Semiconductor nanowires represent a powerful class of nanoscale building blocks for the assembly of functional devices and systems targeted for various applications ranging from electronics, photonics, energy conversion to healthcare. Significantly, high-quality crystalline nanowires can be transferred from the growth substrate to a separate noncrystalline receiving substrate such as plastic and glass. By following a unique and intellectual path of “bottom-up paradigm”, high performance devices and circuits can be subsequently fabricated with standard low temperature processes on the transferred crystalline nanomaterials, which serve as the basis for a large and rapidly growing class of transparent and flexible electronics. These nanowires-based devices offer a feasible, high-efficient approach to resolve the major challenge for the deposition and crystallization of conventional thin film semiconductors on flexible polymeric substrates.
In this thesis, we present our study on the fabrication of high performance metal oxide nanowire transistors targeted for fully transparent and flexible electronics. Focusing on zinc doped indium oxide nanowire (In2O3:Zn) transistors, we review the selection, rational synthesis, and their systematic structural, compositional and transport studies of this class of “in-situ” doped nanowires. Moreover, the fabrication of high performance, fully transparent and flexible transistors, which serve as driving electronics of organic light emitting diodes (OLEDs), is also discussed.
First of all, we briefly review the current development of transparent and flexible electronics, with focus on the device materials selection and corresponding potential application of nanowires-based devices approach. Special for this dissertation study, early exploration to find the best nanowire building blocks for the following device fabrication is presented. Through a general strategy of controlled growth called VLS mechanism, nano-scale metal oxide structures with controlled morphology can be rationally synthesized. Based on the evaluation of their transport properties via standard transistor configuration, In2O3:Zn nanowires with excellent electrical properties are finally defined as the building blocks for the subsequent studies for the fabrication of high performance, fully transparent and flexible transistors.
Secondly, we present our systematic studies on the rational synthesis, structural, compositional and transport characteristics of as-fabricated In2O3:Zn nanowires. Field-effect transistors (FETs) constructed from the In2O3:Zn nanowires exhibit excellent performance characteristics such as high mobility, high “on-state” current, and large on/off current modulation ratio. Moreover, as the first proof-of-concept demonstration, single-nanowire FETs can successfully drive an organic light-emitting diodes (OLEDs) pixel, revealing the application potential of the nanowire-based transistors in high-performance, transparent and flexible displays.
Thirdly, further optimization of the as-fabricated In2O3:Zn nanowire FETs is discussed in detail, with the emphasis on the control and utilization of the hysteresis effect. Hysteresis due to various charge chaps in nanodevices is an important issue for most potential application. In this dissertation, various measurements including changing the operating atmosphere, ozone treatment, and surface passivation were performed to understand and control the hysteresis. More significantly, by utilizing the hysteresis, the as-fabricated devices can function as nonvolatile memory elements with high charge storage stability exceeding 4 hours, which is comparable or longer than many of the reported memory devices. We propose a reasonable mechanism to explain this interesting phenomenon, for which chemical species such as water molecules adsorbed on the devices and surface oxygen defects in the amorphous layer mainly originated from Zn2+ substitution of In3+ are the main causes of the large hysteresis and nonvolatile memory effect.
Lastly, high performance, fully-transparent and flexible nanowire transistors by combining a high-quality In2O3:Zn nanowire channel, a SiNx high-κ dielectric, and conducting Sn-doped In2O3 (ITO) electrodes on a polyethylene terephthalate (PET) substrate are fabricated. The as-fabricated devices show excellent operating characteristics with high carrier mobilities, a drain-source current on/off modulation ratio, a high on-state current, a small sub-threshold gate voltage swing, and a near-zero threshold voltage. The devices further show high reproducibility and stable performance under bending condition. These high-performance nanowire transistors are envisioned to enable new application opportunities in flexible and transparent electronics.
| Date of Award | 2 Oct 2009 |
|---|
| Original language | English |
|---|
| Awarding Institution | - City University of Hong Kong
|
|---|
| Supervisor | Shuit Tong LEE (Supervisor) |
|---|