Charge Transport Properties of Metal / Metal-Phthalocyanine / n-Si Structures

The field of hybrid electronics of molecules and traditional semiconductors is deemed to be a realistic route towards possible use of molecular electronics. Such hybrid electronics finds its potential technological applications in nuclear detectors, near-infrared detectors, organic thin film transistors and gas sensors. Specifically Metal / organic / n-Silicon structures in this regard are mostly reported to have two regimes of charge transport at lower and higher applied voltages in such two terminal devices. The fact is mostly attributed to the change in conduction mechanism while moving from lower to higher applied voltages. These reports describe interactions between the semiconductors and molecules in terms of both transport and electrostatics but finding the exact potential distribution between the two components still require numerical calculations. The challenge in this regard is to give the exact relations and the transport models, towards practical quantification of charge transport properties of metal / organic / inorganic semiconductor devices. Some of the most exiting questions in this regard are; whether the existing models are sufficient to describe the device performances of the hybrid devices or some new models are needed? What type of charge carriers are responsible for conduction at lower and higher applied voltages? What is the source of such charge carriers in the sandwiched organic layer between the metal and inorganic semiconductors? How the transition applied voltage for the change in conduction mechanism is determined? What is the role of dopants in the organic layer semiconductors? What are the possible explanations for observed temperature effects in such devices? In present work the charge transport properties of metal / metal-phthalocyanine / n-Si structures with low (ND = 4×1014 cm-3), medium (ND = 1×1016 cm-3) and high (ND = 2×1019 cm-3) doped n-Si as injecting electrode and the effect of air exposure of the vacuum evaporated metal-phthalocyanine film in these structures is investigated. The results obtained through temperature dependent electrical characterizations of the structures suggest that in terms of dominant conduction mechanism in these devices Schottky-type conduction mechanism dominates the charge transport in low-bias region of these devices up to 0.8 V, 0.302 V and 0.15 V in case of low, medium and high doped n-Silicon devices. For higher voltages, in each case of devices, the space-charge-limited conduction, controlled by exponential trap distribution, is found to dominate the charge transport properties of the devices. The interface density of states at the CuPc / n-Si interface of the devices are found to be lower in case of lower work function difference at the CuPc / n-Si interface of the devices. The results also suggest that the work function difference at the CuPc / n-Si interface of these devices causes charge transfer at the interface and these phenomena results in formation of interface dipole. The width of the Schottky depletion region at the CuPc / n-Si interface of these devices is found to be higher with higher work function difference at the interface. The investigation of charge transport properties of Al / ZnPc / medium n-Si and Au / ZnPc / medium n-Si devices suggest that the Schottky depletion region formed at the ZnPc / n-Si interface of these devices determines the charge transport in the low-bias region of both the devices. Therefore, the Schottky-type (injection limited) and the space-charge-limited (bulk limited) conduction are observed in the low and the high bias regions of these devices, respectively. The determined width of the Schottky depletion region at the ZnPc / n-Si interface of these devices is found to be similar for both the devices, therefore, the higher work function difference at the metal / ZnPc interface of the devices has no influence on the Schottky depletion region formed at the ZnPc / n-Si interface of the devices. The similar diode ideality