BEEM STUDIES ON METAL-ORGANIC INTERFACES YI ZHENG SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY AT DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE c Copyright by YI ZHENG, 2009 NATIONAL UNIVERSITY OF SINGAPORE DEPARTMENT OF PHYSICS The undersigned hereby certify that they have read and recommend to the Faculty of Science for acceptance a thesis entitled “BEEM Studies on Metal-organic Interfaces” by YI ZHENG in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Dated: February 2009 External Examiner: Research Supervisors: Prof. Dr. Andrew T. S. Wee Prof. Dr. N. Chandrasekhar Examining Committee: ii NATIONAL UNIVERSITY OF SINGAPORE Date: February 2009 Author: YI ZHENG Title: BEEM Studies on Metal-organic Interfaces Department: Physics Degree: Ph.D. Year: 2009 Permission is herewith granted to National University of Singapore to circulate and to have copied for non-commercial purposes, at its discretion, the above title upon the request of individuals or institutions. Signature of Author THE AUTHOR RESERVES OTHER PUBLICATION RIGHTS, AND NEITHER THE THESIS NOR EXTENSIVE EXTRACTS FROM IT MAY BE PRINTED OR OTHERWISE REPRODUCED WITHOUT THE AUTHOR’S WRITTEN PERMISSION. THE AUTHOR ATTESTS THAT PERMISSION HAS BEEN OBTAINED FOR THE USE OF ANY COPYRIGHTED MATERIAL APPEARING IN THIS THESIS (OTHER THAN BRIEF EXCERPTS REQUIRING ONLY PROPER ACKNOWLEDGEMENT IN SCHOLARLY WRITING) AND THAT ALL SUCH USE IS CLEARLY ACKNOWLEDGED. iii To my parents. iv Table of Contents Table of Contents v List of Tables viii List of Figures ix Acknowledgements xiv Abstract xv Introduction 1.1 Metal-on-organic and organic-on-metal interfaces . . . . . . . . . . . 1.2 Charge transport in metal-organic diodes: interplay between injectionlimited current (ILC) and space-charge-limited current (SCLC) . . . . 1.2.1 The classical injection-limited current in metal-inorganic semiconductor diodes . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Space charge effects in metal-organic diodes and field-dependent mobility in organic semiconductor . . . . . . . . . . . . . . . . 1.2.3 Hopping injection at metal-organic interface and charge transport in organic thin film: interplay between ILC and SCLC . . 1.3 Ballistic electron emission microscopy (BEEM) and its applications in studying metal-organic interfaces . . . . . . . . . . . . . . . . . . . . 1.3.1 Ballistic electron emission microscopy (BEEM) . . . . . . . . 1.3.2 BEEM studies on metal/organic interfaces . . . . . . . . . . . 1.4 Research objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 11 14 17 Experimental 2.1 BEEM instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Organic thin film preparation . . . . . . . . . . . . . . . . . . . . . . 20 20 22 v 1 4 2.3 2.4 Metal electrode preparation: direct, direct cold, and indirect cold evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ultra-violet photoemission spectroscopy and near edge x-ray adsorption fine structure spectroscopy using synchrotron radiation . . . . . 2.4.1 Ultra-violet photoemission spectroscopy . . . . . . . . . . . . 2.4.2 Near edge x-ray adsorption fine structure spectroscopy . . . . 24 27 27 30 Charge injection at metal-on-pentacene interfaces and charge transport in pentacene thin films 33 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.2 Pentacene thin-film growth on metal substrates . . . . . . . . . . . . 36 3.2.1 Effect of molecule-substrate interactions on thin-film structures and molecular orientation of pentacene on silver and gold . . . 36 3.2.2 Pentacene thin-film growth on oxygen plasma-treated gold surface (Ox -Au) . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.3 Temperature-dependent charge transport study on Ag/pentacene/Ag diodes: transition from ILC and SCLC . . . . . . . . . . . . . . . . . 74 3.4 Charge transport in Ag/pentacene/Ox -Au structures studied by conventional current-voltage characteristics . . . . . . . . . . . . . . . . . 80 3.4.1 Diffusion controlled metal-on-organic interfaces: ideal, slightlydiffuse and heavily-diffuse Ag/pentacene interfaces . . . . . . 