At higher level of indium deposition the metallic film formation ... valence band and the In 4d core level as a function of In coverage on a 70 Ã
CuPc film.
March 8, 2006
Time: 10:09am
jp4132020.tex
J. Phys. IV France 132 (2006) 101–104 C EDP Sciences, Les Ulis DOI: 10.1051/jp4:2006132020
Characterisation of metal-organic semiconductor interfaces: In and Sn on CuPc V.Yu. Aristov1,2 , O.V. Molodtsova1 , V.M. Zhilin2 , D.V. Vyalikh3 and M. Knupfer1 1
Leibniz Institute for Solid State and Materials Research, 01069 Dresden, Germany Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow Distr., 142432, Russia 3 Institute of Solid State Physics, TU Dresden, 01069 Dresden, Germany 2
Abstract. We present an investigation of the chemistry and electronic structure formation during the development of the interfaces between In or Sn and thin films of archetypal organic molecular solid, CuPc (copper phthalocyanine) in ultra high vacuum (UHV) conditions. The photoemission measurements were performed by means of high-resolution photoemission electron spectroscopy (HR-PES), core-level (CL) and valence-band (VB), and using synchrotron-radiation (SR) facility. At room temperature the two stages of the In/CuPc interface formation upon metal deposition are observed. On the first one In atoms penetrate into organic film, modify it’s doping, show obvious chemical interaction with CuPc. The site positions of In ions diffused into the CuPc film are derived to be close to the pyrolle nitrogens of copper phthalocyanine. This stage stops at a stoichiometry of In2 CuPc. At higher level of indium deposition the metallic film formation on the top of organic film takes place (second stage). The electronic structure of the interface, which forms during Sn deposition onto CuPc thin films, shows both similarity and difference as compare to indium.
1. INTRODUCTION Organic molecular thin films (OMTF’s) have received considerable attention over the past two decades because of their potential applications in the development of OMTF’s-based various devices like lightemitting diodes, field effect transistors and photovoltaic cells. The metal-OMTF interface formation carried out in UHV becomes a principal base for organic micro- and nano-device technologies [1]. In spite of the importance of metal-OMTF contacts in general, there is no complete microscopic understanding of this class of interfaces. This is particularly true for contacts that are prepared by deposition of a metal onto an organic film, while in the opposite case, a deposition of organic semiconductors onto metals, there has been quite a large number of experimental and theoretical studies and some progress recently has been achieved as regards basic interfacial properties [1, 2]. The former case (metal deposition onto organic films) is comparatively much less studied [1, 3-5]. The family of the phthalocyanines (Pc’s) (which are archetypal organic molecular semiconductors) plays an important role among other OMTF’s. Their biological significance, catalytic properties and potential technological applications [6] are the good reason for researcher to pay remarkable attention to this materials. Moreover, Pc’s demonstrate good compatibility with UHV and can be successfully grown as thin, ultra-clean, well ordered films on various substrates in standard UHV spectrometers. These films then possess excellent and well defined electronic properties [6]. In the present investigation we have focused on the evolution of the electronic structure of copper phthalocyanine films during metal deposition and on interactions during this process. Indium and tin has been chosen as an example of the metals with a relatively low work function. 2. EXPERIMENTAL CONSIDERATIONS The UHV electron spectrometer on the Russian-German high energy resolution dipole beam line of the Berliner Speicherring f¨ur Synchrotronstrahlung (BESSY) was used for the preparation of the CuPc film, Article published by EDP Sciences and available at http://www.edpsciences.org/jp4 or http://dx.doi.org/10.1051/jp4:2006132020
March 8, 2006
Time: 10:09am
jp4132020.tex
102
JOURNAL DE PHYSIQUE IV
the metal (In and Sn) deposition and the soft x-ray photoemission spectroscopy measurements. The VB and CL photoemission spectra were acquired with a VG-CLAM4 electron-energy analyzer. The CuPc film preparation as well as other experimental details could be found elsewhere [7]. 3. RESULTS AND DISCUSSION The bottom spectra in Figures 1(a) -1(c) show C 1s core level, extended VB, and the top of the VB recorded from a CuPc film as grown on the Au(100) substrate. The C 1s core-level of pristine CuPc is comprised of two components: C-1 which corresponds to the aromatic carbon of the benzene rings of CuPc, and C-2 which is attributed to the carbon linked to nitrogen (pyrolle carbon). In addition each C 1s component has an additional satellite contribution, SC−1 and SC−2 respectively [8]. The onset of the CuPc valence band (peak A) indicates that the initial EF position is located near the middle of the transport gap. According to [9] the spectral features B and C of the valence band can be attributed to: (i) peak B - the benzene C 2p, the bridging aza and pyrolle N 2p; (ii) peak C - the benzene C 2p, the bridging aza and the pyrolle N 2p, and H 1s.
Figure 1. (a) C 1s core level, (b) extended valence band, (c) top of the valence band, and (d) In 4d core level measured using photoemission spectroscopy, as a function of In deposition on CuPc films at RT.