factor, barrier height and the width of the Schottky depletion region, determined for both of these devices, demonstrates that these device characteristics originate from ZnPc / n-Si interface of these devices. Therefore, the work function difference at the metal / ZnPc interface of these devices has no noticeable influence on the device properties originating from ZnPc / n-Si interface in these devices. The investigation of charge transport properties of Al / CuPc / low n-Si devices with and without air exposure of the CuPc film, before depositing metal contact demonstrate that Schottky-type conduction mechanism dominates the charge transport in these devices up to bias of 0.45 V in case devices with the air exposure, and up to 0.8 V in case devices without the air exposure. This decrease in the threshold voltage, for the change in conduction mechanism in the devices, is attributed to wider Schottky depletion width determined at the CuPc / n-Si interface of the devices without the air exposure of CuPc film. For higher voltage the space-charge-limited conduction controlled by exponential trap distribution, is found to dominate the charge transport properties of the devices without the air exposure of CuPc, and in case of devices with the air exposure of CuPc film, the SCLC is controlled by single dominating trap level probably introduced by oxygen impurities.:1 INTRODUCTION 3 1.1 Organic / Inorganic Semiconductor Interfaces 5 1.2 Organic / Metal Interfaces 6 1.3 Organic Material / Semiconductor Interfaces 6 1.4 Interface Dipoles at Organic / Inorganic Interfaces 7 1.5 Objectives of the Study 9 1.6 Research Methodology 10 1.7 References 12 2 BASIC CONCEPTS OF ORGANIC ELECTRONICS 16 2.1 Localized and Delocalized Orbital in Organic Semiconductors 16 2.2 Operating principle of some basic organic / inorganic devices 19 2.3 Electronic Structure of an Organic Solid 20 2.4 Validity Limits of band model and the tunneling model 21 2.5 Dark Electric Conduction 23 2.6 Injection of Carriers from Electrodes 24 2.7 References 26 3 MATERIALS AND DEVICE FABRICATION 27 3.1 Assembly of the hybrid organic / inorganic structures 27 3.2 The Vacuum Systems for Device Fabrication 27 3.3 The n-Si substrates 29 3.4 The Organic semiconductors; CuPc and ZnPc 30 3.5 Sample Fabrication Procedures 32 3.5.1 Experimental Details of Samples Prepared at PCRET labs 32 3.5.2 Experimental details of samples Prepared at TU Chemnitz labs 33 3.6 References 34 4 METHODS FOR DATA ANALYSIS 35 4.1 The Dominant Conduction Mechanisms in the Devices 35 4.1.1 Schottky-type Conduction 35 4.1.1.1 The Standard Characterization Technique 38 4.1.1.2 The R. J. Bennett Technique 39 4.1.1.3 The Cheung and Cheung Technique 42 4.1.1.4 The H. Norde Technique 42 4.1.2 Space Charge Limited Conduction (SCLC) 43 4.1.3 The MIM Models to Determine Dominant Conduction Mechanism 44 4.2 Interface State Energy Distribution 46 4.3 References 48 5 CHARGE TRANSPORT PROPERTIES OF Al / CuPc / n-Si DEVICES IN DARK 50 5.1 Charge Transport Properties of Al / CuPc / low-doped n-Si Devices 51 5.1.1 Interface State Energy Distribution 65 5.2 Charge Transport Properties of Al / CuPc / medium-doped n-Si Devices 67 5.3 Charge Transport Properties of Al / CuPc / High-doped n-Si Devices 75 5.3.1 Charge Transport Properties of Al / CuPc / High-doped n-Si Devices as Metal-Insulator-Metal Structures 82 5.4 Summary 85 5.5 Final Remarks 87 5.6 References 88 6 INFLUENCE OF TOP METAL CONTACT ON CHARGE TRANSPORT PROPERTIES META / ZnPc / n-Si DEVICES IN DARK 89 6.1 Charge Transport Properties of Metal / ZnPc / Medium-doped n-Si Devices 89 6.2 Interface State Energy Distribution 99 6.3 Summary 100 6.4 Final Remarks 101 6.5 References 103 7 INFLUENCE AIR EXPOSURE ON THE CHARGE TRANSPORT PROPERTIES OF Al / CuPc / n-Si DEVICES 104 7.1 Charge Transport Properties of Al / CuPc / low n-Si Devices With (or) without air exposure of CuPc film 104 7.2 Summary 115 7.3 Final Remarks 116 7.4 References 117 8 CONCLUSIONS 118 8.1 Scope of Future Work 120 Index of Figures 121 Curriculum Vitae and List of Publications 12