Energy level alignment and chemical interaction at Alq 3 Õ Co interfaces for organic spintronic devices Y. Q. Zhan, 1, * M. P. de Jong, 1,† F. H. Li, 1 V. Dediu, 2 M. Fahlman, 1 and W. R. Salaneck 1 1 Department of Physics, Chemistry, and Biology, Linköping University, S-581 83 Linköping, Sweden 2 Istituto per lo Studio di Materiali Nanostrutturati, Consiglio Nazionale delle Ricerche (ISMN-CNR), via Gobetti 101, 40129 Bologna, Italy Received 19 March 2008; revised manuscript received 10 June 2008; published 16 July 2008 The electronic structure of the interface between tris8-hydroxyquinoline aluminum Alq 3  and cobalt was investigated by means of photoelectron spectroscopy. As demonstrated recently, this interface is characterized by efficient spin injection in organic spintronic devices. A strong interface dipole that reduces the effective work function of cobalt by about 1.5 eV was observed. This leads to a large barrier for hole injection into the highest occupied molecular-orbital HOMO level of 2.1 eV, in agreement with a previously proposed model based on electron transport in Co-Alq 3 -La 0.7 Sr 0.3 MnO 3 spin valves. Further experimental results indicate that chemical interaction occurs between the Alq 3 molecules and the cobalt atoms, while the latter penetrate the Alq 3 layer upon vapor deposition of Co atoms. The data presented lead to significant progress in understanding the electronic structure of the Co-on-Alq 3 interface and represent a significant step toward the definition of the interface parameters for the efficient spin injection in Alq 3 based spin valves. DOI: 10.1103/PhysRevB.78.045208 PACS numbers: 72.80.Le, 85.75.d, 71.20.b, 73.40.c I. INTRODUCTION Organic/ferromagnetic electrode interfaces have recently become the subject of thorough studies 1–4 because of their applications in organic spintronics. 5–7 In this promising field, the organic semiconductors are mainly used as a spin trans- port layer placed between two ferromagnetic electrodes. The electronic structure of the organic/ferromagnetic electrode interface was not only found to be the main factor determin- ing charge injection but also the possible reason for a nega- tive spin-valve effect. 8 Among many different organic spin- tronic devices, those using tris8-hydroxyquinoline aluminum Alq 3 , shown in Fig. 1 as a spacer between the ferromagnetic La 0.7 Sr 0.3 MnO 3 LSMO and Co electrodes have been mostly used. 6,9,10 Recently, the alignment of en- ergy levels at the Alq 3 -on-LSMO interface was studied 1 and the existence of a strong dipole of about 0.9 eV that shifts down the energy levels of Alq 3 was reported. 1 The results indicate that electrons injected from LSMO into Alq 3 are the dominant charge carriers in the spin-valve device. However, the behavior of these spin-valve devices was only partially understood, mainly because the knowledge about the inter- face between cobalt electrode and the Alq 3 molecular layer was insufficient. Thus far, only the interface formed by ad- sorbing Alq 3 -on-cobalt electrodes Alq 3 / Co has been dis- cussed in the literature. 11 In the standard organic spin-valve devices, however, cobalt is deposited onto a surface of Alq 3 i.e., Co / Alq 3 . It is well known that significantly different interfaces may be formed depending upon the order of depo- sition. For example, if Al atoms are deposited on LiF / Alq 3 surfaces, there is a chemical reaction between the Al atoms and the LiF Ref. 12 that does not occur when Alq 3 atoms are deposited on Al/LiF Ref. 13. In this paper, the results of studies of both Co / Alq 3 and Alq 3 / Co interfaces using ultraviolet and x-ray photoelectron spectroscopy UPS and XPS, respectively are reported. In particular, the interfacial energy level alignment at the inter- face of cobalt and Alq 3 is presented. II. EXPERIMENTAL DETAILS The experiments were carried out using a Scienta ® ESCA 200 spectrometer. The vacuum system consists of an analysis chamber and a preparation chamber. X-ray photoelectron spectroscopy XPS and ultraviolet photoelectron spectros- copy UPS were performed in the analysis chamber at a base pressure of 10 -10 mbar using monochromatized AlK x rays at h =1486.6 eV and He I radiation at h = 21.2 eV, respectively. The experimental conditions were such that the full width at half maximum FWHM of the Au 4 f 7/2 line was 0.65 eV. The binding energies were obtained referenced to the Fermi level with an error of 0.1 eV. Sputtering and material depositions were done in a preparation chamber with a base pressure of 10 -10 mbar. The Alq 3 was purchased from Sigma-Aldrich. Alq 3 was deposited in situ from a Alq3 Co Carbon tape Si Si nm 0 4 m 35 -35 FIG. 1. Color online Schematic of the peel-off technique and the AFM image measured around the edge formed by the peel-off process. PHYSICAL REVIEW B 78, 045208 2008 1098-0121/2008/784/0452086 ©2008 The American Physical Society 045208-1