80 3.4.2 Stressing study of ideal and slightly-diffuse Ag/pentacene interfaces: transitions from ideal to slightly-diffuse and slightlydiffuse to heavily-diffuse M/O interfaces . . . . . . . . . . . . 93 3.5 Charge injection at Ag/pentacene interfaces and electrical transport in pentacene thin films studied by BEEM . . . . . . . . . . . . . . . . . 99 3.5.1 BEEM on Ag/pentacene/Ox -Au with ordered pentacene thin film 99 3.5.2 BEEM on pentacene thin films with disordered structure . . . 115 Comparing ballistic charge injection at metal-inorganic semiconductor, metal-organic semiconductor, and metal-oxide interfaces 118 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 4.2 Hot charge carrier transport in Au-HfO2 -SiO2 -Si structures . . . . . . 122 4.3 Comparing ballistic charge injection at metal-inorganic semiconductor, metal-organic semiconductor, and metal-oxide interfaces . . . . . . . 137 Conclusions and prospective works 139 Bibliography 144 vi Publications 156 vii List of Tables 3.1 Tilt angles of pentacene molecules on air-Au and air-Ag calculated from angle-dependent NEXAFS with different pentacene thin-film thicknesses. 45 viii List of Figures 1.1 Metal diffusion induced doping in pentacene thin film . . . . . . . . . 1.2 Ag/p-Si diode under equilibrium, reverse bias and forward bias . . . . 1.3 Typical crystal structure of planar π-conjugated semiconductors . . . 1.4 Working principles of BEEM . . . . . . . . . . . . . . . . . . . . . . . 11 2.1 RHK UHV STM system . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.2 Modifications of the RHK STM system and BEEM sample connections. 22 2.3 Experimental setup for pentancene thermal evaporation. . . . . . . . 23 2.4 AFM topography and phase images of pentacene thin film on SiO2 . . 23 2.5 Direct and direct cold thermal evaporation of metal electrodes . . . . 25 2.6 Indirect cold thermal evaporation of metal electrodes . . . . . . . . . 25 2.7 Polycrystalline Au and Ag surfaces . . . . . . . . . . . . . . . . . . . 26 2.8 Working principles of XPS and UPS . . . . . . . . . . . . . . . . . . 28 2.9 Schematic layout of synchrotron radiation and beamlines in SSLS . . 29 2.10 Working principles of NEXAFS . . . . . . . . . . . . . . . . . . . . . 31 3.1 UPS spectra of 50 nm pentacene on as prepared air-Ag and air-Au . . 39 3.2 UPS spectra of nm pentacene on air-Ag and air-Au . . . . . . . . . 40 3.3 Hole injection barriers at Pn/air-Au and Pn/air-Ag interfaces probed by UPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.4 NEXAFS spectra of 50 nm pentacene on air-Au . . . . . . . . . . . . 43 3.5 NEXAFS spectra of 50 nm pentacene on air-Ag . . . . . . . . . . . . 44 3.6 STM images of nm pentacene on air-Au . . . . . . . . . . . . . . . 47 ix 3.7 z-V spectra of the “flat-lying phase” and the “thin-film phase” . . . . 3.8 AFM topography and phase images of nominal 50 nm pentacene thin films on PEDOT:PSS, air-Ag, and air-Au. . . . . . . . . . . . . . . . 3.9 50 52 AFM topography and phase images of nominal 50 nm pentacene thin films on air-Ag and air-Au (5 × µm2 ) . . . . . . . . . . . . . . . . . 54 3.10 STM topography of as-prepared Au surface . . . . . . . . . . . . . . . 58 3.11 STM topography of O2 plasma treated Au surface (25W, 120 s) . . . 59 3.12 STM topography of O2 plasma treated Au surface (50W, 120 s) . . . 59 3.13 Work function changes of Au surfaces by O2 plasma treatment. . . . . 60 3.14 Valence band structures of pentacene thin films grown on Ox -Au . . . 61 3.15 Energy level alignment at Pn/air-Au and Pn/Ox -Au interface. . . . . 62 3.16 NEXAFS of pentacene thin films grown on Ox -Au surface . . . . . . . 63 3.17 AFM topography and phase images of pentacene thin film grown on Ox -Au surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 3.18 AFM topography and phase images of pentacene thin film grown on SAM-modified Au surface . . . . . . . . . . . . . . . . . . . . . . . . 65 3.19 STM image of pentacene on Ox -Au prepared without RT N2 cooling . 66 3.20 3D STM image of the lamellar crystallites in Fig. 3.19 . . . . . . . . 67 3.21 STM topography and the corresponding 2D-FFT images of the different crystal surfaces in Fig. 3.20 . . . . . . . . . . . . . . . . . . . . . 