March 8, 2006
Time: 10:09am
jp4132020.tex
ICFSI-10
103
Figure 2. The results of decomposition of the N 1s core level for the spectra recorded from the CuPc films as a function of In (left panel) and Sn (right panel) deposition.
Figs 1(a) – 1(d) also present the CuPc C 1s core level, the extended valence band, the top of the valence band and the In 4d core level as a function of In coverage on a 70 Å CuPc film. The shape of the C 1s core level spectra practically shows no observable changes with increasing In coverage which could indicate that neither pyrolle nor benzene ring carbon atoms show remarkable chemical interaction with the deposited metal. Nevertheless a chemical reaction between In and CuPc becomes evident already upon deposition of smallest amount of the metal −1-2 Å. In the Figs 1(b) and 1 (c) one clearly observes for such small In coverages a VB shift of about 0.26 eV to higher binding energy. At the same time, peaks S1 and S2 appear in the gap of CuPc. A third (S3 ) feature in the gap and another (S4 ) feature which is seen as a shoulder of the peak C appear at a nominal In coverage of ≥ g0 Å. Thus after initial deposition In atoms diffuse into the CuPc film and a strong chemical reaction between In the CuPc molecule takes place. This conclusion is further supported by the evolution of the In core levels (Fig. 1(d)). At the smallest In deposition a relatively strong component of the In 4d core level appears in the spectra being shifted by 1.45 eV to higher BE with respect to metallic In. In addition one can clearly see that the In 4d core level width of the reacted In is very similar to that of the metallic In, which indicates that In atoms being incorporated into the CuPc film are not randomly distributed in the film, but occupy certain, well defined equivalent positions.
March 8, 2006
Time: 10:09am
104
jp4132020.tex
JOURNAL DE PHYSIQUE IV
Figure 3. The A-B distance (open circles) and the B/A intensity ratio (closed circles) between A and B components of the N 1s core level from the decomposition presented in Fig.4 with increasing coverage of In.
Summarizing the discussion presented so far we conclude that indium atoms diffuse into the CuPc film, give up charge to the CuPc molecules and most probably occupy equivalent positions. We now consider Fig. 2 (left panel), where the variations of the N 1s core level spectra upon In deposition are shown. From decomposition of N 1s it is evident, that with deposition of about 4 Å of In, the component A shows a shift and simultaneously an additional component B appears. This B component leads to the overall N 1s asymmetric peak broadening. The B component of the N 1s core level must be attributed to those N atoms which are linked to pyrolle carbon and are involved in a chemical interaction with In atoms. The detailed analysis of A and B component behavior (see Fig. 3 (left panel)) gives us arguments for the conclusions, that the In deposition onto CuPc films is characterized by two stages of the In/CuPc interface formation. The first stage takes place until a nominal In coverage of 5 - 6 Å and is characterized by strong diffusion of the In atoms into the organic film. In atoms occupy sites close to the pyrolle nitrogens, strongly interact and transfer negative charge to CuPc. This stage comes to the end when a stoichiometry of In2 CuPc is reached. The second stage begins just after the first is completed, at ≥ 7 Å: on top of the In2 CuPc compound a metallic indium film is formed. The some results of Sn/CuPc interface formation are presented in Fig. 2 (right panel) and Fig. 3 (right panel). Even simple comparing of the data for In/CuPc and Sn/CuPc interface formation presented in Figs. 2 and Fig. 3 shows evident difference for both systems studied. The detailed discussion will be presented in [10]. Acknowledgments This work was supported by the BMBF under grant No. 01 BI 163 and by the DFG under grant No. 436RUS17/53/05. We are grateful to R. H¨ubel, R. Sch¨onfelder and S. Leger for technical assistance. Two of the authors (V.M.Z & V.Y.A) thank the RFBR (Grant No. 05-02-17390).
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
A. Kahn, N. Koch, and W. Gao, J. Polymer Science: Part B: Polymer Physics 41 (2003) 2529. M. Knupfer, and H. Peisert, Phys. Stat. Sol. (a) 201 (2004) 1055. Y. Hirose, A. Kahn, V. Aristov, and P. Soukiassian, Appl. Phys. Lett. 68 (1996) 217. N. J. Watkins, L. Yan, and Y. Gao, Appl. Phys. Lett. 80 (2002) 4384. C. Shen, A. Kahn, and J. Schwartz, J. Appl. Phys. 90 (2001) 6236. S. R. Forrest, Chem. Rev. 97 (1997) 1793, and references therein. V. Yu. Aristov, O. V. Molodtsova, V. M. Zhilin, D. V. Vyalikh, and M. Knupfer, submitted to PRB. T. Schwieger, H. Peisert, M. S. Golden, M. Knupfer, and J. Fink, Phys. Rev. B 66 (2002) 155207. N. Papageorgiu, Y. Ferro, E. Salomon, A. Allouche, J. M. Layet, L. Giovanelli, and G. Le Lay, Phys. Rev. B 68 (2003) 235105. [10] O. V. Molodtsova, V. Yu. Aristov V. M. Zhilin, and M. Knupfer, in preparation.
