Spectroscopy and imaging of metal–organic interfaces using BEEM

Spectroscopy and imaging of metal–organic interfaces using BEEM

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  + Models Spectroscopy and imaging of metal–organic interfaces using BEEM Linda Kunardi a , Cedric Troadec a , N. Chandrasekhar a,b, * a  Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602, Singapore b  Department of Physics, National University of Singapore (NUS), Science Drive 3, Singapore 117542, Singapore Abstract Charge injection from metal electrodes to organics is a subject of intense scientific investigation for organic electronics. Ballistic electronemission microscopy (BEEM) enables spectroscopy and imaging of buried interfaces with nanometer resolution. Spatial non-uniformity of carrierinjection is observed for both Ag–PPP (poly-paraphenylene) and Ag–MEHPPV (poly-2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylenevinylene)interfaces. BEEM current images are found to correlate only marginally with the surface topography of the Ag film. # 2005 Elsevier B.V. All rights reserved. PACS:  73.20.-r; 73.40.-c; 73.40.Ns Keywords:  BEEM; Hole injection; Schottky barrier; Interface With the growing interest in electronic devices usingpolymers and molecules as active materials, the interfaceproperties of metal–organic (MO) contacts have becomecrucial for understanding the basic physics of their operation[1]. Conventional spectroscopy and current–voltage measure-ments have addressed these properties. However, thesetechniques average over millimeter sized areas and do notprovide local information over experimental device dimen-sions, which are typically in tens of nanometers. In this work,BEEM is used to investigate the properties of a buried MOinterface.The configuration of a BEEM device is as follows. Anorganic semiconductor is overlaid with a thin metal film(typically < 10 nm, termed the base), with an ohmic contact onthe opposite side (termed the collector). The top metal film isgrounded, and carriers are injected from a scanning tunnelingmicroscope (STM) tip into the film at energies sufficiently highabove the metal’s Fermi energy so that they propagateballistically before impinging on the interface. When theenergy of the carriers exceeds the Schottky barrier (SB), theypropagate into the semiconductor and are collected from thebottom contact.PPP and MEHPPV, hole transport materials, are chosen asthe organic semiconductors and silver is used as the base sinceit has been shown to yield injection-limited contacts for theseorganics [2]. A low temperature home-assembled STM systemand a separate sample preparation chamber were used to carryout the experiments. The samples consist of a pre-depositedgold film serving as a collector on glass or silicon. A nominally100 nm thick PPP film is evaporated onto the substrate cooledto 77 K. After removal from vacuum, a mechanical mask isused to define a diode area of    2 mm 2 . A nominally 10 nmthick Ag film isthen evaporatedunder identical conditions. Thesample is then placed in the STM chamber,where it is cooled to77 K for the experiments. The current noise of the setup istypically 1 pA. For MEHPPV, the sample preparation steps aresimilar, except that it is spin coated. The resulting film isestimated to be 500 A˚ in thickness.Fig. 1 shows ballistic hole emission spectroscopy (BHES)after averaging over 200 and 10 individual scans for Ag–PPPand Ag–MEHPPV interfaces, respectively. The spectra aretaken over a range of 0–2 V. The noise level of the Ag–MEHPPV interface in the raw data seems higher. This isattributed to the spin-coated sample that is expected to be moredisordered than an evaporated sample and thus has a largerdensity of trapping sites.Fig. 2(a) shows an STM image of the top Ag film at 0.5 Vbias voltage and 1 nA current. The topography scale is 1.2 nm.The Ag film is well connected, and lateral variations of the www.elsevier.com/locate/apsuscApplied Surface Science xxx (2005) xxx–xxx* Corresponding author. Tel.: +65 6874 8586; fax: +65 6774 4657. E-mail address:  n-chandra@imre.a-star.edu.sg (N. Chandrasekhar).0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2005.09.053APSUSC-13277; No of Pages 3  Fermi level are unlikely. The corresponding BEEM image of the Ag–PPP system (Fig. 2(b)) is obtained at 0.7 V with a fullscale of 3.5 pA. Both images are 100 nm square. We find thatthe BEEM current image doesnotcompletelycorrelatewiththeSTM derivative.Theinterface shows non-uniformtransparencyover the region scanned by the STM. Similar results are foundfor the Ag–MEHPPV interface. Fig. 3 shows an STM imagetaken at 1.5 V, 1 nA and a BEEM image at 1.5 V of anapproximately 150 nm square area for the Ag–MEHPPVsystem. The topography scale on the STM image is 2 nm, andthe full scale on the BEEM image is 5 pA.In conclusion, we demonstrate how BEEM can be used forhigh-resolutionstudiesofburiedmetal–organicinterfaces.BEEMspectroscopy and imaging of a buried Ag–PPP and MEHPPV  L. Kunardi et al./Applied Surface Science xxx (2005) xxx–xxx 2 + Models Fig. 1. (a)  I  – V   and d  I   /d V   for the Ag–PPP interface. (b)  I  – V   for the Ag–MEHPPV interface.Fig. 2. (a) STM and (b) BEEM current images of Ag–PPP.Fig. 3. (a) STM and (b) BEEM current images of Ag–MEHPPV.  interfaces are presented and analyzed. Images of the interfacetransparency show substantial lateral/spatial non-uniformity. Acknowledgments The authors thanks Prof. Narayanamurti, V.I. Arkhipov, C.Joachim, A. Dodabalapur and I. Shalish for discussions. Thiswork was supported by A*STAR and IMRE, Singapore. References [1] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C.Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R.Salaneck, Nature (London) 397 (1999) 121.[2] Y. Yang, Q. Pei, A.J. Heeger, Synth. Met. 78 (1996) 263.  L. Kunardi et al./Applied Surface Science xxx (2005) xxx–xxx  3 + Models
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