68 3.22 STM images of a different lamellar-structure dominant area . . . . . . 70 3.23 STM topography and the corresponding 2D-FFT images of another lamellar-structure area . . . . . . . . . . . . . . . . . . . . . . . . . . 71 3.24 STM image of pentacene thin film on RT Ox -Au substrate . . . . . . 72 3.25 One typical z-V spectrum of pentacene thin film on Ox -Au substrate 73 3.26 Sample geometry of Ag/pentacene/Ag diodes and the top 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Hard breakdown refers to the case of nearly ohmic contact between top and bottom electrodes. [128] R. Ranjan, K. L. Pey, C. H. Tung, D. S. Ang, L. J. Tang, T. Kauerauf, R. Degraeve, G. Groeseneken, S. D. Gendt, and L. K. Bera, in Tech. Dig. - Int. Electron Devices Meet. (2007), p. 1. [129] E. Miranda, J. Molina, Y. Kim, and H. Iwai, Appl. Phys. Lett. 86, 232104 (2005). 155 [130] W. C. Lee and C. M. Hu, IEEE Trans. Electron Devices 48, 1366 (2001). [131] J. G. Simmons, J. Appl. Phys. 34, 1793 (1963). [132] Y. Zheng, A. T. S. Wee, K. L. Pey, C. Troadec, S. J. O’Shea, and N. Chandrasekhar, Appl. Phys. Lett. 90, 142915 (2007). [133] Note that ∼ 0.01 µm is approximately the gate area for 65 nm CMOS techonology and ∼ 104 µm2 is a typical size for a MOS test structure. Publications 1. “Possible transition from space-charge-limited to injection-limited conduction in poly (3-hexylthiophene) thin films ”, Yi Zheng, Linda Kunardi, Cedric Troadec, Andrew T. S. Wee, and N. Chandrasekhar, Appl. Surf. Sci., 252, 4023 (2006) (Proceedings of the 3rd International Conference on Materials for Advanced Technologies/9th International Conference on Advanced Materials, Singapore, 2005 ). 2. “BEEM studies on metal high K-dielectric HfO2 interfaces”, Yi Zheng, Cedric Troadec, Andrew T. S. Wee, K. L. Pey, Sean J. O’Shea, and N. Chandrasekhar, J. Phys.: Conference Series 61, 1347-1351 (2007) (Proceedings of the International Conference on Nanoscience and Technology, Basel, Switherland, 2006 ). 3. “Ballistic electron microscopy of a metal molecule interface”, Linda Kunardi, Yi Zheng, Cedric Troadec, M. H. T. Lwin, W. Knoll, and N. 156 157 Chandrasekhar, IEEE CONFERENCE ON EMERGING TECHNOLOGIES - NANOELECTRONICS 194-196 (2006) (Proceedings of the IEEE Conference on Emerging Technologies - Nanoelectronics, Singapore, 2006 ). 4. “Hot electron transport in Au-HfO2 -SiO2 -Si structures studied by ballistic electron emission spectroscopy”, Yi Zheng, Andrew T. S. Wee, K. L. Pey, Cedric Troadec, S. J. O’Shea, and N. Chandrasekhar, Appl. Phys. Lett. 90, 142915 (2007). 5. “Effect of Molecule-Substrate Interaction on Thin-Film Structures and Molecular Orientation of Pentacene on Silver and Gold”, Yi Zheng, Dongchen Qi, N. Chandrasekhar, Xingyu Gao, Cedric Troadec, and Andrew T. S. Wee, Langmuir 23, 8336 (2007). 6. “Localized breakdown in dielectrics and macroscopic charge transport through the whole gate stack: a comparative study”, Yi Zheng, Andrew T. S. Wee, Yi Ching Ong, K. L. Pey, Cedric Troadec, S. J. O’Shea, and N. Chandrasekhar, Appl. Phys. Lett. 92, 012914 (2008). 7. “Temperature-dependent transition from injection-limited to space-charge-limited current in metal-organic diodes”, Yi Zheng, Andrew T. S. Wee, Cedric Troadec, and N. Chandrasekhar, 158 Appl. Phys. Lett. 95, 143303 (2009). 8. “Metal diffusion controls the electronic structures of metal-on-organic interfaces”, Yi Zheng, Andrew T. S. Wee, Cedric Troadec, and N. Chandrasekhar, in preparation. 9. “Structural analysis of pentacene thin film growth on polycrystalline Ox -Au surfaces using scanning tunneling microscopy”, Yi Zheng, Andrew T. S. Wee, and N. Chandrasekhar, in preparation. [...]