Conjugated p-p bonded polymers offer a wide range of new electronic devices which have developed as a unique niche in the marketplace with an ever-growing need for integration. In particular, polymer-based tunnel diodes (PTDs) which exhibit negative differential resistance (NDR) at room temperature can be integrated with other novel components to realize memory and logic cells for highly manufacturable, roll-to-roll (R2R) printed electronics. The research presented here focuses primarily on the fabrication and operating principles behind PTDs. By incorporating an ultra-thin TiO2 interfacial tunneling barrier into a modified organic light emitting diode (OLED) structure, reproducible NDR can be realized. By varying the properties of the interfacial tunneling oxide, characteristics of NDR such as the peak-valley-current-ratio (PVCR), peak current density (J_peak), and the voltage at the peak density (V_peak) can be improved for memory and logic.
This work successfully demonstrates room temperature NDR in PTDs using ultra-thin TiO2 interfacial tunneling barriers grown via atomic layer deposition (ALD). The intention of this work is to present a viable prototype PTD using ALD to deposit the tunneling barrier. By taking a look at the physical and electrical behavior behind the ALD deposited films, a better understanding can be gained on the nature of interfacial layer.
It is suggested that localized defect states caused by oxygen vacancies induced during oxide growth is behind the tunneling behavior observed in the PTDs. By controlling the oxide growth, the crystal structure can be altered in order modify the oxygen vacancy concentration and therefore improve PVCR. Therefore, a key aspect of this thesis will be to observe how morphology, realized through varying temperature of ALD growth, can affect device characteristics. Additionally, to fully classify these devices, the physics behind the electrical operation needs to be further evaluated. Mapping the properties of the various materials through experimentation and modeling will serve as the starting place for future work to come.
Finally, this thesis is part of an ongoing exploration for low-power, low-cost printed electronics. Therefore, a key aspect of this work is to present an argument for a printable process on a flexible, plastic substrate, and as such the requirement for a low temperature deposition is imperative. Low temperature ALD can come in the form of alternative precursors or alternative tunneling oxides. Moreover, by choosing to use alternative oxides, lower power NDR appearing at lower voltages may be realized. This study begins the work on finding alternative tunneling oxides that demonstrate similar oxygen vacancies observed in TiO2 films. In this case, Ta2O5 replaces TiO2 as the tunneling barrier. The initial data is promising, demonstrating a drop in the NDR voltage by approximately half compared to its TiO2 counterpart. Moreover, this work bolsters the claim that NDR is the result of a trap-based tunneling event through a defined defect band in the ALD deposited tunneling oxides. Though this thesis focuses solely on PTDs, the materials and processes demonstrated can be applied towards research interested in the conductivity properties of metal-oxides in addition to being useful for further work performed in the field of plastic electronics.