first interpretation reveals three different molecular structures on the GaSe passivated Si(111) surface where the second one requires only one structure. The first interpretation reveals three structures, two with flat lying molecules and a third with more bulk-like packing. The second interpretation leads to either flat lying molecules separated by a unusual large gap or a unit cell containing two molecules requiring the molecules to be tilted, probably along their short axis. The second interpretation has the advantage that only one structure is required to explain the LEED pattern and to explain the constant contrast of the reflections in the LEED pattern at different coverages. The formation of a more bulk-like structure at very low coverage is not typical for molecules like tetracene and pentacene on inert substrates5,8,12,13,31,32 but likely results from the weak interaction with the van der Waals-like surface of the GaSe passivation layer on Si(111). The observation of sharp LEED patterns induced by ordered tetracene at room temperature is in contrast to the tetracene structures observed on Ag(111)12 that are only clearly detectable at low temperatures, showing that tetracene on GaSe passivated Si(111) forms a very stable film, even at room temperature. (31) France, C. B.; Schroeder, P. G.; Parkinson, B. A. Nano Lett. 2002, 2, 693. (32) Kang, J. H.; Zhu, X.-Y. Chem. Mater. 2006, 18, 1318.
Deposition of Tetracene
Photoelectron spectroscopy reveals the presents of band bending in the substrate and the absence of band bending in the growing tetracene overlayer. Between the tetracene overlayer and the GaSe passivated Si(111) surface, an interface dipole potential of 0.45V is present. Discontinuities exist between both the HOMO and valence-band maximum as well as between the LUMO and conduction-band minimum which has to be considered in future devices. The inertness of the GaSe half-sheet and the ability to form highly ordered organic layers such as tetracene structures
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demonstrate that the GaSe passivated Si(111) surface is an outstanding substrate for further investigations of the growth of organic molecules on silicon. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft [Contract No. JA899/7_3]. B.A.P. acknowledges support from NSF Grant No. CHE-0518563. LA061361G