... called metal- organic (MO) interfaces, metal- on -organic (M/O) interfaces are usually quite different from organic- on -metal (O/M) interfaces both in physical and electronic structures While abrupt O/M interfaces can be easily prepared by the evaporation of organic molecules on metal surfaces, evaporating metal on organic thin-film surfaces usually lead to diffuse M/O interfaces Compared to metals and inorganic... structures and M/O interfaces is still questionable, since energy level alignment and interfacial electronic structures in these two systems are distinct from the conventional M/S interfaces 14 1.3.2 BEEM studies on metal/ organic interfaces BEEM on Ag/polymer interfaces BEEM studies on metal- organic interfaces were first reported by Chandrasekhar’s group [25–27] In these reports, BEEM experiments were... characterization of this kind of metal/ organic /metal sandwich structures also gives nearly identical behavior for the M/O and O/M interfaces [1–3] In contrast, the effects of metal diffusion on the electronic structures of M/O interfaces without interfacial chemistry are barely known, mainly due to the difficulty in controlling the quality of M/O interfaces One approach in understanding metal diffusion and... Introduction 1.1 Metal- on -organic and organic- on -metal interfaces Despite the fact that organic semiconductors have made a great commercial success in organic light-emitting diode (OLED), metal- organic interfaces are still under extensive investigations and debate due to their complexity and their potential device applications in other fields, especially in organic field-effect transistor (OFET) Though generally... structures of organic semiconductors and oxides can be characterized by BEEM Such a finding greatly improves the versatility of BEEM in studying interfacial electronic structures Based on these results, we point out that the current knowledge of BEEM, derived mainly from metal- inorganic semiconductor interfaces, is insufficient for understanding ballistic charge injection at M/O and metal- oxide interfaces. .. as BEEM signals with enough energy 1.3 Ballistic electron emission microscopy (BEEM) and its applications in studying metal- organic interfaces 1.3.1 Ballistic electron emission microscopy (BEEM) BEEM is a three-terminal modification of scanning tunneling microscopy (STM), pioneered by Kaiser and Bell [19–21] Fig 1.4a shows the typical sample geometry and electronic circuits of a BEEM experiment The BEEM. .. This thesis deals with charge injection at metal- organic (MO) interfaces and electrical transport in organic semiconductor thin films using ballistic electron emission microscopy (BEEM) and other complementary techniques Unlike metal- inorganic semiconductor (MIS) junctions, charge transport in metalorganic diodes is interplay between charge injection at metal- organic interface and space-charge conduction... cold, and indirect cold evaporation are used to examine the effects of sample preparation conditions on the quality of M/O interfaces • to study the same type of M/O interfaces with different molecular orientations at the interfaces by BEEM • to study the effect of metal diffusion on the interfacial electronic structure of M/O interface by BEEM and in-situ charge transport measurements • to investigate the... current at low temperature For BEEM experiments, the space-charge effect requires a voltage on the collector (VBEEM ) to tilt the energy band of organic thin film and to assist charge carriers in hopping towards the collector Using conventional current-voltage characteristics and BEEM spectroscopy with different polarity of VBEEM , we show that metal diffusion modifies metal- on -organic (M/O) interface by creating... generally called as BEEM, ballistic hole injection instead of electron is studied by BEEM for metal/ p-semiconductor interfaces For clarity, in this study, we refer BEEM to ballistic injection of majority charge carriers into semiconductors, meaning electrons for n-type and holes for p-type semiconductors correspondingly The terminology, reverse -BEEM (RBEEM) is used when ballistic injection of minority charge . BEEM STUDIES ON METAL-ORGANIC INTERFACES YI ZHENG SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY AT DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF. OF SINGAPORE DEPARTMENT OF PHYSICS The undersigned hereby certify that they have read and recommend to the Faculty of Science for acceptance a thesis entitled BEEM Studies on Metal-organic Interfaces . (BEEM) and its applications in studying metal-organic interfaces . . . . . . . . . . . . . . . . . . . . 11 1.3.1 Ballistic electron emission microscopy (BEEM) . . . . . . . . 11 1.3.2 BEEM studies