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Energy-Level Control at Hybrid Inorganic/Organic Semiconductor Interfaces

:	Raphael Schlesinger
:	Energy-Level Control at Hybrid Inorganic/Organic Semiconductor Interfaces
:	Springer-Verlag
:	9783319466248
:	1
:	CHF 87.30
:

:	Theoretische Physik
:	English

:	223
:	Wasserzeichen/DRM
:	PC/MAC/eReader/Tablet
:	PDF

This work investigates the energy-level alignment of hybrid inorganic/organic systems (HIOS) comprising ZnO as the major inorganic semiconductor. In addition to offering essential insights, the thesis demonstrates HIOS energy-level alignment tuning within an unprecedented energy range. (Sub)monolayers of organic molecular donors and acceptors are introduced as an interlayer to modify HIOS interface-energy levels. By studying numerous HIOS with varying properties, the author derives generally valid systematic insights into the fundamental processes at work. In addition to molecular pinning levels, he identifies adsorption-induced band bending and gap-state density of states as playing a crucial role in the interlayer-modified energy-level alignment, thus laying the foundation for rationally controlling HIOS interface electronic properties. The thesis also presents quantitative descriptions of many aspects of the processes, opening the door for innovative HIOS interfaces and for future applications of ZnO in electronic devices.

	Supervisor’s Foreword	7
	Abstract	9
	Acknowledgements	11
	Contents	13
	Abbreviations	16
	1 Introduction	18
	References	22
	2 Fundamentals	24
	2.1 Organic Semiconductors: --Conjugated Molecules	24
	2.2 Electronic Structure Theory	27
	2.2.1 Nearly Free Electrons	28
	2.2.2 Model of Tight Binding/LCAO	29
	2.2.3 The Density of States	33
	2.2.4 Gap States	34
	2.3 Models and Processes of Energy--Level Alignment	38
	2.3.1 Doping and Fermi--Level	39
	2.3.2 Band Bending	41
	2.3.3 Vacuum Level Alignment	49
	2.3.4 Fermi--Level Pinning	51
	2.3.5 Push Back	54
	2.3.6 Molecular Film Structure	56
	References	58
	3 Theory of Experimental Methods	63
	3.1 Photoelectron Spectroscopy (PES)	63
	3.1.1 The Three Step Model of Photoemission	66
	3.1.2 Interpretation and Analysis of Photoemission Data	74
	3.2 Near Edge X-ray Absorption Fine Structure (NEXAFS)	79
	3.2.1 Angular Dependence of Molecular NEXAFS Resonances	81
	3.3 Scanning Tunneling Microscopy (STM)	85
	3.4 Low Energy Electron Diffraction (LEED)	87
	References	89
	4 Methodology and Experimental Setups	93
	4.1 Materials and Sample Preparation	93
	4.1.1 ZnO Crystals	93
	4.1.2 Organic Materials	95
	4.2 Experimental Equipment and Settings	98
	4.2.1 PES, NEXAFS and LEED at BESSY II	98
	4.2.2 PES, STM/AFM and LEED Setup at the HU Berlin	100
	4.2.3 PES and LEED at Chiba University	101
	4.2.4 Optical Characterization of HIOS Structures	101
	4.3 Data Analysis	102
	4.3.1 Processing Photoemission Data	102
	4.3.2 Processing NEXAFS Data	103
	4.3.3 Processing AFM/STM Data	104
	4.3.4 PES Peak Fitting and Determination of Positions	105
	References	105
	5 Results and Discussion	107
	5.1 Characterization of the ZnO Faces	108
	5.1.1 Investigation of the Surface Structure	109
	5.1.2 Electronic Characterization of the Clean ZnO Faces	116
	5.2 Donor and Acceptor Molecules on ZnO	119
	5.2.1 Strong Work Function Increases of ZnO Using F4TCNQ	120
	5.2.2 F6TCNNQ: A Similar, but Not as Volatile, Acceptor as F4TCNQ	131
	5.2.3 HATCN, an Application Relevant Acceptor for Tuning the Work Function of ZnO	135
	5.2.4 A Weak Acceptor, Pentacenetetrone, on ZnO	143
	5.2.5 Donors on ZnO	149
	5.3 ZnO Gap State Density of States Limits Adsorption Induced Internal Band Bending	155
	5.4 Universal Energy-Level Alignment of Para-Sexiphenyl on ZnO	162
	5.5 Adjusting HIOS Energy-Level Alignment	168
	5.5.1 Energy-Level Adjustment of 6P on ZnO by HATCN Monolayers	169
	5.5.2 Demonstration of Continuous Energy-Level Adjustment: SP6 on ZnO(0001)	173
	5.5.3 Efficient Light Emission from Inorganic and Organic Semiconductor Hybrid Structures by Energy-Level Tuning	176
	References	186
	6 Conclusion	197
	Appendix A Mathematica Code for Band Bending in Depletion Approximation	201
	Appendix B Main Excitation Lines/Satellites	204
	Appendix C Program Code for Satellite Removal	205
	Appendix D NEXAFS ``Dark Current''/Offset Related Artifact	208
	Appendix E Mathematica Code for Band Bending Using Fermi--Dirac Statistics	211
	Appendix F Additional Experimental Spectra	215
	F.1 Work Functions of Differently Prepared ZnO	215
	F.2 STM of Ex Situ Hot Furnace Annealed ZnO	216
	F.3 XP Spectra of NMA on ZnO(000bar1)	216
	F.4 Relationship Between GDOS and ZnO Energy--Level Shifts / UV Irradiation	218
	F.5 Thick--Film NEXAFS of 6P on ZnO	219
	Curriculum Vitae	221